Prestimulus Alpha Power Enhancement Via Transcranial Alternating Current Stimulation Does Not Influence Visual Contrast Detection

Variability in visual contrast detection has been linked to prestimulus alpha oscillations, yet whether modulating alpha power can causally influence perception remains unclear. In this sham-controlled, single-blinded, within-participant study, we applied transcranial alternating current stimulation (tACS) at individual alpha frequency over the occipital cortex while participants performed a near-threshold visual contrast detection task. Under sham conditions, correlation analyses indicated that higher prestimulus alpha power predicted elevated visual contrast thresholds (VCTs), particularly in the time intervals immediately preceding stimulus onset. When tACS was applied, we observed a modest and temporally selective enhancement of occipital prestimulus alpha power, primarily restricted to specific prestimulus intervals within one experimental block. Importantly, this localized alpha enhancement did not translate into corresponding improvements in visual contrast detection performance. Instead, participants' VCT remained statistically similar between sham and tACS conditions. These findings suggest that although tACS can reliably modulate alpha power, simply elevating occipital alpha amplitude may not be sufficient to alter perceptual outcomes. Factors such as the precise timing of alpha oscillations, the extent and duration of neural modulation, and the interaction with other neural or cognitive processes may be critical for producing measurable behavioral effects. Our findings underscore the nuanced relationship between alpha oscillations and perception, highlight the challenge of establishing direct causal links using neuromodulation, and emphasize the need for more comprehensive stimulation protocols, extended EEG recordings, and investigations into interactions with other confounding factors.