<unk>  can be divided into three categories: #<unk>  caused by<unk>  that is felt but not seen, as in terrestrial<unk> ; #<unk>  caused by<unk>  that is seen but not felt, as in space<unk> ; #<unk>  caused when both systems detect<unk>  but they do not correspond, as in either terrestrial or space<unk> .<unk>  felt but not seen In these cases,<unk>  is sensed by the vestibular system and hence the<unk>  is felt, but no<unk>  or little<unk>  is detected by the visual system, as in terrestrial<unk> . Carsickness A specific form of terrestrial<unk> , being carsick is quite common and evidenced by disorientation while reading a map, a book, or a small screen during travel. Carsickness results from the sensory conflict arising in the brain from differing sensory inputs.<unk>  is caused by a conflict between signals arriving in the brain from the inner ear, which forms the base of the vestibular system, the sensory apparatus that deals with movement and balance, and which detects<unk>  mechanically. If someone is looking at a stationary object within a vehicle, such as a magazine, their eyes will inform their brain that what they are viewing is not moving. Their inner ears, however, will contradict this by sensing the<unk>  of the vehicle. Varying theories exist as to cause. The sensory conflict theory notes that the eyes view<unk>  while riding in the moving vehicle while other body sensors sense stillness, creating conflict between the eyes and inner ear. Another suggests the eyes mostly see the interior of the car which is<unk> less while the vestibular system of the inner ear senses<unk>  as the vehicle goes around corners or over hills and even small bumps. Therefore, the effect is worse when looking down but may be lessened by looking outside of the vehicle. In the early 20th century, Austro-Hungarian scientist Róbert Bárány observed the back and forth movement of the eyes of railroad passengers as they looked out the side windows at the scenery whipping by. He called it "railway nystagmus". Also called "optokinetic nystagmus". It causes nausea and vomiting. His findings were published in the journal Laeger, 83:1516, Nov.17, 1921. Airsickness Air sickness is a kind of terrestrial<unk>  induced by certain sensations of air travel. It is a specific form of<unk>  and is considered a normal response in healthy individuals. It is essentially the same as carsickness but occurs in an airplane. An airplane may bank and tilt sharply, and unless passengers are sitting by a window, they are likely to see only the stationary interior of the plane due to the small window sizes and during flights at night. Another factor is that while in flight, the view out of windows may be blocked by clouds, preventing passengers from seeing the moving ground or passing clouds. Seasickness Seasickness is a form of terrestrial<unk>  characterized by a feeling of nausea and, in extreme cases, vertigo experienced after spending time on a boat. It is essentially the same as carsickness, though the<unk>  of a watercraft tends to be more regular. It is typically brought on by the rocking<unk>  of the craft or movement while the craft is immersed in water. As with airsickness, it can be difficult to visually detect<unk>  even if one looks outside the boat since water does not offer fixed points with which to visually judge<unk> . Poor visibility conditions, such as fog, may worsen seasickness. The greatest contributor to seasickness is the tendency for people being affected by the rolling or surging<unk> s of the craft to seek refuge below decks, where they are unable to relate themselves to the boat's surroundings and consequent<unk> . Some sufferers of carsickness are resistant to seasickness and vice versa. Adjusting to the craft's<unk>  at sea is called "gaining one's sea legs"; it can take a significant portion of the time spent at sea after disembarking to regain a sense of stability "post-sea legs". Centrifuge<unk>  Rotating devices such as centrifuges used in astronaut training and amusement park rides such as the Rotor, Mission: Space and the Gravitron can cause<unk>  in many people. While the interior of the centrifuge does not appear to move, one will experience a sense of<unk> . In addition, centrifugal force can cause the vestibular system to give one the sense that downward is in the direction away from the center of the centrifuge rather than the true downward direction. Dizziness due to spinning When one spins and stops suddenly, fluid in the inner ear continues to rotate causing a sense of continued spinning while one's visual system no longer detects<unk> . Virtual reality Usually, VR programs would detect the<unk>  of the user's head and adjust the rotation of vision to avoid dizziness. However, some cases such as system lagging or software crashing could cause lags in the screen updates. In such cases, even some small head<unk> s could trigger the<unk>  by the defense mechanism mentioned below: the inner ear transmits to the brain that it senses<unk> , but the eyes tell the brain that everything is still.<unk>  seen but not felt In these cases,<unk>  is detected by the visual system and hence the<unk>  is seen, but no<unk>  or little<unk>  is sensed by the vestibular system.<unk>  arising from such situations has been referred to as "visually induced<unk> " (VIMS). Space<unk>  Zero gravity interferes with the vestibular system's gravity-dependent operations, so that the two systems, vestibular and visual, no longer provide a unified and coherent sensory representation. This causes unpleasant disorientation sensations often quite distinct from terrestrial<unk> , but with similar symptoms. The symptoms may be more intense because a condition caused by prolonged weightlessness is usually quite unfamiliar. Space<unk>  was effectively unknown during the earliest spaceflights because the very cramped conditions of the spacecraft allowed for only minimal bodily<unk> , especially head<unk> . Space<unk>  seems to be aggravated by being able to freely move around, and so is more common in larger spacecraft. Around 60% of Space Shuttle astronauts currently experience it on their first flight; the first case of space<unk>  is now thought to be the Soviet cosmonaut Gherman Titov, in August 1961 onboard Vostok 2, who reported dizziness, nausea, and vomiting. The first severe cases were in early Apollo flights; Frank Borman on Apollo 8 and Rusty Schweickart on Apollo 9. Both experienced identifiable and quite unpleasant symptoms—in the latter case causing the mission plan to be modified. Screen images This type of terrestrial<unk>  is particularly prevalent when susceptible people are watching films presented on very large screens such as IMAX, but may also occur in regular format theaters or even when watching TV or playing games. For the sake of novelty, IMAX and other panoramic type theaters often show dramatic<unk> s such as flying over a landscape or riding a roller coaster. This type of<unk>  can be prevented by closing one's eyes during such scenes. In regular-format theaters, an example of a movie that caused<unk>  in many people is The Blair Witch Project. Theaters warned patrons of its possible nauseating effects, cautioning pregnant women in particular. Blair Witch was filmed with a handheld camcorder, which was subjected to considerably more<unk>  than the average movie camera, and lacks the stabilization mechanisms of steadicams. Home movies, often filmed with a cell phone camera, also tend to cause<unk>  in those who view them. The person holding the cell phone or other camera usually is unaware of this as the recording is being made since the sense of<unk>  seems to match the<unk>  seen through the camera's viewfinder. Those who view the film afterward only see the movement, which may be considerable, without any sense of<unk> . Using the zoom function seems to contribute to<unk>  as well since zooming is not a normal function of the eye. The use of a tripod or a camera or cell phone with image stabilization while filming can reduce this effect. Virtual reality<unk>  due to virtual reality is very similar to simulation sickness and<unk>  due to films. In virtual reality the effect is made more acute as all external reference points are blocked from vision, the simulated images are three-dimensional and in some cases stereo sound that may also give a sense of<unk> . The NADS-1, a simulator located at the National Advanced Driving Simulator, is capable of accurately stimulating the vestibular system with a 360-degree horizontal field of view and 13 degrees of freedom<unk>  base. Studies have shown that exposure to rotational<unk> s in a virtual environment can cause significant increases in nausea and other symptoms of<unk> . In a study conducted by the U.S. Army Research Institute for the Behavioral and Social Sciences
Motion sickness
Stroke
Sweating sickness
Benign paroxysmal positional vertigo
Ski sickness
Migraine-associated vertigo
Foreign accent syndrome
Morning sickness