Spatial Frequency Adaptation
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.
The current conventional wisdom to understand this phenomenon is based on spatial frequency channels.
To obtain a more detailed explanation of why adapting to the two gratings on the left makes the ones on the right appear unequal click on more detailed explanation.
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