DISRUPTION OF TRANSTHALAMIC CIRCUITRY FROM PRIMARY VISUAL CORTEX IMPAIRS VISUAL DISCRIMINATION IN MICE.

Layer 5 (L5) of the cortex provides strong driving input to higher-order thalamic nuclei, such as the pulvinar in the visual system, forming the basis of cortico-thalamo-cortical (transthalamic) circuits. These circuits provide a communication route between cortical areas in parallel to direct corticocortical connections, but their specific role in perception and behavior remains unclear. Using targeted optogenetic inhibition in mice of both sexes performing a visual discrimination task, we selectively suppressed the corticothalamic input from L5 cells in primary visual cortex (V1) at their terminals in pulvinar. This suppresses transthalamic circuits from V1; furthermore, any effect on direct corticocortical projections and local V1 circuitry would thus result from transthalamic inputs (e.g., V1 to pulvinar back to V1 (Miller-Hansen and Sherman, 2022). Such suppression of transthalamic processing during visual stimulus presentation of drifting gratings significantly impaired discrimination performance across different orientations. The impact on behavior was specific to the portion of visual space that retinotopically coincided with the V1 L5 corticothalamic inhibition. These results highlight the importance of incorporating L5-initiated transthalamic circuits into cortical processing frameworks, particularly those addressing how the hierarchical propagation of sensory signals supports perceptual decision-making. Appreciation of pathways for transthalamic communication between cortical areas, organized in parallel with direct connections, has transformed our thinking about cortical functioning writ large. Studies of transthalamic pathways initially concentrated on their anatomy and physiology, but there has been a shift towards understanding their importance to cognitive behavior. Here, we have used an optogenetic approach in mice to selectively inhibit the transthalamic pathway from primary visual cortex to other cortical areas and back to itself. We find that such inhibition degrades the animals' ability to discriminate, showing for the first time that specific inhibition of visual transthalamic circuitry reduces visual discrimination. This causal data adds to the growing evidence for the importance of transthalamic signaling in perceptual processing.