This pattern was created by superimposing two drifting gratings, one moving downwards and the other moving leftwards. Here are the two components displayed side-by-side.
When superimposed, the two gratings appear to form a single 'plaid' pattern, drifting rigidly. Adelson and Movshon (1982) argued that the apparent direction of plaid motion can be explained as follows. First, the two grating components generate individual responses in low-level motion detectors. Second, during motion integration the visual system puts together the two component signals to derive a common directional signal for the plaid. The apparent direction seen in plaid patterns does not always agree with that predicted by Adelson and Movshon's explanation. Others have argued that plaid apparent motion can be explained in terms of the movement of the 'blobs' in the pattern, without reference to signals from the two components.
Research on plaid motion is still very active (see, for example, Kim and Wilson, 1993).