Perceiving Motion Empirically Figure 7. Predicting the psychophysical results elicited by image sequences traversing different distances on the image plane at the same speed but having different percentile ranks. (A) Stimuli moving across different distances on the image plane at a particular speed have different percentile ranks. For example, test stimuli traversing the image plane at a speed of 6.5u/s (black arrowhead on the abscissa) have ranks that range from the 65th to the 82nd percentile (colored arrowheads on the ordinate). If motion percepts are generated empirically, then the same image speed should be perceived as slower when traversing distances of 3u, 5u, or 7u in comparison with a 2u reference, but faster when traversing a test distance of 1u. (B) The blue curve indicates the projected speeds at which test stimuli traversing different projected distances (1u, 3u, 5u, and 7u) appeared the same to observers as an image speed traversing 2u at 6.5u/s (data re-plotted from Figure 6C). The area below the curve represents image speed-distance combinations perceived as slower than the reference stimulus, whereas the area above the curve represents combinations perceived as faster than the reference stimulus. Thus, test stimuli presented at 6.5u/s (dashed horizontal line) traversing distances of 3u, 5u or 7u (dashed vertical lines) are seen as moving more slowly than a 2u reference stimulus traveling at the same speed, whereas test stimuli traversing 1u at 6.5u/s are seen as moving faster. doi:10.1371/journal.pone.0006771.g007 PLoS ONE | www.plosone.org 7 August 2009 | Volume 4 | Issue 8 | e6771