Overcoming the Barriers to Effective Transcranial Temporal Interference Stimulation in Humans

Transcranial temporal interference stimulation (TIS) is an emerging novel tool for non-invasive transcranial brain stimulation that holds the potential for focal and steerable neuromodulation, and the possibility to stimulate focally at depth. TIS involves combining two high frequency waveforms to create a waveform with a "beat" frequency that is physiological relevant for neuromodulation. Successful applications to deep brain targets as well as steerability of the stimulation focus have been demonstrated in mice. Numerous recent investigations in humans have shown great clinical potential for this technology, however several questions about the basic mechanism of TIS action remain. The investigators will apply TIS to the motor cortex of humans and use established transcranial magnetic stimulation techniques to assess cortical excitability in relation to the phase of the TIS waveform. Using TMS, the investigators will i) validate that effects of TIS are due to the "beat" frequency and not the high frequency carrier signal, ii) evaluate the effect of the TIS carrier frequency, and iii) evaluate that whether changes in corticospinal excitability outlast the period of stimulation. Knowledge gained from this experiment will provide a basis for the future use of TIS for clinical applications, including informing parameter optimization.