Adaptation is defined as reduced neural activation following repeated stimulus presentation. While numerous previous electroencephalogram studies used either one repetition or multiple repetitions to elicit adaptation in event-related potentials, the adaptation patterns under controlled expectations manifested in the two main auditory components, N1 and P2, are still largely unknown. In addition, multiple repetitions were used in mismatch negativity experiments to measure deviance detection, but how adaptation in different time points contributes to the mismatch negativity remains unclear. Therefore, in the present study, 37 healthy adults participated in an electroencephalogram experiment with a pure tone roving paradigm and a random stimulus arrangement to maintain expectations. The amplitudes were traced along with the first ten tones in the auditory event-related potentials components, N1 and P2, to examine the adaptation patterns. Results showed an L-shaped adaptation in the N1 with a large decrease after the first repetition (N1 initial adaptation), and a continuous, linear amplitude increase in the P2 even after the first repetition (P2 subsequent adaptation), possibly indicating memory trace formation. Regression results showed that the peak amplitudes of the N1 initial adaptation and the P2 subsequent adaptation significantly explained the variance in the mismatch negativity amplitude. The results indicate distinct adaptation patterns for multiple repetitions in different components and suggest that the mismatch negativity combines two processes as indicated by the initial adaptation in the N1 and a continuous memory trace effect in the P2. Separating the two processes may be relevant for models of cognitive processing and clinical disorders.

Repetition suppression (RS) refers to the reduction of neuronal responses to repeated stimuli as compared to non-repeated stimuli. The predictive coding account of RS proposes that its magnitude is modulated by repetition probability (P(rep)) and that this modulation increases with prior experience with the stimulus category. To test these hypotheses, we examined the RS and it’s modulation by P(rep) for three stimulus categories for which participants had different expertise, using EEG methodology. Cantonese speakers watched paired stimuli (S1-S2) of Asian faces, Chinese written words, and animal pictures with the S2 being the same or different from S1. Attributes of S1 (e.g., the sex of the first face) served as a cue for repetition probability of S2. Time-point by time-point Topographic Analyses of Variance (TANOVA) for words showed significant repetition effects across several intervals (92-140, 150-248, 260-488, and 502-560ms), and expectation effects during 789-844ms. Significant repetition effects were also identified for faces (207-358ms), and animals (324-486ms). Timing and topographies suggest N250r effects for all three stimulus categories, but TANOVA comparisons indicate earlier and distinct topographic distributions of repetition effects for words versus faces (151-263, 277-445m) and animals (148-242, 266-437ms), and for faces versus animals (209-316ms). These results suggest that repetition effects differ between stimulus categories, presumably depending on prior experience and stimulus properties, such as spatial frequencies. Importantly, we did not find any EEG evidence for effects of P(rep) potentially manipulating expectancy. Such null findings of P(rep) effects do not support the general predictive coding account of repetition suppression.