Using Virtual Acoustic Space to Investigate Sound Localisation

It is an important task for the future to further close the gap between basic and applied science, in other words to make our understanding of the basic principles of auditory processing available for applications in medicine or information technology. Current examples are hearing aids (Dietz et al., 2009) or sound-localising robots (Calmes et al., 2007). This effort will be helped by better quantitative data resulting from more and more sophisticated experimental approaches. Despite new methodologies and techniques, the complex human auditory system is only accessible in a restricted way to many experimental approaches. This gap is closed by animal model systems that allow a more focused analysis of single aspects of auditory processing than human studies. The most commonly used animals in auditory research are birds (barn owls, chicken) and mammals (monkeys, cats, bats, ferrets, guinea pigs, rats and gerbils). When these animals are tested with various auditory stimuli in behavioural experiments, the accuracy (distance of a measured value to the true value) and precision (repeatability of a given measured value) of the animal’s behavioural response allows to draw conclusions on the difficulty with which the animal can use the stimulus to locate sound sources. An example is the measurement of minimum audible angles (MAA) to reveal the resolution threshold of the auditory system for the horizontal displacement of a sound source (Bala et al., 2007). Similarly, one can exploit the head-turn amplitude of humans or animals in response to narrowband or broadband sounds as a measure for the relevance of specific frequency bands, as well as binaural and monaural cues or perception thresholds (e.g. May & Huang, 1995; Poganiatz et al., 2001; Populin, 2006). The barn owl (Tyto alba) is an auditory specialist, depending to a large extent on listening while localising potential prey. In the course of evolution, the barn owl has developed several morphological and neuronal adaptations, which may be regarded as more optimal solutions to problems than the structures and circuits found in generalists. The owl has a characteristic facial ruff, which amplifies sound and is directionally sensitive for frequencies above 4 kHz (Coles & Guppy, 1988). Additionally, the left and right ear openings and flaps are asymmetrically with the left ear lying slightly higher than the right one. This asymmetry creates a steep gradient of interaural level differences (ILDs) in the owl’s frontal field (Campenhausen & Wagner, 2006). These adaptations to sound localisation are one of the reasons why barn owl hearing was established as an important model system during the last decades.