BME PhD Defense Seminar: Nathaniel Greene

Influence of the Lateral Superior Olive in the Auditory Midbrain

Co-Supervised by Professor Kevin Davis

Abstract

Mammals localize sound along the horizontal plane principally using differences in the time and level of the sound present at the two ears. Evidence suggests that the lateral superior olive (LSO) initiates an excitatory pathway specialized to process interaural level differences (ILDs), and that the projections of LSO cells converge with additional ascending auditory pathways in the central nucleus of the inferior colliculus (ICC). ICC type I units exhibit monaural and binaural response properties similar to those of LSO units in decerebrate cats, thus are supposed to be the midbrain component of an ILD processing pathway, but there have been no direct tests of this hypothesis. The goals of this thesis were thus to identify individual cells in the auditory midbrain whose responses are dominated by LSO influences, and evaluate sound processing within this pathway. In the first part of this thesis I present evidence confirming that LSO projections selectively target type I units, and that the responses of most type I units are indeed dominated by LSO input. I then assess the processing of sounds within this pathway by comparing the responses of LSO and ICC units to amplitude modulated (AM) tones, which are a prominent component of speech and other behaviorally important stimuli. The responses recorded in LSO suggest that AM sensitivity is enhanced at low levels compared to its inputs, and comparisons to ICC type I unit responses reveal additional improvement. These results suggest a previously unidentified role for the LSO to ICC type I pathway in the processing of AM stimuli. In conclusion, this thesis demonstrates that type I units are the midbrain components of an ILD processing pathway, and that additional inputs shape the responses of cells in this pathway.