Motor and cognitive slowing in multiple sclerosis: an attentional deficit?

Action slowing is frequently observed in multiple sclerosis (MS) patients. Several factors may account for response slowing: motor, perceptual, cognitive deficits, global mental slowing. Our objective was to examine mechanisms accounting for action slowing in MS patients.

Twenty MS patients, free of visual impairment and of upper limbs sensory-motor deficit underwent previously validated reaction time (RT) tests using visual stimuli. Three tasks were used: (1) motor tapping, (2) simple reaction time (SRT) in a simple and dual task condition, and (3) choice RT (CRT) with varying response probabilities. Results were compared to those of 20 healthy matched subjects.

MS patients had: (1) lower motor tapping frequency (p=0.02); (2) SRT lengthening (p=0.001) related to a lower proportion of fast responses (p=0.001) indicating attentional deficit whereas perceptuomotor index was spared (p=0.5), without higher sensitivity to dual task (p=0.9); and (3) CRT lengthening (p=0.001) with spared decision time (p=0.7).

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.