tACS

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that delivers a weak, oscillating electrical current to the scalp via surface electrodes. The current is typically sinusoidal and modulates, rather than directly excites, cortical circuits. By imposing rhythmic activity at a chosen frequency, tACS aims to entrain endogenous brain oscillations and bias neural processing in targeted networks.

Stimulation parameters commonly include frequencies from 1 to 100 Hz, intensities around 0.5–2 mA peak-to-peak, and sessions lasting several minutes to about 20–30 minutes. Electrodes are placed to target specific regions; electrode size and montage influence the distribution of stimulation. Experimental designs often use sham stimulation to control for placebo effects. Effects are typically frequency- and phase-dependent and largely depend on individual anatomy and brain state.

Research investigates effects on attention, memory, perception, and motor learning, as well as sleep and fatigue. Some studies report modulation of cognitive or behavioral performance linked to the stimulated frequency, but findings are heterogeneous and often small in magnitude. In clinical contexts, tACS has been explored for mood disorders, schizophrenia, and chronic pain, but there is no consensus on efficacy or approved indications, and results are inconsistent.

Safety and tolerability are generally favorable, with mild side effects such as tingling, itching, or transient skin sensations. Rare adverse events include skin irritation or burns under the electrodes. Contraindications include implanted electrical devices in the head. Methodological challenges—placebo effects, blinding, small samples, and anatomical variability—limit reproducibility and translation to routine clinical use; standardized protocols are still under development.

tACS emerged in the late 2000s from work on oscillatory entrainment in non-invasive stimulation and remains primarily a research tool, with ongoing efforts to refine mechanisms and personalizing approaches.