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.

Remedies for treatment resistance in psychiatric and neurological disorders is a great unmet need. Invasive neuromodulation, such as deep brain stimulation (DBS), applied to a targeted brain region is one option that has helped regulate or modify, abnormal electrical patterns in many patients with neurological deficits; However, invasive approaches carry significant risks while having limited targeting flexibility. Transcranial temporal interference stimulation (TIS) is a non-invasive neuromodulation method with much more favorable focus in deep brain targets than traditional forms of transcranial alternating current stimulation (tACS). tTIS achieves its focal effect by delivering two sinusoidal currents, that are both high in frequency, but that are slightly different from one another (e.g. 1000 kHz and 1020 kHz) through electrodes placed on the scalp. Frequencies that are in the kilohertz range alone do not elicit neuronal response, however the interaction of the two high-frequency currents creates an electric field with a high-frequency carrier (fc, e.g. 1010 kHZ) that is modulated by a low-frequency beat (fb, e.g. 20 Hz). If fc is high enough (>1 kHz) and fb low enough (<100 Hz), neurons respond much more strongly to fb than fc.

Safety and tolerability of TIS has recently been demonstrated in humans with findings indicating that TIS poses no greater risk than other common non-invasive techniques for transcranial current stimulation (TCS). The spatial distribution of the beat field (Eb) is distinct from other noninvasive stimulation methods in three ways: 1) it is focal, 2) it can peak deep in the brain, 3) it can be steered through the brain without moving the electrodes. These features have been demonstrated in mouse experiments, and in human simulation studies.

In humans, motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) applied to the motor cortex provide an established assay of corticospinal excitability. TMS experiments have provided direct evidence that phase-dependent neuromodulation can be induced by transcranial alternating current stimulation (tACS), a form of transcranial current stimulation that is similar to temporal interference stimulation (TIS) but lacks focality and steerability, yet has been key to understanding its neurophysiological effects in humans. For example, recent experimental results showed that TMS pulses time locked to tACS over motor cortex induced MEP amplitude modulation that was dependent on the phase of the tACS oscillatory currents. Building on this finding, the investigators will provide TMS pulses to motor cortex during tTIS and investigate whether neuromodulation of corticospinal excitability depends on the phase of beat frequency (fb). Similar to the experiments that showed neural activation in mice, the investigators will also investigate whether the strength of the effect depends on the carrier frequency (fc). The investigators will use computationally optimized electrode placement, with high- and low-frequency controls, to test that effects are unambiguously due to fb. Finally, the investigators will assess corticospinal excitability PRE and POST stimulation to investigate neuroplasticity induced by TIS. The knowledge gained in this experiment will demonstrate the extent to which modulation effects of TIS in humans are due entrainment of neural activity at fb. This information will provide a basis for the future use of tTIS for clinical applications.

Inclusion Criteria:

• Free of neurological or orthopedic conditions that may impact the ability to participate.
• Right-hand dominant (self-reported)
• Aged 18-65 years

Exclusion Criteria:

• Ferromagnetic metal implants in the brain/skull or elsewhere (dental implants excluded).
• Any implanted electronic device (e.g. cardiac pacemaker, cochlear implant, deep brain stimulator, medication infusion pump; vagal nerve stimulator)
• Diseased or damaged skin on face or scalp
• History of migraines, fainting, seizures/epilepsy, or psychiatric illness
• Neurological disorder or intracranial lesion
• Frequent or severe headaches
• Prior adverse reaction to transcranial magnetic stimulation
• Currently pregnant
• Suspected or diagnosed heart problems (e.g. arrhythmia, cardiac disturbances).
• Insulin-dependent diabetes
• Suspected or diagnosed epilepsy or a familial history of epilepsy
• Use of alcohol or recreational drugs in the prior 12 hours
• Use of prescription and over the counter medications will be grounds for exclusion except for: contraceptive (birth control) drugs (e.g. those containing ethinyl estradiol and/or norethindrone), and seasonal allergy medications (e.g. cetirizine (Zyrtec), fexofenadine (Allegra), and loratadine (Claritin)).