Progress in neuroscience relies on new techniques for investigating 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. Recently introduced methods for holographic control of light propagation in complex media enable the use of a hair-thin multimode optical fibre as an ultranarrow imaging tool. Compared to endoscopes based on graded-index lenses or fibre bundles, this new approach offers a footprint reduction exceeding an order of magnitude, combined with a significant enhancement in resolution. We designed a compact and high-speed system for fluorescent imaging at the tip of a fibre, achieving a resolution of 1.18 ± 0.04 µm across a 50-µm field of view, 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 anaesthetised mice. This study paves the way for modern microscopy to be applied deep inside tissues of living animal models while exerting a minimal impact on their structural and functional properties.

Center for Behavioral Brain Sciences Institute of Cognitive Neurology and Dementia Research German Center for Neurodegenerative Diseases Leipziger Straße 44 Haus 64 Magdeburg 39120 Germany

Centre for Discovery Brain Sciences University of Edinburgh Hugh Robson Building 15 George Square Edinburgh EH8 9XD UK

Institute of Scientific Instruments of CAS Kralovopolska 147 Brno 612 64 Czech Republic

Leibniz Institute of Photonic Technology Albert Einstein Straße 9 Jena 07745 Germany

School of Life Sciences University of Dundee Nethergate Dundee DD1 4HN UK

School of Science and Engineering University of Dundee Nethergate Dundee DD1 4HN UK

Simons Initiative for the Developing Brain University of Edinburgh Edinburgh EH8 9XD UK

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