Lateralization of noise-burst trains based on onset and ongoing interaural delays

Abstract

The lateralization of 250-ms trains of brief noise bursts was measured using an acoustic pointing technique. Stimuli were designed to assess the contribution of the interaural time delay (ITD) of the onset binaural burst relative to that of the ITDs in the ongoing part of the train. Lateralization was measured by listeners' adjustments of the ITD of a pointer stimulus, a 50-ms burst of noise, to match the lateral position of the target train. Results confirmed previous reports of lateralization dominance by the onset burst under conditions in which the train is composed of frozen tokens and the ongoing part contains multiple ambiguous interaural delays. In contrast, lateralization of ongoing trains in which fresh noise tokens were used for each set of two alternating (left-leading/right-leading) binaural pairs followed the ITD of the first pair in each set, regardless of the ITD of the onset burst of the entire stimulus and even when the onset burst was removed by gradual gating. This clear lateralization of a long-duration stimulus with ambiguous interaural delay cues suggests precedence mechanisms that involve not only the interaural cues at the beginning of a sound, but also the pattern of cues within an ongoing sound.

Figure 1

Stimuli and results for Experiment 1-A. (a) The first few ms of the 250-ms binaural noise burst train (when gated abruptly) is illustrated schematically. Each rectangle represents a burst of white noise. The label (‘1’ in this case) indicates the identity of a noise token. In this experiment the noise burst trains were constructed using a single frozen token on each trial. The ITD τ of the first burst in the train is the main experimental variable. In this experiment bursts after the first had ITDs that alternated between −τ and +τ. The time span indicated by ‘repeat’ indicates the pattern of ITDs that is repeated for the remainder of the stimulus. In this experiment the stimuli themselves repeat exactly, but in subsequent experiments that will not necessarily be the case. (b) Results with rectangular gating. Pointer adjustments from four listeners plotted as a function of τ. The diagonal line indicates y=x. At each value of τ tested there are six data points (which sometimes overlay each other). (c) As in (b), but with gradual (125-ms) rise∕fall gating.

Figure 2

Stimuli and results for Experiment 1-B. (a) As in Fig. 1a, but in this experiment the ITDs of the bursts after the first pair alternated between +500 and −500 μs, while τ was varied from −500 to +500 μs in 100-μs steps. The second burst pair in the train, which began the alternation between ±500 μs, was always right leading. Stimuli were abruptly gated. (b) Pointer adjustments from four listeners plotted as a function of τ.

Figure 3

Stimuli and results for Experiment 1-C. (a) As in Fig. 1a, but in this experiment the ITDs of the bursts after the first pair were fixed at 0 μs, while τ was varied from −500 to +500 μs in 100-μs steps. (b) Pointer adjustments from six listeners plotted as a function of τ.

Figure 4

Stimuli and results for Experiment 1-D. (a) As in Fig. 1a, but in this experiment the ITDs of the bursts after the first pair alternated between +α and −α μs, while τ was varied from −500 to +500 μs in 100-μs steps. The second burst pair in the train, which began the alternation between plus and minus α, was always right leading. Stimuli were abruptly gated. (b) Pointer adjustments from four listeners plotted as a function of τ. Only mean matches are shown for each condition. Note that the results for α=0 and α=500 μs were taken from Experiments 1-C and 1-B, respectively.

Figure 5

Stimuli and results for Experiment 1-E. (a) As in Fig. 1a, but in this experiment the ITDs of the bursts after the first pair were fixed at −500 μs, while τ was varied from −500 to +500 μs in 100-μs steps. (b) Pointer adjustments from four listeners plotted as a function of τ. Similar results (not shown) were obtained when the ongoing ITD was fixed at +500 μs.

Figure 6

Stimuli and results for Experiment 2-A. (a) As in Fig. 1a, but in this experiment the noise-burst trains were constructed using “quad sets,” in which a single frozen tokens were repeated only once in each channel before a new token was introduced. ITDs alternated between −τ and +τ, with τ varying from −600 to +600 μs in 100-μs steps. (b) Results with rectangular gating. Pointer adjustments from four listeners plotted as a function of t. (c) As in (b), but with gradual (125-ms) rise∕fall gating.

Figure 7

Stimuli and results for Experiment 2-B. (a) As in Fig. 6a, but in this experiment the quad sets were initiated with the second burst of the train with an ITD of −τ, with τ varying from −600 to +600 μs in 100-μs steps. (b) Pointer adjustments from four listeners plotted as a function of τ.

Figure 8

Stimuli and results for Experiment 2-C with a 6-ms reflection delay. (a) As in Fig. 6a, but in this experiment the quad sets were split up so that the second instance of each token followed 6 ms (rather than 2 ms) after the onset of the first instance of each token, with different tokens interleaved in between. τ varied from −500 to +500 μs in 100-μs steps. (b) Pointer adjustments from three listeners plotted as a function of τ.

Figure 9

Stimuli and results for Experiment 2-C with a 10-ms reflection delay. (a) As in Fig. 8a, but in this experiment the second instance of each token followed 10 ms (rather than 6 ms) after the onset of the first instance of each token, with different tokens interleaved in between. (b) Pointer adjustments from three listeners plotted as a function of τ.

Figure 10

Results for Experiment 2-D with a 10-ms reflection delay. (a) The stimuli are as in Fig. 9a, shown here abruptly gated. Pointer adjustments of two subjects are shown in panel (b) for abruptly-gated stimuli and panel (c) for slowly-gated stimuli.