You probably look in a mirror every day without thinking about it. But mirrors can reveal a great deal about the brain, with implications for psychology, clinical neurology and even philosophy. They can help us explore the way the brain puts together information from different sensory channels such as vision and somatic sensations (touch, muscle and joint sense). In doing so, they can reveal a lot about our sense of self. Would a person who has never looked at his reflection—even in a pool—ever develop a sophisticated self-representation?
Using two bricks, or some duct tape, prop up an 18-inch-square mirror vertically on a table. Sit so that the edge faces you. Now put your left hand on the table at the left side of the mirror (either palm up or down) and match your right-hand position on the right side. If you now look into the right side of the mirror, you will see the right hand’s reflection optically superimposed in the same place where you feel your left hand to be. (You may need to adjust the position of the left hand to achieve this sensation.) It will now look like you are viewing your own left hand, but of course you are not. Now try the following experiments.
While continuing to look in the mirror on the right side and keeping your left hand perfectly still, move your right hand, wiggle its fingers or make a fist. The “left hand” in the mirror will appear to move in perfect synchrony with the right but, paradoxically, feel completely still. The conflict creates a slight jolt; it feels spooky, sometimes mildly uncomfortable. The brain abhors discrepancies.
Now do the opposite; keep the right hand still and move the left hand. The left hand appears still but feels like it is moving. You will feel the same kind of jarring sensation, but it will be less powerful than in the preceding case. The reason for the asymmetry is not clear.
Why the jolt? The answer resides in the right superior and inferior parietal lobules (located above your right ear), where signals from your various senses—visual, somatic—converge to create your internal sense of a body image. Stand up now and close your eyes. Either raise your arms or let them dangle by your side. Obviously you have a vivid sense of being “anchored” in your body except under special circumstances (such as ketamine anesthesia). Now open your eyes, and you have visual confirmation of what your other senses are telling you: you see your hand where you felt it to be. In short, your senses normally blend different sensory inputs to create a vivid dynamic image of your body moving in space and time.
The mirror experiment you did earlier disrupts this consistency of signals in the right superior parietal lobule. The discrepancy is picked up in part by the right insular cortex (buried in the temporal lobe), and that information is then relayed to the right frontal lobe, where it can be picked up through brain imaging (as shown by Richard Frackowiak, Ray Dolan and Chris Frith, all at University College London, and Peter Halligan of Cardiff University in Wales).
Is That My Hand?
You do not need fancy brain-imaging gizmos to try out some additional experiments that can give you insights into brain function.
Return your hands to either side of the mirror. Now have a friend touch, stroke, pinch, tap or rub your right hand while you look at its reflection. Obviously it will look like your own left hand is being stroked, pinched, tapped or rubbed. But because it is not actually being touched, you will experience one or all of the following (the response varies from person to person).
First, the hand may feel numb, anesthetized or asleep, and it will still feel as if it belongs to you. (Your brain is in effect saying, “I see my hand being rubbed but don’t feel it, so it must be asleep.”) This perception is unaffected by your higher-level intellectual knowledge of the optics of the situation. Your perceptual systems integrating vision and touch are on autopilot, as it were, applying their own statistical rules.