Setting standards: Auditory MMN and P3a habituate differentially in a two-syllable oddball task

The Mismatch Negativity (MMN) and the P3a of the event-related EEG potential (ERP) serve as indices of prediction error detection in processing auditory stimuli. The characteristics of both components regarding the significance of the stimuli as standard or deviant have now been comprehensively described, but little is known about how these components behave during the establishment of standards or deviants. Most investigations used paradigms where the average of many instantiations of a stimulus per participant is calculated. Thus, the nascent process of establishing a stimulus as a standard stimulus, i.e., the very initial process of setting a standard, cannot be tracked. The present study used an auditory oddball task, building on a previous study using a two-tone pitch paradigm, but incorporating the speech syllables "ba" (standard) and "pa" (deviant). A rather small number of stimuli (11 deviant instantiations) was applied to a large sample of participants (> 230), to allow for ERP characterization as a function of the standard's or deviant's position in a series of standard and deviant stimuli. We found that, beginning with the fourth instantiation of the standard/deviant pair, the (new) standard was established, and no further modification of the ERP to repeated presentation of the standard stimulus could be detected. Analyzing MMN and P3a components to deviant stimuli revealed a prominent habituation of the P3a between the first and the second instantiation of the deviant, whereas no relevant habituation of the MMN could be detected, indicative of a differential MMN/P3a habituation to repeated speech-like auditory stimuli.