Masking and stimulus intensity effects on duplex perception: a confirmation of the dissociation between speech and nonspeech modes.

Using the phenomenon of duplex perception, previous researchers have shown that certain manipulations affect the perception of formant transitions as speech but not their perception as nonspeech "chirps," a dissociation that is consistent with the hypothesized distinction between speech and nonspeech modes of perception [Liberman et al., Percept. Psychophys. 30, 133-143 (1981); Mann and Liberman, Cognition 14, 211-235 (1983)]. The present study supports this interpretation of duplex perception by showing the existence of a "double dissociation" between the speech and chirp percepts. Five experiments compared the effects of stimulus onset asynchrony, backward masking, and transition intensity on the two sides of duplex percepts. It was found that certain manipulations penalize the chirp side but not the speech side, whereas other manipulations had the opposite effect of penalizing the speech side but not the chirp side. In addition, although effects on the speech side of duplex percepts have appeared to be much the same as in the case of normal (electronically fused) speech stimuli, the present study discovered that manipulations that impaired the chirp side of duplex percepts had considerably less effect on the perception of isolated chirps. Thus it would seem that duplex perception makes chirp perception more vulnerable to the effects of stimulus degradation. Several explanations of the data are discussed, among them, the view that speech perception may take precedence over other forms of auditory perception [Mattingly and Liberman, in Signals and Sense: Local and Global Order in Perceptual Maps, edited by G.M. Edelman, W.E. Gall, and W.M. Cowan (Wiley, New York, in press); Whalen and Liberman, Science 237, 169-171 (1987)].