Effective impulse control presents an essential cognitive function. Impairments can lead to health and socio-economic problems that significantly impact the quality of life. Due to its multifaceted nature and the involvement of a wide spectrum of neural processes, impulsivity is best understood as a multidimensional construct, the occurrence of which can be characteristic of a range of neuropsychiatric and other diseases. One disease where the presence of impulsive behaviour has been observed is Parkinson's disease (PD), a progressive neurological disorder that manifests with both motor and non-motor symptoms. Moreover, in PD patients, it was shown that impulsive behaviour can be even induced and altered by standard therapeutic interventions; including both dopaminergic treatment and deep brain stimulation of the subthalamic nucleus (DBS-STN). In scientific literature, several domains of impulsivity have been recognized, broadly categorized as motor (reactive/proactive), reflection, and cognitive impulsivity. In our study we included 20 PD patients on DBS-STN, who were tested in two sessions, under four different conditions: (i) while ON their regular medication, (ii) while OFF their regular medication (overnight withdrawal), (iii) with DBS-STN ON, and (iv) with DBS-STN OFF. The order of testing was pseudorandomized. A control group of 17 healthy participants was included. To test for motor impulsivity, we used AX Continuous Performance Task (AX-CPT), for reflection Beads Task (BT), and for cognitive Delay Discounting Task (DDT). The goal of the study was to determine how different forms of impulsivity depend on dopaminergic treatment and/or DBS-STN, or on the interaction between medication and DBS-STN, as well as to explore potential differences in the results of the paradigms used between the PD patient group and the group of healthy participants. Results showed that the combination of dopaminergic treatment and DBS-STN leads to increased motor (proactive and reactive) impulsivity in PD patients on DBS-STN. Additionally, healthy participants were less reflectively impulsive than PD patients, but more proactively motor impulsive, which could also be due to the limitations of the study. Therefore, further research is needed to replicate and confirm these findings. Our results support the view of impulsivity as a non-uniform phenomenon. Most previous research has focused on evaluating individually the effect of either dopaminergic treatment or DBS-STN on impulsivity. Our results are the first to suggest that rather the combination of DBS-STN and dopaminergic treatment is more likely to increase both forms of motor impulsivity in PD patients on DBS-STN than the treatments alone. Further research focusing on the interactions and long-term differences in various domains of impulsivity is necessary. Since disturbances in impulsive behaviour affect the quality of life of PD patients, the finding that impulsivity may potentially worsen with the combination of treatments could have important clinical implications.
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