عملية الاتزان

The Responses to angular rotation
The responses of individuals to rotation have been studied quite thoroughly because understanding them is important to controlling performance in high speed aircraft or in spacecraft. When a person, with eyes closed, is submitted to angular rotation, for example in a Barany chair, he will accurately signal the direction of rotation when he first begins to move; however, after a period of rotation at constant velocity, he will report that he has ceased to rotate. This is precisely what is predicted from the rapid adaptation of receptors in the semicircular canals at constant velocity (Figs. 9-6 and 9-7). During the period of acceleration at the beginning of the rotation, the person will also experience a nystagmus in the direction of rotation, the eyes apparently attempting to stay fixated on some target. When the sensation of rotation fades at constant velocity, the nystagmus also disappears.
If the chair is abruptly stopped at this point, the person will have a sensation of rotation in the direction opposite to that he previously experienced, and that too will fade with time. In addition, there will be a postrotatory nystagmus also in the direction opposite to the previous rotation. Again, these are phenomena consistent with the discharge properties of receptors in the semicircular canals. When a rotation stops, the discharge frequency of the canal receptors (on the side toward which the head originally rotated) falls below resting levels and below the level of the contralateral canal, just as it would if there were actually a rotation in the opposite direction. Although rotation has ceased, the central nervous system cannot distinguish this signal from the signal that would occur for an opposite rotation and, therefore, it is interpreted as an opposite rotation. This sensation also fades with time because of receptor adaptation.
Another consequence of rotation in the postrotatory period is past-pointing. If a person is asked to point at a target immediately upon being stopped from a period of constant velocity rotation, he will consistently point inaccurately, with the deviation always in the direction of the previous rotation. Thus, if the previous rotation was clockwise, he will consistently point to the right of the target.
People who must undergo this kind of rotation frequently learn to ignore what their vestibular senses tell them. Figure skaters, for example, can spin for long periods without showing past-pointing or postrotatory nystagmus. Obviously, they must be able to do this if they are to continue skating after the spin. Airplane pilots can suffer what is called a Coriolis effect, a false sense of spinning. A pilot who has put his airplane into a spin, either intentionally or accidentally, may be unaware of it after a while if he relies upon his vestibular sense because of the adaptation of the receptors. When he stops the spin (usually because someone tells him to), he will experience a sensation of spinning in the opposite direction, the Coriolis effect. Were he then to attempt to correct for this illusory spin, he would set himself in a spin again. Many aircraft have been sent spinning to earth by pilots who failed to read their instruments and relied on the Coriolis effect to navigate.