Perceiving Motion Empirically Figure 6. Predicting the psychophysical results elicited by image sequences traversing different distances on the image plane at different speeds but having the same percentile rank. (A) A 2u reference stimulus with an image speed of 6.5u/s (dark blue arrowhead on the abscissa) has a percentile rank at the 76th percentile (black arrowhead on the ordinate). If our hypothesis of motion perception is correct, then test stimuli of 1u, 3u, 5u, and 7u with the same rank should generate perceptions of the same speed, despite their different actual speeds on the image plane (indicated by the other colored arrowheads along the abscissa). (B) The cumulative distribution data from (A) are re-plotted to indicate the predicted motion percepts elicited by the various test stimuli matched to a 2u reference stimulus as a function of the reference image speed. (C) Psychophysical functions produced by the 6 observers for test stimuli relative to the speed of the 2u reference stimulus (dashed blue line). The perceived speed reported for each test stimulus is plotted as a function of the image speed of the reference stimulus, as in (B). Bars indicate61 standard error. doi:10.1371/journal.pone.0006771.g006 an image speed of 6.5u/s traversing distances of 1u, 2u 3u, 5u, and 7u on the retina (Figure 7A). If relative rank determines the speed- distance effect, then stimuli moving over different projected distances with the same image speed should appear to be moving at different speeds. Thus, when a projected image is moving at 6.5u/s, the motion perceived should appear slower when traversing 2u than when traversing 1u; conversely, the same image speed should appear faster when traversing 2u than when traversing larger projected distances (e.g., 3u, 5u and 7u). As shown in Figure 7B, these predictions are also borne out. PLoS ONE | www.plosone.org 6 August 2009 | Volume 4 | Issue 8 | e6771