Change blindness is a fascinating phenomenon in which people do not notice even significant changes in an image they are viewing, as long as the change itself occurs out of view. Our visual processing is sensitive to changes that occur in view, but major changes to a scene can occur from one glance to the next without our noticing in many cases.
(See [this] color changing card trick for an example.)
One group of researchers believe they have a working hypothesis as to why our brains might have evolved in this way. Their idea is that the visual system will essentially merge images over a short period of time in order to preserve continuity – a process they call the continuity field. In essence our brains are sacrificing strict accuracy for perceived continuity.
This is in line with other evidence about how our brains work. Continuity seems to be a high priority, and our brains will happily fill in missing details, delete inconsistent details, and even completely fabricate information in order to preserve the illusion of a continuous and consistent narrative of reality.
Visual continuity is important because otherwise the world would appear jittery to us, constantly morphing as shadows play across an object, or our angle of view changes. This could be highly disruptive and distracting.
The researchers also point out that in the real world objects are fairly stable. They don’t pop in and out of existence, or morph into other objects. So not being perceptive to such changes would not be a big sacrifice and would not be likely to affect fitness. If something is actually moving or changing in our visual field we are very sensitive to that, and our attention will be drawn to it.
Neuroscientists, however, can contrive all sorts of impossible scenarios in order to probe our processing of sensory information. We did not evolve with video or photography, but researchers can use this technology to test how our brains process information.
They also give real world examples, such as the movies. There are often continuity errors in movies, missed by the vast majority of movie-goers.