The role of excitation-pattern cues and temporal cues in the frequency and modulation-rate discrimination of amplitude-modulated tones.
These experiments examine the influence of excitation-pattern cues and temporal-fine-structure cues on frequency difference limens (FDLs) measured as a function of duration. In the first three conditions, listeners were required to detect a change in carrier frequency from a baseline of 250 or 2000 Hz, for stimuli with half-amplitude durations ranging from 5 to 320 ms. In the "steady" condition, duration was manipulated by increasing the steady-state portion of the envelope between two 5-ms linear onset and offset ramps. This resulted in spectra and excitation patterns that broadened with decreasing duration. In the "modulated" condition, the carrier was amplitude modulated with a triangle function (period 10 ms) and duration was manipulated by varying the number of cycles of the modulator. In this case, the spectral envelope did not vary with duration, but the width of individual spectral lobes broadened with decreasing duration. The "low-peak-constant" condition was similar to the modulated condition, except that an increase in carrier frequency was accompanied by a decrease in the period of the modulator, so as to hold constant the frequency of the spectral lobe located roughly 100 Hz below the carrier frequency. In this condition, changes in carrier frequency resulted in minimal changes in excitation level on the low-frequency side of the excitation pattern, but changes on the high-frequency side were larger than for the first two conditions. Both the values of the FDLs, and their variation with frequency and duration, were similar in these three conditions. The fourth, "modulator varying," condition differed in that the carrier frequency was held constant and listeners were required to detect a change in modulator frequency. Thresholds were expressed as the change in frequency of the spectral side lobes adjacent to the carrier frequency ("equivalent" FDLs). Excitation-pattern cues in this condition were at least as large as in the first three conditions. However, equivalent FDLs were significantly higher than for the other three conditions. The higher FDLs are attributed to the lack of temporal fine-structure cues related to the carrier frequency. Overall, the results suggest that FDLs in the first three conditions were determined by temporal cues rather than by excitation-pattern cues. The increase of the FDLs with decreasing duration did not arise from increasing spectral splatter.