Timbral Shepard Illusion

Timbral Shepard-illusion reveals perceptual ambiguity and context sensitivity of brightness perception

BACKGROUND. Recent research has described rapid and long-lasting effects of prior context on the perception of ambiguous pitch shifts of Shepard tones  (Chambers et al., 2017, Nat. Commun.). Here, very similar effects are demonstrated for brightness shift judgments of harmonic complexes with partially cyclic spectral envelopes and fixed fundamental frequency. It is shown that frequency shifts of the cyclic envelopes are perceived as up- or downward shifts of brightness. Completely analogous to Chambers et al., the perceptual ambiguity of half-octave shifts resolves with the presentation of prior context tones. These results constitute a context effect for the perceptual processing of spectral envelope shifts and indicate so-far unknown commonalities of pitch and timbre.

Sound examples

[Please make sure to listen at a sufficient level using headphones with good low-frequency response. Otherwise the sounds might not be properly transmitted.]

Single pairs of tones were used in Exp. 1, and the quasi-cyclic envelopes were shifted by fractions of an octave:

In Exp. 2, target pairs were preceded by a context sequence that induced an upward or downward bias for the target shift (which always was a half-octave step): 

If stepwise shifts (here 1/6ths of an octave, i.e., 2 semitones) follow one another, this gives rise to sort of a timbral Shepard illusion, because  brightness appears to ascend or descend continuously: 

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