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Fast Feet, Slow Brains


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If you have ever run on a treadmill, you’ve felt it: the illusion that you are moving faster than normal when you step off onto solid ground. Adar Pelah and Horace Barlow, physiologists at the University of Cambridge, devised an experiment to find out more about the mechanisms behind this odd sensation.

First the researchers had to establish that the sensation was real, so they asked 14 people--7 of whom did not know of the effect--to walk around a room, walk on a treadmill for 20 minutes, then walk around the room again. All 14 reported a feeling of acceleration when they got off their machines.

To quantify these subjective impressions, the researchers asked the subjects to walk a five-yard course several times just after running on the treadmill, all the while keeping what they perceived to be the pace they had when they first left the treadmill. We timed the laps, and we found that with each additional lap, they had to accelerate, says Pelah. Apparently the runners were walking faster with each lap to compensate for their initial--but gradually diminishing--perception that their speed upon first leaving the treadmill was quite fast.

When someone is on a treadmill, says Pelah, the brain perceives that the limbs are moving in a normal walking mode, but the surroundings remain stationary. After about ten minutes, the brain shifts gears and stops associating the bodily movement of walking with real motion from one location to another. So when the person steps off the treadmill and continues walking at the same pace, the surroundings appear to flow by very swiftly, creating an illusion of acceleration.

But as the brain’s perceptions begin to return to normal, the feeling of acceleration fades. To maintain it, the walkers have to pick up their pace. When we step off and walk around, the brain says, ‘Wow, things are going by very fast,’ and until it readjusts, says Pelah, we have the sensation of accelerated motion.

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