Neural computations leading to space-specific auditory responses in the barn owl.

Abstract

Sound localization is the ability to pinpoint the direction a sound is coming from based on auditory cues alone. Neurons in the brain which mediate this behavior are active only when sound comes from a particular direction. This thesis uses physiological and anatomical methods to investigate the computations which lead to such space-specific neural responses in the barn owl. Chapter 3 studies a behavioral and neural phenomenon called phase ambiguity, which arises from the way in which the auditory nerve and cochlear nuclei encode acoustic information. Phase ambiguity causes errors in sound localization to be made for tonal stimuli, and is resolved through the convergence of information across different frequencies in broadband noise stimuli. Data presented here show that a continuous band of noise is not necessary; a set of tones spaced at the critical bandwidth resolves phase ambiguity just as well as a noise stimulus. This is due to a sub-linear interaction for tones of nearby frequencies. Chapter 4 addresses the head-related transfer function (HRTF) model of sound localization. While traditional barn owl models use linear equations to relate interaural time differences (ITD) to azimuth and interaural intensity differences (IID) to elevation, the HRTF model purports that IID is dependent on frequency to such an extent that pattern recognition is used to match the spectral shape of IID in the stimulus to that characteristic of particular directions in space. Data presented here confirm predictions made by the HRTF model that IID tuning changes with frequency in space-mapped neurons, and that two-tone stimuli whose IIDs match these changes elicit better responses than those which do not. Chapter 5 investigates the computation of space-specificity in the forebrain. Previous anatomical studies have suggested that the space-specificity seen there is not merely inherited from the space map in the midbrain, but rather arises, at least in part, independently. The data presented here reconfirm that the forebrain pathway branches off from the midbrain pathway before the convergence across frequencies leads to space-specific neurons. All previous computations, however, including the formation of ITD-IID combination sensitivity, seem to be shared. Collectively, these three studies expand our knowledge of the neurophysiology of sound localization in the barn owl by detailing specific mechanisms underlying the computation of space-specific neural responses.