Using a balanced Latin-square crossover design, subjects will be allocated, in a double-blind, randomized fashion, to one of the four treatment sequences (Figure 1): 1) amantadine, placebo, modafinil, methylphenidate; 2) placebo, methylphenidate, amantadine, modafinil; 3) modafinil, amantadine, methylphenidate, placebo; and 4) methylphenidate, modafinil, placebo and amantadine. Each medication will be titrated over four weeks to the participants' highest tolerated dose or the pre-defined highest dose. The dosing and titration schedule of the study medications are depicted in Figure 2. Each treatment period will be 6 weeks and there will be a 2-week washout period between each treatment period. At the beginning of the trial, a biostatistician at UCSF will prepare a concealed allocation schedule, randomly assigning the four sequences, in blocks of 4, to a consecutive series of numbers and at the time of enrollment, each participant will be assigned the next consecutive number (and hence the sequence of study medications).