Abstract

Apathy is increasingly appreciated to be a major problem in Parkinson's disease (PD), with up to 70% of cases affected. The mechanisms underlying the condition are poorly understood and objective detection methods are lacking. We used eye movement and pupil modulation in response to rewards as metrics of motivation to assess the relation of reward sensitivity to apathy and influence of dopaminergic medication.

30 patients with idiopathic PD, tested ON and OFF medication, and healthy age-matched participants made saccades for different monetary rewards. Saccadic peak velocity and pupil diameter were measured using an infrared eye-tracker, while apathy was indexed by Lille Apathy Rating Scale scores.

PD patients ON demonstrated increased saccadic velocity and pupil diameter as reward magnitude increased, just like controls. This reward sensitivity was blunted in PD patients OFF dopaminergic medication. Crucially, apathetic PD patients exhibited significantly less pupillary reward sensitivity than more motivated PD cases.

Saccadic velocity and pupil diameter are influenced by reward magnitude, with the degree of modulation varying with motivation levels across individuals. These indices provide novel, objective behavioural measures for assessing clinical apathy in PD. Dopaminergic medication may also be an effective treatment for apathy by increasing reward sensitivity, independent of effects on motor control.