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Agenesis of the corpus callosum (AgCC) is a congenital malformation that can occur in isolation or in association with other neurological conditions. Although the behavioral manifestations associated with AgCC have been widely studied, the effects of complete absence of the corpus callosum (CC) on cerebral cortex anatomy are still not completely understood. In this study, cortical thickness in adults with complete AgCC was compared to a group of healthy controls. Results showed highly variable patterns of cortical thickness in AgCC individuals, with few areas showing significant and consistent alterations including primary visual cortex, primary somatosensory cortex and primary motor cortex. These results suggest relatively limited effects of AgCC on cortical morphology, which are mostly restricted to primary sensory and motor areas.

The ability to temporarily maintain relevant information in mind in the presence of interference or distracting information, also called working memory (WM), is critical for higher cognitive functions and cognitive development. In typically developing (TD) children, WM is underpinned by a fronto-parietal network of interacting left and right brain regions. Developmental absence (agenesis) of the corpus callosum (AgCC) is a congenital brain malformation resulting from disruption of corpus callosum formation. This study aims to investigate functional organisation of WM in children and adolescents with AgCC using functional magnetic resonance imaging (fMRI). Nine children with AgCC and a comparison group of sixteen TD children aged 8-17 years completed an fMRI WM paradigm designed to enable investigation of different WM processes, i.e., encoding, maintenance and retrieval. We found that AgCC children recruited globally similar brain regions as the TD comparison group during the WM task, despite significant disparity in brain development, i.e., bilateral occipito-frontal activations during verbal encoding, and bilateral fronto-parietal executive control network during retrieval. However, compared to their TD peers, children with AgCC seemed less able to engage lateralised brain systems specialised for particular memory material (i.e. less supramarginal activations for verbal material and less fusiform activations for face processing) and particular memory process (i.e. absence of right-predominant activations during retrieval). Group differences in the pattern of activation might also reflect different cognitive strategies to cope with competition in processing resources with different susceptibility to concurrent tasks (verbal vs visual), such as differential recruitment of associative visual areas and executive prefrontal regions in the AgCC compared with the TD group depending on the concurrent task completed during maintenance. This study provides a first step towards a better understanding of functional brain networks underlying higher cognitive functions in children with AgCC.

The role of interhemispheric interactions in the encoding, retention, and retrieval of verbal memory can be clarified by assessing individuals with complete or partial agenesis of the corpus callosum (AgCC), but who have normal intelligence. This study assessed verbal learning and memory in AgCC using the California Verbal Learning Test—Second Edition (CVLT-II). Twenty-six individuals with AgCC were compared to 24 matched controls on CVLT-II measures, as well as Donders’ four CVLT-II factors (i.e., Attention Span, Learning Efficiency, Delayed Memory, and Inaccurate Memory). Individuals with AgCC performed significantly below healthy controls on the Delayed Memory factor, confirmed by significant deficits in short and long delayed free recall and cued recall. They also performed less well in original learning. Deficient performance by individuals with AgCC during learning trials, as well as deficits in all forms of delayed memory, suggest that the corpus callosum facilitates interhemispheric elaboration and encoding of verbal information.

Damage to long white matter pathways in the cerebral cortex is known to affect memory capacity. However, the specific contribution of interhemispheric connectivity in memory functioning is only beginning to become understood. The present study examined verbal and visual memory processing in individuals with agenesis of the corpus callosum (AgCC) using the Wechsler Memory Scale-Third Edition (WMS-III; Wechsler, 1997b). Thirty participants with AgCC (FSIQ >78) were compared against 30 healthy age and IQ matched controls on auditory/verbal (Logical Memory, Verbal Paired Associates) and visual (Visual Reproduction, Faces) memory subtests. Performance was worse in AgCC than controls on immediate and delayed verbal recall for rote word pairs and on delayed recall of faces, as well as on percent recall for these tasks. Immediate recall for thematic information from stories was also worse in AgCC, but groups did not differ on memory for details from narratives or on recall for thematic information following a time delay. Groups also did not differ on memory for abstract figures or immediate recall of faces. On all subtests, individuals with AgCC had greater frequency of clinically significant impairments than predicted by the normal distribution. Results suggest less efficient overall verbal and visual learning and memory with relative weaknesses processing verbal pairs and delayed recall for faces. These findings suggest that the corpus callosum facilitates more efficient learning and recall for both verbal and visual information, that individuals with AgCC may benefit from receiving verbal information within semantic context, and that known deficits in facial processing in individuals with AgCC may contribute to their impairments in recall for faces.

Neuroimaging studies show that permanent peripheral lesions such as unilateral deafness cause functional reorganization in the auditory pathways. However, functional reorganization of the auditory pathways as a result of higher-level damage or abnormalities remains poorly investigated. A relatively recent behavioural study points to functional changes in the auditory pathways in some, but interestingly not in all, of the acallosal individuals that were tested. The present study uses fMRI to investigate auditory activities in both cerebral hemispheres in those same acallosal subjects in order to directly investigate the contributions of ipsilateral and contralateral functional pathways reorganization. Predictions were made that functional reorganization could be predicted from behavioural performance. As reported previously in a number of neuroimaging studies, results showed that in neurologically intact subjects, binaural stimulation induced balanced activities between both hemispheres, while monaural stimulation induced strong contralateral activities and weak ipsilateral activities. In accordance with behavioural predictions, some acallosal subjects showed patterns of auditory cortical activities that were similar to those observed in neurologically intact subjects while others showed functional reorganization of the auditory pathways. Essentially they showed a significant increase and a significant decrease of neural activities in the contralateral and/or ipsilateral pathways, respectively. These findings indicate that at least in some acallosal subjects, functional reorganization inside the auditory pathways does contribute to compensate for the absence of the corpus callosum.

In the lateralized simple reaction time (SRT) task with unimanual responses (Poffenberger paradigm), reaction times (RTs) are faster with ipsilateral (uncrossed) than with contralateral (crossed) response hand-target hemifield combinations. The difference between crossed and uncrossed responses (CUD) has typically been interpreted to reflect callosal transfer time. Indeed, acallosal subjects and split-brain subjects have longer CUDs than control subjects. However, a few recent studies have demonstrated that, contrary to classical findings, the CUD is also affected by non-anatomical factors. Here we show that the CUD is also affected by non-anatomical factors in patients with agenesis of the corpus callosum and complete commissurotomy where interhemispheric transfer must be subcallosal. We tested acallosal subject M.M. and split brain patient A.A. on a lateralized SRT task with their arms alternately uncrossed (natural arms position) or crossed (unnatural arms position) across blocks of trials. The results revealed a significant effect of arms crossing on the size and direction of the CUD as previously found in normal subjects [Mooshagian, E., Iacoboni, M., & Zaidel, E. (2008). The role of task history in simple reaction time to lateralized light flashes. Neuropsychologia, 46(2), 659-664]. This suggests that non-anatomical factors that modulate interhemispheric visuomotor integration may occur in absence of the corpus callosum. Anterior commissure and interhemispheric cortico-subcortical pathways are likely implicated in these effects.

Five people lacking the corpus callosum (two callosotomized, three with agenesis of the corpus callosum) and neurologically normal subjects were shown vertical lines that appeared instantaneously between pairs of rectangles in one or other visual field. When one of the rectangles flashed prior to the presentation of the line, and the line was in the same visual field, all subjects perceived the line as spreading from the flashed rectangle to the other. Normal subjects and one of the callosotomized subjects showed a slight but significant right visual-field advantage, perhaps reflecting a left-hemispheric superiority in processing rapid temporal events. The illusion was also induced when the line and the flash were in opposite visual fields in one of the callosotomized, one of the acallosal subjects, and about half of the normal subjects, implying interhemispheric integration even in the absence of the corpus callosum.

Two split-brained subjects, one (L.B.) with full forebrain commissurotomy and one (R.B.) with callosal agenesis, and a group of twenty neurologically intact subjects were tested in three discrimination tasks: a go-no go task, a two-choice task, and a three-choice task. The discriminations were based on colour in Experiment 1, and on shape in Experiment 2. The stimuli were presented in one or other visual field, and the subjects responded with the fingers of one or other hand, allowing the differences in reaction time between crossed and uncrossed responses (CUD) to be calculated. For the normal subjects the CUD tended to diminish with the complexity of the tasks, suggesting that both hemispheres were increasingly involved. Unlike R.B. and the normal controls, who made virtually no errors, L.B. had increasing difficulty as task complexity increased. He was better able to transfer information from the right to the left hemisphere than vice versa, but an analysis of his accuracy under the crossed conditions showed that the amount transferred was always well under one bit. This confirms previous evidence that L.B. has very limited subcortical transfer of either colour or shape.

The role of the corpus callosum versus other cerebral commissures in the interhemispheric integration of visual information was studied in four individuals with complete agenesis of the corpus callosum, two individuals with partial agenesis, one total commissurotomy patient, and normal individuals. Evoked potential (EP) indices of interhemispheric transmission of visual sensory responses were observed during matching of unilateral and bilateral visual field letters and patterns. Neither the commissurotomy nor any of the acallosal patients had ipsilateral hemisphere visual EPs (P1 and N1), demonstrating that the posterior callosum is necessary for interhemispheric transmission of these components of visual evoked potentials. While the commissurotomy patient could not compare bilaterally presented letters, the anterior commissure of the acallosal patients appeared to be sufficient for interhemispheric comparison of single letters. However, bilateral comparison of more complex visual patterns resulted in considerable difficulty for complete agenesis patients, while comparison of patterns was more nearly normal when anterior callosal fibers were present (partial agenesis).

A 30-year-old right-handed man (W.D.) with total callosal agenesis was examined neuropsychologically and with magnetic resonance imaging. Basic neuropsychological testing revealed normal intelligence and average attentional capabilities. Anatomically, W.D. shows strong leftward perisylvian asymmetry both for the planum temporale and planum parietale, an unusual pattern not found in our database of more than 200 brains of young and healthy individuals. Functionally, W.D. has strong right hand superiority for hand skill and tactile object recognition, indicating unusual left hemisphere dominance for both functions. Our observations could support the hypothesis that callosal connectivity and hemispheric asymmetry may be inversely related.

Although earlier studies have emphasized the absence of ‘split-brain’ symptoms in callosal agenesis patients, the notion of an ‘asymptomatic’ acallosal brain has lately been challenged. We report a number of findings that are indicative of an interruption of interhemispheric communication and integration in individuals lacking the corpus callosum. Several groups of patients with callosal pathology (acallosals, patients with commissurotomy or callosotomy, either complete or partial) were compared to matched controls. Interhemispheric transfer was tested in two different experiments involving pointing to a light source while maintaining central fixation. In the first experiment, a learning paradigm was used to measure transfer of a motor skill from the trained to the untrained hand. In the second experiment, subjects pointed to visual targets at different locations on a perimeter. Midline fusion, a recurrent theme when describing callosal function, was assessed using tasks which included depth perception with binocular and/or monocular cues, two-point discrimination thresholds and sound localization in the peri-central and lateral fields. Subjects with callosal pathology were impaired on all tasks involving transfer of motor and visuo-spatial skills and on some of the tasks requiring sensory integration of visual and tactile information across the body midline. We conclude that these functions require an intact corpus callosum since none of these deficits were seen in controls equated for IQ.

The capability to store and retrieve weight-related information from a lift to scale the force output during a subsequent lift was examined in 10 healthy adults and 50 children (age 2-10 years), as well as a 22-year-old patient with corpus callosum agenesis. Subjects lifted a test object between the thumb and index finger while the isometric fingertip forces were measured. The results suggest that both healthy children and adults can transfer weight-related information between the right and left hand, although a lateralization was found. Also, the storage and retrieval of weight-related information appears to be a dynamic process dependent on both previous sensory information and knowledge of future movements. Late maturation of interhemispheric connections and asymmetric loss of some information during the transfer between hemispheres suggest a lateralization of the internal representation. The patient with a corpus callosum agenesis supported this hypothesis.

a b s t r a c t Studies in patients with an isolated, congenital agenesis of the corpus callosum have documented potentials and limits of brain plasticity. Literature suggests that early reorganization mechanisms can compensate for the absence of the corpus callosum in unisensory tasks that involve interhemispheric transfer. It is unknown, however, how the congenitally acallosal brain processes multisensory infor- mation, which presumably requires interhemispheric transfer of modality-specific input. Therefore, we tested five patients with total and one patient with partial agenesis of the corpus callosum in a visuotactile interference task (the "crossmodal congruency task") with uncrossed and crossed hands and compared their performance to that of 31 healthy controls. We found that congruency effects followed the hands in space not only in healthy, but also in congenitally acallosal individuals. Remarkably, this was also true when patients' hands crossed the vertical visual meridian and stimuli were presented at the same hand. These results suggest that callosal connectivity is not required for remapping of visuotactile space. We conclude that early brain plasticity allows for compensation of the developmental absence of the corpus callosum in a visuotactile interference task.

The aim of this study was to investigate how the absence of the corpus callosum affects somesthetic sensation on the axial midline and in proximal and distal body regions. For this purpose, two-point discrimination ability was evaluated in four acallosal subjects, four callosotomized subjects, six IQ-matched subjects and 10 control subjects with average and above average IQ. Sensory thresholds were established in the distal (index, palm), proximal (forearm), cranio-axial (forehead) and axial (dorsal trunk) body regions. The threshold was defined as the smallest separation at which the two points were perceived at a 70% accuracy level. Results showed that the thresholds of the acallosal and the callosotomized subjects were not significantly different from those of the IQ-matched control groups in the distal, proximal and cranio-axial body regions. However, thresholds in the dorsal trunk were significantly higher in the two experimental groups. It thus appears that the axial regions of the body that are normally densely represented in the corpus callosum function abnormally when this structure is absent or transected. Moreover, compensatory mechanisms normally seen in cases of early brain injury do not seem to apply in the present case since the acallosals showed the same impairments as the callosotomized subjects.

Interference and facilitation of tachistoscopically presented colour stimuli by adjacent incongruent and congruent colour-words (the Stroop effect) was examined in the right and left visual fields, and centrally, in normal subjects and three acallosals. In the central condition, the word and colour were each separated across the fovea. Normal subjects showed a small left hemisphere advantage for colour naming. The combined Stroop effect (CSE), that is the reaction time difference between incongruent and congruent pairs, was the same in both visual fields but reduced centrally. Furthermore, in the central condition, the CSE was the same regardless of the side on which the word or colour appeared. The reduced CSE in the central condition was exaggerated in one of the acallosals, suggesting that the corpus callosum, by acting as a partial barrier, mediates the interhemispheric Stroop effects. The other acallosals while providing some tentative support for this, showed evidence of lateralized dysfunctions to that their performance was less easily interpreted. The methodology described would appear to have applications in studying the functional characteristics of the corpus callosum in a variety of clinical groups.

An abnormal pattern of hemispheric asymmetry, possibly as a result of disturbed interhemispheric communication, is widely, albeit by no means unanimously, held as a major cause of schizophrenia. To behaviourally test interhemispheric communication in schizophrenia we used a task that has been shown to be a reliable indicator of callosal functioning, namely, the redundant signals effect (RSE). It consists of the speeding of simple reaction time when responding to double as opposed to single visual stimuli. When the stimuli in a pair are presented to different hemispheres patients who underwent total commissurotomy or suffer from callosal agenesis show a paradoxically enhanced RSE with respect to healthy controls. Therefore, if schizophrenia patients have a callosal abnormality they ought to show a similar effect. In three experiments we tested a total of 55 patients with a diagnosis of schizophrenia and 51 healthy controls. In Experiment 1 we presented unilateral single stimuli and bilateral simultaneous double stimuli. The RSE was reliably larger in schizophrenics than in controls. In Experiment 2 the temporal interval between the two stimuli in a pair was varied. We found that while in controls the RSE disappeared with interstimulus intervals longer than 17 ms, in schizophrenia patients there was a RSE only for simultaneous double stimuli. Finally, in Experiment 3 we found that there was no enhanced redundancy gain in schizophrenics when the double stimuli were presented to one and the same hemisphere, and therefore, with no need for callosal transmission. All in all, the present results provide evidence of a callosal dysfunction in schizophrenia that impairs interhemispheric integration.

To study the role of the corpus callosum (CC) in midline binocular integration, the effects of late callosotomy and congenital CC agenesis on the ability to perceive dichoptic plaid motion was assessed. Coherent motion was well perceived at all locations in the visual field under dioptic viewing but not along the vertical meridian (VM) when the components were dichoptically presented. This deficit was totally abolished in the agenesis subject and reduced in the callosotomized individual when stimulus size was increased beyond the VM. Electrophysiological correlates were also examined by recording visual evoked potentials and these showed that the P1/N2 components were abnormal for small dichoptic stimuli presented on the midline. These findings attest to the importance of the contribution of CC to midline binocular integration and the effects of cerebral plasticity.

Individuals with agenesis of the corpus callosum (ACC) can, in some cases, perform normally on standardized intelligence tests. Nevertheless, recent studies suggest that individuals with ACC and normal IQ scores have deficits in domains of fluid and social intelligence. Anecdotal reports from families suggest diminished appreciation of the subtleties of social interactions, and deficits in the comprehension of jokes and stories. In this research, both the cartoon and narrative joke subtests of a humor test (developed by Brownell et al. [Brownell, H., Michel, D., Powelson, J., & Gardner, H. (1983). Surprise but not coherence: sensitivity to verbal humor in right-hemisphere patients. Brain and language, 18(1), 20-27] and Bihrle et al. [Bihrle, A. M., Brownell, H. H., Powelson, J. A., & Gardner, H. (1986). Comprehension of humorous and non-humorous materials by left and right brain-damaged patients. Brain and Cognition, 5(4), 399-411]) were given to 16 adults with complete ACC (all with IQs>80) and 31 controls of similar age and IQ. Individuals with ACC performed worse than controls on the narrative joke subtest (p

Horizontal and vertical line bisection was studied in 129 children and adolescents between 8 and 19 years of age, one group (n=32) of typically developing controls and one group (n=97) with spina bifida (SBM), a neurodevelopmental disorder associated with dysmorphology of the corpus callosum, posterior cortex, and midbrain. For each participant, structural brain MRIs were analyzed qualitatively to identify beaking of the midbrain tectum and corpus callosum agenesis and hypoplasia and quantitatively by segmentation and volumetric analyses of regional cortical white and gray matter. Each group showed the line length effect, whereby greater estimation errors are made with longer lines. The group with SBM differed from controls in terms of both accuracy and variability of line bisection. Children with SBM showed pseudoneglect, attending more than controls to left hemispace. The extent of rightward bisection bias was unrelated to right posterior brain volumes, although an intact corpus callosum during development moderated and normalized the exaggerated leftward line bisection bias. More children with SBM than controls attended to inferior hemispace. A normal midbrain and greater posterior cortex volume during development moderated and normalized the downward bias. Children with SBM showed more intra-subject variability than controls. Line bisection in children with SBM reflects three deficits: an exaggerated attentional bias to left hemispace, an abnormal attentional bias to inferior hemispace; and a larger zone of subjective uncertainty in bisection judgments.

Four split-brained subjects, two subjects with agenesis of the corpus callosum, and 14 normal subjects performed two tasks requiring responses to red or green disks, briefly presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. In Experiment 1, simple reaction times to these stimuli, regardless of colour, were recorded (the Go-Both Task), and found to be faster to bilateral-redundant stimulus pairs, than to single stimuli. This so-called "redundancy gain" was much larger for acallosal or split-brained subjects than for normal subjects and exceeded the predictions of a race model, implying neural summation. Experiment 2 used the same stimuli, but subjects were required to respond only to stimuli of a designated colour (the Go/No-Go Task). Redundant target stimuli produced neural summation, while stimuli pairs that included a non-target stimulus did not. These results suggest that neural summation in the acallosal or split brain involves the convergence of response-associated activation, and that redundant sensory processes are not sufficient.

The role of the corpus callosum in spatially selective visual attention is uncertain. Research using commissurotomy and callosotomy patients has attempted to determine if the corpus callosum plays a role in reorienting attention between visual fields, as if spatial attention is unitary or divisible between the cerebral hemispheres. Reorienting of selective visuospatial attention within versus between visual fields was tested in 10 individuals with agenesis of the corpus callosum (ACC) and nine matched controls. Spatially focused attention to the most likely location of target appearance was created using both peripheral sensory cues and central symbolic cues in separate tests. Results demonstrated that individuals with ACC have significantly greater difficulty reorienting attention to an invalidly cued target stimulus occurring in the opposite visual field. However, this effect did not interact with the type of cueing (sensory or symbolic). Individuals with ACC did not differ from controls either with respect to the laterality of within-field reorientation of attention, or with respect to the most efficient direction of between-field shifting of attention. Since congenital absence of the corpus callosum significantly reduces efficiency in the reorienting of attention between visual fields, spatial attention cannot be completely unified based on a subcortical mechanism and the mobilization of attentional resources within each hemisphere must depend on callosal processes.

Background: Based on previous studies and due to the characteristics of dyslexia as an auditory phonological decoding disorder, we predicted that the shape of the posterior corpus callosum (CC) would differ between dyslexic and control subjects. Method: Twenty right-handed boys with developmental dyslexia were selected from a carefully screened general population sample (mean age 11 years) and compared to a matched control group. The CC contour was manually traced on the aligned midsagittal MR slice and total callosal area and its subregions were compared between the groups. A statistical shape analysis and subsequent CC classification was performed using a recently developed shape model method. Results: The shape analysis revealed shorter CC shape in the dyslexic group, localised in the posterior midbody/isthmus region. This region contains interhemispheric fibers from primary and secondary auditory cortices. A shape length difference larger than a fixed threshold in the posterior midbody region could correctly discriminate between control and dyslexic subject in 78% of the cases, where a dyslexic CC was shorter in this region than a control CC. However, there were no significant group differences with respect to overall CC area or subregions. Conclusion: A clear shape difference in the posterior midbody of the CC was found between dyslexic and control subjects. This fits with recent other studies that have reported a strong growth factor in this CC region during the late childhood years, coinciding with literacy acquisition. Our results show that the dyslexic group has not undergone the same growth pattern as the normal reading group.

Three split-brained subjects, one with full forebrain commissurotomy and two with callosotomy, were impaired at judging whether pairs of lights in opposite visual fields were successive or simultaneous. This impairment did not vary with luminance when the lights were grey against a dark background, but was more pronounced when the lights were equiluminant with a yellow background. All three subjects were also better able to discriminate succession from simultaneity when the lights were both in the left visual field than when they were both in the right. A fourth subject with callosal agenesis was only slightly impaired relative to normal subjects, who were virtually errorless.

Symptoms of interhemispheric disconnection are typically much less severe in callosal agenesis than after surgical section of the corpus callosum.Sperry [Sperry R W Plasticity of neural maturation Developmental Biology 1968 2 (Suppl) 306–327] has attributed this difference to two interconnected factors: (1) the callosal section is usually performed after the brain has lost the maximal degree of functional plasticity associated with the early stages of development and (2) the removal of an already formed structure is more disruptive for functional brain organization than the failure of the same structure to develop It has been suggested that functional compensation is less efficient if callosal agenesis is partial rather than complete [Dennis M Impaired sensory and motor differentiation with corpus callosum agenesis: A lack of callosal inhibition during ontogeny? Neuropsychologia 1976 14 455–469] This suggestion is supported by the present findings of partial left-hand anomia partial left-field alexia and poor tactile cross-localization in a subject with a congenital absence of the posterior part of the corpus callosum due to an arteriovenous malformation In agreement with many previous studies similar though more severe symptoms of interhemispheric disconnection were found in a subject with a complete section of the corpus callosum but not in a subject with complete callosal agenesis Praxic control of the left hand on verbal commands was severely deficient in the callosotomy subject but it was normal in the subject with callosal hypogenesis The lesser degree of compensation in partial compared to complete callosal agenesis may be explained by a reduced pressure to develop extracallosal means of interhemispheric communication contingent on the partial existence of callosal connections as well as by the later occurrence in development of the causes of callosal hypogenesis compared to those of total callosal agenesis © 1998 Elsevier Science Ltd All rights reserved

One subject (L.B.) with full forebrain commissurotomy, one (R.B.) with callosal agenesis and 20 normal controls were tested for simple reaction time (RT) with each hand, to visual stimuli in one or the other visual field. RTs for uncrossed conditions (hand ipsilateral to the visual field) were subtracted from RT to crossed conditions (hand contralateral to the visual field) to yield the crossed–uncrossed difference (CUD), taken to be a measure of interhemispheric transfer time. CUDs increased from an average of 4.9 ms among the control subjects, to 23.3 ms for R.B., to 53.1 ms for L.B. Although overall RTs in all subjects increased with decreasing luminance of the stimuli, the CUD was not systematically affected and remained largely unaffected even under equiluminance. The results support previous evidence that interhemispheric transfer, even in the split brain, depends on visually insensitive pathways.

Therapeutic section of the corpus callosum in adult epileptic patients typically results in their incapacity to carry out interhemispheric comparisons of lateralized information. The fact that acallosal and early split-brain subjects display few of these symptoms when tested in the tactile modality has led to the suggestion that these patients may use ipsilateral projections of the somatosensory system more effectively. Compensation, however, is limited by the fact that the lemniscal pathway is strongly lateralized, especially for the distal parts of the body, where few ipsilaterally projecting fibres have been demonstrated. The pathway carrying temperature information has a larger ipsilateral component. Bilateral comparisons within the same hemisphere in subjects who are lacking the corpus callosum should be more common and the development of compensatory mechanisms in early-sectioned or acallosal subjects should be more likely. The objective of the present experiment was to evaluate differential thresholds for thermal stimuli applied on a number of regions either on the same side or on corresponding sites on opposite sides of the body. One subject callosotomized as an adult and one split-brain subject who underwent callosotomy in childhood, as well as three acallosal subjects, were compared to IQ-matched and normal-IQ control subjects. The fingers, forearm and trunk were tested. The comparison temperature was 30 C and the other was varied in an ascending or descending fashion using a modified method of limits. Differential thresholds were similar for within- and between-side comparisons, and comparable to those of the IQ-matched subjects. The results indicate that comparisons involving temperature discrimination for stimuli applied to the two sides of the body do not require the integrity of the corpus callosum.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callsal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.

Simple and choice reaction times (RTs) to lateralized flashes of light were measured in two acallosal subjects, a callosotomized patient with sparing of the splenium, six IQ-matched controls and six controls with above average IQ. The aim of the study was to investigate the influence of stimulus-response factors on visuomotor integration in the absence of the corpus callosum. The results showed that simple-RTs are faster for all groups when sensory input and motor output are coordinated in the same hemisphere, regardless of the spatial relationship between stimulus (S) and response (R). The difference between ipsilateral and contralateral RTs, taken as a measure of interhemispheric transmission time (ITT), was more than three times longer in the patients lacking the corpus callosum than in the controls. In the choice-RT paradigm, the main determinant of the response speed was S-R spatial compatibility rather than the anatomical relationship between the neural structures receiving the visual input and those controlling the motor output. Spatially compatible S-R pairings were always faster than incompatible pairings. The compatibility effect was present in all subjects but it was significantly larger in the callosum-deprived patients and their IQ-matched controls with respect to the high-IQ controls. The latter finding suggests that cognitive factors may be involved in the production of the spatial compatibility effect.

Stereoperception in two acallosal patients and two partial callosotomy patients was compared with that of three normal subjects. All three patients with the splenium missing, whether due to agenesis or surgical intervention, showed midline deficits and broadly similar profiles; namely, they made few uncrossed midline responses. The patient with partial callosal section but with the splenium almost totally spared performed better at the midline than in the periphery. All degrees of disconnection produced some overall loss of performance, confirming the results of H AMILTON and V ERMEIRE (In Two Hemispheres—One Brain, F. L EPORE , M. P ITTO and H.H. J ASPER (Editors), pp. 315–333. Alan R. Liss Inc., New York, 1986) and H AMILTON et al. (Suppl. Invest. Ophthal. Vis. Sci.28, 294, 1987). The results are discussed in the context of earlier reports of human and animal studies of stereoperception. Bearing in mind reports of structural alterations in layer III of the striate cortex in acallosals (S HOUMJRA , K. et al. Brain Res.93, 241–252, 1975. Also, A KERT , K. et al. Trans. Am. Neurol. Assoc.79, 151–153, 1954), it is speculated that the specific difficulties encountered by them in handling uncrossed disparities may be due to a marked reduction or absence of far neurones in acallosal brains (P OGGIO , G.F. and P OGGIO , T. Ann. Rev. Neurosci.7, 379–412, 1984). The likely importance of the anterior commissure in the efficient integration of crossed disparity (near neurones) (C OWEY , A. In Brain Mechanisms and Spatial Vision, D. I NGLE (Editor), NATO Advanced Study Institute Series, Martinus Nijhoff, The Hague, 1985) is seen as a possible explanation of the acallosals relative success in making crossed disparity judgements. The variability of performance in normals documented by H AMILTON and V ERMEIRE (In Two Hemispheres—One Brain, F. L EPORE , M. P ITTO and H.H. J ASPER (Editors), pp. 315–333. Alan R. Liss Inc., New York, 1986) and H AMILTON et al. (Suppl. Invest. Ophthal. Vis. Sci.28, 294, 1987) is, not surprisingly, even more marked amongst acallosals.

Four acallosal subjects, one child, aged 5, and three adults, as well as five epileptic patients who underwent callosotomy between the ages of 6-21 years, were tested on a variety of intra- and intermanual tasks in a study aimed at elucidating the developmental aspects of callosal plasticity. The performance of the clinical sample was compared to that of 48 normal children, aged 5-12 years, an age span generally considered to coincide with the final stages of callosal maturation. As previously reported, interhemispheric integration improved with increasing age in the normal sample. The two patients having undergone callosotomy in childhood performed as well as their normal peers, whereas the three others who had the operation in late adolescence or adulthood showed the typical disconnexion deficits reported in the literature. The acallosal subjects, including the youngest one, outperformed all groups. We speculate that the remarkable plasticity seen in the acallosals and the young callosotomized patients may be related to a critical period in development coinciding with a phase of synaptic overproduction and redundancy that would favor the reinforcement of alternative neural pathways. The compensatory mechanisms appear to become more limited in late adolescence when synaptic distribution presumably assumes adult patterns.

Simple reaction times (RTs) of a young acallosal woman were measured, using three different intensity levels of the visual stimulus. It was found that “crossed” stimulus/hand combinations yielded finger RTs about 20 msec longer, on average, than “uncrossed” combinations. In addition, an inverse relation was found between stimulus intensity and the magnitude of this crossed/uncrossed difference in RT. It was also found that vocal RTs displayed a large advantage (18 msec) for right visual-field stimulation. It is suggested that the stimulus information in crossed finger reactions must cross the midline through visually-coded neurones in the acallosal brain.

Two patients with total callosal agenesis were compared with two control groups on a dichotic listening task using verbal and non-verbal stimuli. Acallosal patients displayed left ear superiority and abnormally long reaction times regardless of the nature of the stimulus. The results are interpreted in terms of hemispheric reorganization in acallosal patients.

Two siblings with total callosal agenesis were compared with two control groups on a number of tachistoscopic tasks involving unilateral and bilateral presentation of verbal and non-verbal stimuli. Acallosals were able to effect intra- and interhemispheric comparisons and to correctly identify all kinds of stimuli in either visual field. However, they responded more slowly, especially in the bilateral condition where they also made more errors than the controls. The results are interpreted in terms of increased use of extra-callosal commissures.

An eight-year-old girl with agenesis of the corpus callosum was tested with the rod orientation test, and the test results were compared with those of ten neurologically normal children (7–9 years). It appeared that this child had a defective performance and the longest mean duration in the tactile part of the test but only when using her right hand. The results in this patient fit in with a disturbed interhemispheric transfer of tactile information.

A young acallosal man was intensively tested in a standard simple reaction time (RT) paradigm using briefly-presented lateralized spots for light. In Experiment 1, findings on previous acallosal patients of a large disadvantage for crossed (e.g. right hemifield-left hand) as against uncrossed (e.g. left hemifield-left hand) RTs were replicated. This crossed-uncrossed difference (CUD), as in previous work, turned out to be smaller in a bimanual response task than in the conventional unimanual task. Experiment 2 was a factorial study of unimanual RTs in which (a) stimulus intensity and (b) spatial S-R compatibility, were varied. As in a previously tested patient, decreased intensity resulted in a greatly increased CUD. S-R compatibility on the other hand had no effect on CUD. The results are interpreted as favouring a role for visual commissural neurones in the acallosal CUD, and as evidence against a spatial compatibility hypothesis.

Two siblings with total agenesis of the corpus callosum were compared with matched epileptic and normal controls on bimanual integration of kinesthetic, somesthetic and motor functions. No differences were found between acallosals and control subjects on any of the transfer tasks. However, the acallosal patients were impaired in bimanual operations with regard to speed. The deficit was more pronounced in the younger subject who also showed impairment of differentiated finger movements. Bilateral organization of speech and cross-cuing strategies could have accounted for facilitated performance on some but not all of the transfer tasks. Increased use of ipsilateral and/or subcortical pathways are a more plausible and more parsimonious explanation for the results of this study.

Studies [Preilowski, B.F.B. in Cerebral Localization, pp. 116-131, Springer, Berlin, 1975; Reynolds, D.M. and Jeeves, M.A. Neuropsychologia 12, 287-290, 1974] of bilateral motor coordination in commissurotomy patients pointed to the importance of direct interhemispheric integration for the fine regulation of the lower motor system within each hemisphere. When external visual feedback was withdrawn in the performance of a bimanual tracking task, partial commissurotomy patients, in whom the anterior portions of the callosum were cut, deviated significantly when drawing lines which required asymmetrical input from the two hands. The task devised by Preilowski was given to two adult acallosals, an 11 yr old acallosal boy, and an 11 yr old girl in whom the centre one-third of the corpus callosum had been sectioned. The acallosals performed in a manner similar to Preilowski's partial commissurotomy patients. The results were interpreted as confirming the interhemispheric integrative function of the corpus callosum. It is argued that they constitute further evidence for the existence of hemispheric dominance for motor control in acallosals. The partial commissurotomy patient did not differ in performance from normals. Her results were consistent with the view that it is the anterior parts of the callosum which are crucial for the interhemispheric integration of the lower motor system in each hemisphere.

A dichotic monitoring task involving detection of target words (e.g., BLACK) in either ear was used. Word pairs that could fuse to produce target word perception (e.g. BACK+LACK=BLACK) were included. Two patients with partially sectioned corpus callosa showed left ear extinction for targets but their fusion rates were normal. A congenitally acallosal patient had unusually high response rates to targets in either ear, as well as high fusion rates. The results indicate that intact commissural connections between the auditory cortices are not required for phonological fusion to occur. An hierarchical model of contralateral suppression of ipsilateral auditory input is proposed.

A dichotic listening test was administered to a young boy with congenital agenesis of the corpus callosum and to 12 normal controls. Unlike patients with surgical section of the corpus callosum, the agenesis case did not show a pronounced laterality effect. These data, taken in conjunction with studies of hemispherectomized patients, indicate that the laterality effect in dichotic listening may be due more to suppression and interference at the cortical level than to the contralateral pathway occluding the ipsilateral pathway at a subcortical level.