Binocular viewing improves several aspects of visual detection compared to viewing with one eye alone, in a process known as binocular summation. A striking example of this perceptual binocular enhancement is how binocular viewing enables the detection of lower contrasts compared to monocular viewing. However, the neuronal substrate of such improved binocular contrast detection in the visual system has so far been poorly understood. The majority of binocular interactions are thought to occur in binocular neurons of the visual cortex. Earlier parts of the image-forming visual pathway have been underappreciated as a potential source of binocular integration, mainly because lateral geniculate (LGN) neurons are mostly monocular, and retinal ganglion cells (RGCs) are considered completely monocular. Recent work in our lab, however, found, that the consensual pupillary response, causing constriction of both pupils when one eye is stimulated elicits a visual response in retinal ganglion cells of the non-stimulated eye, a hitherto undescribed form of binocularity. Here we explore how pupil-induced nondominant responses of RGCs affect binocular vision. Using in vivo 2-photon calcium imaging of RGC boutons in the LGN of mice we first demonstrate that RGC boutons exhibit a robust, cell-type-specific, binocular response. We next show that the binocular information is conveyed to the visual cortex. Furthermore, binocular stimulation elicits facilitation at low contrasts with currently undescribed temporal dynamics. This low-contrast binocular facilitation is consistent with retinal binocularity improving low-contrast detection, thereby suggesting that the retina plays a prominent role in binocular summation.