Ultrasound Brain-Computer Interfaces for Transcranial Closed-Loop Neuromodulation.

Noninvasive brain-computer interfaces (BCIs) have advanced communication, assistive control, and neurorehabilitation, but most remain readout-oriented systems with limited capacity for deep targeting, focal intervention, and response-guided modulation. Ultrasound offers a distinct route to brain interfacing: functional ultrasound imaging provides spatially localized hemodynamic readout, although current human task-related evidence relies on surgically enabled acoustic access; transcranial focused ultrasound enables noninvasive modulation of cortical and subcortical circuits. We define an ultrasound BCI as a system that converts measured brain activity into a functionally useful output, with ultrasound providing readout, state-contingent write-in, or both; in the closed-loop framework considered here, response assessment updates or constrains subsequent modulation. This Perspective argues that ultrasound BCI should be viewed not as a replacement for established BCIs, but as a transcranial closed-loop acoustic neuromodulation platform. We clarify its boundaries, compare current evidence maturity, and outline translational routes for neurorehabilitation, abnormal brain-state intervention, and deep-brain neurofeedback.