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.

Adaptation refers to the decreased neural response that occurs after repeated exposure to a stimulus. While many electroencephalogram (EEG) studies have investigated adaptation by using either single or multiple repetitions, the adaptation patterns under controlled expectations manifested in the two main auditory components, N1 and P2, are still largely unknown. Additionally, although multiple repetitions are commonly used in mismatch negativity (MMN) experiments, it is unclear how adaptation at different time windows contributes to this phenomenon. In this study, we conducted an EEG experiment with 37 healthy adults using a random stimulus arrangement and extended tone sequences to control expectations. We tracked the amplitudes of the N1 and P2 components across the first ten tones to examine adaptation patterns. Our findings revealed an L-shaped adaptation pattern characterized by a significant decrease in N1 amplitude after the first repetition (N1 initial adaptation), followed by a continuous, linear increase in P2 amplitude after the first repetition (P2 subsequent adaptation), possibly indicating model adjustment. Regression analysis demonstrated that the peak amplitudes of both the N1 initial adaptation and the P2 subsequent adaptation significantly accounted for variance in MMN amplitude. These results suggest distinct adaptation patterns for multiple repetitions across different components and indicate that the MMN reflects a combination of two processes: the initial adaptation in the N1 and a continuous model adjustment effect in the P2. Understanding these processes separately could have implications for models of cognitive processing and clinical disorders.