Event-related potentials in an auditory semantic oddball task in humans

Whenever rare target stimuli are presented interspersed by frequent irrelev ant stimuli (standards), the targets elicit a late parieto-central positive wave called P3. Usually standards and targets differ by a simple physical feature (e.g. tone pitch). Less consistent are the data obtained in semanti c tasks, in which standards and targets are represented by different word c lasses. Given an equal number of words in the target and standard categorie s, each individual standard is more frequent than each target, that is, the frequency of the category is confounded with that of individual stimuli. I n the present study participants were presented five semantic classes each being represented by 15 words. Words belonging to one particular class shou ld be counted. Thus targets were rare (20%), although the frequencies of ea ch word and of each semantic class were equal. A highly significant P3 was recorded. Its latency was longer, and the amplitude was smaller, than when standards and targets were two tones. These data indicate that brain waves recorded in semantic tasks are not necessarily manifestations of specific s emantic processing. (C) 2001 Elsevier Science Ireland Ltd. All rights reser ved.