Speed discrimination for vision and audition
Data from the visual and auditory unimodal conditions were analysed separately and are shown in Figure 2. The data for leftward and rightward directions were very similar and did not differ significantly and were therefore pooled. For each subject, trials were sorted by stimulus speed and mean ‘proportion faster’ judgements were calculated for each level of speed. The mean proportions at each speed for all observers were pooled into a super subject analysis and a cumulative Gaussian psychometric function was fitted. The data were bootstrapped 10,000 times to compute 95% confidence intervals. The mean of the best-fitting cumulative Gaussian was taken as the point of subjective equality (PSE) and the Gaussian’s standard deviation (sigma) provided the speed discrimination threshold. Figure 2a shows the visual and auditory data overlaid for comparison. Data for both modalities show the expected pattern of ‘proportion faster’ judgements rising as stimulus speed increases, confirming that the method of single stimuli used here yields reliable and orderly data for auditory motion discrimination – as previously shown for visual motion37,38.