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Abstract

Binocular summation enhances visual discrimination of contrast and luminance during binocular viewing compared to monocular viewing. This improved binocular detection has mostly been described in human psychophysics experiments, and therefore the neural circuitry behind it remains unknown. The majority of other binocular processes, however, originate in the primary visual cortex (V1). Earlier parts of the image-forming visual pathway have not been thoroughly explored as a potential source of binocular integration. While some limited binocular interaction is known to occur in the lateral geniculate nucleus (LGN), retinal ganglion cells (RGC) are considered to be completely monocular. In my PhD thesis, however, I used in vivo 2-photon calcium imaging to demonstrate that RGC boutons in the LGN can respond to stimulation of either eye. Furthermore, these binocular responses were cell-type-specific, with the stimulation of the opposite eye eliciting different responses in ON and OFF RGC boutons. To confirm that binocularity, observed in the RGC boutons was relevant in further visual processing I also recorded responses of the LGN boutons in the V1, showing, that binocular information observed in the RGCs reaches the cortex. Binocularity of the RGCs boutons in the LGN was surprising, as there are no known connections between the two retinas and no known axo-axonal synapses targeting RGC boutons in the LGN. Here, in collaboration with Pietro Micheli I showed, that binocularity, observed in the RGC boutons didn’t originate locally, rather it was a result of consensual pupillary constriction. While it has been known for over a century that due to pupillary light reflex (PLR) pupils constrict consensually, allowing less light to enter the eye, the effect this change of illumination has on visual processing has not been considered. In my thesis, however, we showed, that stimulating one eye leads to constriction of both pupils and a consequent visual response in the RGCs of the non-stimulated eye. To understand the role of this novel form of binocularity in visual processing, I compared RGC responses to binocular and monocular stimulation and observed strong binocular facilitation only at low contrasts, consistent with binocular summation being calculated on the retina.