Hemispheric control of unilateral and bilateral responses to lateralized light stimuli after callosotomy and in callosal agenesis

Abstract

Normally, simple digital or 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. Because of the organization of visual and motor pathways, the integration of uncrossed responses is assumed to occur within a single hemisphere, whereas a time-consuming interhemispheric transfer via the corpus callosum is considered to be necessary for the integration of crossed responses. However, callosal transfer may be dispensable for those crossed responses which can be controlled through ipsilaterally descending motor pathways by the hemisphere receiving the visual stimulus. We investigated crossed-uncrossed differences (CUDs) in speed of simple visuomotor responses to lateralized flashes in one subject with total section of the corpus callosum and two subjects with complete callosal agenesis. We recorded the reaction times as well as the premotor times, as indicated by the electromyographic latencies of the prime movers, of three types of responses: a distal response involving a thumb flexion, a proximal response chiefly involving a forearm flexion and an axial response involving a shoulder elevation. Further, the three types of responses to a single lateralised flash were performed both unilaterally and bilaterally. The three acallosal subjects showed CUDs greatly exceeding normal values on distal responses, either unilateral or bilateral, and on unilateral proximal responses. These abnormally long CUDs stood in sharp contrast to the insignificant CUDs exhibited by the same subjects on bilateral proximal responses and on unilateral and bilateral axial responses in agreement with correspondingly insignificant CUDs reported for normal subjects. These results confirm that a callosal contribution is important for the execution of fast distal and unilateral proximal responses to a visual stimulus directed to the hemisphere ipsilateral to the responding hand. By contrast, the other types of crossed responses appear to be efficiently coordinated across the midline without the aid of the corpus callosum. This is in keeping with the hypothesis that they are governed by a bilaterally distributed motor system which is preferentially activated for the execution of symmetrical bilateral movements, employing axial and proximal limb muscles.

Interhemispheric transfer of colour and shape information in the presence and absence of the corpus callosum

“…Despite some loss of accuracy in the right-hand-RVF condition in two-choice discrimination, his performance remained high under all other conditions, including the three-choice task. The number of trials was perhaps too small to yield accurate measures of the CUD, but the values obtained in the go±no go and two-choice tasks are if anything shorter than expected, given previous evidence for a somewhat prolonged CUD as estimated from simple RT [1,13,21,47]. The prolonged CUD of 51 ms in the three-choice task is tantalising, but was not signi®cantly di erent from zero, perhaps due to the small sample size and high variability.…”

“…There has been no reason for any further neurological testing. Standard neurological examinations have revealed no clear manifestation of interhemispheric disconnection [1], except for a lengthened CUD and some subtle sign of defective visual communication between the hemispheres [13,15,20,21].…”

“…Neurological examination was performed since R.B. complained about headaches leading to a suspicion that he might have inherited a form of optic subatrophy [1]. There has been no reason for any further neurological testing.…”

“…and the acallosal subject R.B. has been found to be largely una ected by variations in luminance, including a condition of equiluminance [1,21], again suggesting that the information transferred is motor rather than sensory. A stronger test of the nature of the information transferred is provided in the present experiments, since the responses are contingent upon the colour and shape of the stimuli.…”

“…In commissurotomised subjects, CUDs based on simple RT vary between about 29 and 96 ms, indicating that transfer via extracallosal pathways is substantially longer than that via the corpus callosum [10,13,21,40]. People with agenesis of the corpus callosum have also been found to have CUDs that are prolonged relative to those in normals, although typically shorter than those with surgical section of the commissures; in various studies, they have ranged between 9.5 and 32 ms [1,13,21,24,28,37,45]. They may also show other subtle de®cits in interhemispheric interaction besides this increase in CUD [15,31,33,36], although they do not show the now classic patterns of disconnection evident in callosotomised or commissurotomized subjects [5,25,30].…”