The N2 component elicited by stimulus matches and multiple targets.

This paper examines two methodological issues concerning the N2 component of human event-related potentials. The first issue concerns the circumstance that the most common way to obtain N2 in discrimination tasks is with an infrequent deviant stimulus that mismatches a frequent, standard stimulus. In these studies it is not possible to disentangle the effects of stimulus probability and stimulus mismatch on N2. In the present study it was found that, if two stimuli regularly alternate, N2 is elicited by infrequent repetitions of either stimulus. Thus, N2 is elicited by infrequent stimulus matches as well as infrequent stimulus mismatches. The second issue concerns the effect of stimulus probability on N2. Whereas previous research has established that the amplitude of N2 is inversely related to stimulus probability, the present study found that the number of possible targets in a visual discrimination task also has effects on N2 amplitude, with the overall probability of targets kept constant. Increasing the number of targets was associated with an increase in the duration of N2 and a differential enhancement of N2 at fronto-central as opposed to posterior-lateral recording sites. The latter results provide further evidence for the existence of two visual N2 components and tentative grounds for differentiating N2 from N400.