BME PhD Defense: Babak Ravazi

Multi-Sensory Integration: Intra- and Extra-Auditory Factors that Influence Sound Localization

Co-Supervised by Co-Supervised by Professor Gary Paige and Professor William O’Neill

Abstract

The auditory system operates in an interactive multi-sensory setting and its representation of space is influenced by physiological as well as environmental factors. In order to obtain an accurate and precise depiction of acoustic space, the brain must interpret auditory spatial cues within the context of these factors and appropriately co-register auditory space with the spatial constructs of other sensory modalities. A dependable multi-sensory map of the external world is crucial for successfully navigating through, interacting with, and surviving in the environment.

The goal of this thesis was to identify, re-evaluate, and elaborate on auditory and non-auditory factors that impact two-dimensional sound localization in humans. The influence of:

1. elevation cues,
2. static eye position,and
3. the sensori-motor context of the pointing method on auditory spatial processing was evaluated behaviorally.

In Part 1, elevation cues, which arise from spectral filtering by the external ear and head, are shown to also impact sound localization in azimuth, which depends on interaural time and intensity difference cues. Therefore, elevation cues potentially contribute to azimuth accuracy to correct and refine errors inherent to the geometry of interaural cues for localizing sounds in two-dimensional space.

In Part 2, evidence is put forth to demonstrate that sound localization adapts to eccentric eye position. The adaptation develops exponentially in time, reaches a maximum within one or two minutes, spans two-dimensional frontal space, and is accompanied by a concurrent adaptation in perceived straight ahead. These findings help reconcile inconsistent reports on this issue in the sound localization literature, and may also have bearing on spatial adaption to vision displaced by optical prisms.

In Part 3, head (or nose) pointing under different sensori-motor contexts reveals that gaze (eye and head) is a more accurate and precise measure of target location than either head or eye position alone. This underscores the importance of quantifying perceived target location using eye in addition to head position even when subjects are specifically instructed to use head pointing.

In conclusion, this thesis demonstrates that two-dimensional sound localization is sensitive to a variety of previously unrecognized or poorly characterized physiological factors. These factors have important implications about how the brain coordinates spatial information across multiple senses, limitations in the methodology used in sound localization studies, and interpretation of the data reported by past studies.