“Ready steady slow”: time slows down when we prepare to move

Not Exactly Rocket ScienceBy Ed YongSep 5, 2012 4:00 AM


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A baseball speeds from the hands of a pitcher, a slave to Newton’s laws. But in the brain of the batter who is watching it, something odd happens. Time seems to dawdle. The ball moves in slow motion, and becomes clearer. Players of baseball, tennis and other ball sports have described this dilation of time. But why does it happen? Does the brain merely remember time passing more slowly after the fact? Or do experienced players develop Matrix-style abilities, where time genuinely seems to move more slowly? According to five experiments from Nobuhiro Hagura at University College London, it’s the latter. When we prepare to make a movement – say, the swing of a bat – our ability to process visual information speeds up. The result: the world seems to move slower. At first glance, this might seem to contradict a now-classic experiment by David Eagleman. He threw volunteers off a tall fairground ride and asked them to stare at a special watch, to see if their perception of time would slow. It didn’t. They merely remembered the experience as being long and drawn out afterwards. (See my earlier post for the details.) But there’s a critical difference between the two studies. Eagleman studied time perception while people were actually undergoing a crisis—in this case, falling to their possible doom. But Hagura showed that time appears more leisurely before an event, rather than during it—when we’re preparing to move, rather than moving. Hagura first asked volunteers to press a key for as long as a white disc appeared on a screen. The disc would then be replaced by a hollow target. In some trials, the volunteers had to release their key and touch the target. In others, they were told to keep pressing the key. In every case, they had to say how long the white disc stayed up for, compared to all the previous trials in the experiment. Hagura found that the volunteers deemed the durations to be longer if they were preparing to move, than if they were planning to keep still. Perhaps the volunteers who were about to reach out were just more excited or attentive? Not so. When Hagura changed the task from pressing (or not pressing) the target, to naming (or ignoring) a letter, the time-slowing effect vanished. Preparing to move makes the difference, rather than just preparing for any old task. In a third variation, the white disc was replaced by two possible targets instead of just one. In some trials, the disc had a line that told the volunteers which of the two targets was correct, allowing them to prepare the right movement. In other trials, there was no line, and the volunteers had to make their move when the two targets appeared. As you might have guessed by now, they thought the white disc stayed up longer if they were preparing to move their arm in a specific direction, but not if they were simply waiting. These three sets of results support the idea that time moves more slowly when we prepare an action. But they could also be explained in the same way that Eagleman’s results were: Time only seemed to pass more slowly because the volunteers remembered it doing so. But two final experiments suggest that, instead, preparing to move actually slows “the flow of visual experience”. First, Hagura replaced the solid white target with one that flickered at different frequencies. The volunteers had to say whether it was flickering faster or slower than usual, compared to previous trials. If they were preparing to hit the screen, they said that the high-frequency flickers were slower than they actually were. Second, Hagura showed his volunteers a stream of rapidly flashing letters, while they held a key. Each letter appeared for just 35 milliseconds, and the whole series went by in less than a second. Somewhere in the stream, there was a C or a G, but never both. Once the sequence had stopped, as before, the volunteers either kept holding their key, or touched the screen. Their task was to say whether they had seen a C or a G. If the volunteers were preparing to reach out, they got the right answer about 66 percent of the time. If they kept still, their success rate was just 59 percent. By readying their arms to touch the screen, they were better able to spot their target amid the zooming letters. This difference was particularly marked if the C or G appeared towards the end of the flashing sequence – the longer the volunteers spent preparing to move, the slower time seemed to pass. How does the slowing effect actually work? We don’t know. Hagura notes that there are certainly connections between the parts of the brain that encode the passage of time, and those that prepare sequences of movement. The details, however, are still unknown. Why does the effect happen? Hagura argues that speeding up our powers of perception allows us to change, tweak and halt our course of action on the fly. He writes: “As expert ballgame players assert, being maximally prepared may allow ‘more time’ to perfect the hit.” That would be a clear benefit, but Andrew Welchman, who studies perception at the University of Birmingham, wonders if there are any drawbacks. “You never get anything in the brain for free, so if you get better at one moment in time, you should get worse at another,” he says. “Take someone who moves a lot versus someone how moves little. They should both be calibrated to the same external time, so the one who moves a lot needs to have more 'downtime' to keep in step.” A bout of Neo-like bullet-time should be followed by a burst of perceptual sluggishness. For example, Welchman says that when we move our eyes around, our visual sensitivity plummets immediately before, during and after the movement. This is called saccadic suppression. The standard interpretation is that we’re “filtering out the junk” – the “smeary visual signals” that we get when our eyes move too quickly. “But framed in light of this paper, it might be a way of resetting the clock so that the person stays calibrated to the visual world around them,” says Welchman. Reference: Hagura, Kanai, Orgs & Haggard. 2012. Ready steady slow: action preparation slows the subjective passage of time. Biology Letters http://dx.doi.org/10.1098/rspb.2012.1339

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