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Motor overflow, also called neurological overflow or synkinesis, is a term used to describe uncontrolled or unconscious movements that have no definite cause, are associated with intentional movements, and are generally not associated with a well-defined neurological condition. These movements may be considered a soft neurological sign, and are often associated with other problems in functioning, such as a lack of fine- or gross motor control, a learning disability, attention problems, or any number of other minor (or major) issues.

There is no one generally acceptable theory to explain what causes these movements. These movements usually accompany planned motor actions that take up a significant portion of the individual’s attention, such as writing, running, imitating another person’s actions, or anything that takes some concentration and focus. (The amount of concentration a task requires is dependent on the individual performing them; some people find simple tasks like touching their nose difficult). The overflow may take the form of trembling, twitches, fidgeting, or any number of other movements. They may affect a completely different part of the body than is being used in the primary task; a mild and common example that could be considered a borderline motor overflow would be sticking your tongue out the corner of your mouth while you are concentrating on reading or writing (I use this example because it is common, not because it is in any way indicative of a neurological problem). It may also cross the body contralaterally, as when an action preformed by one hand or foot causes movement in the other hand/foot.

Children and the elderly may experience these signs even in the absence of a notable neurological condition, but the presence of overflow may alert you to look for other signs of neurological problems. In my experience treatment rarely focuses on motor overflow; if the actions caused by motor overflow are serious enough to warrant treatment they are likely to be labeled more formally. Exceptions include cases in which the overflow causes socially unacceptable behaviors (such as churning the tongue outside of the mouth), or if the overflow movements are taxing the body. It is theorized that overflow movements may weaken kinesthetic memory, since it may dilute the brain’s processing of the intended movement.

Related terms include associated reaction and Brunnstrom approach (in which this effect is used for therapeutic ends), and Raimiste's phenomenon.

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