Phonemes, intensity and attention: differential effects on the mismatch negativity (MMN).

Auditory event-related potentials (ERPs) to speech sounds were recorded in a demanding selective attention task to measure how the mismatch negativity (MMN) was affected by attention, deviant feature, and task relevance, i.e., whether the feature was target or nontarget type. With vowel-consonant-vowel (VCV) disyllables randomly presented to the right and left ears, subjects attended to the VCVs in one ear. In different conditions, the subjects responded to either intensity or phoneme deviance in the consonant. The position of the deviance within the VCV also varied, being in the first (VC), second (CV), or both (VC and CV) formant-transition regions. The MMN amplitudes were larger for deviants in the attended ear. Task relevance affected the MMNs to intensity and phoneme deviants differently. Target-type intensity deviants yielded larger MMNs than nontarget types. For phoneme deviants there was no main effect of task relevance, but there was a critical interaction with deviance position. The both position gave the largest MMN amplitudes for target-type phoneme deviants, as it did for target- and nontarget-type intensity deviants. The MMN for nontarget-type phoneme deviants, however, showed an inverse pattern such that the MMN for the both position had the smallest amplitude despite its greater spectro-temporal deviance and its greater detectability when it was the target. These data indicate that the MMN reflects differences in phonetic structure as well as differences in acoustic spectral-energy structure of the deviant stimuli. Furthermore, the task relevance effects demonstrate that top-down controls not only affect the amplitude of the MMN, but can reverse the pattern of MMN amplitudes among different stimuli.