The network for intracortical communication in mouse visual cortex

New techniques for identifying cell types, tracing their synaptic partners, imaging and manipulating their activity in behaving organisms have made mice a widely used model for linking brain circuits to behavior. Most behaviors are tied to vision: identifying objects, guiding movements of body parts, navigating through the environment, and even social interactions. Reason enough to focus on the mouse visual cortex. To find our way around in the occipital cortex, we needed a map. We took a classic approach and traced in the same animal the outputs from multiple retinotopic sites of primary visual cortex (V1) and compared the relative location of projections in the extrastriate cortex. We found nine extrastriate maps and showed by single unit recordings that each of the connectional maps contained visually responsive neurons whose receptive fields were mapped in orderly fashion and completely covered the visual field. Remarkably, a tiny region of one sixth of a dime contained a two- to three-times larger number of areas than the highly developed somatosensory and auditory cortices. By tracing the connections, we found that each of the ten visual areas projected to 25–35 cortical targets and interconnected virtually all of the areas reciprocally with one another. Although the binary graph density of the connection matrix was nearly complete, the connection strengths between areas within the ventral and dorsal cortex differed, indicating that the information from V1 flowed into distinct but interconnected streams. Unit recordings and calcium imaging studies showed that the ventral and dorsal streams processed different spatiotemporal information, which aligned with known properties of streams in primates. Analyses of the laminar patterns of interareal projections showed that areas were organized at multiple levels, suggesting that each stream represented a processing hierarchy.