Temporal interference stimulation (TI)

Temporal interference stimulation applies two high-frequency current fields at slightly different frequencies (for example 2000 Hz and 2020 Hz) through separate electrode pairs. Neurons cannot follow the individual kilohertz carriers, which are too fast to drive spiking, but they can respond to the low-frequency envelope (here 20 Hz) that arises where the two fields overlap. In principle this steers effective stimulation to a deep focus without strongly stimulating the overlying cortex the currents must pass through.

TI was demonstrated in mice in 2017 (driving hippocampal activity without engaging cortex above it) and has since been explored in humans, including reports of engaging the hippocampus and striatum. Key open questions are how much focality and effective intensity are achievable in the much larger, more conductive human head, the extent of off-target and conventional (non-envelope) stimulation, and whether envelope demodulation by neurons is strong enough to produce clinically meaningful effects.