Transcranial alternating current stimulation affects the BOLD signal in a frequency and task-dependent manner

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Brain Stimulation

Background:

• Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique.
• tACS modulates ongoing brain oscillations, but its precise effects on neural activity, particularly the blood oxygenation level dependent (BOLD) signal, are not fully understood.
• Previous research has shown frequency-specific cognitive effects of tACS, but the underlying neural mechanisms require further investigation.

Purpose of the Study:

• To investigate how tACS influences regional fMRI BOLD signal.
• To determine the impact of tACS frequency, current strength, and task conditions on BOLD activity.
• To explore the spatial distribution of tACS effects and their relationship to electrode placement and current density models.

Main Methods:

• Healthy human subjects received tACS over the posterior cortex.
• fMRI BOLD signal was measured during rest and various task conditions (visual perception, passive video viewing, motor task).
• tACS was applied in a blockwise manner at frequencies of 10, 16, 60, and 80 Hz.

Main Results:

• The most significant tACS effects on BOLD activity were observed at alpha (10 Hz) and beta (16 Hz) frequencies.
• tACS at 16 Hz led to increased BOLD activity in fronto-parietal areas.
• tACS effects were frequency- and task-dependent, often appearing in regions not directly targeted by electrodes and not activated by the task.
• Observed modulated regions were poorly predicted by current density modeling.

Conclusions:

• tACS effects on brain activity are complex and not limited to regions directly beneath the electrodes.
• The strongest tACS influences on BOLD signals occur at lower frequencies (alpha and beta bands).
• Region-specific effects of tACS may arise from the differential susceptibility of brain regions to entrain to specific stimulation frequencies.