Scientists have long noted that people suffering from Parkinson's disease commonly exhibit a specific personality type characterized by, among other things, a lower-than-average tendency to seek out new experiences. In explanation, investigators suggested that this trait was rooted in an inability to reap the pleasurable rewards of increased dopamine levels normally brought about by new stimuli because the disease destroys the neurotransmitter. Previous studies of personality and dopamine activity in Parkinson's patients involved subjects receiving treatment for their disease. Now, for the first time, scientists have examined the link between personality and Parkinson's in patients yet to receive medication. The report, published in the current issue of the Proceedings of the National Academy of Sciences indicates that, in fact, dopamine is not involved in regulating the low-novelty-seeking trait associated with Parkinson's.
Valtteri Kaasinen of the University of Turku in Finland and colleagues studied 61 patients who had been diagnosed with Parkinson's within the past five years but were not taking medication and compared them to a control group of 45 healthy subjects. Using personality questionnaires, the scientists compared traits such as novelty seeking and harm avoidance between the two groups. Unlike previously reported studies involving medicated patients, the correlation between low novelty seeking and Parkinson's was not highly significant. The authors suggest the previously demonstrated link may have been due in part to the medication administered to treat the disease. Parkinson's patients did, however, score significantly higher than control subjects for the harm avoidance personality trait, often associated with anxiety and depression. But the authors note that this trait is not disease-specific and "possibly reflects a psychological response to a chronic disease."
The team next investigated the explicit role of dopamine in mediating the personality of a Parkinson's patient. Using positron emission tomography (PET), the scientists studied the uptake of radioactively labeled L-DOPA (a precursor to dopamine) in the brains of 47 of the Parkinson's patients. Participants with high scores for harm avoidance demonstrated increased uptake of L-DOPA in the brain's right caudate region. But patients with low scores for novelty seeking did not show a similar reaction to L-DOPA. "Although the results of this study are not in disagreement with the concept of low-novelty-seeking personality type in Parkinson's disease," the authors conclude, "the personality type does not seem to be dopamine dependent."