For this study, 25 healthy controls, 25 early patients and 25 mid-stage patients will be included. The sample size was predicted by a power-analysis based on our own pilot study and the study of Dan et al., using a β=0.20, α=0.05. Mean movement time after the first 18 blocks of the SSP-task were available for healthy controls and PD patients in the mid-stage and predicted for early-stage patients. To counter potential dropout from the studies or data loss, the recruitment numbers are at least 30% higher than the ones resulting from the power analysis. Patients will be tested during the OFF phase of the medication cycle, i.e. approximately 12 h after last medication intake. The distinction between early and mid-stage patients is based on a combined criterion of disease duration and upper limb items of the MDS-UPDRS-III (while OFF medication). Patients are considered as early when ≤ 2 years have passed since the first symptoms emerged (note this is not years since diagnosis) in combination with maximal score of 2 on each of the upper limb MDS-UPDRS-III items. Patients are considered as mid-stage when either one of these criteria is not fulfilled.

Participants will first undergo an inclusion session at home. During this session, they will undergo an extensive behavioral test battery, assessing cognitive and motor skills. Patients will start this session in the OFF phase of the medication phase and will therefore be asked to postpone their morning medication. First, motor skills will be assessed by means of the Movement Disorders Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) and Clinch token transfer test (C3T). Cognitive assessment includes the Mini-Mental State Examination (MMSE), Montreal Cognitive Assessment (MoCA) and Trail Making Test (TMT). Additionally, tablet skills will be tested extensively using a newly developed test battery, including performance of tapping between two spots, swiping in a single direction and swiping in multiple directions in random order, in single and dual task conditions. Additionally, a mobile phone task will be performed in which participants have to type in a pre-defined phone number on a smartphone. This part of the assessment will take approximately 60 min. Afterwards patients will be able to take their regular medication and several questionnaires will be filled out. These include the New Freezing Of Gait Questionnaire (NFOG-Q), the non-gait freezing questionnaire, the Dexterity questionnaire (DextQ-24), the Mobile Device Proficiency Questionnaire (MDPQ-16), questions regarding smartphone and tablet use and remaining parts of the MDS-UPDRS. Furthermore, daily levodopa doses will be recorded. Healthy controls will undergo a similar protocol, though disease-specific assessments and questionnaires will not be performed (i.e. MDS-UPDRS, NFOG-Q, non-gait freezing questionnaire and medication intake).

Following inclusion, participants will be invited to the radiology dept. UZ Leuven. They will train the SSP-task on an MRI-compatible tablet, while their hemodynamic responses are measured using fMRI. Training will include two runs of 7 min 50 s. The SSP-task is based on the finger movements that have to be made to unlock smartphones or tablets or the trajectory that can be used to quickly form words using a keyboard on a smartphone. During this test, participants will have to make different pre-defined patterns. To reduce cognitive load, the pattern will be visible in one of the upper corners of the screen. During the task, participants will be able to see the lines they are drawing. Every pattern will begin in one of nine circles and will consist of equally long movements. Participants will be asked to move the hand without fully lifting the finger stylus from the screen to maintain the online trace. In addition, participants will be instructed to return to a fixed starting point when the pattern is complete. An MRI-compatible version of the touch-sensitive tablet will be used. Participants will see the trace of their pen on the tablet by means of a built-in mirror on top of the head coil. In addition to the task-based fMRI, a high-resolution T1-weighted anatomical scan and diffusion weighted imaging will be performed. Before and after the scan, participants will perform the mobile phone task, a single- and dual-task version of the SSP (to test automatization) and the tapping between two spots test outside the scanner. Additionally, participants will fill out the Hospital Anxiety and Depression Scale (HADS) and Pittsburgh Sleep Quality Index (PSQI).

From day 2 till 5, participants will continue practice of the SSP-task at home. For this, participants will be asked to perform the SSP-task on a tablet each morning, for patients this will be just before taking their regular medication. These training sessions will be limited to 10 minutes and contain the pattern that was learned on day 1, as well as two new patterns to allow variation. The patterns will be offered in a random order, as research has shown that random practice can improve retention and transfer in both healthy elderly adults and patients with PD. On day 5, the researcher will go to the participants' home and perform an immediate retention test, involving the single- and dual task version of the SSP-task. On days 6 and 7 participants will not practice to allow for a retention period. On day 8, all participants will have a (delayed) retention/transfer scan, consisting of two runs: (i) a run containing the learned pattern; and (ii) a run including a new pattern to assess transfer. Again, ST and DT performance on the SSP-task will be assessed outside the scanner, using a different pattern to avoid learning. Participants will also perform the mobile phone task and tapping between two spots test. To minimize head movements during the scan itself, a vacuum fixation pillow to accommodate these difficulties will be used.