The influence of aging, memory, and stimulus characteristics on human auditory and visual spatial localization

This thesis aims to quantify the accuracy and precision (repeatability) of localizing free-field auditory and visual targets across a variety of controlled experimental conditions in young and aging adults. Investigations address the influence of stimulus bandwidth and level on sound localization as well as the effect of central ocular fixation and spatial memory with or without distraction on both auditory and visual target localization. Assessing the contribution of advancing age to spatial processing constitutes an important applied theme.;First, sound localization of ongoing low-pass, high-pass, and broadband white-noise targets was quantified in young adults across a wide range of stimulus levels. Subjects generally showed horizontal overestimation and vertical underestimation of auditory space, and this effect varied with frequency band and sound level. A particular surprise was an extraordinarily large horizontal overshoot (40%) in localizing narrow high-pass (3-10 kHz) targets, which declined at lower sound levels. Results demonstrate that band-pass sound localization relies upon distinct known neural channels for spatial processing that are uniquely influenced by stimulus level.;Second, the influence of advancing age on sound localization of ongoing low-pass, high-pass, broadband, and narrowband targets was examined. Localization of band-passed targets demonstrated increase in horizontal overshoot and vertical undershoot with age, coupled with deterioration in precision in both spatial planes, mostly related to age-related high-frequency hearing loss. Horizontal accuracy and precision of narrowband targets dramatically worsened with age for frequencies between 1250-1500 Hz, reflecting age-related difficulty in utilizing inter-aural time differences. These results suggest contributions of both peripheral and central aging to impaired auditory processing.;Third, the influence of central ocular fixation on localization of ongoing or transiently presented (memorized) auditory and visual targets was assessed in young and aged adults. The effect of spatial memory (up to 10 s) on auditory, visual and combined auditory-visual (bimodal) localization was also quantified. Sound localization of ongoing or transient auditory targets guided by fovea) vision (target fixation) resulted in an overestimation of horizontal target position in young adults. However, maintaining the eyes centrally, thereby using peripheral vision to guide localization (central fixation), largely abolished this error, reflecting the influence of eye position. In contrast, visual localization with central fixation resulted in undershoot for transient targets and deterioration in precision for ongoing targets, relative to the respective target fixation tasks. Localization with central ocular fixation in the elderly demonstrated increased overshoot for ongoing targets and poorer precision for ongoing or transient targets across sensory modalities, relative to young adults. When required to localize memorized targets after delays up to 10 s, both young and elderly subjects showed surprisingly small deterioration in their localization accuracy and precision.;Fourth, follow-up studies investigated the influence of spatial (auditory or visual) or non-spatial (auditory pitch) distraction presented simultaneously with, immediately following, or 1 s after the primary memorized auditory or visual target. Visual spatial and auditory pitch distraction, regardless of timing relative to the target, resulted in modest overshoot of horizontal sound localization in young listeners. Interestingly, the elderly showed decline in horizontal spatial performance relative to baseline only when auditory pitch distraction was presented simultaneously with an auditory target. This effect was dependent upon the distance between the target and the distracter, manifested as spatial distortion away from the distracter when localizing proximal auditory targets.;In conclusion, the present findings document deterioration of human spatial localization with age when challenged by carefully manipulated parameters aiming to unveil shortcomings of peripheral and central auditory processing. Enhanced awareness and understanding of age-related deficits in spatial orientation serves as an essential step toward identifying preventive and palliative measures to improve the health and safety of a growing aging population.