Reward sensitivity deficits modulated by dopamine are associated with apathy in Parkinson's disease.
Level 3 - non-randomized controlled study
Controlled observational study with a counterbalanced within-subject crossover drug state comparison
PubMed 27452600 · doi:10.1093/brain/aww188
What was done
The authors evaluated whether reward insensitivity contributes to apathy in Parkinson's disease using novel ocular metrics (pupillary dilation and saccadic peak velocity responses to monetary incentives). The study tested 40 patients with Parkinson's disease, 31 elderly age-matched controls, and 20 young healthy volunteers. To assess the role of dopamine, 30 patients with Parkinson's disease were examined in both ON and OFF dopaminergic medication states across two counterbalanced sessions. Autonomic dysfunction was controlled for using overnight heart rate variability measures.
What was found
The abstract reports no numerical values, effect sizes, or p-values. Qualitatively, pupillary dilation increased with higher monetary rewards across healthy participants and patients, but pupillary reward sensitivity declined with age. In Parkinson's disease, reduced pupillary response to incentives predicted apathy severity independently of motor impairment and autonomic dysfunction. Additionally, reward sensitivity in both pupillary response and saccadic peak velocity was blunted when patients were OFF dopaminergic medication compared to ON medication.
Why it matters
The study links dopamine-dependent reward processing deficits directly to clinical apathy in Parkinson's disease. It identifies pupillary and saccadic responses to reward as potential objective physiological biomarkers for measuring motivational impairment.
Limits
The abstract provides no quantitative data, effect sizes, or confidence intervals. The sample size is modest (n=40 patients, with n=30 in the ON/OFF medication sub-analysis), and testing relied entirely on laboratory monetary incentives rather than real-world behavioral rewards.
Cited by
- supports In Parkinson's disease, eye movement velocity increases when the incentive reward is increased, and administering dopamine increases saccadic velocity even further.