Discrimination of frequency steps linked by glides of various durations.

Thresholds were measured for detecting steps in frequency linked by glides of various durations. The goals were to assess the relative importance of place and temporal information for this task, and to determine whether there is a mechanism for detecting dynamic frequency changes per se, as opposed to comparing the initial and final frequencies of the stimuli. Subjects discriminated a 500-ms sinusoid of constant frequency from a sinusoid with three parts: an initial part with constant frequency, a downward frequency glide, and a final part with constant frequency. The overall duration was 500 ms, and the glide duration was varied from 5 to 500 ms. In one special case, the portion of the stimuli when a glide might occur was replaced by a brief silent interval. The center frequency was fixed at 0.5, 1, 2, 4, or 6 kHz (condition 1), or varied randomly from one stimulus to the next over a 4-ERB range around the nominal center frequency (condition 2). The randomization impaired performance, but thresholds remained lower than the best that could be achieved by monitoring either the initial or final frequency of the stimuli. Condition 3 was like condition 2, but for each stimulus a glide in level was added at the time when a frequency glide might occur, so the initial and final levels differed; the glides in level varied randomly in extent and direction from one stimulus to the next over the range +/- 20 dB. This impaired performance, but thresholds remained lower than the best that could be achieved by monitoring changes in excitation level on only one side of the excitation pattern. Excitation-pattern models of frequency discrimination predict that thresholds should not vary across center frequency when expressed as the change in ERB number, delta E. For all conditions, delta E values increased at 6 kHz, suggesting a role for temporal information at lower frequencies. The increase was smallest for the longest glide duration, consistent with a greater relative role of place information when there was no steady state portion. Performance was better when a brief glide was present than when no glide was present, but worsened with increasing glide duration. The results were fitted well by a model based on the assumption that information from the steady parts of the stimuli (perhaps extracted mainly using temporal information) was combined with information from the glides (perhaps extracted mainly using place information).