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The rhythm created by spacing a series of brief tones in a regular pattern can be disguised by interleaving identical distractors at irregular intervals. The disguised rhythm can be unmasked if the distractors are allocated to a separate stream from the rhythm by integration with temporally overlapping captors. Listeners identified which of 2 rhythms was presented, and the accuracy and rated clarity of their judgment was used to estimate the fusion of the distractors and captors. The extent of fusion depended primarily on onset asynchrony and degree of temporal overlap. Harmonic relations had some influence, but only an extreme difference in spatial location was effective (dichotic presentation). Both preattentive and attentionally driven processes governed performance.

The effects of subunit formation on adult listeners' ability to notice changes in a continuous spectral gradient of sound were studied. Results of this experiment support the idea that the auditory system processes information differently within a unit, and that this processing does not occur unless the perceptual system detects unit boundaries. In this experiment, silences were inserted into a continuously changing sound to cause the formation of short units. Listeners noticed the change earlier in conditions with silences inserted than in to conditions where the transition was either unbroken or broken by loud noise bursts. Results are discussed in terms of two processes, one that accentuates stimulus properties present at moments of onset and offset, and a second that uses onsets and offsets to signal the beginnings and ends of units and reduces the change perceived within units.

The perceptual grouping of a four-tone cycle was studied as a function of differences in fundamental frequencies and the frequencies of spectral peaks. Each tone had a single formant and at least 13 harmonic. In Experiment 1 the formant was created by filtering a flat spectrum and in Experiment 2 by adding harmonics. Fundamental frequency was found to be capable of controlling grouping even when the spectra spanned exactly the same frequency range. Formant peak separation became more effective as the sharpness (amplitude of the peak relative to a spectral pedestal) increased. The effect of each type of acoustic difference depended on the task. Listeners could group the tones by either sort of difference but were also capable of resisting the disruptive effect of the other one. This was taken as evidence for the presence of a schema-based process of perceptual grouping and the relative weakness of primitive segregation. RESUME Nous avons etudie le regroupement perceptuel d'un cycle de quatre sons en fonction des differences dans les fequences fondamentals et les frequences de pics spectraux. Chaque son avait un seul formant et au moins 13 harmoniques. Dans l'experience 1 le formant etait cree par le filtrage d'un spectre plat et dans l'experience 2 par l'addition d'harmoniques. Nous avons trouve que la frequence fondamentale etait capable de controler le regroupement meme quand les spectres traversaient exactement la meme etendue de frequences. La separation entre les pics de formant devenait plus efficace a mesure que l'acuite (l'amplitude du pic relatif au piedestal spectral) augmentait. L'effet de chaque type de difference acoustique dependait de la tache. Les sujets pouvaient regrouper les sons par l'une ou l'autre des differences et etaient aussi capables de resister a l'effet perturbateur de l'autre difference. Ceci a ete considere comme une evidence de la presence d'un processus schema-dependant du regroupement perceptuel et de la faiblesse relative de la segregation primitive.