Motor sequence awareness is impaired in dystonia despite normal performance

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Dystonia is a movement disorder characterized by involuntary movements and abnormal postures. The range of clinical manifestations in dystonia is highly heterogeneous. On one extreme, the involuntary movements may affect multiple body parts and be present at rest. On the other, the movements may be isolated to a single body part and appear only during specific tasks, as in musician's focal hand dystonia. Dystonia has been associated with impaired motor sequence learning, with evidence that this impairment is caused by underlying abnormalities in sequence cognition.1
Sequence learning has mostly been studied in dystonia through paradigms similar to the serial reaction time task,3 in which subjects respond to individual stimuli that are cyclical (ie, the stimuli appear in either nonrandom or fixed order). For example, manifesting and nonmanifesting carriers of the DYT1 gene mutation, but not the DYT6 mutation, lack the ability to predict the target location in a sequential radial target task.1 Performance with random stimuli is normal in people with dystonia, indicating that the differences are not due to impaired ability to perform the task. Sequence learning in dystonia does not seem to be caused by the dystonic movements themselves.
There are 2 main components to the serial reaction time task: the stimulus and the response. That subjects with dystonia have difficulty predicting upcoming target locations does not distinguish whether learning impairment exists at the level of the stimulus or the level of the response. In other words, beyond unawareness of a pattern in the stimulus, are people with dystonia also unaware of that they are moving in a pattern? It has been shown that subjects with dystonia have difficulty reporting a sequence even outside of motor performance (ie, with only observation of the stimulus).1 We wanted to know whether subjects with dystonia have abnormal knowledge of patterns in the motor response in addition to the stimulus.
We predicted that awareness of patterns in the motor response would be abnormal in people with dystonia. Dystonia is associated with a reduced sense of agency, meaning that people with dystonia are not always aware that they are controlling their own movements.4 This is likely related to the well‐described impairment in sensorimotor integration,5 with specific deficits in spatial and temporal discrimination7 or proprioception.8 There is evidence that performance monitoring is abnormal in dystonia.9 We tested our hypothesis that subjects with dystonia would have reduced motor awareness with a common motor pattern prompted by changing stimulus‐to‐response mappings.
Using a finger sequence tapping task designed to separate motor performance from awareness of the motor pattern, we have shown in healthy subjects that awareness of the underlying motor pattern is predicted by tapping consistency in the temporal and kinematic domains.10 In the present study, we used this task to explore motor awareness in individuals with dystonia. Because movement variability is a hallmark of dystonia, we hypothesized that subjects with dystonia would have decreased motor awareness due to decreased temporal and kinematic consistency. Our results show that subjects with dystonia have decreased motor awareness, but that those differences are not due to greater performance variability.
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