Abnormal motor control effort costs in Parkinson's disease patients with apathy

Background In Parkinson's disease, abnormalities in reward-effort trade-offs are thought to underlie both apathy and bradykinesia. However, it remains unclear whether effort cost computations contribute to these symptoms independently of reward, and which exact effort costs are involved. Objective To assess whether effort-based decision-making in the absence of any explicit rewards differs between Parkinson patients and healthy controls and to explore whether putative differences are related to apathy or bradykinesia. Methods To address this, we developed a novel decision-making task that isolated physical effort costs from reward processing. Twenty-one Parkinson patients tested on and off dopaminergic medication, and 20 healthy participants, had to choose between two options that differed only in their requirements of force effort, either requiring larger total amounts of force or greater changes in force. We quantified participants’ preference for these different effort types and their sensitivity to changes in relative effort. Results Patients differed from healthy participants in both overall effort preference ( P = 0.009) and sensitivity to changing efforts ( P = 0.036), being more likely to choose options requiring overall lower force levels and more influenced by increasing requirements of changes in force. These effects were unaffected by dopaminergic medication. Importantly, sensitivity to increasing requirements for changes in force was associated with apathy ( P = 0.004), but not bradykinesia. Conclusion Our findings suggest that, beyond well-known diminished responses to potential rewards, apathy in Parkinson's disease is related to aberrant effort cost computations, in particular those involving dynamic changes in movement, during decision-making.