If you want some thrills this Halloween, you should try tricking yourself for a change. In this Spotlight, we look at some spooky experiments that'll trick the brain and ask intriguing questions about consciousness and perception.

Treat yourself to some scientific 'spooks' this Halloween.

In Act One, Scene Five of William Shakespeare's play Hamlet, the main character, after meeting his father's ghost, remarks to his best friend, "There are more things in heaven and earth, Horatio,/Than are dreamt of in your philosophy."

Hamlet is referring to the world being filled with spooky mysteries that we may have a hard time even imagining.

Perhaps one of the most mysterious things on earth is, in fact, the human brain.

How does our consciousness work? Can we rely on our senses, or do they — and the brain — often trick us?

In this Spotlight, we look at a series of spooky experiments that shed some light on how our brains work, and which might make you question your own senses.

So, if you are in the mood to test the limits of your perception this Halloween, why not try to trick your own brain by replicating one of the experiments below?

1. The ghost in the mirror

One legend that used to be popular among schoolchildren has it that if you look into a mirror by the light of a candle and recite "Bloody Mary" three times, a woman's specter will appear in the glass.

In the past, young women supposedly performed other, similar rituals in the hope that they'd catch a glimpse of their future husbands in the dimly lit surface of the mirror.

It turns out that while peering into a mirror in a dimly lit room will bring about no supernatural events, it will likely reveal to the viewer one or several strange faces — sometimes with a fearsome, and at other times a benevolent, expression. How so?

What will you see if you peer at your own reflection in a mirror placed in a dimly lit room?

This is what Giovanni Caputo, in the Department of Psychology at the University of Urbino in Italy, set out to answer.

He reported his findings in a paper published in the journal Perception in 2010.

In his study, Caputo recreated a visual illusion that occurs when a person stares at their own face in the mirror in a room with poor lighting.

The researcher used "a relatively large mirror" of 0.5 x 0.5 meters, which he placed in a room lit by "a 25-Watt incandescent light," although he notes that to recreate this experiment, the exact same conditions are not necessary.

Each volunteer sat 0.4 meters away from the mirror, and they had about 10 minutes to peer into it; though the illusion, Caputo says, usually manifested within about 1 minute.

At the end of the session, the participants wrote down what they had seen in the mirror, and their descriptions varied greatly. Out of a total of 50 participants:

·         66 percent reported seeing "huge deformations" of their own faces

·         18 percent saw "a parent's face with traits changed," with 10 percent of these seeing the faces of deceased parents, and 8 percent those of parents who were still alive

·         28 percent saw "an unknown person"

·         another 28 percent reported seeing "an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor"

·         18 percent saw the face of an animal

·         48 percent saw "fantastical and monstrous beings"

The Troxler effect or spectral apparitions?

First and foremost, this visual illusion seems to appear due to the fact that the eyes are forced to fixate on a single point. In this respect, the faces in the mirror illusion could be compared to an optical illusion called "Troxler's fading" or the "Troxler effect."

If you stare at the red dot in the center for long enough, the blue circle will soon start to fade away.
Image credit: Mysid, Wikimedia Commons

This phenomenon — which Ignaz Paul Vital Troxler discovered in 1804 — occurs when someone stares fixedly at a single point.

When it starts to happen, anything surrounding that point, particularly splashes of color, will begin to fade away.

As a result, it might seem as though we have temporarily lost our ability to perceive colors.

This likely happens as a result of "neural adaptation," in which our nerve cells ignore stimuli that are not essential to perceiving the object of our focus.

Therefore, we end up seeing the one thing that we are fixing our gazes on and little or nothing else. This, however, is not the case with the faces in the mirror illusion, Caputo says.

"[This] explanation," he writes, "would predict that face traits should fade away and eventually disappear, whereas the apparitions in the mirror consist of new faces having new traits."

Instead, what may happen is that by staring continually at our own faces, the stimuli initially stop connecting in a meaningful way, so that we are unable to "string together" the facial traits we perceive.

This may result in a spontaneous reassembly of these traits, so it may seem to us that our faces have become deformed or uncanny. However, this fails to explain all, Caputo suggests.

"[The] frequent apparition of fantastical and monstrous beings," he writes, "and of animal faces cannot [...] be explained by any actual theory of face processing."

Dissecting terror: How does fear work?

How much do you really know about fear?

The 'other' that we project

So, what happens? It seems likely that once our vision is disrupted, our brains start projecting fears or desires onto the distorted traits in the mirror, giving them new identities and purposes.

Caputo inferred this when analyzing the participants' emotional responses to their individual mirror "apparitions." Depending on what they thought they saw, the volunteers often felt either scared or happy.

"Some participants saw a malign expression on the 'other' face and became anxious. Other participants felt that the 'other' was smiling or cheerful, and experienced positive emotions in response. The apparition of deceased parents or of archetypal portraits produced feelings of silent query."

Giovanni Caputo

According to him, the apparition of strange faces in the mirror, to which we then respond in such a strong emotional way, is probably due to the fact that the complex process of self-identity construction — which we go through each time we see our reflections — is disrupted.

This, he thinks, may cause a "potential breakdown of self-identity" that we experience as a spooky dissociation.

2. Is that your hand?

There is little that we are as certain of as the fact that we own every inch of our bodies. Well...this is true for most of us, at least.

The rubber hand illusion can make you think that an artificial hand has replaced your real one.

Following serious health events such as brain lesions, a person may experience something called "somatoparaphrenia."

This is a sense of dissociation from a part of or even the entire body.

In other words, a person will believe that a limb, some other body part, or their entire body does not belong to them.

These may seem like extreme cases, but some simple experiments have shown that pretty much all of us can be tricked into dissociating from our bodies, or claiming artificial body parts or even "ghost" limbs as our own.

The most famous experiment conducted in this sense is that of the rubber hand. In this experiment, a dark screen shields one of the participant's arms from their sight.

Instead, the researchers place a rubber arm in front of the participant. Then, they repeatedly tickle both the rubber hand and the participant's hidden real hand at the same time.

At this point, the volunteer has surprisingly taken ownership of the rubber arm and seems to react as though their own real hand has been tickled. In the video below, put together by National Geographic, you can see a variation of the "rubber hand illusion" experiment:

Movement and the sense of self

In a study focused on the rubber hand illusion, a team of researchers from the University of Milan, the University of Milan Medical School, and the University of Turin — all in Italy — wanted to see what happens in the brain when a person experiences this strange illusion.

The investigators found "that body ownership and the motor system are mutually interactive and both contribute to the dynamic construction of bodily self-awareness in healthy and pathological brains."

In other words, MRI scans showed that when participants began to believe that the rubber hand was their own, the brain networks that coordinated movement in the real hand began to slow down.

"The present findings," they explain, "which shed new light on our understanding of the different aspects that contribute to the formation of a coherent self-awareness, suggest that bodily self-consciousness strictly depends on the possibility of movement."

3. What the brain hears

Our sense of hearing helps us navigate the world. It is fairly easy to trick this sense — but particular experiences can tell us a lot about how our brains can actually control what we hear.

Do we only hear what we have already learned to hear?

Earlier this year, a cryptic audio track went viral. The catch? People could not agree on whether the recorded voice was saying the word "Yanny" or the word "Laurel."

Why do people hear different names, however? One explanation has to do with pitch, or audio frequency, and how each person's ears are "tuned."

So, some people may hear "Yanny" while others will hear "Laurel."

However, according to Prof. Hugh McDermott — at Melbourne's Bionics Institute in Autralia — who spoke to the newspaper The Guardian, the story is more complex than that; it may have to do with how our brains process information.

Because the track is auditively ambiguous, our brains have to choose their own "interpretation" — but how do they do that?

"When the brain is uncertain of something, it uses surrounding cues to help you make the right decision," explains Prof. Mc Dermott.

"If you heard a conversation happening around you regarding 'Laurel' you wouldn't have heard 'Yanny.' Personal history can also give an unconscious preference for one or another. You could know many people named 'Laurel' and none called 'Yanny.'"

Prof. Hugh McDermott

Your brain, the anticipator

In other words, our brains are able to make sense of things by anticipating them. That is, if we have already learned something, only then are we able to identify it. That is what makes the difference between hearing gibberish and hearing a sentence that makes actual sense.

This is why our brains make choices when presented with ambiguous stimuli or information. A good example of this is sine-wave speech, which consists of computer-altering voices so they are almost unrecognizable.

Take these examples that researchers at the University of Sussex in the United Kingdom created. If you listen to this track, it is unlikely that you will be able to make head or tail of it.

However, if you listen to the original unaltered recording first, and then to the sine-wave track, you will have no trouble in understanding the sentence, despite the distortion.

Perhaps the reason ghosts so easily spook us is that we do not have a clear understanding of how our consciousness works. Some of the discoveries surrounding the workings of our brains are, in themselves, spooky.

A 1992 survey revealed that 10–15 percent of the respondents, based in the United States, have experienced some sort of sensory hallucination at one point in their lives.

When our bodies and minds can be so easily tricked, there is little wonder that the ghosts and ghouls of Halloween still hold such fascination over so many of us.

Souce: MedicalNewsToday