What bilateral displays do

Bilateral presentations of stimuli produce asymmetries larger than those of unilateral presentations, an effect that exists across modalities. In the case of visual field differences at least 13 hypotheses of the effect have been proposed, 6 of which suggest that some form of hemispheric interaction is responsible for the bilateral advantage. Previous research has eliminated all 7 noninteraction hypotheses, has supported the class of hemispheric interaction mechanisms, and has ruled out two specific instances of the latter. Here four experiments test the remaining hypotheses. The results support the homolog activation hypothesis, which states that bilateral stimulation by similar stimuli activates homologous areas of the two hemispheres, disrupting communication between the areas. The other hypotheses were eliminated.