Assessing the possible role of frequency-shift detectors in the ability to hear out partials in complex tones

The possible role of frequency-shift detectors (FSDs) was assessed for a task measuring the ability to hear out individual "inner" partials in a chord with seven partials uniformly spaced on the ERBN-number (Cam) scale. In each of the two intervals in a trial, a pure-tone probe was followed by a chord. In one randomly selected interval, the frequency of the probe was the same as that of a partial in the chord. In the other interval, the probe was mistuned upwards or downwards from the "target" partial. The task was to indicate the interval in which the probe coincided with the target. In the "symmetric" condition, the frequency of the mistuned probe was midway in Cams between that of two partials in the chord. This should have led to approximately symmetric activation of the up-FSDs and down-FSDs, such that differential activation provided a minimal cue. In the "asymmetric" condition, the mistuned probe was much closer in frequency to one partial in the chord than to the next closest partial. This should have led to differential activation of the up-FSDs and down-FSDs, providing a strong discrimination cue. Performance was predicted to be better in the asymmetric than in the symmetric condition. The results were consistent with this prediction except when the probe was mistuned above the sixth (second highest) partial in the chord. To explain this, it is argued that activation of FSDs depends both on the size of the frequency shift between successive components and on the pitch strength of each component.