French philosopher Henri Bergson has a famous quote: "The eye sees only what the mind is prepared to comprehend." Bergson probably meant it metaphorically, but it seems to be literally true according to research by psychologist Martin Rolfs and colleagues. Rolfs studies the role of rapid eye movements in visual perception.
"It turns out, to see one thing, we have to not see many other things. Our brains can't process everything in our field of view, so our visual system has to come up with ways to pick and choose what we perceive." ~ Flora Lichtman
"The visual scene is full of so much information, we cannot analyze everything at the same time...Our results show that shifts of visual attention precede rapid eye movements, improving accuracy in identifying objects in the visual field and speeding our future actions to those objects...And that's actually how you can tell imagery from a real visual experience, because if you move your eyes, things change on the retina. If you move your eyes while you're imagining, while you're hallucinating, nothing changes. So if you want to know if you're dreaming or not, you should just move your eyes or your body." ~ Martin Rolfs, from "Looking at What the Eye Sees," NPR's Science Friday, Feb. 25, 2011
“Hold your eyes still, and the image of a moving target is quite likely to flash by individual photoreceptors in milliseconds. If it moves past very quickly, your photoreceptors won’t have a chance to respond, and you won’t see the target at all. Even if it’s moving quite slowly, the target is likely to be blurred. This is why humans fixate so strongly on objects moving across their field of view — if they didn’t, they wouldn’t be able to see them. If you concentrate on a moving object, the eye will turn smoothly in an attempt to track it. The more you are interested in what you are looking at, the more it will try to hold on. But there is a limit: move your pen too fast, and the image will blur. The moment that happens, the eye will give up its ‘tracking shot’ and revert to ‘angle shots’. It will saccade, snapping from location to location, capturing a selection of ‘stills’.
In French, le saccade, describes the way a sail snaps in the wind. There is nothing subtle about a saccade; nothing delicate. The movement is sudden, ballistic, and powerful enough that the eyeball commonly overshoots its target; a barrage of smaller correcting saccades follows any large eye movement.
The eye exists to detect movement. Any image, perfectly stabilized on the retina, vanishes. Our eyes cannot see stationary objects, and must tremble constantly to bring them into view.”
"The Mind's Eye," from Education in the Digital Age