Event-related potentials recorded during the discrimination of improbable stimuli.

In three separate experiments using auditory stimuli, subjects detected improbable targets in an on-going train of standard stimuli. In experiment I there were two equally improbable target stimuli, one difficult to discriminate from the standard stimuli and one easy to discriminate. Experiment II investigated the effects of discrimination difficulty using four different targets, each equal in probability. Experiment III evaluated the effects of both target discriminability and probability on the target-evoked potentials. The subjects in all three experiments consistently displayed delayed reaction times and higher percentages of misses with the more difficult targets. The amplitude of the N1 wave to target stimuli increased as target discriminability was increased. The latencies of the target-evoked N2 and P3 waves increased and the amplitude of the P3 decreased as target discriminability decreased. The amplitude of the N2 wave increased as the target discrimination became more difficult. The amplitude of the N2 wave elicited by an easy target was enhanced by making the target highly improbable whereas probability did not affect the N2 elicited by a difficult target.