Anyone who’s been on a treadmill at the gym has probably had that strange perceptual experience afterwards – once you start to walk on stable ground again, it feels for a time as though you’re moving forward more quickly than you really are. The illusion, which is especially striking for treadmill newbies, was first documented scientifically in a Nature paper 20 years ago. Since then psychologists have come to better understand what’s going on and the ways the effects can manifest.
It mostly has to do with the mismatch between optic flow – the speed the visual world is moving past, which is zero when you’re on a treadmill – and locomotor activity, be that walking or running. Essentially your brain has to perform a perceptual recalibration. Usually when you walk forwards, the visual world moves past at a predictable rate that depends on your speed. But on the treadmill this usual relationship is decoupled. Your brain comes to expect that to stay still (that is, to experience zero optic flow) it has to walk or run at a certain speed.
One way this can manifest is that if you blindfold someone who’s been running on a treadmill, then ask them to run on the spot, they will actually jog forwards without realising it. The blindfold stops them recalibrating back to normal and their brain is still acting as if it needs to run to remain stationary.
Now a study in Psychonomic Bulletin and Review has added another piece to the treadmill illusion jigsaw. It tackles the question of what measure your brain uses to calibrate your locomotion with optic flow. Let’s say you’re walking at 3 miles per hour on the treadmill. Your brain recalibrates, now figuring that this is the act of walking that results in zero optic flow. But what is “this act of walking” – how is it measured, by speed or by effort?
Jonathan Zadra and Dennis Proffitt, at the Universities of Utah and Virginia, tested this by first asking 41 undergrads to march on the spot blindfolded for twenty seconds – this was to get a baseline sense of how much they drifted forwards involuntarily (most people drift a bit when blindfolded). Next, the students took off the blindfold and spent ten minutes walking at three mph on a treadmill. Half of them did this with the treadmill set to be level (i.e. no incline), the other half walked the same speed but with the treadmill set as a slope with a ten degree incline. Finally, they got off, put the blindfold back on, and tried another 20 seconds marching on the spot.
Both groups of students showed extra forward drift after being on the treadmill compared with baseline, but the students who’d been on the inclined treadmill showed significantly more drift. This suggests that when recalibrating to the effects of zero optic flow on a treadmill, our brains factor in amount of walking or running effort. If our brains relied only on speed, then the two groups should have shown the same amount of drift.
A second study ruled out the possibility that there could be some other more mechanical explanation, for example to do with the hill walking affecting the muscles and joints in some way, and this then affecting the subsequent marching. The whole procedure was repeated, but with students wearing a virtual reality headset on the treadmill, to induce artificial optic flow (the display showed streets passing by as they walked). Now the effects of treadmill walking on marching drift disappeared, both for those who’d been on a flat treadmill and those on the inclined treadmill. This confirms that greater drift shown by the inclined walkers in the first experiment must have been to do with making a larger optic flow recalibration, factoring in their larger energy expenditure.
Everyday life is full of fun examples, like the treadmill illusion, that suddenly make us aware of the remarkable unconscious work our brains do, to help us navigate safely through the physical world. This study is a reminder that we’re still figuring out exactly how it does it!