This pattern contains two fields of drifting dots. One set of dots is moving vertically upwards, and the other set is moving 45 degrees clockwise from vertical. Both sets of dots are visible, and appear to move over each other transparently. Notice that the angular difference in their directions is apparently enlarged, so that the vertically drifting field appears to be moving slightly anticlockwise, and the obliquely drifting field appears to be moving almost horizontally.
This well known effect can be explained by inhibitory interactions between motion detectors tuned to different directions (eg. Mather and Moulden, 1980; Hiris and Blake, 1996). These competitive interactions are thought to play a role in motion integration.