May 2011
Voon V, Gao J, Brezing C, Symmonds M, Ekanayake
V, Fernandez H, Dolan RJ, Hallett M
Impulse control disorders are common in Parkinson's; disease, occurring
in 13.6% of patients. Using a pharmacological manipulation
and a novel risk taking task while performing
functional magnetic resonance imaging, we investigated the relationship
between
dopamine agonists and risk taking in patients with
Parkinson's; disease with and without impulse control disorders.
During
functional magnetic resonance imaging, subjects
chose between two choices of equal expected value: a ‘Sure’ choice and a
‘Gamble’
choice of moderate risk. To commence each trial, in
the ‘Gain’ condition, individuals started at $0 and in the ‘Loss’
condition
individuals started at −$50 below the ‘Sure’
amount. The difference between the maximum and minimum outcomes from
each gamble
(i.e. range) was used as an index of risk (‘Gamble
Risk’).
Sixteen healthy volunteers were behaviourally tested. Fourteen
impulse control disorder (problem gambling or
compulsive shopping) and 14 matched Parkinson's; disease controls were
tested
ON and OFF dopamine agonists. Patients with impulse
control disorder made more risky choices in the ‘Gain’ relative to the
‘Loss’ condition along with decreased orbitofrontal
cortex and anterior cingulate activity, with the opposite observed in
Parkinson's; disease controls.
In patients with
impulse control disorder, dopamine agonists were associated with
enhanced
sensitivity to risk along with decreased ventral
striatal activity again with the opposite in Parkinson's; disease
controls.
Patients with impulse control disorder appear to
have a bias towards risky choices independent of the effect of loss
aversion.
Dopamine agonists enhance sensitivity to risk in
patients with impulse control disorder possibly by impairing risk
evaluation
in the striatum.
Our results provide a potential
explanation of why dopamine agonists may lead to an unconscious bias
towards
risk in susceptible individuals.
