New Lab Technology
Team members in the Kara lab have extensive experience in two-photon imaging, which provides sub-micron spatial resolution in vivo. However, the usable maximum imaging depth with two-photon microscopy is typically about 600-800 μm. While the mouse visual cortex is only about 750 μm thick, non-rodent mammals have a visual cortex that is 2 mm deep. Thus, much of the circuitry is missed when using two-photon microscopy. For example, layer 4 imaging of the non-rodent neocortex is of paramount importance in understanding cortical computation because it is the layer in which almost all visual cortical receptive field properties emerge de-novo. Just one synapse preceding layer 4 in the primary visual cortex, i.e., in the lateral geniculate nucleus of the thalamus, neurons do not have elongated oriented receptive fields and are not tuned for stereoscopic binocular vision. Therefore, for imaging cortex from 800–2000 μm below the brain surface, our new laboratory at the University of Minnesota has setup 3-photon imaging. See figures of the 3p laser light generation, a comparison of image quality of labeled neurons for 2p and 3p microscopy, and volumetric 3p imaging.