Transcranial Magnetic Stimulation in Different Current Directions Activates Separate Cortical Circuits

Abstract

cranial magnetic stimulation (TMS) to the primary motor cortex (M1) produces a series of corticospinal descending waves, with a direct (D) wave followed by several indirect (I) waves. TMS inducing posterioranterior (PA) current in the brain predominantly recruits the early I1-wave, whereas anterior-posterior (AP) directed current preferentially recruits the late I3-wave. However, it is not known whether I-waves elicited by different current directions are mediated by the same neuronal populations. We studied the neuronal mechanisms mediating I-waves by examining the influence of short-latency afferent inhibition (SAI) on various I-waves. SAI was tested with electrical median nerve stimulation at the wrist followed by TMS to the contralateral M1 at different current directions. Surface electromyograms and single motor units were recorded from the first dorsal interosseous muscle. SAI was weaker for the AP compared with that for the PA current direction. With increasing median nerve stimulation intensities, SAI increased for the PA direction but showed a U-shaped relationship for the AP direction. SAI produced more inhibition of late I-waves generated by PA than those generated by AP current direction. We conclude that late I-waves generated by PA and AP current directions are mediated by different neuronal mechanisms.

Modulation of Short-Latency Afferent Inhibition Depends on Digit and Task-Relevance

“…We and others have shown that SAI is reduced during both the onset of muscle activity [6], [8], [9] and during sustained muscle contraction [6], [7]. Specifically, we observed reductions in SAI as early as movement preparation between an auditory “warning” and “go” cue and these reductions are likely cortically or sub-cortically mediated [6].…”

“…The functional significance of SAI to hand control remains largely unknown yet an active muscle can modify the magnitude of SAI [6], [7]. We and others have shown that SAI is reduced during both the onset of muscle activity [6], [8], [9] and during sustained muscle contraction [6], [7].…”

Physiological Basis of Transcranial Magnetic Stimulation

“…The tendency for LM magnetic stimulation to produce D wave activation of corticospinal fibers probably accounts for the reduced sensitivity of the evoked EMG responses to changes in the level of cortical excitability. Short-latency responses from LM stimulation were also reported in the first dorsal interosseous muscle by Ni and colleagues in 2011 [35].…”

“…Further exploration has been made to elucidate more precise differences between PA and AP stimulation. As described above, SAI has been shown to be weaker for the AP compared with that for the PA current direction (Ni et al [35]). The authors emphasized that MEP generated by the AP current direction in which more late I waves were produced were less inhibited compared with MEPs generated by the PA direction.…”

“…In addition to determining which coil orientations produce D or I waves, some studies have used coil orientation to explore the mechanisms that produce the early and late I waves. Ni et al [35] studied the neuronal mechanisms mediating I waves by examining the influence of short-latency afferent inhibition (SAI) on various I waves. SAI was tested with electrical stimulation of the median nerve at the wrist followed by TMS to the contralateral motor cortex at different current directions.…”