Hemispheric independence in word recognition: evidence from unilateral and bilateral presentations.

We compared behavioral laterality effects in a lexical decision task using cued unilateral or bilateral presentations of different stimuli to normal subjects. The goals were to determine the effects of lexical variables on word recognition in each hemisphere under conditions of maximal independence of information processing in the two hemispheres and to assess the degree of residual interhemispheric effects that can still exist then. Bilateral presentations increased hemispheric independence in word recognition, indexed by a significant interaction of response hand with target visual field. Bilateral presentations also selectively impaired word decisions, suggesting that word processing benefits from interhemispheric interactions, whereas nonword processing is done independently in each hemisphere. Indeed, there was a significant congruity effect for word targets only, whereby the wordness of the unattended stimulus affected the speed of processing of attended word targets. Word frequency and regularity affected both hemispheres equally, arguing against the hemispheric interpretation on the dual route model of word recognition. Length affected the processing of nonwords more than words and in the left visual field more than in the right visual field. Taken together, the data support the conclusion that each normal hemisphere can control word recognition independently of the other.