Cognitive function and fMRI in patients with multiple sclerosis: evidence for compensatory cortical activation during an attention task

Mild cognitive impairment has frequently been reported for patients in the early stages of multiple sclerosis. The aim of the present study was to measure whether altered cortical activation during a sustained attention task occurs along with limited extent of neuropsychological problems. Expanded brain activation of multiple sclerosis patients with normal motor function compared with healthy controls during a finger tapping paradigm has previously been reported. Compensatory brain activation in patients with multiple sclerosis compared with normal controls may also be observed when the subjects are performing cognitive functions. In 21 patients with clinically definite relapsing-remitting multiple sclerosis, a psychometric assessment was performed using the Wechsler Memory Scale (WMS) and the Multiple Sclerosis Functional Composite Score (MSFC). In addition, functional MRI was performed during a Paced Visual Serial Addition Task (PVSAT), a visual analogue of the Paced Auditory Serial Addition Task (PASAT). All patients were within 3 years of diagnosis and were not suffering from a relapse at the time of investigation. The multiple sclerosis patients were compared with a control group of 21 healthy volunteers matched for handedness, age, years of education and sex. With regard to psychometric results, the WMS general memory score showed statistically significant differences between patients and controls. We did not find differences for either the MSFC or the PASAT scores. A group analysis of the functional imaging data during the PVSAT revealed different activation patterns for patients compared with control subjects. In healthy volunteers, the main activation was found in the frontal part of the right gyrus cinguli (Brodmann area 32). In patients, the main activation was detected at the right hemispheric frontal cortex (Brodmann areas 6, 8 and 9). In addition, the left hemispheric Brodmann area 39 was activated. We interpret the different patterns of activation, accompanied with intact performance in a sustained attention task of our multiple sclerosis sample compared with healthy controls, as the consequence of compensatory mechanisms. This is an expression of neuronal plasticity during early stages of a chronic disease.