Ultrasound Directly Activates Sparse Neurons and Modulates Visual Circuits in Deafened Mice
Focused ultrasound neuromodulation (FUN) is widely regarded as a next-generation technology for neural modulation, with applications spanning rodents, non-human primates, and humans. While mechanistic studies are advancing, a persistent confound-auditory interference-casts doubt on whether ultrasound exerts direct mechanical effects or merely indirect auditory responses. To resolve this, we engineered a circular ultrasound transducer compatible with two-photon calcium imaging and examined its effects on the primary visual cortex (V1) of surgically deafened mice, eliminating auditory contributions. Our results reveal that ultrasound directly activates a sparse subset of ultrasound-sensitive neurons (USSN) (response rate >30%) in V1, comprising only a small fraction of the total population and exhibiting a spatially sparse distribution. The proportion of USSN scale with ultrasound pressure, confirming a direct neuromodulatory effect independent of audition. Intriguingly, despite this sparse activation, ultrasound significantly modulates V1 circuitry: it alters the dynamics of light-sensitive neurons (LSN), with subsets showing excitation, inhibition, or no change in response to visual stimuli. These findings provide the first rigorous in-vivo evidence that FUN induces direct mechanical effects on neural activity, disentangling them from auditory confounds. By demonstrating both the specificity and broader circuit-level impact of ultrasound in deafened mice, this study reframes our understanding of FUN's mechanisms and strengthens its potential as a precise neuromodulatory tool.