This arm involves a within-subject comparison of different frequencies of active TIEFS stimulation and sham stimulation, all within the same session. Patients will already have been admitted to SEEG surgery, and will be approached while still having the electrodes implanted. Each participant will receive multiple types of stimulation at different envelope frequencies (ranging from 1-130 Hz). During the session, electrodes will be placed on the scalp to administer TIEFS at varying frequencies. Each frequency will be tested in separate trials within the session, with multiple trials per frequency. The results from these trials will be averaged to determine the overall impact of each frequency on brain activity (measured through SEEG) and/or cognitive functions (evaluated using tasks related to memory, language, motor function, and perception). Sham stimulation will also be delivered within the same session, and participants will be blinded to the type of stimulation (active vs sham).

This arm involves a within-subject comparison of active and sham TIEFS stimulation in healthy control subjects. Participants will receive both active and sham stimulation in a single session, with different frequencies of TIEFS applied during active stimulation. The goal is to measure the changes in brain activity using EEG and assess the impact of TIEFS on cognitive functions such as memory, motor function, language, and perception. Participants will be blinded to the type of stimulation they are receiving during the session.

Temporal Interference Electrical Stimulation (TIEFS) is a non-invasive brain stimulation technique that uses two high-frequency electrical fields (>1 kHz) to generate an interference pattern at a specific brain region, creating a low-frequency envelope (Δf = f2 - f1) at the target depth. Unlike traditional stimulation methods, which affect only superficial brain regions, TIEFS can reach deeper structures without the need for implanted electrodes. The stimulation is applied through scalp electrodes. It has the potential for targeted brain interventions in both functional studies and clinical applications for neurological conditions.

This outcome measures the changes in brain activity, either measured through scalp EEG electrodes (in Healthy Participant controls), or intracranial stereotaxic EEG electrodes (for SEEG pre-surgical evaluation epilepsy patients). Local field potential (LFP) power and EEG power in response to active TIEFS stimulation will be compared to sham stimulation. Electrical signals recorded from the brain reflect neural activity. Intracranial signals will be measured across different locations in the brain using implanted SEEG electrodes. Additionally, SEEG electrodes may have tetrodes tips to capture single neuron activity. We will compare how the brain's electrical activity changes during active TIEFS at various frequencies and during the sham stimulation to assess the impact on brain and neural function.