Specificity and Asymmetry in the Functional Organization of Inter-Hemispheric Connections in Mouse Visual Cortex
Lesions studies and developmental disorders have taught us that interhemispheric inputs profoundly influence the functional development and tuning of the primary visual cortex. What signals are provided to V1 from the opposite hemisphere remains unknown, and the precise horizontal and laminar structure of these projections is unclear. Here we apply modern viral tracing, in-vivo 2-photon Ca2+ imaging, and whole-brain imaging techniques to identify the functional patterning of interhemispheric projections from mouse V1. In addition, we have developed a novel statistical tool for regional analysis of large 3D images of mouse brains called BrainQuant3D. Using a test case of immediate early gene (IEG) induction to readout neuronal activity following light exposure, we confirm the sensitivity of our method in mice. We find asymmetries in results using different IEGs, cFos, and Npas4 that may arise from differences in specificity for Ca2+-dependent activity. Applying BQ3D to interhemispheric tracing data, we find these projections predominantly target bV1 with projections extending to mV1 and HVAs. We used 2-photon calcium imaging to determine the functional tuning of interhemispheric boutons in bV1 from bV1 in the opposite hemisphere. These projections are strongly biased towards the ipsilateral eye and high spatial frequency tuned, with a clear preference for stimuli crossing the midline. The functional signals carried by callosal inputs diverge from the target population, the source population, and the thalamocortical input. Together, These findings provide a functional basis for the observations made in lesion studies and suggest a role for interhemispheric projections in spatial acuity and depth processing. Finally, we show that proper spatial frequency and binocularity tuning of these projections depends on early visual experience.