My off-cuff shot at explaining the spiral-illusion.
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- My fuller comments on the stimulus.
"This is a complicated stimulus! And that makes it even difficult to characterize the full extent of what is being misperceived. ...much less why.
But, to the extent that one is referring to the fact that one perceives the circles as spirals, here's a plausible shot. Although each ring of squares is a circle, the tilt of the squares is consistent with a spiral, providing conflicting cues to your visual system. Alternating the color of the squares from black to white reduces the salience of the larger-scale circularity of the stimulus, so that the local consistent-with-a-spiral tilt cues is winning. (Notice that squinting eventually kills the illusion, in part because the tilts go away, but also in part because the difference in luminance contrast of the black and white squares is muted.) Once your visual system guesses that they may actually be spirals at the larger scale, it actually creates a perception of it being that way.
Also, in thinking through stimuli such as this, it's easy to think only in 2D. But your (unconscious visual) brain doesn't think in 2D. When it's trying to make sense of complex stimuli, it places its money on 3D scenes it might actually be standing in front of. So, when your brain here decides that those rings are actually spirals, it's probably not deciding they're spirals lying within a flat surface in front of you. Instead, spirals in real life are more often due to something going in a circle while simultaneously changing in distance from you. For example, imagine walking through a tube, and there are stripes painted down the side. But instead of the stripes going straight down the tunnel, they twist around. Looking down the tunnel, you'll see spirals. If some twist in opposite directions, you'll see spirals that cut through one another (which is what's perceptually going on in this illusion). I'm not suggesting that tubes are natural, but it is an illustration of a more natural occurrence where something is visible going off into the distance, but rather than it staying at some particular angle relative to you, tends to revolve in angle in this way.
Not only will you see spirals in such a tube, but when you move forward, they'll spin around you. In a stimulus where your brain is being tricked into seeing spirals, then if your brain thinks the spirals are going off into the distance (rather than being flat on a surface in front of you), looming toward the center of the stimulus should create some spinning in the spirals, because that's what would happen in the next moment if you really were in front of the 3D scene your brain guesses is out there. That's an example of perceiving-the-present. Note that this does indeed happen here (although not nearly as strongly as found in this great illusion -- http://www.perrorist.com/images/PinnaBrelstaff.jpg ), for when you loom forward toward the center, some of the rings rotate a little."Sep 11, 2012
- ah,... just see your post nowSep 11, 2012
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