Repeated presentation of the same stimulus typically leads to reduced neural activation, which is called neural adaptation or repetition suppression (Grill-Spector et al., 2006). It can be observed across different stimuli in both visual and auditory modalities, for example, in the repetition of faces (see Schweinberger & Neumann, 2016 for a review), symbols (e.g., Soltész & Szűcs, 2014), and tones (e.g., Todorovic & de Lange, 2012). However, it is unclear whether and how the repetition number, specifically one repetition or several repetitions, influences adaptation, as previous studies adopted designs with different approaches. For example, some studies only used two tones with one repetition to measure adaptation (e.g., Jaffe-Dax et al., 2017; Peter et al., 2019; Todorovic & de Lange, 2012; Todorovic et al., 2011), while other studies adopted long trains of tones with many repetitions to examine adaptation (Rosburg, 2004; Zhang et al., 2011), especially when testing its association with the mismatch negativity (MMN) (e.g., Garrido et al., 2008; Takasago et al., 2020). Therefore, the current study aims to compare the adaptations elicited by the initial tone repetition and the subsequent repetitions and examine their relationships with the MMN.
Adaptation can be measured by an amplitude decrement of components of the event-related potential (ERP) that is recorded from the scalp using electroencephalography (EEG). In the auditory modality, ERP components are typically measured at fronto-central electrodes, and include the P1, a positive component peaking at around 50 ms after the stimulus onset (de Wilde et al., 2007), the N1, a negative component occurring between around 60–160 ms (Woods, 1995), and the P2, a positive component peaks at around 150–250 ms (Jaffe-Dax et al., 2017). Distributed over fronto-central areas (Crowley & Colrain, 2004; Pratt et al., 2008; Woods, 1995), these components typically have a polarity reversal over inferior posterior electrodes, when measured against an average reference, as their sources are mainly in the temporal lobe (Crowley & Colrain, 2004; Fogarty et al., 2020; Näätänen & Picton, 1987). The auditory P1, N1, and P2 adaptations, as reflected by amplitude decrement, were found in a vast number of previous studies using pairs of tones or long trains of tones (e.g., P1: de Wilde et al., 2007; Todorovic & de Lange, 2012; N1: Bourbon et al., 1987; Budd et al., 1998; Lagemann et al., 2012; Näätänen & Picton, 1987; Rosburg, 2004; Rosburg & Mager, 2021; P1 and N1: Boutros et al., 1999; Rosburg et al., 2006; N1 and P2: Hari et al., 1982; Herrmann et al., 2016; Peter et al., 2019; Polich, 1986; Rosburg et al., 2022; Rosburg et al., 2010; P1, N1, and P2: Sambeth et al., 2004). However, inconsistent results on the adaptation pattern were found. Some studies showed an N1 decrease stabilized after the 2nd or 3rdsound in a stimulus sequence but no further decrease for subsequent sounds (e.g., Bourbon et al., 1987; Boutros et al., 1999; Budd et al., 1998; Lagemann et al., 2012; Rosburg, 2004). However, some studies found a gradual response decrease (e.g., Herrmann et al., 2016; Öhman & Lader, 1972). Therefore, the relationship between stimulus repetition and N1 adaptation is unclear. In addition, fewer studies were conducted in P1 and P2, so the adaptation effects associated with these two components are even more obscure. One study found that the P1 amplitude decrease caused by repetition mainly happened in the first two tones (Boutros et al., 1999). A similar finding with an initial amplitude decrease was shown in the P2 in other studies (Rosburg & Sörös, 2016; Rosburg et al., 2010). However, some studies also reported an increase of a slow positive wave from 50 to 250 ms post-stimulus onset due to stimulus repetition, which was termed “repetition positivity” (RP; Cooper et al., 2013; Recasens et al., 2015; more details of RP will be discussed below). Since the RP is conceptualized as an increase of P1 and P2 and a decrease of N1, it seems to be contradictory to the diminishing P1 and P2. Therefore, more research on this topic is necessary to understand the adaptation patterns in different components.
Compared with amplitudes, less attention has been paid to latencies in adaptation research. In general, most studies found a latency decrease only in the first two stimuli in the N1, but it did not diminish further in later stimuli (Bourbon et al., 1987; Budd et al., 1998; Rosburg, 2004; Rosburg et al., 2006). Additionally, albeit less investigated, a continuous latency decrease along trains of five repeated tones was found in the P2 in one study (Rosburg et al., 2010). In contrast, no P1 latency differences between repeated tones were found (Rosburg et al., 2006).
Focusing on the comparison between stimulus pair and continuous train, the present study used “initial adaptation” to represent the amplitude decrease from the 1st to the 2ndtone and “subsequent adaptation” to capture the following decrease from the 2nd to the final tone in each sequence of the same stimuli. To our best knowledge, a study that examines the adaptation effects in first and subsequent stimulus repetitions in terms of both amplitude and latency in all three components in the same participants has not yet been conducted. The findings of such a comparison may be informative as previous studies used different paradigms making comparisons across studies difficult. Hence, the first research question of this study was to examine whether auditory adaptation mainly happens between the first two tones, or whether it occurs continuously along a train of tones reflected by three ERP components: P1, N1, and P2. This may enhance the understanding of whether and how each component is affected by neural adaptation. Importantly, repetition suppression can emerge in a variety of ERP components (Schweinberger & Neumann, 2016) so it is crucial to include multiple components when examining adaptation comprehensively.