M.A. Jeeves

Bilateral field advantage and evoked potential interhemispheric transmission in commissurotomy and callosal agenesis.

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).

Directional asymmetries in interhemispheric transmission time: Evidence from visual evoked potentials.

The hypothesis was tested that interhemispheric transfer time (IHTT), as measured in the latency of bilaterally recorded visual evoked potentials, is directionally asymmetric, i.e. that an IHTT is faster for transmission from right-to-left hemisphere, than from left-to-right. A meta-analysis of 18 experiments within the published literature reporting visual evoked potential IHTTs indicates a significant experiment-wise predominance of faster right-to-left IHTTs. A new experiment is also reported in which significantly faster right-to-left IHTT was found in visual evoked potentials recorded from parietal electrodes to lateral visual field presentations while subjects performed a task requiring complex stimulus recognition and analysis, and a choice response.

Bimanual co-ordination in callosal agenesis and partial commissurotomy.

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 Preilowskis 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.

Bilateral visual field processing and evoked potential interhemispheric transmission time.

The relationship between the efficiency of interhemispheric interactions via the corpus callosum and the speed and accuracy in making comparisons of information simultaneously presented to the right and left visual fields was studied by comparing bilateral (vs unilateral) advantages in matching letters, with evoked potential measures of interhemispheric transmission time (EP-IHTT). The primary finding was a strong correlation suggesting that larger bilateral field advantages in reaction time are associated with faster EP-IHTT. However, the association between EP-IHTT and bilateral advantage was strong only for transmission speed from left hemisphere to right hemisphere, but not for speed of transmission in the opposite direction. The data are consistent with a hypothesis of asymmetric homologue enhancement, i.e. a directionally asymmetric callosal influence which facilitates processing of letter stimuli in the right hemisphere allowing for increased response speed and accuracy of bilateral visual field comparisons.

Role of the corpus callosum in the development of a bimanual motor skill

Normal adults quickly become highly skilled on a task of bimanual control of the movement of a pen on a vertical screen, described by Preilowski (1972) as an X‐Y plotter. When visual feedback is denied, the performance of highly practiced normal adults remains unchanged. In contrast, patients with surgical division of the anterior portion of the corpus callosum (Preilowski, 1972) or with callosal agensis (Jeeves, Silver, & Jacobson, in press) show marked deterioration of performance without visual feedback. Although groups of 10‐ and 20‐year‐olds performed as normal on Preilowskis task, a group of 6‐year‐olds performed more like acallosals. These results are seen as evidence of reduced functional efficiency of the corpus callosum before myelination is complete at age 10.

Reading in callosal agenesis

It has been suggested that deficits in explicit phonological processing are causal in developmental dyslexia. Deficits in such skills have been reported in developmental phonological dyslexia, though not in developmental surface dyslexia. The reading performance of two children with callosal agenesis, who have been previously shown to have impairments on rhyming tasks, are reported. Neither child is dyslexic in the traditional sense, since word reading levels are appropriate for age. However, both children have impaired development of the phonological reading route despite normal lexical skills. The pattern of their reading is therefore comparable to developmental phonological dyslexia. Problems in explicit phonological processing may be causal in the failure to establish an efficient phonological reading route but this is insufficient to create difficulty with word recognition itself. The corpus callosum may be essential for the normal development of a phonological reading route.

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

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.

A developmental study of hemisphere specialization for recognition of faces in normal subjects.

In a developmental study of hemisphere lateralization for recognition of faces in normal subjects three groups of subjects (mean ages: 7 years 9 months; 13 years 10 months; and 19 years 6 months) were tested. The procedure was a go-no go reaction time task in which the subject responded to faces tachistoscopically presented to the right or left visual field. A left visual field superiority was found for the two older age groups, which is interpreted as a right hemisphere dominance for recognition of faces. However, the 7 and 8 year old group did not demonstrate a significant visual field difference. These results indicate that the hemisphere specialization (lateralization) for recognition of faces involves processes which develop with increasing age through childhood and is not well-established until some time after the age of 8 and before age 13.

Ten pen men: rhyming skills in two children with callosal agenesis.

Cases of callosal agenesis provide unique opportunities to investigate the normal role of the corpus callosum in the development of cognitive functions, including language. The only language impairment which has been consistently observed in three acallosal patients is on the retrieval of words from rhyming cues. Two new cases of callosal agenesis in children of normal intelligence are presented. Their performance on a variety of rhyming tasks involving both production and recognition of rhyme is reported. Both children display deficits and possible explanations are discussed.

Further Studies of Tactile Perception and Motor Coordination in Agenesis of the Corpus Callosum

A 12-year-old female with total agenesis of the corpus callosum has been tested on several tasks involving tactile perception and motor coordination. Her performance has been compared with a group of normals of the same age and sex and a group of subjects matched for age, sex and I.Q. On 3 of the bimanual motor coordination tasks the acallosals performance was not clearly distinguishable from the controls. She was, however, slower than the controls on the pegboard task. On a transfer of training task (formboard) the acallosal was slower than normals and did not manifest transfer. However, transfer of training was shown on a maze-learning task. No deficit was found in the ability to tactually cross-identify objects, but a deficit in tactile cross-localization was evidenced.