GRIN-lens microendoscope & miniature microscope (miniscope)

To read optically from a deep structure without waiting for light to fight through the overlying tissue, a gradient-index (GRIN) rod lens is implanted as a relay that carries the image from depth up to the surface. Paired with a head-mounted miniature fluorescence microscope — a miniscope, weighing only a few grams — this yields single-cell calcium imaging of hundreds of neurons in deep regions such as the hippocampus or amygdala in freely-moving animals over weeks to months.

The cost is that implanting the lens damages a column of tissue directly above the target and provides a fixed, limited field of view. Open-source miniscope platforms have nonetheless made deep, chronic, single-cell imaging a widespread systems-neuroscience tool, though it remains an animal-research method with no direct human analogue.