ERPs to Laterally Presented Pictures and Words in a Semantic Categorization Task

Event‐related potentials (ERPs) were recorded from subjects reacting to visual stimuli (pictures or words) presented in the left and right visual fields. Subjects were instructed to match these stimuli on the basis of physical or categorical identity with stimuli of a previously presented memory list. The ERPs elicited by the lateral stimuli consisted of a sequence of topographically and functionally separable, negative and positive components, including N200, P300, N540, P720, and SW. On left and right hemispheric locations N200 was consistently larger, and P720 consistently smaller to stimuli presented in the contralateral fields than to stimuli presented in the ipsilateral fields. In addition, the amplitude of P720 to ipsilaterally presented stimuli showed a marked variation in amplitude 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. The latter results are discussed in relation to possible intracranial sources and the functional significance of the P3 component elicited by laterally presented stimuli.