title: Orientation-specific fast rTMS maximizes corticospinal inhibition and facilitation.
abstract: Specific stimulation of neuronal circuits may promote selective inhibition or facilitation of corticospinal tract excitability. Monophasic stimulation is more likely to achieve direction-specific neuronal excitation. In 10 healthy subjects, we compared four types of repetitive transcranial magnetic stimulation (rTMS), monophasic and biphasic stimuli with the initial current in the brain flowing antero-posteriorly ("posteriorly directed") or postero-anteriorly ("anteriorly directed"). We applied rTMS over the primary motor cortex contralateral to the dominant hand, using 80 stimuli at 5 Hz frequency at an intensity yielding baseline motor evoked potential (MEP) amplitudes of 1 mV. Monophasic stimulation was always more efficient than biphasic. Facilitation was induced by intracerebral anteriorly directed current flow and inhibition by posteriorly oriented current flow, although only initially for approximately 30 pulses. The early inhibition was absent when studied during a tonic muscle contraction. Several mechanisms could account for these findings. They include a more efficient excitation of inhibiting circuits by posteriorly oriented pulses, and a back-propagating D-wave inhibiting early I-waves and thus inducing early inhibition of MEP amplitude. In any case biphasic rTMS results can be explained by a mixture of monophasic opposite stimulations. We propose the use of monophasic pulses for maximizing effects during rTMS.
authors.affiliation: Department of Clinical Neurophysiology, University of Göttingen, Robert-Koch-Str. 40, 37075, Göttingen, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Tings
authors.forename: Nicolas
authors.initials: N
authors.lastname: Lang
authors.forename: Frithjof
authors.initials: F
authors.lastname: Tergau
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
authors.forename: Martin
authors.initials: M
authors.lastname: Sommer
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D004307:Dose-Response Relationship, Radiation; D019054:Evoked Potentials, Motor / Q000502:physiology / Q000528:radiation effects*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology / Q000528:radiation effects; D009119:Muscle Contraction / Q000528:radiation effects; D009433:Neural Inhibition / Q000528:radiation effects*; D009949:Orientation; D011712:Pyramidal Tracts / Q000502:physiology / Q000528:radiation effects*; D050781:Transcranial Magnetic Stimulation