A single complex V2 as an alternative hypothesis for the organization of mouse higher visual cortex

The mouse has become a major model for studies of the visual system, creating a need for principles of areal delineation that can be compared across species. In many well-studied mammals, V1 is bordered by a single second visual area, V2, whereas the cortex immediately surrounding mouse V1 is commonly parcellated into roughly eight higher-order visual areas (HVAs), with boundaries defined in part by reversals in the progression of the visual field. However, work in other species has shown that such reversals can occur within a single visual area when retinotopic mapping is complex. This raises the possibility that some conventionally defined mouse HVAs may instead be subdivisions of a single complex V2. We use simulations to show that complex retinotopy within a single visual area can generate multiple apparent areal borders beyond mouse V1, and show that visual-field position contributes to reported functional differences among HVAs. We also release a large-scale Allen Institute OpenScope dataset comprising electrophysiological and two-photon recordings across V1. As an initial application of this resource, we examined response properties associated with visual cortical hierarchy and found that they clearly distinguished V1 from higher visual cortex, while variation among individual HVAs did not significantly exceed variation across sampled locations within V1. Together, these results motivate the unified-V2 hypothesis as an alternative model of mouse higher visual cortex while emphasizing that areal boundaries are most strongly supported when independent topographic, functional, architectonic, and connectional features exhibit reproducible, spatially coincident transitions. The OpenScope dataset provides a resource for independent tests of this and alternative models of mouse visual cortical organization.