 Testing of cortical function will occur both before and after theta-burst stimulation, and will involve a standard protocol using suprathreshold single and paired-pulse magnetic stimulation with measurement of the induced motor evoked potential MEP in the hand and arm muscles. The resting MEP threshold and input-output curve will be measured for the first dorsal interosseus muscle. Paired-pulse inhibition of the MEP will be tested using a subthreshold pulse 80 of resting motor threshold followed by a suprathreshold pulse at 2.5msec interval. Paired-pulse facilitation will be tested using a 12msec interval. Silent period duration in response to a pulse at 120 of resting motor threshold will be measured. 