Transcranial electrical stimulation (tDCS / tACS)

Transcranial electrical stimulation passes weak currents (typically 1 to 2 mA) between scalp electrodes. Transcranial direct current stimulation (tDCS) applies a constant current that shifts resting membrane potential subthreshold, so it modulates the probability of firing rather than triggering it: anodal stimulation generally increases and cathodal generally decreases cortical excitability, though the picture is montage- and state-dependent. Transcranial alternating current stimulation (tACS) applies an oscillating current intended to entrain endogenous rhythms at a chosen frequency.

Only a fraction of the scalp current reaches cortex, and the field is broad rather than focal, so effects are small and vary substantially across individuals. The literature is contested on rigor grounds: sham blinding is imperfect, dosing is often not individualized, and reproducibility is uneven. Realistic subject-specific current-flow modeling, individualized dosing, and better sham control are current priorities for turning tES from a promising but noisy modulator into a reliable one.