Uncovering how the mind constructs our sense of time could help treat and prevent some psychological disorders.
by Carinne Piekema
December 8, 2012
The brain’s master clock. Suprachiasmatic nuclei: These clusters of cells are located at the base of the brain, just above where the optic nerves enter. They play a central role in keeping time in the brain. Credit: Russell Foster
When David Eagleman was just eight years old, he fell from the roof of a house under construction. The short journey from rooftop to ground—which he later calculated could have taken no more than a second—changed his life. "It seemed to take a very long time to get from the roof to the floor and I appeared to have very clear thoughts of what was happening," Eagleman recalls. "I was really surprised by that, because I couldn’t understand how it could feel so long even though it was such a short physical duration of time."
Many who have had the misfortune of meeting with a life-threatening situation have reported a similar experience: Time seems to slow down during perilous circumstances. Today, Eagleman heads the Neuroscience Laboratory for Perception and Action at Baylor College of Medicine in Houston, Texas. With his team, he is trying to understand why time appears to slow in this way and how the brain deals with automatic, involuntary perceptions of time. An accurate sense of timing is essential for almost all behavior, from walking and talking to waking and sleeping, and Eagleman’s is one of a number of groups around the world investigating the brain’s timekeeping mechanisms. Disturbances in timing processes can contribute to the symptoms of a number of major disorders, such as Parkinson’s disease, schizophrenia, and depression. The hope is that by understanding the fundamental nature of time at the biological level, it may be possible to treat and even prevent certain medical conditions.
"We need to understand everything, from the basics of neurons to the nuclei and up to the systems in the brain, to know what actually goes on," says Eagleman. "We still have a long way to go in terms of having a model of how it all fits together."
At his lab, Eagleman has been investigating, among other things, whether time really does slow down when your life is under threat—or whether that’s an illusion. Since Einstein put forward his theories of relativity around a century ago, physicists have been comfortable with the idea that clocks—including our internal body clocks—literally run at different rates, depending on motion and gravity. This is exemplified in the famous twin paradox: Two sisters may start out at the same age, but if one of them stays on Earth, while the other takes a trip on a spacecraft traveling close to the speed of light, the stay-at-home sister will age more than her intrepid twin. That’s a real physical effect. But are the biological examples of time dilation and contraction experienced by people everyday—effects Eagleman describes as examples of "neural relativity"—also real phenomena?
Although time appears to be a single, unified entity, the emerging picture indicates that may be an illusion.
- David Eagleman
There are two possible explanations for the effect of time-slowing down during stressful events. The first is that during the event—the fall from roof to ground, say—people experience what’s known as "increased time resolution," marking time in sharper, narrower intervals, enabling them to observe more details than usual. The second hypothesis is that memories are laid down differently during the fall, so that later, when looking back on the event, they are fooled into think that time passed more slowly than it did. In order to test between these two possibilities, Eagleman’s team built a wristwatch for volunteers that flickered a series of numbers. The display changed at a rate that was slightly too fast for people to be able to distinguish the individual numbers on the screen, under normal circumstances; instead they just saw a blur. The idea was that if time really does slow while you are under threat—that is, if you experience increased time resolution during this scary period—you should be able to read the digits on the watch as they flash past.
To recreate a realistic life-threatening situation, Eagleman dropped volunteers wearing the wristwatches from a 150 ft tower. While in free fall, they looked at their watch to check whether they could make out the digits. The answer was they could not; people were no better at reading the watch-face when falling—meaning that time did not literally slow down for them. Eagleman also asked the volunteers, after having landed, to retrospectively estimate the length of their own fall compared to the drop time of other volunteers. People consistently felt that their own fall lasted longer.
Eagleman investigated this duration dilation effect further to find out why it may occur. In one lab task, volunteers look at pictures presented one after the other in the centre of a computer screen, each of which stays on the screen for exactly the same amount of time. Nonetheless, when, after several repetitions of the same shoe picture, for example, a ball appears on the screen, people usually report that the ball stayed on the screen for much longer than the shoe. Eagleman’s explanation for this effect is that while the shoes keep appearing, you can easily successfully predict what will come next, so your brain gradually expends less effort—and thus less energy—paying attention to the images. In turn, your diminished attention makes it seem as if time is passing more quickly. Indeed, by measuring brain activity, the team showed activation in parts of the brain declines with repeated viewing of the same image. Then, the sudden appearance of a novel ball amongst all the shoes grabs the brain’s full attention again, consuming more energy and causing it to seem as though the ball was present on the screen for longer.
Duration dilation can be seen in experiments carried out by other researchers too: When images that are brighter, bigger, or more complex are flashed up, for instance, the brain will perceive that they were displayed for a longer period of time because, says Eagleman, they forced the brain to burn more energy in observing them. This would also explain why we think that terrifying events are longer in duration—because our brains burn more energy than usual, as they unfold.
Experiments such as those described by Eagleman demonstrate that to the brain, time is not one simple thing, but is multi-faceted: "Although time appears to be a single, unified entity, the emerging picture indicates that may be an illusion," he explains. "Instead, many different aspects of time may be underpinned by different neural mechanisms."
This has been borne out by brain-imaging tests that show that animals, including humans, do not have a dedicated set of regions for processing time information. At Warren Meck’s laboratory at Duke University in North Carolina, researchers use a wide range of brain-imaging techniques to monitor how different animal species—including monkeys, rats, mice and people—keep time while carrying out the same psychophysical tasks. In one experiment, subjects are trained to press a button when they think a certain amount of time has passed. (Animals can be taught to do this if given food rewards for accurate predictions.) Both humans and other animals can learn to be very accurate at this.
Meck and colleagues employ a variety of techniques to monitor brain activity and, in some cases, to selectively interfere with it—by safely applying electrical stimulation using electrodes, for instance—during the timing task. Through these experiments, they have discovered that neurons in multiple parts of the brain fire when processing timing information (Allman, M. J., & Meck, W. H., Brain, 135, 656-677 (2012)). This sets it apart from our major senses like vision, hearing and touch, which each have their specific corners of the brain. Instead, time appears to be represented within an extended set of brain regions centering around the frontal cortex (important for planning) and the basal ganglia (which supports movement and several other functions).
This lack of a specific brain region dedicated to timing, equivalent to the visual cortex for vision, does not mean that timing is merely a by-product of other complex computations: Meck believes the cortical-basal ganglia system is the core mechanism that promotes timing together with localized systems, which are specific to controlling sense and motion. "This sort of satellite system with a ’mother ship’ ensures that the organism’s timing ability is never completely lost," explains Meck. "Timing is so crucial that it is embedded in other systems such as motor control and memory, but the presence of the core system is fundamental to time perception."
Warren Meck Duke University
It is not just the structure of the brain that plays a role in our sense of timing. Much research over the years has shown that the balance of neurochemicals within these structures is vitally important too. For instance, if you give animals drugs that change the level of dopamine in their brains, they will behave in their timing tests as though the speed of their internal clocks has been altered. Similar changes might also affect patients who suffer from neurological or psychiatric disorders. We tend to associate Parkinson’s disease, which is caused by a loss of dopamine in the basal ganglia, with movement problems such as uncontrollable tremor. (Parkinson’s disease is diagnosed by characteristic motor abnormalities—and so is not classically thought of a neuropsychiatric disorder. However, Parkinson’s disease may be preceded by, and is frequently accompanied by, a wide range of cognitive and neuropsychiatric features.) However, Meck and his collaborators are beginning to find evidence that these patients’ timing abilities are also impaired.
Eagleman’s work with patients suffering with schizophrenia as well as with non-sufferers also backs the idea that timing plays a key role. In one test, he asked users to press a button and then, after a very slight delay, a flash of light appeared on the screen. Volunteers’ brains learned to ’edit out’ this time-delay, instead perceiving that the push and flash occurred simultaneously. When Eagleman suddenly removed the tiny delay so that the button-press and the flash genuinely happened simultaneously, most users were fooled into thinking that the flash had occurred before they pressed the button—and they then wrongly stated that they did not cause the flash. This reminded Eagleman of the way that many schizophrenic patients argue that they do not cause some of their own actions.
The Master Clock
Our health and well-being is also affected by another important internal timing mechanism: the approximately 24-hour circadian rhythm that regulates our sleep patterns. Every individual’s circadian rhythm depends on the intricate collaboration between brain regions and hormones, as well as on neurotransmitters like dopamine. It is controlled by a master clock located in a small structure lying in the middle of our brain called the suprachiasmatic nucleus. This nucleus acts as a reference point for other clocks in the body determining such things as our heart rate and hormones. While this system can be very robust, even small changes can have a significant impact on our sleep, wakefulness and ultimately our mental health. Russell Foster, at the University of Oxford, UK, leads a research group investigating whether bigger disruption to our circadian clocks could be linked to mental and neurological illnesses, such as depression, schizophrenia and Parkinson’s.
"Many different brain neurotransmitters and brain structures are involved in sleep," explains Foster. "So if you have a defect in one system in the brain that predisposes you to neurological illness, you are almost always going to find an impact on sleep."
Foster’s interest in the link between circadian rhythms, sleep and neurological disorders started with a casual conversation in an elevator with a psychiatrist, who told him that "everybody knows" that the reason that patients with schizophrenia have appalling sleep patterns is because they don’t have a jobs, so they go to bed late, get up late, and have a disrupted social life. "That did not make sense to me," says Foster, who immediately questioned which was the cause and which was the effect in the behavior patterns described. "I wanted to find out what is going on there," he says.
Russell Foster Oxford University
Katharina Wulff, a chronobiologist in Foster’s lab, studied the sleep and wake cycles of patients with schizophrenia using a small device placed around the patients’ wrist that monitors their patterns of activity and rest. The results showed the worst disruption Foster and Wulff had ever seen: "This clearly wasn’t the result of the lack of a job and we came to the realization that schizophrenia and other neurological conditions share common and overlapping mechanisms with the generation of normal patterns of sleep and wake," says Foster.
On the basis of recent studies using mice, Foster and colleagues think that in some forms of schizophrenia, the problem arises because the master clock itself is running on normal time, but the output it produces is somehow pushed forward in time and this sends an abnormal signal to the rest of the body (Oliver, P.L et al, Current Biology 22(4):314-319). This distortion has some of the features of jet-lag. By using cognitive behavioral therapy and advice about how to achieve good sleep, psychiatrist Daniel Freeman, a close collaborator of Foster’s at Oxford, was able to greatly improve the quality of sleep in schizophrenia patients and in parallel reduced the psychotic symptoms of these individuals. These preliminary studies are now being expanded to include large numbers of subjects, and hopes are high that this approach will represent a real breakthrough in the treatment of psychosis, says Foster.
If you have a defect in one system in the brain that predisposes you to neurological illness, you are almost always going to find an impact on sleep.
- Russell Foster
Further support for an overlap between the mechanisms that generate normal sleep and mental health come from the observation that abnormal sleep often precedes mental illness. Foster’s group has looked at people who have a high risk of developing bipolar disorder but are not yet ill. They found clear disturbances in their sleeping patterns, suggesting that these sleep abnormalities act as an early warning for the full-blown disorder. The knowledge that these problems happen even in the pre-clinical state might help clinicians pre-empt the onset of the illness.
The nature of time in biology has turned out to be complicated, but the work of Foster’s group, as well as that of Eagleman, Meck and others, has shown that getting a handle on it could fundamentally improve the quality of life of many, and in some cases help to delay or even prevent the onset of psychiatric illness. "This is a big wish," says Foster, "but clearly worth exploring."
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