In vivo transcranial imaging of connections in mouse visual cortex

From the moment the mouse model took center stage for studies of cortical arealization and map formation, there was an urgent need for methods to identify areal borders in the living animal. The need was met in part by intrinsic optical signal imaging, which has been successfully applied to map topographic representations in primary visual, auditory and somatosensory cortex. However, the challenge remains to register these maps to the underlying structure. This is especially important for studies of the mouse brain in which cortical areas are often only a few hundred microns across. Here, we show that in visual cortex neuronal tracing with fluororuby and fluoroemerald can be used for transcranial imaging through the intact skull of callosal connections from the opposite side of the brain, and for mapping of topographic striate-extrastriate cortical pathways in living mice. Because callosal connections are important landmarks for cortical areas, the new method will allow registration of functional maps to underlying structures and facilitate targeted single-unit recordings in identified cortical areas.