Functions of Interoception: From Energy Regulation to Experience of the Self

Abstract

We review recent work on functions of interoceptive processing, by which the nervous system anticipates, senses, and integrates signals originating from the body. We focus on several exemplar functions of interoception, including energy regulation (ingestion and excretion), memory, affective and emotional experience, and the psychological sense of self. We emphasize two themes across these functions. First, the anatomy of interoceptive afferents makes it difficult to manipulate or directly measure interoceptive signaling in humans. Second, recent evidence shows that multimodal integration occurs across interoceptive modalities, and between interoceptive and exteroceptive modalities. Whereas exteroceptive multimodal integration has been relatively extensively studied, fundamental questions remain regarding multimodal integration that involves interoceptive modalities, and future empirical work is required to better understand how and where multimodal interoceptive integration occurs.

The Multiple Functions of Interoception

The last two decades have seen rapid growth in research activity devoted to interoception and interoceptive processing [1]. Some of the functions of interoception have a long scientific history (e.g., heartbeat perception), whereas others have more recently coalesced into a robust and substantial literature (e.g., the psychological sense of self). In this review we focus on a set of exemplar functions of interoceptive processing. In particular, we examine functions that support the regulation of energy intake and outflow, as well as mental phenomena such as memory, affective and emotional experience and the ‘sense of self’. Arguably, even these relatively high-level mental processes have their origins in what is every organism’s ultimate concern, namely the efficient regulation of energy to support life [2–4].

In this article, we use the term interoception to refer to “the integrative interpretation of internal and external stimuli, in the cognitive/emotional context, to derive an overall physiological representation of the state of the body, including conscious and unconscious aspects” [5]. Throughout, we will emphasize two themes. First, viscerosensory signaling is difficult to directly measure and experimentally manipulate in humans because interoceptive afferents are thin, typically unmyelinated, and diffusely arrayed, making it difficult to use traditional peripheral nerve stimulation approaches. Although findings from animal models face questions regarding generalizability to humans, such studies are instrumental in motivating researchers to develop new experimental methods, some of which might ultimately be applicable in humans as well. Second, using distinctions drawn by anatomists and neurophysiologists between interoceptive and exteroceptive sensory systems, it is now clear that multisensory integration can occur not only across two exteroceptive modalities, but also across diverse interoceptive inputs as well as across combinations of interoceptive and exteroceptive modalities. Accordingly, in this article, we use the term multisensory integration in a broader way than the more traditional sense of integration across exteroceptive modalities [6]. We provide illustrative examples of this broader view of multisensory integration. We conclude with the example of affective touch, which illustrates the idea of interoceptive and exteroceptive multisensory integration, and also the feasibility of manipulating interoceptive sensory signaling in humans.

Interoception and Energy Regulation

Allostasis and Homeostasis

Evolutionary considerations have led neuroscientists to suggest that mammalian brains evolved to control the motor functions of their bodies within the context of their current array of exteroceptive and interoceptive sensory signals [4, 7, 8]. Some have even suggested that a key function, if not the key function of exteroceptive sensory processing, is support of motor control [7–9]. Anatomically, the interoceptive system has a similar organization to that of skeletomotor control, with viscerosensory inputs directly linked to visceromotor outputs [5, 10]. Finally, all sensory systems are subject to evolutionary selection pressures that drive the need to minimize neural energetic costs [11]. In this way, energy regulation is a key concern for a brain, and the optimization of energy efficiency is a key constraint for both brain and body [2, 12]. We suggest that a key function of interoception is to support efficient mobilization of crucial resources to where they are needed most, as well as efficient removal of waste. In this perspective, the body can be seen as the brain’s effector and sensor, enabling a delicate balance of resource intake and outflow, and efficiently controlling movements that support resource intake, include foraging, ingestion, breathing, and circulation [4, 13]. The body must also remove waste products via movements that include breathing, circulation, elimination, and defecation.

To optimally support these major bodily functions, the brain also maximizes overall energy efficiency by anticipating imminent resource needs, a process referred to as allostasis [2, 14]. The body is also supported by “in-the-moment” local, regulatory processes or homeostasis. Homeostasis can operate independent of predictive or anticipatory regulation by the brain, but can also be modulated by the brain when necessary. Homeostasis provides local regulation that fine-tunes peripheral functions or makes adjustments in cases where error-correction is needed, when predictive regulation fails to precisely match local needs. Together, allostasis and homeostasis enable the body’s most metabolically-expensive organ, the brain, to operate near peak efficiency [2].

Energetic regulation occurs via both neural and humoral means, with many of the chemical energetic mediators functioning in both the peripheral body and the brain where they modulate both short- and longer-term shifts in energy mobilization and utilization (e.g., ghrelin, leptin, and lactate; [e.g., ghrelin, leptin, and lactate; 15, 16-21]. For example, plasma ghrelin is high before feeding and low after it [15], and ghrelin receptors are found in all major tissues of the body [22]. Leptin, by contrast, is released predominantly by white adipose tissue stores, and its serum levels change with the cycle of the circadian rhythm and is lower before feeding [18, 23]. Together these two mediators provide interoceptive signals of the peripheral energetic state. Lactate also serves as a hormonal energetic signal [21]. Mediators such as ghrelin and leptin appear to have not only direct endocrine effects in the central nervous system (CNS), but also indirect effects via vagal and spinal visceral afferent signaling [23–25]; see also Interoception and Feeding). Recent evidence in animal models also suggests that interoceptive signaling by cytokine release (e.g., IL-1β, IL-6) can occur via the carotid body, a critical peripheral chemosensor for energetically-relevant mediators, such as oxygen, carbon dioxide, and potentially even glucose [26, 27]. This suggests an emerging and important role for the immune system in interoceptive signaling to the brain about energetic status [e.g., 28]. These newer lines of evidence parallel classical findings [29] showing that pro-inflammatory cytokines in the immune system can convey, via the vagus nerve, interoceptive signals required for generating fever and sickness behaviors. An ongoing challenge however is the difficulty of assessing such interoceptive mechanisms in humans, where one typically cannot safely manipulate these important, life-sustaining, energy regulation signals. Finally, the multiplicity of energetic mediators also reveals considerable multisensory integration both across different interoceptive modalities ([e.g., glucose & pro-inflammatory cytokines; 28]), and also across interoceptive and exteroceptive modalities ([e.g., vision & ghrelin in feeding; 30]).

Interoception and Feeding

Food procurement and consumption are important energy-regulation functions that are strongly influenced by the detection and interpretation of interoceptive signals of energy need, appetite, satiation, and satiety. These signals dynamically fluctuate based on energy status, and can also affect hedonic aspects of food intake control independent of actual energetic need [31]. The most prominent biological pathways by which energy status-related interoceptive states are communicated from the periphery to the brain are (i) the vagus nerve, which communicates gut-derived within-meal signals directly to the caudal brainstem [32], and (ii) endocrine-mediated signaling from the periphery to the central nervous system [33]. Interoceptive energy status cues do not, however, reflexively or obligatorily orchestrate feeding behavior. Rather, the capacity for energy need, appetite, satiation, or satiety to initiate, terminate, or delay feeding behavior requires [a] appropriate interpretation of and attention to these signals, [b] integration of these signals with previous learned experiences, and [c] integration with exteroceptive sensory information [30]. The latter is particularly relevant to foraging behavior, as the integration of interoceptive signals of energetic deficit to initiate foraging must be integrated with visuospatial, olfactory, and other environmental sensory stimuli to successfully locate a food source. Thus, a key feature of energy regulation is the requirement for multisensory integration of interoceptive and exteroceptive cues. Moreover, brain regions that process both interoceptive and exteroceptive energy status signals, including the orbitofrontal cortex, nucleus accumbens, and lateral hypothalamus, are also critical in regulating reward and affective behavior (see Affective and Emotional Experiences), suggesting that such multisensory integration can simultaneously influence multiple, interrelated behavioral outcomes [34].

The hippocampus is a brain region classically studied in the context of visuospatial and episodic memory processing. The hippocampus appears also to be important for the interpretation of energy status signals, and their integration with previous learned experiences and with exteroceptive sensory signals. A growing body of literature has been implicating the hippocampus in the learned control of feeding behavior [30, 35]. For example, rats with selective hippocampal lesions are unable to use interoceptive energy status cues (e.g., 0 vs. 24hr food restriction) as discriminative stimuli for reinforcement [36, 37]. Relatedly, human amnesic patients with nonselective bilateral damage to the hippocampus and adjacent medial temporal lobe structures do not seem to appropriately use hunger, satiation, and satiety cues, as exemplified by their failure to adjust hunger ratings after eating as well as their consumption of multiple consecutive meals [38, 39]. Endocrine signaling, in addition to input via the vagus nerve [40] (see Interoception and Memory), also communicates energy status to the hippocampus, with the hippocampus containing receptors for many feeding-related endocrine and neuropeptide mediators (reviewed in [30]), including ghrelin, which was shown to act in hippocampus as an interoceptive signal of energy need in otherwise food-sated rats [41]. These recent findings of multiple neural and humoral mechanisms of interoceptive signaling from gut to brain also emphasize the continued need for animal model studies for advancing our understanding of the role of interoceptive signaling in the control of feeding, particularly given the lack of effective methods for assessing interoceptive signals from the gut in humans.

Interoception and Urinary Function

Urinary function provides an illustrative case of the role of interoception in a waste removal function, another facet of energy regulation. Regulation of the lower urinary tract (LUT) engages autonomic neural control, including both parasympathetic and sympathetic branches, as well as somatic neural control, and thus is yet another suitable system for studying how interoceptive feedback regulates a complex behavior [42]. The role of pressure feedback in signaling bladder fullness is well established, and this information is integrated with other exteroceptive signals to regulate behavior. For example, urinary urgency is influenced by proximity to the entrance to one’s home, and such cues generated stronger urge responses in persons with overactive bladder than in healthy individuals [43]. More generally, the integration of bladder fullness and other interoceptive and exteroceptive signals influences perception of urinary urgency, and symptoms of urinary incontinence are amplified by psychological stress [44]_, anxiety [45], and depression [46].

Regulation of the LUT also relies on multisensory integration, and in addition to bladder pressure or fullness, recent results also highlight the importance of feedback from the urethra in regulating LUT function. The sensory innervation of the urethra transmits information related to flow [47], and disruption of flow-related feedback reduces the efficiency of bladder voiding [48, 49]. Electrical nerve stimulation can restore bladder emptying efficiency following disruption of flow-related feedback [50], and this has potential clinical implications for urinary retention – the inability to empty the bladder completely [51] – because loss of urethral sensitivity may contribute to inefficient voiding [52].

Flow-related feedback from urethral afferents accommodates rapidly [47], illustrating a ubiquitous feature of primary sensory neurons (both exteroceptive and interoceptive) – adaptation during a persistent stimulus. How is it then that only transient flow-related feedback has such profound effects on efficient bladder emptying? Many species, including mice, rats and dogs, paradoxically exhibit increased phasic activity of the external urethral sphincter (EUS) during voiding which transiently occludes the urethra. Surprisingly, disruption of this phasic EUS activity reduces voiding efficiency in the rat [48, 49] and dog [53], revealing that phasic bursting of the EUS serves as a sensory amplifier. Specifically, the transient closure of the urethra increases activity in flow-responsive afferents, i.e., reduces accommodation, [47], and this increased sensory activity enables more efficient voiding [49]. Further, the imposition of artificially-induced phasic bursting of the EUS in cats, which normally exhibit EUS relaxation during voiding, promoted more efficient voiding by increasing sensory feedback [49]. These data suggest a possible approach to enhance bladder emptying in urinary retention in humans [51].

Emerging research tools enable addressing the challenge of establishing the role of interoceptive feedback on LUT function without the confounding effects of anesthesia. These include the use of awake cystometry in lightly-restrained animals [54], the use of implanted sensors and metabolic cages to quantify the effects of phasic EUS activity on voiding efficiency [55], and wireless implanted telemeters to monitor bladder pressure [56]. The state-of-the-art for rodent studies combines a strain gauge to measure bladder distension and integrated light-emitting diodes opposed to the bladder to illuminate light-sensitive optogenetic channels and modulate sensory nerve activity [57]. In humans, the tools available for assessing interoceptive function in the LUT are much more limited, and include primarily urethral stimulation [58] and anesthesia [59]. Further, epidural electrodes implanted for restoration of function [60] may also enable mapping of human LUT function [61].

Interoception and Memory

Emerging evidence reveals that learning, memory, and other cognitive processes are influenced by interoceptive signaling [62]. Peripheral signals of energy status preferentially target neurons in the hippocampus, thereby promoting learning and memory. The hippocampus is critical for remembering the location and features of extra-personal physical spaces (visuospatial and contextual memory, respectively) and for declarative memory, the memory for facts and episodic events. These memory processes are crucial to foraging and eating behaviors because animals must be able to accurately return to the location of a food source and recall episodic features of feeding events, such as whether a particular food was nutritive, as well as diurnal and seasonal features that influence predator and prey dynamics. Thus, it is reasonable to expect that the central neural systems that receive interoceptive energy status signals are tightly linked anatomically and functionally with those involved in remembering and learning about features of the environment that optimize nutritive intake.

During food intake, afferent vagal fibers convey signals from the upper and lower gastrointestinal tract that directly communicate satiation to the caudal brainstem and initiate meal termination [32]. Signals of gastric distension and intestinal nutrient infusion in rodents also cause increased cerebral blood flow (CBF) to the hippocampus [63, 64], and hippocampal CBF is increased following electrical gastric vagal nerve stimulation (VNS) in humans [65]. The relevance of the gut-hippocampal pathway to learning and memory function is also suggested by neurophysiological and neurochemical alterations in hippocampal neurons following VNS in rodents, including induction of long-term potentiation [66] and increased expression of neurotrophic and neurogenic markers related to memory [67]. A recent study confirmed a physiological role for gut-derived vagal afferent signaling in learning and memory function by revealing that rats with selective ablation of vagal sensory inputs from the upper gastrointestinal tract are impaired in hippocampal-dependent visuospatial working memory and contextual episodic memory [40]. These findings illustrate the idea of multisensory integration of information from both interoceptive (energy status signals from the gut) and exteroceptive (visuospatial navigational and contextual cues) sources. A conceptual framework for these findings posits that meal-derived vagal signaling primes the hippocampus to encode mnemonic details associated with eating behavior (e.g., location of food source, episodic memory) to more efficiently direct future feeding behaviors [35].

Other hormonal systems also provide interoceptive information about energy status and promote enhanced memory function via direct action on hippocampal neurons. Receptors for leptin, ghrelin, glucagon-like peptide-1 (GLP-1), and insulin are expressed in the hippocampus (see also Allostasis and Homeostasis above), fluctuate with energy status and influence feeding and metabolism. Further, direct action of these signals on hippocampal neurons enhances hippocampal-dependent learning and memory [see 35 for further review]. Of these signals, ghrelin is unique in that it is a peripherally-derived appetitive meal anticipation signal, suggesting that enhancement of hippocampal-dependent memory processes is not mediated exclusively by interoceptive signals related to satiation (vagal signaling, GLP-1) but also includes interoceptive signals of energy need [68]. Recent data thus illustrates that multisensory integration of interoceptive and exteroceptive signaling regulates feeding behavior, including important features of memory for prior feeding experiences that optimize the efficiency of energy regulation.

Affective and emotional experience

Affective and emotional experience represent phenomena that have been proposed to be importantly related to interoceptive signaling for at least the last 100 years [for reviews see 69, 70]. We refer to affective feelings as the lower-dimensional qualia of experience that are most commonly described as having features of valence (pleasant vs. unpleasant) and arousal (activated vs. deactivated). We consider emotional experience as arising from interoceptive sense data that are categorized in a specific situation using an emotion concept such as ‘anger’ or ‘happiness’. Although virtually all hypotheses about the nature of emotion view interoception as playing at least some role in affect and emotion, the experimental evidence for the specific role is incomplete because the phenomena are experiential and subjective, and thus only measurable via self-report. A key issue is that the design of many studies has blurred the important distinction between what participants believe or expect about their bodily changes vs. actual changes in interoceptive signaling. This has led to confusion about the specific role of interoceptive signals in affective and emotional experience [71]. Early work such as [72] used physiological manipulations to alter interoceptive signaling and thereby affect, but on the whole, this work has been difficult to replicate, likely due to the crucial additional role of the external context [73]. These studies also illustrate how difficult it is with current interoceptive methods in humans to study the specific impact of interoceptive signals on affective and emotional experience. Several emotion hypotheses, including recent construction-based approaches, propose that interoception is a basic function of the nervous system and is not specific to emotional events or emotional experiences [74–76]. Rather, they propose that interoceptive and exteroceptive sense data are continually predicted and used by the CNS to maintain allostasis and, in the process, make meaning about the relationship between one’s body, its predicted needs, and the external world [for a review of these constructionist approaches see 77]. Specifically, the active inference-based views of Barrett and colleagues [75, 76] propose significant multimodal integration across multiple interoceptive and exteroceptive signals. It was further proposed that interoception is achieved by a combination of prediction and encoding of ascending interoceptive signals [75, 76], such that the brain is always running an internal model of the body to estimate body status, a model that is continually updated by changing ascending sensory signals (i.e., interoceptive prediction errors). Other recent theoretical perspectives also suggest that instances of emotion arise from active inference about the presumed causes of interoceptive signals [78, 79].

The strongest evidence for the role of interoception in emotional and affective experience is provided by the striking overlap in the neuroanatomical substrates that underlie interoceptive function, body regulation, and emotional and affective experience [76, 80–83]. Other evidence for a role for interoception in affective and emotional experience comes in the form of temporal covariation of changes in peripheral physiology (such as heart rate or skin conductance) and changes in affective experience while viewing affectively-evocative pictures or other stimuli [see Study 4; 84]. A current methodological limitation is that one is constrained to using either temporal or magnitude covariations, and these provide only indirect evidence for the role of interoceptive signaling and interoceptive processes in affective experience. This reliance on correlational evidence is a major limitation in understanding the role of interoceptive sense data in affect and emotion. Two recent papers ([77] and [85]) proposed that the accessibility of C-tactile afferent nerve endings in the skin that mediate so-called affective touch may provide an externally-accessible experimental channel for manipulating sensory input that, by virtue of its anatomy, appears “interoceptive” (see Box 1 for a description of affective touch). Applying this approach and perhaps other new methods for directly manipulating interoceptive sense data, would be instrumental in examining how exactly interoceptive signals impact affect and emotional experience.

Box 1: Affective Touch.

Studies demonstrate that the hairy skin of mammals contains C-tactile afferent (or CT) fibers that are preferentially activated by stroking at a force, velocity and temperature similar to the ones typically used when primates stroke one another [113]. Importantly, stroking that activates these fibers is rated as affectively pleasant. These fibers have many features consistent with those of interoceptive afferents (i.e., they are thin, unmyelinated, and diffusely arrayed). Critically, in a patient with a selective loss of large-diameter myelinated sensory afferents in hairy skin, activation of intact CT afferent fibers activated the dorsal posterior insula, much like other interoceptive afferents, but did not activate primary or secondary somatosensory cortex, as would be expected with exteroceptive signals [114]. Because CT afferents are located in skin, these sensors are much more accessible to experimental manipulation, and recordings from these afferents can be made in humans using microneurography. Manipulation of affective touch presents a potentially promising way in which researchers can study interoceptive signaling in humans [85]. The comingling of many different afferent fibers, both myelinated and unmyelinated, in the hairy skin of mammals also serves as an example of how touch sensations can easily blur the classical distinction between interoceptive and exteroceptive processes.

Interoception and the Psychological Sense of Self

From the perspective of psychology, the question of selfhood is intimately linked to the question of body-awareness. However, much of the early work on body-awareness focused on the perception of one’s own and others’ bodies from the outside, in terms of exteroceptive, observable sensory and motor events. A prototypical example of this exteroceptive route to selfhood is mirror self-recognition [86], an ability considered a hallmark of self-awareness. The sense of body-ownership has also been studied in the context of exteroceptive multisensory integration, in particular visual and tactile integration. In the Rubber Hand Illusion (RHI), an influential experimental model of embodiment, watching a rubber hand being stroked synchronously with one’s own unseen hand causes a person to experience the rubber hand as part of their body ([6]; see Box 1 for a consideration of how the rubber hand illusion could arise from multimodal interoceptive-exteroceptive sensations). Such evidence suggests that self-awareness is highly malleable and subject to the perception of the body from the outside.

More recently, research has increasingly begun to consider the role that interoceptive signals and their awareness play in shaping self-awareness. From the somatic markers hypothesis [87] and the neuroanatomical and functional focus on the insula [88] to more recent predictive coding and computational models of interoception [89, 90], current views on the bodily basis of selfhood instead consider the visceral body and its cortical representation to be a core element of the self.

A study that aimed to examine the relationship between interoceptive and exteroceptive awareness of the body reported observing a negative correlation between levels of interoceptive awareness and the malleability of self-awareness as a result of multisensory exteroceptive input in the classic RHI [91]. Participants with lower interoceptive awareness experienced a stronger alteration in body-ownership, suggesting that in the absence of accurate interoceptive representations one’s model of self is dominated by exteroceptive input (see also [92–95]). Beyond the conscious aspect of interoception, other studies used a combination of concurrent interoceptive and exteroceptive signals to study how these are integrated in body awareness [96–98], and also how interoceptive signals may impact self-processing [99, 100], as well as a range of cognitive [101–104] and social-affective processes [105, 106].

Within predictive coding accounts of bodily self-awareness, individuals may differ in interoceptive accuracy and awareness in part due to the relative precision of incoming interoceptive vs. exteroceptive signals used to compute the relative probabilities that a given sensory signal is related to one’s own body [107]. In this context, multisensory integration contributes to the perception of exteroceptive signals alongside an ever-present flow of interoceptive sense data. Although interoceptive signals are typically in the background of phenomenal awareness (i.e., one does not focus their attention on activity in their gut during all waking hours), interoceptive signals can sometimes be more or less foregrounded in attentional awareness.

Despite the growing research on interoception in adulthood, questions of when and how interoceptive awareness emerges in early life remain unexplored, partly because of the limited range interoceptive measures for use in humans, especially in children [108]. However, with new measures [109] and experimental tasks [108] and insights from studies of psychophysiological coupling between caregivers and infants [110], researchers can now begin to test hypotheses [111, 112] about the inherently social origins of interoceptive awareness. Given that in infants, key allostatic and homeostatic processes are still emerging, caregivers play a key role in both inferring the infant’s hidden interoceptive perturbations that give rise to behavioral expressions of affective feelings and in providing an appropriate response to assist in regulating the infant’s needs. It is through such embodied interactions that the infant’s brain will slowly start mentalizing her own interoceptive sensations and eventually take appropriate actions [111].

Concluding Remarks

As the preceding attests, interoception plays a role in a wide array of human functions, from how we gather resources and remove waste to optimally support our energetic needs to how we view ourselves as distinct from others and the world around us. Although there are distinctive features of interoceptive and exteroceptive sensory anatomy (see [5]), real-world sensations occur in the context of continual and extensive multisensory integration across both interoceptive and exteroceptive systems with impacts on every function reviewed here. Recent studies have broadened the scientific perspective on the important role that interoceptive modalities play in multisensory integration in addition to the more well-studied exteroceptive modalities. Our review, while selective rather than comprehensive, examined research across a number of interoceptive functions to illustrate both how interoceptive sense data are integrated with other sensory modalities as well as the difficulties inherent to manipulating and measuring interoceptive signaling in humans. The hope is that new techniques, such as electrical stimulation of the vagus nerve or the use of methods to manipulate affective touch may provide additional tools for studying interoceptive processes in humans.

Outstanding Questions.

• How does the brain use interoceptive signals from across different organ systems to estimate the current energetic status of the body?

• How does the brain integrate its ongoing (predictive) model of the body with ascending interoceptive signals?

• How does the brain transform interoceptive signals into affective feelings and emotional episodes, with and without emotional experiences?

• How is interoceptive signaling affected by the widespread neural, hormonal, and other physiological changes occurring across different developmental periods? Specifically, what are key developmental features of interoceptive signaling and interoceptive awareness during life stages such as infancy and childhood, old age, or during puberty, a period that is especially critical for mental health?

Highlights.

• A key function of interoception is signaling about the body’s energy status, which then drives the behaviors needed to renew energy resources.

• Methodologies to manipulate and measure interoceptive signaling in humans are currently limited, which presents a challenge for fully understanding some of the functions of interoceptive processing.

• Recent work suggests that the accessibility of C-tactile afferent nerve endings and their fibers in mammalian hairy skin, a pathway which mediates affective touch and shares anatomical features with classical interoceptive afferents, may provide an externally accessible experimental channel for manipulating interoceptive signaling in healthy humans.

• In real-world situations, sensations (e.g., touch) are usually multimodal, with the co-location of sense data coming from activity in both interoceptive (affective touch) and exteroceptive (tactile) afferents.

Glossary

Affective feelings

The lower-dimensional qualia of experience that are most commonly described as having features of valence (pleasant vs. unpleasant) and arousal (activated vs. deactivated). Many theories of emotion suggest that affective feelings arise from interoceptive processes

Affective touch

Touch that preferentially activates C-tactile afferent (or CT) fibers and is reported by humans to be affectively pleasant. Sensations associated with affective touch most often occur when hairy skin is stroked at a moderate force and velocity, with stimuli at body temperature. These sensations mimic those used in common interactions among primates such as grooming or caressing

Allostasis

Physiological regulatory processes that enhance an organism’s efficiency by anticipating future needs and preparing to meet them before they arise

Cystometry

A technique for measuring the contractile force exerted by the bladder during the retention or voiding of urine

Emotion

Emotions constitute events that arise from the brain’s efforts to give meaning to sense data, including interoceptive data, which are categorized in a given situation using an emotion concept such as ‘anger’ or ‘happiness’. Emotion concepts are used to categorize when affective feelings are strong but also based on current external, situational cues. Emotional events always include experience (e.g., the experience of the world as threatening or novel) but only sometimes include the conscious experience of emotion (e.g., an awareness that one is fearful)

External Urethral Sphincter

The external urinary sphincter, made of skeletal muscle, is controlled by motor fibers of the pudendal nerve. When activated, the external urinary sphincter inhibits voiding of the bladder

Ghrelin

A peptide released in substantial amounts in the gut, and which is a key regulator of nutrient sensing (including glucose regulation), meal initiation, and appetite

Homeostasis

Physiological regulatory processes that react to current needs, and can occur with or without central nervous system (CNS) input. In this view, homeostatic changes can be highly regular or temporally-patterned, and in such cases, regulation may require little modulation by the CNS

Lactate

Lactate is a salt produced as a byproduct of the breakdown of glucose under anaerobic conditions. In addition, lactate, although not typically a primary fuel like glucose, may operate as a glucose-sparing alternative fuel under conditions such as intense exercise where lactate levels in blood and brain increase

Leptin

A peptide released predominantly by adipose tissue, which is a key regulator of meal initiation and appetite. Circulating leptin levels are typically higher in those with a higher body mass index (BMI), and the brain is less sensitive to leptin in those with higher BMI than those with lower BMI

Lower Urinary Tract

The lower urinary tract consists of the bladder, urethra, and in males, the prostate

Microneurography

Method for recording from peripheral nerves that lie close to the skin’s surface in humans. An experimenter uses a very small diameter needle to impale a nerve underlying the skin. The needle has a small exposed tip, and is used to record multiunit neural activity without the need for anesthesia

Multisensory integration

The process by which sense data from two or more modalities are combined to form a product that is distinct from, and thus cannot be easily ‘deconstructed’ to reconstitute, the components from which it is created (also called multimodal integration)

Satiety

The physical feeling of being fed to or beyond one’s capacity, i.e., a feeling of being full

Satiation

Satiation refers to the cessation of additional desire to continue eating