What determines the perceived speed of dots moving within apertures?

Whereas it is a well known fact that objects appear to move faster in small er 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 movin g behind small apertures. In these experiments we varied the size and the s hape of the aperture and its location in the visual field, as well as the s timulus duration. We report that the field-size effect does not depend on t he overall duration of the stimulus, which does influence the typical path length of individual dots in the display. It is, however, affected by the s hape 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 furtherm ore depends on the location of the stimulus in the visual field. Our combin ed results are best described as an increase in perceived speed that is con sistently elicited when a motion sink, i.e. a boundary of disappearing dots , is located close to the fovea. Such a description of the relevant stimulu s parameters is discussed with respect to possible high-level mechanisms, r elating back to classic Gestalt psychology explanations of the field-size e ffect, and with respect to well-known aspects of neuronal processing that m ay underlie speed perception and motion integration.