The COMBO window: A chronic cranial implant for multiscale circuit interrogation in mice

Neuroscientists studying the neural correlates of mouse behavior often lack access to the brain-wide activity patterns elicited during a specific task of interest. Fortunately, large-scale imaging is becoming increasingly accessible thanks to modalities such as Ca2+ imaging and functional ultrasound (fUS). However, these and other techniques often involve challenging cranial window procedures, and are difficult to combine with other neuroscience tools. We address this need with an open-source 3D-printable cranial implant - the COMBO ( C hr O nic M ultimodal imaging and B ehavioral O bservation) window. The COMBO window enables chronic imaging of large portions of the brain in head-fixed mice while preserving orofacial movements. We validate the COMBO window stability using both brain-wide fUS and multi-site two-photon imaging. Moreover, we demonstrate how the COMBO window facilitates the combination of optogenetics, fUS and electrophysiology in the same animals to study the effects of circuit perturbations at both the brain-wide and single-neuron level. Overall, the COMBO window provides a versatile solution for performing multimodal brain recordings in head-fixed mice. ### Competing Interest Statement The authors have declared no competing interest.