Our theory assumes that (1) the bar's lateral edges and (2) the edges in the background both influence the perceived speed of the leading edge. A study by Anstis (
2003a) supports the first assumption. In this investigation, Anstis considered the perceived movement of a graded-contrast line that in reality moved vertically downward. When the contrast of the ends of the line was larger than the contrast of the sides of the line (
Figure 4a), the motion signals from the ends dominated the perceived motion of the line, and hence, the bar's true downward motion was perceived. However, when the contrast of the ends was smaller than the contrast of the sides (
Figure 4b), the motion signals of the sides dominated the percept. The situation was therefore analogous to the motion aperture problem (Marr,
1982; Wallach,
1935; Wuerger, Shapley, & Rubin,
1996), which occurs when a bar is viewed through an aperture so that its ends cannot be seen and, thus, cannot contribute to the perceived motion of the bar (
Figure 4c). As in the aperture problem, the line in
Figure 4b was perceived to move orthogonally to its orientation. Anstis therefore provided direct evidence that, consistent with our theory, the sides of a bar contribute to the bar's perceived motion and that the degree to which the sides influence the motion percept is determined by the luminance contrast of the sides relative to the luminance contrast of the ends. We note that this finding is consistent with a number of other studies (Castet, Lorenceau, Shiffrar, & Bonnet,
1993; Chey, Grossberg, & Mingolla,
1997; Lorenceau, Shiffrar, Wells, & Castet,
1993).