Dear Mr. Alice:
Do you know when you swing a movie camera around fast, you get a blurry picture? I wonder why when I move my head around fast I don't see the same thing. It doesn't make things blurry, even when you're on a roller coaster or in a car going fast. I hope you can explain why that is.
-- Maryjo S., San Diego
How did an observation this sophisticated fall into the M.A. mail bin? The P.O. must be mixing up our letters with Marilyn vos Savant's again. Every time that happens, Marilyn calls the cops thinking she's suddenly being stalked by the dangerously uninformed. But we checked with Dr. Peeper, our eye guy, and it turns out he can answer this one.
A camera records everything that comes through the lens. Our eyes edit what we see. Dr. Peeper assures me that if our eyes were cameras, we'd be falling down and puking all day long from all the dizziness and confusion. Unless we're staring at a fixed point or smoothly tracking an object with our eyes, we view the world in what is known as saccades (sah-cahds). That's a series of rapid jumps from point to point, like riffling through a big pile of snapshots. During a saccade, even though light continues to strike the retina, there's no visual signal to our brains. Movement of the target across the retina signals the need for a saccade, the ocular system hits the saccade button, our eyes suddenly speed ahead, then slow and stop when the target is centered again. Despite all the image-shifting, we "see" the world as continuous and static. There's beaucoup research into how and what we see; according to Dr. Peeper, we look with our eyes, but we see with our brains, which process raw visual signals into a picture of a comprehensible world.
See Here, Matthew...
It seems like 10, 12 years ago, or maybe it was just last month, we got a question about why we don't ever see a smeary, blurred view of the landscape the way a movie camera does. I dutifully explained saccades -- the little jumps our vision makes so that we see the world as a series of snapshots, not a continuous film. Not good enough for Brian Foster, who's spent decades studying vision and has finally found a use for it: bugging me. "You included nothing of the contribution of the middle ear," he sez. There's a difference between your head remaining still with the viewed object moving (the movement of the object across your retina trips the saccade), and the viewed object remaining still and your head and eyes moving. "The inner ear's vestibular canals stimulate saccades in response to head movements.... The loss of vestibular inputs...gives the patient the smearing images."