Interhemispheric collaboration in response to simultaneous bilateral input.

Two experiments tested whether processing is speeded when inputs are divided between the hemispheres rather than projected to a single hemisphere. Bilateral (bihemispheric) vs unilateral (single hemisphere) word pairs were tachistoscopically presented. Subjects responded when the words semantically matched. Irrespective of distance, visual angle and compatibility with reading biases, bilateral/bihemispheric performance was as fast as the better of the two individual unilateral/single hemisphere conditions and faster than their average. This pattern occurred only at the beginning of each experiment. With practice, performance significantly improved in the single but not the bihemispheric conditions. The excellent initial performance during the bihemispheric condition and its lack of improvement with practice are discussed with reference to models of interhemispheric collaboration.