Role of primary sensorimotor cortices in generating inhibitory motor response in humans

To clarify the mechanism by which inhibitory motor responses such as cortic al negative myoclonus are generated in humans, three patients with medicall y intractable partial epilepsy (two with frontal lobe epilepsy and one with parietal lobe epilepsy) were studied by means of direct cortical stimulati on with a single electric pulse through subdural electrodes. All underwent chronic long-term video/EEG monitoring, cortical mapping by 50 Hz electric cortical stimulation and recording of cortical somatosensory evoked potenti als with chronically implanted subdural grid electrodes (3 mm in diameter a nd centre-to-centre distance of 1 cm) to map both epileptogenic and functio nal zones. After these clinical evaluations, cortical stimulation by single electric pulse (0.3 ms duration, 1 Hz) was carried out through pairs of su bdural electrodes located at the primary sensorimotor area (MI-SI), pre-sup plementary motor area (pre-SMA) and lateral negative motor area (lateral NM A), while surface EMG tvas recorded from the muscles of the contralateral h and. The results showed that (i) in all subjects, single pulse stimulation of hi-SI elicited a motor evoked potential (MEP) followed by a silent perio d (SP) in the contralateral distal hand muscles, the latter lasting 300 ms after the stimulus. The duration of SP was proportional to the size of the preceding MEP, In one subject, SP without any preceding MEP was elicited, a nd, in another subject, there was a short SP immediately before MEP in the contralateral thenar muscle, (ii) Following the stimulation of either pre-S MA or lateral NMA, no SP was observed. It is concluded that the inhibitory mechanism within the MI-SI, but probably not in the non-primary motor areas , either closely linked to or completely independent of excitation, most li kely plays an important role in eliciting brief negative motor phenomena su ch as cortical negative myoclonus or SP.