speed. (B) The psychophysical functions from the 6 subjects for a 4u reference stimulus. (C) The cumulative distribution data for a 6u reference stimulus plotted as a function of image speed. (D) The psychophysical functions from the subjects for a 6u reference stimulus. As with the 2u reference stimulus, the amount of variance explained by the simulation for 4u and 6u reference stimuli was quite good (4u reference = 1u test: 80.9%, 3u test: 99.4%, 5u test: 98.7%, 7u test: 95.9%; 6u reference = 1u test: 31.2%, 3u test: 95.3%, 5u test: 99.7%, 7u test: 98.9%). The single outlier (6u reference, 1u test) arises from small variations in the cumulative distribution at these distances, resulting in a slight downward shift of the 1u function. Smoothing the cumulative distribution corrects this anomaly; however, the uncorrected results are presented. Bars in (B) and (D) indicate61 s.e.m. Found at: doi:10.1371/journal.pone.0006771.s002 (0.58 MB TIF) Figure S2 Figure S2. 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Nature 447: 1075–1082. PLoS ONE | www.plosone.org 11 August 2009 | Volume 4 | Issue 8 | e6771