Why don't we move faster? Parkinson's disease, movement vigor, and implicit motivation

Pietro Mazzoni, Anna Hristova, John W. Krakauer

Research output: Contribution to journalArticlepeer-review

257 Scopus citations

Abstract

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, Nc). Difficulty was reflected in endpoint accuracy and Nc. Patients were as accurate as control subjects in all conditions (i.e., PD did not affect the speed-accuracy trade-off). However, Nc was consistently higher in patients, indicating reluctance to move fast although accuracy was not compromised. Specifically, the dependence of Nc on movement energy cost (slope SN) was steeper in patients than in control subjects. This difference in SN 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. SN was less steep, but statistically significant, in control subjects, which demonstrates a role for energetic cost in the normal control of movement speed. Wepropose 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.".

Original languageEnglish
Pages (from-to)7105-7116
Number of pages12
JournalJournal of Neuroscience
Volume27
Issue number27
DOIs
StatePublished - Jul 4 2007

Keywords

  • Bradykinesia
  • Dopamine
  • Motor planning
  • Movement time
  • Reward
  • Speed-accuracy trade-off

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