Comodulation masking release : is comodulation sufficient ?

Common amplitude modulation across frequency facilitates comodulation masking release (CMR) and may also contribute to auditory grouping. The purpose of the present investigation was to determine whether there is an association between these two phenomena; that is, does the occurrence of CMR require that the comodulated noise bands making up the composite masker be grouped together? Three experiments were undertaken, each manipulating one variable designed to hinder the fusion of the comodulated noise bands. In experiment 1, the variable of onset/offset asynchrony was observed to have a marked effect: CMR could be completely abolished if the asynchrony between the on-signal band and the flanking bands was sufficiently great (> or = 50 ms). In experiment 2, an auditory streaming paradigm was employed and this was observed to disrupt CMR significantly. However, a substantial CMR remained. Experiment 3 attempted to hinder the fusion of the comodulated noise bands by drawing the various noise bands into separate harmonic series. However, the small effect observed did not appear to be due to a parsing of the fused image. Results from experiments 1 and 2 suggest that the CMR mechanism does not act in isolation from the processes that give rise to auditory object formation. In particular, the cue of onset/offset synchrony appears to be of paramount importance.