How the hand sees what your eye can't


As a child, if not more recently, you’ll have seen the Ebbinghaus illusion. When a circle is surrounded by other, much smaller, circles it appears bigger than when it is surrounded by large circles.


Distorting illusions like this are fun, but they potentially make dealing with our surroundings tricky. If we don’t see things the way they are, how can we manipulate them?


When we reach for something, we open our hand before we get to it. In mid-flight we separate our fingers and thumb in readiness to grasp it. When we reach for a pencil we sensibly separate them less than when we reach for a tennis ball or a box of tissues. So if someone reached for a poker chip surrounded by smaller discs you might expect that they’d open their fingers wider when reaching for it than when it is surrounded by bigger discs. When they get to the chip their fingers would be in the wrong place and they’d have to fiddle around adjusting them by touch.


But they don’t. Visual illusions are for consciousness. The visual system which does useful things, like guide our hand to pick something up, is different to that of consciousness, which imagines that it does such things. The system which really picks things up isn’t fooled by illusions; it can’t afford to be.


Dr Chris Paley is the author of Unthink, which has been published in six languages.

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