Reaction times to lateralized visual stimuli in callosal agenesis: stimulus and response factors

Inter-hemispheric integration of tactile-motor responses across body parts

In simple detection tasks, reaction times (RTs) are faster when stimuli are presented to the visual field or side of the body ipsilateral to the body part used to respond. This advantage, the crossed-uncrossed difference (CUD), is thought to reflect inter-hemispheric interactions needed for sensorimotor information to be integrated between the two cerebral hemispheres. However, it is unknown whether the tactile CUD is invariant when different body parts are stimulated. The most likely structure mediating such processing is thought to be the corpus callosum (CC). Neurophysiological studies have shown that there are denser callosal connections between regions that represent proximal parts of the body near the body midline and more sparse connections for regions representing distal extremities. Therefore, if the information transfer between the two hemispheres is affected by the density of callosal connections, stimuli presented on more distal regions of the body should produce a greater CUD compared to stimuli presented on more proximal regions. This is because interhemispheric transfer of information from regions with sparse callosal connections will be less efficient, and hence slower. Here, we investigated whether the CUD is modulated as a function of the different body parts stimulated by presenting tactile stimuli unpredictably on body parts at different distances from the body midline (i.e., Middle Finger, Forearm, or Forehead of each side of the body). Participants detected the stimulus and responded as fast as possible using either their left or right foot. Results showed that the magnitude of the CUD was larger on the finger (~2.6 ms) and forearm (~1.8 ms) than on the forehead (≃0.9 ms). This result suggests that the interhemispheric transfer of tactile stimuli varies as a function of the strength of callosal connections of the body parts.

The role of task history in simple reaction time to lateralized light flashes

In lateralized simple reaction time (SRT) tasks with unimanual responses, 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) is typically interpreted to reflect callosal transfer time. Indeed, split brain patients have much longer CUDs than control subjects. However, while many studies have supported the hypothesis that the CUD reflects callosal transmission time, a few studies have suggested that the CUD may be affected by non-anatomical factors. We investigated the nature of these inconsistent results in two experiments. In the first, we asked half of our subjects to cross their arms while performing the task. The CUD was not affected by arms crossing, supporting the anatomical model of the CUD. In the second experiment, however, all subjects were asked to cross their arms in half of the trials. In this experiment, arms crossing significantly affected the CUD, thus showing that spatial attention modulates the CUD. These latter results cannot be readily explained by a simple callosal relay interpretation of the CUD. Rather, the CUD seems to reflect a mix of anatomical and non-anatomical factors produced by task history. Thus, the seemingly inconsistent results of previous studies can be reconciled by taking into account differences in task history across studies.

Behavioral evidence of prolonged interhemispheric transfer time among psychopathic offenders

Several lines of evidence suggest the possibility of abnormal interhemispheric communication in psychopathy, but there have been few direct empirical studies. To address this gap in the literature, the authors examined one important aspect of interhemispheric communication, the efficiency with which information is transferred across the corpus callosum. Using A. T. Poffenberger's (1912) paradigm for estimating interhemispheric transfer time (IHTT) from simple motor responses to lateralized stimuli, the authors found a substantially prolonged IHTT among psychopathic criminals relative to nonpsychopathic criminals. This prolonged IHTT was somewhat more pronounced when participants were using their right hand to respond. This study provides initial behavioral evidence of slowed interhemispheric transfer in psychopathy.

Interhemispheric transfer following callosotomy in humans: Role of the superior colliculus

It is now common knowledge that the total surgical section of the corpus callosum (CC) and of the other forebrain commissures prevents interhemispheric transfer (IT) of a host of mental functions. By contrast, IT of simple sensorimotor functions, although severely delayed, is not abolished, and an important question concerns the pathways subserving this residual IT. To answer this question we assessed visuomotor IT in split-brain patients using the Poffenberger paradigm (PP), that is, a behavioral paradigm in which simple reaction time (RT) to visual stimuli presented to the hemifield ipsilateral to the responding hand is compared to stimuli presented to the contralateral hemifield, a condition requiring an IT. We tested the possibility that the residual IT is mediated by the collicular commissure interconnecting the two sides of the superior colliculus (SC). To this purpose, we used short-wavelength visual stimuli, which in neurophysiological studies in non-human primates have been shown to be undetectable by collicular neurons. We found that, in both totally and partially callosotomised patients, IT was considerably longer with S-cone input than with L-cone input or with achromatic stimuli. This was not the case in healthy participants in whom IT was not affected by color. These data clearly show that the SC plays an important role in IT of sensorimotor information in the absence of the corpus callosum.

Exaggerated redundancy gain in the split brain: A hemispheric coactivation account

Recent studies of redundancy gain indicate that it is especially large when redundant stimuli are presented to different hemispheres of an individual without a functioning corpus callosum. This suggests the hypothesis that responses to redundant stimuli are speeded partly because both hemispheres are involved in the activation of the response. A simple formal model incorporating this idea is developed and then elaborated to account for additional related findings. Predictions of the latter model are in good qualitative agreement with data from a number of sources, and there is neuroanatomic and psychophysiological support for its underlying structure.

Callosal lesions and behavior: history and modern concepts

Callosotomy has played a unique role in the treatment of epilepsy and in the understanding of human brain function. The pioneering work of Dejerine and Liepmann presenting the first findings of callosal lesion pathology at the turn of the 20th century was accepted but then quickly forgotten. Two schools resurrected the phoenix of callosal syndromes: Roger Sperry and Michael Gazzaniga leading in experimental neuroscience, and Norman Geschwind leading in clinical neurology. Callosotomy remains an effective technique to treat atonic, tonic, and tonic-clonic seizures, especially in patients with symptomatic generalized epilepsies such as Lennox-Gastaut syndrome. Neurologic, cognitive, and behavioral complications limit its use given that precise characterization of these complications as well as their frequency is difficult. The high frequencies of developmental delays, severe seizures, head injuries, antiepileptic drug burden, and other factors limit the ability to attribute a specific change to surgical intervention, since surgery can change multiple factors. For example, subtle behavioral changes in executive function and personality are difficult to delineate in a population with preexisting neurologic and psychiatric disorders. Despite this, a clearer picture of the effects of callosotomy, as defined by clinical neurology and neuropsychology as well as cognitive neuroscience, is emerging.

Permanent or transitory effects on neurocognitive components of the CNV complex induced by brain dysfunctions, lesions and ablations in humans

Since the mid-1960s, essentially using electrophysiological methods, our research group has examined the effects of different brain diseases in humans, both on first- and second-order conditioned responses and on some types of neurocognitive potentials of the CNV complex. This didactic lecture will focus on our various attempts to identify and understand the neuroanatomical and neurophysiological substrates involved in cognitive information processing followed by the conception and execution of sensory-motor and behavioural responses evoked by significant acoustic stimuli, in both pathological situations and normal control subjects. Great interest was, e.g. aroused in the early 1970s by the rare, fortunately unrepeatable, opportunity of examining the CNV patterns in various psychiatric patients treated with psychosurgical Freeman-Watts bilateral prefrontal 'radical' lobotomy, also with repeated recordings (The Responsive Brain (1976) 158; Multidisciplinary Perspectives in Event-Related Brain-Potentials Research (1978) 376) or bimedial bifrontal cingulotomy (Multidisciplinary Perspectives in Event-Related Brain Potential Research (1978) 383). In the same period, investigations into CNV activity recorded in patients submitted to complete callosotomy ('split brain': Attention and Performance, vol. IV (1972) 221; Electroenceph. Clin. Neurophysiol. Suppl. 33 (1973) 161) were also begun and were continued into the 1980s, also with regard to other types of ERP (Brain 111 (1988) 553; J. Cog. Neurosci. 2 (1990) 258). All these data furnished unique information about the sub-second dynamics of unilateral or bihemispheric cortico-cortical and cortico-subcortical interconnections in humans. In recent years, with a classic method of analysis based on sequential scalp-topographic bidimensional neuroelectric mapping and 21/19 electrodes connected to three different references, and binaural/monaural clicks as warning signals (S1), we have repeatedly examined the CNV activity of 11 selected patients submitted to complete ablation of the damaged cortical areas, with uni- or bilateral lesions restricted to the prefrontal or associative parieto-temporal areas. We have always used the standard CNV paradigm (S1-S2 motor-response) which evokes a complex of neurocognitive potentials, including the P300 from S1, which are well-known, since they are certainly among the most studied ERPs in the various ages and races of normal subjects, psychiatric patients and subjects with different brain diseases. The most important results have been, (1) In normal subjects the MRI and the latency differences of CNV component measurements along the bidirectional pathways functionally interconnecting ipsilateral distant associative cortical areas (e.g. the arcuate-superior longitudinal complex bundle) were accounted for by the transcortical conduction time, which varies in our scalp recordings from 1 cm/0.74 to 1.28 ms ( approximately 9.8 m/s). (2) Constantly, no true auditory S1-elicited N1a, b, c, P2, N2, P300 components or CNV slow waves (O- and E-wave) were recordable over the whole of the ablated cortical areas, but only clearly identifiable volume-conducted EP/ERPs generated in other hemispheric structures. (3) The post-S1 ERP/CNV complexes on the intact hemisphere were found to be within the normal limits. (4) Effects of severe disruption on the S1 ERP/CNV complexes evocable on the site and on remote ipsilateral apparently normal anatomo-functionally interconnected brain regions were observed in 5 patients, 4 of whom had extensive frontocortical ablations. In two of the latter the distant disruptive action on the CNV components over the neuroradiologically normal ipsilateral two-way connected post-rolandic sensory and association areas was seen to be partially reversible, showing aspects of a probable slowly evolving diaschisis-like effect. Similar deactivation of some ERP components was observed in reverse on the ipsilateral dorsolateral frontocortical region in the fifth patient with a large parieto-temporal cortex ablation. These data require confirmahese data require confirmation, and when this phenomenon is observable, it must be appropriately monitored with different methods of functional neuroimaging. This will serve not only for medical and neuropsychophysiological diagnosis purposes, but also particularly for a correct and really useful planning of neuro-rehabilitation activities in selected cases.

Spatial attention in agenesis of the corpus callosum: shifting attention between visual fields

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.

Changes in interhemispheric transfer rate and the development of bimanual coordination during childhood

In this study, we investigated the effect of the development of interhemispheric communication on age-related change in bimanual coordination. Interhemispheric communication was assessed by comparing the latency of a manual response to a visual stimulus when the hemisphere perceiving the stimulus and the hemisphere controlling the manual response were the same (uncrossed condition) to the latency when they were different (crossed condition). In the first experiment (5- to 10-year-old children) we used a two-choice response-time task, and in the second experiment (3- to 7-year-old children) we used a simple response-time task. In both studies, bimanual coordination was tested on a line-drawing task, and we compared performance on mirror and parallel movements. The crossed-uncrossed difference decreased with age in both experiments. When estimated on the simple response-time task, the crossed-uncrossed difference was related to the difference in performance between mirror and parallel movements on the bimanual task. Thus, improved interhemispheric communication contributes to progress in bimanual coordination, especially that which requires resisting the attraction of mirror movements in order to rotate both hands with parallel movements.

Interhemispheric transfer of colour and shape information in the presence and 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.

Right hemispheric contribution to motor programming of simultaneous bilateral response

The purpose of this study was to verify the hypothesis that the motor program which integrates the left- and right-hand responses in the bilateral reaction time (RT) task is located in the right hemisphere. 8 female and 50 male students performed bilateral simultaneous RT tasks to lateralized light stimuli. Uncrossed RT based on the relationship between the unifying center in the right hemisphere and hemifield stimulus was shorter than crossed RT for the bilateral response. Therefore, the hypothesis was supported. Some areas of the right hemisphere contributed to unification of the movements of the right and left hands for bilateral movements in reaction time.

Interhemispheric transmission times in the presence and absence of the forebrain commissures: effects of luminance and equiluminance

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.

Increased axon number in the anterior commissure of mice lacking a corpus callosum

Relatively few behavioral deficits are apparent in subjects with hereditary absence of the corpus callosum (CC). The anterior commissure (AC) has been suggested to provide an extracallosal route for the transfer of interhemispheric information in subjects with this congenital defect. Anterior commissure size, axon number, axon diameter, and neuronal distribution were compared between normal mice and those with complete CC absence. No difference in midsagittal AC area was found between normals and acallosals, nor were differences found in the numbers or diameters of myelinated axons. However, axon counts indicated an 17% increase or about 70,000 more unmyelinated axons in the AC of acallosal mice, and the mean diameter of unmyelinated axons was slightly less than in normal mice (0.24 vs 0.26 microm). This decrease in axon diameter enabled more axons to pass through the AC without increasing its midsagittal area. The topographical distribution of neurons sending axons through the AC, assessed with lipophilic dyes, was qualitatively similar for almost all the known regions of origin of the anterior commissure in normal and acallosal mice. There was a pronounced deficit of AC cells in the anterior piriform cortex of BALB/c mice, but this occurred whether or not the mouse suffered absent CC. Although the increase in AC axon number is far smaller than the number of CC axons that fail to reach the opposite hemisphere, the higher number of axons present in the AC of acallosal mice may contribute to the functional compensation for the loss of the CC.

Interhemispheric transfer time differences related to aging and gender

The difference in simple reaction times to unstructured stimuli contralateral to the hand used for response ('crossed' responses) and those stimuli ipsilateral to the hand used for response ('uncrossed' responses)--or crossed-uncrossed difference (CUD) is assumed to be a reliable index of interhemispheric transfer time (IHTT). Studies using the CUD paradigm with acallosal patients as well as a variety of populations with known variations in callosal size or functioning have demonstrated that such callosal differences are reflected in differences in recorded CUD times. Recent studies have suggested that elderly individuals show a reduction in size of the corpus callosum, particularly in the anterior region. In order to assess any potential change in callosal transfer efficiency with aging. CUDs were obtained from elderly (60+ years) and younger subjects (18 30 years). The elderly subjects showed a significantly elongated CUD compared to younger subjects, with elderly females contributing the greatest increase. No significant gender differences were found for younger subjects. In addition, an unexpected trend for an overall right field reaction time advantage was found for all subgroups, accompanied by a larger calculated CUD for right hand responses than for left hand responses.

Maternal phenylketonuria: magnetic resonance imaging of the brain in offspring

Phenylketonuria (PKU) produces white matter changes identifiable by magnetic resonance imaging. These changes occur postnatally. Offspring of untreated mothers with PKU also have a brain effect, expressed as microcephaly and mental retardation. This effect occurs prenatally. To determine whether the white matter changes seen in PKU are also present in maternal PKU offspring, despite the different developmental stages of exposure to PKU, we performed brain magnetic resonance imaging studies in seven maternal PKU offspring, five from essentially untreated pregnancies and two from treated pregnancies. None had white matter changes, although the one offspring with PKU had delayed myelination. However, hypoplasia of the corpus callosum was present in three of the four offspring from untreated pregnancies and in the offspring from a maternal PKU pregnancy not treated until the third trimester. Unlike PKU, white matter changes are not a feature of the brain effect in maternal PKU. However, hypoplasia of the corpus callosum is a feature of maternal PKU and is probably a result of inhibition of corpus callosum development at 8 to 20 weeks of gestation. The hypoplastic corpus callosum could be a marker for brain effect in maternal PKU and may have implications for the cognitive deficits in these offspring.

Temporal integration of events within and between the cerebral hemispheres

The effect of interhemispheric transmission time (IHTT) on temporal perception was investigated by comparing simultaneity thresholds under unimanual and bimanual stimulation conditions. In the unimanual conditions the tactile stimuli were delivered to the same hand and were received by the same hemisphere, but in bimanual conditions, where stimuli were delivered to different hands, interhemispheric communication was necessary in order to compare the timing of the two stimuli. Randomised order of stimulating the fingers of the hands was compared with consistent stimulation of each hand. Previous studies have used only consistent stimulation. Thirty-two undergraduate university students (16 males and 16 females) were tested. Bimanual simultaneity thresholds were significantly higher than unimanual. Previous research showing an absence of laterality effects for simultaneity judgements was confirmed, supporting a hemispheric equivalence model of temporal processing. Simultaneity thresholds were not affected by randomisation of stimulus location, indicating that expectancy effects were not responsible for the difference between bimanual and unimanual thresholds. The implication of these findings is that temporal perceptions are affected by the process of interhemispheric transmission.

Spatial stimulus-response compatibility in callosotomy patients and subjects with callosal agenesis

Subjects with partial or complete defects of the corpus callosum, either congenital or acquired, performed a choice reaction time (RT) task involving a right or left key-press response to a light presented at random in the right or left hemifield. Like normal subjects, all of them exhibited two additive effects typical of these tasks: the spatial stimulus-response compatibility effect (faster RT for stimuli and responses matched for side), and the hand placement effect (longer RT for responses performed with crossed hands). Two subjects with a complete callosal defect, one acquired and the other congenital, showed a third effect, not present in normal subjects, consisting of a marked advantage for RT of responses with hand anatomically ipsilateral to the stimulus, independent of both stimulus-response compatibility and hand placement. These findings can be interpreted according to a hierarchical model of information processing assuming that, in the absence of the corpus callosum, the matching of the mental codes for the stimulus and response sets takes place solely in the hemisphere receiving the stimulus, with a subsequent rapid-intrahemispheric or slow-interhemispheric transmission of the response command to the appropriate motor centers.

Interhemispheric integration of simple visuomotor responses in patients with partial callosal defects

Because of the organization of visual and motor pathways, simple manual responses to a light stimulus in the right or left visual hemifields are performed faster with uncrossed hand-field combinations than with crossed hand-field combinations. Uncrossed responses can be integrated within a single hemisphere, whereas crossed responses require a time-consuming interhemispheric transfer via the corpus callosum which is reflected in the difference between crossed and uncrossed reaction times. We investigated crossed-uncrossed differences (CUDs) in speed of simple visuomotor responses to lateralized flashes in seven subjects with an anterior section of the corpus callosum sparing the splenium and in one subject with an agenetic absence of the splenium due to a cerebrovascular malformation. There was no evidence of an abnormal prolongation of the CUDs in any of these subjects, in sharp contrast with the very long CUDs exhibited by an epileptic subject with a complete callosal section and two subjects with total callosal agenesis tested in the same experimental situation [1]. The normality of the CUDs in the subjects with partial callosal defects was not due to a postoperatory reorganization of interhemispheric communication, since there was no indication of an increased CUD in a patient tested as early as 5 days after the anterior callosotomy. These results are compatible with the assumption that both anterior and posterior callosal routes may subserve the integration of speeded manual responses to a visual stimulus directed to the hemisphere ipsilateral to the responding hand.

Behavioral estimates of interhemispheric transmission time and the signal detection method: a reappraisal

On the basis of a review of the literature, Bashore (1981) concluded that only simple reaction time experiments with manual responses yielded consistent behavioral estimates of interhemispheric transmission time. A closer look at the data, however, revealed that these experiments were the only ones in which large numbers of observations were invariably obtained from many subjects. To investigate whether the methodological flaw was the origin of Bashore's conclusion, two experiments were run in which subjects had to react to lateralized light flashes. The first experiment dealt with manual reactions, the second with verbal reactions. Each experiment included a condition without catch trials (i.e., simple reaction time) and two conditions with catch trials. Catch trials were trials in which no stimulus was given and in which the response was to be withheld. Both experiments returned consistent estimates of interhemispheric transmission time in the range of 2-3 msec. No differences were found between the simple reaction time condition and the signal detection conditions with catch trials. Data were analyzed according to the variable criterion theory. This showed that the effect of catch trials, as well as the effect of interhemispheric transmission, was situated at the height of the detection criterion, and not in the rate of the information transmission.

Interhemispheric transfer in children with early-treated phenylketonuria

Phenylketonuria (PKU) is a genetic disorder of amino acid metabolism that is associated with brain catecholamine depletion and deficient myelination. Although neuropsychological deficits have been documented in children with early-treated PKU (ETPKU), no study to date has examined possible effects of impaired myelination in this population. In the present study, interhemispheric transfer time was assessed for 14 children with ETPKU, 22 children with attention deficit-hyperactivity disorder, and 48 normal children, using a manual reaction time paradigm previously validated with callosal agenesis patients (Milner, 1982). Children with ETPKU demonstrated slowed interhemispheric transfer from the left to the right hemisphere as compared with the two other groups. The magnitude of slowing was correlated with age and phenylalanine levels at birth. Results support the hypothesis that abnormal myelination disrupts the development of interhemispheric connections in ETPKU, and suggest that left hemisphere projections may be particularly susceptible to such disruption.

Estimation of interhemispheric dynamics from simple unimanual reaction time to extrafoveal stimuli

This essay reviews research on interhemispheric transfer time derived from simple unimanual reaction time to hemitachistoscopically presented visual stimuli. Part 1 reviews major theoretical themes including (a) the significance of the eccentricity effect on interhemispheric transfer time in the context of proposed underlying neurohistological constraints; (b) the significance of gender differences in interhemispheric transfer time and findings in dyslexics and left-handers in the context of a fetal brain testosterone model; and (c) the significance of complexity effects on interhemispheric transfer time in a context of "dynamic" vs. "hard-wired" concepts of the underlying interhemispheric communication systems. Part 2 consists of a meta-analysis of 49 published behavioral experiments, in view of drawing a portrait of the best set of experimental conditions apt to produce salient, reliable, and statistically significant measures of interhemispheric transfer time, namely (a) index rather than thumb response, (b) low rather than high target luminance, (c) short rather than prolonged target display, and (d) very eccentric rather than near-foveal stimulus location. Part 3 proposes a theoretical model of interhemispheric transfer time, postulating the measurable existence of fast and slow interhemispheric channels. The proposed mechanism's evolutionary adaptive value, the neurophysiological evidence in its support, and favorable functional evidence from studies of callosotomized patients are then presented followed by proposals for critical experimental tests of the model.

Influence of anatomical factors and spatial compatibility on the stimulus-response relationship in the absence of 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.

Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis

Using a meta-analytic procedure we have analysed 16 studies employing a simple unimanual reaction time (RT) paradigm and lateralized visual stimuli to provide an estimate of interhemispheric transfer time in normal right-handed subjects. We found a significant overall RT advantage of the left visual field over the right and of the right hand over the left. These asymmetries can be explained by a superiority of the right hemisphere for the detection of simple visual stimuli and by a corresponding superiority of the left hemisphere for the execution of the manual response, respectively. Alternatively, they may be interpreted as related to an asymmetry of interhemispheric transmission of visuomotor information, with transfer from the right hemisphere (side of stimulus entry) to the left (side of response generation) faster than in the reverse direction. Although a direct test of these hypotheses is still lacking, we think that the evidence available is more in keeping with the latter possibility.

Bilateral hemispheric control of foot distal movements: evidence from normal subjects

Normal subjects have been tested for interhemispheric transfer (IT) of visuo-motor information using a simple reaction time (RT) paradigm and lateralized stimuli and responses (the so-called Poffenberger paradigm). In this paradigm IT time is assumed to correspond to the RT difference between crossed and uncrossed stimulus-response combinations (CUD). In Experiment 1, two types of movements were used: a unilateral flexion of the thumb and a unilateral plantar flexion of the big toe. A reliable CUD (7.4 msec) was found only with manual responses. Changing stimulus retinal eccentricity (10 degrees vs. 70 degrees) or attentional demands (blocked vs. random stimulus presentation) did not result in any reliable effect on the CUD. In Experiment 2 the number of RTs for each subject was considerably increased and several visual field sites (from areas close to the vertical meridian to the monocular crescent) were tested. Notwithstanding these modifications, this experiment confirmed the lack of CUD found for foot responses in Exp. 1. Taken together, these results are in keeping with a less lateralized hemispheric control of distal foot movements in comparison to hand movements.

Limitations of interhemispheric extracallosal transfer of visual information in callosal agenesis

A 57-year-old patient with callosal but not anterior commissure agenesis was investigated with a visual interfield comparison and a naming task. Stimuli were presented tachistoscopically either bilateral-simultaneously or unilaterally in the LVF and/or RVF. The stimuli presented bilaterally differed with respect to their degree of similarity. Whereas the patient was able to detect gross differences between stimuli presented in the left and right half-field, he was impaired at discriminating similar and identical stimuli across the half-fields. Identification by naming was correct on unilateral presentation of a stimulus, while with bilateral presentation of two stimuli, errors increased considerably with the stimulus named second. The data are thought to indicate a limited capacity of the extracallosal commissures, probably the anterior, for the transfer of visual information.

Genetic and developmental defects of the mouse corpus callosum

Among adult BALB mice fewer than 20% usually have a small or absent corpus callosum (CC) and inheritance is polygenic. In the fetus at the time when the CC normally forms, however, almost all BALB mice show a distinct bulge in the interhemispheric fissure and grossly retarded commissure formation, and inheritance appears to result from two autosomal loci, provided the overall maturity of fetuses is equated. Most fetuses recover from the early defect when the CC axons manage to cross over the hippocampal commissure, and thus there is developmental compensation for a genetic defect rather than arrested midline development. The pattern of interhemispheric connections when the adult CC is very small is topographically normal in most respects, despite the unusual paths of the axons. The proportion of mice which fail to recover completely can be doubled by certain features of the maternal environment, and the severity of defects in adults can also be exacerbated by new genetic mutations which create new BALB substrains. The behavioral consequences of absent CC in mice are not known, nor have electrophysiological patterns been examined. The mouse provides an important model for prenatal ontogeny and cortical organization in human CC agenesis, because these data are not readily available for the human condition.

Interhemispheric transfer of spatial tactile information in callosal agenesis and partial commissurotomy

The De Renzi Rod Test was used to investigate the transfer of spatial tactile information between hands in acallosals and partial commissurotomy patients. The right-handed performance of young acallosals was impaired, confirming the results of an earlier study by Meerwaldt (1983). The adult acallosals, with one exception, showed no difference between right and left hand performances. Two partial callosotomy patients with sparing of the most caudal parts of the body of the callosum showed no difference between the hands. A third callosotomy patient with little, if any, sparing of the most caudal part of the body of the callosum showed a marked impairment when performing the tactile task with the right hand. The unimpaired performances of the adult acallosals are attributed to the use of ipsilateral pathways and the development of behavioural strategies. The callosotomy patients' results are consistent with the mapping by Pandya and Seltzer (1986) of the interhemispheric callosal pathways between the cortical parietal areas in non-human primates.