Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Communication Sciences and Disorders

First Advisor

John K. Cullen

Abstract

The comodulation masking release (CMR) effect involves improved detection of a pure-tone signal in an amplitude-modulated (AM) masker by an addition of AM noise spectrally distant from the signal (i.e, outside the critical-band of the signal). The underpinning of CMR has been elusive, and despite many previous investigations of this phenomenon it remains unexplained. A plausible explanation of CMR involves perceptual, auditory-grouping of acoustic signals. The purpose of this project was to investigate an hypothesis based on auditory-grouping to explain the CMR effect. A series of three experiments was completed towards this purpose. In the first, signal detection thresholds were obtained for a 2000-Hz pure-tone centered in narrow-band, AM maskers. Narrow-band, AM "flankers" centered at 1700-Hz were presented concurrently with the masker-band. The AM of the flanker-band was either identical to (correlated condition), or independent of (uncorrelated condition) the AM of the masker-band. Thresholds for the tone were about 3 dB better (lower) in the correlated condition, overall--a CMR effect of about 3 dB. In comparisons of specific correlated/uncorrelated noise-band pairs the magnitude of the CMR effect was found to vary substantially. CMR magnitude appeared to be related to the degree of envelope correlation of the "uncorrelated" noise-band pairs. In experiment two, strength of vertical-fusion of correlated and uncorrelated noiseband pairs was inferred from the interstimulus-interval (ISI) necessary to "capture" the flanker-band into a horizontal-stream. Shorter ISIs were needed to capture the flanker-band in correlated noise-band pairs than in uncorrelated noise-band pairs. This suggested that the strength of vertical-fusion of noise-band pairs was greater in correlated conditions than in uncorrelated conditions. In addition, the ISIs needed to capture the flanker-band were shorter for the "uncorrelated" noise-band pairs where the absolute value of the correlation coefficient was largest. In the third experiment, thresholds for the pure-tone were obtained in correlated noisebands. Two conditions were created, and denoted "weakly-fused" condition and "strongly-fused" condition. According to the auditory-grouping hypothesis of CMR, thresholds should have been better in the strongly-fused condition than in the weakly-fused condition. A significant difference for threshold was not found, however, between the two conditions. This finding suggests that auditory-grouping does not play a dominant role in CMR. Alternative explanations of why a difference for threshold was not found between the two conditions are offered.

Pages

104

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