Project Summary Interactions between the auditory and visual systems are among the most well-established cases of crossmodal interplay, yet the overwhelming bulk of the sensory physiology literature reflects studies examining processing confined to a single modality and we understand comparatively little about the circuitry mediating crossmodal interplay, or under what conditions such circuits are active. Audiovisual interactions exist even in primary auditory cortex (AC), with latencies too low to be mediated through purely top-down connections. How does this information influence sound processing? Which connections relay visual information to the auditory cortex? What are the functional consequences of these early crossmodal interactions, and what general computational principles account for interactions within this system that could extend our understanding of crossmodal processing elsewhere in the brain? Our proposal will leverage the strengths of the awake transgenic mouse model to address the core aspects of these overarching questions. We will combine translaminar recording in auditory and visual cortices of awake mice with optogenetic manipulation techniques, quantitative modeling, and behavior. We will determine how visually driven inputs affect responses, dynamics, and functional connectivity across auditory cortical layers and cell types, identify and functionally characterize circuitry relaying visual information to auditory cortex, and determine the extent to which behavioral goals modulate visual influence on auditory cortex and communication from visual to auditory cortex.