Auditory segregation: stream or streams?

When auditory material segregates into "streams," is the unattended stream actually organized as an entity? An affirmative answer is suggested by the observation that the organizational structure of the unattended material interacts with the structure of material to which the subject is trying to attend. Specificially, a to-be-rejected stream can, because of its structure, capture from a to-be-judged stream elements that would otherwiise be acceptable members of the to-be-judged stream.     

Negating the effects of binaural cues: competition between auditory streaming and contralateral induction

When a pair of monaural pure tones, A and B, are repeatedly alternated in one ear, with noise bursts presented in synchrony with B in the other ear, the noise sometimes delateralizes B. This is presumably a case of Warren and Bashford's (1976) contralateral induction effect. However, the present experiment shows that the degree of contralateral induction is proportional to the separation in frequency between A and B. It was also found that the degree to which the noise bursts influenced B's timbre was proportional to the separation in frequency between A and B. The combined results suggest that cues that govern the sequential organization of sounds influence the use of binaural cues not only during the assignment of position to auditory events but during the assignment of timbre.     

An experimental evaluation of three theories of auditory stream segregation

Three theories of auditory stream segregation were evaluated. In two-part trials, subjects heard an induction sequence, whose effects upon an immediately subsequent test sequence were measured. The rhythm and total duration of Induction Sequence tones were varied in two experiments. The similarity between induction and test sequences aided segregation, but rhythmic predictability and longer tone durations did not. Frequency alternation during the induction sequence was not necessary to induce segregation in the test sequence. Furthermore, peripheral processes inadequately account for the segregation effects found. The data suggest that, once a distinct percept emerges from an auditory scene, properties derived from the percept (particularly changes) are fed back to control the ongoing analysis of that auditory scene. A neural adaptation to stimuli with constant properties may form part of this analysis.     

The perceptual segregation of simultaneous vowels with harmonic,shifted, or random components

This experiment was an investigation of the ability of listeners to identify the constituents of double vowels (pairs of synthetic vowels, presented concurrently and binaurally). Three variables were manipulated: (1) the size of the difference in FO between the constituents (0, 1/2, and 6 semitones); (2) the frequency relations among the sinusoids making up the constituents: harmonic, shifted (spaced equally in frequency but not integer multiples of the FO), and random; and (3) the relationship between the F0 contours imposed on the constituents: steady state, gliding in parallel, or gliding in opposite directions. It was assumed that, in the case of the gliding contours, the harmonics of each vowel would “trace out” their spectral envelope and potentially improve the definition of the formant locations. It was also assumed that the application of different FO contours would introduce differences in the direction of harmonic movement (common fate), thus aiding the perceptual segregation of the two vowels. The major findings were the following: (1) For harmonic constituents, a difference in FO leads to improved identification performance. Neither tracing nor common-fate differences add to the effect of pitch differences. (2) For shifted constituents, a difference between the spacing of the constituents also leads to improved performance. Formant tracing and common fate contribute some further improvement (3) For random constituents, tracing does not contribute, but common fate does.     

Auditory streaming and the continuity illusion

In the present experiment, auditory stream organization was investigated in the presence of perceptually restored continuity. It was found that auditory streaming processes tend to yield the same perceptual organization independently of the presence or absence of perceptual restoration. Other observations include the dominance of frequency proximity over trajectory as a perceptual organization principle, and the effect of harmonic enrichment on perceptual grouping.     

Stream segregation of narrow-band noise bursts

Normally hearing adults heard rapid alternations of a pair of band-limited noise bursts that had flat spectra (in terms of equal-loudness weighting of components) and sharp band edges. The bursts differed in center frequency (CF), but were matched on overall intensity, on bandwidth (BW) on a log frequency scale, and (roughly) on pitch strength. Listeners judged the ease with which the sequence could be held together perceptually in a single auditory stream (vs. forming separate high and low streams). Involuntary segregation was examined as a function of the following measures of frequency separation of the alternating noise bands: (1) the closest band edges, (2) the most remote band edges, (3) the CFs of the bands on a logarithmic scale, and (4) the BWs. Segregation was best predicted from the separation of the two CFs on a log-frequency scale (very strong effect). Increasing the BWs of the two alternating bursts (the same size, in log frequency, for both bands) also led to greater segregation (very weak effect).