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Figure 2-24. The stepping feet illusion, with the striped background
Even though they're still moving in the same direction, the blocks now appear to be alternately jerking forward, like little stepping feet. Like a lot of illusions, the effect is stronger in your peripheral vision; fix the center of your gaze at the cross off to the side and the stepping feet will be even clearer.
How It Works
The easiest way to see why the stepping feet occur is to look at the same pattern, but without any colorthe yellow becomes white and the blue becomes black. Michael Bach's animation of stepping feet (http://www.michaelbach.de/ot/mot_feet_lin; Flash) allows you to remove the color with a click of the Color Off button.
With no color, there's no illusion: the moving blocks appear like stepping feet even when you look straight at them. When the black (previously blue) block overlaps a black stripe, you can't see its leading edge so it isn't apparent that it's moving. Given no cues, your motion processing centers assume no movement. Then as the black block begins to move over a white stripe, you can suddenly see the leading edge again, and it's moved from where you brain had thought it was. That's when you see the block apparently jump forward and then move normallyat least until it overlaps the black stripe again. The same is true for the white (previously yellow) block over white stripes, only it moves when the black block looks still and vice versa.
So that's what the blocks look like in black and white. Losing the movement information of the leading edge over one stripe in two makes the blocks look like stepping feet. And that's what the motion-sensitive and color-insensitive magnocellular pathway sees. The color information is added back in only later, reattached in the visual cortex after the motion has been computed. In the end, you're able to simultaneously see the stepping feet motion via one pathway and the colors via the other.
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End Notes
1. Anstis, S. M. (2003). Moving objects appear to slow down at low contrasts. Neural Networks, 16, 933-938.
2. Thiele A., Dobkins, K. R., & Albright, T. D. (2000). Neural correlates of contrast detection at threshold. Neuron, 26, 715-724.
See Also
· Stuart Anstis's publications online (http://psy.ucsd.edu/~sanstis/SAPub.html).
· Anstis discusses the effect of contrast on motion perception (http://psy.ucsd.edu~sanstis/PDFs/YorkChapter.pdf).
Shading in pictures combined with the continuous random jiggling our eyes make can generate compelling movement illusions.
We've all seen optical illusions in which parts of a completely static picture appear to drift and swirl. One of the most famous examples is Professor Akiyoshi Kitaoka's rotating snake illusion (Figure 2-25), commonly passed around via email, but, sadly, rarely with explanation.
Figure 2-25. The rotating snake illusion, Akiyoshi Kitaoka © 2003, is available in color at http://www.ritsumei.ac.jp/~akitaoka/index-e.html
This is really a story about why you don't see everything moving all the time rather than about why you see movement sometimes when it isn't there. Your eyes constantly move in your head [Hack #15], your head moves on your body, and your body moves about space. Your brain has to work hard to disentangle those movements in incoming visual information that are due to your movement and those due to real movement in the world.