Factors affecting footsteps: contrast can change the apparent speed, amplitude and direction of motion

Abstract

Contrast can affect the apparent speed of a moving stimulus [P. Thompson, Ph.D. thesis, University of Cambridge, Cambridge, UK, 1976; Vis. Res. 22 (1982) 377; Perception 28 (1999) 33]. Specifically, when a grey square drifts steadily across stationary black and white stripes, it appears to stop and start as its contrast changes––the so-called `footsteps illusion' [Perception 30 (2001) 785; Neural Networks 16 (2003a) 933; S.M. Anstis, Levels of motion perception, in: L. Harris, M. Jenkin (Eds.), Levels of Perception, Springer, New York, 2003b, p. 75]. We now show that what matters is the contrast of the leading and trailing edges, not of the lateral edges. The stripes act by altering the stimulus contrast, and are not merely stationary landmarks. Back and forth apparent motion appears smaller in amplitude at low contrasts, even on a spatially uniform (non-striped) surround, and this is a specific motion phenomenon, not a result of misjudging static position. Contrast also affects the perceived direction of a moving stimulus. A vertically jumping grey diamond on a surround of black and white quadrants appears to change its direction of movement depending on the relative contrast of its left-oblique versus right-oblique edges against the surround. Thus, the perceived direction, amplitude and speed of moving objects depend greatly on their luminance contrast against the surround. A model of motion coding is proposed to explain these results.