Emerging evidence suggests the central importance of top-down influence in cognitive processing. Despite our detailed understanding of the anatomy of long-range top-down projections, the operation of top-down signals for perception remains unclear. At the Grass Laboratory, I will perform a topographical mapping of top-down projections in a mouse whole-brain, as well as in vivo wide-field calcium imaging of long-range axonal projections to the primary sensory cortex while a mouse performs a perceptual task. These experiments will provide new insights into how top-down modulation is regulated in space and time in the cortex during dynamic perceptual behaviors.