Abstract
Callosal agenesics and callosotomized epileptics manifest markedly increasing simple visual reaction time (SVRT) from conditions of ipsilateral to contralateral stimulus-response relation (SRR). In the contralateral SRR, a response is presumed possible because of presence of other commissures (anterior, intercollicular). The SRR effect is prolonged presumably because the remaining commissures are less efficient than the corpus callosum in relaying necessary visual or motor information. Consequently, the SRR effect is believed to correspond to callosal relay time (CRT) in the normal subject. However, both callosal agenesics and callosotomy patients manifest general slowing of SVRT in addition to a prolonged SRR effect. These patients have massive extra-callosal damage which could plausibly cause both the SVRT and the CUD prolongation. If such were the case, the CRT inference would be in jeopardy. A test of the CRT inference is therefore required where patients with massive diffuse extra-callosal brain damage and normal callosi would show marked general SVRT prolongation and a normal SRR effect. Four trisomy-21 (T21) males were compared to age and sex-matched normal controls. General SVRT was highly significantly prolonged in T21, but the CUD was nearly identical in both groups.
Highlights
Over 50 published studies have been designed to investigate the difference between simple visual reaction time (SVRT) to targets ipsilateral versus contralateral to the responding hand, in normal humans
In all of the reports reviewed by us, the condition requiring interhemispheric relay, namely the contralateral stimulus-response relation (SRR condition), always generated, on the average, a longer SVRT than the condition not requiring any interhemispheric relay
The T-21 subjects made more than 17 times more errors than the normal control subjects (Chi square goodness of fit test: X = 355.16, p < 0.0000)
Summary
Over 50 published studies have been designed to investigate the difference between simple visual reaction time (SVRT) to targets ipsilateral versus contralateral to the responding hand, in normal humans. CUD is an index of callosal relay time (CRT) in the normal subject Both callosal agenesics and callosotomy patients manifest general slowing of SVRT in addition to prolonged CUDs. For different reasons and in different ways (foetal hypoplasic brain development, post-natal progressive degenerative brain disease, etc.) these patients have relatively massive and diffuse extra-callosal damage in addition to not having a callosal commissure. The extracallosal damage could plausibly be the sole or major cause of both the SVRT and CUD prolongation, rather than only of general SVRT If this were to be the case, the ITT To help consolidate the SVRT-ITT inference it would be useful to demonstrate that massive diffuse extra-callosal brain damage (not affecting the corpus callosum) can heavily prolong general SVRT while leaving unaffected the CUD.
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