Abstract
Cortical layers (L) 5 and 6 are populated by intermingled cell-types with distinct inputs and downstream targets. Here, we made optogenetically guided recordings from L5 corticofugal (CF) and L6 corticothalamic (CT) neurons in the auditory cortex of awake mice to discern differences in sensory processing and underlying patterns of functional connectivity. Whereas L5 CF neurons showed broad stimulus selectivity with sluggish response latencies and extended temporal non-linearities, L6 CTs exhibited sparse selectivity and rapid temporal processing. L5 CF spikes lagged behind neighboring units and imposed weak feedforward excitation within the local column. By contrast, L6 CT spikes drove robust and sustained activity, particularly in local fast-spiking interneurons. Our findings underscore a duality among sub-cortical projection neurons, where L5 CF 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.
Highlights
Corticofugal (CF) neurons broadly fall into two classes: intratelencephalic and sub-cortical (Harris and Mrsic-Flogel, 2013; Harris and Shepherd, 2015)
We show that L5 CF neurons utilize dense, non-linear coding of sound features and have little influence on intra-columnar processing whereas L6 CT neurons have sparse selectivity for sound features, and strongly modulate local processing within auditory cortex (ACtx)
As a first step towards highlighting the differences in L5 CF and L6 CT neurons, we wanted to confirm that the well-established patterns of sub-cortical connectivity in other brain areas and species could be reprised in the mouse auditory system
Summary
Corticofugal (CF) neurons broadly fall into two classes: intratelencephalic and sub-cortical (Harris and Mrsic-Flogel, 2013; Harris and Shepherd, 2015). The largest compartment of the auditory CF projection system comes from neurons in layer 5 and 6 of the cortex that innervate the medial geniculate body (MGB) (Ojima, 1994; Prieto and Winer, 1999; Winer et al, 2001). L5 CF neurons deposit giant axon terminal collaterals in the dorsal division of the medial geniculate body (MGBd, (Ojima, 1994; Bajo et al, 1995; Rouiller and Welker, 2000)), en route to additional downstream targets in the tectum, striatum, and amygdala (Deschenes et al, 1986; Bourassa and Deschenes, 1995; Bourassa et al, 1995; Rockland, 1998; Veinante et al, 2000; Kita and Kita, 2012; Asokan et al, 2018). Unlike L5 CF neurons, auditory L6 CT axons collateralize
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