4.3 P1 Adaptation: A Slight Increasing Effect
Our hypothesis that no latency differences in P1 between tones based on previous studies (e.g., Rosburg et al., 2006) was supported by the current finding. Furthermore, according to the RP, the P1 amplitude was expected to increase instead of decrease along repetitions (Recasens et al., 2015), and this pattern was found in the current study. First, an increasing trend from the 4th to the 10th tones was observed in the P1 adaptation pattern (Figure 2a). In addition, the P1 subsequent adaptation effect was positively correlated with the N1 and P2 subsequent adaptation effects (Table 2). These results are generally consistent with that the RP is a slow potential that incorporates not only N1 and P2 but also P1 (Haenschel et al., 2005). Importantly, the current study shows that this RP pattern could also be found when expectations were minimized.
Nonetheless, the above finding was contradictory to our hypothesis and the decreased amplitude appeared in the first repetition found in previous studies (Boutros et al., 1999; Rosburg et al., 2006; Todorovic & de Lange, 2012; c.f. Recasens et al., 2015). This discrepancy may be attributed to the different experimental designs. Previous studies either used a pair of tones (Todorovic & de Lange, 2012) or a stable train of tones (Boutros et al., 1999; Rosburg et al., 2006) to elicit the repetition suppression. On the contrary, the current study adopted a roving paradigm, in which the tones were played continuously. Hence, intervals were inserted between each stimulus train in previous studies showing a P1 adaptation effect, but they did not exist in the current study and the study by Recasens et al. (2015), in which they also did not find any P1 differences between tones. P1 adaptation effect may therefore be seen only when there was enough time for adaptation recovery. Another potential explanation is that expectations may modulate the adaptation pattern in P1. In previous studies, participants could predict the frequency of the next tone due to a fixed pair or a train of tones to a large extent, but in the current study, they could not. Hence, the P1 adaptation effect described by previous studies may comprise the congruence of expectations and stimuli.