Why do we see one thing in more than one way?
*Spinning ballerina. By now, almost everyone has seen the spinning ballerina illusion (aka the silhouette illusion, first created by Nobuyuki Kayahara), in which the dancer can be seen spinning either clockwise or counterclockwise. Here's a great version that can help you see it either way essentially on command: dancing ballerina / spinning dancer optical illusion made easy.
*It's not a personality test, or a test of hemispheric dominance. Although various folks on teh internetz have suggested the direction you see the ballerina spin is a test of some aspect of your personality or the balance between right and left brain processing, those folks are just plain wrong. It's really just an indication of how the visual system solves problems in the face of ambiguity - what we do when the solution to a problem is under-constrained.
*What makes stimuli ambiguous? By the very nature of our visual systems and the world around us, many 2D representations of the world are ambiguous - meaning that the visual information received that projects onto our retinas can be interpreted by our brains in more than one way. The images you see on a computer screen really only have 2-dimensions, but silhouettes, like the spinning ballerina, are ambiguous because they don't contain the same occlusion cues as would a real ballerina spinning in the real world. IRL, we can tell which arm is in front and which is behind, but in a silhouette, that information is missing. So our brains flip back and forth between the two possible interpretations. This version of the spinning ballerina illusion introduces explicit occlusion cues into the stimulus, which enables you to see motion in either direction quite easily: dancing ballerina / spinning dancer optical illusion made easy.
*What makes the sphere ambiguous? As in the spinning ballerina illusion, there are no perfect occlusion cues in the +s and •s that make up the sphere. If you see the +s in front, you'll perceive the sphere rotating to the left. If you see the •s in front, you'll perceive the sphere rotating to the right. Of course, there is a bias to see the +s in front because they are larger than the •s, so you might tend to see leftward motion.
*What factors affect perceived direction in ambiguous motion stimuli? A number of factors can determine whether you'll see the sphere (or the ballerina) moving in one direction or another, including neuronal adaptation, occlusion cues, temporal transients in the visual system, priming, and attention. Although most people have a bias to see motion to the left (as in the spinning ballerina illusion), you may be able to change the perceived direction of motion by directing your attention to the •s, and "pulling" them to the front - if you can modulate your percept via attention, you'll be able to flip the direction of the sphere back and forth. A good example of how to do this in the spinning ballerina illusion is shown in
's terrific website, where he provides visual markers for the eyes to help your attentional system along (http://goo.gl/EvpDRS). Other factors, like blinking (short interruptions of the display), can lead to alternations in the perceived direction as well, an effect nicely illustrated on Richard van Wezel's website (http://goo.gl/F3Fya6).
*Motion is not the only ambiguous percept Of course, even casual observers of psychology and neuroscience know that our brain can see ambiguity even in static stimuli, like the Necker Cube (http://goo.gl/SCDgJo), Rubin's Face/Vase Illusion (http://goo.gl/Nf6Ax3), or even our interpretation of objects with shadows (http://goo.gl/tNU3e6). The fact that we can interpret some objects in more than one way isn't a failure of our visual systems, it's a sign of the flexibility and power of our brains to make the most out of impoverished information in the real world.
In addition to getting more information on ambiguous percepts at the websites linked above, you can read a bit more about some recent scientific studies in the field in these open source articles:
Visual working memory contents bias ambiguous structure from motion (2013) Scocchia L, Valsecchi M, Gegenfurtner KR, Triesch J. PLoS One, 8(3):e59217. doi: 10.1371/journal.pone.0059217
Ambiguous figures - what happens in the brain when perception changes but not the stimulus (2012) Kornmeier J, Bach M. Frontiers in Human Neuroscience, 6:51. doi: 10.3389/fnhum.2012.00051
The role of attention in ambiguous reversals of structure-from-motion (2012) Stonkute S, Braun J, Pastukhov A. PLoS One, 7(5):e37734. doi: 10.1371/journal.pone.0037734.
United we sense, divided we fail: context-driven perception of ambiguous visual stimuli (2012) Klink PC, van Wezel RJA, van Ee R. Philosophical Transaction of the Royal Society:B Biological Sciences, 367(1591): 932–941. doi: 10.1098/rstb.2011.0358
And, for a super cool double-illusion, combining a Necker Cube with an anamorphic display, check out this video I shared a while back: http://goo.gl/i5icSO
image credit: / Davidope - who makes some of the world's most beautiful animated gifs. You can see his work at http://dvdp.tumblr.com/ and read more about him at (http://goo.gl/E5hgUh)
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