An explanation of why component contrast affects perceived pattern motion

Component contrast is an essential element in computing spatio-temporal motion energy, and has been shown to bias perceived motion (Thompson, 1982). More recently, Champion, Hammett, and Thompson (2007) concluded that two-dimensional features in the stimulus was the explanation for this motion bias. Here a method was used that eliminated two-dimensional features as the source of the bias. Bowns (1996) showed that Type II plaids shifted from the intersection of constraints direction (IOC) to the vector average direction (VA) as a function of the speed ratio of the components at short durations. It was therefore argued that if the speed of the components could be increased or decreased by varying the component contrast, then this should be reflected in the change from the IOC to the vector average. Perceived direction was markedly affected by contrast. Contrast can bias perceived motion even when two-dimensional features are controlled for, but the source of the bias is not from computing the IOC from motion energy, or by tracking two-dimensional features, but instead is predicted by the Component Level Feature Model developed to be predominantly invariant to contrast.