Lexical decision after left, right and bilateral presentation of function words, content words and non-words: evidence for interhemispheric interaction.

Function words, content words and pronounceable non-words (pseudowords) were presented tachistoscopically either in the left or the right visual field or with identical copies flashed simultaneously to both visual half-fields. Consistent with earlier studies [10], function words were found to show a right visual field advantage, whereas for content words the right visual field advantage was absent. Compared to either of the unilateral modes of presentation, bilateral presentation of identical word stimuli improved accuracy and latency significantly. The bilateral (Bi) advantage was largest for content words, and was also highly significant for function words in both latency and accuracy. The Bi gain was absent for non-words (significant interaction of Wordness x Visual Field). These results indicate that the lexicons of the left and right hemisphere can "collaborate" rather than inhibit each other or act independently when processing the same linguistic stimuli. Our findings are consistent with the view that the neuronal counterparts of words are Hebbian cell assemblies consisting of strongly connected excitatory neurons of both hemispheres. Since function words show a right visual field advantage in addition to their Bi gain, their assemblies are likely to have most of their neurons located in the left hemisphere. Neuronal assemblies corresponding to content words may be less strongly lateralized.