Dynamic frequency change among stimulus components: effects of coherence on detectability.

Experiments were performed to determine the effect of coherent dynamic frequency change among stimulus components on detection of one of those components. Detectability of a frequency glide signal, centered at 660 Hz, was measured in the presence of two additional frequency glides centered at 220 and 440 Hz. Results show that the signal is most difficult to detect when it is changing coherently with the other stimulus components, and that detectability gradually improves as the frequency change among stimulus components becomes less coherent. A control experiment indicated that detectability of the signal is influenced somewhat by the average spectral distance between the signal and the other stimulus components. In order to separate the effects of dynamic frequency change from those of altering harmonic relationships, the experiment was repeated with stimulus components not harmonically related. Results are similar in pattern, though with somewhat smaller shifts. A final experiment was performed to determine if the observed shifts in detectability might be due to changes in the maximum spectral distance between signal and masker when their frequencies are changing incoherently. Results of this experiment indicate that detectability improves when frequency change of signal and masker is not coherent, even when maximum spectral distance does not change. These data indicate the likelihood that the auditory system is able to use coherent dynamic frequency change among stimulus components.