Sequence-Specific Implicit Motor Learning: Influences of Cognitive Feedback and Aerobic Exercise Training in Parkinson’s Disease Individuals

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that is characterized by degeneration of dopamine-producing cells within the substantia nigra, which results in bradykinesia, rigidity, tremor, postural instability, and gait disability (Tolosa E., 2006). In particular, this disorder has been implicated in the degeneration of motor learning, which refers to the process of acquiring new motor skills or relearning existing ones through practice and experience (Shmuelof L., 2011). It has been evaluated that certain aspects of learning in PD patients are impaired, especially those targeting automatic responses to feedback (i.e., walking), causing learning and motor control to rely on feedforward systems (Nieuwboer A., 2009; Olson M., 2019). Evidence shows the potential relationship between the deficit of learning and the loss of motor control in PD, suggesting the crucial need to assess motor learning as the disease evolves (Tremblay PL., 2010). Although this ability to learn or refine a movement is typically assessed by a paradigmatic task called Serial Reaction Time Task (SRTT; Nissen MJ., 1987), it is unclear whether motor learning is interchangeable with respect to the body district used. Indeed, this test is commonly administered by a somewhat simplistic finger-pressing movement task, where participants’ reaction times in response to visual stimuli were recorded. However, understanding whether motor learning behaves differently depending on the body district used is of fundamental importance for research and clinical applications. In this context, the assessment of motor learning using alternative body districts (i.e., arms, feet) has the potential to make the SRTT an outcome measure to evaluate the effectiveness of rehabilitation and exercise treatments in different body districts (i.e., gait training in PD patients). Furthermore, recent studies showed that this alteration in motor learning in individuals with PD can be remedied by administering exercise therapy (Abbruzzese G., 2016; Ferrazzoli D., 2018; Olson M., 2019). Early, regular, and tailored exercise-therapy interventions effectively attenuate the deterioration of gait and balance in PD, and those targeting the mechanism of motor learning appear very promising in decreasing symptom progression (Abbruzzese G., 2016; Nieuwboer A., 2009). Cognitive feedback training has been proposed as a method to enhance the compromised motor learning mechanism in PD (Abbruzzese G., 2016). Visual and auditory feedback could facilitate the development of new neuronal pathways or the use of alternative pre-existing ones, thus bypassing the faulty basal ganglia (Nieuwboer A., 2009; Ferrazzoli D., 2020). This would explain the larger effects of cognitive feedback training vs. classical therapy. Several studies promote the use of interventions that, by combining exercise and cognitive challenges, improve motor learning in PD, with the final aim to attenuate the impact of the loss of automated movements and to empower the ability of re-learning and coping with progressive movement deficits (Abbruzzese G., 2016; Nieuwboer A., 2009; Ferrazzoli D., 2018). In particular, treadmill training combined with cognitive feedback (CFTT) appears to improve motor learning, thereby improving rehabilitation efficacy compared to traditional treadmill training (TTT) (Abbruzzese G., 2016; Muthukrishnan N., 2019). These devices allow performing gait training in combination with feedback on selected gait parameters (e.g., step length, cadence). However, the extent of the possible advantage of CFTT over traditional exercise therapy and identifying the optimal intensity dose of this treatment to enhance motor learning remains unclear and needs further investigation to individualize the best exercise prescription.