ERP correlates of the bilateral redundancy gain for words.

Neurophysiological correlates of hemispheric asymmetry and interhemispheric interaction in lexical processing were investigated in a lexical decision task with tachistoscopic stimulus presentation either unilaterally, to the right or left visual field, or bilaterally, with identical stimulus copies to each visual hemi-field. Behavioral data confirmed both right visual field advantage and bilateral redundancy gain for words but not for pronounceable orthographically regular pseudowords. ERPs showed a significant amplitude increase 160-200 after stimulus presentation specifically for words after bilateral redundant stimulation, which was present in the recordings from both hemispheres. Localization of cortical sources using minimum norm estimation indicated stronger cortical activity for words in temporal regions of both hemispheres after bilateral presentation compared with each of the unilateral stimulation conditions individually. Pseudoword presentation did not lead to a general increase of cortical activation in the bilateral condition compared with unilateral presentation. The specific activation increase for words in the bilateral redundant condition relative to unilateral stimulation and the absence of this effect for pseudowords, which became manifest in a significant interaction of the factors lexicality and presentation mode, is best explained by summation of neuronal activation from both hemispheres within distributed lexical circuits. Source estimation indicates that temporal areas, particularly in the left hemisphere, are the primary cortical loci where such stimulus-specific activity increases occurred.