Interhemispheric interaction in the normal brain: Comparisons within and between the hemispheres

The Bilateral Distribution Advantage (BDA) results when comparisons of stimuli presented to opposite hemispheres (bilateral trials) are performed more quickly or accurately than comparisons made within a single hemisphere (unilateral trials) despite the costs associated with interhemispheric communication. We studied a three-item version of the Posner-Mitchell shape and name letter matching task in which matching items are presented either unilaterally or bilaterally. This task was designed to overcome confounds with scanning direction and unequal load on the hemispheres in unilateral and bilateral trials which may cause the BDA in two-item tasks. We addressed the claim that task difficulty and an imbalance in perceptual load in the hemispheres are the source of the BDA. We manipulated task parameters in a series of twelve experiments to examine the roles of difficulty, parallel processing, attention, and perceptual imbalance in producing the BDA.; We showed that post-exposural scanning of the display was affecting performance, and that scanning was reduced or eliminated with attentional manipulation. We also demonstrated that the BDA could occur in the absence of this scanning. We studied a four-item task which removes the imbalance in perceptual load in the hemispheres and found a smaller BDA, demonstrating a role for perceptual imbalance in the advantage. However, the residual BDA implicates an imbalance in cognitive load as an important separate contributor. Scanning and perceptual imbalance appear to be indexed by response bias (beta), whereas the imbalance in cognitive load is reflected in sensitivity ({dollar}dspprime{dollar}).; The BDA was not indexed by any simple measure of task difficulty. Instead, it may depend on the number of component processes in the task. This is supported by the inability of a simple horserace model to describe the data, although processes in the two hemispheres may dominate at different stages.; The perceptually balanced four-item task with attentional cues provides a promising tool for studying individual differences in the capacity of the corpus callosum to mediate parallel processing in the hemispheres. It is also useful for studying patients thought to possess abnormal interhemispheric relations, such as schizophrenics or patients with circumscribed callosal lesions.