Azimuth and envelope coding in the inferior colliculus of the unanesthetized rabbit: effect of reverberation and distance.

Recognition and localization of a sound are the major functions of the auditory system. In real situations, the listener and different degrees of reverberation transform the signal between the source and the ears. The present study was designed to provide these transformations and examine their influence on neural responses. Using the virtual auditory space (VAS) method to create anechoic and moderately and highly reverberant environments, we found the following: 1) In reverberation, azimuth tuning was somewhat degraded with distance whereas the direction of azimuth tuning remained unchanged. These features remained unchanged in the anechoic condition. 2) In reverberation, azimuth tuning and envelope synchrony were degraded most for neurons with low best frequencies and least for neurons with high best frequencies. 3) More neurons showed envelope synchrony to binaural than to monaural stimulation in both anechoic and reverberant environments. 4) The percentage of envelope-coding neurons and their synchrony decreased in reverberation with distance, whereas it remained constant in the anechoic condition. 5) At far distances, for both binaural and monaural stimulation, the neural gain in reverberation could be as high as 30 dB and as much as 10 dB higher than those in the anechoic condition. 6) The majority of neurons were able to code both envelope and azimuth in all of the environments. This study provides a foundation for understanding the neural coding of azimuth and envelope synchrony at different distances in reverberant and anechoic environments. This is necessary to understand how the auditory system processes "where" and "what" information in real environments.