Precedence-effect thresholds for a population of untrained listeners as a function of stimulus intensity and interclick interval.

Data are reported from 127 untrained individuals under lag- and single-click conditions in a precedence-effect task. In experiment I, each subject completed ten runs in a two-interval forced-choice design under a lag-click condition and three runs under a single-click condition. The cue to be discriminated was an interaural time difference (ITD). Stimuli were 125-micros rectangular pulses and the interclick interval (ICI) was 2 ms. Subjects were randomly assigned to three groups of approximately 30. Each group was tested at one stimulus intensity (43, 58, or 73 dB). Mean threshold within each group was greater than 500 micros for lag-click ITD conditions, although substantial intersubject variability and a clear effect of stimulus intensity on lag-click ITD thresholds were observed, with lower thresholds for higher intensities. In experiment II, the ICI was varied from 0.3 to 10 ms, and thresholds were obtained from groups of approximately 20 untrained subjects. Data were also collected from three highly experienced observers as a function of ICI. The best naive subject produced mean thresholds near, but not as low as those obtained from experienced subjects. Analysis of adaptive-track patterns revealed abrupt irregularities in threshold tracking, consistent with either losing the cue or listening to the wrong cue in an ambiguous stimulus.