A population study of the precedence effect

Data are reported from a population of untrained individuals under lag- and single-click conditions in a discrimination suppression precedence-effect task. The cue to be discriminated was an interaural level-difference (ILD). Each of 91 observers completed 10 runs in a two-interval forced-choice design under a lag-click condition and three runs under a single-click condition. Stimuli were 125-micros rectangular pulses and the interclick interval was 2 ms. Observers were randomly assigned to three groups of approximately 30. Each group was then tested at one stimulus intensity (43, 58, or 73 dB). Mean threshold within each group was greater than 15 dB for the lag-click condition and 6 dB for the single-click condition, although there was substantial interobserver variability. In contrast to [J. Acoust. Soc. Am. 114 (2003) 420] who reported a strong effect of intensity on lag-click ITD discrimination, no effect of intensity was observed on lag-click ILD thresholds. Analysis of over 50,000 near-threshold trials from 302 observers pooled across studies showed a spatial asymmetry in response patterns and a small, but statistically significant effect of gender. A model is proposed which shows that decay of sensory memory and increases in auditory filter bandwidths with intensity may predict the different findings for ILD versus ITD lag-click thresholds.