The Role of Layer 6 Corticothalamic Circuits in Vision: Plasticity, Sensory Processing, and Behavior.

Layer 6 corticothalamic (L6 CT) pyramidal neurons send feedback projections from the primary visual cortex to both first- and higher-order visual thalamic nuclei. These projections provide direct excitation and indirect inhibition through thalamic interneurons and neurons in the thalamic reticular nucleus. Although the diversity of L6 CT pathways has long been recognized, emerging evidence suggests multiple subnetworks with distinct connectivity, inputs, gene expression gradients, and intrinsic properties. Here, we review the structure and function of L6 CT circuits in development, plasticity, visual processing, and behavior, considering computational perspectives on their functional roles. We focus on recent research in mice, where a rich arsenal of genetic and viral tools has advanced the circuit-level understanding of the multifaceted roles of L6 CT feedback in shaping visual thalamic activity.