Structural and functional evidence supports re-defining mouse higher order visual areas into a single area V2

The mouse has become one of the main organisms for studies of the visual system. As a result, there is increased effort to understand universal principles of visual processing by comparing the mouse visual system to that of other species. In primates and other well-studied species including cats and tree shrews, the visual cortex is parcellated into an area V1 and several higher order areas defined by structural and functional differences, and a near complete map of the visual field. In mice, the visual cortex beyond V1 is parcellated into several higher order areas, with less notable structural and functional differences, partial coverage of the visual field, and areal boundaries defined by reversals in progression of the visual field. Notably, recent work in tree shrews and primates has shown that reversals in progression of the visual field can be a hallmark of complex retinotopic mapping within a single visual area. This, and other lines of evidence discussed here, provides a compelling case that the apparent existence of multiple higher order visual areas in the mouse is related to the false assumption of simple retinotopy. Specifically, we use simulations to show that complex retinotopy within a single visual area can recapitulate the appearance of multiple areal borders beyond mouse V1. In addition, we show that many reported differences in functional properties between higher order visual areas can be better explained by retinotopic differences rather than areal identity. Our proposal to reclassify some of the higher order visual areas in the mouse into a single area V2 is not mere semantics because areal definitions influence experimental design and data analysis. Furthermore, such a reclassification would produce a common set of rules for defining areal boundaries among mammals and would bring the mouse visual system into agreement with evolutionary evidence for a single area V2 in related lineages.