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 and 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 between crossed and uncrossed responses (CUD) to be calculated. For the normal subjects the CUD tended to diminish with the complexity of the tasks, suggesting that both hemispheres were increasingly involved. Unlike R.B. and the normal controls, who made virtually no errors, L.B. had increasing difficulty as task complexity increased. He was better able to transfer information from the right to the left hemisphere than vice versa, but an analysis of his accuracy under the crossed conditions showed that the amount transferred was always well under one bit. This confirms previous evidence that L.B. has very limited subcortical transfer of either colour or shape. (C) 1999 Elsevier Science Ltd. All rights reserved.