Cross-sensory calibration of spatial hearing

Abstract

This thesis describes experiments designed to investigate cross-sensory spatial coordination. In particular, it deals with sound localization and its relation to the visual system. The main questions and conclusions are: What is the effect of blindness on sound localization? Barn-owl studies have shown that vision is used to spatially calibrate the auditory system. Relatively little is known about this process in humans. We show that visual feedback may be required to optimally extract sound elevation, but not azimuth, in the presence of background noise. Further, the results suggest that vision shifts the origin for arm pointing from the shoulder to the cyclopean eye. What is the effect of compressed vision on sound localization? Shifting prisms have been used to study cross-sensory spatial plasticity. We compressed vision with 0.5x lenses. The results show that azimuth localization, but not elevation, was compressed accordingly within the visual field of the lenses. The changes in localization behavior outside this field are consistent with a model in which azimuth localization is encoded by recruitment rather than by a population code. How is sound location encoded in the IC? The inferior colliculus is the first nucleus in the ascending auditory pathway that may encode 2D sound position. Response properties of auditory IC neurons to sound position, level and eye position are described. Besides ample tuning to sound position and level, the results show that, albeit in a relatively weak 'gain-field' like fashion, a small fraction of the population was modulated by eye position. How are spectro-temporal properties encoded in the IC? We used sounds with spectro-temporal modulations ('ripples') to determine the excitatory and inhibitory response properties of auditory IC neurons (STRFs). The results show that a wide variety of STRFs is found and that these can be used to linearly predict responses to various sound stimuli