The effect of amplitude comodulation on auditory object formation in sentence perception.

To comprehend speech in most environments, listeners must combine some but not all sounds from across a wide range of frequencies. Three experiments were conducted to examine the role of amplitude comodulation in performing an essential part of this function: the grouping together of the simultaneous components of a speech signal. Each of the experiments used time-varying sinusoidal (TVS) sentences (Remez, Rubin, Pisoni, & Carrell, 1981) as base stimuli because their component tones are acoustically unrelated. The independence of the three tones reduced the number of confounding grouping cues available compared with those found in natural or computer-synthesized speech (e.g., fundamental frequency and simultaneity of harmonic onset). In each of the experiments, the TVS base stimuli were amplitude modulated to determine whether this modulation would lead to appropriate grouping of the three tones as reflected by sentence intelligibility. Experiment 1 demonstrated that amplitude comodulation at 100 Hz did improve the intelligibility of TVS sentences. Experiment 2 showed that the component tones of a TVS sentence must be comodulated (as opposed to independently modulated) for improvements in intelligibility to be found. Experiment 3 showed that the comodulation rates that led to intelligibility improvements were consistent with the effective rates found in experiments that examined the grouping of complex nonspeech sounds by common temporal envelopes (e.g., comodulation masking release; Hall, Haggard, & Fernandes, 1984). The results of these experiments support the claim that certain basic temporal-envelope processing capabilities of the human auditory system contribute to the perception of fluent speech.