Objective: To compare the selection of stepping actions during a probabilistic reaction time task in the ON state with the OFF state in people with Parkinson’s disease (PD).

Background: Dopamine is thought to be critical in encoding the precision of sensory information used to build representations of possible actions, during action selection. In PD dopamine levels are reduced and we propose that in the OFF state this affects the action selection process during step initiation.

Methods: We aim to test 30 PD subjects, once in OFF and once in ON (randomized). 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 ON and OFF states within subjects.

Results: Preliminary results for the first eight PD subjects showed no general slowing in OFF when compared to ON, p = 0.137. RT for Probable trials [ON 967±66 ms; OFF 977±64 ms] was significantly faster than for Unpredictable trials [ON 1026±66 ms; OFF 1011±54 ms], in both ON, p = 0.012, and OFF, p = 0.050. Also, RT for Probable trials was significantly faster than for Improbable trials [ON 1040±57 ms; OFF 1022±46], in ON, p = 0.012, and in OFF, p = 0.036. The difference between Improbable and Unpredictable trials was not significantly different between ON and OFF. All interaction effects were not significant.

Conclusions: Preliminary analysis suggests that subjects in both ON and OFF dopaminergic medication are faster to react to Probable than Unpredictable trials showing they implicitly learn the sequence. In both conditions subjects are slower to react to Improbable than Probable trials, showing there is an effect of task. So far the results do not show any difference between ON and OFF in how PD subjects react to selecting the most appropriate stepping action. Although, in a similar paradigm with finger pressing actions a difference in RT between ON and OFF has been shown (1). 1 Galea JM, Bestmann S, et al (2012). J Neurosci 32(2):542-50.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/does-dopamine-affect-the-selection-of-stepping-actions-in-pd/