The processing of envelope information in comodulation masking release (CMR) and envelope discrimination

In comodulation masking release (CMR), thresholds for a signal masked by a narrow-band noise are reduced when additional noise is present. To demonstrate CMR, the additional noise must have similar amplitude envelope fluctuations over time as the primary noise band masking the signal. The specific source of information that provides the reduction in masked threshold remains unclear, although it is the focus of recent interest. The envelope of the on-frequency masking band (OFB) changes with the addition of the signal at threshold: in the present work, the importance of these changes in providing the CMR has been investigated. First, CMR thresholds were collected in sinusoidally amplitude-modulated noises which varied either in the phase relationship of their envelopes or in their modulation depth. Then envelope discrimination (ED) thresholds were collected for changes in envelope phase disparity (i.e., envelope correlation) and for changes in modulation depth per se. The patterns of CMR thresholds with envelope phase disparity and with modulation depth are not similar to those of the ED thresholds. A computer simulation was conducted in which the stimulus waveforms were processed through an auditory model which comprised bandpass auditory filters, a square-law nonlinearity, and a sliding temporal window. The envelopes were then extracted from the processed waveforms to determine whether the envelope changes that occur in the signal intervals in the CMR and ED tasks may be similar. The results of this analysis indicate that discriminability of envelope correlation due to the addition of the signal at threshold in the CMR task was insufficient to explain CMR. However, the discriminability of changes in modulation depth due to the addition of the signal is in agreement with thresholds obtained from the CMR task.