A weighted graph of the projections to mouse auditory cortex

Abstract

The projections to individual cortical areas from extrinsic sources are a major determinant of the areas function, but we lack comprehensive quantitative input maps even for primary sensory areas in most model species. To quantify all input sources to the mouse primary auditory cortex (Au1), we made localized injections of modified rabies virus (SAD{Delta}G-mCherry) into Au1 of five C57BL/6 mice and identified all the cortical and subcortical areas containing retrogradely labeled cells. Of all neurons projecting to Au1 from extrinsic areas, 27 % were located in the ipsilateral cortex, 14 % in the contralateral cortex, and 58 % in subcortical regions (almost exclusively ipsilateral, predominantly in the medial geniculate nucleus). Although 90 % of the labeled cells in the ipsilateral cortex were located within 1 mm of Au1, most cortical areas projected to Au1, including visual, somatosensory, motor, rhinal, cingulate and piriform cortices. The hierarchical relations of the cortical areas projecting to Au1 were determined based on the proportion of cell bodies in superficial versus deep layers. Feedback projections (from deep layers 5/6) dominated, but temporal association and auditory cortices were on the same hierarchical level, providing input from both superficial and deep layers. Au1 is embedded in a densely connected network that involves a high degree of cross-modal integration.