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 a nd 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 be tween crossed and uncrossed responses (CUD) to be calculated. For the norma l subjects the CUD tended to diminish with the complexity of the tasks, sug gesting that both hemispheres were increasingly involved. Unlike R.B. and t he normal controls, who made virtually no errors, L.B. had increasing diffi culty as task complexity increased. He was better able to transfer informat ion from the right to the left hemisphere than vice versa, but an analysis of his accuracy under the crossed conditions showed that the amount transfe rred was always well under one bit. This confirms previous evidence that L. B. has very limited subcortical transfer of either colour or shape. (C) 199 9 Elsevier Science Ltd. All rights reserved.