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How do you really know what time it is?

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Why can't you tell when an hour has passed without looking at a watch? Why are you able to do three things at once? Does coffee make time go faster? Neuroscientsts explain how our brains tell time - or don't.

It was a familiar feeling of surprise and dismay. Looking at the clock on my computer, I realized that the five minutes I'd spent finishing up an article had actually been 25 minutes. Now I was going to be late for my lunch meeting. As I raced down one of those insanely tall San Francisco hills to get to the restaurant, I wondered for probably the millionth time why I am always running late.


This time, I vowed, I was going to find out.

I turned to an obscure field of neuroscience for answers. The scientists who work on the problem of time in the brain sometimes refer to their area of expertise as "time perception" or "clock timing." What they've discovered is that your brain is one of the least accurate time measurement devices you'll ever use. And it's also the most powerful.


Why your perception of time will never be exact
When you watch the seconds tick by on a digital watch, you are in the realm of objective time, where a minute-long interval is always 60 seconds. But to your brain, a minute is relative. Sometimes it takes forever for a minute to be over. That's because you measure time with a highly subjective biological clock.

Your internal clock is just like that digital watch in some ways. It measures time in what scientists call pulses. Those pulses are accumulated, then stored in your memory as a time interval. Now, here's where things get weird. Your biological clock can be sped up or slowed down by anything from drugs to the way you pay attention. If it takes you 60 seconds to cross the street, your internal clock might register that as 50 pulses if you're feeling sleepy. But it might register 100 pulses if you've just drunk an espresso. That's because stimulants literally speed up the clock in your brain (more on that later). When your brain stores those two memories of the objective minute it took to cross the street, it winds up with memories of two different time intervals.

Image via Warren Meck.

And yet, we all have an intuitive sense of how long it takes to cross a street. But how do we know, if every time we do something it feels like it a slightly different amount of time? The answer, says neuroscientist Warren Meck, is "a Gaussian distribution" - in other words, the points on a bell curve. Every time you want to figure out how long something is going to take, your brain samples from those time interval memories and picks one. "You randomly sample from it," says Meck. "So you might pull a 25 out of distribution, or a 36. You're only accurate in the mean."


The good news is that, on average, you will predict correctly how long it takes to cross the street. The bad news is that occasionally, you'll pull an outlier memory from that bell curve and decide to cross the street much more slowly than you should.

Your intuitive sense of how much time something will take is taken at random from many distorted memories of objective time. Or, as Meck puts it, "You're cursed to be walking around with a distribution of times in your head even though physically they happened on precise time."


Why you can do three things at the same time
Your internal clock may be the reason why you can multitask. Because nobody - not even the lowly rat - has just one internal clock going at the same time.


At the very least, you've got two internal clocks running. One is the clock that tracks your circadian rhythms, telling you when to go to sleep, wake up, and eat. This is the most fundamental and important of all your internal clocks, and scientists have found it running even in organisms like green algae. The other clock you've likely got running is some version of the interval time clock I talked about earlier - the one that tells you how long a particular activity is going to take.

Working with Meck, neuroscientist Catalin V. Buhusi discovered in experiments with rats is that your brain can keep multiple interval time clocks going in a pretty complicated way. He trained rats to press three levers for food at three different intervals: one at 10 seconds, one at 30, and one at 90. After a learning phase, the rats were able to simultaneously time the intervals for all three levers, pressing one every 10 seconds, one every 30 seconds, and so on. More amazingly, the rats could stop, start, and reset those clocks. If the 30 second lever stopped, they would continue right along with the 10 and 90 second ones. And when the 10 second lever stopped and then started again, the rats could recalibrate the time of each interval and start pressing that lever at the appropriate speeds again. What this demonstrated to Buhisi was that rats can run many internal clocks at once. And humans can too.


Right now, it's likely that you are running at least three clocks: Circadian, plus a clock that's timing how long it's taking you to read this article, plus a clock telling you how long you have until you get home from work. And who knows what else you are keeping track of? If you're tapping your foot and doing a task in the background, that would add two more clocks.

Your ability to do many tasks at the same time hinges on this talent for juggling multiple clocks. It should come as no surprise, then, that the neural networks in your brain that assist in time perception are the same networks that allow you to plan and coordinate your physical movements. Your sense of time and your ability to act are connected at a very deep level in your brain. Put simply, timing two things at once and doing two things at once are, from your brain's point of view, pretty much the same thing.


Why coffee makes time go faster, and Alzheimer's makes it slower
I already told you that I'm late all the time, so I might as well admit more of my foibles: I drink coffee and I smoke pot. Not surprisingly, given my problems with being on time, both of these drugs are known to affect the speed of internal clocks.

It turns out that changing the speed of your internal clock affects your memories, too. Let's start with caffeine, which makes your internal clock go faster. If your brain normally stores 60 pulses for 60 seconds, your brain on caffeine stores 100 pulses. Two things happen as a result. First, when you retrieve your time memory, that minute will seem shorter than the 60 seconds it actually took. So a speedy clock means that time gets faster. Second, your memories get more granular. In one minute, you are storing 100 pulses, which add up to more data storage in your brain per second. Coffee and other stimulants make you remember more. On the flip side, an antipsychotic drug like haloperidol slows down your internal clock and makes that minute seem much longer (though far less memorable).


Caffeine and haloperidol affect only clock speed, mostly by messing with the dopamine system in your brain. But you can distort time just by manipulating memory, too. The acetylcholine system in your brain regulates how time-based memories are stored. It turns out that one of the primary causes of memory loss in Alzheimer's comes from a lack of the memory-saving enzyme acetylcholine in the brain. People with Alzheimer's have internal clocks that are running just fine, but they are saving memories from those clocks much more slowly. That's why people with Alzheimer's have a hard time judging how long things will take. And why their memories are so vague.


One common treatment for Alzheimer's memory loss is to take supplements that boost acetylcholine in the brain. These supplements can sharpen anybody's memory without ever giving you that "speedy" effect of caffeine because they never touch the dopamine system. They send more chunks of time to memory without ever speeding up your internal clock.

The interesting thing about smoking pot is that marijuana is one of those rare drugs that seems to interact with both the dopamine and the acetylcholine system, speeding up the former and slowing down the latter. That's why when you get stoned, your heart races but your memory sucks.


Why you hear faster than you see
It's easy to distort the way your brain perceives time, but this organ is also remarkably accurate when it comes to figuring out what's happening to you millisecond by millisecond. Virginie van Wassenhove is a biologist who has studied how the brain figures out the order of events that happen in under a second's time - and has discovered that what you see and hear can change the way you perceive time.


One of the weirdest aspects of time perception is that your brain sees things much more slowly than it hears them. As van Wassenhove put it, "If you present a beep and a flash to somebody, then record from their cortex, you'll find that activity in the visual area will respond 50 milliseconds later. But the auditory cortex responds 12 milliseconds later." So your brain processes what you see more slowly than it processes what you hear.

Nobody is sure why this is. Van Wassenhove speculates, "Maybe it's about the difference between the speed of sound and light. The auditory system involves transduction, and doesn't take much time. Maybe it's about photochemistry in the eyes. There may just be differences in processing time required."


And yet, despite this discrepancy, your brain can still perceive the order of flashes and beeps that are only 20 milliseconds apart. Despite the fact that there's a 38 millisecond time lag between what you hear and what you see, your brain can still figure out if a something burst into flame 25 milliseconds before there was a loud popping noise.

Remember, this is all happening in under a second. So this same brain that isn't sure how long it takes to cross the street is able to pinpoint the order of events down to a few milliseconds.


In a series of experiments, Van Wassenhove and her colleagues found that what you see can change time perception. For example, if an object is looming in your vision and appears to be getting closer, perceptive time gets slower. The same goes for a sound that gets louder. The reason for this is simple: When you pay close attention to something, time is distorted.

It seems that paying attention to visual inputs can even distort the meaning of sounds. Say you're looking at an enormous airplane zooming toward you. Time will dilate even if you hear a sound that suggests the object is receding, such as the engine getting fainter.


Why I am often late
So let's return to my original burning question, as I plummet down the hill to my lunch meeting. Why am I always late? One possibility is that all my coffee-drinking and pot-smoking has permanently affected my ability to figure out how long things will take. I'm speeding up my clock so often, and slowing down my memory-storing acetylcholine system so frequently, that my memories of time intervals have gone random. When I sample an interval from that Gaussian distribution, I get something truly misleading, which causes me to miscalculate how much time it will take to get to lunch.


Though this scenario would probably make "just say no to coffee" campaigners very happy, it turns out to be unlikely. In experiments with rats, Meck observed that the rodents' brains began to compensate for time distortions caused by drugs that slowed or sped up internal clock time. Once my internal clock is used to operating at a faster speed due to caffeine or marijuana, it adjusts and provides me with a reliable average time interval for events.

In fact, it seems more likely that the culprit in my case can be explained by van Wasserhove's experiments with attention and time distortion. Remember, the looming object that occupies your attention can cause time to slow down subjectively. Paying attention to something can make time seem to speed up, too.


When I asked van Wasserhove to offer some ideas about why I'm always late, she said it could have to do with what I'm paying attention to. She explained:

If you go on your first romantic date, time is going to be very fast because you're not paying attention to it. You're having an interesting discussion or something and you don't think about time. But at the doctor's office you really keep track as you wait for an appointment. If you pay attention to time it slows down.


In my case, I was paying attention to writing an article, and what I thought was five minutes turned out to be 25. From the perspective of my brain, focused attention is like a drug. It sped my internal clock up, giving me a distorted sense of how many minutes had passed.

So how do I prevent myself from being late? Even though my brain is running many internal clocks, it turns out there's a good reason why I carry an external, objective clock too. By consulting my digital watch, I correct the distortions I'm doomed to experience as a creature whose temporal sense is generated by an imprecise, biological mechanism.


Which made me realize that clocks may well be humankind's oldest brain-enhancing technology. They allow our brains to experience something we never could without machines: Objective time.

Image by Daniel Spitzer

This io9 Flashback originally appeared in November 2010.