Parallel feedback pathways in visual cortex of cats revealed through a modified rabies virus.

The visual cortex of cats is highly evolved. Analogously to the brains of primates, large numbers of visual areas are arranged hierarchically and can be parsed into separate dorsal and ventral streams for object recognition and visuospatial representation. Within early primate visual areas, V1 and V2, and to a lesser extent V3, the two streams are relatively segregated and relayed in parallel to higher order cortex, although there is some evidence suggesting an alignment of V2 and V3 to one stream over the other. For cats, there is no evidence of anatomical segregation in areas 18 and 19, the analogs to V2 and V3. However, previous work was only qualitative in nature. Here we re-examined the feedback connectivity patterns of areas 18/19 in quantitative detail. To accomplish this, we used a genetically modified rabies virus that acts as a retrograde tracer and fills neurons with fluorescent protein. After injections into area 19, many more neurons were labeled in putative ventral stream area 21a than in putative dorsal stream region posterolateral suprasylvian complex of areas (PLS), and the dendrites of neurons in 21a were significantly more complex. Conversely, area 18 injections labeled more neurons in PLS, and these were more complex than neurons in 21a. We infer from our results that area 19 in cat is more aligned to the ventral stream and area 18 to the dorsal stream. Based on the success of our approach, we suggest that this method could be applied to resolve similar issues related to primate V3.