In Audrey Niffenegger's novel The Time Traveler's Wife, Henry DeTamble is a man with a rare disorder that causes him to involuntarily travel through time. His wife Clare experiences life linearly, but never knows when or where she will see her husband next. "Each moment that I wait feels like a year," Clare says. "Through each moment I can see infinite moments lined up, waiting."

You don't need a time-traveling husband to have a warped experience of time like Clare's — think of how long a Monday back at work seems to stretch out, and how quickly the weekend flashes by. Time should march steadily, but that doesn't always match our perception. Neuroscientists and psychologists are searching for answers to this conundrum, using psychophysical experiments and brain scanning technology to unlock just how human brains track the passage of time. Researchers are gaining insights, and some of these findings could help us to better understand, among other things, disorders like schizophrenia and Parkinson's where sufferers have trouble perceiving time.

"Timing is fundamental, and it's involved in many different tasks," says John Wearden, a psychologist at the University of Keele in the U.K. "You wouldn't be able to tap on a keyboard or move your body unless you could program actions in time." Wearden has spent nearly 25 years researching timing in animals using the Scalar Expectancy Theory, which hypothesizes that animals have internal clocks. He was also the first person to apply this theory to human timing.

As it turns out, we humans can be pretty decent at keeping time. In one of Wearden's experiments, volunteers were trained to distinguish between a short tone of two seconds and a long tone of eight seconds. After participants learned to decipher between the two sounds, they were presented with tones of varying lengths and asked to identify which was short and which was long. Wearden found that although participants had a slight bias for responding long, they were fairly accurate in their assessment of long or short sounds based on memory.

More directly addressing our subjective sense of time — when we get the feeling things are moving in slow motion or going by in a flash — neuroscientist David Eagleman had volunteers measure their perception of time as they plummeted 15 stories down on a free-fall ride at an amusement park. Eagleman gave each volunteer a wristwatch that displayed numbers flickering by at faster-than-normal speeds, hypothesizing the riders would be able to see the numbers if their perception of time was slowing. As it turned out, they couldn't see the numbers, but participants did report that the fall seemed to last nearly twice as long as the actual 2.6 seconds it took to drop.

Some researchers speculate that during novel situations, time feels slower because the brain pays more attention. To assess this, in 2004, Dartmouth College neuroscientist Peter Tse performed a computer-based experiment in which a repetitive image flashed on the screen followed by a unique one. All of the images were shown for the same duration of time, but participants mistakenly believed that the unique image appeared on the screen for longer. Neuroscientists refer to this as the "oddball effect," which occurs when the brain pays less attention to the mundane and more attention to novel stimuli. (A similar test was performed on patients with schizophrenia, but none of the participants experienced the oddball effect, presumably because their brains were less able to focus on targets or ignore distractions — essentially, everything was novel to them.)

To fully understand how the brain keeps time, researchers say that it's important to start with a view of the brain as a complex network of circuits. "The greatest insight is that the brain doesn't use a man-made clock," says Dean Buonomano, a professor in the Departments of Neurobiology and Psychology at UCLA. "It uses patterns."

The brain takes in an overwhelming amount of sensory information about the present moment, rearranges it and presents it as a particular coherent story of the world. FMRI studies have shown that clock time and subjective time activate various parts of the brain, including the cerebellum, motor cortex and basal ganglia.

Each time the brain takes in a sensory event it sets off a chain of reactions between brain cells. And every reaction leaves a time stamp of sorts, so that the brain cell network can translate time, Buonomano explained in a paper in the journal Neuron. These patterns, say Buonomano, shape how we relate to the world. "The brain evolved for animals to predict and anticipate when it is going to be dark, when the seasons change," he says. "We can't understand the brain without understanding how to tell time."

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