SonoMod Focuses Ultrasound on a Cortical Target While Recording Its Response in Freely Moving Mice

SonoMod Focuses Ultrasound on a Cortical Target While Recording Its Response in Freely Moving Mice

On August 18, the authors published a preprint describing SonoMod, a head-mounted module for mice that directs focused ultrasound at a cortical target while a miniature camera records the cortical response. They also released the assembly files, optical components, analysis software, and data in a public repository.

Focused ultrasound concentrates the energy of a sound wave at a small point in the brain. Testing a neural network requires researchers to stimulate a selected point while observing what happens around it. Previous systems often blocked the optical recording path, required the animal's head to remain fixed, or separated stimulation and imaging into different sessions.

“In neuroscience, instruments for perturbing neural circuits and instruments for reading their responses have traditionally relied on incompatible physical principles,” the authors write.

In SonoMod, light and sound pass through the same module. A transparent lithium niobate disc converts an electrical signal into ultrasound. A printed acoustic lens delays different parts of the wave by different amounts and focuses the sound. The lens is surrounded by a liquid with nearly the same refractive index, which allows the camera to retain a clear view of the cortex. The lens can be replaced or rotated to target a new site. The module mounts onto the UCLA Miniscope v4, an open miniature microscope designed for experiments in freely moving animals.

The authors tested the device in stages. First, a tail pinch produced a sensory response in the recording, confirming that the camera could image the cortex through the module. They then directed ultrasound at the secondary motor cortex (M2). At all three tested pulse repetition frequencies, the calcium signal in M2 was higher than when the ultrasound was focused on another region. This comparison included four mice. When the focus was moved to the retrosplenial cortex, a region in the posterior cortex, the calcium signal increased there but did not change in M2. This series included five animals. In both series, the response followed the target.

In a recent human study, researchers likewise moved the ultrasound focus between adjacent points in the brain and assessed the effects using self-reported pain and mood. SonoMod makes it possible to observe the same shift in focus within a laboratory experiment. At the same time, the camera shows how the calcium signal spreads across the cortex.

The mice expressed a fluorescent calcium indicator. When the calcium concentration inside the cells changes, the indicator's brightness also changes, giving the camera an indirect measure of neuronal activity. SonoMod's field of view covers about 4,5 millimeters of the cortical surface, compared with approximately one millimeter for the original miniscope. Researchers can therefore observe both the target and neighboring cortical regions during stimulation.

The next experiment can be designed around this map. Researchers can change the target or ultrasound settings and immediately observe where the response begins and how it spreads across the portion of the cortex visible to the camera.