Acoustic cues underlying auditory distance in barn owls

Conclusions. We conclude that: (1) among several cues examined, the monaural cue of direct-to-reverberant (D/R) ratio in the ipsilateral ear provides the most information about sound-source distance; (2) interaural level difference (ILD) provides less information about sound-source distance; and (3) a comprehensive theory of three-dimensional auditory localization must incorporate the fact that all of the major acoustic cues change with distance. Objective. Neural mechanisms underlying auditory localization of distance are poorly understood. The present study was an initial step toward filling this gap in knowledge. Materials and methods. The binaural room impulse responses of adult barn owls were measured. The sound source was placed at various distances (up to 80 cm) and azimuths (0-90) relative to the owl's head, with the elevation kept at 0. Results. We determined the value of each cue for a 3-10 kHz band, and found that: (1) D/R ratio of signal amplitudes provided the most information about sound-source distance; (2) the ipsilateral D/R ratio represented distance more clearly than the contralateral or binaural-average D/R ratios; (3) ILD of direct signals increased with decreasing distance under certain conditions; (3) interaural time difference (ITD) of direct signals increased with decreasing distance at 90 azimuth; and (4) the spectral patterns of ILD and the monaural direct signals changed with distance in complex ways.