Selective connectivity limits functional binocularity in the retinogeniculate pathway of the mouse

Eye-specific segregation of retinal ganglion cell (RGC) axons in the dorsal lateral geniculate nucleus (dLGN) is considered a hallmark of visual system development. However, a recent anatomical study showed that nearly half of the neurons in dLGN of adult mice still receive input from both retinae, but functional data about binocularity in mature dLGN is conflicting. Here, we found that a variable but small fraction of thalamocortical neurons is binocular in vivo. Using dual-channel optogenetics in vitro we correspondingly found that dLGN neurons are dominated by retinogeniculate input from one eye only, although most neurons also received small but detectable input from the non-dominant eye. Anatomical overlap between RGC axons and dLGN neuron dendrites did not explain this strong bias towards monocularity. Our data rather suggest that functional input selection and refinement, leaving the remaining non-dominant eye inputs in a juvenile-like state, underlies the prevalent monocularity of neurons in dLGN.