Motivational modulation of bradykinesia in Parkinson's disease off and on dopaminergic medication

Abstract

Motivational influence on bradykinesia in Parkinson's disease may be observed in situations of emotional and physical stress, a phenomenon known as paradoxical kinesis. However, little is known about motivational modulation of movement speed beyond these extreme circumstances. In particular, it is not known if motivational factors affect movement speed by improving movement preparation/initiation or execution (or both) and how this effect relates to the patients' medication state. In the present study, we tested if provision of motivational incentive through monetary reward would speed-up movement initiation and/or execution in Parkinson's disease patients and if this effect depended on dopaminergic medication. We studied the effect of monetary incentive on simple reaction time in 11 Parkinson's disease patients both "off" and "on" dopaminergic medication and in 11 healthy participants. The simple reaction time task was performed across unrewarded and rewarded blocks. The initiation time and movement time were quantified separately. Anticipation errors and long responses were also recorded. The prospect of reward improved initiation times in Parkinson's disease patients both "off" and "on" dopaminergic medication, to a similar extent as in healthy participants. However, for "off" medication, this improvement was associated with increased frequency of anticipation errors, which were eliminated by dopamine replacement. Dopamine replacement had an additional, albeit small effect, on reward-related improvement of movement execution. Motivational strategies are helpful in overcoming bradykinesia in Parkinson's disease. Motivational factors may have a greater effect on bradykinesia when patients are "on" medication, as dopamine appears to be required for overcoming speed-accuracy trade-off and for improvement of movement execution. Thus, medication status should be an important consideration in movement rehabilitation programmes for patients with Parkinson's disease.

Fig. 1

Illustration of the simple reaction time task, showing events presented on the computer screen at different stages of the unwarned and warned trials

Fig. 2

Initiation time (IT) and Movement time (MT) data for PD patients “off”and “on”medication and for healthy participants in the unwarned simple reaction time (uSRT) and warned simple reaction time (wSRT) task. Error bars are standard error of the mean. Data presented are the raw data. Blocks 1 and Block 2 are unrewarded blocks; Blocks 3 and Block 4 are rewarded blocks

Fig. 3

PD patients “off”medication showed a significant positive correlation between improvement of IT in response to prospect of monetary reward and the number of anticipation errors, suggestive of “speed-accuracy trade-off”. This association was not present in PD patients “on” medication or in healthy participants. Percentage of IT improvement is plotted on the x-axis and larger values correspond to greater improvement in IT in response to prospect of reward. Difference in anticipation errors between rewarded and unrewarded blocks is plotted on the y-axis and a larger value indicates more anticipation errors in rewarded trials