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Mismatch negativity and the P300 have been investigated as electrophysiological indices of behavioral auditory discrimination of duration. Using an oddball paradigm, responses were evoked to stimuli that had been behaviorally demonstrated to be either perceptible or imperceptible. The results indicated P300 events were present with the perceptible contrast but absent for the imperceptible contrast with all participants, while mismatch negativities were present in approximately 80 and 20% of participants to the perceptible and imperceptible contrasts, respectively. The present study extended the findings by applying the same paradigm to the discrimination of spectral and amplitude contrasts. 10 young adults with normal hearing were participants. Assuming that auditory assessment should involve representation and processing in all acoustic domains (i.e., temporal/duration, spectral/frequency, and amplitude/intensity), the effects seen with temporal contrasts were predicted to be similar for spectral and amplitude contrasts. The findings generally illustrated that the P300 was more accurate than the mismatch negativity in reflecting behavioral discrimination. Together, these studies challenge the use of mismatch negativity as an electrophysiological correlate for behavioral discrimination of auditory perceptible contrasts.