Once a week.
Subscribe to our weekly newsletter.
Sleep paralysis: a terrifying encounter with our own mind
Neuroscience explains terrifying ordeals, from out-of-body experiences to alien abductions.
- Sleep paralysis, which 20 percent of people experience at least once, can be terrifying.
- Though it is a neurological phenomenon, our culture and beliefs can make the experience worse.
- One potential treatment is to learn to control the content of our dreams.
Imagine waking up in the middle of pitch darkness, only to realize you are completely paralyzed. You suddenly notice out of nowhere, an aggressive and horrendous human-like cat is on your bed. Next, the worst-case scenario unfolds: The creature viciously attacks you, and you vividly feel its razor-sharp teeth penetrating your flesh. Next morning, you wake up with a bruise on your body.
It sounds like something out of a Stephen King horror novel, but the events describe a real-life case of sleep paralysis, as my colleagues and I recently reported in a study conducted in Italy.
Sleep paralysis is a condition in which a person awakens from sleep but is temporarily paralyzed, unable to move or speak. The phenomenon, in fact, is not uncommon. Around 20 percent of people experience sleep paralysis at least once in their life.
Though the episodes of sleep paralysis are brief, lasting a few seconds to minutes, they are rich with mystery and potential insight into the nature of the human brain. How does sleep paralysis happen, and why does it accompany the strangest hallucinations?
Neurological origins of sleep paralysis
Credit: Albert Anker via Wikipedia / Public domain
Sleep paralysis often occurs when we take a nap during the day, when jet lagged, or in any way, when sleep deprived. It happens when we wake up while still in a stage of sleep, called rapid eye movement sleep (REM), during which most vivid dreams occur. During REM, a part of the front brain called the dorsolateral prefrontal cortex, central to our ability to plan and think logically, turns off. This explains why our dreams during REM seem so real, and why the fabric of reality is so out of control when we dream — with warped landscapes and abruptly changing times, places, and people. (The Hollywood blockbuster Inception brilliantly captures the surreal flavor of dreams.)
I was once able to slide into a lucid dream during my own sleep paralysis. Lucky for me, no terrifying intruders were present.
To prevent us from acting out such intensely "real" dreams during REM and potentially hurting ourselves, our brain has a brilliant solution: it makes our bodies temporarily paralyzed.
REM is also the stage that most resembles wakefulness. Our blood pressure and heartbeat increase, and our breathing quickens. Even brain waves speed up, becoming virtually indistinguishable from wakefulness.
Sometimes, however, we actually do wake up while still in REM sleep. In a sense, we have a "switch" in the brain that tilts us between REM and wakefulness. And all it takes is a few neurochemicals to leave us stuck in this borderline state between parallel "realities" — sleep and wakefulness.
As if being paralyzed and unable to speak when waking up isn't chilling enough, occasionally, the vivid and sometimes threatening dreaming of REM can "spill over" into conscious awakening. This state — in medical jargon referred to as "sleep paralysis accompanied by hypnopompic hallucinations" — can best be described as a dream, or worse yet, a nightmare coming alive before our eyes.
Becoming a ghost
Sleep paralysis can sometimes cause eerie sensations of floating outside one's body or looking down upon oneself from the bedroom ceiling. In certain cultures, such out-of-body experiences are attributed to the "soul" — a type of "astral travel" — where the spiritual self projects itself into an alternative realm of existence.
But out-of-body experiences originate in the brain. In fact, they can reliably be produced in the laboratory. We simply have to disrupt the activity of a brain region called the temporoparietal junction. This region helps us build a "body image" in the parietal lobes (the top-middle part of the brain) or a type of neural representation of the self, based on the inputs it receives from the senses. The temporoparietal junction, which is also critical for our ability to distinguish between "self" and "other," is normally turned off during REM sleep. This is why there is a loosening of the sense of self when we dream: we sometimes see ourselves from a third-person perspective, and other times the self occupies another person's body.
It is thought that similar disturbances in the temporoparietal junction can occur during sleep paralysis. When we realize we are paralyzed, the motor cortex in the brain immediately sends signals to the rest of the body to move and to overcome the paralysis. It also sends additional signals (sort of like "cc'ing" when emailing) to the parietal lobes. Normally, there is feedback from the limbs telling the brain how to build our body image but not during sleep paralysis.
The confusing signals received by the brain can influence how the brain builds our sense of "self," and the result is all kinds of bizarre bodily hallucinations, such as out-of-body experiences or seeing one's limbs or entire body rotate in the air like a tornado or sink deep into the bed as if drowning in quicksand.
Seeing a ghost
Perhaps more distressing than becoming a ghost is seeing one. Sleep paralysis is arguably most infamous for the sinister shadowy "bedroom-intruder" that sometimes attacks the sleeper. The "creature" is usually lurking in the distant dark, slowly approaching in on its victim.
From here, all kinds of ominous things can happen, as far as the imagination can stretch. Commonly, the intruder chokes and suffocates the person by crushing his chest or pressing on his neck. And occasionally, the creature brutally rapes the paralyzed sleeper. The figure often appears simply as a dark shadow, similar to the human size and shape. But, it can also include detailed features, say, a scary demonic face with animal characteristics, like sharp teeth and cat eyes.
This figure goes by different names around the world. My colleague Devon Hinton of Harvard Medical School and I found that in Egypt, the creature is thought to be a Jinn (an "evil genie") — a spirit-like entity that may hunt down, terrorize, and even kill its victims. In another study, we've discovered that among some Italians, it is believed to be a malevolent witch or a terrifying human-like cat, known locally as the Pandafeche. Some space alien abduction cases also fit the sleep paralysis scenario: the person is laying in his bed paralyzed; suddenly the alien appears and begins to experiment on the sleeper's sexual organs, collecting eggs and semen.
A disturbance in the brain's body map
UC-San Diego neuroscientist VS Ramachandran and I recently proposed a neurological explanation for why we see this shadowy creature during sleep paralysis.
The idea was sparked by research showing that people who are born with a missing arm may experience phantom limbs, meaning that they feel the presence of missing limbs. This led to the idea that there might be a "hardwired" template, or map, of a person's body surface in the right parietal lobe of the brain. So when a person born with no arm is experiencing a phantom arm, he is really feeling the presence of the "arm" that is part of his internal body map. This map would be connected to emotional and visual centers in the brain, causing us to be attracted to body shapes similar to our own. In other words, causing us as humans to be innately attracted to other humans, and not to, say, pigs (at least for most of us!).
More clues about such a hardwired body map come from a rare disorder called apotemnophilia, in which a person has a desire to have a limb amputated and is attracted to people with missing limbs.
Ramachandran and I suggested that a disturbance in the processing of "self" and "other" — at the temporoparietal junction — results in a hallucinated projection of one's own body map; the mind literally casts a shadow, just like the body does. As the barrier between self and other dissolves, the person mistakes his own "shadow" (or body template) for a separate entity.
Compare this to an out-of-body experience: here your sense of self is shifted and you identify with your "ghostly self," not your "bodily self." When you see a "ghost," on the other hand, your vantage point doesn't get shifted, and you identify with your "bodily self," instead of your "ghostly self." And with the "threat detection system" of the brain on high alert (also known as threat hypervigilance), we are even more likely to interpret the human-like shadow as an evil, other entity.
Moreover, our brain regards it as highly improbable that chest pressure, sensations of suffocation, rapid breathing (which are caused by REM physiology), and — on top of everything — seeing a human-like shadow, occur by random chance. When REM dreaming becomes activated as well, the shadowy figure can take on all kinds of sophisticated shapes and dimensions, and the entire plot thickens. At this point, memory and the narrative abilities of other brain regions play a role in the evolving hallucination.
While our neurological explanation for the shadowy figure has yet to be proven, it fits well with previous observations. It has been shown that occasionally when the temporoparietal junction is disrupted using an electric current, instead of having an out-of-body experience, the person senses the presence of a shadowy figure. This figure is perceived to stand behind the person and to mimic his posture; even though the person is aware that the postural features of the shadowy figure are similar to his, he still regards it as a separate person. Based on this, the scientists who conducted the study concluded that they had created a "ghost-like" double.
Fear feeds terrifying sleep paralysis
In the 1986 Stephen King horror Novel It, the clown-like killer exploits the fears of its victims to hunt down its prey — young children who fear monsters. Although fictional, literature is sometimes closer to science than one would think. Our own research suggests that one's beliefs about sleep paralysis can profoundly shape the experience.
In one study, Devon Hinton and I found that in Denmark, people regard their sleep paralysis as something trivial caused by the brain. In sharp contrast, we found Egyptians often hold very specific cultural and supernatural beliefs about theirs. In another study, we discovered that Egyptians experiencing sleep paralysis not only fear it much more than Danes do — to the extent that many fear dying from it — but they also have longer episodes and on average experience sleep paralysis three times more often.
These findings strongly indicate that for Egyptians, beliefs have radically transformed the experience — a form of mind-body interaction — causing not only psychological fear but also conditioned physiological fear of sleep paralysis. When they go to bed, they fear the "cultural creature" might attack them. Ironically, this will activate fear centers in the brain (such as the amygdala), making them more likely to wake up during REM and have sleep paralysis. And once they have sleep paralysis, they interpret it in light of their cultural beliefs, thinking, "I am being attacked by an evil spirit," making them even more terrified. Escalating fear and arousal would worsen sleep paralysis by prolonging the episode and resulting in more intense bodily hallucinations, as they are more likely to try to move during the paralysis, causing body image disturbances.
It doesn't end there. Now, they notice that they are experiencing sleep paralysis more often and that episodes are longer and more horrifying. They become convinced that they are targeted, perhaps even possessed, by a supernatural creature. This, in turn, makes them even more afraid, and the vicious cycle continues to feed on itself.
It is still unclear whether the fear generated by sleep paralysis can be pathological. But in our recent study, we found that experiencers of sleep paralysis in Egypt have greater symptoms of trauma and anxiety compared to those who have never experienced it. Intriguingly, we also found that those who experience hallucinations during their sleep paralysis have more trauma and anxiety symptoms. These findings point to the possibility that sleep paralysis, if accompanied by certain beliefs, might be a traumatizing experience. This is also consistent with the findings of Richard McNally, also at Harvard, that sleep paralysis interpreted as alien abduction can elicit physiological fear comparable to patients with post-traumatic stress disorder.
Control your dreams: a cure for sleep paralysis?
In the 1984 horror movie A Nightmare on Elm Street, the ghost Freddy Krueger ferociously terrorizes and kills young teenagers during their nightmares. But the protagonist Nancy is finally able to make Krueger vanish from her nightmares; she follows the advice of her friend Glen (played by a young Johnny Depp), who insists that if you turn your back on the monster, you "take away its energy and it disappears."
Indeed, dispelling the fear surrounding sleep paralysis is an important means to help people cope with their experience and, crucially, to prevent escalating fear cycles that can lead to worse and more frequent episodes. One way to do this is to disseminate scientific knowledge about the experience. This often works. People are genuinely relieved to hear that they aren't "crazy," that they aren't the only ones experiencing it, and that the phenomena seem to originate in the brain.
A more radical approach to overcome the fear of sleep paralysis is by "literally" turning your back on the terrifying monster, by sliding into a lucid dream — that is, a dream in which you are aware that you are dreaming. It is not surprising that sleep paralysis can be a gateway to lucid dreaming. Both sleep paralysis and lucid dreaming are consciousness states that lie between REM and wakening; the former is dreaming while awake; the latter, being awake while dreaming.
Neural circuitry associated with wakefulness is more likely to become activated during sleep paralysis, such as the dorsolateral prefrontal cortex that helps us organize our logical thoughts when awake (and which is normally turned off during REM). When the dorsolateral prefrontal cortex becomes active during sleep, we enter a type of hybrid consciousness that combines the surrealness of dreams and the rationality of wakefulness. And so, we become aware that we are dreaming — and like a great Michelangelo we can create our own fantasy worlds composed of colorful landscapes and creatures of all kinds conjured by our minds. Being able to manipulate the content of one's sleep paralysis hallucinations and REM-dream imagery could give the experiencer a sense of control over the situation and might therefore be therapeutic.
I was once able to slide into a lucid dream during my own sleep paralysis. Lucky for me, no terrifying intruders were present. When I became aware that my dreaming "self" was walking around in my bedroom, it occurred to me to do an "experiment." I found a piece of scrap paper on the floor and put it in my pocket. I thought to myself, if it's still there upon awakening, I would have to reconsider some of my own scientific theories about the role of the brain in favor of more uncanny explanations. My pocket was empty when I woke up.
On a different occasion, I wanted to test whether by deliberately trying to move during the paralysis (causing disturbances to my body image) and imagining that a sinister creature was present in my bedroom (activating dream imagery), I could create my own hallucinated "Frankenstein monster." I wasn't able to complete the "experiment" out of sheer horror, but I still joke with my colleagues telling them that we're among a select group of people who can say we're working while sleeping.
Based on my scientific work on sleep paralysis around the world and our proposed neurological explanation for why people hallucinate ghosts during the episode, I thought to myself, "How do I create a simple, yet effective therapy for sufferers?" Meditation-relaxation (MR) therapy was recently designed as a direct treatment for sleep paralysis. The treatment, which includes techniques of cognitive reappraisals and emotional distancing, meditation, and muscle relaxation, aims to minimize current attacks and decrease the frequency, severity, and duration of future ones. In a recent pilot study, we found that MR therapy reduced sleep paralysis episodes by 50+ percent when applied for eight weeks in patient with narcolepsy.
As we are just beginning to probe this fascinating condition and unlock its neural basis, the mystery remains. Here is a single phenomenon that can make us see and become ghosts, have encounters with space aliens from distant galaxies, and plunge us into far and exotic lands of lucid dreaming, where we are the sculptors of our own realities, all the while laying silently in our beds. It shows us firsthand how the feeling of a sense of self as a unified entity separate from others arises in the brain and how vulnerable this feeling is to disruption.
Dr. Baland S. Jalal is a researcher at Harvard University, Department of Psychology and visiting researcher at Cambridge University, Department of Psychiatry. He obtained his PhD at Cambridge University in the School of Clinical Medicine (Trinity College Cambridge) and was a Fellow at Harvard University (2016, 2018). He is a close collaborator and co-author on 10 papers with the renowned neuroscientist VS Ramachandran (2011 TIME magazine 100 most influential people in the world).
- There's No Such Thing as Ghosts. Instead, One of These Scientific ... ›
- Big Think Interview with Shelby Harris - Big Think ›
- Sleep Paralysis Is the Most Terrifying State of Consciousness - Big ... ›
- Scientists can induce out-of-body experiences without drugs - Big Think ›
A Harvard professor's study discovers the worst year to be alive.
- Harvard professor Michael McCormick argues the worst year to be alive was 536 AD.
- The year was terrible due to cataclysmic eruptions that blocked out the sun and the spread of the plague.
- 536 ushered in the coldest decade in thousands of years and started a century of economic devastation.
The past year has been nothing but the worst in the lives of many people around the globe. A rampaging pandemic, dangerous political instability, weather catastrophes, and a profound change in lifestyle that most have never experienced or imagined.
But was it the worst year ever?
Nope. Not even close. In the eyes of the historian and archaeologist Michael McCormick, the absolute "worst year to be alive" was 536.
Why was 536 so bad? You could certainly argue that 1918, the last year of World War I when the Spanish Flu killed up to 100 million people around the world, was a terrible year by all accounts. 1349 could also be considered on this morbid list as the year when the Black Death wiped out half of Europe, with up to 20 million dead from the plague. Most of the years of World War II could probably lay claim to the "worst year" title as well. But 536 was in a category of its own, argues the historian.
It all began with an eruption...
According to McCormick, Professor of Medieval History at Harvard University, 536 was the precursor year to one of the worst periods of human history. It featured a volcanic eruption early in the year that took place in Iceland, as established by a study of a Swiss glacier carried out by McCormick and the glaciologist Paul Mayewski from the Climate Change Institute of The University of Maine (UM) in Orono.
The ash spewed out by the volcano likely led to a fog that brought an 18-month-long stretch of daytime darkness across Europe, the Middle East, and portions of Asia. As wrote the Byzantine historian Procopius, "For the sun gave forth its light without brightness, like the moon, during the whole year." He also recounted that it looked like the sun was always in eclipse.
Cassiodorus, a Roman politician of that time, wrote that the sun had a "bluish" color, the moon had no luster, and "seasons seem to be all jumbled up together." What's even creepier, he described, "We marvel to see no shadows of our bodies at noon."
...that led to famine...
The dark days also brought a period of coldness, with summer temperatures falling by 1.5° C. to 2.5° C. This started the coldest decade in the past 2300 years, reports Science, leading to the devastation of crops and worldwide hunger.
...and the fall of an empire
In 541, the bubonic plague added considerably to the world's misery. Spreading from the Roman port of Pelusium in Egypt, the so-called Plague of Justinian caused the deaths of up to one half of the population of the eastern Roman Empire. This, in turn, sped up its eventual collapse, writes McCormick.
Between the environmental cataclysms, with massive volcanic eruptions also in 540 and 547, and the devastation brought on by the plague, Europe was in for an economic downturn for nearly all of the next century, until 640 when silver mining gave it a boost.
Was that the worst time in history?
Of course, the absolute worst time in history depends on who you were and where you lived.
Native Americans can easily point to 1520, when smallpox, brought over by the Spanish, killed millions of indigenous people. By 1600, up to 90 percent of the population of the Americas (about 55 million people) was wiped out by various European pathogens.
Like all things, the grisly title of "worst year ever" comes down to historical perspective.
Quantum theory has weird implications. Trying to explain them just makes things weirder.
- The weirdness of quantum theory flies in the face of what we experience in our everyday lives.
- Quantum weirdness quickly created a split in the physics community, each side championed by a giant: Albert Einstein and Niels Bohr.
- As two recent books espousing opposing views show, the debate still rages on nearly a century afterward. Each "resolution" comes with a high price tag.
Albert Einstein and Niels Bohr, two giants of 20th century science, espoused very different worldviews.
To Einstein, the world was ultimately rational. Things had to make sense. They should be quantifiable and expressible through a logical chain of cause-and-effect interactions, from what we experience in our everyday lives all the way to the depths of reality. To Bohr, we had no right to expect any such order or rationality. Nature, at its deepest level, need not follow any of our expectations of well-behaved determinism. Things could be weird and non-deterministic, so long as they became more like what we expect when we traveled from the world of atoms to our world of trees, frogs, and cars. Bohr divided the world into two realms, the familiar classical world, and the unfamiliar quantum world. They should be complementary to one another but with very different properties.
The two scientists spent decades arguing about the impact of quantum physics on the nature of reality. Each had groups of physicists as followers, all of them giants of their own. Einstein's group of quantum weirdness deniers included quantum physics pioneers Max Planck, Louis de Broglie, and Erwin Schrödinger, while Bohr's group had Werner Heisenberg (of uncertainty principle fame), Max Born, Wolfgang Pauli, and Paul Dirac.
Almost a century afterward, the debate rages on.
Einstein vs. Bohr, Redux
Two books — one authored by Sean Carroll and published last fall and another published very recently and authored by Carlo Rovelli — perfectly illustrate how current leading physicists still cannot come to terms with the nature of quantum reality. The opposing positions still echo, albeit with many modern twists and experimental updates, the original Einstein-Bohr debate.
Albert Einstein and Niels Bohr, two giants of 20th century science, espoused very different worldviews.
I summarized the ongoing dispute in my book The Island of Knowledge: Are the equations of quantum physics a computational tool that we use to make sense of the results of experiments (Bohr), or are they supposed to be a realistic representation of quantum reality (Einstein)? In other words, are the equations of quantum theory the way things really are or just a useful map?
Einstein believed that quantum theory, as it stood in the 1930s and 1940s, was an incomplete description of the world of the very small. There had to be an underlying level of reality, still unknown to us, that made sense of all its weirdness. De Broglie and, later, David Bohm, proposed an extension of the quantum theory known as hidden variable theory that tried to fill in the gap. It was a brilliant attempt to appease the urge Einstein and his followers had for an orderly natural world, predictable and reasonable. The price — and every attempt to deal with the problem of figuring out quantum theory has a price tag — was that the entire universe had to participate in determining the behavior of every single electron and all other quantum particles, implicating the existence of a strange cosmic order.
Later, in the 1960s, physicist John Bell proved a theorem that put such ideas to the test. A series of remarkable experiments starting in the 1970s and still ongoing have essentially disproved the de Broglie-Bohm hypothesis, at least if we restrict their ideas to what one would call "reasonable," that is, theories that have local interactions and causes. Omnipresence — what physicists call nonlocality — is a hard pill to swallow in physics.
Credit: Public domain
Yet, the quantum phenomenon of superposition insists on keeping things weird. Here's one way to picture quantum superposition. In a kind of psychedelic dream state, imagine that you had a magical walk-in closet filled with identical shirts, the only difference between them being their color. What's magical about this closet? Well, as you enter this closet, you split into identical copies of yourself, each wearing a shirt of a different color. There is a you wearing a blue shirt, another a red, another a white, etc., all happily coexisting. But as soon as you step out of the closet or someone or something opens the door, only one you emerges, wearing a single shirt. Inside the closet, you are in a superposition state with your other selves. But in the "real" world, the one where others see you, only one copy of you exists, wearing a single shirt. The question is whether the inside superposition of the many yous is as real as the one you that emerges outside.
To Einstein, the world was ultimately rational... To Bohr, we had no right to expect any such order or rationality.
The (modern version of the) Einstein team would say yes. The equations of quantum physics must be taken as the real description of what's going on, and if they predict superposition, so be it. The so-called wave function that describes this superposition is an essential part of physical reality. This point is most dramatically exposed by the many-worlds interpretation of quantum physics, espoused in Carroll's book. For this interpretation, reality is even weirder: the closet has many doors, each to a different universe. Once you step out, all of your copies step out together, each into a parallel universe. So, if I happen to see you wearing a blue shirt in this universe, in another, I'll see you wearing a red one. The price tag for the many-worlds interpretation is to accept the existence of an uncountable number of non-communicating parallel universes that enact all possibilities from a superstition state. In a parallel universe, there was no COVID-19 pandemic. Not too comforting.
Bohm's team would say take things as they are. If you stepped out of the closet and someone saw you wearing a shirt of a given color, then this is the one. Period. The weirdness of your many superposing selves remains hidden in the quantum closet. Rovelli defends his version of this worldview, called relational interpretation, in which events are defined by the interactions between the objects involved, be them observers or not. In this example, the color of your shirt is the property at stake, and when I see it, I am entangled with this specific shirt of yours. It could have been another color, but it wasn't. As Rovelli puts it, "Entanglement… is the manifestation of one object to another, in the course of an interaction, in which the properties of the objects become actual." The price to pay here is to give up the hope of ever truly understanding what goes on in the quantum world. What we measure is what we get and all we can say about it.
What should we believe?
Both Carroll and Rovelli are master expositors of science to the general public, with Rovelli being the more lyrical of the pair.
There is no resolution to be expected, of course. I, for one, am more inclined to Bohr's worldview and thus to Rovelli's, although the interpretation I am most sympathetic to, called QBism, is not properly explained in either book. It is much closer in spirit to Rovelli's, in that relations are essential, but it places the observer on center stage, given that information is what matters in the end. (Although, as Rovelli acknowledges, information is a loaded word.)
We create theories as maps for us human observers to make sense of reality. But in the excitement of research, we tend to forget the simple fact that theories and models are not nature but our representations of nature. Unless we nurture hopes that our theories are really how the world is (the Einstein camp) and not how we humans describe it (the Bohr camp), why should we expect much more than this?
Maybe eyes really are windows into the soul — or at least into the brain, as a new study finds.
- Researchers find a correlation between pupil size and differences in cognitive ability.
- The larger the pupil, the higher the intelligence.
- The explanation for why this happens lies within the brain, but more research is needed.
What can you tell by looking into someone's eyes? You can spot a glint of humor, signs of tiredness, or maybe that they don't like something or someone.
But outside of assessing an emotional state, a person's eyes may also provide clues about their intelligence, suggests new research. A study carried out at the Georgia Institute of Technology shows that pupil size is "closely related" to differences in intelligence between individuals.
The scientists found that larger pupils may be connected to higher intelligence, as demonstrated by tests that gauged reasoning skills, memory, and attention. In fact, the researchers claim that the relationship of intelligence to pupil size is so pronounced, that it came across their previous two studies as well and can be spotted just with your naked eyes, without any additional scientific instruments. You should be able to tell who scored the highest or the lowest on the cognitive tests just by looking at them, say the researchers.
The pupil-IQ link
The connection was first noticed across memory tasks, looking at pupil dilations as signs of mental effort. The studies involved more than 500 people aged 18 to 35 from the Atlanta area. The subjects' pupil sizes were measured by eye trackers, which use a camera and a computer to capture light reflecting off the pupil and cornea. As the scientists explained in Scientific American, pupil diameters range from two to eight millimeters. To determine average pupil size, they took measurements of the pupils at rest when the participants were staring at a blank screen for a few minutes.
Another part of the experiment involved having the subjects take a series of cognitive tests that evaluated "fluid intelligence" (the ability to reason when confronted with new problems), "working memory capacity" (how well people could remember information over time), and "attention control" (the ability to keep focusing attention even while being distracted). An example of the latter involves a test that attempts to divert a person's focus on a disappearing letter by showing a flickering asterisk on another part of the screen. If a person pays too much attention to the asterisk, they might miss the letter.
The conclusions of the research were that having a larger baseline pupil size was related to greater fluid intelligence, having more attention control, and even greater working memory capacity, although to a smaller extent. In an email exchange with Big Think, author Jason Tsukahara pointed out, "It is important to consider that what we find is a correlation — which should not be confused with causation."
The researchers also found that pupil size seemed to decrease with age. Older people had more constricted pupils but when the scientists standardized for age, the pupil-size-to-intelligence connection still remained.
Why are pupils linked to intelligence?
The connection between pupil size and IQ likely resides within the brain. Pupil size has been previously connected to the locus coeruleus, a part of the brain that's responsible for synthesizing the hormone and neurotransmitter norepinephrine (noradrenaline), which mobilizes the brain and body for action. Activity in the locus coeruleus affects our perception, attention, memory, and learning processes.
As the authors explain, this region of the brain "also helps maintain a healthy organization of brain activity so that distant brain regions can work together to accomplish challenging tasks and goals." Because it is so important, loss of function in the locus coeruleus has been linked to conditions like Alzheimer's disease, Parkinson's, clinical depression, and attention deficit hyperactivity disorder (ADHD).
The researchers hypothesize that people who have larger pupils while in a restful state, like staring at a blank computer screen, have "greater regulation of activity by the locus coeruleus." This leads to better cognitive performance. More research is necessary, however, to truly understand why having larger pupils is related to higher intelligence.
In an email to Big Think, Tsukahara shared, "If I had to speculate, I would say that it is people with greater fluid intelligence that develop larger pupils, but again at this point we only have correlational data."
Do other scientists believe this?
As the scientists point out in the beginning of their paper, their conclusions are controversial and, so far, other researchers haven't been able to duplicate their results. The research team addresses this criticism by explaining that other studies had methodological issues and examined only memory capacity but not fluid intelligence, which is what they measured.