THREE-PHOTON HOLOGRAPHIC MICROSCOPY FOR DEEP PRECISE OPTOGENETICS

Optogenetics—the use of light to control genetically modified neurons—has transformed neuroscience by allowing highly specific manipulation of neuronal circuits^​ in vivo. While single-photon widefield illumination allows reliable activation or inhibition of genetically defined neuronal populations, it lacks spatial precision. Two-photon (2P) optogenetics, combined with 2P imaging, has significantly improved spatial precision to near single-cell control in vivo, but remains limited to depths of a few hundred micrometers due to tissue scattering. Consequently, deeper brain regions typically require invasive optical implants in rodents and remain inaccessible in smaller organisms. Three-photon (3P) excitation microscopy, using longer wavelengths (1300–1700 nm), offers deeper tissue penetration and reduced scattering, making it a promising approach for probing deep brain regions. Despite its widespread use in functional and structural imaging, 3P optogenetic activation has remained largely unexplored, with only one prior demonstration in cultured cells. Here, we present the first in-depth validation of 3P optogenetics in vivo at depths up to 800 µm in the mouse visual cortex. We further characterize 3P optogenetic activation in organotypic brain slices, targeting three excitatory and one inhibitory opsin. Using Computer-Generated Holography (CGH) and temporal focusing (TF), we achieve soma-targeted activation with 1300 nm or 1700 nm illumination, confirm a genuine 3P excitation regime and demonstrate reliable neuronal responses. Notably, we show enhanced spatial confinement and reduced out-of-focus activation compared to 2P stimulation, with near single-cell resolution. This approach combining deep tissue penetration with holographic precision opens new avenues for probing deep neuronal circuits with unprecedented accuracy.