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.