Escaping the nocturnal bottleneck, and the evolution of the dorsal and ventral streams of visual processing in primates.

Early mammals were small and nocturnal. Their visual systems had regressed and they had poor vision. After the extinction of the dinosaurs 66 mya, some but not all escaped the 'nocturnal bottleneck' by recovering high-acuity vision. By contrast, early primates escaped the bottleneck within the age of dinosaurs by having large forward-facing eyes and acute vision while remaining nocturnal. We propose that these primates differed from other mammals by changing the balance between two sources of visual information to cortex. Thus, cortical processing became less dependent on a relay of information from the superior colliculus (SC) to temporal cortex and more dependent on information distributed from primary visual cortex (V1). In addition, the two major classes of visual information from the retina became highly segregated into magnocellular (M cell) projections from V1 to the primate-specific temporal visual area (MT), and parvocellular-dominated projections to the dorsolateral visual area (DL or V4). The greatly expanded P cell inputs from V1 informed the ventral stream of cortical processing involving temporal and frontal cortex. The M cell pathways from V1 and the SC informed the dorsal stream of cortical processing involving MT, surrounding temporal cortex, and parietal-frontal sensorimotor domains. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.