Objective: This study assessed kinematic profiles of individuals with Parkinson’s on and off dopaminergic medication to investigate whether the effect of dopaminergic modulation on speed extends to the modulation of speed as a function of curvature.

Background: Parkinson’s has been robustly associated with decreased movement speed (“reduced vigour”), which is hypothesised to be due to reduced motivation to move [1]. Few studies, however, have assessed whether the modulation of speed is affected in Parkinson’s. Across multiple species, and multiple effectors, speed is modulated as a function of the curvature of a movement trajectory [2], according to the trajectory’s angular frequency [3], such that individuals move more slowly when executing more tightly curved movements (e.g., when drawing the “corners” as opposed to “straights” of an ellipse). Importantly, the relationship between speed and curvature (the “speed-curvature gradient”) is independent of movement speed. Assessing differences in speed and speed-curvature gradients in Parkinson’s both on and off dopaminergic medication has the potential to uncover selective movement impairments in Parkinson’s and shed light upon the mechanistic role of dopamine in these processes.

Method: Participants used a stylus and touch-screen device to trace four shapes. Speed and speed-curvature gradients were extracted from participants’ recorded movements for each shape. Linear mixed models were employed for each parameter with medication (on vs off) and shape as fixed effects.

Results: As expected, dopaminergic medication affected speed: participants moved faster ON medication compared to OFF medication (p<.001). A reaction time task also demonstrated quicker reaction times in the ON medication condition (p=.007). By contrast, there were no differences ON vs OFF medication for speed-curvature gradients.

Conclusion: These results demonstrate that dopaminergic medication increases movement vigour in Parkinson’s; however, modulation of speed as a function of curvature was unaffected. This highlights the impact and limitations of dopaminergic medication on improving motor functioning in Parkinson’s. More broadly, the results reinforce the link between dopamine and vigour but fail to find evidence in support of a role for dopamine in the modulation of speed as a function of curvature.

Previous meeting: British Association of Cognitive Neuroscience 25/5/22

References: [1] Mazzoni, P., Hristova, A., & Krakauer, J. W. (2007). Why don’t we move faster? Parkinson’s disease, movement vigor, and implicit motivation. Journal of Neuroscience, 27(27), 7105-7116. https://doi.org/10.1523/JNEUROSCI.0264-07.2007
[2] Lacquaniti, F., Terzuolo, C., & Viviani, P. (1983). The law relating the kinematic and figural aspects of drawing movements. Acta psychologica, 54, 115-130.
[3] Huh, D., & Sejnowski, T. J. (2015). Spectrum of power laws for curved hand movements. Proceedings of the National Academy of Sciences, 112(29), E3950-E3958. https://doi.org/10.1073/pnas.1510208112

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MDS Abstracts - https://www.mdsabstracts.org/abstract/dopaminergic-medication-improves-movement-vigour-in-parkinsons-but-not-the-adaptation-of-movement-speed-to-curvature/