In short, change blindness is an issue of attention and representation. If we fail to represent an object in a scene either before or after a change, then we won’t notice the change, and we tend not to represent objects that aren’t important to the meaning of what we’re looking at, because we’re just not paying attention to them (though paying attention to them doesn’t guarantee representation).
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At this point, you’re thinking, “This is insane! I would notice if the stranger I was talking to suddenly looked like a completely different person!” I thought the same thing, and we’re not alone. In fact, this disbelief is so common that it has its own name: change blindness blindness.