Transcranial alternating current stimulation selectively modulates aperiodic EEG component: Unveiling alternative mechanisms of modulation.

OBJECTIVE

Transcranial alternating current stimulation (tACS) mechanisms remain poorly understood. While tACS is expected to modulate periodic electroencephalography (EEG) activity due to the imposition of oscillatory alternating currents, it may also modulate the aperiodic EEG component, suggesting alternative tACS mechanisms that have not yet been explored. This study examined whether 20-minute tACS session delivered at 40 Hz over C3/C4 can selectively modulate periodic and aperiodic components.

METHODS

We implemented a paired, counterbalanced, and sham-controlled design consisting of two sessions during which tACS was administered over the sensorimotor regions. EEG was recorded before and after tACS under three EEG conditions: resting state, active task (i.e., the serial reaction time task), and passive task (i.e., 40-Hz auditory stimulation).

RESULTS

We found that tACS led to selective modulation of the aperiodic component, specifically a broadband downward shift in the power spectrum (i.e., reduction in the offset), independent of the EEG conditions and with no evidence of an effect on periodic activity.

CONCLUSION

This result may reflect a general effect on neural spiking rather than direct modulation of oscillatory activity.

SIGNIFICANCE: Disentangling the contribution of the aperiodic component from periodic activity represents an opportunity to identify alternative neurophysiological mechanisms underlying neural modulation and provides a clearer interpretation of tACS mechanisms.