Cortical responses to conflicting binocular stimuli in mouse primary visual cortex

Binocular vision requires that the brain integrate information coming from each eye. These images are combined (fused) to generate a meaningful composite image. Differences between images, within a range, provide useful information about depth (stereopsis). Interocular disparities that are not effectively combined result in diplopia and rivalry. The neural mechanisms underlying these binocular interactions remain poorly understood. Using a combination of visually evoked potential (VEP) recordings, unit recordings, and 2-photon calcium imaging in the binocular region of mouse primary visual cortex (bV1), we probed the neural mechanisms underlying the processing of two distinct forms of disparate binocular signals. Using a dichoptic display, introduction of a spatial interocular phase disparity in grating stimuli reduced VEP magnitude through decreased neuronal firing in the early phase of the response (40-80 ms after stimulus onset, corresponding to the VEP negativity). Introduction of an interocular orientation disparity also decreased VEP magnitude, but this difference was driven by an increase in firing in the late portion of the visual response (100-200 ms after stimulus onset, corresponding to the VEP positivity). This increase in activity was observed for both regular-spiking (putative excitatory) and fast-spiking (putative parvalbumin-positive inhibitory) units. By contrast, visually evoked calcium responses of somatostatin-positive interneurons decreased with introduction of the interocular orientation disparity. Based on these results, we propose that interocular phase differences largely suppress bV1 responses via feedforward thalamocortical interactions, whereas interocular orientation differences prolong visually evoked activity in bV1 through somatostatin-positive interneuron-mediated disinhibition.