Mind Hacks

Hack #29. Turn Gliding Blocks into Stepping Feet

Motion detection uses contrast information first, not color.

The moral of this story is that if you want people to see moving objects, make them brighter or darker than the background, not just a different color.

Motion is important stuff for the brain. Information about movement gets routed from the eye to the visual cortex—the final destination for all visual information—along its own pathway (you can take a tour round the visual system [[Hack #13]]), the magnocellular pathway. (Like a lot of things in neuroscience, this sounds more technical than it is; magnocellular means “with large cells.”)

Color and form information travels along the parvocellular pathway (yup, “small cells”) to the visual cortex, which means any motion has to be processed without access to that information. This functional division makes sense for a brain that wants to know immediately if there’s a movement, and only secondly what exactly that moving something looks like. Problems arise only when movement processing is trying to figure out what sort of motion is occurring but the clues it needs are encoded in color and so not available.

In Action

Stuart Anstis has constructed just such a problematic situation, and it leads to the nifty stepping feet illusion ¹ ( http://psy.ucsd.edu/~sanstis/Foot.html ; Shockwave). Blue and yellow blocks move smoothly in tandem from side to side. Click the Background button to bring up the striped background, and look again. It ...