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.

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Listeners were presented with 300-ms ‘‘chords’’ of five synchronous pure tones, followed after a 0.5–8-s silence by a single pure tone. The frequencies of each chord’s components were selected randomly, but spaced by intervals of between 6 and 10 semitones. In one condition (‘‘up/down’’), the single tone following a chord was 1 semitone higher or lower than one of the chord’s three intermediate components; on each trial, the corresponding component was selected randomly and the task was to indicate the direction in which its pitch changed. In another condition (‘‘present/absent’’), the single tone following a chord was either identical to one of the three intermediate components or halfway in frequency between two components; the task was to indicate if the single tone was present in the chord or not. Performance was much better in the up/down condition than in the present/absent condition, even though the opposite trend was predictable for an ideal ‘‘analytic’’ listener. Ten listeners reported that, in the up/down condition, they could often perceive the appropriate pitch change without having heard out the relevant component of the chord. These results provide strong evidence for the existence of pitch change detectors in the auditory system.