ERPs to laterally presented pictures and words in a semantic categorization task.

Recorded event-related potentials (ERPs) from 10 adult males reacting to visual stimuli (pictures or words) presented in the left and right visual fields. Ss were instructed to match these stimuli on the basis of physical or categorical identity with stimuli of a previously presented memory list. ERP elicited by lateral stimuli consisted of a sequence of topographically and functionally separable, negative and positive components. On left and right hemispheric locations, N200 was consistently larger and P720 was consistently smaller to stimuli presented in the contralateral fields than to stimuli presented in the ipsilateral fields. P720 amplitude to ipsilaterally presented stimuli showed a marked variation as a function of stimulus type, level of practice, and the hemispheric site of measurement, while contralaterally presented stimuli produced only minimal variation in P720 amplitude.