Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) is the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6 kHz that increased gain by 12 to 23 dB depending on source azimuth; and a notch at 7.6 kHz and peak at 9.8 kHz visible for most azimuths. Overall, frequencies above 4 kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5 kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16 kHz varied monotonically with azimuth over {+/-}31 dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5 kHz and 1.5 kHz also varied monotonically with azimuth over {+/-}358 s and {+/-}260 s, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs, whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs. Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.