Transcranial Ultrasonic Neuromodulation of Primary Visual Cortex and Primary Auditory Cortex in Humans

This study will evaluate the effects of low-intensity transcranial focused ultrasound (tFUS) on brain activity in healthy adult volunteers. Transcranial ultrasound is a non-invasive technique that delivers acoustic energy through the skull to a small, targeted region of the brain. Unlike other forms of non-invasive brain stimulation, ultrasound can reach deep and superficial brain structures with millimeter-scale precision. Transcranial ultrasound is currently being explored as a potential treatment for neurological and psychiatric conditions, including chronic pain, addiction, and depression. However, the optimal stimulation parameters for increasing versus decreasing brain activity in humans are not yet well established. The purpose of this study is to determine which ultrasound pulse parameters produce measurable excitation or suppression of brain activity in the human cortex. The study will focus on two brain regions: the primary visual cortex and the primary auditory cortex. These regions were selected because stimulation of these areas may produce perceptual experiences, such as seeing flashes of light or hearing tones. These perceptual reports, together with brain recordings, allow researchers to directly assess how ultrasound affects neural activity. Participants will complete three visits. During Visit 1, participants will undergo informed consent, baseline testing of visual or auditory function, and magnetic resonance imaging (MRI). MRI scans will be used to identify each participant's individual brain anatomy and precisely localize the stimulation target. During Visit 2, participants will undergo ultrasound stimulation while brain activity is recorded using electroencephalography (EEG). EEG is a non-invasive method that measures electrical signals from the scalp. Baseline brain responses to visual or auditory stimuli will be recorded before stimulation. Ultrasound will then be delivered in brief bursts while EEG continues to record brain activity. Participants may be asked to report any sensations experienced during stimulation. After stimulation, additional EEG recordings and behavioral tests will be performed to evaluate short-term effects. Three different ultrasound pulse durations will be tested in randomized order. Each stimulation condition will consist of short bursts separated by rest periods. Total stimulation time will be limited, and rest intervals will be included between conditions. During Visit 3, which will occur at least one week later, participants will repeat behavioral testing and undergo a follow-up structural MRI scan to confirm that no structural changes occurred. The ultrasound intensities used in this study are below established U.S. Food and Drug Administration (FDA) safety limits for diagnostic ultrasound. The spatial peak pulse average intensity (Isppa) and spatial peak temporal average intensity (Ispta) will be maintained below FDA exposure limits, even before accounting for attenuation of the signal by the skull. Mechanical index (MI) values will also remain below recommended safety thresholds. Based on prior human studies involving thousands of participants, the risk of serious adverse effects is considered very low. Potential risks include temporary scalp discomfort, mild skin irritation from ultrasound gel, or transient sensory effects such as light flashes or sounds. Although seizures have not been reported in human studies using these parameters, ultrasound does modulate brain activity, and a theoretical risk exists similar to other non-invasive brain stimulation methods. Participants will be monitored throughout all procedures. There is no direct medical benefit to participants. However, the findings from this study may help guide the development of ultrasound-based treatments for neurological and psychiatric disorders and improve understanding of how targeted brain stimulation affects sensory perception. Data collected will include MRI images, EEG recordings, behavioral performance measures, and participant-reported experiences. All data will be coded and stored securely in accordance with institutional policies.