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.