Temporal Interference Brain Stimulation

This is an investigational early phase testing of temporal interference (TI) stimulation in humans. The overall aim of the study is to assess the safety, feasibility, focality, and steerability of TI stimulation by selectively modulating activity in subregions of a cortical area (calcarine cortex - the primary visual cortex) Healthy subjects who meet inclusion and exclusion criteria will be entered into the study. The study will recruit up to 20 subjects with the aim to complete 12 subjects. Study Visits: The study will consist of up to 6 study visits. The screening and baseline visit, the MRI visit, and up to 4 TI study. The screening and baseline visit and TI visits will occur at Beth Israel Deaconess Medical Center in the Berenson-Allen Center. The MRI visit will take place at the Boston University Cognitive Neuroimaging Center. After Informed Consent is obtained, the following screening and baseline procedures will be completed: * Inclusion and exclusion criteria review * Subject demographics * Handedness assessment * Medical history and medication review * Physical and Neurological exam conducted by a Neurologist or Neurologic Nurse Practitioner * Baseline perimetry assessment * Baseline EEG * The Mini International Neuropsychiatric Interview (MINI) assessment * All female subjects will undergo a pregnancy test and pregnant women will be excluded * Screening for retinotopic mapping - assessing the participant's ability to hold fixation with their eyes for experimental trials * MRI safety review The MRI session will take place at the Boston University Cognitive Neuroimaging Center under a Boston University submitted and approved protocol that is specific to this study. An MRI scan of the brain will be conducted while the participant views visual stimuli to obtain each individual's retinotopic map. This data will be provided to the study team at Beth Israel Deaconess Medical Center (BIDMC) to conduct the study visit and for analysis. Each subject will then undergo up to 4 TI stimulation sessions (2 minimum) separated by at least 2 days to minimize the risk of carry over effects of the stimulation. In each visit, the participant will receive TI stimulation to one of four regions of retinotopic representation in the calcarine fissure: 1. peripheral visual field in the deep region of the fissure 2. foveal visual field in the polar region of the fissure 3. superior quadrant of the visual field in the lower bank of the fissure 4. inferior quadrant of the visual field in the upper bank of the fissure The cortical targets will be defined by electrical field modelling that will be used to optimize the electrode placement. Regions #1 and #2 will be stimulated in the first two visits with the order of stimulation regions to be counterbalanced between participants. If an effect is noted, participants will be asked to complete the additional 2 visits in which regions #3 and #4 will be stimulated. Each visit will consist of up to 4 blocks of stimulation paired with a visual discrimination task and assessment of visual disturbance with an Amsler grid. The stimulation blocks will each be completed at a different frequency - a control stimulation where TI visual effect is not anticipated (e.g 2 or 20 hertz (Hz)), a no offset stimulation (e.g. matched carrier stimulation frequencies such as no envelope modulation is anticipated) and up to 2 frequencies ranging from 8 to 12. The most common signal from visual cortex during wakeful relaxation is in the frequency range (8-12 Hz). It is hypothesized that TI with a residual effective stimulation frequency of 1-20 Hz will be ideally suited for activation of the targeted visual cortex. Participants will be monitored throughout he visit for any adverse effects and a tES side-effect questionnaire will be administered at the beginning and end of each stimulation visit to additionally track any adverse effects. Although any visual disruption induced by the stimulation is expected and anticipated to be transient in nature, a visual perimetry assessment will be completed to compare to baseline.