On the binding of successive sounds: perceiving shifts in nonperceived pitches.

It is difficult to hear out individually the components of a "chord" of equal-amplitude pure tones with synchronous onsets and offsets. In the present study, this was confirmed using 300-ms random (inharmonic) chords with components at least 1/2 octave apart. Following each chord, after a variable silent delay, listeners were presented with a single pure tone which was either identical to one component of the chord or halfway in frequency between two components. These two types of sequence could not be reliably discriminated from each other. However, it was also found that if the single tone following the chord was instead slightly (e.g., 1/12 octave) lower or higher in frequency than one of its components, the same listeners were sensitive to this relation. They could perceive a pitch shift in the corresponding direction. Thus, it is possible to perceive a shift in a nonperceived frequency/pitch. This paradoxical phenomenon provides psychophysical evidence for the existence of automatic "frequency-shift detectors" in the human auditory system. The data reported here suggest that such detectors operate at an early stage of auditory scene analysis but can be activated by a pair of sounds separated by a few seconds.