Cognitive impairment is common in multiple sclerosis (MS) and has devastating impact on functional activities. There is great demand for medications that will enhance cognitive capacity in MS patients. To date, there is no evidence for improvement in cognition following treatment with aminopyridines, but the few studies on the topic included neuropsychological (NP) tests as secondary or tertiary outcomes, and were methodologically flawed. Dalfampridine may enhance cognition by direct pharmacological mechanisms, and should have effects on motor outcomes as in prior studies. By combining cognition and motor outcomes in the proposed study, the investigators will evaluate if the same patients with positive effects show beneficial responses on motor outcomes including physical activity and if such motor outcomes mediate and/or moderate cognitive improvements with dalfampridine