Cathodic tDCS (1 mA for 20 minutes) will be sequentially applied over the motor cortex of both sides, for five consecutive days every month for twelve months. The control group will receive a sham stimulation that reproduce tactile sensation of real stimulation but has no effects on central nervous system. For stimulation, researchers will employ a CE-certified medical device acting as a micro-processor-controlled constant current source. All patients will take riluzole during the entire period of the study.