Deep brain stimulation of the subthalamic nucleus and the temporal discounting of primary and secondary rewards

Although deep brain stimulation of the subthalamic nucleus is an effective surgical treatment for Parkinson's disease, it may expose patients to non-motor side effects such as increased impulsivity and changes in decision-making behavior. Even if several studies have shown that stimulation of the subthalamic nucleus increases the incentive salience of food rewards in both humans and animals, temporal discounting for food rewards has never been investigated in patients who underwent STN-DBS. In this study, we measured inter-temporal choice after STN-DBS, using both primary and secondary rewards. In particular, PD patients who underwent STN-DBS (in ON medication/ON stimulation), PD patients without STN-DBS (in ON medication) and healthy matched controls (C) performed three temporal discounting tasks with food (primary reward), money and discount vouchers (secondary rewards). Participants performed also neuropsychological tests assessing memory and executive functions. Our results show that STN-DBS patients and PD without DBS behave as healthy controls. Even PD patients who after DBS experienced weight gain and/or eating alterations did not show an increased temporal discounting for food rewards. Interestingly, patients taking a higher dosage of dopaminergic medications, fewer years from DBS surgery and, unexpectedly, with better episodic memory were also those who discounted rewards more. In conclusion, this study shows that STN-DBS does not affect temporal discounting of primary and secondary rewards. Furthermore, by revealing interesting correlations between clinical measures and temporal discounting, it also shed light on the clinical outcomes that follow STN-DBS in patients with PD.