How to Defeat This Optical Illusion: Grab it!

November 04, 2009

This week's guest blogger is John Pavlus, a science writer and filmmaker based in Brooklyn. You can follow him on Twitter @xjparker.

Everyone's seen this visual trick (known to nerds and academics as the Müller-Lyer illusion):

You perceive the top line as being longer than the bottom line -- even though both are exactly the same length. Ho hum.

But here's the interesting part. If you reach out to "grasp" the line -- that is, if you spread your thumb and index finger apart as if trying to pick it up by touching the tips -- your hand is not fooled. You'll likely spread your finger and thumb the same length to "grab" either line, even though you're literally seeing them as being different lengths.

Why? How can you be consciously perceiving one thing -- the illusion -- while your hand is "perceiving" another, and more accurately at that?

According to Melvyn Goodale, a cognitive psychologist at the University of Western Ontario, this is because there are literally two separate systems in your brain (well, the visual cortex, to get technical) that are processing that optical illusion. One is called the ventral stream (so named because it goes in a strip from the back of your brain down towards the underside), and the other is the dorsal stream (which is a strip that points over the top).

The ventral stream processes things in terms of "what and where" -- recognizing objects and placing them within a scene. Or, in simpler terms: it tells you what you consciously see, and it's accurate most of the time. But in the Müller-Lyer illusion, the ventral stream is telling you that there are two lines of different length.

The dorsal stream, however, uses visual input to compute actions, like accurate reaching and grasping. It doesn't so much care what something is as much as it cares about what you can do with it, and how. So when you reach out to "grasp" the line, the dorsal stream is telling your fingers, "okay, start spreading this far apart so that we'll be in position to snatch that thing by the time the hand gets there." But this processing happens below the level of consciousness -- and somewhat separately from what's going on in the ventral stream -- which is why your hand "knows better" even while you're being fooled by what you see.

There's a lot of experimental data to support this idea of two separate visual systems, but it's still under vigorous debate by neuroscientists and cognitive psychologists. And no one believes that the two systems are completely separate -- brains just don't work that way; different networks are always crosstalking and flinging feedback and feedforward signals around.

But it's pretty cool to have a hack that lets you be able to see your "two minds" in action... literally.

Special thanks to Mel Goodale, who lent me his images and has a great book on vision and consciousness called Sight Unseen.

Comments

[Sunny]

I'm fairly certain my boss just walked by and caught me with one eye pinched closed, trying to grab the lines on my computer screen. Whoops!