Improving working memory by electrical stimulation and cross-frequency coupling

Abstract

Working memory (WM) is essential for the temporary storage and processing of information required for complex cognitive tasks and relies on neuronal theta and gamma oscillations. Given the limited capacity of WM, researchers have investigated various methods to improve it, including transcranial alternating current stimulation (tACS), which modulates brain activity at specific frequencies. One particularly promising approach is theta-gamma peak-coupled-tACS (TGCp-tACS), which simulates the natural interaction between theta and gamma oscillations that occurs during cognitive control in the brain. The aim of this study was to improve WM in healthy young adults with TGCp-tACS, focusing on both behavioral and neurophysiological outcomes. Thirty-one participants completed five WM tasks under both sham and verum stimulation conditions. Electroencephalography (EEG) recordings before and after stimulation showed that TGCp-tACS increased power spectral density (PSD) in the high-gamma region at the stimulation site, while PSD decreased in the theta and delta regions throughout the cortex. From a behavioral perspective, although no significant changes were observed in most tasks, there was a significant improvement in accuracy in the 14-item Sternberg task, indicating an improvement in phonological WM. In conclusion, TGCp-tACS has the potential to promote and improve the phonological component of WM. To fully realize the cognitive benefits, further research is needed to refine the stimulation parameters and account for individual differences, such as baseline cognitive status and hormonal factors.

Keywords: Electroencephalography (EEG); Power spectral density (PSD); Theta-gamma peak-coupled transcranial alternating current stimulation; Transcranial alternating current stimulation (tACS); Working memory (WM).

Fig. 1

Experimental setup of the study. Session # 1 and session # 2 are separated by at least 72 h. TGCp-tACS: theta/gamma peak coupled transcranial-alteranating current stimulation; WM:working memory; EEG: electroencephalography

Fig. 2

Effect of TGCp-tACS on accuracy across various cognitive tasks. This bar graph illustrates the change in accuracy due to TGCp-tACS across various cognitive tasks. The error bars represent the 95% confidence intervals for each task condition. The change in accuracy was only statistically significant for the 14-item Sternberg task. TGCp-tACS: Theta/gamma peak coupled-transcranial-alternating current stimulation; VWM2: visuospatial working memory (2-stimulus); VWM4: visuospatial working memory (4-stimulus); DSST: Digit Symbol Substitution Task; Sternberg10: Sternberg task (10-item); Sternberg14: Sternberg task (14-item)

Fig. 3

Topographic Maps of PSD Distribution in Delta Range: Sham Condition OE. Topographic 2D maps depicting PSD distribution across different electrodes in the delta range. These maps highlight statistically significant differences observed before and after the sham condition with eyes open (red dots). Statistical analysis was conducted using paired t-tests with false FDR correction for multiple comparisons (p < 0.05) utilizing the Fieldtrip toolbox. OE: eyes open; CE: eyes closed; Hz: hertz; PSD: Power spectral density; FDR: false discovery rate

Fig. 4

Topographic Maps of PSD Distribution in Delta Range: Verum Condition OE. Topographic 2D maps depicting PSD distribution across different electrodes in the delta range. These maps highlight statistically significant differences observed before and after the verum condition with eyes open (red dots). Statistical analysis was conducted using paired t-tests with FDR correction for multiple comparisons (p < 0.05) utilizing the Fieldtrip toolbox. OE: eyes open; CE: eyes closed; Hz: hertz; PSD: Power spectral density; FDR: false discovery rate

Fig. 5

Topographic Maps of PSD Distribution in Delta, theta and high-gamma Ranges: Verum Condition CE. These maps highlight statistically significant differences observed before and after the verum condition with eyes closed (red dots). Statistical analysis was conducted using paired t-tests with false discovery rate (FDR) correction for multiple comparisons (p < 0.05) utilizing the Fieldtrip toolbox. OE: eyes open; CE: eyes closed; Hz: hertz; PSD: Power spectral density; FDR: false discovery rate