No aftereffect of transcranial alternating current stimulation (tACS) on theta activity during an inter-sensory selective attention task.

Background: Selective attention is essential to filter the constant flow of sensory information reaching the brain. The contribution of theta neuronal oscillations to attentional function has been the subject of several electrophysiological studies, yet no causal relationship has been established between theta rhythms and selective attention mechanisms.

Objective And Hypotheses: We aimed to clarify the causal role of theta oscillations in inter-sensory selective attention processes by combining transcranial alternating current stimulation (tACS) and electrophysiology (EEG) techniques. We hypothesized that modulation of theta activity by tACS enhances selective attention, with greater behavioral efficiency and theta power over fronto-central regions after theta-tACS compared to control conditions.

Methods: In a double-blinded within-subject study conducted in young adults (n = 20), three stimulation conditions were applied prior to a cued inter-sensory (auditory and visual) selective attention task. The frequency of theta stimulation was individualized to match the endogenous theta peak of each participant. In addition to a sham condition, stimulation at an off-target frequency (20 Hz) was also applied. We analyzed behavioral efficiency and variability measures and performed spectral and time-frequency power analyses.

Results: No statistically significant differences in task performance or theta EEG activity were found between theta-tACS and control-tACS conditions (ps > 0.05).

Conclusions: The results of our study suggest that theta-tACS did not modulate performance or offline oscillations in the context of inter-sensory attention. These findings challenge the design of tACS protocols for future studies aiming to understand the contribution of theta oscillations in attentional processes.