Temporal weighting functions for interaural time and level differences. II. The effect of binaurally synchronous temporal jitter

Abstract

Recent work has demonstrated that sensitivity to interaural time differences (ITD) carried by high-rate cochlear implant pulse trains or analogous acoustic signals can be enhanced by imposing random temporal variation on the stimulus rate [see Goupell et al. (2009). J. Acoust. Soc. Am. 126, 2511-2521]. The present study characterized the effect of such "temporal jitter" on normal-hearing listeners' weighting of ITD and interaural level differences (ILD) applied to brief trains of Gabor clicks (4 kHz center frequency) presented at nominal interclick intervals (ICI) of 1.25 and 2.5 ms. Lateral discrimination judgments were evaluated on the basis of the ITD or ILD carried by individual clicks in each train. Random perturbation of the ICI significantly reduced listeners' weighting of onset cues for both ITD and ILD discrimination compared to corresponding isochronous conditions, consistent with enhanced sensitivity to post-onset binaural cues in jittered stimuli, although the reduction of onset weighting was not statistically significant at 1.25 ms ICI. An additional analysis suggested greater weighting of ITD or ILD presented following lengthened versus shortened ICI, although weights for such "gaps" and "squeezes" were comparable to other post-onset weights. Results are discussed in terms of binaural information available in jittered versus isochronous stimuli.

Figure 1

Schematic illustration of stimuli (not to scale). Each trial consisted of a diotic reference stimulus followed by a probe stimulus. In isochronous (left) conditions (tested in a previous investigation), the reference stimulus was comprised of 16 equal-amplitude Gabor clicks presented synchronously to the left and right earphones. Following a 550 ms silent interval, an ITD or ILD probe stimulus was presented, comprised of 16 Gabor click pairs with random ITD or ILD imposed on each. In the ITD probe, each click pair carried an ITD drawn from a uniform distribution of ±100 μs about a base ITD of −100, 0, or 100 μs (0 in the above illustration); clicks were presented at equal amplitude to the two earphones in this condition. In the ILD probe, each click pair carried an ILD drawn from a uniform distribution of ±2 dB about a base ILD of −1, 0, or 1 dB (0 in the above illustration); clicks were presented synchronously to the two earphones in this condition. The ICI, which corresponded to the rate of click presentation, was held constant within and between trials of a single run. The jittered conditions (right) were identical to the isochronous conditions with the exception that the ICI in both the reference and probe stimuli were varied randomly about the nominal ICI according to the parameter k (see text).

Figure 2

TWFs averaged across subjects for isochronous (left column) and jittered (right column) conditions across ICI (rows) for both ITD (upper panels) and ILD (lower panels). Within each panel, per-click ITD or ILD weights (AUC₁…AUC₁₆, filled circles) are plotted against click number to form the TWF; the weight of mean ITD or ILD (AUC_mean, dashed line) and the weight of ITD or ILD following the longest (AUC_maxICI, upward triangle) and shortest (AUC_minICI, downward triangle) ICI per click train are plotted for comparison (see text). The shaded region plots the range of chance classification (see text).

Figure 3

Individual TWFs for isochronous and jittered conditions. Legend as in Fig. 2.

Figure 4

Cross-subject averages of AUC₁ − AUC_mean against ICI for isochronous (black) and jittered (gray) conditions for both ITD (left panel) and ILD (right panel). Error bars indicate one standard error of the mean. Data evidenced a significant reduction of onset dominance with jitter at 2.5 ms ICI.