Functional Monitoring and Imaging in Deep Brain Structures

Optical techniques are capable of cell-specific targeting, concurrent multimodal measurements, and multiscale measurements with high temporal and spatial resolution. However, the biggest limiting factor of optical techniques for functional brain monitoring is depth penetration, due to the highly scattering nature of the brain tissue. Without physical access into the brain, optical methods are limited to measuring from the surface, in general, down to about a millimeter in depth. This poses a great limitation for brain monitoring in commonly used animal models, considering a mouse brain is over 6mm deep. By implanting small-diameter optical fiber probes, or gradient refractive index (GRIN) lenses to deliver and collect light from the brain regions of interest, optical methods can measure from the deep brain structures without hindering the animal from moving and behaving freely. In this chapter, we review hardware enabling optical functional monitoring of deep brain regions. We outline system design considerations, including optical contrast options, spatial resolution, hardware choices, considerations for applications in freely moving animals, and safety considerations. We review some recent work as examples of different system schemes for monitoring calcium signals, voltage signals, and hemodynamics from deep brain structures.