Slashdot Mirror
It's Surprisingly Hard To Notice When Moving Objects Change
An anonymous reader writes "Scientists at Harvard have found that people are remarkably bad at noticing when moving objects change in brightness, color, size, or shape. In a paper published yesterday (PDF) in Current Biology, the researchers present a new visual illusion that 'causes objects that had once been obviously dynamic to suddenly appear static.' The finding has implications for everything from video game design to the training of pilots."
IAAVN (I am a Visual Neuroscientist). It's a compelling illusion. I have not read the original paper, but will speculate nevertheless in true Slashdot fashion. The change that's perceived before the ring rotates is not so much due to the colors changing -- if you pay close attention -- but something that's called apparent motion. The classic example of apparent motion is the sequencing of lights around a movie marquis -- they appear to move, although the lights themselves are not actually moving. In the same way, the static ring has internal apparent motion as the colors change, because your brain is interpreting, for example, one dot turning yellow next to a dot that was previously yellow, as motion of a yellow dot, even though the underlying dots do not move. While apparent motion can be very strong, it is not the same as true motion.
Then, when the ring starts to rock back and forth, there is a true motion signal that swamps the apparent motion. If you pay attention to a given dot while holding your gaze still fixed at the central white point (not as hard as it sounds), you can clearly still see the colors changing.
So without having read the paper, I reserve some skepticism that they have not actually measured what they think they have. Change is still perceptible, but it would seem that real motion interferes with apparent motion.
Put my fist through my alarm clock with its ding-dong death inside my ear. - The Blackjacks.
That depends on what you call a processor. The eyes do a bit of preprocessing - the raw output of the rods and cones isn't fed directly into the optic nerve; intermediate cells inhibit and excite each other, altering the image in-eye. One example of retinal filtering is how Mach bands are created: Lateral inhibition between the cells causes edges to appear more pronounced than they actually are. The brain is not involved.
(Essentially, the more light one cell receives, the more its neighboring cells are inhibited. At an edge between a light and a dark area, a "light" cell close to the edge will receive less inhibition than entirely bright-surrounded cells due to its "dark" neighbors and thus the light side of the edge will be perceived as brighter. Conversely, the "dark" cells closest to the edge will receive stronger inhibition than other "dark" cells due to their "bright" neighbors, causing that side of the edge to appear darker.)
USE HOT GRITS WITH STATUE OF NATALIE PORTMAN (NAKED AND PETRIFIED)
Uh, did you read the Wikipedia link you posted? "Cone cells are densely packed in the fovea, but gradually become sparser towards the periphery of the retina." They aren't located only in the fovea, but all across the retina. They're merely more densely packed in the fovea than towards the edge. What most likely happened to your classmate is cone bleaching: the longer you stare at a particular image, the more the particular cones bleach their photoreceptors, and the harder it is to figure out the correct color. Depending on what color the chalkboard was, it's quite possible he simply had stared at it for too long.
The retina still perceives color at the edges, it just does so less effectively than if you focus on the center.
Those who can, do. Those who can't, sue.