The effects of cognitive load on cognitive fatigue in Multiple Sclerosis - Preliminary results

Background. Cognitive fatigue is one of the most frequent and debilitating symptom of Multiple Sclerosis (MS). While it is well established that MS patients show a decrement of performances during/following a long cognitive task, only few studies investigated the effects of cognitive load on MS-related fatigue. Objective. To explore the effects of cognitive load on performance decrement and pupil variations in MS. Methods. Thirteen patients with early MS (disease duration <5) and 12 matched healthy subjects performed a dual-task in cognitive load conditions (low and high). High cognitive load was adjusted to individual resources by determining the fastest presentation rate leading to at least 85% accuracy; the low cognitive load condition is 50% slower. Additionally, pupil size variations were obtained during the task for 6 patients and 7 controls. Repeated measures ANOVA were performed to determine the group, time and cognitive load effects on averaged performances and pupil size. Supplementary analysis were conducted on performances using Bayesian statistics. Results. ANOVA on performances revealed an effect of condition (F(1,20)=37.79, p<.0001) and time (F(3,20)=13.32, p<.0001) but no effect of group nor interaction effects. Bayesian statistical analysis revealed an absence of difference in performances’ evolution between groups (BF01=5.71). ANOVA for pupil size revealed a tendency to significance for the group*condition interaction (p=0.077). Discussion. MS patients did not demonstrate larger fatigue effects on performances than control when the task is adapted to individual resources. The lack of pupillary response to cognitive load seems to reflect cognitive fatigue in patients despite behavioral compensation