How the human auditory system treats repetition amongst change

Occasional repetitions embedded in a sequence of ever-changing sounds elicit a frontally negative event-related potential peaking between 100 and 200 ms from the onset of sound repetition (repetition negativity, RN). Three interpretations have been suggested for the emergence of RN: (1) RN is a mismatch negativity (MMN) response elicited because the auditory system detected sound change as a regular feature of the stimulus sequence, which is violated by repetitions. (2) RN reflects short-term sensitization of the auditory system to the specific acoustic parameters of each sound, a process subserving the formation of durable memory traces. (3) The difference between responses elicited by repetition and change reflects differential refractoriness of the P2 component. In order to distinguish between the MMN-based and the other two explanations of RN, we varied the global sequential probability of tone-repetitions (at 5, 20, and 50%) in sequences composed of tones of five different frequencies. Whereas all three explanations of the RN predict a local repetition-probability effect on the RN amplitude, MMN should also be affected by the global repetition-probability, because MMN is sensitive to the global deviant-probability, independently of local deviant-probability. To eliminate differences in local repetition-probabilities, we compared the responses elicited by tones that followed the same micro-sequence (change-change-repetition) across the three different global-probability conditions. Global repetition probability had a significant effect on RN: it was only elicited with 5%, but not with the two higher repetition-probabilities. This result supports the MMN interpretation of the RN.