Around half of MS patients have cognitive issues. The test most frequently used to measure cognition is the so-called Symbol Digit Modalities Test (SDMT), in which the tested person is asked to decode a list of symbols as fast as possible using a code at the top of the page. In this study, as well as performing the SDMT, we shall also be measuring and quantifying subtle eye coordination problems. This will improve our knowledge of the extent to which eye trouble contributes to poorer test results. We shall also investigate whether new digital cognitive tests are less sensitive to eye problems.