Multiple Sclerosis patients develop cognitive impairment at the earliest stages of the disease, with changes in cortical recruitment related to cognitive tasks. We evaluated the relation between information-processing speed performance with different diffusivity measurements in subcortical regions between healthy subjects and RRMS patients. We evaluated the FA, MD, RD, and AD diffusivity maps of U-fibers with PASAT test scores in healthy subjects and RRMS patients. PASAT test scores are significantly linear related to Frontal, Temporal, and Parietal cortical areas in healthy subjects. There is no linear relationship between PASAT test scores and diffusivity maps in RRMS patients.