The dorsal lateral geniculate nucleus (dLGN) in carnivores and primates is a laminated structure, where each layer gets visual input from only one eye [1, 2]. By contrast, in rodents such as mice and rats, the dLGN is not overtly laminated, the retinal terminals from the two eyes are only partially segregated [3, 4], and many cells in the binocular segment of dLGN get excitatory inputs from both eyes [5, 6]. Here, we show that the evolutionary ancient koniocellular (K) division of primate dLGN, like rodent dLGN, forms a subcortical site of binocular integration. We recorded single-cell activity in dLGN of anesthetized marmoset monkeys. As expected, cells in the parvocellular (P) and magnocellular (M) layers received monocular excitatory inputs. By contrast, many cells in the K layers received excitatory inputs from both eyes. The specialized properties of distinct K sub-populations (for example, blue-yellow color selectivity) were preserved across the two eye inputs, and where tested, the contrast sensitivity of each eye input was roughly matched. The results argue that evolutionarily widely separated orders such as rodents and primates have a shared strategy of integrating signals from the two eyes in subcortical circuits.
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