Low voltage bifocal electrical stimulation of the motor cortex.
Currently, non-invasive cerebral motor area stimulation is performed by magnetic or high voltage bifocal electrical stimulation. These techniques require non-conventional stimulators. The present study, performed on 28 normal subjects between 18 and 73 years of age, was designed to standardize a method of bifocal stimulation of motor cortex which uses low voltage conventional stimulators and therefore can be routinely used in electrophysiological laboratories. The hand motor area was stimulated by surface electrodes applied in Cz (cathode) and 8 cm lateral on the bimetal line (anode). Under the electrodes infiltration with Xylocaine 2% was made. Duration and intensity of the rectangular pulses were 150 microseconds and 200-250 Volts, respectively. The motor evoked potential (MEP) was recorded from opponens pollicis muscle by concentric needle electrodes and a slight voluntary contraction was made to facilitate the motor response. MEP latency and central motor conduction time (CMCT) were calculated for both sides. A significant relation with age and height in normal subjects was found only for the MEP latency. We computed also the equations expressing the normal variability between the sides of MEP latency and of CMCT.