Chamber implant for chronic optical recordings from the cerebral cortex of marmosets

Marmosets are becoming an increasingly important animal model in Neuroscience and optical approaches such as two-photon microscopy are expected to provide data significantly contributing to our understanding of the brain, especially when performed chronically.

We describe a novel imaging chamber that provides an optical window onto the marmoset cerebral cortex over several months, together with the surgery needed for its implantation and that of an associated headpost. MRI data allow optimal positioning on the skull. The chamber is tightly inserted into a craniotomy and its sealing system combines a thread with a silicone elastomer minimizing infection risk and CSF leakage. Continuous contact between the window and the cortex delays tissue regrowth. Opening and sterile re-sealing are easy to perform whenever access to the cortex is needed.

Functional data from longitudinal two-photon imaging using genetically encoded fluorescent calcium sensors reveal high optical quality over months. We also demonstrate the feasibility of two-photon imaging of genetically encoded voltage sensors in the marmoset cortex in-vivo.

The chamber provides a field of view twice as large as current designs yet can be machined with standard equipment. Using a curved rather than flat glass window together with easy adjustment of the window's distance with respect to the underlying cortex allows achieving gentle but continuous contact with the underlying cortex.

Currently, our chamber already provides a durable solution for long-term imaging in marmosets. Yet, we discuss a few straightforward modifications that may improve its performance even further.