SPECTRAL CUES FOR SOUND LOCALIZATION IN CATS - A MODEL FOR DISCHARGE RATE REPRESENTATIONS IN THE AUDITORY-NERVE

Neural representations of pinna-based spectral cues for sound localiza tion were modeled by simulating auditory nerve discharge rates to nois e bursts that had been shaped by filtering properties of the cat's hea d-related transfer functions (HRTFs) at 179 locations in the frontal f ield. The auditory nerve model transformed spectral differences betwee n HRTFs into simulated neural rate differences. Linear equations for t his transformation were developed from actual auditory nerve responses to a limited subset of HRTF-filtered noise bursts [Rice ct al., J. Ac oust. Sec. Ani. 97, 1764-1776 (1995)]. Signal detection methods were u sed to investigate simulated neural responses to pairwise changes betw een HRTFs. The quality of neural representation for these changes, in terms of dl values, declined when the reference HRTF was moved from a central location (0 degrees AZ, 0 degrees EL) to a large positive azim uth in the horizontal plane (75 degrees AZ, 0 degrees EL) or a high el evation in the median plane (0 degrees AZ, 75 degrees EL). Most simula ted responses exhibited large d' values for comparisons of contralater al versus ipsilateral azimuths, or eccentric versus frontal elevations . This rate information resulted from directionally dependent changes in the overall gain of HRTFs. In addition, fibers with best frequency (BF: the frequency of greatest sensitivity for individual fibers) betw een 5 and 18 kHz showed large d' values for HRTF contrasts in the imme diate frontal held because of the effects of spectral notches (i.e., s harp drops in gain over a narrow frequency range). Spectral notches al so played a prominent role in simulations that required identification of HRTF location in the absence of a fixed reference stimulus. These modeling results correspond well with previously described patterns in the cat's localization behaviors. (C) 1997 Acoustical Society of Amer ica.