Waveforms of cortical discharge demonstrated by jerk-locked back-averaging time-locked to the involuntary EMG discharge of the left dorsal interosseous muscle (DIO) in a patient with familial cortical tremor (myoclonus). The positive-negative biphasic spike (arrow) is maximal at the right central area (C2), which precedes the EMG onset by 30 ms. There is no slow EEG shift preceding the EMG onset, although the same filter setting as that used for the movement-related cortical potential recording (time constant of 3 s) was used.