  Progress in neuroscience constantly relies on the development of new
techniques to investigate the complex dynamics of neuronal networks. An ongoing
challenge is to achieve minimally-invasive and high-resolution observations of
neuronal activity in vivo inside deep brain areas. A perspective strategy is to
utilise holographic control of light propagation in complex media, which allows
converting a hair-thin multimode optical fibre into an ultra-narrow imaging
tool. Compared to current endoscopes based on GRIN lenses or fibre bundles,
this concept offers a footprint reduction exceeding an order of magnitude,
together with a significant enhancement in resolution. We designed a compact
and high-speed system for fluorescent imaging at the tip of a fibre, achieving
micron-scale resolution across a 50 um field of view, and yielding 7-kilopixel
images at a rate of 3.5 frames/s. Furthermore, we demonstrate in vivo
observations of cell bodies and processes of inhibitory neurons within deep
layers of the visual cortex and hippocampus of anesthetised mice. This study
forms the basis for several perspective techniques of modern microscopy to be
delivered deep inside the tissue of living animal models while causing minimal
impact on its structural and functional properties.
