Spatial organization of long-range inhibition in binaural hearing

Abstract

We are able to follow our friend's voice as she calls across a crowded plaza or whispers in a darkened theater. Amazingly our sense of hearing can identify and localize sounds ranging eight orders of magnitude in loudness. A highly specialized circuit in the auditory brainstem uses submillisecond differences in the arrival time of sound to the two ears, interaural time differences (ITDs), to localize a sound source. The avian nucleus laminaris (NL), analogous to the mammalian medial superior olive, processes ITDs. In NL, ITD sensitivity is maintained by sound-level-dependent inhibition arising from the superior olivary nucleus (SON). By studying the spatial organization of SON input to NL, this dissertation examines how inhibition may shape the precise activity patterns in NL and influence in binaural hearing. Chapter 1 provides an overview of topographic organization in the nervous system and introduces the chicken auditory brainstem system containing NL and SON. Chapter 2 describes quantitative analyses of the coarse tonotopic organization in SON. Chapter 3 demonstrates a broad topographic projection from SON to NL. Further, we present preliminary data from examining the widespread arborization patterns of single SON axons in NL in Chapter 4. This broad innervation pattern of inhibitory input is in stark contrast to the precise, narrow innervation pattern of excitatory inputs to NL. Finally Chapter 5 summarizes the main findings reported in this dissertation, proposes some functions of broadly arranged inhibition in auditory brainstem processing, and offers potential avenues for future study.