Effects of pinna position on head-related transfer functions in the cat

To measure the effects of the pinna position on spectral sound localization cues, the head-related transfer function (HRTF) from the free-field to a point in the ear canal was measured for anesthetized cats with their pinnae in three positions: the relaxed, anesthetized position; pulled forward into an approximation of the "alert cat" posture; and pulled back. The general features of HRTFs are not changed by moving the pinna, although the mapping of particular HRTF features onto directions in space is changed. As an approximation, the pinna behaves like a fixed-shaped sound collector, so that HRTFs shift with the pinna when it moves; however, pinna movement changes some quantitative details of HRTFs beyond what is predicted by this approximation. When viewed as directional gain, pinna movements serve to optimize listening conditions. However, when considering sound localization, pinna movements lead to ambiguities regarding source location. If pinna position is not incorporated into the computation, and spectral cues alone are used for localization, the ambiguity is about 60 degrees in azimuth and 30 degrees in elevation. Pinna movements produce similar azimuthal ambiguity in interaural level differences. Interaural time difference cues could be used to reduce the ambiguity in azimuth, but a knowledge of pinna position seems to be necessary to resolve ambiguities in elevation.