Theta transcranial alternating current stimulation over the prefrontal cortex enhances theta power and working memory performance

Abstract

Introduction: Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain oscillations. This study investigated whether 5 Hz tACS could modulate neural oscillations in the prefrontal cortex and how this modulation impacts performance in working memory (WM) tasks.

Method: In two sessions, 28 healthy participants received 5 Hz tACS or sham stimulation over the left dorsolateral prefrontal cortex (DLPFC) while performing tasks with high and low WM loads. Resting-state EEG was recorded before and after stimulations for 5 minutes. EEG power was measured at electrodes surrounding the stimulation site.

Results: The results showed that tACS significantly improved reaction time (RT) compared to sham stimulation. This effect was task-specific, as tACS improved RT for hit responses only in high WM load trials, with no impact on low-load trials. Moreover, tACS significantly increased EEG power at 5 Hz and in the theta band compared to pre-stimulation levels.

Discussion: These findings demonstrate that tACS applied over left DLPFC modulates post-stimulation brain oscillations at the stimulation sites - known as tACS after-effects. Furthermore, the results suggest that 5 Hz tACS enhances response speed by elevating task-related activity in the prefrontal cortex to an optimal level for task performance.

Conclusion: In summary, the findings highlight the potential of tACS as a technique for modulating specific brain oscillations, with implications for research and therapeutic interventions.

Keywords: EEG; dorsolateral prefrontal cortex; theta power; transcranial alternating current stimulation; working memory.

Figure 1

An illustration of (A) the delayed match-to-sample task used in the experiment and (B) stimulation and EEG data recording protocol. (A) Each trial began with a fixation cross for 2 seconds, followed by either 2 (low-load) or 4 (high-load) visual objects. Then, another fixation cross was presented for 2 seconds, followed by a probe stimulus. Participants responded by pressing a button to indicate whether they had seen the probe stimulus earlier. Each block lasted approximately 20 minutes. (B) Resting state EEG was collected for 5 minutes before and after the stimulation.

Figure 2

Effects of Sham and tACS stimulations on working memory performance, measured by d-prime (A) and reaction time (RT) for hit responses (B). The violin plots display the distribution of individual data points shown as jittered dots. Red diamonds mark the mean and red error bars show the standard error. Significant differences between specific conditions are denoted by asterisks above the plots. * p <.05.

Figure 3

Spectral power at 5 Hz and theta band in Sham and tACS stimulation conditions. The upper (A, B) show individual power values with lines connecting baseline (pre-stimulation) and post-stimulation powers for each participant. (A) represents 5 Hz power, while (B) shows Theta power, with statistical significance indicated by an asterisk (*) between the baseline and post-stimulation powers in the tACS condition. The lower (C, D) present bar plots of mean power with standard error for each condition. (C) shows 5 Hz power, and (D) shows Theta power, with an asterisk (*) highlighting significant differences between baseline and post-tACS power. * p <.05.