Electrophysiological evidence of implicit and explicit categorization processes

Event-related brain potentials (ERPs), which provide on-line measures of categorization processes, were used to assess the implicit and explicit categorization of stimuli along evaluative (positive and negative) and nonevaluative (people and no-people) dimensions. Participants were exposed to stimuli that simultaneously varied along both dimensions, but were explicitly instructed to categorize along only one of them. Consistent with prior research, the late positive potential (LPP) of the ERP was sensitive to participants' explicit categorization task. However, the LPP also revealed implicit categorization along the non-task-relevant dimension. Additionally, there was evidence of an implicit negativity bias in which negative stimuli spontaneously received greater processing than positive stimuli and a people/no-people analog in which stimuli containing people received greater processing than stimuli without people. Results suggest the operation of adaptively beneficial implicit categorization processes, triggered by significant proximal stimuli, serving broad, cross-situational goals. (C) 2000 Academic Press.