The effects of an occluding aperture on perceived direction of motion. In this didactic illustration, the linear object in the aperture is moving horizontally from left to right (yellow arrow). However, when seen through the aperture, the line is perceived to be moving downward to the right (red arrow). A particularly impressive way to experience the effect of an aperture is to create a sequence in which an aperture like the one shown here can be applied or removed from the same moving line stimulus translating horizontally from left to right. In the absence of the aperture, the line is of course seen as moving horizontally from left to right; the moment the aperture is applied, however, the direction of movement changes. See for yourself demonstrations of this effect in apertures with different shapes.