Interhemispheric neural summation in the split brain with symmetrical and asymmetrical displays.

The present study, investigates interhemispheric integration in the split brain. Four split-brained, two acallosal and 14 normal subjects carried out a simple reaction time task in which they responded to stimuli presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. Stimuli were white against a black background and bilateral stimuli were either symmetrical or asymmetrical around the central vertical meridian. For unilateral stimuli, the difference in response time (RT) between crossed and uncrossed hand-field combinations (crossed-uncrossed difference, or CUD) measured interhemispheric transfer time. RTs to bilateral and unilateral stimulus displays were compared to provide a measure of redundancy gain (RG). Normal subjects exhibited small CUDs and RGs. Split-brained and acallosal subjects were found to have much longer CUDs, and to show enhanced RGs which could not be explained by a probability (race) model, implying subcortical neural summation. This summation did not depend on the preservation of symmetry, suggesting that it may not occur at the retinotopically organized superior colliculus, but at another site such as the pons or reticular formation.