Mechanisms of sound localization investigated with head-related transfer functions

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Sound-localization behavior is an important orienting reaction of humans and animals that serves to fixate interesting objects. The barn owl is a model system for sound localization because of the nocturnal ecological niche it occupies. This led to adaptations that allow the barn owl to hunt mainly by listening. Many mechanisms underlying sound localization have been worked out through work in the barn owl. So far experimental approaches have used single stimulus parameters. Such experiments showed that the interaural time difference plays the main role in azimuthal sound localization, while the interaural level difference is important for elevational sound localization. However, these experiments have also suggested that further, hitherto unknown cues influence sound-localization behavior and the responses of neurons extracting sound-localization information. It specifically remains unclear whether and how positions in space may be disambiguated that have equal broadband interaural time and level differences. To tackle such questions we propose a new approach, using head-related transfer functions (HRTFs) for stimulation. The HRTF recorded at the eardrum contains the information originating from one position in space, albeit distorted by the body, head, and the outer-ear of the listener. HRTFs should, thus, contain the most complete information a listener can use in sound localization. We propose a close interaction of theory and experiment, proceeding in five steps: 1) record HRTFs, 2) analyze the HRTFs and find out whether the position where the HRTF was recorded can be reconstructed by reverse modelling, 3) use normal and manipulated HRTF-stimuli in behavioral experiments to find out which information contained in the HRTF is used by the animal and how, 4) use HRTF-stimuli in electrophysiological recordings to find out which information drives the spiking responses, and 5) use the spiking responses to try to reconstruct the position of the source that elicited a given response. We expect that the new procedures and results will significantly advance our understanding of sound localization, not only in the barn owl, but also in other animals, including humans. Our studies may also yield results that may be implemented in sound-localizing systems, like autonomic agents.