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The Rotating Snakes Are All In Your Mind

Story courtesy of NPR:

Like a detective, the vision scientist knows how to recognize clues, especially when they’re staring him in the face. Says Backus:

  • “The first thing to notice about Rotating Snakes is that the motion grinds to a halt if you stare at just one part of the image. On the other hand, it keeps going if you keep looking around. So eye movements are important. However, the illusory motion is not actually caused by the motion of the image across your retina. Instead, what matters is that the image be at different positions on the retina from time to time. In fact, a briefly flashed image of Rotating Snakes appears to rotate like crazy, even if there’s no time to move your eye across the image.”

This is a first clue to what’s driving the illusion of motion. Here’s another:

  • “Notice that any given disk within the image always rotates in the same direction. Kitaoka, who created this version of the illusion, has included both clockwise and counter-clockwise rotating disks, but if you pay attention, you will notice that each disk in the image is faithful to its own direction of rotation. The illusory motion is always in the same direction as this progression of colors: black, blue, white, yellow.”

And that’s a second clue. So the illusion has something to do with the visual processing that occurs when the image first hits the retina at a particular location, sending signals to the brain’s visual cortex, and also with the progression of colors (black, blue, white, yellow), which determines the direction of rotation. These clues tell us something about the visual processes that give rise to the illusion, but what?

There are (at least) three important facts about the visual system at work in this illusion. Backus explains:

  • “The first fact is that neurons fire very rapidly whenever the image on the retina changes, then quickly slow down. You might think this would cause the image to fade, but that’s not what happens. The fast initial firing rates allow visual perception to be fast. Then neurons go into ‘maintenance mode’ until the image changes again. This decrease in firing rate has been called ‘adaptation’ in the sense that neurons ‘adapt (stop responding) over time, but it’s better to think of it as an efficient coding scheme that saves energy by not making neurons fire more than necessary.”

One implication of this fact is that when Rotating Snakes first projects onto a particular location on the retina, there’s a lot of neural activity. That activity is highest for the regions with the highest contrast (black and white), and lower for regions with moderate contrast (blue and yellow). The activity for the high contrast regions also dies down (i.e., adapts) at a faster rate than the activity for the moderate contrast regions. The resulting differences in firing and adaptation turn out to be crucial:

  • “The difference in the rate at which neurons adapt in the black vs. blue regions, and in the white vs. yellow regions, causes a shift in the location of peak neural activity from black towards blue, and from white towards yellow. There could still be more activity at black and white than there is at blue and yellow, but the ratio changes, and this causes movement in the location of the center of mass of the neural activity, which is detected by motion mechanisms.”

This shift in the ratio of activity across the image generates the perception of motion, even though the sharp edges of the colored sections are decidedly static:

  • “The second fact is that the visual system decomposes images into separate representations at a variety of spatial scales, from coarse to fine. Motion is measured separately within each of these representations before they are recombined. In Rotating Snakes, we see motion even though the fine details (the crisp edges) aren’t moving, because motion detectors at intermediate spatial scales get activated.”

In other words, it’s predominantly sensors at a coarser scale that generate the illusion of motion, and they do so despite the fact that motion sensors at a finer scale are mostly keeping mum.

A final fact about the visual system helps explain why we seem to perceive rotating disks, and not individual, shifting patches:

  • “The third critical fact is that large scale ‘global’ motions, like disk rotations, have their own separate detectors at a secondary stage of processing within the visual system. These detectors look for larger patterns of motion in the image, and they are very sensitive. Thus, a small amount of illusory motion at many different places in the disk can cause the entire disk to rotate — provided the motion is consistent throughout the disk. There’s hardly any illusory rotation in a disk if some of it is colored black-blue-white-yellow in the clockwise direction, and some of it is colored in the opposite, counter-clockwise direction.”

And that, in a nutshell, is how some cleverly arranged blobs of color can generate a compelling (though illusory) sense of motion.

Read the full article here.
Source:
The Rotating Snakes Are All In Your Mind
Tania Lombrozo, March 24, 2014
Retrieved, January 20, 2017 from http://www.npr.org/sections/13.7/2014/03/24/293740555/the-rotating-snakes-are-all-in-your-mind

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