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.
"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." explains Martin Rolfs in this short video.