Behavioral sensitivity to interaural time differences in the rabbit

Abstract

An important cue for sound localization and separation of signals from noise is the interaural time difference (ITD). Humans are able to localize sounds within 1-2 degrees and can detect very small changes in the ITD (10-20micros). In contrast, many animals localize sounds with less precision than humans. Rabbits, for example, have sound localization thresholds of approximately 22 degrees . There is only limited information about behavioral ITD discrimination in animals with poor sound localization acuity that are typically used for the neural recordings. For this study, we measured behavioral discrimination of ITDs in the rabbit for a range of reference ITDs from 0 to +/-300micros. The behavioral task was conditioned avoidance and the stimulus was band-limited noise (500-1500Hz). Across animals, the average discrimination threshold was 50-60micros for reference ITDs of 0 to +/-200micros. There was no trend in the thresholds across this range of reference ITDs. For a reference ITD of +/-300micros, which is near the limit of the physiological window defined by the head width in this species, the discrimination threshold increased to approximately 100micros. The ITD discrimination in rabbits less acute than in cats, which have a similar head size. This result supports the suggestion that ITD discrimination, like sound localization [see Heffner, 1997. Acta Otolaryngol. 532 (Suppl.), 46-53] is determined by factors other than head size.

Figure 1

Examples of individual tracks used to determine the behavioral JND via a staircase tracking procedure. A. Example of a single track. The reference ITD was 0 μs. The last six reversals (open circles) were used to compute the threshold for the track (dashed line at 60 μs). B. All tracks used to compute the JND for this reference ITD. The JND (dashed line) for a given reference ITD was the difference between the reference ITD and the average of the thresholds (arrow), and in this case was 56 μs. C. The psychometric function derived from the tracks in B. The parameters of the function are given, including the midpoint of the function (m), the slope (s), the lapse (P_L) the y-intercept (y(0)), and the percent variance accounted for (r²).

Figure 2

JNDs for different reference ITDs in different animals. A. JNDs for one subject tested for reference ITDs lateralized to both sides. The arrows indicate the direction of tracking. Most points were taken from tracks using 20 μs steps; two tracks (asterisks) used 10 μs steps. One point (open circle) was taken while tracking the normal instead of the warning sound. B. The JNDs measured in two other animals. C. All of the data from the three animals, plotted as if lateralized to the same side in each case. The average JNDs are shown by the thick line.

Figure 3

Psychometric functions. A. Functions for the positive reference ITDs for animal 1. B. Functions for the negative reference ITDs for animal 1. C. Functions for the positive reference ITDs for animal 2. D. Functions for a reference ITD at 0 μs for animal 1 using either the best 50% of tracks (filled circles) or all of the tracks (open circles).