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.

Low-contrast patterns in general produce a less vigorous response from the motion-sensitive parts of the brain,² which may explain why objects seen in fog appear to drift serenely, even though they may actually be moving quite fast.

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).

Hack 30. Understand the Rotating Snakes Illusion

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.

The Vestibular-Ocular Reflex One way your brain cuts down on confusion is shutting down visual input during rapid eye movements [Hack #17] . Another mechanism is used to cancel out visual blur that results from head movements. Signals from how your head is moving are fed to the eyes to produce opposite eye movements that keep the visual image still. Try this experiment. Hold the book in one hand and shake your head from side to side. You can still read the book. Now shake the book from side to side at the same speed at which you shook your head. You can't read a word, even though the words are moving past your head in the same way, and at the same speed, as when you were shaking your head. The vestibular-ocular reflex feeds a signal from your inner ear [Hack #47] to your eyes in such a way that they move in the opposite direction and at the correct rate to correct the visual displacement produced by the movement of your head. You can readily demonstrate that this is a reflex hardwired to your inner ear, rather than a clever compensatory mechanism that depends on the motor signals you are sending to shake your head. If you get a friend to move your head from side to side while you relax completely (be sure your friend is careful and gentle!), you'll see that you can still read. This compensation doesn't depend on your knowing to where your head is going to move.

Another source of confusion for our visual system is a constant random drift in the exact focus of our eyes.1 This happens between saccades (see Figure 2-5, for example, in [Hack #15] ). Our muscles are constantly sending little corrective signals to keep our eyes in the same place. These signals never keep the eyes exactly still, producing so-called fixational movements. This is a good thing. If visual input is completely constant (i.e., if your eyes become paralyzed), the neurons in the eye stop responding to the constant input (because that is what they do [Hack #26] ) and everything fades out.

Normally your brain uses the structure of the current scene combined with the assumption that small random movements are due to eye movement so as not to get distracted by these slight constant drifts. To actually see these fixational movements, you have to look at something without any structure and without any surrounding frame of reference.

In Action

We need to get a handle on various principles of vision and motion computation before we can understand the rotating snakes illusion. Fortunately, each step comes with a practical demonstration of the principle.

Date: 2015-12-11; view: 1850

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