Klinische Neurophysiologie 2012; 43 - P052
DOI: 10.1055/s-0032-1301602

Influence of current direction on short-interval intracortical facilitation probed with paired-pulse TMS

I Delvendahl 1, H Lindemann 1, A Pechmann 1, H Siebner 2, V Mall 3
  • 1Zentrum für Kinder- und Jugendmedizin, Universitätsklinikum Freiburg, Freiburg
  • 2Danish Research Center for Magnetic Resonance, Copenhagen University Hospital Hvidovre, Hvidovre, DK
  • 3Universitätsklinikum Carl Gustav Carus an der Technischen Universität Dresden, Dresden

Background: Transcranial magnetic stimulation (TMS) of the human primary motor hand area (M1-HAND) can produce multiple descending volleys in fast-conducting corticospinal neurons, especially so-called I waves which result from trans-synaptic excitation. The facilitatory interaction between these I waves can be studied with paired-pulse TMS using a conditioning-test paradigm referred to as short interval intracortical facilitation (SICF). Here, we examined whether SICF depends on the current direction of the TMS pulses. Methods: In ten healthy volunteers aged 22–29, we applied paired-pulse TMS to right M1-HAND at intensity slightly above resting motor threshold to probe the impact of current direction on SICF. SICF was tested at inter-stimulus intervals (ISIs) ranging from 1.0 to 5.0 ms by recording the motor evoked potential (MEP) from the left abductor pollicis brevis muscle. In a first experiment, we probed SICF with monophasic stimuli evoking an anterior-posterior (AP-AP condition) or posterior-anterior (PA-PA condition) current in M1-HAND. In a separate experiment, the two TMS pulses had a half-sine wave configuration and evoked tissue currents with opposing directions (PA-AP condition and AP-PA condition). Results: For all four combinations of current directions, paired-pulse TMS yielded three facilitatory peaks at ISIs of 1.2, 2.6 and 4.8 ms separated by distinct troughs without facilitation. The SICF peaks were comparable with respect to magnitude and periodicity. Conclusion: The results indicate that, at slightly suprathreshold intensities, all types of sagittally oriented tissue currents induce a similar amount of facilitatory I wave interaction in human M1-HAND when using a conventional SICF paradigm.