Investigating human non-lemniscal inferior colliculus contributions to auditory learning with 7T MRI

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PROJECT SUMMARY Human inferior colliculus (IC) plays a critical role in auditory processing. However, the anatomy and function of the lemniscal (primary) and non-lemniscal subdivisions of IC in living humans are poorly understood due to the technical challenges of in vivo magnetic resonance imaging (MRI) of the small midbrain structures deep within the brain. In particular, despite predominant top-down and bottom-up theories of auditory learning, the neural systems underlying human speech category learning is unknown. Recent advances in MRI acquisition open the door for focused investigations into the anatomy and functional processing of human auditory midbrain. My research environment and mentor team will allow me to gain the expertise necessary to independently investigate the subcortical auditory system in living humans using MRI. In this project, we will use ultra-high field 7T MRI to quantify anatomical midbrain tissue contrast in a sub- structure dependent manner. We will also map the structural connections from each IC subdivision throughout the auditory system. Quantifying the specific anatomical MRI contrasts and connectivity patterns in living human midbrain will enable future clinical applications for investigating hearing disorders such as sensorineural hearing loss and tinnitus. A possible functional role for non-lemniscal IC is in learning novel speech sound categories. We will collect 7T functional MRI at multiple timepoints during a sound-to-category learning program to assess the contribution of IC and auditory cortex to sound category learning. Our results will elucidate whether novel categories are learned via cortically driven plasticity (cortex represents categorical features at an earlier stage than IC’s relevant acoustic enhancement) or stimulus feature enhancement (IC and cortex have similar time courses of plasticity). Existing methods for probing auditory processing, such as the scalp-recorded frequency following response (FFR), have both subcortical and cortical generators, but their relative contributions throughout the auditory learning process have not been investigated in humans. Participants in our sound-to-category learning paradigm will also undergo FFR recordings. Using representational similarity analysis, we will assess whether sound category feature representation in FFRs primarily follows that of auditory cortex or that of IC, suggesting the relative contribution of each generator at each phase of sound-to-category learning. This project implements state-of-the-art anatomical and functional 7T MRI techniques to quantify foundational characteristics of the human inferior colliculus, a key but poorly investigated subcortical auditory structure. The methods we utilized can be adapted to investigate other small, deep structures throughout the human auditory system and will enhance our understanding of IC contributions to tinnitus and optimal placement of auditory brain implants for individuals with sensorineural hearing loss.