Revisiting post-stimulus theta activity: evidence for an aperiodic rather than oscillatory origin.

The aperiodic, 1/f-like component of electrophysiological activity is increasingly recognized as a meaningful feature of neural function, rather than background noise. In parallel, many EEG studies report transient changes in oscillatory power following stimulus onset and interpret these effects as signatures of attention, salience, or cognitive control. However, such conclusions usually rely on baseline normalization procedures that assume aperiodic activity remains stable from pre- to post-stimulus periods. Using high-density EEG recordings from typically developing children, we tested this assumption in two paradigms: an audiovisual simple reaction-time task (n = 36) and a visual oddball task (n = 38). For each task, conventional spectral analyses were compared with analyses that explicitly modeled and removed the aperiodic component in both pre- and post-stimulus windows. Across tasks, stimulus onset was associated with robust increases in aperiodic exponent and offset, indicating systematic changes in the 1/f component of the spectrum. In the audiovisual task, these changes were modality-specific, with central, parieto-occipital, or combined topographies depending on stimulus type. These effects were reduced but remained significant after ERP removal, indicating that they were not fully explained by phase-locked activity. Critically, once aperiodic activity was accounted for, the apparent poststimulus increase in theta power was largely abolished in both tasks, including the canonical fronto-central theta enhancement to infrequent targets in the oddball paradigm. The conventional method also overestimated the magnitude of beta desynchronization, particularly in the induced (ERP-removed) signal. The apparent gamma desynchronization detected by conventional analyses was reversed after aperiodic correction, revealing either synchronization or no change, indicating that it reflects a spurious consequence of spectral slope steepening rather than a true suppression of gamma oscillatory activity. In contrast, alpha desynchronization remained robust after aperiodic correction and was in fact enhanced, suggesting it reflects genuine oscillatory suppression. Together, these findings indicate that a substantial portion of conventional time-frequency effects, particularly apparent theta synchronization, may reflect changes in aperiodic activity in response to stimulation rather than genuine periodic oscillations, challenging core assumptions of conventional time-frequency analyses.