Can you wipe away a friend's face?

A cream and pink exhibit table with a pink information panel in the centre.

This exhibit demonstrates how your brain ‘fills in’ blind spots and your preference to notice movement rather than faces.

How it works

This exhibit needs two people and the illusory effect may be too subtle for some people. Together with the help of a friend, use a blank wall and a mirror to create the illusion that you are slowly erasing parts of your friend’s face, leaving only their eyes or smiling mouth floating in mid-air.

This is based on The Cheshire Cat exhibit by The Exploratorium in San Francisco, USA. 'Cheshire Cat' refers some people can only see their friend’s smiling mouth hanging in mid-air, similar to the Cheshire Cat in Alice in Wonderland. Other people get the impression of their friend’s mouth disappearing.

Background

The back of your eyeballs (retinas) have millions of light receptors, except in one spot where your optic nerve and attaches to your eye ball. This spot—known as your blind spot—cannot detect any light, but you don’t notice this gap in your vision, because your brain ‘patches over’ the gap by duplicating the colours and patterns being detected by receptors surrounding the blind spot.

In the exhibit, your left eye sees your friend’s face, while your right eye sees the white wall and your hand in the mirror, so each eye feeds your brain with two very different views. Your brain tries to put these two views together in a way that makes sense, choosing part or all of the view from one eye or the other. When your brain combines and waivers between these views of your friend’s face, it’s called perceptual or binocular rivalry.

Your right eye sees the white wall in the mirror and your brain ‘fills in’ your right eye’s blind spot with white. Also, movement attracts your brain’s attention, so when you sweep your hand across the white wall, your brain pays more attention to your moving hand in the mirror than it does to your friend’s stationary face—and your brain erases parts of their face by your moving hand.