Illusions of self-motion

Illusions of self-motion refers to a phenomenon that occurs when someone feels like their body is moving when no movement is taking place. One can experience illusory movements of the whole body or of individual body parts, such as arms or legs.

The vestibular system is one of the major sources of information about one's own motion. Disorders of the visual system can lead to dizziness, vertigo, and feelings of instability. Vertigo is not associated with illusory self-motion as it does not typically make you feel as though you are moving; however, in a subclass of vertigo known as subjective vertigo one does experience their own motion. People experience themselves being pulled heavily in one direction There are also specific self-motion illusions that can occur through abnormal stimulation of various parts of the vestibular system, often encountered in aviation. This includes an illusion of inversion, in which one feels like they're tumbling backwards. Through various stimuli, people can be made to feel as if they are moving when they are not, not moving when they are, tilted when they are not, or not tilted when they are.

When a large part of the visual field moves, a viewer feels like he has moved and that the world is stationary. For example, when one is in a train at a station, and a nearby train moves, one can have the illusion that one's own train has moved in the opposite direction. Common sorts of vection include circular vection, where an observer is placed at the center of rotation of a large vertically-oriented rotating drum, usually painted with vertical stripes; linear vection, where an observer views a field that either approaches or recedes; and roll vection, where an observer views a patterned disk rotating around his or her line of sight. During circular vection, the observer feels like he is rotating and the drum is stationary. During linear vection, the observer feels like he has moved forwards or backwards and the stimulus has stayed stationary. During roll vection, the observer feels like he has rotated around the line of sight and the disk has stayed stationary.

Inducing vection can also induce motion sickness in susceptible individuals.

After being on a small boat for a few hours and then going back onto land, it may feel like there is still rising and falling, as if one is still on the boat. It can also occur on other situations, such as after a long train journey. It is not clear whether sea legs is a form of aftereffect to the predominant frequency of the stimulation (e.g., the waves or the rocking of the train), whether it is a form of learning to adjust one's gait and posture, or whether it is a form of the Tetris effect. Sea legs needs to be distinguished from mal de debarquement, which is much more disturbing and long-lasting.

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