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. The two sets of dots 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 to the right.
This well known effect can be explained by inhibitory interactions between motion detectors tuned to different directions.
See:
Mather, G. & Moulden, B. (1980) A simultaneous shift in apparent direction: evidence for a distribution-shift model of direction coding. Quarterly Journal of Experimental Psychology, 32, 325-333.
Benton, C.P. & Curran, W. (2003) Direction repulsion goes global. Current Biology, 13, 767-771.