PROJECT SUMMARY How our brain achieves coherent perception by integrating information from parallel sensory pathways distributed across space and time remains a central question in neuroscience. In the auditory system, sound information reaches the cortex via the lemniscal (“primary”) and non-lemniscal (“secondary”) pathways. The non-lemniscal pathways have often been described as slower integrators of multi-sensory information, in contrast to the roles of the lemniscal pathways as fast and reliable relays for sound inputs. However, the contribution of the non-lemniscal pathways in driving fast cortical responses and how they interact with the lemniscal pathways during sound processing are still matters of debate. Our preliminary electrophysiology experiments show that layer 6 (L6) of not only the primary but also the secondary auditory cortex receives sound inputs whose latency can be shorter than the L4 lemniscal inputs. Surprisingly, our retrograde tracing demonstrates that this short-latency L6 input originates from the non-tonotopic parts of the auditory thalamus, supporting the role of the non-lemniscal pathway in fast sensory processing. Building on this exciting finding, we will combine anatomical tracing, in vitro/in vivo electrophysiology, optogenetics, and behavior to delineate this non-classical pathway and determine how it interacts with the lemniscal pathway to regulate cortical sensory processing. Specifically, we will examine the hypothesis that short-latency non-lemniscal inputs onto L6 regulate cortical sound processing in a timing-dependent manner and control the tuning and temporal fidelity of sound responses. To achieve this goal, this project aims to (1) Delineate the anatomy of the fast non- lemniscal pathway from the cochlear nucleus to the auditory cortex using both anatomical tracing and in vivo unit recordings, (2) Determine the synaptic impact of the non-lemniscal input onto cortical cell types by performing targeted whole-cell recordings in cortical slices while simultaneously activating L6-targeting thalamic inputs, and (3) Identify the roles of the fast non-lemniscal input in cortical sound processing in vivo by optogenetically manipulating thalamic inputs onto L6 during unit recordings in the mice performing sound- guided behaviors. Through our research, we seek to provide a more holistic understanding of auditory processing across the two major ascending pathways. Since parallel thalamocortical inputs onto L4 and L6 are conserved across sensory modalities, results from this project will provide insights into the generalizable principles underlying the cortical circuitry of sensory integration. Ultimately, these studies will help the future development of targeted treatments for not only hearing disorders but also other sensory integration dysfunctions.