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</div>Adapt to either of the side images for about a minute (just look at the horizontal bar) and then look at the middle image. The middle image will appear tilted (the gratings will appear to tilt in the opposite direction from the adapting gratings) if you adapted to the image on the left. The middle image will look to be of two different spatial frequencies (the upper grating will appear coarser and the lower grating will appear finer) if you adapted to the image on the right.