Aperiodic and oscillatory neural activity underlying external and internal attention

Our attention constantly fluctuates between the external environment and internal milieu in daily life. Thus far, the attention literature has extensively investigated external attention and its neural basis; however, our knowledge of internal attention remains limited. In particular, direct comparisons between these two attentional states have been largely overlooked, with existing EEG studies focusing on composite spectral measures that do not differentiate between oscillatory and aperiodic activity. In this study, we therefore examined whether external and internal attentional states differentially modulate the aperiodic component of EEG signals, its relationship with oscillatory measures, as well as its functional significance. During an attention task, participants were asked to either attend externally (to task-relevant tones) or internally (to their own thoughts) as we record their EEG. Our results revealed a smaller aperiodic offset and smaller aperiodic exponent (i.e., flatter slope) during internal compared to external attention. We also found differential attentional modulations of the relationships between aperiodic parameters and alpha and beta activity. Finally, aperiodic parameters predicted accuracy during the external condition, whereas oscillatory alpha activity predicted attention ratings during the internal condition. Overall, our results offer insights into the electrophysiological underpinnings of external and internal attentional states, highlighting the distinct functional role of aperiodic and oscillatory features of the EEG signal.