Illusions of self-motion

Illusions of self-motion occur when one moves or feels one has moved without feeling that one has made the movement. Motions can include movements of the whole body or movements of single limbs. Illusions include vestibular illusions, vection, sea legs, the ideomotor effect, and alien hand syndrome.

Vestibular illusions

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. There are also specific self-motion illusions that can occur through abnormal stimulation of various parts of the vestibular system. These include "vestibular/somatogyral Illusions" and "vestibular/somatogravic Illusions" in aviation. These are described in Sensory illusions in aviation.

Vection

When a large part of the visual field moves, a viewer feels that he or she has moved and that the world has stayed stationary. [ [http://www.hitl.washington.edu/publications/r-98-11/node17.html Vection Measures ] ] For example, when one is in a train at a station, and another neighbouring 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 centre 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 that he or she is rotating and the drum is stationary, during linear vection, the observer feels he or she has moved forwards or backwards and the stimulus has stayed stationary, and during roll vection, the observer feels that he or she has rotated around the line of sight and the disk has stayed stationary.

Inducing vection can also induce motion sickness in susceptible individuals.

ea legs

After being on a small boat for a few hours, when one comes back onto land, one may feel that one is still rising and falling as if still on the boat. [ [http://www.newscientist.com/backpage.ns?id=mg18524832.400 Sea legs - 22 January 2005 - New Scientist ] ] 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., of the waves or the rocking of the train), whether it is a from 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.

Ideomotor effect

The ideomotor effect occurs when someone moves his or her voluntary muscles without being aware of doing so. One example is in moving the planchette on an Ouija board. There are at least two theories. One is that an unconscious consensus guides all the individuals to make similar, very slight movements, none of which are enough to be conscious in any individual, but together make it seem that the planchette is under external control. The other is that the consensus emerges from the sum of the various unconscious movements made by the participants, and that once this motor consensus begins to be established, it recruits confirming slight movements in each participant, again less than is conscious to any individual.

Other examples include automatic writing, dowsing, and facilitated communication.

Alien hand syndrome

Alien hand syndrome occurs when an individual makes a movement of one part of his or her body without being aware of doing so. It is usually associated with different sorts of brain damage.

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