Cortical layers (L) 5 and 6 are populated by a spatially intermingled menagerie of 28 neurons with distinct inputs and downstream targets. Here, we made optogenetically guided recordings from L5 and L6 corticothalamic (CT) neurons in the mouse auditory cortex to discern 30 underlying patterns of functional connectivity and sensory processing in the largest sub-cerebral projection system. Whereas L5 CT neurons showed broad stimulus selectivity with sluggish 32 response latencies and extended temporal non-linearities, L6 CTs exhibited sparse sound feature selectivity and rapid temporal processing. L5 CT spikes lagged behind neighboring units and 34 imposed weak feedforward excitation within the local column. By contrast, L6 CT spikes drove robust and sustained activity in neighboring units. Our findings underscore a duality among CT 36 projection neurons, where L5 CT units are canonical broadcast neurons that integrate sensory inputs for transmission to distributed downstream targets, while L6 CT neurons are positioned to 38 regulate thalamocortical response gain and selectivity.40