Repetitive Transcranial Alternating Current Stimulation to Improve Working Memory: An EEG-fNIRS Study

Area of Science:

• Neuroscience
• Cognitive Science
• Biomedical Engineering

Background:

• Transcranial electrical stimulation, including tACS, is a non-invasive technique explored for cognitive enhancement.
• Periodic stimulation has been shown to augment brain neuroplasticity.
• Understanding the precise effects of tACS parameters on cognitive function and brain activity is crucial.

Purpose of the Study:

• To investigate the effects of repetitive transcranial alternating current stimulation (tACS) on cognitive function, functional connectivity, and brain activity.
• To evaluate the impact of specific tACS parameters (1 mA, 5 Hz, 2 min duration) on working memory and neurophysiological markers.
• To explore the utility of EEG and fNIRS in quantifying tACS-induced changes.

Main Methods:

• Fifteen healthy subjects underwent tACS and sham stimulation targeting the frontal cortex.
• Working memory tasks, EEG, and fNIRS were used to measure brain activity before, during, and after stimulation.
• Analysis included behavioral scores, EEG band-wise powers, EEG topography, oxygenated hemoglobin concentration, and functional connectivity (FC).

Main Results:

• tACS significantly increased working memory scores and EEG band-wise powers (theta and beta bands) compared to sham.
• Enhanced functional connectivity was observed in both EEG-fNIRS signals, particularly in large-scale networks and interhemispheric connections.
• Hemodynamic response, measured by fNIRS, increased post-stimulation compared to pre-stimulation and sham conditions.

Conclusions:

• Repetitive theta-band tACS effectively enhances working memory capacity, supporting behavioral and neuroplasticity improvements.
• Proposed fNIRS biomarkers and EEG measures can serve as neurofeedback indices for closed-loop brain stimulation.
• tACS presents a promising non-invasive method for cognitive enhancement and neuromodulation.