Abstract :
[en] Fatigue is the most invaliding symptom in 65% of patients with multiple sclerosis (MS). However, few data are available on the effect of mental fatigue on their cognitive efficiency. This study aimed to determine if mental fatigue alters performance according to task difficulty in MS patients. Eleven early MS patients (disease duration <5 y) and eleven matched healthy controls (HC) performed a working memory task (N-back) in a situation of high or low mental fatigue (experimental induction of the fatigue state). Repeated measures ANOVA (p<0.05) were conducted to determine the effects of fatigue (high vs. low), difficulty (1 to 3-back) and group (MS, HC) on hit rates (HR), decision criterion and sensitivity index (d’). Analysis of HR showed effects of group (F(1,20)=7.39, p<.05), difficulty (F(2,40)=33.86, p<0.001) and a difficulty*group interaction (F(2,40)=3.78, p<.05), with lower performance in MS, particularly in the most demanding condition. Decision criterion analysis only showed more conservative answers in MS patients (F(1,20)=7.10, p<.05). For d’, effects of group (F(1,20)=5.27, p<.05) and difficulty (F(2,40)=47.83, p<.001) were observed, with a smaller sensitivity index as difficulty increased and in the MS group. MS patients showed lower performance and were more conservative than HC, especially as the difficulty of the task increased. This suggests difficulty for allocation of attentional resources in MS. By contrast, we did not show fatigue-related effects on task performance in MS.
