Parallel systems for sound processing and functional connectivity among layer 5 and 6 auditory corticothalamic neurons
Cortical layers (L) 5 and 6 are populated by a spatially intermingled menagerie of 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 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 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 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 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 regulate thalamocortical response gain and selectivity.