Head-related transfer functions of the Rhesus monkey

Head-related transfer functions (HRTFs) are direction-specific acoustic fil ters formed by the head, the pinnae and the ear canals. They can be used to assess acoustical cues available for sound localization and to construct v irtual auditory environments. We measured the HRTFs of three anesthetized R hesus monkeys (Macaca mulatta) from 591 locations in the frontal hemisphere ranging from -90 degrees (left) to 90 degrees (right) in azimuth and -60 d egrees (down) to 90 degrees (up) in elevation for frequencies between 0.5 a nd 15 kHz. Acoustic validation of the HRTFs shows good agreement between fr ee field and virtual sound sources. Monaural spectra exhibit deep notches a t frequencies above 9 kHz, providing putative cues for elevation discrimina tion. Interaural level differences (ILDs) and interaural time differences ( ITDs) generally vary monotonically with azimuth between 0.5 and 8 kHz, sugg esting that these two cues can be used to discriminate azimuthal position. Comparison with published subsets of HRTFs from squirrel monkeys (Saimiri s ciureus) shows good agreement. Comparison with published human HRTFs from t he frontal hemisphere demonstrates overall similarity in the patterns of IL D and ITD, suggesting that the Rhesus monkey is a good acoustic model for t hese two sound localization cues in humans. Finally, the measured ITDs in t he horizontal plane agree well between -40 degrees and 40 degrees in azimut h with those calculated from a spherical head model with a radius of 52 mm, one-half the interaural distance of the monkey. (C) 2000 Elsevier Science B.V. All rights reserved.