HAVE you ever worked out on a treadmill and then been disturbed by a
sensation of moving at immense speed when you walk away afterwards? If so, don鈥檛
worry. The effect is a natural consequence of the way the human brain adapts to
discrepancies in the environment, says Adar Pelah of the University of
Cambridge.
When a person walks or runs, the brain is kept well-informed about the body鈥檚
rate of progress from a variety of sources: from muscles telling it how hard
they are working, from eyes that watch the scenery go by and from nerve signals
verifying that the brain鈥檚 instructions are being carried out. But when running
on a treadmill, some of this information is at odds. The eyes report that the
body is going nowhere, but the muscles and nerves insist otherwise.
The brain soon adapts, however. 鈥淥ver time your normal associations change,鈥�
says Pelah. After about 10 minutes, the brain has recalibrated the information
from eye and muscles to equate strenuous running with zero motion.
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Pelah and his Cambridge colleague Horace Barlow have now studied the
illusions created by this recalibration once people climb off a treadmill. They
asked 14 volunteers to jog vigorously on a treadmill for 10 minutes, and then
climb off and walk, maintaining a constant 鈥減erceived鈥� speed as they did so.
After a couple of minutes, the subjects鈥� walking speeds had increased by an
average of a third. This shows that they were initially overestimating their
rate of progress, presumably because the treadmill-calibrated input from their
eyes overrode the information coming in from the muscles (Nature, vol
381, p 283).
Many everyday experiences create similar illusions, says Pelah. For instance,
riding for a few minutes on an airport travelator makes you feel especially
sluggish when you step off, he says.