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The Journal of Neuroscience, July 4, 2007, 27(27):7105-7116; doi:10.1523/JNEUROSCI.0264-07.2007

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Behavioral/Systems/Cognitive
Why Don't We Move Faster? Parkinson's Disease, Movement Vigor, and Implicit Motivation

Pietro Mazzoni, Anna Hristova, and John W. Krakauer

Motor Performance Laboratory, Department of Neurology, Columbia University College of Physicians and Surgeons, New York, New York 10032

Correspondence should be addressed to Dr. Pietro Mazzoni, Department of Neurology, Columbia University College of Physicians and Surgeons, 710 West 168th Street, Box NI-175, New York, NY 10032. Email: pm125{at}columbia.edu

People generally select a similar speed for a given motor task, such as reaching for a cup. One well established determinant of movement time is the speed–accuracy trade-off: movement time increases with the accuracy requirement. A second possible determinant is the energetic cost of making a movement. Parkinson's disease (PD), a condition characterized by generalized movement slowing (bradykinesia), provides the opportunity to directly explore this second possibility. We compared reaching movements of patients with PD with those of control subjects in a speed–accuracy trade-off task comprising conditions of increasing difficulty. Subjects completed as many trials as necessary to make 20 movements within a required speed range (trials to criterion, N_c). Difficulty was reflected in endpoint accuracy and N_c. Patients were as accurate as control subjects in all conditions (i.e., PD did not affect the speed–accuracy trade-off). However, N_c was consistently higher in patients, indicating reluctance to move fast although accuracy was not compromised. Specifically, the dependence of N_c on movement energy cost (slope S_N) was steeper in patients than in control subjects. This difference in S_N suggests that bradykinesia represents an implicit decision not to move fast because of a shift in the cost/benefit ratio of the energy expenditure needed to move at normal speed. S_N was less steep, but statistically significant, in control subjects, which demonstrates a role for energetic cost in the normal control of movement speed. We propose that, analogous to the established role of dopamine in explicit reward-seeking behavior, the dopaminergic projection to the striatum provides a signal for implicit "motor motivation."

Key words: movement time; speed–accuracy trade-off; bradykinesia; dopamine; motor planning; reward

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Received Jan. 19, 2007; revised May 23, 2007; accepted May 24, 2007.

Correspondence should be addressed to Dr. Pietro Mazzoni, Department of Neurology, Columbia University College of Physicians and Surgeons, 710 West 168th Street, Box NI-175, New York, NY 10032. Email: pm125{at}columbia.edu

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