THE OFFSET CORTICAL POTENTIAL - AN ELECTRICAL CORRELATE OF MOVEMENT INHIBITION IN MAN

Nine normal subjects were asked to make either a brisk isometric pinch of a force transducer held between the forefinger and thumb, or to ho ld a pinch for approximately IS s and then release the force suddenly without any avert antagonist contraction. EEG activity was averaged ab out the onset or offset of EMG activity and movements were made In the subjects' own time, All subjects found the task simple. The EEG activ ity preceding offset of contraction (offset cortical potential) was si gnificantly smaller in lateral leads than that seen before onset of co ntraction, Midline activation was similar in related to the act to be performed contributes substantially to the EEG potentials in lateral l eads. This activity is absent in the offset task which requires only w ithdrawal of tonic input to motor cortex, Midline activity, common to both onset and offset tasks. could reflect timing, attentional, or oth er processes. The results are discussed in relation to previous data f rom an isotonic relaxation task.(1)