Interhemispheric facilitation and inhibition studied in man with double magnetic stimulation

Double magnetic stimulation was used to investigate the influence of a conditioning magnetic shock, applied to the hand motor area, on the size of the EMG response of the first dorsal interosseous (FDI) muscle elicited by magnetic test stimulation, applied to the same area of the opposite hemisphere, in 55 subjects unaffected by neurological disease. Both hemispheres were studied and compared at different intervals between shocks, ranging from 1-15 ms. With short intervals between shocks (1-5 ms), the EMG response to the target muscle of the test shock was increased (facilitation); it diminished after 6 ms (inhibition). Facilitatory effects started at 1-2 ms between shocks and lasted up to 5 ms between shocks. They were mainly present on one side; only 11/35 subjects tested from 1 to 5 ms between shocks had bilateral facilitatory effects. No significant differences between right and left hemispheres and sexes were noted. The conduction time to mediate facilitation between hemispheres was estimated at 8-9 ms. No facilitation occurred when the electrical test shock was conditioned by a magnetic shock, or when the magnetic test shock was conditioned by an electrical shock. We conclude that facilitation takes place at the cortical level since electrical stimulation is known to act on axons and magnetic stimulation on neurons. Facilitation did not occur when the magnetic conditioning coil was moved away from the motor area, suggesting that it requires pathways that link these areas, e.g. the corpus callosum. Facilitation was observed with muscles at rest; it disappeared when the FDI muscle was contracted. Inhibition occurred after 6 ms between shocks. It was noted in both hemispheres, in all subjects tested with no gender differences observed. The depth of inhibition increased with an increase in the interval between shocks. It clearly started at 6 ms between shocks in both hemispheres. The conduction time between hemispheres for inhibitory phenomena was estimated at 13 ms. We concluded that interhemispheric facilitatory and inhibitory phenomena are present in man, as suggested by experimental data in animals. For clinical purposes, we propose an interval of 4 ms between shocks to investigate facilitatory interhemispheric influences and 11 ms for inhibitory effects.