Optical investigation of microcircuit computations in mouse primary visual cortex

Abstract

A fundamental challenge of visual cortical neuroscience is to understand how sensory representations are transformed within and across layers of primary visual cortex (V1). Visual signals are thought to propagate in a feedforward manner from the retina to the thalamus and then through the layered structure of V1, from layer 4 (L4) to layer 2/3 to layer 5. Here, we address one major step of visual processing: how are representations transformed within L4, the primary input layer of V1? We first developed a strategy to label neurons in L4. To understand computations during visual processing in awake animals, we then employed influence mapping, an approach developed by our lab to causally manipulate neural activity using in vivo single-neuron optical perturbations. We performed simultaneous two-photon optogenetic photostimulation of targeted individual neurons while imaging the responses of neighboring populations. By relating the influence of a neuron on other cells to their visual tuning properties, the computational function of neural activity patterns can then be inferred. Using this technique, we found that excitatory neurons in L4 strongly recruited activity from nearby but not distant cells, which was dependent on similarity in activity and certain tuning features such as preferred orientation and receptive field overlap. This result supports the idea that L4 amplifies visual signals via a feature-specific like-excites-like motif. These experiments have advanced our understanding of local transformations within L4 of V1. In addition, the technical approaches developed serve as a foundation for future studies of laminar cortical mechanisms that underlie visual processing, including comparing the transformations that occur within and between layers.