Perceptual and Electrophysiological Metrics of Fixed and Moving Auditory Targets in the Azimuthal Plane

Sound localization is particularly important for maintaining situational awareness and overall safety, yet this listening skill is often neglected during clinical evaluations of the auditory system. Many people experience sound localization difficulties, including those with hearing loss (peripheral and central), the aging adult population (with or without substantial hearing loss), and those who have experience traumatic brain injury, such as concussion or stroke, for example. There are two main cues that are used to localize sounds on the horizontal plane: interaural level differences (ILD) and interaural timing differences (ITD). In both cases, comparisons are made by the central auditory nervous system for the sound arriving at both ears; these are called “binaural cues”. Much is known about how listeners utilize ILDs and ITDs for the purposes of sound localization for static or fixed location sound sources. However, some basic questions remain regarding the use of these binaural cues for auditory movement perception. The practical implication here, is that for the most part, our auditory environment is dynamic in the sense of relative movement; that is, either the listener and/or the sound sources are moving. While there is a substantial literature on localization, electrophysiology, and central auditory processing of fixed-position binaural cues, the literature about perception and electrophysiology for moving or dynamic cues is rather thin. The studies in this dissertation fill a gap in knowledge with systematic comparisons of fixed vs. moving sound sources, using both psychoacoustic and electrophysiologic measures. The motivation for the studies in this dissertation was born from both scientific as well as a clinical perspective. The clinical perspective was creating some novel stimulus paradigms that could be translated into methods that could be used in an auditory processing assessment. The specific aims of this dissertation were to 1) Evaluate the effects of fixed vs. moving auditory targets on perception of laterality; 2) Evaluate the effects of fixed vs. moving auditory targets on the latency and amplitude of obligatory and cognitive event related potentials; 3) Calculate the difference in hemispheric activity involved in the perception of fixed vs moving auditory targets; and 4) Correlate the metrics of perception with event related potential latency and amplitude and determine how much variance in perception is predicted by the electrophysiology. To address these aims, two levels of cortical auditory processing were evaluated. In the first experiment, the P300 event related potential, associated with perception and cognition, was used, and in the second study, the acoustic change complex event related potential, linked to stimulus characteristics, was used. The results from these studies indicate that 1) Movement perception is highly dependent on temporal processing and requires a longer duration window of analysis relative to fixed-position sound source perception; 2) Cortical electrophysiology is uniquely tied to perception for cognitive and obligatory evoked responses; and 3) There are hemispheric differences in auditory spatial processing that are dependent on whether the stimulus is moving, and if it is moving, how fast it is moving. The implications of these foundational studies are that electrophysiological and perceptual methods may have a place in the neuroaudiology clinic to assess specific deficits in sound localization function. These tests assessments could be used to guide the implementation of deficit-specific binaural processing auditory training or rehabilitation. They also offer a means to objectively and subjectively measure rehabilitation outcomes of ILD and ITD training. Moreover, this research may also lead to some signal processing strategies for hearing aids and cochlear implants to ultimately improve hearing capabilities for individuals with binaural deficits