Interhemispheric cooperation: Left is left and right is right, but sometimes the twain shall meet.

Abstract

Interhemispheric cooperation in the processing of nonsense syllables projected simultaneously to both hemispheres was investigated in 2 experiments. Stimuli were projected unilaterally in the left and right visual fields (LVF, RVF) and bilaterally (the same syllable in both fields, BVF; Experiment 1, 64 right-handed subjects) or centrally (CVF; Experiment 2, 32 right-handed subjects). Accuracy and error patterns differed for the LVF and RVF. Error patterns were intermediate for the BVF-CVF and were partly shifted toward the RVF for subjects with large RVF advantages and toward the LVF for those with small asymmetries. Regression analyses showed that variance in BVF-CVF error patterns was jointly predicted by LVF and RVF variances. Both hemispheres, as demonstrated by means and regression analyses, contribute to the processing of bihemispherically presented syllables. In focusing on the functional differences of the two halves of the human brain, investigators sometimes lose sight of the fact that both hemispheres contribute to and collaborate in producing most behaviors. Some of the central questions regarding interhemispheric integration are as follows: Do the functionally distinct cerebral hemispheres share the processing load? If they do, under what conditions? What kinds of information and tasks are processed cooperatively, and when is information shared? Studies of interhemisphe ric collaboration have investigated the speed with which two stimuli, presented in the same or opposite visual fields, can be compared. This design can yield information on the efficiency of intra- versus interhemispheric processing because bihemispheric integration must occur when the two stimuli to be compared are presented in opposite visual fields (crossed-field condition) but is not necessary when stimulus pairs are presented in the same visual field (within-field condition). Processing load is spread more evenly across the hemispheres on crossed-field trials, which may result in a processing advantage. This has