The influence of coil-skull distance on transcranial magnetic stimulation motor-evoked responses

We have investigated the effects of changing the coil-to-skull distance on the motor-evoked responses (MEP) induced with two different magnetic stimulator coils (80 mm round and figure-of-eight coil) at rest and during voluntary muscle contraction. The changes in MEP latency, amplitude and silent period (SP) duration induced by stimulation directly upon the skull, and 1 cm away from the skull were analyzed by computing the probability density distribution (PDD) for the responses obtained from all subjects. This measure corresponds to the finite probability that the event occurs within a given area. Overall, the results were consistent with a distance-induced decrease in magnetic field strength. However, the increase in coil-to-skull distance induced a higher probability of longer latencies in active muscle when stimulating with either coil. Also, stimulating at a distance with the figure-of-eight coil increased the probability of a longer SP duration. The stimulation strength at the two distances was comparable because it was set based on the motor threshold obtained for each distance. Therefore, our results are not entirely compatible with the established exponential drop in magnetic field with increasing distance. Rather, they suggest that a more complex set of interactions occurs in the cortex. The results imply that distinct patterns of cortical network activation may exist related to the distance-induced alterations when the coil is moved away from the skull. Further studies are required to elucidate the precise nature of the distance-related interactions of the magnetic field with the cortex.