It is controversial whether mouse extrastriate cortex has a "simple" organization in which lateral primary visual cortex (V1) is adjoined by a single area V2 or has a "complex" organization, in which lateral V1 is adjoined by multiple distinct areas, all of which share the vertical meridian with V1. Resolving this issue is important for understanding the evolution and development of cortical arealization. We have used triple pathway tracing combined with receptive field recordings to map azimuth and elevation in the same brain and have referenced these maps against callosal landmarks. We found that V1 projects to 15 cortical fields. At least nine of these contain maps with complete and orderly representations of the entire visual hemifield and therefore represent distinct areas. One of these, PM, adjoins V1 at the medial border. Five areas, P, LM, AL, RL, and A, adjoin V1 on the lateral border, but only LM shares the vertical meridian representation with V1. This suggests that LM is homologous to V2 and that the lateral extrastriate areas do not represent modules within a single area V2. Thus, mouse visual cortex is "simple" in the sense that lateral V1 is adjoined by a single V2-like area, LM, and "complex" in having a string of areas in lateral extrastriate cortex, which receive direct V1 input. The results suggest that large numbers of areas with topologically equivalent maps of the visual field emerge early in evolution and that homologous areas are inherited in different mammalian lineages.
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