Multisensory mental imagery of fatigue in patients with multiple Sclerosis. Preliminary evidence from a fMRI study

Fatigue, defined as a subjective lack of physical and/or mental energy, is a clinical symptom highly characterizing multiple sclerosis (MS). The present study utilized a novel approach to the study of fatigue, examining first person-mental imagery of the symptom. Eighteen right-handed patients with MS (14F, 4 M, mean age 45.8 ± 8.15 years) were evaluated and were compared to nineteen healthy controls (10F, 9 M, mean age 43.15 ± 8.34 years) Patients were all in relapsing remitting form and no patient had presented relapses in the 6 months prior to inclusion in the study. We evaluated their behavioral performance and fMRI activations. We used an fMRI paradigm used to trigger first person-mental imagery of fatigue, through short sentences describing the principal manifestations of fatigue. Participants were asked to imagine the corresponding sensations (Sensory Imagery, SI). As a control, they had to imagine the visual scenes (Visual Imagery, VI) described in short phrases. They made a vividness rating by pressing the corresponding button. Behaviorally, we found that patients’ mean scores at the Multidimensional Fatigue Symptom Inventory for the general scale, physical scale, and mental scale were significantly higher than healthy controls (p = 0.05, p = 0.002, p = 0.006 respectively), but not for the emotional scale and for vigor scale (p = 0.207, n.s., p = 0.06, n.s.). In the imagery fMRI task, patients were significantly slower (mean reaction times and standard deviation: 2.24 s ± 0.33) than controls (mean reaction times and standard deviation: 1.918 s ± 0.455) for the SI task (Z=-2.058, p = 0.040), while no significant difference was found for the VI task. Regarding brain mapping, our main result is a group by task interaction. The SI task (vs. VI task) in healthy controls (relative to patients) increased activation in the left inferior parietal lobule. These preliminary results indicate that fatigue is related to dysfunctions in higher-order aspects of motor control, given the role of the posterior parietal lobe in motor planning and multisensory integration.