Patients with Parkinson's disease (PD) can make rapid responses to avoid adverse stimuli, but cannot speed up their actions to gain a reward, report UK researchers.
Tamara Shiner (University College London) and colleagues say that their research "supports proposals that tonic dopamine levels control the rate and vigour of movements, possibly by signalling the average reward rate in the environment."
They suggest that bradykinesia "is not simply related to movement," but to how a hypodopaminergic brain determines the importance of an action.
The 12 PD patients in the study completed the trials while off medication. They had idiopathic early or moderate stage PD and were an average of 67 years old. The patients undertook computer-based trials that involved completing set tasks in as short a time as possible. Overall, they were slower to complete these tasks than 11 control participants without PD.
In one type of trial, the participants had to react fast to avoid an electric shock, delivered to the back of the hand. Both patients and controls performed this trial faster than the reward trial, in which they could win a small amount of money.
However, the difference in reaction time between the two types of trial was significantly more pronounced in patients than controls, show the findings in PLoS One.
If patients or controls received an electric shock, then their reaction time tended to be faster at the next attempt. If control participants failed to win money, they significantly increased their reaction time at the next attempt, but the PD patients did not.
"Patients did not speed up their movements after failing to win a reward despite physically being able to move faster, a fact they clearly demonstrated in the shock avoidance trials," say Shiner et al.
They note: "In the future it would be interesting to see whether varying the magnitude of both the positively and negatively valenced outcomes might modulate this behavioural effect."
Some trials contained a distraction - a flashing green square in the middle of the screen. This had no notable effect on patients' and controls' performances in the reward trials, whereas as it had a significant adverse impact on patients', but not controls', efforts in the shock-avoidance trials.
The researchers suggest that the hypodopaminergic brain favors avoiding adverse stimuli over responding to rewarding stimuli, but that this comes at the expense of increased sensitivity to distractions.
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