The role of envelope information in CMR tasks

Abstract

In comodulation masking release (CMR), thresholds for a signal masked by a narrow‐band noise are reduced when additional noise is present, provided that the additional noise has the same amplitude envelope fluctuations over time as the original masking band. Recently, some speculation has appeared in the literature as to the specific source of information which provides the reduction in masked threshold. In the present work, the importance of changes in the envelope of the masker band, due to the addition of the signal at threshold, 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 and for changes in modulation depth per se. The data indicate that the patterns of CMR thresholds with envelope phase disparity and with modulation depth vary differently from that of the ED thresholds. A simulation was then conducted in which the stimulus waveforms were processed through a model that includes an auditory filter [equivalent rectangular bandwidth (ERB) filtering], a square‐law nonlinearity, and a sliding temporal window [an equivalent rectangular duration (ERD) filter]. The envelopes were then extracted from the processed waveforms to determine what actual changes in the envelopes were occurring in the signal intervals in both CMR and ED. The results of this waveform analysis indicate that discriminability of envelope changes due to the addition of the signal at threshold in the CMR task are insufficient to explain CMR.