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Peripheral-channeling theorists argue that differences in excitation pattern between successive sounds are necessary for stream segregation to occur. The component phases of complex tones comprising unresolved harmonics (F0=100 Hz) were manipulated to change pitch and timbre without changing the power spectrum. In experiment 1, listeners compared two alternating sequences of tones, A and B. One sequence was isochronous (tone duration=60 ms, intertone interval=40 ms). The other began isochronously, but the progressive delay of tone B made the rhythm irregular. Subjects had to identify the sequence with irregular rhythm. Stream segregation makes this task more difficult. A and B could differ in passband (1250-2500 Hz, 1768-3536 Hz, 2500-5000 Hz), component phase (cosine, alternating, random), or both. Stimuli were presented at 70 dB SPL in pink noise. Dissimilarity in either passband or phase increased discrimination thresholds. Moreover, phase differences raised threshold even when there was no passband difference. In experiment 2, listeners judged moment-by-moment the grouping of long ABA-ABA-... sequences. The measure was the proportion of time a sequence was heard as segregated. The factors that increased segregation were very similar to those that increased threshold in experiment 1. Overall, the findings indicate that substantial stream segregation can occur without differences in power spectrum. It is concluded that differences in peripheral channeling are not a requirement for stream segregation.

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Although the Ebbinghaus illusion is commonly used as an example of a simple size-contrast effect, previous studies have emphasised its complexity by identifying many factors that potentially influence the magnitude of the illusion. Here, in a series of three experiments, we attempt to simplify this complexity. In each trial, subjects saw a display comprising, on one side, a target stimulus surrounded by inducers and, on the other, an isolated probe stimulus. Their task was to indicate whether the probe appeared larger or smaller than the target. Probe size was adjusted with a one-up, one-down staircase procedure to find the point of subjective equality between probe and target. From these experiments, we argue that the apparent effects of inducer size are often confounded by the relative completeness of the inducing surround and that factors such as the similarity of the inducers and target are secondary. We suggest a simple model that can explain most of the data in terms of just two primary and independent factors: the relative size of the inducers and target, and the distance between the inducers and the target. The balance between these two factors determines whether the size of the target is underestimated or overestimated.

The tendency to hear a tone sequence as 2 or more streams (segregated) builds up, but a sudden change in properties can reset the percept to 1 stream (integrated). This effect has not hitherto been explored using an objective measure of streaming. Stimuli comprised a 2.0-s fixed-frequency inducer followed by a 0.6-s test sequence of alternating pure tones (3 low [L]-high [H] cycles). Listeners compared intervals for which the test sequence was either isochronous or the H tones were slightly delayed. Resetting of segregation should make identifying the anisochronous interval easier. The HL frequency separation was varied (0-12 semitones), and properties of the inducer and test sequence were set to the same or different values. Inducer properties manipulated were frequency, number of onsets (several short bursts vs. one continuous tone), tone:silence ratio (short vs. extended bursts), level, and lateralization. All differences between the inducer and the L tones reduced temporal discrimination thresholds toward those for the no-inducer case, including properties shown previously not to affect segregation greatly. Overall, it is concluded that abrupt changes in a sequence cause resetting and improve subsequent temporal discrimination.