PREFERENTIAL ACTIVATION OF DIFFERENT I-WAVE BY TRANSCRANIAL MAGNETIC STIMULATION WITH A FIGURE-OF-8-SHAPED COIL

Transcranial magnetic stimulation (TMS) over the human primary motor c ortex (MI) evokes motor responses in the contralateral limb muscles. T he latencies and amplitudes of those responses depend on the direction of induced current in the brain by the stimuli (Mills et al. 1992, We rhahn et al. 1994). This observation suggests that different neural el ements might be activated by the differently directed induced currents . Using a figure of-eight-shaped coil, which induces current with a ce rtain direction, we analyzed the effect of direction of stimulating cu rrent on the latencies of responses to TMS in normal subjects. The lat encies were measured from surface electromyographic responses of the f irst dorsal interosseous muscles and the peaks in the peristimulus tim e histograms (PSTHs) of single motor units from the same muscles. The coil was placed over the M1, with eight different directions each sepa rated by 45 degrees. Stimulus intensity was adjusted just above the mo tor threshold while subjects made a weak tonic voluntary contraction, so that we can analyse the most readily elicited descending volley in the pyramidal tracts. In most subjects, TMS with medially and anterior ly directed current in the brain produced responses or a peak that occ urred some 1.5 ms later than those to anodal electrical stimulation. I n contrast, TMS with laterally and posteriorly directed current produc ed responses or a peak that occurred about 4.5 ms later. There was a s ingle peak in most of PSTHs under the above stimulation condition, whe reas there were occasionally two peaks under the transitional current directions between the above two groups. These results suggest that TM S with medially and anteriorly directed current in the brain readily e licits Il waves, whereas that with laterally and posteriorly directed current preferentially elicits 13 waves. Functional magnetic resonance imaging studies indicated that this direction was related to the cour se of the central sulcus. TMS with induced current flowing forward rel ative to the central sulcus preferentially elicited Il waves and that flowing backward elicited 13 waves. Our finding of the dependence of p referentially activated I waves on the current direction in the brain suggests that different sets of cortical neurons are responsible for d ifferent I waves, and are contrarily oriented. The present method usin g a figure-of-eight-shaped coil must enable us to study physiological characteristics of each I wave separately and, possibly, analyse diffe rent neural elements in M1, since it activates a certain I wave select ively without D waves or other I waves.