title: The effect of current flow direction on motor hot spot allocation by transcranial magnetic stimulation.
abstract: The objective of this study was to investigate the significance of pulse configurations and current direction for corticospinal activation using transcranial magnetic stimulation (TMS). In 11 healthy subjects (8 female), a motor map for the motor evoked potentials (MEPs) recorded from the first dorsal interosseus (FDI), abductor digiti minimi (ADM), extensor carpi radialis, and biceps brachii (BB) muscles of the dominant side was established. Starting from a manually determined hot spot of the FDI representation, we measured MEPs at equal oriented points on an hexagonal grid, with 7 MEPs recorded at each point, using the following pulse configurations: posteriorly directed monophasic (Mo-P), anteriorly directed monophasic (Mo-A), biphasic with the more relevant second cycle oriented posteriorly (Bi-P) as well as a reversed biphasic condition (Bi-A). For each pulse configuration, a hot spot was determined and a center of gravity (CoG) was calculated. We found that the factor current direction had an effect on location of the CoG-adjusted hot spot in the cranio-caudal axis but not in the latero-medial direction with anteriorly directed pulses locating the CoG more anteriorly and vice versa. In addition, the CoG for the FDI was more laterally than the cortical representations for the abductor digiti minimi (ADM) and extensor carpi radialis (ECR) which were registered as well. The results indicate that direction of the current pulse should be taken into account for determination of the motor representation of a muscle by TMS.
authors.affiliation: Department of Clinical Neurophysiology, University of Göttingen, Göttingen, Germany cstephani@med.uni-goettingen.de.
authors.forename: Caspar
authors.initials: C
authors.lastname: Stephani
authors.affiliation: Department of Clinical Neurophysiology, University of Göttingen, Göttingen, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
authors.affiliation: Department of Clinical Neurophysiology, University of Göttingen, Göttingen, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Sommer
mesh_terms: D000328:Adult; D001931:Brain Mapping / Q000379:methods*; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Motor mapping; TMS; pulse configuration