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The human visual system is very good at extracting sudden changes (brightness, motion, etc) from the visual environment. One aid in detecting these changes is adaptation to the unchanging background, in a sense "tuning out" what isn't likely to be useful.
The visual system can adapt to color, brightness, density, orientation, consistent motion, size, etc. The examples below demonstrate adaption to orientation; and spatial frequency. In each case, focus on the bar between the left-hand sets of lines, allowing your gaze to drift along it, for 20-30 seconds. Then transfer your focus to the dot between the right-hand sets of lines. In the orientation case, the straight lines on the right will appear tilted; in the spatial frequency case, the upper set of right-hand lines will look thinner than the identical lower set.
This experiment is not yet available.