Parkinson's disease (PD) is characterized by severe motor and non-motor symptoms, including upper limb dysfunction. Due to the degradation of dopaminergic neurons in the striatum, PD patients experience difficulties with motor learning and more specifically with the consolidation of motor memory. Recent work showed that intensive writing training improved writing skills in PD. Although consolidation effects were present, difficulties with retention were also still apparent. Besides impacting writing, manual dexterity deficits in PD can also affect the use of touchscreens. Researchers from our lab demonstrated that impairments were most pronounced in multi-direction sliding motions, indicating the need for training of these motor skills. Our lab demonstrated the classic difficulties with retention in PD after one session of training of a swipe and slide pattern on a tablet (SSP-task) as single task (ST), although immediate gains were demonstrated. Therefore, in this study the investigators will examine whether a two-week home-based training program of a tablet-based SSP-training program will lead to immediate and consolidated improvements that are retained in time. This program will combine ST and dual task (DT) training to provide variation during the training period, but also to increase the cognitive challenge during learning, thereby stimulation consolidation of learning. The primary aim of this study consists of investigating the learning effects after two weeks of targeted touchscreen training. Secondary, the investigators will examine whether these effects will also be retained after four weeks without practice and whether targeted training results in consolidated improvements, in terms of automaticity and transfer towards an untrained task. Given the objective recording of compliance to the training protocol, the investigators will explore the association between compliance rates and learning effects.

Given the impaired touchscreen skills and the espoused difficulties with retention in PD, th e investigators will examine the training of touchscreen sliding motions delivered on a tablet enabling practice in the home setting. Both ST and DT conditions will be offered in a random order and feedback will be provided by means of knowledge of performance to enhance motivation and retention, as proved successful in previous work. The objective of this study is to examine the effects of this innovative training program immediately after two weeks of targeted training as the primary outcome in a randomized controlled trial (RCT). To resemble daily life as much as possible, all test and training sessions will be carried out in the ON-phase of the medication-cycle, i.e., approximately 1 hour after last medication intake. The effect of a 2-week home-based training program with a 4-week follow-up will be examined by including 36 PD patients, who will be randomized (strata: age, H&Y stage) in the experimental or control group. Sample size was based on the pilot study using the SSP-task showing an average movement time of 7.4s ± 3.43 in PD for sequence swiping at baseline in ST. Based on literature, the researchers assumed an improvement of 20.2% following training at retention. For the control group, the researchers expect a deterioration in performance of 5.6% after a period without training based on the changes observed in the control groups of recent studies investigating manual dexterity parameters over time. Using an α = 0.05 and β = 0.20, a sample size was calculated for repeated measures ANOVA analysis with a within-between factor interaction design (within: pre - post - retention; between: experimental group - control group). Consequently, total sample size was estimated at 30 participants. Taking a 20% dropout rate into account, this resulted in a total of 18 participants in each group.

The experimental group will train the SSP task for a duration of two weeks, whereas the control group will not receive any intervention. On day 1 (T0), all participants will undergo an extensive screening session, including motor and non-motor screening. Testing can be performed at the subject's home or in our laboratory according to patients' preference.

The average slide duration (in ms) and the accuracy (as the number of correctly formed patterns in %) on the SSP-task will be recorded as the main digitised outcomes, objectively recorded via the tablet. In addition to evaluating the immediate effects of touchscreen training, the investigators will also test retention effects after four weeks without training as a secondary outcome. Further, consolidation of learning will be assessed using secondary outcomes such as the SSP-test in ST and DT conditions to assess automaticity of learning, the mobile phone task to test transfer of learning (MPT, typing a predefined telephone number on a smartphone). As tertiary outcomes, specific questionnaires to capture motor and cognitive performance will be administered. Following the baseline session, the experimental group will receive 10 training sessions of the SSP-training (5 days/week, 10 min/session) over a period of two weeks. The training is home-based and unsupervised but is extensively explained by an expert trainer. The control group does not receive training. Both immediately after training (T1) and after a four-week retention period (T2), touchscreen motor skills will be re-assessed at home. The most appropriate linear mixed models will be used to analyse SSP performance, with group (intervention - control) as between-subject factor and time (baseline - post - retention) as within-subject factor. LEDD and other confounders will be included as covariates.

PD is a highly heterogeneous disorders, not only affecting motor but also cognitive functions. In fact, up to 80% of patients develops dementia over the disease course. Previous studies have identified global cognitive function as important determinants of training success. Therefore, cognitive function will be assessed extensively during the baseline session to investigate the association with training responsiveness. A global cognitive screen will consist of the Montreal Cognitive Assessment. Moreover, 2 specific tests will be used to assess each cognitive subdomain. Attention and working memory will be assessed using a digit and visual span forward and backward. The trail making test and alternating names test will be used for executive function. Visuospatial function will be examined using the short form of the Benton's judgement of line orientation and the Rey Osterrieth Complex figure. The 30-min recall of the latter test will also be used to assess memory, together with the Rey Auditory Verbal Learning test. The Boston naming test and the Animal fluency test of the Controlled Oral Word Association test will be used to assess language. Other non-motor features, such as anxiety, depression, and sleep quality, will be tested using validated questionnaires for PD.

Another important determinant of rehabilitation success is training compliance. However, previous studies have used self-reported logbooks of home-based exercise, which show high inter-participant variability. As such, our training program overcomes this limitation as training compliance is automatically recorded via the digitized training tablet. The investigators will explore whether compliance to training has an impact on learning effects within the training group by using a correlation analysis between compliance rates and training-related changes in performance.

Inclusion Criteria:

• Diagnosis of Parkinson's disease based on the 'UK Brain Bank' criteria
• Hoehn and Yahr (H&Y) stage I-III, participants in H&Y stage I should have the right side as the most affected side
• Right handed, or right-handed use of touchscreen devices.

Exclusion Criteria:

• Cognitive decline (Mini Mental State Examination < 24)
• Comorbidities of the upper limb that could interfere with the study and are not caused by Parkinson's disease
• Other neurological disorders besides Parkinson's disease
• Color blindness as determined by the Ishihara test for color deficiency