Thesis (Ed.D.)--Boston University.

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Thesis (Ph.D.) -- University of Texas at Dallas, 2006 / Includes vita. Includes bibliographical references (leaves 135-148)

Thesis (MTech (Elec. Eng.))--Peninsula Technikon, 2000. / Word processed copy. Summary in English. Includes bibliographical references. Also available online.

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published_or_final_version / abstract / toc / Speech and Hearing Sciences / Master / Master of Science in Audiology

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The effects of sub-unit formation on adult listeners' ability to notice changes in a continuously changing timbre (with fixed fundamental frequency) were studied. In Experiment 1, reaction times were recorded when listeners were asked to detect a change in the timbre of a 5-sec sound with spectral content that gradually shifted across time producing a perceptual change from "dull" to "bright." Sixteen silences were inserted in some conditions to cause the formation of short units. Listeners noticed the change sooner in these conditions than when the transitions were unbroken or broken by loud noise bursts. Experiments 2 and 3 looked at the individual unit and considered two separate processes as possible explanations for the previous results. Process 1 accentuates stimulus properties present at moments of onset and offset. Process 2, instead, uses onsets and offsets to signal the beginnings and ends of units and reduces the change perceived within a unit. The former would imply that listeners made onset comparisons in a point-to-point manner, whereas the latter would imply that the onsets, which formed units, enabled a unit-to-unit comparison. In Experiments 2 and 3 listeners were asked to distinguish between 1-sec sounds that both started with Timbre 1 and ended with Timbre 2, but were different in their internal rates of timbral change. Listener's performance was significantly poorer when asked to distinguish between two sounds that had identical onset and offset information in comparison to their performance when asked to distinguish either the first halves or second halves of these, which would differ in onset or offset information. Evidence suggests the presence of both Process 1 and Process 2.

The cross-spectral grouping of complex sounds was investigated with the rhythmic masking release (RMR) paradigm. RMR involves the discrimination of one of two possible rhythms, despite perceptual masking of the rhythm by an irregular sequence of sounds identical to the rhythmic sounds that are interleaved among them. The rhythm can be released from masking by inducing the perceptual fusion of the irregular interfering sounds with concurrent "flanking" sounds situated in different frequency regions. The accuracy of the identified rhythm and its rated clarity in a two-alternative forced-choice procedure measured the degree of cross-spectral fusion of the interfering sounds with the flanking sounds. / The results obtained in six experiments suggest that temporal synchrony is sufficient for the fusion of (i) brief noise bursts that are widely apart in frequency (large DeltaF), that have uncorrelated high-rate intensity changes (HRIC), and that are spatially separated; and (ii) brief tones that are not harmonics of a common fundamental frequency (F0) and that are spatially-separated. An asynchrony of 20--40 ms is sufficient for the segregation of: (i) brief temporally-overlapping noise bursts with correlated HRIC and without spatial separations; and (ii) brief overlapping tones sharing a common F0 and without spatial separations. Intermediate asynchronies of 10--20 ms produce ambiguous cases of grouping and the presence of other segregation or fusion cues is critical to disambiguate them. / Thus, whereas uncorrelated ERIC, large DeltaF's, different F0's, and dichotic presentation all significantly affect the fusion of simultaneous or nearly simultaneous sounds, they are not sufficient by themselves to fully segregate these sounds; however, when a group of them act together, their synergetic action reinforces the effect of a small asynchrony in promoting segregation. When two sources are simultaneously active and emit short-duration sounds, separation of the sound sources in space has a negligible effect on their segregation. This research demonstrates the dominance of spectro-temporal cues (e.g., temporal synchrony) over spatial cues (e.g., common sound-source location) for the grouping of brief concurrent sounds. It also demonstrates that the auditory system is highly sensitive to any deviation from temporal synchrony. Further research is needed to establish whether the present conclusions apply to sounds of a longer duration.

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