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. 2020 Jan 24;9:e47895. doi: 10.7554/eLife.47895

Damage to the right insula disrupts the perception of affective touch

Louise P Kirsch 1,2,✉, Sahba Besharati 3, Christina Papadaki 1, Laura Crucianelli 1,4, Sara Bertagnoli 5, Nick Ward 6, Valentina Moro 5, Paul M Jenkinson 7, Aikaterini Fotopoulou 1
Editors: Stephen Liberles8, Christian Büchel9
PMCID: PMC7043887  PMID: 31975686

Abstract

Specific, peripheral C-tactile afferents contribute to the perception of tactile pleasure, but the brain areas involved in their processing remain debated. We report the first human lesion study on the perception of C-tactile touch in right hemisphere stroke patients (N = 59), revealing that right posterior and anterior insula lesions reduce tactile, contralateral and ipsilateral pleasantness sensitivity, respectively. These findings corroborate previous imaging studies regarding the role of the posterior insula in the perception of affective touch. However, our findings about the crucial role of the anterior insula for ipsilateral affective touch perception open new avenues of enquiry regarding the cortical organization of this tactile system.

Research organism: Human

Introduction

Increasing evidence points to the importance of affective touch to human development and health (McGlone et al., 2014). It has been proposed that humans, like other mammals, have a specialized neurophysiological system for tactile affectivity (in particular, pleasant sensations arising from the skin; called the ‘CT system’, McGlone et al., 2014; Croy et al., 2016), separate from that for touch discrimination (Vallbo et al., 1999; Essick et al., 1999; Olausson et al., 2002). Specifically, in the peripheral nervous system, affectivity of touch has been linked to the activation of unmyelinated, mechanosensitive C-tactile fibers (CTs) that are present only in hairy skin and respond preferentially to low pressure, slow stroking touch at skin temperature (Löken et al., 2009; Ackerley et al., 2014), in opposition to fast conducting myelinated (Aβ) fibers that provide the brain fast sensory information about tactile stimulations, including their duration, texture, force, velocity and vibration (Johansson and Vallbo, 1979; Vallbo and Johansson, 1984). Microneurography studies found that CTs are velocity tuned, responding optimally to a stimulus moving over their receptive field at between 1 and 10 cm/s, with discharge frequencies that correlate with subjective ratings of stimulus pleasantness as measured psychophysically (Vallbo and Hagbarth, 1968; Nordin, 1990; Vallbo et al., 1999; Löken et al., 2009). Functional neuroimaging studies have demonstrated a functional segregation, with primary and secondary somatosensory cortices most commonly associated with discriminatory touch (Aβ mediated), while tactile pleasantness (CT mediated) is associated with other areas such as the posterior insula (Björnsdotter et al., 2009; McGlone et al., 2012; Morrison, 2016), parietal operculum, orbitofrontal cortex and superior temporal sulcus (Perini et al., 2015; Björnsdotter, 2016). C-tactile afferents have been shown to take a distinct ascending pathway from the periphery to the posterior insula (Olausson et al., 2002; Morrison et al., 2011, but see also Marshall et al., 2019), which is understood to support an early convergence of sensory and affective signals about the body that are then re-represented in the mid- and anterior insula, the proposed sites of integration of interoceptive information with other contextual information (Critchley et al., 2004; Craig, 2009; Evrard and Craig, 2015). However, these studies are correlational. Only two neuromodulatory, repetitive transcranial magnetic stimulation (rTMS) studies (Case et al., 2016; Case et al., 2017) have investigated causal relationships, finding that the right primary and secondary somatosensory cortex are not necessary for the perceived affectivity of touch. The causative role of the insular cortex, subcortical structures and white matter connections has not yet been studied, as virtual lesion methods have limited validity when applied to these deeper regions (Lenoir et al., 2018). By contrast, lesion studies allow for direct examination of the functional role of both superficial and deep brain areas.

Accordingly, we aimed to investigate for the first time the right hemisphere regions which are necessary for the perceived affectivity of CT-optimal touch, applying a voxel-based lesion symptom mapping approach (VLSM; Bates et al., 2003) in a large, consecutively recruited cohort of patients (N = 59) with recent, first-ever, right hemisphere lesions following a stroke. Contrary to other neuropsychological approaches that employ diagnostic, group comparisons, the VLSM method uses continuous measures in a single sample, and identifies which regions of the brain are crucial to a specific behavior (e.g. here CT pleasantness perception), without assuming that all patients show the same tactile profile. The selection of right-hemisphere patients restricts any laterality interpretations, but it also avoids the possibly confounding sequelae of left hemisphere lesions, such as language and depression symptoms (Robinson et al., 1984; Whyte and Mulsant, 2002).

We used a previously validated tactile stimulation paradigm (Crucianelli et al., 2013; Crucianelli et al., 2018; Gentsch et al., 2015; Mohr et al., 2017; Kirsch et al., 2018), together with standardized neuropsychological, somatosensory and mood assessments. Our affective touch paradigm required blindfolded patients (N = 59, RH) and age-matched healthy controls (N = 20, HC), to rate the intensity and pleasantness of brushing stimuli delivered at velocities known to activate the CT-system optimally (3 cm/s; CT-optimal affective touch) or not (18 cm/s; CT-suboptimal neutral touch) to both the left (contralesional) and the right (ipsilesional) forearm (Vallbo et al., 1999; Löken et al., 2009). This touch on the forearm stimulates both Aβ and CT fibers; one cannot stimulate one type of fiber without stimulating the other simultaneously (except in patients without Aβ afferents, as studied by Olausson et al., 2002; Olausson et al., 2008). However, our paradigm is optimized to stimulate CT fibers differentially based on velocity, and the resulting difference in pleasantness, that is CT pleasantness sensitivity, is assumed to be at least partly linked to the differential involvement of these CT fibers (even if not restricted to it). Specifically, Aβ fiber activation is known to linearly increase with increases in velocity, while the mean frequency firing rate of CT fibers follows an inverted U shape with higher firing being in the 1–10 cm/s range, and have been shown to be the only unit types for which firing patterns correlate with average psychophysiological ratings, that is pleasantness (Löken et al., 2009). In addition to the affective touch paradigm, to control for general pleasantness deficits (specific to touch), participants had to imagine being touched by pleasant (i.e. velvet) and unpleasant (i.e. sandpaper) materials and rate the associated pleasantness.

Given that right hemisphere and particularly right perisylvian regions have been previously associated with somatosensory and interoceptive perception (Dijkerman and de Haan, 2007; Preusser et al., 2015), we expected our patients to have, on average, reduced ratings of both touch intensity and pleasantness in comparison to healthy controls, and particularly in the contralesional left forearm. An overall reduced tactile pleasantness in patients (both in actual touch and imagined touch pleasantness ratings) would suggest tactile anhedonia linked to general right hemisphere lesions. Crucially, given the assumed neurophysiological specificity of the CT system, we expected that more specific lesions to the posterior insula (Morrison, 2016) would reduce the affective sensitivity of these patients to CT-optimal touch, over and above general effects of anhedonia, tactile acuity and other neuropsychological deficits caused by the broader lesion profile of our whole sample. In other words, an intact posterior insula should be necessary for the added affective sensitivity that the CT fibers are conveying during touch optimally activating the CT system versus an identical touch and social context that does not activate this afferent pathway optimally. Moreover, this would give further substance to the hypothesis that the CT afferent pathway is a specialized system that allows individuals to distinguish a range of velocities that are likely to have social-affective relevance, for the purposes of further integration with sensory and affective information in the insula (Olausson et al., 2008; see Morrison et al., 2010 for discussion).

Results and discussion

In the present study, we used a previously validated affective touch protocol in stroke patients to investigate, for the first time, the right hemisphere regions which are necessary for the perceived affectivity of CT-optimal touch, applying a voxel-based lesion symptom mapping approach.

First, we investigated the effect of right hemisphere lesions on the perception of touch intensity and pleasantness, on the contralesional and ipsilesional forearm separately, by comparing stroke patients’ and healthy controls’ intensity and pleasantness ratings in turn. In line with the high percentage of contralesional tactile deficits in right hemisphere stroke patients (including in our patients’ sample, see Materials and methods), patients, as compared to healthy controls, perceived touch, regardless of velocity, as less intense on the contralesional forearm (contralesional: F(1,57)=55.918, p<0.001, ηp2=.495; BF10 = 1.480*107; ipsilesional: F(1,38)=0.834, p=0.367, ηp2 = .021, BF10 = 0.759; see Figure 1A and B). Most interestingly, we observed a main effect of stroking type on pleasantness ratings, with both patients and controls rating CT-optimal affective touch as more pleasant than CT-suboptimal neutral touch on both forearms (contralesional: F(1,53)=22.444, p<0.001, ηp2 = .297, BF10 = 3526.340; ipsilesional: F(1,59)=11.519, p=0.001, ηp2 = .163, BF10 = 38.833; Figure 1C and D). Moreover, patients perceived touch as less pleasant than controls on both forearms (contralesional: F(1,53)=14.074, p<0.001, ηp2=.210, BF10 = 62.636; ipsilesional: F(1,59)=7.100, p=0.010, ηp2=.107, BF10 = 4.992; Figure 1C and D). This was also the case when considering only patients that had intact tactile perception on the contralesional forearm (i.e. could feel all the touch trials; N = 25, F(1,43)=9.880, p=0.003, ηp2 = .187, Figure 1—figure supplement 1; see Materials and methods section for details). A similar general tactile anhedonia (reduced pleasantness ratings) was observed in our patients as compared to the controls for imagined tactile pleasantness, when patients had to rate how pleasant it would be to be touched by pleasant and unpleasant fabric (F(1,70) = 22.348, p<0.001, ηp2=.242, BF10 = 550.118, Figure 1—figure supplement 2). However, no interaction between touch type and group was found (contralesional: F(1,53)=0.393, p=0.533, ηp2 = .007, BF10 = 0.371, Figure 1C; ipsilesional: F(1,59)=0.073, p=0.788, ηp2 = .001, BF10 = 0.287, Figure 1D; imagined tactile pleasantness: F(1,70)=.061, p=0.806, ηp2=.001, BF10 = 0.270), suggesting that right hemisphere lesions in general do not necessarily lead to reduced CT pleasantness sensitivity, and confirming that any differential deficits in the pleasantness perception of CT-optimal versus CT-suboptimal touches at the individual level would relate to specific lesions rather than general stroke effects.

Figure 1. Behavioural Results.

(A) Average intensity ratings on the contralesional left forearm (NRH = 39, NHC = 20), (B) Average intensity ratings on the ipsilesional right forearm (NRH = 20, NHC = 20), (C) Average pleasantness ratings on the contralesional left forearm (NRH = 35, NHC = 20), (D) Average pleasantness ratings on the ipsilesional right forearm (NRH = 41, NHC = 20), for CT-optimal and CT suboptimal touch. Stroke patients (RH) are depicted in dark gray, Healthy controls (HC) in light gray. Error bars represent the standard error of the mean. *depicts significant comparison, p<0.05.

Figure 1.

Figure 1—figure supplement 1. Average pleasantness ratings on the contralesional left forearm for patients with intact tactile perception in dark gray (NRH = 25).

Figure 1—figure supplement 1.

Results for healthy controls are the same as presented in Figure 1. C, in light gray (NHC = 20). Error bars represent the standard error of the mean. Average pleasantness ratings on the contralesional left forearm for patients with intact tactile perception in dark gray (NRH = 25). Results for healthy controls are the same as presented in Figure 1. C, in light gray (NHC = 20). Error bars represent the standard error of the mean.

Figure 1—figure supplement 2. Average pleasantness ratings for imaginary touch.

Figure 1—figure supplement 2.

Patients rated how pleasant it would be to be touched by a typically pleasant material (i.e. velvet) and a typically unpleasant fabric (i.e. sandpaper). Error bars represent the standard error of the mean.
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The present study aimed to investigate the lesion patterns and neuropsychological deficits that may be associated with the inability of certain stroke patients to distinguish the pleasantness of CT-optimal versus CT-suboptimal touches. Accordingly, CT pleasantness sensitivity was calculated as the difference between the pleasantness of CT-optimal and CT-suboptimal touches. As a convention, CT pleasantness sensitivity inferior or equal to zero is considered as low in CT pleasantness sensitivity (i.e. low CT affective touch perception; Crucianelli et al., 2018). Interestingly, none of the patients’ demographic characteristics or, neuropsychological deficits correlated significantly with their CT pleasantness sensitivity, including education, anxiety and depression scores, as well as memory as measured by the MOCA memory subscale, and working memory as measured by the Digit Span (all p>0.1 and all BF10 <1). Thus, low CT pleasantness sensitivity was not explained by other general cognitive and emotional deficits, as assessed in the present study. Moreover, there was no correlation between CT pleasantness sensitivities and tactile anhedonia on either forearm (as measure by the difference between the imagined pleasantness of pleasant and unpleasant material; r31 = -.104, p=0.578, BF10 = 0.259 for the contralesional forearm; r36 = -.086, p=0.618, BF10 = 0.234, for the ipsilesional forearm), nor with tactile acuity as measured by intensity ratings.

A VLSM analysis with CT pleasantness sensitivity on the contralesional forearm (differential pleasantness scores) as continuous predictor, controlling for lesions size, with a 0.01 FDR-corrected threshold, and considering only regions lesioned in at least 10 patients, revealed specific lesions in the rolandic operculum (see Figure 2A, Figure 2—figure supplement 1A, and Table 1A). Subcortically, the tracts of the superior corona radiata were involved. Importantly, running the same analysis including only patients without sensory deficit on the left forearm (i.e. participants that rated all the trials as more intense than 2; N = 25) involves the same area but also extends to the posterior part of the insula (see Figure 2B, Figure 2—figure supplement 1B and Table 1B). This corroborates the importance of the posterior insula and the rolandic operculum in perceiving CT-optimal touch on the contralateral forearm as more pleasant than CT-suboptimal touch, particularly when other tactile pathways are intact.

Figure 2. Lesions associated with decreased CT pleasantness sensitivity.

(A) Lesions associated with decreased CT pleasantness sensitivity on the contralesional left forearm, in all patients (N = 35). (B) Lesions associated with decreased CT pleasantness sensitivity on the contralesional left forearm, only in patients without sensory deficit on the left (N = 25). (C) Lesions associated with decreased CT pleasantness sensitivity on the ipsilesional right forearm (N = 41). The numbers above the brain slices indicate the corresponding MNI axial coordinates. L = Left; R = Right; The second row represents heat maps of the voxels with power enough to detect significant results, at α = 0.01, FDR-corrected. Different colors represent the area under the ROC curve (AUROC) scores, ranging from 0.2 to 0.6.

Figure 2.

Figure 2—figure supplement 1. Lesions Overlaps.

Figure 2—figure supplement 1.

(A) Lesions overlap map for patients with negative CT pleasantness sensitivity on the left contralesional forearm, among all patients (N = 10). Out of the 10 patients that showed a negative CT pleasantness sensitivity on the contralesional forearm (among all patients, N = 35), 8 of them had a lesion to the rolandic operculum cluster (X = 48, Y=-9, Z = 15); and for the two remaining patients, one had a more focal deep lesions (amygdala, putamen, thalamus), that could still be on the posterior insula track; and the other had an insula lesion but more frontal. (B) Lesions overlap map for patients with negative CT pleasantness sensitivity on the left contralesional forearm, only in patients without sensory deficit (N = 6). When taking into account only patients without sensory deficit, out of the six patients that showed a negative CT pleasantness sensitivity on the contralesional forearm (among patients without sensory deficit, N = 25), five had a lesion to the posterior insula cluster (X = 38, Y=-12, Z = 12), and the other had a more focal deep lesion (amygdala, putamen, thalamus) (C) Lesions overlap map for patients with negative CT pleasantness sensitivity on the right ipsilesional forearm (N = 18). Out of the 18 patients with a negative CT pleasantness sensitivity on the ipsilesional forearm, 15 had a lesion of the anterior insula cluster (X = 45, Y = 3, Z = 8); and the three remaining had a lesion to the insula, but not on that specific cluster. An overlay heat map of participants’ lesions was calculated from all lesions and superimposed on the chi2bet template brain using MRICron (Rorden et al., 2007).
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Table 1. Number of significant voxels (from the atlas of gray matter – AAL – and white matter – JHU – and NatBrainLab’s atlas) resulting from the VLSM analyses.

A. with the CT pleasantness sensitivity scores for the contralesional left forearm as predictor, in all patients (N = 35); B. with the CT pleasantness sensitivity scores for the contralesional left forearm as predictor, only in patients without sensory deficit, N = 25; C. with the CT pleasantness sensitivity scores for the ipsilesional right forearm as predictor (N = 41).

A. Lesions associated with decreased CT pleasantness sensitivity on the contralesional left forearm, in all patients (N = 35)
Region NVoxels X Y Z T-value
AAL Unclassified 104 43 1 19 2.88
Rolandic_Oper 63 48 -9 15 2.59
JHU Unclassified 120 43 1 19 2.88
Superior_corona_radiata 45 24 8 30 2.59
NatBrainLab Unclassified 69 43 1 19 2.88
Arcuate_Anterior_Segment 72 48 -9 15 2.59
Corpus_Callosum 11 22 7 28 2.56
Internal_Capsule 15 25 5 27 2.56
B. Lesions associated with decreased CT pleasantness sensitivity on the contralesional left forearm, only in patients without sensory deficit (N = 25)
Region NVoxels X Y Z T-value
AAL Unclassified 446 33 16 -4 3.08
Frontal_Inf_Oper 8 49 9 6 2.55
Frontal_Inf_Orb 8 35 25 -8 2.77
Rolandic_Oper 88 37 -4 20 2.57
Insula 598 38 −12 12 3.06
Putamen 118 33 -4 8 3.27
Heschl 24 44 −17 8 2.65
JHU Unclassified 1254 33 -4 8 3.27
Superior_corona_radiata 8 26 8 24 2.57
External_capsule 22 33 -5 7 3.06
Superior_longitudinal_fasciculus 6 32 -6 24 2.57
NatBrainLab Unclassified 1277 33 -4 8 3.27
Arcuate_Anterior_Segment 11 37 -5 21 2.57
Inferior_Occipito_Frontal_Fasciculus 1 37 2 -8 2.54
Internal_Capsule 1 26 8 24 2.57
C. Lesions associated with decreased CT pleasantness sensitivity on the ipsilesional right forearm (N = 41)
Region NVoxels X Y Z T-value
AAL Frontal_Inf_Oper 59 42 9 9 2.76
Rolandic_Oper 79 45 4 9 2.76
Insula 32 45 3 8 2.70
JHU Unclassified 170 45 4 9 2.76
NatBrainLab Unclassified 170 45 4 9 2.76
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In contrast, deficits in CT pleasantness sensitivity on the ipsilesional forearm were associated with lesioned voxels in the anterior part of the insula (including the adjacent regions, rolandic and frontal inferior operculum – see Figure 2C, Figure 2—figure supplement 1C and Table 1C). As patients’ perception of the discriminatory, emotionally-neutral aspects of touch on the ipsilesional forearm was not affected (verified by the lack of difference in intensity ratings between healthy controls and patients, as well as patients’ performance on a standardized somatosensory assessment; see Materials and methods, and Figure 1B), and as the left insula and somatosensory cortex of these patients were intact, these results suggest that the right anterior insula has a necessary role in the CT pleasantness sensitivity, even for the ipsilateral side of the body.

Additionally, as a control for a general pleasantness deficit, patients rated how pleasant it would be to be touched by a typically pleasant material and a typically unpleasant fabric. As done for CT pleasantness sensitivity, imagined tactile pleasantness sensitivity was computed as the difference between pleasant and unpleasant materials pleasantness ratings, for each patient. We considered the same patients as for the CT pleasantness sensitivity VLSM analysis (N = 36 as we had missing data for 5 of them), and ran a VLSM analysis with this top-down tactile pleasantness sensitivity as predictor. This yielded significant voxels subcortically in the caudate, thalamus, putamen and pallidum, but crucially, not the insula, suggesting that the above results are specific to applied tactile stimuli and not more general pleasantness comparisons (see Supplementary file 1).

This lesion study aimed to investigate deficits in the perceived affectivity of CT-optimal touch. Our results suggest a causal role of the posterior contralateral opercular-insular cortex for the perception of CT-optimal touch as more pleasant than CT-suboptimal touch, offering support to previous, correlational, functional neuroimaging findings on the CT system (Olausson et al., 2002; Morrison, 2016). In addition, our findings reveal that the right anterior fronto-insular junction is necessary to perceive the pleasantness of CT-optimal touch as more pleasant than CT-suboptimal touch on the ipsilateral forearm. Thus, even when the left insula and somatosensory cortex are intact and hence presumably contralateral stimuli are processed in the left cortex (as also revealed by the intact detection of ipsilesional tactile stimuli in our patients), a right anterior insula lesion is enough to cause deficits in the perception of affective touch on the right forearm.

The present study has considered CT pleasantness sensitivity as the difference between the pleasantness of CT-optimal slow touch (3 cm/s) and CT-suboptimal fast touch (18 cm/s). Future studies should investigate whether the present findings replicate when using very slow touch instead of fast touch as CT-suboptimal touch, as very slow touch (<1 cm/s) also leads to suboptimal activation of the CT fibers (Löken et al., 2009). Moreover, the specificity of the present findings to CT fibers should be further investigated by comparing tactile stimulation on hairy (e.g. forearm) vs. glabrous skin (e.g. palm, that do not contain any CT fibers).

Taken together, our findings support previous findings about the functional organization and role of the human insula (Craig, 2010; Cauda et al., 2011; Kurth et al., 2010; Heydrich and Blanke, 2013; Ronchi et al., 2015; Salomon et al., 2018); see review by Evrard (2019), on recent findings on the organization of the insula in non-human primates), consisting of specialized substrates organized in a posterior to anterior structural progression, with posterior parts representing the primary cortical representations of interoceptive stimuli from contralateral body parts and more anterior parts, tested here in the right hemisphere, acting as integration areas for sensory signals and contextual cues ultimately leading to interoception. Indeed, present findings are consistent with the growing evidence considering CT-afferents as sharing more characteristics with interoceptive (i.e. related to the sense of the physiological condition of one's own body; Ceunen et al., 2016), rather than exteroceptive, modalities (Björnsdotter et al., 2010), in light of their contribution to the maintenance of our sense of self (Crucianelli et al., 2018). Moreover, our findings address existing debates about hemispheric laterality and interoception, with a right-hemisphere dominance in interoceptive integration of both contra- and ipsilateral signals (Kann et al., 2016; Khalsa et al., 2009; Salomon et al., 2016; Garfinkel and Critchley, 2013), although the VLSM method has known intrinsic limitations, and we cannot exclude the possible role of the left insula in affective touch perception, nor the impact of lesions of the right hemisphere in disconnecting tracts towards the left hemisphere. Furthermore, as VLSM methods preclude direct comparison between CT pleasantness sensitivity deficits on the contralesional and ipsilesional forearm at the brain level, future studies should investigate further the posterior-anterior insula segregation in relation to affective touch as an interoceptive modality.

Materials and methods

Subjects and clinical investigation

Fifty-nine, unilateral, right-hemisphere-lesioned stroke patients (mean age: 65.86 ± 14.12 years; age range: 38–88 years; 31 females) were recruited from consecutive admissions to seven stroke wards as part of a larger study using the following inclusion criteria: (i) imaging-confirmed first ever right hemisphere lesion; (ii) contralateral hemiplegia; (iii) < 4 months from symptom onset; (iv) no previous history of neurological or psychiatric illness; (v) > 7 years of education; (vi) no medication with significant cognitive or mood side-effects (e.g. pregabalin, lamotrigine); (vii) no language impairments that precluded completion of the study assessments; and (viii) right handed. All participants gave written, informed consent to take part in the study. The local National Health System Ethics Committees approved the study, which was carried out in accordance to the Declaration of Helsinki.

All patients underwent a thorough neurological and neuropsychological examination. Premorbid intelligence was assessed using the Wechsler Test of Adult reading (WTAR; Wechsler, 2001). Post-morbid, general cognitive functioning, including long-term verbal recall was assessed using the Montreal Cognitive Assessment (MoCA; Nasreddine et al., 2005). The Medical Research Council scale (MRC; Saunders, 1986) was used to assess limb motor strength. Proprioception was assessed with eyes closed by applying small, vertical, controlled movements to three joints (middle finger, wrist and elbow), at four time intervals (correct = 1; incorrect = 0; Vocat et al., 2010). Working memory was assessed using the digit span task from the Wechsler Adult Intelligence Scale III (Wechsler, 1997). The Hospital Depression and Anxiety Scale (HADS; Zigmond and Snaith, 1983) was used to assess anxiety and depression. Executive and reasoning abilities were assessed using the Frontal Assessment Battery (FAB; Dubois et al., 2000). Four subtests of the Behavioural Inattention Test (BIT; Wilson et al., 1987) were used to assess visuospatial neglect. Personal neglect was assessed using the ‘one item test’ (Bisiach et al., 1986) and the ‘comb/razor’ test (McIntosh et al., 2000).

Twenty age-matched healthy control subjects were recruited and tested with the same behavioural paradigm in order to assess the specificity of deficits in the patient group (healthy control group; 63.05 ± 12.12 years; age range: 46–87 years; 11 females). Patients’ demographic characteristics and their performance on standardized neuropsychological tests and how they compared to the healthy sample are summarized in Table 2.

Table 2. Summary of demographics and neuropsychological data.

Description: Nottingham = Light Touch subscale of the Revised Nottingham Sensory Assessment (rNSA; Lincoln et al., 1998; score overall for each arm with 0: no sensation; 1: slightly impaired; 2: no deficit); MRC = Medical Research Council scale (Saunders, 1986); MOCA = The Montreal Cognitive Assessment (Nasreddine et al., 2005); FAB = Frontal Assessment Battery (Dubois et al., 2000); Premorbid IQ-WTAR = Wechsler Test of Adult Reading (Wechsler, 2001); HADS = Hospital Anxiety and Depression scale (Zigmond and Snaith, 1983); Comb/razor test = tests of personal neglect (McIntosh et al., 2000); Bisiach one item test = test of personal neglect; line crossing, star cancellation, copy and representational drawing = conventional sub-tests of Behavioural Inattention Test (Wilson et al., 1987). Dashed line indicates not applicable. Due to several clinical constraints (e.g. fatigue, acceptance and time constraints), we have a number of missing data on these tests. Specific numbers are indicated in the right column. NRH = number of right hemisphere stroke patients having fully completed the corresponding test. NHC = number of healthy controls having fully completed the corresponding test. * Significant difference between groups, p<0.05.

Stroke Patients –RH (N = 59; 31 females) Healthy Controls - HC (N = 20, 11 females) Mann-Whitney Test NRH/NHC
Mean SD Mean SD
Age (years) 65.86 13.87 63.05 12.12 U(78)=514.00, Z = -.857, p=0.391 N = 59/20
Education (years) 11.40 2.87 14.75 2.82 U(70)=211.50, Z = −3.906, p<0.001* N = 52/20
Days from onset 16.95 18.68 - -
Orientation 2.80 0.41 - -
Nottingham on left arm (max 2) 0.66 0.78 - -
Nottingham on right arm (max 2) 2 0 - -
Proprioception (max 9) 5.10 2.64 - -
MRC Left upper limb 0.30 0.75 - -
Digit span forwards 5.95 1.40 6.58 1.83 U(66)=279.50, Z = 0.936, p=0.349 N = 56/12
Digit span backwards 3.50 1.55 4.75 1.28 U(66)=177.00, Z = −2.621, p=0.009* N = 56/12
MOCA 19.85 5.18 28.19 1.92 U(45)=5.50, Z = −4.271, p<0.001* N = 39/8
MOCA memory subscale 2.92 1.78 4.00 1.60 U(45)=95.00, Z = −1.769, p=0.077 N = 39/8
Premorbid IQ-WTAR 34.00 9.35 49.11 1.69 U(25)=3.00, Z = −4.037, p<0.001* N = 18/9
HADS depression 5.75 3.49 3.13 2.19 U(50)=150.00, Z = −2.593, p=0.010* N = 37/18
HADS anxiety 8.02 4.33 6.06 3.01 U(50)=208.00, Z = −1.409, p=0.159 N = 37/18
FAB total score 11.38 4.02 - -
Comb/razor test bias (%bias) −23.37 27.06 - -
Bisiach one item test 0.47 0.68 - -
Line crossing (max 36) 22.56 11.85 - -
Star cancelation (max 54) 29.93 18.23 - -
Copy 0.87 1.20 - -
Representational drawing 0.62 0.93 - -
Line bisection 2.87 3.05 - -
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Design and Predictions

The present study aimed to investigate the neuroanatomical bases of affective touch. To this aim, we compared a large cohort of right hemisphere stroke patients to healthy controls, and explored how deficits in affective touch perception are linked with specific brain lesions. We applied an affective touch paradigm that manipulated three factors: i) the velocity of the touch applied (slow, CT-optimal, affective touch vs. fast, CT-suboptimal, neutral touch); ii) the forearm the touch was applied to (right, ipsilesional vs. left, contralesional); iii) and the group of participant (Stroke patients vs. Healthy controls). For each type of touch we recorded two measures: 1) intensity ratings and 2) pleasantness ratings. We also asked participants to rate the pleasantness of imagined touch with either a smooth material (velvet) versus a rough material (sandpaper), to control for top-down effects; and general tactile anhedonia due to right hemisphere stroke.

To investigate the neuroanatomical bases of affective touch, we conducted two main voxel-based, lesion-symptom mapping analyses, separately for each forearm, using as predictors the CT pleasantness sensitivity (difference between average pleasantness ratings for CT-optimal touch and CT-suboptimal touch). In addition to the main analyses we also ran a control analysis, using the difference between imagined pleasantness ratings of pleasant (velvet) and unpleasant (sandpaper) material as predictors, to control for potential top-down affective deficit. Finally, a lesion overlap was calculated to create a color-coded overlay map of lesioned voxels across participants with negative or null CT pleasantness sensitivity on each forearm.

Given our patients’ lesions to several perisylvian regions of the right hemisphere previously associated with somatosensory perception (Dijkerman and de Haan, 2007; Preusser et al., 2015), we expected that our patients would have, on average, reduced ratings of both touch intensity and pleasantness in comparison to healthy controls, and specifically in the contralesional left forearm. However, we did not expect a general right stroke effect on pleasantness sensitivity to CT affective touch (defined as the pleasantness difference between CT-optimal and CT-suboptimal velocities), given the assumed neurophysiological specificity of the CT system. Instead, we expected that lesions involving mainly the right posterior insula (Morrison, 2016) would lead to a lack of CT pleasantness sensitivity, particularly on the contralesional forearm. Moreover, as some authors have proposed that the right hemisphere, and particularly the right anterior insula, has a crucial role in interoceptive awareness for the entire body (Craig, 2009; Critchley et al., 2004; Kann et al., 2016; Khalsa et al., 2009; Salomon et al., 2016), we expected also to find some causal role of ipsilateral areas (right hemisphere regions after touch on the right forearm) and particularly the right anterior insula in the perception of affective touch on the ipsilesional forearm.

Affective touch protocol

Tactile stimulation followed a previously validated protocol (Crucianelli et al., 2013; Crucianelli et al., 2018; Gentsch et al., 2015; Mohr et al., 2017; Kirsch et al., 2018), including both ‘imagined’ and actual touch questions. Specifically, first a 9 cm x 4 cm area of skin stimulation was marked on both forearms and patients were familiarized with the vertical rating scales (to minimize the effects of neglect; we also always ensured the participants could see the scale and read it aloud to facilitate them), anchored at ‘0 - not at all’ and ‘10 - extremely’. We first sampled top-down, prior beliefs about tactile pleasantness by asking two hypothetical questions about imagined touch: ‘How pleasant would it be to be touched by velvet on your skin’ (typically pleasant) and ‘How pleasant would it be to be touched by sandpaper on your skin?’ (typically unpleasant). Participants were asked to answer using the vertical 0 to 10 pleasantness scale. No other instruction was given to the participants (neither speed nor pressure of the imagined touch).

We then explained that actual tactile stimuli would be delivered on the marked forearm areas, while participants were blindfolded, and instructed to remain still and to focus on both the intensity and pleasantness of the touch they were experiencing (Figure 3). Tactile stimuli were administrated by a 4 cm wide soft make up brush made from natural hair (Natural hair Blush Brush, No. 7, The Boots Company). Brush strokes were administered by a trained female experimenter in proximal-to-distal direction with the brush held in a perpendicular position, with the edges of the brush tracking the width of the testing area to control for pressure. Every touch condition lasted for 3 s; with an inter-stimuli interval of at least 30 s. After each touch, participants were asked to answer two questions: first ‘How well did you feel the touch?’ (i.e. touch intensity rating), and if they felt the touch (i.e. reporting an intensity rating >0), they were asked ‘How pleasant was the touch?’ (i.e. touch pleasantness rating), using the above described 0 to 10 vertical scale. Tactile stimuli were delivered at two different velocities on the participant’s left and right forearm: CT-optimal speed (3 cm/s, known to activate CT fibers optimally; one stroke over the 9 cm long area) and CT-suboptimal speed (18 cm/s, known to activate CT fibers to a lesser degree, suboptimally; Gentsch et al., 2015; six strokes). Each condition was repeated 6 times, leading to a total of 24 trials – delivered in a pseudorandomized order. The experiment was split into three blocks to avoid fatigue; short breaks were taken after a set of 8 trials (2 repetitions of each condition in each block).

Figure 3. Experimental design and timeline.

Figure 3.

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1. Participants were first asked to answer two hypothetical questions about imagined touch: ‘How pleasant would it be to be touched by velvet on your skin’ (typically pleasant) and ‘How pleasant would it be to be touched by sandpaper on your skin?’ (typically unpleasant). Participants were asked to answer using the vertical 0 to 10 pleasantness scale. 2. Participants were then asked to put on a blindfold at the onset of each trial before the experimenter delivered the touch on the left or right forearm at CT-optimal (CT touch) or CT-suboptimal velocities (nonCT touch; pseudorandomized), each touch lasted for 3 s and was repeated twice with a one second break in between. After each touch, blindfold was removed so participants could rate the touch on two scales: Intensity = How well they felt the touch; and Pleasantness = How pleasant was the touch, each on a vertical scale ranging from 0, not at all, to 10, extremely. After ratings were recorded, the participant was asked to put the blindfold back before starting the next trial.

All patients had intact sensation on the right ipsilesional forearm (i.e. rated the intensity of tactile stimuli as greater than zero in all the trials, irrespective of velocity, and had intact sensation on this side according to a standardized assessment; the Revised Nottingham Sensory Assessment [rNSA; Lincoln et al., 1998]) but as predicted, on the contralesional side, some patients (40.7%, N = 24) were not able to perceive the tactile stimuli (corroborated also by the above standardized somatosensory assessment), and therefore gave a rating of zero on the intensity scale, and were not asked to provide pleasantness ratings. Thus, pleasantness ratings were available only from the remaining 35 patients who were able to perceive the intensity of most contralesional tactile stimuli in our paradigm.

Behavioural data analysis

We investigated the effect of right hemisphere lesions on the perception of touch intensity and pleasantness, on the contralesional and ipsilesional forearm separately, by comparing stroke patients and healthy controls intensity and pleasantness ratings in turn. As the data were normally distributed, separate ANOVAs were run with touch type (CT-optimal vs. CT-suboptimal) and group (stroke patient vs. healthy controls) as factors, for each rating type and each forearm. An additional ANOVA comparing stroke patients and healthy controls was conducted for the imagined tactile pleasantness ratings (velvet vs. sandpaper).

We were able to collect contralesional touch intensity ratings on only 39 out of the total sample of 59 patients due to an administrative error (the experimenter took binary, ‘yes’ or ‘no’ responses to the tactile stimuli instead of using the rating scale in the remaining patients). For the same reason, we only had ipsilesional touch intensity ratings for CT-optimal touch on 36 and CT-suboptimal touch on 20 patients. This unfortunately meant that our sample was reduced to 39 patients for the analyses of intensity ratings on the contralesional forearm and of 20 patients for the ipsilesional forearm.

We were able to record pleasantness ratings for contralesional forearm touch on 35 and 39 patients for CT-optimal and CT-suboptimal touch velocities respectively (data of 21 and 13 patients were missing due to the fact that patients did not feel the touch and gave an intensity rating of 0; the remaining 3 and 8 missing data were due to an administrative error). For the right ipsilesional forearm, pleasantness ratings of 56 and 41 patients were recorded at CT-optimal and CT-suboptimal touch velocities respectively. Thus, the sample of the analysis of touch pleasantness was of 35 patients for the contralesional forearm and of 41 patients for the ipsilesional forearm.

Moreover, as supplementary analyses, we also considered patients with intact tactile perception on the contralesional forearm. For these analyses, only patients that gave intensity ratings above two were included (N = 25).

We used both frequentist and Bayesian statistics to assess the observed effects, depending on the aim and hypothesis in each case. The complementary use of these two statistical approaches is recommended by a number of authors to facilitate a fuller understanding of the data (see e.g. Dienes and Mclatchie, 2018; Dienes, 2014; Jarosz and Wiley, 2014; Howard et al., 2000). Bayesian statistics were performed in order to allow further interpretation of the observed effects, in particular, the extent to which data provided support for the alternative versus null hypotheses. Bayes Factors (BF10) provide a continuous measure that indicates the relative strength for the null versus alternative hypotheses (i.e. the number of times more likely the data are under the alternative than the null hypothesis), and were used as a means of interpreting evidence for each hypothesis, using benchmarks provided by Jeffreys (1939). We interpreted a BF10 >3 as substantial evidence for the alternative hypothesis, a BF10 <0.3 as substantial evidence in favour of the null hypothesis, and 0.3 < BF10<1 as anecdotal evidence in favour of the null hypothesis (see Dienes, 2014). Bayes Factor were computed using JASP (JASP Team, 2019). JASP (Version 0.10).

Lesion mapping methods and analyses

Routinely acquired clinical scans obtained on admission (<2 days post stroke) were collected for the 59 patients (49 via computerized tomography, CT; and 10 via magnetic resonance imaging, MRI). We note that testing patients in the acute post-stroke phase entails challenges but avoids any confounds relating to plasticity and functional reorganization (Baier et al., 2014; de Haan and Karnath, 2018). The patient’s lesion was mapped by means of the MRIcron software (Rorden and Brett, 2000) on the standard T1-weighted MRI template (ICBM152) of the Montreal Neurological Institute (MNI) coordinate system. Lesions from these scans were segmented and co-registered using a manual procedure, as this method remains the best methods to date for lesion mapping of clinical scans and shown to be more accurate than automatized methods (Maier et al., 2015; de Haan and Karnath, 2018; Liew et al., 2018). Two expert clinicians, blind to the hypotheses of the study, outlined the lesions. In the case of disagreement of two lesion plots, the opinion of a third, expert anatomist was requested. Scans were registered to the T1-weighted MRI scan template (ICBM152) of the Montreal Neurological Institute, furnished with the MRIcron software (ch2, http://www. cabiatl.com/mricro/mricron/index.html). First, the standard template (size: 181 × 217×181 mm, voxel resolution: 1 mm2) was rotated on the three planes in order to match the orientation of the patient's MRI or CT scan. Lesions were outlined on the axial slices of the rotated template. The resulting lesion volumes were then rotated back into the canonical orientation, in order to align the lesion volumes of each patient to the same stereotaxic space. Finally, in order to exclude voxels of lesions outside white and gray matter brain tissue, lesion volumes were filtered by means of custom masks based on the ICBM152 template.

The statistical contribution of lesion location to CT pleasantness sensitivity and imagined tactile pleasantness deficits was tested using voxel-based lesion symptom mapping (VLSM), using the behavioral scores as continuous predictor. The statistical process performed in voxel-based lesion–symptom mapping (Bates et al., 2003) consists of the following steps: at each voxel of the spatially standardized scan images, patients are divided into two groups according to whether they did or did not have a lesion affecting that voxel. Behavioral scores are then compared for these two groups with a t-test, yielding a single-tailed p-value for each voxel. Normal t-tests were used as the behavioural data entered in the VLSM models were normally distributed (de Haan and Karnath, 2018). This method allows controlling for lesion size, which is included as a covariate of non-interest. Note that to avoid spurious results due to low numbers of lesioned voxels, only voxels lesioned in at least 10 participants were tested. This results in color-coded VLSM maps that represent voxels where patients with lesions show a significantly different behavioral score from those whose lesions spared that voxel at an α level of 0.01 after correction for multiple comparisons using the false discovery rate (Curran-Everett, 2000). Software to perform VLSM (Bates et al., 2003; https://aphasialab.org/vlsm/) was run using MATLAB R2016b (Mathworks, Inc). It is to note, that in accordance with recent recommendation by de Haan and Karnath (2018), as no correlations were found between CT pleasantness sensitivity (or pleasantness ratings) and any of the neuropsychological scores that differed between healthy controls and right hemisphere stroke patients (HADS Depression scale, Digit Span backward, MOCA memory scale, and Premorbid IQ-WTAR), none of these variables could be considered as nuisance variables and were not considered in the VLSM lesion analyses.

Each analysis was conducted separately for the contra- and the ipsilesional forearm, and only regions of more than 10 voxels that passed the set 0.01 FDR-corrected threshold were considered in the discussion. VLSM results were visualized in MRIcron. Three anatomical templates served to identify gray and white matter region labels: the ‘automated anatomical labelling’ (AAL) template (Tzourio-Mazoyer et al., 2002), the JHU white-matter tractography atlas, (Mori et al., 2005), and the ‘NatBrainLab’ template of the ‘tractography based Atlas of human brain connections Projection Network’ (Natbrainlab, Neuroanatomy and Tractography Laboratory; Catani and de Schotten, 2012; Thiebaut de Schotten et al., 2011).

Acknowledgements

We thank the all the stroke patients for their kindness and willingness to take part in the study, as well as the healthy participants. We are also particularly grateful to Sonia Ponzo, Amanda Hornsby, and Arturo Kerbel, for their help with patient recruitment and testing.

Funding Statement

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Contributor Information

Louise P Kirsch, Email: kirsch.lou@gmail.com.

Stephen Liberles, Harvard Medical School, United States.

Christian Büchel, University Medical Center Hamburg-Eppendorf, Germany.

Funding Information

This paper was supported by the following grants:

  • European Research Council ERC-2012-STG GA313755 to Aikaterini Fotopoulou.

  • Ministry of Education, University and Research PRIN 20159CZFJK to Valentina Moro.

  • University of Verona Bando di Ateneo per la Ricerca di Base 2015 project MOTOS to Valentina Moro.

Additional information

Competing interests

No competing interests declared.

Author contributions

Conceptualization, Formal analysis, Investigation, Visualization, Methodology, Project administration.

Conceptualization, Investigation, Methodology.

Conceptualization, Investigation, Methodology.

Conceptualization, Investigation, Methodology.

Investigation, Drawing of patients' lesion.

Helped recruiting the stroke patients on his ward.

Investigation, Drawing of patients' lesions.

Conceptualization.

Conceptualization, Supervision, Funding acquisition, Methodology.

Ethics

Human subjects: All participants gave written, informed consent to take part in the study and to publish. The local National Health System Ethics Committees approved the study (REC:05/Q0706/218), which was carried out in accordance to the Declaration of Helsinki.

Additional files

Supplementary file 1. Number of significant voxels (from the atlas of gray matter – AAL – and white matter – JHU – and NatBrainLab’s atlas) resulting from the VLSM analysis with the general pleasantness sensitivity scores (velvet-sandpaper average pleasantness ratings), N = 36.

As control for a general pleasantness deficit, patients rated how pleasant it would be to be touched by a typically pleasant material (i.e. velvet, Mpleasantness rating = 6.91, SD = 1.88) and a typically unpleasant fabric (i.e. sandpaper, Mpleasantness rating = 0.33, SD = 0.93). Similarly, as for CT pleasantness sensitivity, top-down tactile pleasantness sensitivity was computed as the difference between pleasant (velvet) and unpleasant pleasantness ratings (sandpaper), for each patient. We considered the same patients as for the CT pleasantness sensitivity VLSM analysis (N = 36 as we had missing data for 5 of them) and ran a VLSM analysis with this top-down tactile pleasantness sensitivity.

elife-47895-supp1.docx (17.4KB, docx)
Transparent reporting form

Data availability

The data that support the findings of this study are available on the Open Science Framework (https://osf.io/fyrwc/).

The following dataset was generated:

Kirsch LP, Besharati S, Papadaki C, Crucianelli L, Bertagnoli S, Ward N, Moro V, Jenkinson PM, Fotopoulou A. 2019. Affective Touch Lesion Study. Open Science Framework. fyrwc

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Decision letter

Editor: Stephen Liberles1

In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses.

Acceptance summary:

The authors map brain circuits involved in the perception of affective touch, the tactile sensation associated with affiliative social contact. The authors identified a large cohort of patients with brain lesions in the insular cortex, and used clinical behavioral assays across patients to map regions involved in affective touch sensation. These studies advance our understanding of neural circuits that process sensations of positive valence in the somatosensory system.

Decision letter after peer review:

Thank you for submitting your article "Damage to the right insula disrupts the perception of affective touch" for consideration by eLife. Your article has been reviewed by Christian Büchel as the Senior Editor, a Reviewing Editor, and two reviewers. The reviewers have opted to remain anonymous.

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

Summary:

This manuscript examines whether sensations of pleasant touch are altered in stroke patients with lesions in the insular cortex. A large patient cohort was analyzed, and a correlation was observed, with differences observed between anterior and posterior cortical regions.

Essential revisions:

Reviewer #1:

The paper shows that stroke patients with insular lesions rate the pleasantness of arm-stroking less than patients with lesions elsewhere. Arm stroking was conducted in a manner known to optimally stimulate firing of unmyelinated 'C-tactile' fibres, a channel of cutaneous somatosensation most associated with affective experience, including pleasant stroking, and less pleasant tickle and itch. Previous studies (e.g. Olausson et al., 2002) have shown the preservation of C-tactile fibres in individuals with peripheral cutaneous somatosensory (demyelinating) neuropathy can lead to patients with limb anesthesia to experience cutaneous sensation, an effect related to insular rather than primary somatosensory cortical activation. This paper adds to this knowledge but nevertheless can be improved.

1) The nomenclature could be tighter: The 3-10cm/s stroking stimulation is called CT optimal or suboptimal, based on electrophysiological recording (can this evidence be cited). However, calling reported pleasantness 'CT pleasantness sensitivity' is stretching this link as there is no direct evidence that people are reporting an exclusive readout of CT fibres. Better to refer to touch (or stroking) pleasantness.

2) The papers cited relating pleasantness to optimal CT stimulation (e.g. Gentsch, 2015) were undertaken in non-clinical individuals. Patients may not show the same profile of stroking rate / pleasantness judgment, e.g. may feel pleasantness at a different rate of stroking but it is assumed that the normative relationship holds.

3) It is not actually that clear what the higher-level predictions (hence conclusions) actually were. Some experiences of touch are more neutral than other experiences of touch; while somatosensory cortex is not associated with construction / appraisal of emotional meaning, parts of insula and other regions are implicated in affective judgements, in part through association with interoceptive processes (e.g. bradycardia associated with affiliative interaction). Is the question then whether there is selective deficit in the experience of positive affect related to speed of touch in a brain-damaged patient group that can be localised to areas implicated in affect? There are multiple aspects to this question, so it ought to be clearly motivated.

4) Continuing the point above, does the investigation of right hemisphere regions associated with the perceived affective impact of CT-optimal touch condense to whether affective experience is compromised by focal (insular) lesions; whether stroking sensation, rather than not discriminative touch is compromised by focal lesions, or whether the affective aspects of stroking sensation can uncoupled from non-affective intensity of such sensation. This is relevant as, for example, there are no equivalent (exteroceptive or interoceptive) control for pleasant sensation, so general or specific inferences are constrained. Affective reactivity to other external stimuli may also be an important individual difference, although medication with significant cognitive or mood side effect were excluded (it would be useful to state explicitly that this included β blockers, pregabalin, lamotrigine). Nevertheless, mood was recording and it is unclear how tactile anhedonia related to negative affectivity in the patients.

5) The hypothetical touch rating (velvet vs sandpaper) protocol could be mentioned in the Introduction as it pops out unexpectedly in the Results section. The rationale should be more explicit. (Is it a sub hypothesis about perceptive loss leading to imaginative loss for this channel of sensation?). In the Materials and methods section, it could be usefully described in more detail: were people asked to imagine velvet and sandpaper drawn across the skin at different velocities in order to mirror the effects of CT optimal / suboptimal stimulation. Would an interaction be expected?

6) There is mention of interoception and interoceptive awareness e.g. in the Discussion section. The consensus definition of interoception is the sensation of the internal state of the body. If the authors maintain Craig's proposal that stroking and tickle sensation are honorary interoceptive sensations by virtue of having un- or poorly- myelinated fibres and sharing in part the same spinal and perhaps central pathways as viscerosensory afferents then this should be strongly and explicitly argued. If this is argued that the core aspect is that these sensations are affective/emotive then the issue about prediction / hypothesis underlying the experimental design is relevant. The term interceptive awareness has evolved since Garfinkel, 2013 in the light of consciousness science equating awareness with metacognitive insight, interoceptive sensibility being one term used for reported perception or misperception of visceral sensation.

7) Anatomical understanding of insula has been enhanced greatly through the work of Evrard. This could be reasonably cited.

Reviewer #2:

The manuscript describes a lesion study in 59 patients investigating the impact of insular damage on CT functioning. The study is very interesting, very timely and of potential high contribution for the field. The methods are sound. The Introduction, Discussion section and Results section are quite short. This could be a benefit, but it makes the paper hard to understand at several places.

My main concern is the mismatch between hypothesis, result and discussion. The authors hypothesize the pleasantness difference (CT optimal- suboptimal velocities) to be higher when patients are contralateral stroked than in other conditions (healthy controls or patients ipsilateral), This would be a 3-way interaction (velocity by group by side) which is not tested. In addition, Figure 1 does not support this hypothesis. Anyhow, the authors write in the Discussion section, that the hypothesis is supported.

Support for the hypothesis comes from the VLSM approach. I needed to reread the results several times in order to understand this. Especially as the methods are described after the results, it is unclear what the authors actually did. It would be much easier, if the authors would introduce the approach and the corresponding hypothesis in the Introduction. Results and Discussion section: what kind of patients are the ones with intact sensation? How did you measure this? In the Results and Discussion section, the authors introduce a comparison between hypothetical velvet stroking and sandpaper stroking. This is very surprising and could also be introduced better in the introduction part. I would actually remove this part, which is in my eyes unnecessary for the hypothesis but confusing for the reader.

The generally reduced pleasantness scores in lesion patients are most likely due to the enhanced depression and reduced attention in this group, a confounder analysis would clarify this.

[Editors' note: further revisions were suggested prior to acceptance, as described below.]

Thank you for submitting your article "Damage to the right insula disrupts the perception of affective touch" for consideration by eLife. Your article has been reviewed by Christian Büchel as the Senior Editor, a Reviewing Editor, and two reviewers. The following individuals involved in review of your submission have agreed to reveal their identity: Hugo Critchley (Reviewer #1).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

The reviewers noted a substantial improvement in the revised manuscript, but a few issues persisted that should be addressed. Please see the reviewer comments below.

Reviewer #1:

This is an interesting and novel study of regional brain lesions and aspects of touch perception preferentially associated with unmyelinated peripheral cutaneous sensory nerve transmission.

I had previously raised a number of points, which the authors have taken time and consideration to address in full.

I suggest no further changes and anticipate the paper will attract interest with impact from eLife readership.

Reviewer #2:

The revised manuscript improved in terms of understandability and the authors clearly put effort in the rebuttal. However, my concerns are not eliminated.

I still have some questions about the "pleasantness sensitivity"

- It is not clear to me why the authors used 18cm/s as a control for suboptimal CT activation. The microneurography studies are far from clear for this velocity and 30cm/s would be the standard with supporting microneurography data.

- How did the authors deal with those cases where roof or bottom effects were observed (e.g. rating all touch highly positive or negative)?

- How many of the healthy controls have a "negative pleasantness sensitivity"? And how many do not seem to differentiate between 3 and 18cm/s (no difference or very minor difference in pleasantness ratings)?

I may have missed it, but I cannot find the place where the authors show that the construct they call CT pleasantness sensitivity differs between patients with posterior insula lesions and healthy controls. Instead, the authors report differences between patients with various lesions. This does not relate to the abstract or to the hypothesis.

I don't understand why the authors did not control for potential sources of variance in the VTLM. The authors argue that "However, as explained in point 4 above, no correlations were found between CT pleasantness sensitivity (or pleasantness ratings) and any of the neuropsychological tests that differed between healthy controls and right hemisphere stroke patients (HADS Depression scale, Digit Span backward, MOCA and Premorbid IQ_WTAR). " However, the main observation the authors report is not between healthy controls and patients, but within groups of patients.

eLife. 2020 Jan 24;9:e47895. doi: 10.7554/eLife.47895.sa2

Author response

Essential revisions:

Reviewer #1:

1) The nomenclature could be tighter: The 3-10cm/s stroking stimulation is called CT optimal or suboptimal, based on electrophysiological recording (can this evidence be cited). However, calling reported pleasantness 'CT pleasantness sensitivity' is stretching this link as there is no direct evidence that people are reporting an exclusive readout of CT fibres. Better to refer to touch (or stroking) pleasantness.

We thank the reviewer for the positive assessment of our study’s scope and their suggestions for improving the manuscript.

We had omitted citations to microneurography studies due to the limit on citations the journal requires as well as word restrictions for Short Reports, but following the reviewers’ suggestion we now cite the following studies: Vallbo and Hagbarth, 1968; Vallbo et al., 1993, 1999; Nordin, 1990 (Introduction).

We agree that there is no evidence or current technical way to provide a tactile stimulus to individuals that activates only the CT fibers, and we now explicitly acknowledge this in the manuscript (Introduction and below). However, in this study we have used a psychophysical procedure, with control procedures and measures for top-down effects (see manuscript and below) and a related differential measure (the felt pleasantness difference between tactile stimuli that differ only in speed), determined by well-established facts about the mean frequency of CT activity in microneurography studies (Vallbo et al., 1993, 1999; Löken et al., 2009), by clinico-anatomical and behavioural evidence by the peripheral neuropathies the reviewer mentions (Olausson et al., 2002, 2008), two neuromodulation studies using rTMS (Case et al., 2016, 2017), and several functional neuroimaging studies (Morrison et al., 2011; McGlone et al., 2012) and also by psychophysiological studies to approximate ‘CT pleasantness sensitivity’ (Crucianelli et al., 2013, 2018; Gentsch et al., 2015; von Mohr et al., 2017; Kirsch et al., 2018) and hence we wish to maintain this term, albeit with now a more explicit acknowledgement that this is an inference on our part based on prior convergent evidence.

Following the spirit of the reviewer’s suggestion, in the Discussion section, we also now explicitly acknowledge the additional controls that future studies could implement to substantiate our claims.

Introduction. “This touch on the forearm stimulates both Aβ and CT fibers; one cannot stimulate one type of fiber without stimulating the other simultaneously (except in patients without Aβ afferents, as studied by Olausson et al., 2002, 2008). […] Specifically, Aβ fiber activation is known to linearly increase with increases in velocity, while the mean frequency firing rate of CT fibers follows an inverted U shape with higher firing being in the 1-10cm/s range, and have been shown to be the only unit types for which firing patterns correlate with average psychophysiological ratings, i.e. pleasantness (Löken et al., 2009).”

Results and Discussion section. “The present study has considered CT pleasantness sensitivity as the difference between the pleasantness of CT-optimal slow touch (3cm/s) and CT-suboptimal fast touch (18cm/s). […] Moreover, the specificity of the present findings to CT fibers should be further investigated by comparing tactile stimulation on hairy (e.g. forearm) vs. glabrous skin (e.g. palm, that do not contain any CT fibers).”

2) The papers cited relating pleasantness to optimal CT stimulation (e.g. Gentsch, 2015) were undertaken in non-clinical individuals. Patients may not show the same profile of stroking rate / pleasantness judgment, e.g. may feel pleasantness at a different rate of stroking but it is assumed that the normative relationship holds.

We thank reviewer 1 for this comment that allows us to clarify the benefits of our neuropsychological/lesion approach, which does not follow the older logic of comparing lesions between a diagnostically-defined and a healthy group, but is a more advanced method using continuous measures in a single sample. This has many general neuropsychological advances and specifically in relation to the question the reviewer asks, we agree that patients, presumably based on their particular lesion profile, may not show the same profile of stroking rate/pleasantness judgement and this is precisely what our study is testing by a VLSM approach, i.e. we recruit a relatively large sample in which we did not expect all patients to show the same tactile profile and hence we examine the lesioned voxels that correspond to different behavioural deficit profiles.

We now clarify this in our manuscript as follows:

Introduction. “Contrary to other neuropsychological approaches that employ diagnostic, group comparisons, the VLSM method uses continuous measures in a single sample, and identifies which regions of the brain are crucial to a specific behavior (e.g. here CT pleasantness perception), without assuming that all patients show the same tactile profile.”

3) It is not actually that clear what the higher-level predictions (hence conclusions) actually were. Some experiences of touch are more neutral than other experiences of touch; while somatosensory cortex is not associated with construction / appraisal of emotional meaning, parts of insula and other regions are implicated in affective judgements, in part through association with interoceptive processes (e.g. bradycardia associated with affiliative interaction). Is the question then whether there is selective deficit in the experience of positive affect related to speed of touch in a brain-damaged patient group that can be localised to areas implicated in affect? There are multiple aspects to this question, so it ought to be clearly motivated.

We thank reviewer 1 in giving us the opportunity to further specify the more higher-level, theoretical aspects of our hypothesis in relation to some of the interesting and wider issues they raise. Our study addresses the question of whether the activation of CT afferents (a bottom-up rather than a top-down evaluative/appraisal mechanism; please see our specification of control and differential measures for the latter aspect), with its critical, primary cortical processing in the posterior insula, increases the likelihood of touch being perceived as positive. This hypothesis is distinguished from a number of other hypotheses assessed with control comparisons, now specified below. Our answer here also relates to the next three points raised by the reviewer so we will refer back to it.

Introduction: “C-tactile afferents have been shown to take a distinct ascending pathway from the periphery to the posterior insula (Olausson et al., 2002; Morrison et al., 2011), which is understood to support an early convergence of sensory and affective signals about the body that are then re-represented in the mid- and anterior insula, the proposed sites of integration of interoceptive information with other contextual information (Critchley et al., 2004; Craig, 2009; Evrard and Craig, 2015).”

Introduction.: “Given that right hemisphere and particularly right perisylvian regions have been previously associated with somatosensory and interoceptive perception (Dijkerman and de Haan, 2007; Preusser et al., 2015), we expected our patients to have, on average, reduced ratings of both touch intensity and pleasantness in comparison to healthy controls, and particularly in the contralesional left forearm. […] Moreover, this would give further substance to the hypothesis that the CT afferent pathway is a specialized system that allows individuals to distinguish a range of velocities that are likely to have social-affective relevance, for the purposes of further integration with sensory and affective information in the insula (Olausson et al., 2008; see Morrison et al., 2010 for discussion).

4) Continuing the point above, does the investigation of right hemisphere regions associated with the perceived affective impact of CT-optimal touch condense to whether affective experience is compromised by focal (insular) lesions; whether stroking sensation, rather than not discriminative touch is compromised by focal lesions, or whether the affective aspects of stroking sensation can uncoupled from non-affective intensity of such sensation. This is relevant as, for example, there are no equivalent (exteroceptive or interoceptive) control for pleasant sensation, so general or specific inferences are constrained. Affective reactivity to other external stimuli may also be an important individual difference, although medication with significant cognitive or mood side effect were excluded (it would be useful to state explicitly that this included β blockers, pregabalin, lamotrigine). Nevertheless, mood was recording and it is unclear how tactile anhedonia related to negative affectivity in the patients.

We thank reviewer 1 for this comment, and we now better specify the rationale and implications of our hypothesis, consistently with the higher-level theoretical aspects explained above. Specifically, the investigation of the specific right-hemisphere lesions associated with reduced CT pleasantness specificity was motivated by the hypothesis that the CT system has the unique functional role in picking the socio-affective value of gentle touch among the potential noise of other tactile and sensory information and conveying it to the posterior and anterior insula for further integration and affective processing. Indeed, as the reviewer notes in their following point we have controlled for more general effects of tactile anhedonia, as well as for more general tactile, mood and sensory deficits in our sample and we have now revised our manuscript to clarify these control procedures and analyses earlier in the manuscript (Introduction, Results and Discussion section).

Introduction. “In addition to the affective touch paradigm, to control for general pleasantness deficits (specific to touch), participants had to imagine being touched by pleasant (i.e. velvet) and unpleasant (i.e. sandpaper) materials and rate the associated pleasantness.

Given that right hemisphere and particularly right perisylvian regions have been previously associated with somatosensory and interoceptive perception (Dijkerman and de Haan, 2007; Preusser et al., 2015), we expected our patients to have, on average, reduced ratings of both touch intensity and pleasantness in comparison to healthy controls, and particularly in the contralesional left forearm. An overall reduced tactile pleasantness in patients (both in actual touch and imagined touch pleasantness ratings) would suggest tactile anhedonia linked to general right hemisphere lesions.”

Results and Discussion section. “Interestingly, none of the patients’ demographic characteristics or, neuropsychological deficits correlated significantly with their CT pleasantness sensitivity, including education, anxiety and depression scores, as well as memory as measured by the MOCA memory subscale, and working memory as measured by the Digit Span, all p>0.1 and all BF10 <1. Thus, low CT pleasantness sensitivity was not explained by other general cognitive and emotional deficits, as assessed in the present study. Moreover, there was no correlation between CT pleasantness sensitivities and tactile anhedonia on either forearm (as measure by the difference between the imagined pleasantness of pleasant and unpleasant material; r31=-.104, p=0.578, BF10=.259 for the contralesional forearm; r36=-.086, p=.618, BF10=.234, for the ipsilesional forearm), nor with tactile acuity as measured by intensity ratings.”

Materials and methods section. “(vi) no medication with significant cognitive or mood side-effects (e.g. pregabalin, lamotrigine);”

5) The hypothetical touch rating (velvet vs sandpaper) protocol could be mentioned in the Introduction as it pops out unexpectedly in the Results section. The rationale should be more explicit. (Is it a sub hypothesis about perceptive loss leading to imaginative loss for this channel of sensation?). In the Materials and methods section, it could be usefully described in more detail: were people asked to imagine velvet and sandpaper drawn across the skin at different velocities in order to mirror the effects of CT optimal / suboptimal stimulation. Would an interaction be expected?

We thank the reviewer for this suggestion, and we have now included this scope of measuring this dimension of imagined tactile pleasantness, as well as all the other control measures of our study which we think further specify the relation between our results and our hypotheses (see point 4 and additions below).

Moreover, we were not trying to match imagined touch to actual touch at an embodied level, but we aimed to ensure that patients do not have deficits in imaging or, expressing tactile pleasure beyond the level of actual tactile stimulation, which is controlled for by our main task by the faster trials.

Introduction: “In addition to the affective touch paradigm, to control for general pleasantness deficits (e.g. tactile anhedonia), participants had to imagine being touched by pleasant (i.e. velvet) and unpleasant (i.e. sandpaper) materials and rate the associated pleasantness.”

(…)

“An overall reduced tactile pleasantness in patients (both in actual touch and imagined touch pleasantness ratings), would suggest tactile anhedonia, linked to general right hemisphere lesions.”

Results and Discussion section:” Additionally, as a control for a general pleasantness deficit, patients rated how pleasant it would be to be touched by a typically pleasant material and a typically unpleasant fabric. As done for CT pleasantness sensitivity, imagined tactile pleasantness sensitivity was computed as the difference between pleasant and unpleasant materials pleasantness ratings, for each patient. We considered the same patients as for the CT pleasantness sensitivity VLSM analysis (N=36 as we had missing data for 5 of them) and ran a VLSM analysis with this top-down tactile pleasantness sensitivity as predictor. This yielded significant voxels subcortically in the caudate, thalamus, putamen and pallidum, but crucially, not the insula, suggesting that the above results are specific to applied tactile stimuli and not more general pleasantness comparisons (see Supplementary file 1)”

Materials and methods section. “No other instruction was given to the participants (neither speed nor pressure of the imagined touch).”

6) There is mention of interoception and interoceptive awareness e.g. in the Discussion section. The consensus definition of interoception is the sensation of the internal state of the body. If the authors maintain Craig's proposal that stroking and tickle sensation are honorary interoceptive sensations by virtue of having un- or poorly- myelinated fibres and sharing in part the same spinal and perhaps central pathways as viscerosensory afferents then this should be strongly and explicitly argued. If this is argued that the core aspect is that these sensations are affective/emotive then the issue about prediction / hypothesis underlying the experimental design is relevant. The term interceptive awareness has evolved since Garfinkel, 2013 in the light of consciousness science equating awareness with metacognitive insight, interoceptive sensibility being one term used for reported perception or misperception of visceral sensation.

We thank reviewer 1, giving us the opportunity to specify our view. As mentioned in previous comments we made several additions in the introduction and in the discussion to clarify our definitions of interoception, and justification of including affective touch as an interoceptive modality.

Moreover, we have changed our terminology, to be more accurate and in line with recent findings (from interoceptive awareness, that refers more to confidence in judgements, to interoceptive perception, that captures more sensing the body sensations; Garfinkel et al., 2016; Critchley and Garfinkel, 2017).

Results and Discussion section: “Taken together, our findings support previous findings about the functional organization and role of the human insula(Craig, 2010; Cauda et al., 2011; Kurth et al., 2010; Heydrich and Blanke, 2013; Ronchi et al., 2015; Salomon et al., 2018; see review by Evrard, 2019, on recent findings on the organization of the insula in non-human primates), consisting of specialized substrates organized in a posterior to anterior structural progression, with posterior parts representing the primary cortical representations of interoceptive stimuli from contralateral body parts and more anterior parts, tested here in the right hemisphere, acting as integration areas for sensory signals and contextual cues ultimately leading to interoception. […] As VLSM methods preclude direct comparison between CT pleasantness sensitivity deficits on the contralesional and ipsilesional forearm at the brain level; future studies should investigate further the posterior-anterior insula segregation in relation to affective touch as an interoceptive modality.”

7) Anatomical understanding of insula has been enhanced greatly through the work of Evrard. This could be reasonably cited.

We thank reviewer 1 for the advice; we have now included reference to Evrard work in the Introduction and Results and Discussion section (Results and Discussion section Evrard 2019; Introduction Evrard and Craig, 2015).

Results and Discussion section. “Taken together, our findings support previous findings about the functional organization and role of the human insula (Craig, 2010; Cauda et al., 2011; Kurth et al., 2010; Heydrich and Blanke, 2013; Ronchi et al., 2015; Salomon et al., 2018; see review by Evrard, 2019 on recent findings on the organization of the insula in non-human primates), consisting of specialized substrates organized in a posterior to anterior structural progression, with posterior parts representing the primary cortical representations of interoceptive stimuli from contralateral body parts and more anterior parts, tested here in the right hemisphere, acting as integration areas for sensory signals and contextual cues ultimately leading to interoception.”

Reviewer #2:

The manuscript describes a lesion study in 59 patients investigating the impact of insular damage on CT functioning. The study is very interesting, very timely and of potential high contribution for the field. The methods are sound. The Introduction, Discussion section and Results section are quite short. This could be a benefit, but it makes the paper hard to understand at several places.

My main concern is the mismatch between hypothesis, result and discussion. The authors hypothesize the pleasantness difference (CT optimal- suboptimal velocities) to be higher when patients are contralateral stroked than in other conditions (healthy controls or patients ipsilateral), This would be a 3-way interaction (velocity by group by side) which is not tested. In addition, Figure 1 does not support this hypothesis. Anyhow, the authors write in the Discussion section, that the hypothesis is supported.

Support for the hypothesis comes from the VLSM approach. I needed to reread the results several times in order to understand this. Especially as the methods are described after the results, it is unclear what the authors actually did. It would be much easier, if the authors would introduce the approach and the corresponding hypothesis in the Introduction.

We thank reviewer 2 for these positive comments and giving us the opportunity to improve our manuscript. We have restructured the manuscript along your comments and reviewer 1’ comments and hope it to be clearer. Please also note the strict word limit of the journal for Short Report.

Indeed, as the reviewer notes the operationalization of our hypothesis and the chosen method is a VLSM study where the interaction the reviewer mentions does not apply. VLSM (Voxel-based, Lesion-Symptom Mapping) studies differentiate patients in a single sample based on the voxels that are damaged in the sub-set of patients that have a behavioural deficits versus those that do not. We apologize for any lack of clarity on our behalf and we have now specified our approach and hypotheses in the introduction, also in response to reviewer 1 (points 1 to 3 in particular). As we say in the manuscript, we are not expecting such a general effect of the stroke on CT pleasantness sensitivity in our patients, but rather we were looking for the specific lesion-symptom relation, which is optimally examined by a VLSM methodology.

Introduction “Contrary to other neuropsychological approaches that employ diagnostic, group comparisons, the VLSM method uses continuous measures in a single sample, and identifies which regions of the brain are crucial to a specific behavior (e.g. here CT pleasantness perception), without assuming that all patients show the same tactile profile.”

Introduction. “An overall reduced tactile pleasantness in patients (both in actual touch and imagined touch pleasantness ratings) would suggest tactile anhedonia linked to general right hemisphere lesions. […] Moreover, this would give further substance to the hypothesis that CT afferent pathway is a specialized system that allows individuals to distinguish a range of velocities likely to have social-affective relevance, for the purposes of further integration with sensory and affective information in the insula (Olausson et al., 2008; see Morrison et al., 2010 for discussion).”

Results and Discussion section. “First, we investigated the effect of right hemisphere lesions on the perception of touch intensity and pleasantness, on the contralesional and ipsilesional forearm separately, by comparing stroke patients’ and healthy controls’ intensity and pleasantness ratings in turn. In line with the high percentage of contralesional tactile deficits in right hemisphere stroke patients (including in our patients’ sample, see Materials and methods section), patients, as compared to healthy controls, perceived touch, regardless of velocity, as less intense on the contralesional forearm.”

Results and Discussion section. “However, no interaction between touch type and group was found (contralesional: F(1,53)=0.393, p=0.533, ηp2 = .007, BF10=0.371, Figure 1C; ipsilesional: F(1,59)=0.073, p=0.788, ηp2 = .001, BF10=0.287, Figure 1D; hypothetical imagined tactile touch pleasantness: F(1,70)=.061, p=0.806, ηp2=.001, BF10=0.270), suggesting that right hemisphere lesions in general do not necessarily lead to reduced CT pleasantness sensitivity, and confirming that any differential deficits in the pleasantness perception of CT-optimal versus CT-suboptimal touches at the individual level would relate to specific lesions rather than general stroke effects.”

Results and Discussion section. “The present study aimed to investigate the lesion patterns and neuropsychological deficits that may be associated with the inability of certain stroke patients to distinguish the pleasantness of CT-optimal versus CT-suboptimal touches. Accordingly, CT pleasantness sensitivity was calculated as the difference between the pleasantness of CT-optimal and CT-suboptimal touches. As a convention, CT pleasantness sensitivity inferior or equal to zero is considered as low in CT pleasantness sensitivity, i.e. low CT affective touch perception (Crucianelli et al., 2018).”

Results and Discussion section: what kind of patients are the ones with intact sensation? How did you measure this?

We thank reviewer 2 for this comment, we have now clarified this in the manuscript. Patients with intact sensations are a subsample of the 59 patients, patients who rated all touch trials above 2 on the intensity scale (i.e. could feel all the trials).

Introduction. “This was also the case when considering only patients that had intact tactile perception on the contralesional forearm, i.e. could feel all the touch trials (N=25, F(1,43)=9.880, p=0.003, ηp2 = .187, Figure 1—figure supplement 1; see Materials and methods section for details).”

Materials and methods section: Moreover, as supplementary analyses, we also considered patients with intact tactile perception on the contralesional forearm. For these analyses, only patients that gave intensity ratings above 2 were included (N=25).

In the Results and Discussion section, the authors introduce a comparison between hypothetical velvet stroking and sandpaper stroking. This is very surprising and could also be introduced better in the introduction part. I would actually remove this part, which is in my eyes unnecessary for the hypothesis but confusing for the reader.

We thank reviewer 2 for this comment. We agree that this analysis was not well introduced and justified. As in response to reviewer 1 who wanted more precision on this analysis, we have now clarified and expanded it. We hope it is less confusing.

Introduction. “In addition to the affective touch paradigm, to control for general pleasantness deficits (specific to touch), participants had to imagine being touch by pleasant (i.e. velvet) and unpleasant (i.e. sandpaper) materials and rate the associated pleasantness.”

Results and Discussion section. “A similar general tactile anhedonia (reduced pleasantness ratings) was observed in our patients as compared to the controls for imagined tactile pleasantness, when patients had to rate how pleasant it would be to be touched by pleasant and unpleasant fabric (F(1,57) = 55.918, p<0.001, ηp2=.495, Figure 1—figure supplement 2).”

The generally reduced pleasantness scores in lesion patients are most likely due to the enhanced depression and reduced attention in this group, a confounder analysis would clarify this.

We thank reviewer 2 for this comment, as for reviewer 1’s comment 4, we have run correlations between depression scores (as measured by the HADS), memory deficits (as measured by the MOCA memory subscale) and averaged pleasantness ratings; this yielded non-significant correlations (all p-values>0.50, BF10<1). Moreover, as explained to reviewer 2, given non-significant correlations with CT pleasantness sensitivities and neuropsychological tests, these variables were not included in the VLSM analyses as nuisance covariates (as advised by de Haan and Karnath, 2018)

Results and Discussion section. “Interestingly, none of the patients’ demographic characteristics or, neuropsychological deficits correlated significantly with their CT pleasantness sensitivity, including education, anxiety and depression scores, as well as memory as measured by the MOCA memory subscale, and working memory as measured by the Digit Span, all p>0.1 and all BF10 <1. Thus, low CT pleasantness sensitivity was not explained by other general cognitive and emotional deficits, as assessed in the present study. Moreover, there was no correlation between CT pleasantness sensitivities and tactile anhedonia on either forearm (as measure by the difference between the imagined pleasantness of pleasant and unpleasant fabric; r31=-.104, p=0.578, BF10=.259 for the contralesional forearm; r36=-.086, p=.618, BF10=.234 for the ipsilesional forearm), nor with tactile acuity as measured by intensity ratings.”

[Editors' note: further revisions were suggested prior to acceptance, as described below.]

Reviewer #2:

The revised manuscript improved in terms of understandability and the authors clearly put effort in the rebuttal. However, my concerns are not eliminated.

I still have some questions about the "pleasantness sensitivity"

We thank the reviewer for their careful comments however we wish to stress that such comments seem to correspond to and thus perhaps be motivated by a different, older methodological approach to lesion studies. Namely, one that relies on comparisons between diagnosis-based clinical groups and/or between clinical groups and healthy controls. Please note that as we explained in the text, in the previous revision and our points below, the VLSM method is not based on such comparisons, i.e. it does not use patients and controls to be grouped by either lesion, diagnosis or behaviour, but rather makes use of continuous behavioural and lesion information (e.g. Bates et al., 2003) and this is what we have applied to our relatively, large sample of right hemisphere stroke patients.

- It is not clear to me why the authors used 18cm/s as a control for suboptimal CT activation. The microneurography studies are far from clear for this velocity and 30cm/s would be the standard with supporting microneurography data.

We use the 18cm/s velocity on the basis of both behavioural reasons and microneurography indications:

a) Behavioural: While we agree with the reviewer that several studies use 30cm/s as a control velocity, this psychophysical choice is not without limitations in behavioural studies for a number of reasons including (1) the very poor ecological validity of applying this kind of speed on human skin in comparison to 3cm/s and 18cm/s (2) the very different amount of strokes or time required to administer this kind of touch on the same skin area in comparison to 3cm/s and to 18cm/s, (3) the related differences in the kind of emotions and mental states people consciously associate with increasing velocities, already present at 18cm/s (Kirsch et al., 2018), (4) the very different attentional demands this kind of velocity generates in comparison to 3cm/s and 18cm/s and (5) the related difficulties in administering such stimuli manually and by the bedside in right-hemisphere patients with known attentional, arousal and visuospatial difficulties. The above reasons may be of lesser relevance in microneurography studies, but they are of great importance in behavioural and clinical studies. Thus, our lab has for the past 6 years conducted more than 12 published studies using the 18cm/s velocity as a CT non-optimal velocity, all of which showed that this velocity is subjectively perceived as significantly less pleasant (e.g. Crucianelli et al., 2013; Gentsch et al., 2015; Kirsch et al., 2018; von Mohr, Kirsch and Fotopoulou, 2019) and it has different, indirect, implicit effects on behavioural and cognition (e.g. von Mohr et al., 2017; Krahé et al., 2016; 2018; Panagiotopoulou et al., 2017; 2018; Crucianelli et al., 2018) than the 3cm/s velocity.

b) Microneurography studies have indeed tested 3cm/s versus the 30cm/s velocity. However, they have also tested intermediate velocities (i.e. 9cm/s and 10cm/s) and they have consistently found that these generate both CT and subjective responses that fall somewhere between the 3cm/s and the 30cm/s (see Loken et al., 2009; and Ackerley et al., 2014). Hence, the upper component of the inverted U shape and hence to our knowledge, no studies has ever considered any velocity faster than 10cm/s as optimal CT velocity. Our choice of a sub-optimal velocity is thus well above the accepted optimal range of CT fibers.

In sum, taking both behavioural and microneurography considerations and evidence into account we elected 18cm/s as the best control velocity for our paradigm.

- How did the authors deal with those cases where roof or bottom effects were observed (e.g. rating all touch highly positive or negative)?

Our data did not show ceiling or floor effects of the kind the reviewer is concerned about (only 1 patient out of the 59 rated both touch velocities at the maximum and none of the patients and healthy controls rated the touch as not pleasant at all). In all cases, our continuous differential measure, motivated by our specific hypothesis, is designed to indeed take this into account, i.e. touch insensitive ceiling or floor responses would lead to a low sensitivity, i.e. a very small differential.

- How many of the healthy controls have a "negative pleasantness sensitivity"? And how many do not seem to differentiate between 3 and 18cm/s (no difference or very minor difference in pleasantness ratings)?

Individual differences are only meaningful in the patient group and it is these differences only that are taken into account in our lesion analyses. Any comparison involving the smaller group of healthy participants for behavioural validity purposes is meaningful only at the group level and it does not relate to our lesion analysis. For the reviewers’ interest, we note here that for the contralesional arm, one out of 20 healthy participants rated both velocities as similar in pleasantness and two others found the 18cm/s velocity slightly more pleasant than the optimal one.

I may have missed it, but I cannot find the place where the authors show that the construct they call CT pleasantness sensitivity differs between patients with posterior insula lesions and healthy controls. Instead, the authors report differences between patients with various lesions. This does not relate to the abstract or to the hypothesis.

We apologize if our response in the previous round of revision was not clear. We had changed both the abstract and our hypotheses to be clearly about lesion-patients and not about healthy controls vs. patients. Our aim is not to compare patients with posterior insula lesions to healthy controls, but to find which region is crucial to CT pleasantness perception. For this reason, we used a continuous VLSM approach in patients with right hemisphere lesion.

Indeed, as written in the Introduction “Contrary to other neuropsychological approaches that employ diagnostic, group comparisons, the VLSM method uses continuous measures in a single sample, and identifies which regions of the brain are crucial to a specific behavior (e.g. here CT pleasantness perception), without assuming that all patients show the same tactile profile.”. For this reason, the VLSM method do not need to compare healthy controls and patients, and in fact looking whether CT pleasantness sensitivity differs between patients with posterior insula lesions and healthy controls would be double dipping of the data, and make the analyses circular. For this reason, we would like to argue that it would not be scientifically sound to do so.

Note: Below is how we introduced our aim and hypotheses:

Abstract: " We report the first human lesion study on the perception of C-tactile touch in right hemisphere stroke patients (N = 59), revealing that right posterior and anterior insula lesions reduce tactile, contralateral and ipsilateral pleasantness sensitivity, respectively.

Aim (Introduction): "Accordingly, we aimed to investigate for the first time the right hemisphere regions which are necessary for the perceived affectivity of CT-optimal touch, applying a voxel-based lesion symptom mapping approach (VLSM; Bates et al., 2003) in a large, consecutively recruited cohort of patients (N=59) with recent, first-ever, right hemisphere lesions following a stroke.

Hypotheses (Introduction): "Crucially, given the assumed neurophysiological specificity of the CT system, we expected that more specific lesions to the posterior insula (Morrison, 2016) would reduce the affective sensitivity of these patients to CT-optimal touch, over and above general effects of anhedonia, tactile acuity and other neuropsychological deficits caused by the broader lesion profile of our whole sample. In other words, an intact posterior insula should be necessary for the added affective sensitivity that the CT fibers are conveying during touch optimally activating the CT system versus an identical touch and social context that does not activate this afferent pathway optimally."

I don't understand why the authors did not control for potential sources of variance in the VTLM. The authors argue that "However, as explained in point 4 above, no correlations were found between CT pleasantness sensitivity (or pleasantness ratings) and any of the neuropsychological tests that differed between healthy controls and right hemisphere stroke patients (HADS Depression scale, Digit Span backward, MOCA and Premorbid IQ_WTAR). " However, the main observation the authors report is not between healthy controls and patients, but within groups of patients.

We apologize if our answer was not clear, it might have been confusing as we were also referring to our response to reviewer’1 comment. Indeed, we are focusing mainly on patients as a continuous group in the present study.

In accordance with recent recommendations on how to conduct lesion-behaviour mapping by de Haan and Karnath, (2018), it is advised to include only nuisance covariates in the VLSM analyses, such as the severity of frequently co-occurring deficits, that may correlate with the cognitive function of interest. For this reason, we first determined whether variables that differed between right hemisphere stroke patients and healthy controls are nuisance variable worth including by looking if there is a correlation between the variable of interest and the nuisance variable. If no significant correlation is found (within the patient group), the variable does not need to be included – and including it would reduce power unnecessarily.

As written in our previous rebuttal, given non-significant correlations between CT pleasantness sensitivities and neuropsychological tests in patients (and not considering healthy controls here), these variables were not included in the VLSM analyses as nuisance covariates (as advised by de Haan and Karnath, 2018).

More precisely:

Results and Discussion section. “Interestingly, none of the patients’ demographic characteristics or, neuropsychological deficits correlated significantly with their CT pleasantness sensitivity, including education, anxiety and depression scores, as well as memory as measured by the MOCA memory subscale, and working memory as measured by the Digit Span, all p>0.1 and all BF10 <1.”

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Data Citations

    1. Kirsch LP, Besharati S, Papadaki C, Crucianelli L, Bertagnoli S, Ward N, Moro V, Jenkinson PM, Fotopoulou A. 2019. Affective Touch Lesion Study. Open Science Framework. fyrwc [DOI] [PMC free article] [PubMed]

    Supplementary Materials

    Supplementary file 1. Number of significant voxels (from the atlas of gray matter – AAL – and white matter – JHU – and NatBrainLab’s atlas) resulting from the VLSM analysis with the general pleasantness sensitivity scores (velvet-sandpaper average pleasantness ratings), N = 36.

    As control for a general pleasantness deficit, patients rated how pleasant it would be to be touched by a typically pleasant material (i.e. velvet, Mpleasantness rating = 6.91, SD = 1.88) and a typically unpleasant fabric (i.e. sandpaper, Mpleasantness rating = 0.33, SD = 0.93). Similarly, as for CT pleasantness sensitivity, top-down tactile pleasantness sensitivity was computed as the difference between pleasant (velvet) and unpleasant pleasantness ratings (sandpaper), for each patient. We considered the same patients as for the CT pleasantness sensitivity VLSM analysis (N = 36 as we had missing data for 5 of them) and ran a VLSM analysis with this top-down tactile pleasantness sensitivity.

    elife-47895-supp1.docx (17.4KB, docx)
    Transparent reporting form
    elife-47895-transrepform.pdf (246KB, pdf)

    Data Availability Statement

    The data that support the findings of this study are available on the Open Science Framework (https://osf.io/fyrwc/).

    The following dataset was generated:

    Kirsch LP, Besharati S, Papadaki C, Crucianelli L, Bertagnoli S, Ward N, Moro V, Jenkinson PM, Fotopoulou A. 2019. Affective Touch Lesion Study. Open Science Framework. fyrwc

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