When I returned home after a day of sailing lessons, I still had the feeling that the room was moving up and down. Why is this?
鈥� In order for you to estimate your location, your brain combines information from a variety of sources, including sight, touch, joint position, the inner ear and its internal expectations. Internal expectations, in particular, provide a useful predictive component to this mental motion processing. Under most circumstances, the senses and internal expectations all agree. When they disagree, there is imprecision and ambiguity about motion estimation, which can result in loss of balance and motion sickness.
On boats, seasickness may develop because of conflict between sensory input and internal expectations about motion. Developing 鈥渟ea legs鈥� is nature鈥檚 cure for seasickness: you become accustomed to anticipating the boat鈥檚 movements and prepare to adjust your posture accordingly. When you finally go ashore, you may feel your body continuing to do this for hours or even days afterwards, making it seem as if the room is moving and sometimes even leading to nausea.
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A few unfortunate people experience persistent symptoms lasting months or even years. This is known as mal de debarquement syndrome. Exactly why their symptoms persist so long isn鈥檛 understood, but they can be treated.
Sailing isn鈥檛 the only activity that causes illusory motion after-effects. Overnight rail passengers sometimes say they can feel the 鈥渃lickety-clack鈥� of the track in their legs after they leave the train. And astronauts returning to Earth commonly experience vertigo, nausea, difficulty walking and sensory flashbacks. In all cases, the longer one is exposed to the unfamiliar motion environment, the more prominent and long-lasting are the after-effects.
Timothy Hain, Department of physical therapy and human movement sciences, Northwestern University, Chicago, Illinois, US
Charles Oman, Man Vehicle Laboratory, MIT, Cambridge, Massachusetts, US