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.
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.
· 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).