OUTREACH: “Ghosts in the Brain”

I’ve been meaning to put together a post on creepy sensory illusions for months. It all started when Ammon and I volunteered at the Texas Memorial Museum’s Fright at the Museum Halloween event. Thousands of kids came to visit and spent the afternoon reaching their hands into creepy jars, handling snakes, and wandering around the spookily decorated museum. Our booth was called “Is That A Ghost Or Is Your Mind Playing Tricks On You?,” and demonstrated a bunch of sensory illusions that teach people things about our brains that we’re not normally aware of. This post is a curated list of some of the more interesting sensory illusions we put together and their cognitive explanations. Enjoy!

Print, cut out and tape together this dragon statue. Then close one eye (or dim the lighting in the room) and look at the dragon as you move around. It will seem like its head is tracking your every movement.

1) Why is that dragon following your every move?

You may have seen the video before. A handheld camera zooms around a tiny paper dragon statue, which seems to follow the camera’s every move with its eyes.

What is going on?

The dragon’s face is folded in such a way that it is “hollow,” or concave. Our brains have had many years to get used to the faces of animals and humans being convex and “popping out” at us. So when our eyes feed the information to our brain that no matter where we walk, the dragon’s face is still completely visible to us, our brain interprets this to mean that the dragon must be moving its head along with us.

This illusion is shockingly robust. Even once we know that the dragon’s head isn’t following us and that its face is hollow, we continue to perceive it in a way that doesn’t reflect reality.

2) Convince yourself that you have a third arm (or two noses)

With just a cheap plastic arm, a cardboard box, and a couple of brushes (we used some thick paint brushes), you can be convinced in 5 minutes or less that you have a third arm. The basic steps of the demonstration are nicely laid out in the video below.

If you’re feeling like doing a simpler tactile illusion, close your eyes, cross your fingers, and then line your fingers up so they are on either side of your nose. Then start stroking your nose with your crossed fingers. You should start feeling an eerie sensation that you have two noses instead of one. 

What is going on?

Two Noses Illusion: This illusion is commonly known as Aristotle’s Illusion, because the Greek philosopher wrote about it at least twice. In Metaphysics he wrote:

Touch says there are two objects when we cross our fingers, while sight says there is one.

The trick to this illusion again rests on what our brain is used to sensing in the world. By crossing our fingers, we are ensuring that the outside of each finger is stimulated by its brush against our nose. In our day-to-day existence, having only the outside of two fingers feel something in this way normally tells us that we are touching two separate objects. So even though we know that we don’t have two noses, our brain quickly interprets the incoming tactile information as just the opposite!

Hand Illusion: This one seems to work so well because two senses are being tricked at once. As you watch the fake hand get stroked by the brush, and feel that stroking on your hidden real hand, your brain starts adopting its new plastic arm surprisingly quickly. It seems that the multi-sensory trickery is key here. Our brains appear to decide on “ownership” of body parts by integrating information from all of the senses. Trick a couple of your senses at once, and your brain effectively throws up its metaphorical hands and decides you must suddenly have an extra appendage. The brain’s quick adaptability to altered stimuli has been observed in other odd situations concocted by scientists. For example, people who are asked to wear mirrored goggles that effectively flip the world up-side-down report that after a few days of wearing the goggles they start to see the world right-side-up again. 

3) Get a virtual haircut

Want to feel like you’re getting a haircut without even stepping away from your computer? Simply start playing the video below after plugging some headphones into your computer. Then, close your eyes, and prepare to get a little creeped out.

What is going on?

When an object vibrates, it pushes nearby air molecules away, which collide with other molecules and start a chain reaction that moves outward like a wave. The mechanical movements caused by these waves within our ears gets translated into neural signals, which our brain interprets as sound.

Because our two ears are placed on each side of our head we can use minuscule time delays in how long it takes sound waves to reach them to interpret the direction of sounds. The barbershop recording was made using the snazzy looking plastic head in this picture. He has a microphone in each of his plastic ears, which can record the directions of sounds just like our ears do. Then the resulting recordings got mixed together into a single stereo recording, so you can enjoy a free virtual haircut whenever you please.

Stay tuned for Part II of the sensory illusions series!

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