Characterizing binocularity in adult and developing mouse primary visual cortex

Our visual systems are able to perceive 2-dimensional images in the eyes and combine them to perceive a 3-dimensional environment. This process, stereopsis, is a key component of depth perception and at the cellular level originates from disparity selective neurons in the primary visual cortex (V1). Unlike most other visual response properties, disparity selectivity cannot be assessed with monocular stimulation; it only emerges after the integration of binocular inputs. This makes disparity selectivity an excellent model for studying how neurons converge multiple information streams into a singular output.
Here, I performed a thorough characterization of different visual response properties that have been used to assess binocular vision: ocular dominance, interocular matching, and disparity selectivity (Chapter 2). Data was acquired using electrophysiological multichannel extracellular recordings in the V1 of awake mice as they viewed dichoptic stimuli via a polarized projector system. I demonstrated that the three binocular visual response properties were independent of each other, and that their distributions indicated that binocularity in mouse V1 was much more widespread than previously known. Furthermore, the binocular responses I observed could not be fully predicted by summation of the responses to monocular inputs, and waveform analysis showed that fast-spiking putatively inhibitory neurons also responded to binocular stimulation. Altogether, these results indicated that binocularity is prevalent throughout its namesake region in V1, but manifests in a complex manner that requires thoughtful stimulus design to uncover. Inhibitory neurons in intracortical circuits likely also contribute to binocular disparity computations performed by V1 neurons.
Experience-dependent activity during the critical period refines neural circuitry, in the visual system and beyond. Disrupted visual experience has been shown to shift ocular dominance distributions, while normal visual experience has been shown to be necessary in producing matching orientation selectivity in V1 neurons. In order to characterize the effect of visual experience on binocular development, I performed acute electrophysiological experiments on young mice starting from P14 when they opened their eyes for the first time (Chapter 3). My data showed disparity selectivity to be present at eye-opening, albeit weaker than in mature neurons, indicating that functional circuits were wired prior to eye opening but still needed experience in order to strengthen selectivity.
Together, these findings present a thorough investigation of binocularity in the adult and developing mouse visual cortex, and form a foundation for further examination of disparity selectivity computation and development, as well as future studies of binocular integration in other model systems.