What you see is often affected by what you expect to see.
When your brain expects to see one thing and is presented with something quite different, you may feel some peculiar sensations.
Note: This activity can be done with a glass mirror (12-inch glass mirror tile is commonly available in home improvement stores), but we highly recommend using plastic mirror because it is much safer. If you must use a glass mirror, tape the edges to prevent cuts and glue one entire surface to a backing of cardboard or wood to minimize shards flying if the mirror cracks or breaks. It's also not practical to drill a hole in a glass mirror, but you can use epoxy, Gorilla Glue, or another strong adhesive to glue the handles to the glass.
Square piece of acrylic plastic mirror, available from plastics stores (mirror shown in the photo is 10 inches [25 centimeters] per side, but exact size is not critical)
Two wooden dowels, 1 in (2.5 cm) in diameter × 1 foot (30 cm) long
Dowel screw (double-ended screw), 3/16 in x 3 in works well
Two drill bits, one slightly larger than the dowel screw and one slightly smaller
Use the larger drill bit to drill a hole in the middle of the mirror.
Use the smaller drill bit to drill a hole straight into one end of each of the wooden dowels (the dowels will be your handles).
Insert the double-ended dowel screw in the mirror hole and screw on the dowels (one on either side of the mirror) until tight.
To Do and Notice
Grab a dowel with each hand. While looking at the reflection of your hand in the mirror, move the other hand, the one that is out of view behind the mirror.
Hold one of your arms on each side of the mirror so the reflection of one arm looks like the continuation of the other arm. Snap the fingers on both your hands simultaneously, then stop snapping the fingers on only one hand. Or have someone drop an object (such as a set of keys) into the hand behind the mirror.
What's Going On?
When you look at the reflection of your hand in the mirror, your brain is fooled into thinking that the image it sees in the mirror is actually your other hand. When you move that hand, your brain naturally expects to see the hand move. After all, messages from the nerves in that hand tell your brain that the hand is moving. The hand’s apparent failure to move can be profoundly disturbing to your brain, which doesn’t enjoy having its assumptions trifled with!
Some amputees who experience phantom limb pain in their amputated limbs have successfully reduced their pain using a kind of mirror therapy that uses the same principles demonstrated by this experiment.
Mirror therapy is also used to help stroke patients recover function in their limbs, particularly the hands and arms.