A tic is a sudden, repetitive, nonrhythmic motor movement or vocalization involving discrete muscle groups. Tics can be invisible to the observer, such as abdominal tensing or toe crunching. Common motor and phonic tics are, respectively, eye blinking and throat clearing.

Tics may increase as a result of stress, fatigue, boredom, or high-energy emotions, which can include negative emotions, such as anxiety, as well as positive emotions, such as excitement or anticipation. Relaxation may result in a tic increase (for instance, watching television or using a computer), while concentration on an absorbing activity often leads to a decrease in tics.

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