tDCS and Cognition in Adults With Multiple Sclerosis or Encephalitis

Participants enrolled into this study may be asked to do the following: * Come to the investigators' testing office at the Johns Hopkins Hospital where the tDCS equipment is located for 10 study visits. * Grant permission for the researchers to view medical records associated with the participant's diagnosis of multiple sclerosis or encephalitis (if applicable) and general state of health. * Complete a questionnaire and provide a health history in order to verify eligibility to participate and be able to safely undergo the experimental procedures. * Complete several computerized, written, and/or aural tasks (i.e., saying words out loud) that assess different cognitive functions such as attention, memory, language, or processing speed. * Wear electrodes that will be placed on the scalp with a large rubberized band. These electrodes will administer very weak electrical current (tDCS) for 30 minutes. * Participate in several study conditions. The exact conditions and the order will be randomized. That is, the conditions and order will vary by chance (like the flip of a coin). Under some conditions, the participant might receive active stimulation (tDCS) and under other conditions, the participant might receive placebo (or sham) stimulation. Placebo stimulation is similar to active tDCS but lasts only a few seconds. However, all groups will wear the electrodes for the same length of time to prevent the participant from knowing whether participants are receiving active tDCS or sham stimulation. The study doctor and research staff will know which group the participant is in. * Permission to audio and/or video-tape test sessions for later scoring and observation. These tapes will not be viewed by anyone not affiliated with the study without the participant's consent. * Have structural, resting-state and functional Magnetic Resonance Imaging scans as a part of the study.