Objective: To compare the selection of stepping actions during a probability reaction time task in people with Parkinson’s disease (PD) in the ON state with that of healthy elderly controls (EL).

Background: Before initiating a step the brain selects the “best-at-the-time-action” from a set of possible actions – a process known as action selection, for which dopamine is thought to be critical. If this is the case, we propose that the selection of stepping actions in PD in the ON state should not be different from that of healthy elderly controls.

Methods: We aim to include 30 PD subjects in ON and 15 EL subjects. The participants were first trained to make different steps (left or right – backward or forward) in response to four different visual stimuli. Then they performed a probabilistic reaction time stepping task which consisted of: two Unpredictable blocks where the likelihood of each stimulus appearing was random; and two Predictable blocks where the likelihood of each stimulus appearing was such that subjects experienced an implicit sequence (Probable trials) which, at times, was violated (Improbable trials). Reaction times (RT; as defined by lift-off of the stepping foot) were compared between the different groups.

Results: Our preliminary results for the first eight PD subjects showed a general slowing in RT compared to EL (n=15), which bordered on statistical significance (p=0.059). RT for Probable trials [EL 907±117 ms; PD 967±66 ms] was significantly faster than for Unpredictable trials [EL 963±106; PD 1026±66], in both EL (p=0.005) and PD (p=0.012). Also, RT for Probable trials was significantly faster than for Improbable trials [EL 992±121; PD 1040± 57], for EL, p = 0.001, and PD, p = 0.012. All interaction effects were not significant. The difference between Improbable and Unpredictable trials was not significantly different for either group.

Conclusions: From subjects collected so far, it is clear that PD subjects in ON can implicitly learn the sequence as well as elderly controls, they are faster to react to Probable than Unpredictable trials. There is an effect of task in both groups, meaning that subjects are affected by Improbable trials but that PD in ON are not more affected than elderly. Preliminary analysis reveals that PD in ON seem just as able as elderly controls to react to selecting the most appropriate stepping action, suggesting that action selection in PD in the ON state is not impaired.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/is-the-selection-of-stepping-impaired-in-pd-in-on-state/