 For assessment of active motor threshold, subjects will be asked to contract the FDI muscle against resistance while being given biofeedback to maintain average integrated EMG at 20 of maximal voluntary isometric EMG. For assessment of resting motor threshold, EMG of the FDI must be less than 5 of maximal prior to stimulation. Single-pulses at increasing intensity will be used to determine the intensity at which 5 out of 10 pulses evoke an EMG response greater than 200mV active threshold or 50mV resting threshold above baseline. Active motor threshold is virtually always less than the resting motor threshold. If this is not the case in a subject, then stimulation will be adjusted so that intensity never exceeds 80 of the resting motor threshold. Thresholds will be assessed both before and after theta-burst stimulation, and these thresholds will be used for the cortical excitability testing as described above. 