1. Look at the two gratings on the right side of the figure. Do you see any differences between them?

2. Now for about 60 seconds scan your gaze back and forth along the horizontal red bar between the two gratings on the left.

3. When you are finished scanning back and forth along the horizontal red bar, fixate on the short red bar between the gratings on the right.

4. What differences do you see between these two gratings on the right compared to the first time you looked at these gratings?

 

explanation

The two gratings on the right, when you looked at them the first time, undoubtedly appeared identical. When you scanned the red bar between the two gratings on the left you allowed your visual system to adapt to these gratings. The adaptation was not the same for the top and bottom grating because they differ in spatial frequency. The one on the bottom has a higher spatial frequency than the one on top. Then after adapting to the gratings on the left for approximately 60 seconds the ones on the right if viewed while fixating between them on the red square probably no longer appear identical. This non-identity will not last for too long.

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