Noncanonical Short-Latency Auditory Pathway Directly Activates Deep Cortical Layers.

Auditory processing in the cerebral cortex is considered to begin with thalamocortical inputs to layer 4 (L4) of the primary auditory cortex (A1). In this canonical model, A1 L4 inputs initiate a hierarchical cascade, with higher-order cortices receiving pre-processed information for the slower integration of complex sounds. Here, we identify alternative ascending pathways in mice that bypass A1 and directly reach multiple layers of the secondary auditory cortex (A2), indicating parallel activation of these areas alongside sequential information processing. We found that L6 of both A1 and A2 receive short-latency (<10 ms) sound inputs, comparable in speed to the canonical A1 L4 input but transmitted through higher-order thalamic nuclei. Additionally, A2 L4 is innervated by a caudal subdivision within the traditionally defined primary thalamus, which we now identify as belonging to the non-primary system. Notably, both thalamic regions receive projections from distinct subdivisions of the higher-order inferior colliculus, which in turn are directly innervated by cochlear nucleus neurons. These findings reveal alternative ascending pathways reaching A2 at L4 and L6 via secondary subcortical structures. Thus, higher-order auditory cortex processes both slow, pre-processed information and rapid, direct sensory inputs, enabling parallel and distributed processing of fast sensory information across cortical areas.