Electrophysiological evidence for the typicality effect of human cognitive categorization.

Event-related brain potentials (ERPs) were recorded from 14 normal subjects during a category verification task. Stimulus words were selected from 17 semantic categories (e.g. 'vegetables'). Half of the words were typical category members (e.g. 'carrot', 'spinach') and the other half were atypical (e.g. 'parsley', 'asparagus'). Subjects were required to judge whether each stimulus belonged to a target category ('vegetables' or 'sports') or a non-target category. For the non-target category, the typicality effect was neither found in ERPs nor in reaction times. For the target category, typical words were responded to more quickly than were atypical words and the ERP amplitudes between a 300-450 ms period were more negative after the atypical words than after the typical words (typicality effect). These results suggested that typical words of the target were more primed by a target category than were the atypical words of the target and thus that a concept is represented by a prototype, the central tendency of all members of the category.