Sequential effects on the ERP in discriminating two stimuli.

Two auditory stimuli differing in pitch were presented in random order and equal probability with the constant inter-stimulus interval of 1 sec. The subject's task was to count one of these stimuli. The event-related brain potentials (ERPs) to each stimulus were averaged according to the immediately preceding stimulus sequence. It was found that when a few consecutive repetitions of one stimulus occurred, the ERP to the other stimulus immediately after those repetitions included features resembling those of the ERP to the infrequent stimulus usually observed in the so called 'oddball paradigm'. These features included, among other things, the mismatch negativity usually regarded as a scalp reflection of the neuronal mismatch process with an existing neuronal model. The mismatch negativity was accompanied by an 'N2b', a sharper and later negativity. Interestingly, N2b only occurred when the not-to-be-counted stimulus succeeded (one or several) counted stimuli but not when the order was reversed. This suggests that N2b reflects template mismatch, the occurrence of a stimulus mismatching with the mental image of the target stimulus voluntarily held by the subject.