Rhythmic Auditory Stimulation (RAS) is a music therapy technique that provides rhythmic auditory cues (like a beat) to help improve patients' movements, especially when walking.

The purpose of this study is to compare the effect on walking performance of a home based walking program (HBWP) with Rhythmic Auditory Stimulation (RAS), to that of a HBWP without RAS, or to RAS without walking exercise.

A second part of this study will assess the effects of Rhythmic Auditory Stimulation (RAS) on brain activity in patients with Multiple Sclerosis while performing mental imagery of walking.

Inclusion Criteria:

1. age 18 years or older
2. diagnosis of MS per Mc Donald criteria
3. Ambulation Index score from 2 to 6 inclusive (clinically observable gait disturbance, whether the subject walks with no, unilateral, or bilateral support)
4. spastic paresis is the main neurologic impairment causing the gait disturbance, per investigator's judgment.

Exclusion Criteria:

1. neurologic impairments other than spastic paresis (e.g. cerebellar ataxia or sensory ataxia), or non-neurologic impairments (e.g. musculoskeletal problems) play a major role in the subject's gait disturbance, per investigator's judgment;
2. treatment for an MS exacerbation in the past 30 days;
3. severe co morbidity precluding participation in the study per investigator's judgment (e.g. severe cardiac or respiratory failure);
4. severe cognitive deficits precluding informed consent or preventing the subject from following study procedures safely
5. contraindication to MRI such as severe claustrophobia and implanted devices such as neurostimulators, pacemakers, aneurysm clips etc.
6. pregnancy.