Understanding how the brain perceives the passage of time could lead to treatments for mental illnesses. Why does time seem to slow down during a life-threatening situation? Our reporter falls 15 storeys to find out.
A few moments ago, I was strapped into a harness and winched 46m into the air. Four massive steel girders support my weight, and I can see that I'm the highest object around for miles. I am about to become the fastest-moving man in science, and I can barely keep my breakfast down.
This contraption is called the Suspended Catch Air Device, but the folks at the Zero Gravity Thrill Amusement Park in Dallas prefer the more colloquial "Nothin' But Net". That's because when the operator releases my rope, I will fall, untethered, until I plop into a modified circus net. The terrifying free fall will last less than three seconds, but to me it will feel much longer. And in this experiment, that is exactly the point.
The study of how the brain perceives the passage of time is no longer just the work of philosophers. In the past few decades, medical scanners and computers have improved such that scientists can monitor the brain's activity millisecond by millisecond. Sorting out how the brain handles time-related information could reveal the cause of several mental illnesses. But some basic information still eludes researchers, in particular an explanation for "time dilation", the notion that time seems to slow during life-threatening situations. My impending fall is the latest in a series of experiments designed by David Eagleman, a neuroscientist at Baylor College of Medicine, to crack this nut.
Attached to my wrist is a perceptual chronometer, basically two LED screens, each blinking random digits between one and nine. Before I was hoisted up here, the chronometer was set so that the numbers alternate just fast enough that I cannot read them. If Eagleman is correct, and the brain's perception of time slows down during disaster, then I should see the numbers on the chronometer flicker in a readable slow-mo, sort of like how characters in The Matrix films see bullets. That is, if I can keep my eyes open.
The Time Keeper: The walls of David Eagleman's lab are covered with scribblings related to studying time, including the equation for entropy and analysis of scenes from science-heavy films. John B. Carnett
Your Brain on Time Travel
In recent years, scientists have learned that the circadian rhythms that control our 24-hour sleep/wake cycle are governed by a cluster of 10,000 brain cells called the suprachiasmatic nucleus. Sorting out what happens moment to moment is the focus of Eagleman's work, and his Baylor-based Laboratory for Perception and Action is one of the only facilities dedicated to running experiments that produce hard data on how we perceive time.
Eagleman began his career researching vision, and in 2000 he became interested in the flash-lag effect, an optical illusion that scientists had never satisfactorily explained. On a computer screen, a blue doughnut-like ring circles a fixed point. Every so often, the ring's hole turns white for a split second. Sometimes, the white centre and the blue ring, which has continued on its path, appear to overlap. After running dozens of students through this test, Eagleman posited that it might be a temporal illusion, and that it tricks the brain, not the eyes. In addition to interpreting the white flash, the brain is also predicting where the blue ring should be a few milliseconds in the future, and that is being lumped in with the experience that reaches your consciousness. This was the first evidence that our perception of time is not an exact representation of what is occurring in the moment we consider to be the present.
A day before my jump, I visited Eagleman's lab to try out some of his temporal illusions. Eagleman is 38 years old but looks younger by a decade. He has short brown hair, an athletic build and an affable manner. In 2009 his book Sum: Forty Tales from the Afterlives became a quick best seller. His lab looks like a traditional office, with cubicles and coffeepots and such, except the walls have been painted with a light blue dry-erase paint and are covered floor-to-ceiling with the markings of his research. But I'm staring instead at a computer screen, trying to click on a flitting green square.
In 9 Square, nine blue squares are arranged like a noughts-and-crosses board, and every now and again one of them turns green. My job is to click on the green square. At first, it jumps at a steady rate, moving to the next spot 200 milliseconds after I click the mouse. But after a while, the rate begins to vary and, when the green square moves faster, it seems to jump before I click on it.
"This is because your brain is constantly calibrating duration," Eagleman explains. "If every time you flip on the lights there is a 200-millisecond delay, your brain recognises the pattern and edits out the delay. Flip the switch, and the lights seem to turn on instantaneously. But if you moved to a funky house where the lights really did come on instantaneously, it would appear that they came on before you flipped the switch. Your brain is temporarily stuck on the old pattern."
Eagleman had dozens of people play 9 Square while he scanned their brains with a functional-MRI machine. He found that when people experience the time delays, there is a boost of activity in the anterior cingulate cortex, which activates only when different parts of the brain process conflicting information. This suggested that the brain maintains at least two separate versions of time, a master clock that feeds you a perception of the now, and another that is constantly at work tidying up that perception.
Similar tests backed up his results, indicating that-unlike speech, which is processed in Broca's area, or vision, which the occipital lobe handles-our sense of time is not centralised. Because of this, most scientists in the field have moved on to solving how parts of the brain work together to produce a single representation of time. But they first need to know if the system is truly capable of varying the rate at which it interprets the data. Eagleman remembered falling off a roof as a kid and how time seemed to stretch out forever in what was really only an instant. That's when he decided that dropping people off a tower could be the way to figure all this out.
Free Falling: From top to bottom, the 46m drop lasts 2.6 seconds. Most subjects report that it feels longer than four seconds. John B. Carnett
Explaining the Oddball
"Three, two, one, go!" counts the operator, and I'm falling. The top of the tower starts to recede, my stomach rushes into my throat and, as Eagleman predicted, I feel time begin to crawl. Using every bit of willpower, I focus on the chronometer, and even though the seconds stretch out, those blinking numbers are still too blurry to read.
I hit the net harder than I had prepared myself for, but I'm intact, and I sheepishly report my experience to Eagleman. "Just as I expected," he says. "Another null effect." In the initial round of testing, he ran 23 subjects through the high dive (one was excluded because she shut her eyes on the way down). Every subject felt as if the experience lasted longer than it really did-the average estimate was closer to four seconds than the actual 2.6 - but they could read the chronometer no better while falling than they had with two feet on the ground. Their brains didn't actually perceive time at a slower rate. At first Eagleman was disappointed, but then he realised that the results indicated that time dilation is, in fact, a misremembered experience. The fall doesn't seem to take longer while you're falling; you just remember it that way.
Not everyone agreed with his conclusion. Dartmouth College neuroscientist and time researcher Peter Tse offered another explanation. Evolution trained our brain to notice novelty. Moving shadows in the jungle could mean dinner or it could mean becoming dinner-either way, it pays to pay attention. "When we are paying attention," Tse says, "the brain processes more information per second." It's a survival tactic. When faced with a life-threatening situation, like falling 46 metres, your brain is presented with more information per second than it is accustomed to, so it recalibrates on the fly. Like with the fast-acting lightbulb, your brain is stuck in the old pattern. If you're made aware of what would normally be four seconds' worth of data, then you think your fall lasts for four seconds.
In 2004 Tse performed an experiment in which he flashed repetitive images on a computer screen, followed by a novel one, as in: coffee cup, coffee cup, flower. Even though the images are all on the screen for the same amount of time, participants reported that the novel one seemed to last longer. Tse concluded that the brain stretches time out during novel experiences.
If this were true, then Eagleman's jumpers should have seen the numbers on the chronometer flashing at a slower rate-and they would need to flash only a hair slower to be readable-while falling. (Tse contends that the retina can't process images fast enough for this to work under any circumstance; Eagleman counters that studies show that the retina processes images 100 times per second, well within the range required to read his chronometer.) So Eagleman reran Tse's oddball experiment with a twist. If attention is responsible for this effect, a more emotionally stirring "oddball"-like a guy pointing a gun at you, which tests have shown is much more salient than a flower-should seem to stay on the screen even longer. But people found the gun-toting man no more novel than a flower.
In trying to puzzle out why, Eagleman read studies reporting diminished electrical activity in the brain when it viewed a familiar image. "It's a boring rule of thumb called repetition suppression," he says. "But for this experiment, it was the Rosetta stone." Eagleman proposed that instead of slowing down time in response to a novel flower, as Tse believes, the brain speeds it up during the repetition of the coffee cup-it recognises the cup immediately and spends less time and energy inspecting it. "In a perfect system, the brain would know what was coming next and use zero energy to represent it," Eagleman says. "The diminished energy of the repeated image is just a lesser example of this."
Time Travel as Medicine
Out in the real world, this research has some interesting ramifications, among them figuring out the causes of mental illness. Everyone has a near-constant internal monologue in their head. It's a two-stage process: You generate a voice and hear that voice. "This occurs simultaneously," Eagleman says, "but if the timing of those two processes got out of sync, it could sound like you were hearing someone else's voice." It's possible that this could be the root of the auditory hallucinations that many people with schizophrenia experience. In the past year, Eagleman ran 30 schizophrenic patients through the oddball test and found that they don't exhibit repetition suppression-to them, every experience is novel. "Just like we recalibrate the brain's timing mechanisms in the 9 Square experiment," he says, "we could use games to recalibrate the brains of schizophrenic patients to make their auditory hallucinations go away."
Deana Davalos, a psychologist at Colorado State University who works on timing and mental illness, agrees. "Sensory gating, the process by which the brain filters out repeated stimuli, is a problem with schizophrenia," she says. "Most people think it's a breakdown in their ability to inhibit responses to repeated stimuli, but Eagleman's work points to a timing malfunction." To this end, Eagleman, with the help of psychologists, is designing a videogame that would recalibrate the brains of patients. He hopes to begin testing it in the next few years.
For now, he continues devising new temporal illusions, hoping to force another odd flash-lag type of result that will help unlock the brain's secrets. And although his work focuses on producing scientific methods for measuring time, he can't help but ponder what it all means. When Albert Einstein showed that time was relative, he said that a person in a spaceship travelling at the speed of light experiences time differently than one standing on Earth. But Eagleman is finding that time might be relative even if the two observers are standing next to each other. He has a long way to go to prove that time is not the objective constant we think it to be, and that each person instead experiences time's passage on an individual basis, but, he says, "it does make one wonder what else we're going to learn along the way."
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