Perceptual fusion and fragmentation of complex tones made inharmonic by applying different degrees of frequency shift and spectral stretch.

Global pitch depends on harmonic relations between components, but the perceptual coherence of a complex tone cannot be explained in the same way. Instead, it has been proposed that the auditory system responds to a common pattern of equal spacing between components, but is only sensitive to deviations from this pattern over a limited range [Roberts and Brunstrom, J. Acoust. Soc. Am. 104, 2326-2338 (1998)]. This hypothesis predicts that spectral fusion will be largely unaffected either by frequency shifting a harmonic stimulus (because equal spacing is preserved), or by small degrees of spectral stretch (because significant deviations from equal spacing only cumulate over large spectral distances). Complex tones were either shifted by 0%-50% of F0 (200 Hz+/-10%) or stretched by 0%-12% of F0 (100 Hz+/-10%). Subjects heard a complex followed by a pure tone in a continuous loop. One of the components 2-11 was mistuned by +/- 4%, and subjects adjusted the pure tone to match its pitch. Broadly consistent with our hypothesis, frequency shifts had relatively little effect on hit rates and only large degrees of stretch reduced them substantially. The implications for simultaneous grouping are explored with reference to an autocorrelation model of auditory processing.