Illusions of self-motion (or "vection") occur when one perceives bodily motion despite no movement taking place. One can experience illusory movements of the whole body or of individual body parts, such as arms or legs.

Vestibular illusions

The

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating motor coordination, movement with balance. Together with the cochlea, a part of the auditory sys ...

is one of the major sources of information about one's own motion. Disorders of the visual system can lead to

dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to Balance disorder, disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a ...

vertigo Vertigo is a condition in which a person has the sensation that they are moving, or that objects around them are moving, when they are not. Often it feels like a spinning or swaying movement. It may be associated with nausea, vomiting, perspira ...

, and feelings of instability. Vertigo is not associated with illusory self-motion as it does not typically make one feel as though they 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.

Visual illusions

When a large part of the visual field moves, viewers feel like they have 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 their line of sight. During circular vection, the observer feels like they are rotating and the drum is stationary. During linear vection, the observer feels like they have moved forwards or backwards and the stimulus has stayed stationary. During roll vection, the observer feels like they have rotated around the line of sight and the disk has stayed stationary. Inducing vection can also induce

motion sickness Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, dizziness, tiredness, loss of appetite, and increased salivation. Complications may rarely include ...

in susceptible individuals. Susceptibility changes over the lifetime: young children and older adults seem to be more susceptible (at least to a circular vection stimulus) than adolescents and young adults. Furthermore, females seem to be slightly more susceptible to circular vection stimuli than males.

Auditory illusions

Compared to visually-induced vection, auditorily-induced vection is generally weaker. Auditory-induced vection can only be elicited in about 25% to 75% of the participants under laboratory conditions, and only when participants are

blindfold A blindfold (from Middle English ') is a garment, usually of cloth, tied to one's head to cover the eyes to disable the wearer's sight. While a properly fitted blindfold prevents sight even if the eyes are open, a poorly tied or trick blindfo ...

ed. Most of the research has focused on eliciting circular vection horizontally about the body. Researchers have induced circular vection by mechanically rotating a buzzer around a subject in the dark or by presenting sound sequentially in one of several speakers arranged in a circular array. Adding auditory stimuli can significantly enhance visual, vestibular, and biomechanical vections.

Biomechanical illusions

Sea legs, dock rock, or stillness illness

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 journey by train or by aircraft, or after working up a swaying tree. It is not clear whether sea legs are 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 Gait is the pattern of Motion (physics), movement of the limb (anatomy), limbs of animals, including Gait (human), humans, during Animal locomotion, locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on s ...

and

posture Posture or posturing may refer to: Medicine * List of human positions ** Abnormal posturing, in neurotrauma **Spinal posture * Posturography, in neurology Other uses * Posture (psychology) * Political posturing Political posturing, also known a ...

. The "sea legs" condition needs to be distinguished from mal de debarquement, which is much more long-lasting.

Treadmills

Subjects report a strong sense of self-rotation from stepping along a circular treadmill in the dark, which can be further enhanced through auditory cues. After spending more than 10 minutes on a linear treadmill, it is common to experience the visual illusion of moving at a substantially accelerated pace for 2-3 minutes once back on solid ground.

References

{{reflist Self-motion Motor control