Objective and subjective psychophysical measures of auditory stream integration and segregation

Abstract

The perceptual organization of sound sequences into auditory streams involves the integration of sounds into one stream and the segregation of sounds into separate streams. "Objective" psychophysical measures of auditory streaming can be obtained using behavioral tasks where performance is facilitated by segregation and hampered by integration, or vice versa. Traditionally, these two types of tasks have been tested in separate studies involving different listeners, procedures, and stimuli. Here, we tested subjects in two complementary temporal-gap discrimination tasks involving similar stimuli and procedures. One task was designed so that performance in it would be facilitated by perceptual integration; the other, so that performance would be facilitated by perceptual segregation. Thresholds were measured in both tasks under a wide range of conditions produced by varying three stimulus parameters known to influence stream formation: frequency separation, tone-presentation rate, and sequence length. In addition to these performance-based measures, subjective judgments of perceived segregation were collected in the same listeners under corresponding stimulus conditions. The patterns of results obtained in the two temporal-discrimination tasks, and the relationships between thresholds and perceived-segregation judgments, were mostly consistent with the hypothesis that stream segregation helped performance in one task and impaired performance in the other task. The tasks and stimuli described here may prove useful in future behavioral or neurophysiological experiments, which seek to manipulate and measure neural correlates of auditory streaming while minimizing differences between the physical stimuli.

FIG. 1.

Mean proportion of “two streams” responses measured as a function of frequency separation (ΔF) in experiment 1. Each panel shows results obtained under a different instructions condition: neutral instructions (top panel), integration-promoting instructions (middle panel), and segregation-promoting instructions (lower panel). Different symbols are used to indicate results obtained using different sequence lengths: N = 2 (circles), N = 4 (diamonds), and N = 8 (squares). Data points corresponding to results obtained using a T of 50 ms (fast presentation rate) are shown as solid symbols connected by solid lines. Data points corresponding to results obtained using a T of 100 ms (slow presentation rate) are shown as open symbols connected by dashed lines. The error bars show standard errors of the mean. To avoid overlap, some error bars are not shown.

FIG. 2.

Schematic spectrograms of example stimulus sequences presented on a trial in experiments 2 and 3. The A and B tones are labeled only for the first triplet. Two of the three main stimulus parameters, frequency separation (ΔF) and the within-triplet inter-tone interval (T), are indicated explicitly on the schema. The third parameter, number of triplets (N), is not shown; in these examples, it was equal to 4. A. A In experiment 2, the duration of the interval between the A and B tones within each triplet, which is labeled T and shown using double gray arrows, was constant throughout the sequence, except in the last triplet, where it was either reduced or increased (at random with equal probability) by Δt. In this example, the interval was decreased, so that the B tone was shifted toward the leading A tone and away from the trailing A tone. On other trials, the shift could be in the opposite direction. In this experiment, the inter-triplet interval was roved independently for each pair of triplets, including the last pair. B In experiment 3, the duration of the interval between consecutive B tones, which is labeled 6T and shown using double gray arrows, was constant throughout the sequence except for the last pair of B tones, where it was reduced or increased (at random with equal probability) by Δt. In this experiment, the timing of the A tones relative to the B tones was roved, with both A tones from a triplet shifted forward or backward coherently by T ms.

FIG. 3.

Mean thresholds in experiment 2. The different bar shadings correspond to different ΔF’s, as indicated in the top legend. The numbers 1, 2, 4, and 8 along the abscissa refer to the number of tones in the sequence, N. Bars on the left half of the plot indicate thresholds obtained with a T of 50 ms (fast presentation rate). Bars on the right half of the plot indicate thresholds obtained with a T of 100 ms (slow presentation rate). The error bars indicate plus one standard error above the mean.

FIG. 4.

Mean thresholds in experiment 3. The different bar shadings correspond to different ΔF conditions, as indicated in the top legend. Open bars indicate thresholds measured in the absence of A tones. The numbers 2, 4, and 8 along the abscissa refer to the number of elements (ABA tone triplets or B-B tone pairs) in the sequence, N. Bars in the left half of the plot indicate thresholds obtained with a T of 50 ms (fast presentation rate). Bars in the right half of the plot indicate thresholds obtained with a T of 100 ms (slow presentation rate). The error bars indicate plus one standard error above the mean.