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Neuropsychological studies of callosal agenesis.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Milner, Derek J. |
| Copyright Year | 1983 |
| Abstract | A developmental error or arrest during the foetal growth of the telencephalic midline can lead to partial or complete agenesis of the corpus callosum. A person who is born with no corpus callosum at all is likely also to lack the hippocampal commissure, but unlikely to lack the anterior commissure (Loeser & Alvord, 1968 a). Indeed, some congenitally acallosal individuals appear to have an enlarged anterior commissure (Bossy, 1970; Loeser & Alvord, 19686). Their other commissures, as far as is known, do not differ appreciably from those of a normal brain. Against this background, behavioural investigations of totally acallosal subjects have been largely directed towards answering three neuropsychological questions. First, is an intact corpus callosum necessary for cerebral dominance to be established? Secondly, given that in most respects the 'disconnection syndrome' well known in patients who have undergone surgical section of the cerebral commissures (Sperry, 1974) is absent in cases of agenesis, how does the brain compensate for such a drastic structural deficiency? And, thirdly, what are the limits of compensation, both in regard to the cross-integration of sensory information and in relation to behavioural capacities which may depend in a less obvious way upon interhemispheric collaboration? In recent years some progress has been made towards answering all three of these questions, although complete answers are still not available. The first question relates to the ontogeny of cerebral dominance. Some theorists (e.g. Kinsbourne & Hiscock, 1977) have argued that dominance may essentially be wired-in at birth, and that there is no good evidence that the degree of functional asymmetry between the cerebral hemispheres grows with age. Others (Seines, 1974; Moscovitch, 1977; Denenberg, 1981) have argued that, even if there is the seed of cerebral asymmetry present at birth, this can only develop into the normal adult pattern through an inhibitory callosal interaction, by which one side can dominate and suppress the other. In support of their argument such theorists cite what they take to be evidence of weak or even non-existent lateralization of cerebral function in the acallosal brain. In particular, they cite small behavioural asymmetries in three types of test: tachistoscopic recognition (of words, letters and pictures), tactual naming of objects, and dichotic listening (to digits, syllables or words). It is true that following surgical callosal section in adulthood, even when the anterior commissure is spared, extreme asymmetries in these tasks are found (Sperry, 1974; McKeever et al. 1981). However, the fact that congenital absence of the callosum does not have these consequences could be fully explained by the compensatory development of other commissures, decussations, and/or ipsilateral sensory projections. Such re-wirings might easily co-exist with normal degrees of cerebral lateralization in callosal agenesis (Milner & Jeeves, 1979, 1981). That some such development of neural pathways must have occurred is demonstrated by the existence of visual and tactile cross-matching abilities in acallosal subjects which are not present in patients with callosal section. These abilities (e.g. Ettlinger et al. 1972) cannot be explained by incomplete cerebral lateralization. The question as to whether acallosal brains are incompletely lateralized cannot therefore be answered by comparing acallosal with callosotomized patients and demonstrating smaller behavioural asymmetries. On the other hand, it can be demonstrated that some asymmetry of function is present in acallosal brains. For example, there generally seems to be better tachistoscopic recognition of verbal and pictorial items presented in the right visual field which projects to the left hemisphere (Ettlinger et al. 1972; Jeeves et al. 1983). Furthermore, vocal responses to a photic stimulus have |
| Starting Page | 4301 |
| Ending Page | 4304 |
| Page Count | 4 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033291700051424 |
| PubMed reference number | 6665090v1 |
| Volume Number | 13 |
| Issue Number | 4 |
| Journal | Psychological medicine |
| Language | English |
| Access Restriction | Open |
| Subject Keyword | Agenesis of corpus callosum Auditory Perception Body of uterus Brain Childhood Cerebral Astrocytoma Congenital absence Cortical Cell Layer of the Cerebral Cortex Digit structure Dominance, Cerebral Enlargement procedure Granuloma, Lethal Midline Growth MATCHING Neural Pathways Partial Patients Physical object Projection Defense Mechanism Small Structure of anterior commissure Syllable |
| Content Type | Text |
| Resource Type | Article |
