TMS stimulation will be applied to the corresponding region of the contralateral primary motor cortex to determine motor threshold and to examine the motor cortical excitability measures after Ketamine. The optimal coil position will be determined by moving the TMS coil in 1-cm increments over the motor cortical area while delivering single or paired magnetic pulses and by observing maximal contraction of the contralateral abductor pollicis brevis (APB). Electromyography readings will be obtained from the APB muscle. TMS stimulation will then be applied to Left DLPFC or Left Brodmann Area 6 to investigate cortical excitability changes after ketamine. Electroencephalography(EEG) recordings will be concurrent with TMS procedure.