Parkinson's disease (PD) is a common and fast-growing neurological disease, clinically characterized by motor slowing (bradykinesia), stiffness (rigidity) and resting tremor. The pathological hallmark of PD is nigro-striatal dopamine depletion, but the noradrenergic (stress) system is also affected. Indeed, the prevalence of stress-related neuropsychiatric symptoms in PD is high and many PD patients suffer from reduced health-related quality of life. Also, (chronic) stress worsens many motor symptoms and may have detrimental long-term consequences by accelerating disease progression, as suggested by animal models.

There is no cure for PD, and currently no treatments to slow down disease progression. Therefore, the development of new and effective treatments is crucial. Given the large role of stress on PD symptoms, stress reduction might improve motor as well as non-motor symptoms. Intriguingly, recent evidence suggests that mindfulness training, where mindfulness is the trainable capacity to experience the present moment on purpose and without judgment, is an effective way to achieve such stress reduction. In fact, the effects of mindfulness practice have gained much interest as a topic of scientific research and clinical practise recently, where Mindfulness-Based Cognitive Therapy (MBCT) is one of the most commonly applied interventions, shown to be effective for a variety of somatic and psychiatric disorders. Importantly, previous trials investigating the effect of mindfulness-based interventions (MBIs) on clinical symptoms in PD showed positive effects on depression in 6/8 trials, on anxiety in 4/7 trials and on motor symptoms in 2/3 studies. Also, a large online survey on patients' experiences with stress and mindfulness showed that on one hand, patients experienced considerably more stress than controls, and significant stress-related worsening of PD symptoms; on the other, PD mindfulness users reported positive effects of mindfulness on anxiety and depression. In summary, current evidence suggests a positive effect of MBIs on psychological distress in PD, but clinical evidence is inconclusive. Also, to date, there is no research on the (cerebral) mechanisms underlying the (positive) effects of mindfulness in PD. Insight to the cerebral mechanisms of MBIs can pave the way for developing new, mechanism-based interventions, and can help to uncover the nature of the effects of stress on Parkinson's disease. Specifically, a mechanism based approach allows us to disentangle the symptomatic (stress as an amplifying factor on motor dysfunction) as opposed to neurodegenerative (nigro-striatal cell loss) effects of stress.

In this study, the investigators will test the effect of MBCT on the clinical (symptomatic) and neurodegenerative course of PD. If proven to be effective, MBCT can be applied as a new and cost-effective therapy to PD patients. The investigators will perform a randomized controlled trial with MBCT as intervention and a treatment as usual (TAU) control group. The investigators will evaluate whether a MBCT mindfulness course can lead to clinically relevant reductions in psychological distress (measured with the Hamilton Anxiety and Depression Scale) in PD patients with mild to moderate symptoms of psychological distress. Also, the investigators will evaluate the effects of a MBCT mindfulness course on other PD symptoms (e.g. motor dysfunction), cerebral markers of neurodegeneration, and neuroplasticity, and explore whether the intervention lowers systemic inflammatory tone in PD. The total duration of data acquisition per participant will be 12 months, consisting of a baseline measurement (T0), an intervention period of 2 months followed by a post-measurement (T1), and a final measurement (T2) that takes place 12 months after T0.