Differential Effects of Practice and Signal Intensity on Temporal Discrimination of Brief and Long Intervals

Two experiments were performed to investigate differential effects of practice and signal intensity on auditory duration discrimination. In Experiment 1, an adaptive forced choice procedure was used to elucidate the effects of practice. Two base durations of either 50 ms and 1,000 ms were employed. As a measure of performance, the difference limen (DL) was computed. Twenty naive volunteers performed on the same task on 5 consecutive days. Results indicate a significant decrease in DL as a function of testing session for the 1,000-ms but not for the 50-ms base duration. In Experiment 2, 48 volunteers were tested with a duration discrimination task similar to the one of Experiment 1, except that the intervals were presented at different levels of intensity. There were three experimental conditions: In the control condition both intervals to be compared within one trial were presented at the same level of intensity (50 dB). In two conditions intervals within one trial were presented at different intensity, i.e. 50 dB and 75 dB. Either the long interval or the short interval was presented at the higher level of intensity. A significant effect of intensity was demonstrated only for duration discrimination in the range of milliseconds, where performance was less accurate for trials in which the short interval was presented at a higher intensity than the long interval. Results of both experiments provide converging evidence for the existence of dissociable timing mechanisms underlying duration discrimination of intervals in the range of milliseconds and seconds. To date, there is a growing body of evidence supporting the notion of distinct timing mechanisms underlying duration discrimination in the range of seconds and milliseconds, respectively (e.g., Rammsayer, 1997; Rammsayer & Lima, 1991). While the processing of longer durations is cognitively mediated, temporal processing of very brief durations appears to be beyond cognitive control and based on basic sensory mechanisms. In the present study, two different experimental paradigms were employed to provide additional experimental evidence for the notion of distinct timing mechanisms.