Zusammenfassung
Das Ziel der vorliegenden Studie ist die Analyse der prämotorischen Veränderungen
der Erregbarkeit der motorischen Hirnrinde vor einer ballistischen Willkürreaktion
mit der rechten (dominaten) Hand (isometrische Abduktion des Zeigefingers) in einem
Reaktionszeitexperiment. Dazu wurde die transkranielle Magnetstimulation (TMS) verwendet,
wobei sowohl Einzelimpuls- als auch Doppelimpuls-TMS-Reize mit unterschiedlicher Verzögerung
nach dem Erscheinen des visuellen go signal über der linken motorischen Hirnrinde appliziert wurden. Die Intensität der TMS-Test-Reize
betrug 120 % der motorischen Schwelle, die der konditionierenden Reize bei Doppelimpuls-TMS
war 70 %, und als Interstimulusintervall (ISI) wurden 3 ms bzw. 13 ms verwendet. Gemessen
wurden das EMG des M. interosseus dorsalis I und die laterale Kraft. Die Parameter
des TMS-induzierten motorisch-evozierten Potenzials (MEP) wurden zur Beschreibung
der kortikalen Erregbarkeit bestimmt. Im prämotorischen Zeitbereich, etwa 80 ms vor
dem Einsetzen der Willkürreaktion im EMG (burst), findet man generell deutlich vergrößerte MEP-Amplituden. Gegenüber der Situation
bei Einzelimpuls-TMS, tritt bei Doppelimpuls-TMS mit ISI = 13 ms diese kortikale Fazilitation
etwas früher, aber mit geringerer Stärke auf; bei ISI = 3 ms ist erwartungsgemäß zunächst
die intrakortikale Inhibition zu beobachten, die aber dann ebenfalls übergeht in eine
Fazilitation, wobei diese im Vergleich zur Einzelimpuls-TMS etwas mehr zum Einsatzzeitpunkt
des burst hin verschoben ist. In allen TMS-Experimenten ist ein minimales Intervall (dead band)
zwischen dem Beginn des MEP und des burst erkennbar. Weiterhin zeigt sich ein wesentlicher Einfluss der TMS-Applikation auf
die Verteilungseigenschaften der Reaktionszeiten.
Abstract
The aim of the present study was to investigate the effect of transcranial magnetic
stimulation (TMS) on reaction time (RT) and on motor cortex excitability preceding
voluntary movements. Single pulse and paired-pulse TMS with two interstimulus intervals
(ISI) of 3 and 13 ms were applied to the left motor cortex at different delays after
a visual go command for a voluntary isometric index finger abduction of the right
(dominant) hand. The intensity of the testing and conditioning stimuli were 120 %
and 70 % of motor threshold at rest, respectively. Changes of the parameters of motor
evoked potentials (MEP) recorded from the first dorsal interosseous muscle (FDI) were
used to assess motor cortex excitability. The MEPs in response to single pulse TMS
were augmented in a period around 80 ms before the voluntary myoelectrical activity
(EMG) onset. In paired-pulse TMS at an ISI of 13 ms, the MEP augmentation was smaller
but started earlier before the voluntary EMG. In case of ISI = 3 ms, the expected
intracortical inhibition was evident only when TMS preceded the voluntary muscle activity
by a premovement interval of more than 80 ms, but at shorter intervals, rather some
MEP augmentation was observed. In all TMS experiments, a minimum interval between
MEP onset and burst onset of the voluntary response was observed (dead band). Moreover,
TMS influence on RT was accompanied by essential changes of RT distribution.
Key words
Transcranial magnetic stimulation - reaction times - motor evoked potentials - intracortical
inhibition - intracortical facilitation
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Prof. Dr. Andon R. Kossev
Institute of Biophysics Bulgarian Academy of Sciences
Acad. G. Bontchev Str., Bldg. 21
1113 Sofia · Bulgaria
Email: kossev@bio.bas.bg