What determines the perceived speed of dots moving within apertures?

Whereas it is a well known fact that objects appear to move faster in smaller stimulus fields, the reason for such a misjudgement of speed is still a matter of debate. We present four experiments to characterise the stimulus parameters that are important for the apparent speed increase of dots moving behind small apertures. In these experiments we varied the size and the shape of the aperture and its location in the visual field, as well as the stimulus duration. We report that the field-size effect does not depend on the overall duration of the stimulus, which does influence the typical path length of individual dots in the display. It is, however, affected by the shape of the aperture in such a way that the aperture size along the motion path is crucial for the speed misjudgement. The field-size effect furthermore depends on the location of the stimulus in the visual field. Our combined results are best described as an increase in perceived speed that is consistently elicited when a motion sink, i.e. a boundary of disappearing dots, is located close to the fovea. Such a description of the relevant stimulus parameters is discussed with respect to possible high-level mechanisms, relating back to classic Gestalt psychology explanations of the field-size effect, and with respect to well-known aspects of neuronal processing that may underlie speed perception and motion integration.