Near-field discrimination of sound source distance in the rabbit.

The acoustical cues and physiological processing mechanisms underlying the perception of the distance of sound sources are not well understood. To understand the relation between physiology and behavior, a first step is to use an animal model to study distance sensitivity. The goal of these experiments was to establish the capacity of the Dutch-belted rabbit to discriminate between sound sources at two distances. Trains of noise bursts were presented from speakers that were located either directly in front of the rabbit or at a 45 ° angle in azimuth. The reference speaker was positioned at distances of 20, 40, and 60 cm from the subject, and the more distant test speaker was systematically moved to determine the smallest difference in distance that could be reliably discriminated by the subject. Noise stimuli had one of three bandwidths: wideband (0.1-10 kHz), low-pass (0.1-3 kHz), or high-pass (3-10 kHz). The mean stimulus level was 60 dB sound pressure level (SPL) at the location of the rabbit's head, and the level was roved over a 12-dB range from trial to trial to reduce the availability of level cues. An operant one-interval two-alternative non-forced choice task was used, with a blocked two-down-one-up tracking procedure to determine the distance discriminability. Rabbits were consistently able to discriminate two distances when they were sufficiently separated. Sensitivity was better when the reference distance was 60 cm at either azimuth (distance ratio = 1.5) and was worse when the reference distance was 20 cm (distance ratio = 2.4 at 0 ° and 1.75 at 45 °).