This study showed that action slowing of MS patient is mainly related to (1) attentional deficit resulting in inability to maintain high level of rapid actions, and (2) subtle motor slowing even in patients without motor deficit on clinical examination, whereas (3) divided attention and decisional process are preserved.
El Sankari, S., Bottin, C., Roussel-Pieronne, M., & Godefroy, O. (2010). Motor and cognitive slowing in multiple sclerosis: an attentional deficit? Clinical Neurology and Neurosurgery, 112(3), 226-232. https://doi.org/10.1016/j.clineuro.2009.11.017 (Original work published 2010)