Aberrant Somatosensory–Motor Adaptation in Musicians' Dystonia

Background: Some forms of movement disorders are characterized by task-specific manifestations of symptoms. However, its underlying mechanisms are poorly understood. Here we addressed this issue through a novel motor adaptation experimental paradigm. Methods: Pianists with and without focal task-specific dystonia learned to play the piano with a key whose weight can be modified by a novel robot system. Results: The result clearly demonstrated a significantly larger error between the target and produced keystroke velocities in the patients than the controls following a repetition of keystrokes of the weighted key. This adaptation failure was not correlated with the variability of timing and velocity of the keystroke when the patients were playing unloaded piano keys, which suggests distinct effects of focal task-specific dystonia on motor adaptation and fine motor control. Immediately after a repetition of the strikes of the heavy key with keeping the fingers adducted, the error of the keystroke velocity when striking the key with the fingers more abducted was maintained in both the patients and controls. This generalization of the adaptation across different hand postures suggests that motor memory of dynamics of the piano key is independent of biomechanical properties of the hand. Importantly, a lack of difference in the finger muscular strength between the groups indicated that the adaptation failure was not attributed to deficit of muscular strength in the patients. Conclusions: These findings suggest that task-specific manifestation of dystonic movements in focal task-specific dystonia is associated with malfunctions of internal representation of mechanical properties of a well-trained tool.