Disease activity on MRI will be assessed using several MRI measures, focusing on the number of new on-study lesions but also including the number of contrast-enhancing lesions, the total number of MRI-visible lesions, brain atrophy, and more recently defined MRI measures of tissue damage, such as quantitative magnetic relaxation mapping, diffusion-weighted imaging (DWI), magnetization transfer imaging (MTI), and MR spectroscopy (MRS). Additionally, participants will be assessed with other imaging modalities, specifically optical coherence tomography (OCT) and fluorescein angiography of the eye, and they will also be studied clinically and with neurophysiologic tests.