Bifocal alpha-band tACS modulates temporal sampling in visual perception
Alessia Santoni, Giuseppe Di Dona, Riccardo Gironi, Sara Stottmeier, Luca Battaglini, Luca Ronconi
Abstract
• We investigated the role of alpha oscillations in visual temporal processing using bifocal high-definition transcranial Alternating Current Stimulation (tACS). Specifically, we delivered tACS at either the Individual Alpha Frequency or off-peak frequencies (±2 Hz) during a 2 Flash Fusion Task. • We found that tACS at higher frequencies improved temporal sampling, while stimulation at lower frequencies decreased performance, specifically for stimuli presented on the right hemifield. • Investigating the offline neurophysiological correlates of these effects, we found no shift in the Individual Alpha Frequency following stimulation but a general decrease of alpha power as well as a modulation of the aperiodic exponent of the power spectrum. • These results support the deterministic role of alpha oscillations in providing a timeframe for visual temporal perception and add novel insights into the neurophysiological modulations following alpha-band tACS. Alpha oscillations (7–13 Hz) have been historically considered to shape temporal sampling processes in the visual modality, with faster alpha frequencies linked to finer temporal resolution. However, evidence is mostly correlational, and findings are not always replicated. Here, we investigated the role of alpha oscillations in temporal sampling using transcranial alternating current stimulation (tACS) and electroencephalography (EEG). Over three consecutive blocks, bilateral high-definition tACS was applied over extra-striate visual areas including V5/MT at either the individual alpha frequency (IAF), IAF+2, or IAF-2 Hz while participants performed a two flash fusion task. In line with previous findings, baseline resting-state IAF correlated with temporal segregation accuracy, with higher IAF predicting better segregation ability. Importantly, stimulation at IAF+2 Hz improved temporal segregation accuracy, whereas IAF-2 Hz decreased performance. This effect was specific for stimuli in the right hemifield, highlighting important hemispheric differences in alpha-mediated visual temporal sampling. Following stimulation, EEG data revealed no shift in the IAF and a decrease of alpha power independently from stimulation condition. Nonetheless, specifically for the IAF+2 stimulation, we observed a modulation of the aperiodic exponent of the power spectrum, indicating possible changes in cortical excitability as a short-term plasticity mechanism that persists beyond online tACS effects. Taken together, these results corroborate the role of alpha oscillations in defining temporal sampling processes and add novel evidence on the neurophysiological effects of tACS.