The left demo above shows a horizontal and a vertical grating, superimposed to form a ‘plaid’. The combined checkerboard pattern appears to move in a diagonal direction. One grating also masks the visibility of the other, making it harder to detect changes in its contrast. This masking effect is though to occur at an early stage of visual processing. These things are already known.

We found two new things (Freeman & Verghese, 2009

). Firstly the strength of this masking is greatly enhanced for drifting versus static gratings. Secondly, one grating is made larger (see right demo), it becomes possible to see the each grating’s own horizontal and vertical motion again, and masking for the smaller grating is eliminated.

This illusion demonstrates that cues influencing scene segmentation can modulate contrast masking. Motion and surround cues seem act in opposite directions: common motion binds the components together, enhancing masking, while context helps to peel them apart again, thus relieving masking.

Such segmentation might help explain how, while driving in bad weather, we are still able to see the road safely through a wet and muddy windscreen and sweeping windscreen wipers.