Mechanisms underlying apathy in Parkinson's disease

Area of Science:

• Neuroscience
• Behavioral Science

Background:

• Apathy is prevalent in Parkinson's disease (PD), affecting up to 70% of patients.
• Underlying mechanisms of apathy remain unclear, and objective detection methods are lacking.
• This study investigates novel objective measures of motivation and reward sensitivity in PD patients with apathy.

Purpose of the Study:

• To use saccadic velocity and pupil modulation as objective metrics for motivation in Parkinson's disease.
• To compare reward sensitivity in Parkinson's disease patients on and off dopaminergic medication.
• To assess the relationship between reward sensitivity and apathy severity.

Main Methods:

• Recruited 20 idiopathic Parkinson's disease patients and 21 healthy controls.
• Monitored saccadic eye movements (velocity) and pupil diameter during reward-based tasks using an infrared eye tracker.
• Assessed apathy using the Lille Apathy Rating Scale (LARS) and tested patients on and off dopaminergic medication.

Main Results:

• Healthy controls and medicated PD patients showed increased saccadic velocity with higher rewards; this was lost in unmedicated PD patients.
• Pupil dilation increased with reward magnitude in all groups, with a trend for greater pupil reward sensitivity in medicated vs. unmedicated PD patients.
• Apathetic PD patients (low motivation) exhibited significantly less reward sensitivity in saccadic peak velocity compared to non-apathetic patients.

Conclusions:

• Saccadic velocity and pupil diameter changes reflect reward magnitude and individual motivation levels.
• These objective measures offer a novel approach to studying motivation deficits in neurological disorders like Parkinson's disease.
• Dopaminergic medication may treat apathy by enhancing reward sensitivity, potentially independent of motor control.