Comodulation masking release and auditory grouping

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Abstract
The detectability of a pure-tone signal masked by a band of noise centered on the signal can be improved by the addition of flanking noise bands, provided that the temporal envelopes of the flanking bands are correlated with that of the on-signal band. This phenomenon is referred to as comodulation masking release (CMR). The present study examined CMR in conditions in which some flanking noise bands were comodulated with the on-signal band, but other flanking bands (termed "deviant" bands) were not. Past research has indicated that CMR is often substantially reduced when deviant bands are present at spectral locations close to the signal frrquency. An investigation was undertaken to determine whether the disruptive effects of such bands could be reduced by factors related to auditory grouping. The signal frequency was 1000 Hz. In one condition, only 20-Hz-wide comodulated bands, centered on 400, 600, 800, 1000, 1200, 1400, and 1600 Hz, were present. The CMR for this condition, referenced to threshold for the on-signal band only, was approximately 15 dB. In a second condition, two deviant bands were added at 900 and 1100 Hz; their presence reduced the CMR to only 3-4 dB. The number of deviant bands was then increased progressively, from two to eight bands. Deviant bands either shared a common envelope (codeviant), or had unique envelopes (multideviant). The number of bands that were comodulated with the on-signal band was held constant at six. The rationale was that as more deviant bands were added, the auditory system would be more likely to group the deviant bands near the signal frequency with the other deviant bands, rather than with the on-signal band, and therefore the disruptive effects of the proximal deviant bands would be reduced. A second manipulation was to provide an onset/offset asynchrony between the on-signal and comodulated bands, and the deviant bands. The rationale was that the asynchrony would enhance the segregation of the comodulated bands from the deviant bands. Both of the above stimulus manipulations (increasing the number of distally placed deviant bands, and providing onset/offset asynchrony) reduced the disruptive effects of the deviant bands. The results of dichotic conditions showed that the disruptive effects of deviant bands placed at 900 and 1100 Hz could also be reduced by adding codeviant bands (placed at 300, 500, 700, 1300, 1500, and1700 Hz) in the ear contralateral to the signal. These results indicate that CMR may occur subsequent to stages of auditory grouping or object formation.

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