Is transcranial alternating current stimulation effective in modulating brain oscillations?

Abstract

Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain oscillations, as well as a possible therapeutic intervention. However, the lack of conclusive evidence on whether tACS is able to effectively affect cortical activity continues to limit its application. The present study aims to address this issue by exploiting the well-known inhibitory alpha rhythm in the posterior parietal cortex during visual perception and attention orientation. Four groups of healthy volunteers were tested with a Gabor patch detection and discrimination task. All participants were tested at the baseline and selective frequencies of tACS, including Sham, 6 Hz, 10 Hz, and 25 Hz. Stimulation at 6 Hz and 10 Hz over the occipito-parietal area impaired performance in the detection task compared to the baseline. The lack of a retinotopically organised effect and marginal frequency-specificity modulation in the detection task force us to be cautious about the effectiveness of tACS in modulating brain oscillations. Therefore, the present study does not provide significant evidence for tACS reliably inducing direct modulations of brain oscillations that can influence performance in a visual task.

Figure 1. Schematic illustration of the temporal structure of a trial.

A Gabor patch at different contrast levels appeared for 30 ms inside one of two lateral placeholders after a variable interval (450–750 ms) from a warning signal (the fixation cross became larger for 50 ms). Participants had to provide two consecutive responses: first, to report whether they believed the Gabor patch was present or not (detection) and then to select its orientation (discrimination). For every response, a fixed interval of 1500 ms was available.

Figure 2. Results relative to the detection response (R1) in terms of accuracy.

(A) Accuracy (proportion of correct responses) of every tACS frequency (Sham, 6 Hz, 10 Hz and 25 Hz) is shown during the baseline session (in black) and tACS session (in grey). (B) Normalised accuracy of every tACS frequency is shown as the difference between the % of correct responses during the tACS session and % of correct responses during the baseline session. Vertical bars correspond to the standard error of the mean. * indicates p<0.05.

Figure 3. Results relative to the discrimination response (R2) in terms of accuracy.

(A) Accuracy (proportion of correct responses) of every tACS frequency (Sham, 6 Hz, 10 Hz and 25 Hz) is shown during the baseline session (in black) and tACS session (in grey). (B) Normalised accuracy of every tACS frequency is shown as the difference between the % of correct responses during the tACS session and % of correct responses during the baseline session. Vertical bars correspond to the standard error of the mean. * indicates p<0.05.