Some factors affecting the magnitude of comodulation masking release

Abstract

This paper examines some of the factors that can affect the magnitude of comodulation masking release (CMR). In experiment I, psychometric functions were measured for the detection of a 1-kHz sinusoidal signal in a "multiplied" narrow-band noise centered at 1 kHz (reference condition) and the same noise with two comodulated flanking bands added. The functions were slightly steeper for the comodulated than for the reference masker. Thus CMRs measured at a high percent correct point were slightly (0.4 dB) larger than CMRs measured at a low percent correct point. Large individual differences were found for the reference masker but not for the comodulated masker. Experiment II compared CMRs obtained with narrow-band Gaussian noise and multiplied noise, using a single flanking band. For a flanking band remote from the signal frequency, the CMRs were smaller and more variable for the multiplied noise than for the Gaussian noise. This variability arose mainly from individual differences in the reference condition. Experiment III compared growth-of-masking functions for a signal centered in Gaussian noise and multiplied noise. Thresholds were lower for the multiplied than for the Gaussian noise, and the differences were greatest at high noise levels. The results are consistent with the idea that, for multiplied noise, some subjects can detect a change in the distribution of the envelope of the stimulus, when the signal is added to the masker. Such subjects have low thresholds in the reference condition, and give small CMRs. Other subjects are relatively insensitive to this cue. They have higher thresholds in the reference condition, and give larger CMRs. For Gaussian noise, thresholds for the reference condition are relatively stable across subjects and CMRs tend to be substantial, even for flanking-band frequencies remote from the signal frequency.