Effects of visual fields, stimulus degradation, and level of practice on event-related potentials of the brain

12 male undergraduates were instructed to indicate whether letter pairs that were randomly presented in the center and peripheral (left and right) visual fields were semantically same or different. Letter pairs could be either intact or perceptually degraded, and all Ss participated in 3 consecutive sessions. Results show that event-related potential (ERP) components were strongly affected by the visual field of presentation of stimuli. Center field stimuli elicited late positive components (P630, slow wave) of larger amplitude than peripheral field stimuli, at all electrode locations. At lateral sites, N200 components were always larger (more negative) and late positive components were consistently smaller (less positive) to contralaterally than ipsilaterally presented stimuli. However, the contralateral negativity of N200 was much larger over the right than left regions of the scalp, while positive ERP components showed a more symmetrical distribution of the contra- vs ipsilateral stimulation effect. Results are discussed in relationship to cognitive aspects of information processing, as well as the neural origin of endogenous ERP components.