This optical illusion is called Simultaneous Brightness Contrast, and the dot appears to become brighter as it is moved up and darker as it is moved down, even though its color never changes. Surprisingly, we still don’t know what causes it - all we know is that somehow, somewhere in the brain, we forfeit reality for exaggerated contrast difference. Recent studies have suggested that we may be born to make this mistake, and it makes sense. It may be an error, but it may also help us more quickly distinguish features in the environment. Natural selection may have favored this over accuracy because sometimes it’s more important to live than to see reality as it is. Thank you.