Compromised inter-hemispheric transfer of information partially mediates cognitive function deficits in adolescents with Fetal Alcohol Syndrome

Abstract

Background:

Prenatal alcohol exposure (PAE) has been associated with compromised inter-hemispheric transfer of tactile stimuli in childhood and structural changes to the corpus callosum (CC). In this study we used a finger localization task (FLT) to investigate whether inter-hemispheric transfer deficits persist in adolescence, whether effects of PAE on perceptual reasoning, working memory, and executive function are mediated by deficits in inter-hemispheric transfer of information, and whether CC size in childhood predicts FLT performance in adolescence.

Methods:

Participants were 16- to 17-year-olds from the Cape Town Longitudinal Cohort, whose mothers were recruited during pregnancy and interviewed regarding their alcohol use using the timeline follow-back approach. Diagnoses of fetal alcohol syndrome (FAS) and partial FAS (PFAS) were determined by two expert dysmorphologists; non-syndromal exposed children were designated as heavily exposed (HE); those born to abstainers or light drinkers, as controls. The FLT was administered to 74 participants (12 FAS, 16 PFAS, 14 HE and 32 controls). CC size at age 9–12 years was available for 35 of these participants (7 FAS, 13 PFAS, 5 HE and 10 control).

Results:

Although degree of PAE was similar in the FAS, PFAS and HE groups, only the adolescents with FAS showed more transfer-related errors than controls in conditions in which one finger was stimulated. FLT performance mediated effects of FAS on perceptual reasoning and executive function. In the subsample for whom neuroimaging data from childhood were available, there was an association among those with PAE of smaller CC volumes with more transfer-related errors on the one-finger/hand hidden condition, suggesting that CC damage previously seen in childhood continues to impact function through adolescence.

Conclusions:

This study provides evidence of compromised inter-hemispheric transfer of information in adolescents with FAS, while those with PFAS or heavy exposed non-syndromal individuals are apparently spared. It is the first to show that PAE effects on important aspects of cognitive function are partially mediated by deficits in inter-hemispheric transfer of information.

Introduction

Fetal alcohol spectrum disorders (FASD) is an umbrella term used to encompass the range of deficits associated with prenatal alcohol exposure (PAE) (Hoyme et al., 2005, 2016). Within FASD, fetal alcohol syndrome (FAS) is the most severe of the disorders and is characterized by growth deficits, microcephaly, and a distinctive pattern of craniofacial dysmorphic features as well as a broad range of lifelong cognitive and behavioral deficits (Glass et al., 2014; J. Jacobson et al., 2021). Partial FAS (PFAS) is characterized by presence of at least two of the three sentinel craniofacial dysmorphisms and height, weight, or microcephaly. The Western Cape Province of South Africa has one of the highest prevalence rates of FAS worldwide (May et al., 2000, 2013; Viljoen et al., 2005). Historic socio-cultural factors have had profound effects on this population, perpetuating the prevalence of heavy PAE (May et al., 2013, 2008). Numerous studies on structural brain changes have been conducted in those prenatally exposed to alcohol (see reviews, e.g., Lebel et al., 2012; Donald et al., 2015) and how these are linked to specific behavioral deficits (Riley et al., 2003).

Previous studies suggest that white matter may be particularly susceptible to the teratogenic effects of PAE (Archibald et al., 2001; Fan et al., 2016), and several diffusion tensor imaging (DTI) studies have highlighted associations of microstructural changes with cognitive deficits (Ma et al., 2005; Lebel et al., 2008; 2011; Sowell et al., 2008; Fryer et al., 2009; Wozniak et al., 2009; 2011; Fan et al., 2016). The corpus callosum (CC) is one of the most researched white matter structures (Lebel et al., 2012; Riley and McGee, 2005) with early autopsy studies of FAS reporting abnormal shapes, hypoplasia or, in extreme cases, agenesis (Clarren et al., 1978; Jones and Smith, 1973; Peiffer et al., 1979; Wisniewski et al., 1983). Neuroimaging of individuals with FASD has demonstrated CC hypoplasia on mid-sagittal magnetic resonance (MR) images (Mattson et al., 1992; Sowell et al., 2001; Autti-Rämö et al., 2007; Jacobson et al., 2017), CC displacement (Sowell et al., 2001; Bookstein et al., 2002), and disorganized white matter fibre tracts and myelination disturbances (Fan et al., 2016; Ma et al., 2005; Sowell et al., 2008; Wozniak et al., 2009). Functionally, thickening of the CC has been linked to deficits in executive function, whereas a thinner CC has been linked to deficits in motor function (Bookstein et al., 2002) and CC shape abnormalities with verbal memory impairment (Sowell et al., 2001). These findings point to an important role of the CC and associated WM tracts in behavioral and functional outcomes associated with FASD and PAE.

The CC, which is the largest commissural tract in the brain, plays an important role in transferring information from one side of the cortex to the homologous region of the brain in the opposite hemisphere (Quinn and Geffen, 1986). CC damage may lead to impaired inter-hemispheric functional connectivity, which has been demonstrated in children aged 10–17 years with FASD in medial parietal (para-central) regions connected by posterior callosal fibre projections (Wozniak et al., 2011) and in neonates with PAE between the left and right somatosensory networks (Donald et al., 2016). Inter-hemispheric transfer of tactile information, which relies on an intact CC and somatosensory areas, has been investigated in both normal and clinical populations using the finger localization task (FLT). In this task, the participant is asked, after stimulating the tip of his/her finger (or series of fingers), to report which finger(s) were touched (Quinn and Geffen, 1986; Pipe, 1991; Roebuck et al., 2002; Dodge et al., 2009). During crossing conditions, when participants are required to report on the opposite hand from the one touched, the signal is transmitted to the opposite hemisphere via the CC. Specifically, the touch is registered in the ipsilateral primary somatosensory cortex within the parietal lobe from where it is transmitted to the somatosensory association area in the posterior parietal cortex. From there it crosses over via the CC to the contralateral somatosensory association area and is transmitted to the contralateral motor cortex for physical report by the participant (i.e., indicating on the opposite hand which finger(s) were stimulated) (Quinn and Geffen, 1986). While the error rate for normal adults is 7% higher on the FLT in the crossed conditions (Geffen et al., 1985), it was 28% higher in participants with a partially sectioned midline CC and 82% higher in those with full commissurotomy. Performance on the FLT can, therefore, serve as a good proxy for inter-hemispheric transfer of tactile information and can provide insight into the functional integrity of the CC (Geffen et al., 1985).

Roebuck and associates (2002) found that children (aged 8–15 years) prenatally exposed to alcohol made more errors than nonexposed controls when the FLT increased in complexity, and, in a subset for whom neuroimaging data were available, that smaller CC size was related to poorer FLT performance. In a cross-sectional sample of school-aged children (aged 9–12 years) from Cape Town, Dodge et al. (2009) reported that those diagnosed with FAS showed greater impairment in inter-hemispheric transfer of tactile information on the FLT compared to other heavily exposed children (including four with partial FAS (PFAS)) and controls. In a small subset (7 PAE and 7 controls) of children from this heavily exposed Cape Town cohort that had previously been assessed on a 1.5T Siemens Symphony scanner, greater differences between performance on uncrossed and crossed trials were associated with smaller CC size for this group. Dodge et al. also reported findings from an independent study of young adults (aged 19 years) from the Detroit Longitudinal Cohort: a binge pattern of drinking during pregnancy was associated with more transfer-related errors on the FLT.

Roebuck et al. (2002, p. 1870) noted that, given the links between CC function and attention (Banich, 1998; Hynd et al., 1991; Roeltgen and Roeltgen, 1989), learning disabilities (Davidson et al., 1990; Njiokiktjien et al., 1994), and behavioral functioning (O’Brien, 1994), and that these domains are compromised in many children with heavy PAE, “Inefficient processing between the two cerebral hemispheres is likely to have significant implications for cognitive and psychosocial functioning in FASD, particularly as complexity of information increases.” The authors suggest that future studies examine the extent to which effects of PAE on these domains are attributable to deficits in inter-hemispheric transfer of information. Performance on the FLT task depends heavily on perceptual function (perceiving which finger is touched by the examiner); working memory (retention of that information and utilizing it to respond by indicating which finger was touched); and executive function, which is needed for selecting and successfully monitoring behaviors that facilitate the attainment of the task goals.

Given our recent findings of an inverse relation between extent of PAE and CC size in newborns (Jacobson et al., 2017) and in 9- to 11-year-old children from our Cape Town Longitudinal Cohort (Biffen et al., 2018), and of poorer performance on the FLT by children with FAS in our independent, cross-sectional Cape Town-based school-aged sample cited above (Dodge et al., 2009), we hypothesized that PAE and smaller CC size would both be related to poorer performance on the FLT in adolescence. In this paper we examine (i) whether the impairment in inter-hemispheric transfer of tactile information seen in FASD at school age persists into adolescence pointing to permanent damage rather than developmental delay, (ii) the degree to which effects of PAE on perceptual reasoning, working memory, and executive function are attributable to or mediated, in part, by deficits in inter-hemispheric transfer of information, and (iii) whether CC size measured in childhood in a subset of the children predicts FLT performance in adolescence.

Methods

Participants

Participants were 74 16- to 17-year-old adolescents from our Cape Town Longitudinal Cohort (Jacobson et al., 2008; S. Jacobson et al., 2021). The Cape Town Cohort consists of children born to mothers recruited between 1998–2002 at their first visit to an antenatal clinic in a historically disadvantaged community where the prevalence of alcohol use is unusually high (Croxford and Viljoen, 1999). Consents and interviews were conducted in the participants’ preferred language (Afrikaans or English). Approval was obtained from the ethics committees at Wayne State University and the University of Cape Town Faculty of Health Sciences. 35 of these adolescents had completed MRI scans when they were 9–12 years of age (e.g., see Biffen et al., 2018; Jacobson et al., 2017).

Ascertainment of maternal alcohol, cigarette smoking, and drug use.

At time of enrolment, pregnant women were interviewed about their daily alcohol use during the previous 2 weeks using the “gold standard” timeline follow-back (TLFB) approach (Jacobson et al., 2002, 2008). For each type of alcoholic beverage consumed, volume of alcohol consumed each day was recorded and converted to ounces of absolute alcohol (AA; 1 oz AA ≈ 2 standard drinks) using the following weights: liquor = 0.4, beer = 0.05, wine = 0.12, cider = 0.06 (adapted from Bowman et al. (1975) to reflect values for beer, wine and hard liquor sold in Cape Town). Binge drinking was defined as 5 or more standard drinks per occasion at that time. Any woman who reported alcohol consumption of at least 1 oz AA/day or binge drinking on at least 2 occasions within the first trimester was invited to participate in the study. A comparable number of women from the same clinic were recruited at the same time to participate as controls if they reported abstaining or drinking minimally and no binge drinking. Exclusion criteria for participation were women younger than 18 years of age or those with chronic medical problems, including diabetes, epilepsy or cardiac problems. Infants from multiple births were excluded, as well as infants presenting with major chromosomal anomalies, seizures and neural tube defects.

The women were again administered the TLFB interview in mid-pregnancy regarding their alcohol use during the previous 2 weeks and at 1-month postpartum, regarding a typical 2-week period during the latter part of pregnancy. Data from the three TLFB interviews were averaged to provide three summary measures: oz AA/day averaged across pregnancy, oz AA/occasion, and frequency (no. drinking days/week). We have previously validated the TLFB interview in relation to fatty acid ethyl esters (FAEEs) and biologically stable metabolites of alcohol that are deposited in meconium (Bearer et al., 2003), as well as to infant outcomes (Jacobson et al., 2002). At each of the interviews the women were also interviewed regarding number of cigarettes smoked/day and how many days/month they used marijuana, methaqualone (“mandrax”), cocaine, or other illicit drugs during pregnancy.

FASD diagnosis.

The children were examined by two expert dysmorphologists (H.E. Hoyme (HEH), M.D., and L.K. Robinson (LKR), M.D.) in a clinic held in 2005 (Jacobson et al., 2008). Children were examined for growth and FAS-related dysmorphic features using a standard protocol (Hoyme et al., 2005) based on the Revised Institute of Medicine (IOM) criteria. The determination of which children met criteria for diagnosis with FAS or PFAS was made during case conferences conducted with the dysmorphologists, SWJ, JLJ, and CDM. A final consensus diagnosis was reached based on maternal alcohol history, growth, dysmorphic features, and neurobehavioral assessments. If children did not meet criteria for either of these syndromal groups they were designated, based on maternal alcohol consumption, as either non-syndromal heavily exposed (HE) or controls. Five participants could not attend the clinic in 2005 and were seen by another expert dysmorphologist (N. Khaole, M.D.) There was substantial agreement between HEH and LKR on their assessments of the dysmorphic features, including palpebral fissure length and philtrum and vermilion ratings based on the Astley and Clarren (2001) rating scales (r-values = 0.80, 0.84, and 0.77, respectively) as well as between them and the Cape Town-based dysmorphologist (N. Khaole; median r = 0.78). The diagnoses were subsequently confirmed when the children were re-examined by HEH and LKR in 2009 and by a team of dysmorphologists led by HEH in follow-up clinics in 2013 and 2016 (see S. Jacobson et al., 2021).

Finger Localization Task (FLT)

The FLT was administered to the adolescents in this cohort by examiners blind regarding PAE exposure and FASD diagnosis, except in a few extreme cases where FAS was clearly apparent. In the FLT, the examiner gently touches the fingertips of the participant with a pencil (Fig. 1). Participants are asked to report which finger was touched by touching the stimulated finger with their thumb. In uncrossed trials, the participant touches his/her thumb to the actual finger stimulated, while for crossed trials s/he indicates which finger was touched on the opposite hand. The task comprises uncrossed and crossed trials for each of three conditions: (1) one finger touched with hands visible to the participant, (2) one finger touched with hands hidden from the participant, and (3) two fingers touched consecutively with hands hidden. In the condition where two fingers are touched, the participant is asked to report the stimuli in the same order as they were presented. Within each condition there were four 16-trial blocks: (1) right hand, uncrossed; (2) left hand, uncrossed; (3) right hand, crossed; (4) left hand, crossed. Each condition always began with the uncrossed trials, but the starting hand was counterbalanced across subjects. Each condition was repeated twice – once with the fingers on the left hand being stimulated, and once with those on the right hand being stimulated. Each trial always began with the uncrossed condition, but the starting hand (right, left) was counterbalanced across subjects.

Figure 1:

Pictures showing the middle finger on the left hand being stimulated by touch (top), and responses for uncrossed (left bottom) and crossed (right bottom) trials, respectively. On uncrossed trials there is no inter-hemispheric transfer of tactile information.

A cardboard box with windows on both sides (one covered with a cloth for the participants to insert their hands into and one for the researcher to administer the test through) was used for the hidden conditions. Before each condition the participants were told whether one or two fingers would be touched and whether they were expected to report back on the same or opposite hand. Instructions were administered in the participant’s first language (Afrikaans or English) to make the task as clear as possible. Self-corrections were allowed. No feedback regarding performance was provided by the examiner.

For each condition and each hand, the number of errors on uncrossed and crossed trials were recorded separately.

Cognitive and Behavioral Outcomes

The cognitive and behavioral outcome measures were comprised of data collected on the Wechsler Intelligence Test for Children, 4^th edition (WISC-IV) at age 10.2 ± 1.0 (mean ± SD) years and the Delis-Kaplan Executive Function System (D-KEFS) at age 16.4 ± 0.8. The WISC-IV assessments were conducted by two and the D-KEFS by three MA-level neuropsychology students in Afrikaans or English, depending on the language of instruction in the child’s school. The principal cognitive outcome measures were WISC-IV IQ, the WISC-IV Perceptual Reasoning Index, WISC-IV Digit Span Backward (a measure of working memory), and Executive Function. An Executive Function composite score was constructed by converting to z scores and averaging the five measures from the D-KEFS (Stroop Color-word Interference Test inhibition and shift; Verbal Fluency letters and categories; and total achievement score on the Tower Planning Test).

Neuroimaging

Neuroimaging data from childhood (mean age ± SD: 10.6 ± 0.7 years) were available for 35 (7 FAS, 13 PFAS, 5 HE and 10 control) of the 74 adolescents assessed on the FLT. On average, the 5 children in the HE group were 13 months older than those in the other groups at the time of scanning. MRI scans were acquired at the Cape Universities Brain Imaging Centre (CUBIC) using a 3 T Allegra MRI scanner (Siemens Medical Systems, Erlangen, Germany). High-resolution T1-weighted images were acquired using a volumetric navigated (Tisdall et al., 2012) multi-echo magnetization prepared rapid gradient echo (MPRAGE) sequence optimized for morphometric neuroanatomical analysis using FreeSurfer software (van der Kouwe et al., 2008). Imaging parameters were: FOV 256×256mm, 128 sagittal slices, TR: 2530mm, TE:1.53/3.21/4.89/6.57mm, TI: 1100ms, flip angle: 7°, voxel size: 1.3×1.0×1.3 mm³, acquisition time: 8:07 minutes. The 3D EPI navigator provided real-time motion tracking and correction to reduce artifacts resulting from motion. CC volumes were determined by performing automated segmentation with FreeSurfer v 6.0 (http://surfer.nmr.mgh.harvard.edu/).

Statistical Analyses

All statistical analyses were performed using SPSS version 25 (SPSS Inc, Chicago, IL). Two maternal (smoking during pregnancy and education) and five child (age, sex, total intracranial volume (TIV), lead exposure, and IQ) characteristics were considered as potential confounders. Any control variable found to be related even weakly (p ≤ 0.10) to an outcome measure was considered a potential confounder and adjusted for statistically using analysis of covariance (ANCOVA) or multiple regression.

For each condition (one-finger/hand visible; one-finger/hand hidden; two-finger/hand hidden), we ran two factor repeated measures analyses of variance (ANOVA) for the sample as a whole to examine main effects of inter-hemispheric transfer (crossed/uncrossed trials) and stimulated hand (right/left) on the number of errors, as well as interaction effects of stimulated hand on inter-hemispheric transfer.

Since the difference between the number of errors on crossed and uncrossed trials of each condition (crossed-uncrossed differences, CUDs) represent the increase in transfer-related errors compared to other errors, this measure was used as a proxy for inter-hemispheric transfer deficiency. CUDs were computed for each of the three conditions: CUD1 for one-finger/hand visible; CUD2 for one-finger/hand hidden; CUD3 for two-fingers/hand hidden. One-way ANOVAs were then run for each condition to examine the effect of FASD group (Controls/HE/PFAS/FAS) on crossed minus uncrossed errors (CUDs). ANCOVAs were used to control for potential confounders. Spearman correlation was used to examine the relation between the CUDs and the continuous measures of PAE.

One-way ANOVA and ANCOVA were used to examine the effects of FASD group on the four cognitive outcome measures described above: WISC-IV IQ, the WISC-IV Perceptual Reasoning Index, Working Memory (assessed on WISC-IV Digit Span Backwards), and an Executive Function composite score. The degree to which inter-hemispheric transfer deficiency (crossed minus uncrossed errors) mediated the effects of FASD diagnosis on each of those outcomes was examined using the MEDIATE macro for SPSS (Hayes and Preacher, 2014), a multivariate extension of the Sobel (1982) method that permits the use of multi-categorical independent variables. Mediation is determined based on the size of the indirect effect, which is the product of the regression coefficients for (1) the FASD effects on the mediating variable (inter-hemispheric transfer), and (2) the effect of the mediating variable on the outcome variable. Statistical significance of the indirect effects is determined based on 95% bootstrap percentile confidence intervals. Indirect effects are deemed statistically significant when the confidence intervals do not overlap with 0. We used t-tests to determine p-values for the regression coefficients that measured the indirect effects.

Within the subset of adolescents for whom structural MRI data from childhood were available, we examined whether reduced CC size in childhood predicts inter-hemispheric transfer deficiency in adolescence. Pearson correlation was used to examine associations of FreeSurfer CC volumes with CUDs. Multiple linear regression was used to control for potential confounders. Because the direction of the association between CC size and FLT performance had been documented in two prior studies (Roebuck et al., 2002; Dodge et al., 2009), 1-tailed significance tests were applied in these analyses.

Results

Sample characteristics are summarized in Table 1. Groups did not differ by sex. Adolescents with FAS or PFAS were, on average, 7 months older than controls, and had lower IQs than those in the HE and control groups. Mothers of adolescents with FAS or PFAS had poorer socioeconomic status and fewer years of formal education than mothers of adolescents in the control group, and a higher number of previous live births (parity) than mothers of adolescents in the HE and control groups. All alcohol exposed adolescents had higher levels of lead exposure than controls, and fewer of their mothers were married. Per design of the study, heavy alcohol users consumed more alcohol during pregnancy than mothers in the control group. Mothers of children with FAS, PFAS and HE drank on average 7–9 standard drinks/occasion. All but one mother in the control group abstained from drinking during pregnancy; the one exception was a light drinker who consumed 2 drinks/occasion on 2–3 days during pregnancy. Mothers of the alcohol exposed children smoked more cigarettes during pregnancy. The mothers of nine adolescents (1 FAS, 1 PFAS, 5 HE, 2 controls) used marijuana during pregnancy; of these, three (1 FAS, 1 PFAS, 1 HE) were in the sub-sample for whom neuroimaging data from childhood were available. The mothers of two adolescents (both in the HE group) used methaqualone during pregnancy – one mother used methaqualone an average of 3.2 times/month and the other 2.5 times/month; neither of these adolescents were scanned during childhood.

Table 1.

Sample characteristics

	Control (n = 32)	HE (n = 14)	PFAS (n = 16)	FAS (n = 12)	Total (N = 74)	F or χ2
Child characteristics
Sex: male n (%)	19 (59.4)	8 (57.1)	10 (62.5)	6 (50.0)	43 (58.1)	0.48
Age at FLT (yr)a	16.1 (0.8)	16.4 (0.7)	16.7 (0.7)	16.7 (0.7)	16.4 (0.8)	2.62†
WISC-IV IQb	76.9 (12.4)	79.9 (10.7)	64.3 (10.1)	65.2 (10.2)	72.8 (12.8)	8.07***
Lead (ug/dl)c	6.8 (2.9)	10.3 (3.2)	10.8 (6.3)	11.8 (4.7)	9.1 (4.6)	6.06***
Intracranial volume (m3)d	1303.1 (117.5)	1331.1 (127.3)	1330.7 (142.1)	1141.5 (120.9)	1289.2 (140.2)	6.23***
Maternal characteristics
Age at delivery (yr)e	25.2 (4.6)	25.1 (5.7)	27.6 (7.3)	31.2 (7.5)	26.7 (6.2)	3.37*
Socioeconomic statusf	23.4 (8.4)	19.1 (5.6)	16.5 (6.4)	16.5 (10.9)	19.7 (8.4)	3.28*
Parityg	1.6 (0.9)	1.8 (1.0)	2.8 (2.1)	3.3 (1.9)	2.2 (1.6)	5.14**
Married n (%)h	14 (43.8)	1 (7.1)	2 (12.5)	2 (16.7)	19 (25.7)	9.96*
Education (yr)i	10.0 (1.8)	9.1 (2.5)	6.9 (2.4)	8.3 (2.2)	8.9 (2.4)	7.82***
Alcohol across pregnancy
AA/day (oz)j	0.0003 (0.002)	1.2 (1.0)	1.1 (0.7)	1.5 (2.0)	0.7 (1.1)	11.43***
AA/occasion (oz)j	0.04 (0.2)	4.8 (3.5)	3.6 (1.2)	4.5 (1.7)	2.4 (2.8)	36.27***
Frequency (days/week)j	0.002 (0.1)	1.9 (1.0)	2.2 (1.0)	1.8 (1.7)	1.1 (1.3)	29.77***
Other prenatal exposure
Cigarettes/dayk	4.9 (1.9)	8.7 (5.7)	8.1 (6.2)	8.4 (5.3)	7.5 (5.2)	1.42
Marijuana (days/month)l	2.2 (0.7)	3.8 (2.5)	3.1 (--)	0.9 (--)	3.0 (2.1)	0.55

Values are mean (SD). HE=heavily exposed non-syndromal; FAS=fetal alcohol syndrome; PFAS=partial FAS; WISC-IV=Wechsler Intelligence Scales for Children-Fourth Edition.

^aControl < FAS, PFAS (p’s ≤ 0.01)

^bFAS < HE and Control (p’s ≤ 0.003); PFAS < HE and Control (p’s ≤ 0.001)

^cControl < FAS, PFAS and HE (p’s ≤ 0.01)

^dFAS < PFAS, HE, and Control (p’s ≤ 0.001)

^eFAS > HE and Control (p’s ≤ 0.01)

^fHollingshead (2011). FAS and PFAS < Control (p’s ≤ 0.02)

^gFAS and PFAS > HE and Controls (p’s ≤ 0.05)

^hControl > FAS, PFAS, HE (p’s ≤ 0.05)

ⁱControl > FAS and PFAS (p’s ≤ 0.02); HE > PFAS (p’s ≡ 0.006)

^jControl < FAS, PFAS, and HE (p’s ≤ 0.001)

^kSmokers only. 12 Controls, 11 HE, 15 PFAS, 9 FAS.

^lUsers only. 2 Controls, 5 HE, 1 PFAS, 1 FAS.

^†p < 0.10

^*p < 0.05

^**p < 0.01

^***p < 0.001

Performance on the FLT for the sample as a whole is summarised in Table 2. A significant main effect of inter-hemispheric transfer (crossed/uncrossed) was seen in all three conditions; in all conditions more errors were made when information had to cross the CC. No effects of hand or trial type-by-hand interactions were detected. Given the absence of effects of hand, the number of errors on left- and right-hand trials were summed for subsequent analyses.

Table 2.

Effects of inter-hemispheric transfer (crossed vs uncrossed) and stimulated hand (left vs right), as well as transfer by hand interactions, on the number of errors per trial (N = 74).

	Inter-hemispheric transfer			Stimulated hand			Transfer × hand interaction
	Uncrossed	Crossed	F(p)	Left	Right	F(p)	F(p)
One-finger/hand visible	0.1 (0.2)	0.3 (0.6)	10.11 (0.002)	0.2 (0.3)	0.2 (0.4)	0.02 (0.90)	0.86 (0.36)
One-finger/hand hidden	0.1 (0.3)	0.7 (1.1)	36.94 (<0.001)	0.4 (0.6)	0.5 (0.9)	1.71 (0.20)	0.40 (0.53)
Two-finger/hand hidden	2.2 (2.1)	4.1 (2.8)	64.73 (<0.001)	3.2 (2.4)	3.0 (2.4)	0.82 (0.37)	0.02 (0.88)

Values are mean (SD); Bold denotes significance at p≤0.05.

Figure 2 shows the number of errors (summed for left- and right-hand trials) by FASD diagnostic group separately for the uncrossed and crossed trials of each condition. While performance on the uncrossed trials was similar across groups in the two one-finger conditions, adolescents with FAS or PFAS made more errors than controls in the most difficult two-fingers/hand hidden condition. On the crossed trials, in all three conditions, only adolescents with FAS performed more poorly than controls.

Figure 2:

Box-and-whisker plots of the number of errors by diagnostic group on uncrossed and crossed trials of each condition. Errors on left- and right-hand trials were summed. Any group showing significantly more errors than controls on any condition are indicated (one-tailed student’s t-test).

Two of the control variables examined, higher blood lead levels and poorer IQ were both associated with higher CUDs on both the one-finger/hand hidden and two-fingers/hand hidden conditions (Table 3).

Table 3.

Association of crossed/uncrossed differences (CUDs) in each FLT condition with potential confounders. Left- and right-hand trials were summed prior to computing crossed/uncrossed differences. (N = 74)

	Age at assessment	Sex	Lead	WISC IQ	Maternal education	Cigarettes per day	TIVa
One-finger/hand visible (CUD1)	0.09 (0.43)	−0.06 (0.64)	0.08 (0.51)	−0.02 (0.88)	−0.10 (0.39)	−0.09 (0.44)	−0.18 (0.31)
One-finger/hand hidden (CUD2)	0.08 (0.48)	0.08 (0.52)	0.24 (0.04)	−0.35 (0.002)	0.06 (0.61)	0.09 (0.44)	−0.03 (0.86)
Two-finger/hand hidden (CUD3)	0.18 (0.13)	−0.06 (0.62)	0.20 (0.08)	−0.30 (0.009)	−0.15 (0.21)	−0.002 (0.99)	−0.06 (0.75)

Values are Spearman rho (p values). WISC-IV=Wechsler Intelligence Scales for Children-Fourth Edition; Bold denotes significance at p≤0.10.

^aTotal intracranial volume (TIV) only available for 35 participants who were scanned.

Children with FAS made significantly more transfer-related errors (i.e., had larger CUDs) on both the one-finger conditions (hand visible and hand hidden) than children in all other groups (Table 4 and Fig. 3). These differences persisted after controlling for lead and IQ. No between-group differences were seen on the CUD3 measure, apparently because all the groups performed poorly on the more challenging two-fingers/hand hidden condition. There was a relation between alcohol drinks consumed per occasion and CUDs on the one-finger/hand hidden condition that fell short of statistical significance (r = 0.19, p = 0.10); no associations of CUDs with extent of PAE were found on any of the other FLT conditions (all p’s > 0.20).

Table 4.

Comparison by diagnostic group of crossed/uncrossed differences (CUDs) for each FLT condition. Left- and right-hand trials were summed prior to computing crossed/uncrossed differences. (N = 74)

	Control (n = 32)	HE (n = 14)	PFAS (n = 16)	FAS (n = 12)	F (p)	Fa (p)	Fb (p)
One-finger/hand visible (CUD1)c	0.2 (0.6)	0.2 (0.7)	0.2 (0.8)	1.8 (2.2)	6.94 (<0.001)	6.80 (<0.001)	6.61 (0.001)
One-finger/hand hidden (CUD2)c	0.9 (1.3)	0.6 (1.2)	1.1 (1.8)	3.3 (2.4)	8.06 (<0.001)	5.96 (0.001)	5.94 (0.001)
Two-finger/hand hidden (CUD3)	3.8 (4.1)	3.4 (4.0)	3.9 (5.4)	4.4 (4.1)	0.14 (0.94)	0.22 (0.88)	0.32 (0.81)

Values are mean (SD). HE=heavily exposed non-syndromal; FAS=fetal alchol syndrome; PFAS=partial FAS; Bold denotes significance at p≤0.05.

^aControlling for IQ

^bControlling for IQ and lead

^cFAS > PFAS, HE and control groups (all p’s≤0.009).

Figure 3:

Box-and whisker plots of crossed/uncrossed differences (CUDs) by diagnostic group for each condition. Any group showing significantly more transfer-related errors than controls on any condition are indicated (one-tailed student’s t-test).

Performance on all four cognitive measures assessed was significantly worse for participants in the FAS or PFAS groups compared to the HE and control groups (Table 5). Mediation of these effects by inter-hemispheric transfer of information was examined using crossed minus uncrossed errors for the one-finger/hand hidden condition (CUD2) as the mediating variable. The FAS, PFAS, and HE groups were indicator coded using the controls as the reference group. Indirect effects were estimated for all three contrasts (FAS vs Controls; PFAS vs Controls; HE vs Controls). CUD2 was a significant mediator of the FAS vs Controls contrast on the WISC-IV Perceptual Reasoning Index and the Executive Function composite measure (Table 5). Mediation by CUD2 of the effect of FASD group on the two other outcomes fell short of statistical significance. CUD2 did not mediate either the PFAS vs Controls or the HE vs Controls contrast.

Table 5.

Mediation by transfer-related errors on the one-finger/hand hidden condition (CUD2) of the effects of FASD diagnosis on cognitive outcomes

	Control	HE	PFAS	FAS		Indirect effects mediated by CUD2a
	(n = 32)	(n = 14)	(n = 16)	(n = 12)	F or χ2	FAS vs Ctl	PFAS vs Ctl	HE vs Ctl
WISC IQ
Full Scale IQb	76.9 (12.4)	79.9 (10.7)	64.3 (10.1)	65.2 (10.2)	8.07***	−0.32†	−0.03	0.03
Perceptual Reasoning Indexb	82.7 (13.4)	87.5 (11.3)	69.8 (11.6)	71.0 (11.3)	7.83***	−0.36*	−0.04	0.04
Working memoryc	6.8 (1.7)	7.2 (2.6)	5.6 (1.9)	5.3 (1.7)	3.26*	−0.31†	−0.03	0.03
DKEFS composited	0.1 (0.6)	0.3 (0.5)	−0.5 (0.8)	−0.2 (0.7)	4.52**	−0.37*	−0.06	0.04

^†p<0.08

^*p<0.05

^**p<0.01

^***p<0.001

^aValues for group comparisons are means (standard deviations). Values for indirect effects are standardized regression coefficients.

^bWechsler Intelligence Scales for Children--4^th edition (WISC-IV).

^cWISC-IV Digit Span Backwards.

^dDelis-Kaplan Executive Function System composite of Stroop-word Interference Test inhibition and shift measures; Verbal Fluency letters and categories measures; and Tower Planning Test.

FAS=fetal alcohol syndrome; PFAS=partial FAS; HE=heavily exposed non-syndromal

Among the adolescents for whom neuroimaging data from childhood were available, the effects of CC volume on the FLT measures fell short of conventional levels of statistical significance (Table 6). Among those with PAE, the number of transfer-related errors on the one-finger/hand hidden condition (CUD2) increased with decreasing CC volume, a finding that continued to be significant after controlling for differences in age at scanning, IQ and lead exposure. This effect was most clearly seen in the FAS/PFAS group.

Table 6.

Associations of crossed/uncrossed differences (CUDs) on each FLT condition with corpus callosum (CC) volume in the subset of participants with neuroimaging data at age 9–12 years. Associations are presented both across and by diagnostic groups.

	All adolescents (N= 35)		Controls (n=10)		PAE (n=25)		FAS/PFAS (n=20)
	r	β a	r	β a	r	β a	r	β a
One-finger/hand visible (CUD1)	−0.13 (0.23)	−0.26 (0.08)	0.01 (0.48)	−0.04 (0.47)	−0.16 (0.48)	−0.33 (0.08)	−0.09 (0.35)	−0.38 (0.04)
One-finger/hand hidden (CUD2)	−0.26 (0.06)	−0.26 (0.08)	0.22 (0.27)	0.06 (0.46)	−0.37 (0.04)	−0.45 (0.04)	−0.41 (0.04)	−0.49 (0.04)
Two-finger/hand hidden (CUD3)	0.04 (0.41)	0.03 (0.43)	0.20 (0.28)	0.20 (0.38)	0.01 (0.48)	0.17 (0.25)	0.09 (0.35)	0.20 (0.23)

Values are Pearson correlation (r) and standardized regression coefficients (β) (1-tailed p levels).

^aAfter controlling for age at scan (yr), IQ and lead.

PAE=adolescents with prenatal alcohol exposure; FAS=fetal alcohol syndrome; PFAS=partial FAS

Discussion

This study examined whether fetal alcohol-related impairment in inter-hemispheric transfer of tactile information observed at younger ages continues to be evident in adolescence, the extent to which the effects of FASD on cognitive development are mediated by a deficit in inter-hemispheric transfer, and whether CC size in childhood predicts inter-hemispheric transfer in adolescence.

We found that only adolescents with FAS, the most severe form of FASD, made more transfer-related errors compared to all other groups when one finger was stimulated – in either the visible or hidden condition. There were no differences between the groups on number of transfer-related errors in the more complex two-fingers/hand hidden condition, probably due to the greater number of errors that were seen in all groups on the uncrossed trials in this condition. Given that the adolescents with FAS, PFAS, and HE did not differ in terms of degree of PAE (see Table 1), these findings suggest that the effect on inter-hemispheric transfer in the more severely affected FAS group is not attributable to higher levels of PAE in that group and may instead reflect differences in timing of exposure in the FAS group and/or greater genetic or other vulnerability. Using continuous measures of PAE, only AA/occasion showed a weak association, below conventional levels of significance, with CUD2. AA/occasion is a measure of binge-like drinking that assesses the extent to which the individual concentrates his/her alcohol intake on a few days during the week. Among the alcohol-exposed adolescents for whom neuroimaging data from childhood were available, there was an inverse relation between CC volume and CUD2.

Our finding of an inter-hemispheric transfer deficit in adolescents only in the FAS group is consistent with that reported previously in an independent cross-sectional Cape Town cohort of children aged 8–12 years, in which only those with FAS demonstrated a transfer deficit on the one-finger/hand hidden condition (CUD2) (Dodge et al. 2009). In a study of 8- to 15-year-old children, Roebuck and associates (2002) found more transfer-related errors across one- and two-finger hand hidden trials (but not in the one-finger/hand visible condition) in a group comprised of children with PAE compared to controls. Since 64% of the children in their PAE group had been diagnosed with FAS, it is possible that their finding was largely attributable to those more severely affected children. In our Detroit Longitudinal cohort, where only 2 of the 85 young adults with PAE had been diagnosed with FAS (Dodge et al., 2009), more hemispheric transfer errors were made by the participants whose mothers reported binge drinking (≥5 standard drinks/occasion or 2.5 oz AA/occasion) during pregnancy than those who drank regularly (M ≥1 drink/day) without binge drinking. Although binge drinking, where the drinking is concentrated on 1–3 days over the weekends, is the predominant pattern seen across all three PAE groups in the heavy drinking women from this Cape Town community, the inter-hemispheric transfer deficit was not seen in the adolescents with PFAS nor those from the HE group.

Our finding that more drinks/occasion (reflecting binge-like drinking) is weakly associated with more transfer errors on the one-finger/hand hidden condition (CUD2) supports that of Dodge et al. (2009), who found increasing transfer-related errors on both the one- and two-finger/hand hidden conditions (CUDs 2 and 3) in young adults from the Detroit Longitudinal cohort whose mothers reported more concentrated drinking (i.e., more drinks/occasion). Previous human and animal studies have shown that binge drinking is particularly detrimental to the brain’s structural and functional maturation (e.g., Flak et al., 2014). Furthermore, evidence from animal models suggests that a more concentrated dose/occasion can be more harmful than higher total alcohol exposure delivered in lower doses (Bonthius and West, 1990; Goodlett et al., 1987). We have also found that concentrated alcohol exposure of 4–5 or more drinks/occasion is related to an increased risk of deficits on a broad range of other neuroimaging and cognitive outcomes (Carter et al., 2005; Jacobson and Jacobson, 1994; Jacobson et al., 1998; Jacobson et al., 2008; Robertson et al., 2016; Lewis et al., 2021).

Similar to findings from Dodge et al. (2009) and Roebuck et al. (2002), participants across all groups made more errors on the most complex condition in which two fingers compared to only one were stimulated. While all groups performed similarly on uncrossed trials of conditions in which one finger was stimulated, children with FAS made more errors than control children on both uncrossed and crossed trials for this more challenging condition. Roebuck et al. (2002) also reported more errors in children with PAE than controls on the two-finger trials. The fact that, in this condition, children with FAS made more errors not only on the crossed trials involving inter-hemispheric transfer but also on the uncrossed trials, resulting in similar crossed/uncrossed differences across groups, raises the question whether poor performance on this condition may be related to lower IQ. Notably, group differences in CUDs on the two one-finger conditions remained significant after control for IQ and lead, confirming that these differences are not attributable to the lower IQ of children with FAS but are specific to alcohol-related deficits in inter-hemispheric transfer.

Anatomically, the CC undergoes the greatest growth before age 10 years but continues its maturation until adulthood (Quinn and Geffen, 1986). Thus, normally developing children have been shown to perform better on the more complex FLT conditions with increasing age (Pipe, 1991; Quinn and Geffen, 1986). Improved performance with age on the more complex condition is also seen when comparing the errors reported in the three FLT studies examining effects of PAE, with the younger children from the Cape Town cross-sectional cohort (Dodge et al., 2009) and the Roebuck et al. (2002) study making more errors on the two-finger trials than the young adults from the Detroit Longitudinal cohort (Dodge et al., 2009) and the adolescents studied here.

Roebuck et al. (2002) suggested that future studies examine the degree to which effects of PAE on cognitive and behavioral outcomes are mediated by inter-hemispheric transfer. In this study, we examined effects of FASD group on three cognitive domains likely to be important in successful performance on an inter-hemispheric transfer task: perceptual reasoning, working memory, and executive function. Because the effect of FAS on inter-hemispheric transfer was seen only on the CUD2 measure, we examined the degree to which CUD2 performance mediated the effects of FAS on these outcomes. In this first study to assess mediation by inter-hemispheric transfer of information of the effects of FAS on cognitive outcomes, we found that the effects of FAS on perceptual reasoning and executive function were mediated, in part, by CUD2. Because four of the five executive function tasks were time limited or assessed in terms of time to task completion, these data suggest that efficient inter-hemispheric transfer of information may be particularly important for executive function tasks that depend heavily on information processing speed. Mediation of the effect on working memory fell short of statistical significance, possibly due to the light memory load in the one-finger/hand hidden condition; stronger mediation by working memory might be seen in the two-finger condition in individuals who successfully perform that task.

Although we did not find associations of CC volumes in childhood with inter-hemispheric transfer deficiency at this later age on the visible one-finger condition, there was an association among alcohol exposed adolescents of smaller CC volumes in childhood with more transfer-related errors on the one-finger/hand hidden condition – effects that got stronger after excluding the five non-syndromal HE children (Table 6). The fact that CC reductions evident during childhood impact performance on inter-hemispheric transfer 6–7 years later, suggests that the effects of PAE-related damage to the CC may be permanent rather than a transitory developmental delay. This should be further investigated in future studies involving longitudinal brain imaging data.

Dodge et al. (2009) also found negative associations of CUDs with areas of the isthmus and splenium on the same condition in a cross-sectional cohort of younger children from Cape Town. In contrast to the two Cape Town studies, Roebuck et al. (2002) did not find an association of CUDs on the one-finger/hand hidden condition (CUD2) with total CC area, but instead found that smaller CC areas were associated with increasing CUDs on the more complex two-finger condition, effects that got stronger after excluding the 6 control children (37.5%) in their sample. With respect to the directionality of inter-hemispheric transfer, as in the Roebuck et al. (2002) study, we did not find any effect of hand or any transfer by hand interaction, confirming that the deficits in FLT are bidirectional and not influenced by which hand was stimulated.

One limitation of this study is that the MRI scans were obtained at a mean age of 10.6 (± 0.7) years, while the FLT was performed at mean age 16.4 (± 0.8) years. Although it would have been good to have age-matched MRIs as well, this design allowed us to make some inferences as to the extent to which effects of early CC damage persist and affect future function. Notably, associations of CC volume with FLT performance survived after controlling for group differences in age at scanning. Although our power to detect associations of FLT performance with CC volume was limited in that the MRI data were only available for a sub-sample of the participants who were assessed on the FLT, the data from this sub-sample confirm findings from two previous independent samples (Roebuck et al., 2002; Dodge et al., 2009).

This study supports and expands upon previous findings that PAE causes differences in inter-hemispheric transfer of tactile information in severely affected children. Our findings in adolescence support previous evidence in a pre-adolescent Cape Town cohort (Dodge et al., 2009) that alcohol exposure-related deficits in inter-hemispheric transfer are seen in children with FAS. As Roebuck et al. (2002) suggested, subtle alcohol-related deficits in inter-hemispheric transfer are likely to pose a more challenging problem when individuals are confronted with more complex tasks which require integration across the cerebral hemispheres. Our findings suggest that the PAE-related deficit in inter-hemispheric transfer of information impacts particularly strongly on perceptual reasoning and an executive function measure that is dependent on speed of information processing. Moreover, among adolescents with PAE, reduced CC size in childhood is related to long-term poorer inter-hemispheric transfer that is more likely permanent than a developmental delay.