Effects of visual-field and matching instruction on event-related potentials and reaction time.

Vertical letter pairs were presented randomly in the left and right visual hemifields of 20 right-handed male undergraduates in physical-identity match and name-identity match conditions. Reaction times (RTs) showed a right visual-field superiority for name matches and a left visual-field superiority for physical matches. Event-related potentials to letter pairs showed a sequence of 3 waves: a negative wave (N2) at around 270 msec, a positive wave (P3) at around 500 msec, and a broad positive slow wave (SW) at around 600-700 msec, respectively. P3 and SW amplitudes were consistently larger at the left hemisphere than at the right hemisphere, regardless of the field of stimulation. At both hemispheres, N2 waves were always larger to stimuli presented in the visual field contralateral to a hemisphere than stimuli presented in the visual field ipsilateral to a hemisphere. The positive waves (P3 and SW) showed smaller amplitudes to stimuli that were presented contralaterally than stimuli that were presented ipsilaterally to a given hemisphere. Results are attributed to a shift in sustained negativity on the directly stimulated hemisphere relative to the indirectly stimulated hemisphere, reflecting either sensory or attentional processes in the posterior cerebral hemisphere. (26 ref)