4.4 Implications, Limitations, and Future Studies
This study holds theoretical significance because it elucidates the relationship between stimulus repetition, adaptation, and MMN. While previous studies often examined the adaptation and MMN separately (e.g., Budd et al., 1998; Haenschel et al., 2005), this study sheds light on their relationship by tracing the adaptation pattern across a sequence of trials. Furthermore, the practical implications are noteworthy as understanding these adaptation patterns and the mechanisms underlying MMN could potentially enhance the effectiveness of clinical applications. For instance, the diagnosis of dyslexia may benefit from utilizing adaptation patterns and MMN, given previous studies indicating weaker adaptation and smaller MMN in dyslexic individuals, compared with healthy controls (e.g., Baldeweg et al., 1999; Jaffe-Dax et al., 2017).
It is important to note that the adaptation effect and MMN can be influenced by various factors, including stimulus characteristics such as tone frequency difference (e.g., Butler, 1968), tone duration (e.g., Lanting et al., 2013), number of repetitions (e.g., Baldeweg, 2007) and ISI (e.g., Budd et al., 1998; Herrmann et al., 2016; Lanting et al., 2013; Pereira et al., 2014). Additionally, participant-related variables such as expectations and attention also play a role (e.g., Costa-Faidella et al., 2011a; Hari et al., 1979; Herholz et al., 2009; Todorovic et al., 2011). While the present study did not manipulate some of these variables, it is worth noting that almost all of these factors remained constant throughout the experiment, minimizing potential bias in the results. However, future studies could examine how these factors may modulate the MMN and adaptation findings.
A limitation of the present study is that the contribution of the N1 initial adaptation to MMN might be overestimated. Previous research has indicated that pitch differences between standards and deviants can result in contamination of the MMN by N1 response recovery, attributed to the frequency-specificity of some N1 generators (Butler, 1968, 1972). Therefore, the observed association between the initial N1 adaptation and MMN amplitude might be partly contaminated due to the overlapping time window of these two components. While the present study shed light on the contribution of the N1 initial adaptation to MMN, albeit including the contamination, future studies should consider adopting more sophisticated experimental designs and analysis methods to obtain a more precise estimate of the contribution of the N1 initial adaptation to MMN.
Another limitation is that we only examined frequency differences, leaving it unclear whether the observed adaptation patterns and the relationship between adaptation effects and MMN can be generalized to deviants with other features, such as intensity, duration, or abstract pattern. However, based on previous findings, we would expect adaptation to play a less prominent role in MMN when the repetition rule is not involved (Carbajal & Malmierca, 2018). Furthermore, future studies should consider including a control condition where the same tones as the standards in the experimental condition are presented but embedded within different tones to avoid adaptation. This control condition would help distinguish the adaptation and prediction error components (Carbajal & Malmierca, 2018). Overall, more research is needed to examine how the relationship between adaptation and MMN is modulated by different stimulus features, particularly when expectations are controlled.