Figure 2. Unimodal data showing velocity discrimination of a set of velocities around a standard velocity with a geometric mean of 54.03 °/s. (A) The data points show mean ‘proportion faster’ for each level of velocity for all observers pooled into a single super subject and error bars show 95% confidence intervals produced by bootstrapping the data 10,000 times. The continuous lines show the best-fitting cumulative Gaussian psychometric function. (B) Parameters from the best-fitting psychometric functions for audition and vision. The means (left) and bandwidth (right) are plotted with errors bars showing 95% confidence intervals from 10,000 bootstraps. The dashed line shows the geometric mean speed.
Figure 2b compares the PSEs and discrimination thresholds for auditory and visual motion. The PSE effectively indicates the mean perceived speed of the stimulus set and for comparison the dashed horizontal line in 2b shows the geometric mean of the set of speeds used (54.03°/s). While the arithmetic mean speed is 60°/s, velocity is represented logarithmically in cortex (Nover, Anderson & DeAngelis, 2005) and thus the geometric mean is more appropriate. As the confidence intervals show, the PSE for vision (46.625°/s; CI95 = [45.196–47.909]) was significantly lower than the mean stimulus speed and was also significantly lower than the PSE for audition (53.526°/s; CI95 = [52.133–54.867]). This represents a reliable underestimation of speed in vision relative to audition. Moreover, mean perceived speed for auditory motion was surprisingly veridical and did not differ from the geometric mean speed. An interesting feature of the unimodal psychometric functions is the closely matched discrimination thresholds for auditory and visual motion. Motion discrimination is generally considered poor for audition40 yet here the speed discrimination thresholds (i.e., the bandwidth or standard deviation of the psychometric function) for vision (20.488°/s; CI95 = [18.501–22.295]) and audition (20.823°/s; CI95 = [19.174–22.666]) were very similar and not significantly different.