ti
c
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Masashi Hamada , Ritsuko Hanajima, Yasuo Terao, Noritoshi Arai, Toshiaki Furubayashi,
Satomi Inomata-Terada, Akihiro Yugeta, Hideyuki Matsumoto, Yuichiro Shirota,
Results: Motor cortical MEPs were facilitated to about 190% of baseline (p < 0.001) for 10 min post rPPS intervention and returned to
et al., 2000a,b; Sommer et al., 2002), although there is a
moderate interindividual variability in the direction of
effect (Maeda et al., 2000a). So far, most of the studies have
that monophasic pulses preferentially activate population
of neurons oriented in the same direction so that their
effects readily summate. In the same way, it is possible that
monophasic rTMS would result in a more powerful after-
effect than biphasic rTMS. Indeed, Thickbroom and
coworkers (2006) produced a powerful after-effect by repet-
*
Corresponding author. Tel.: +81 3 5800 8672; fax: +81 3 5800 6548.
E-mail address: mhamada-tky@umin.ac.jp (M. Hamada).
Clinical Neurophysiology 1the baseline at 10–15 min post intervention. Brainstem MEPs were not affected by the intervention.
Conclusions: The facilitation of MEPs after rPPS at an interval of 1.5 ms occurs at the motor cortex.
Significance: rPPS at an interval of 1.5 ms is an effective method for increasing motor cortical excitability.
 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
Keywords: Repetitive transcranial magnetic stimulation (rTMS); Paired pulse stimulation; Motor cortex; Monophasic pulse; I wave periodicity
1. Introduction
Repetitive transcranial magnetic stimulation (rTMS)
protocols have been reported to be a potential tool to mod-
ify human motor cortical excitability. High frequency
rTMS tends to induce facilitatory after-effects and low fre-
quency rTMS inhibitory effects (Chen et al., 1997; Maeda
used biphasic rTMS (Berardelli et al., 1998, 1999; Chen
et al., 1997; Di Lazzaro et al., 2002; Huang et al., 2005;
Maeda et al., 2000a,b; Pascual-Leone et al., 1994; Romeo
et al., 2000).
We have previously shown that monophasic TMS has a
stronger short-term effect during repetitive stimulation
than biphasic TMS (Arai et al., 2005). This finding suggestsYoshikazu Ugawa
Department of Neurology, Division of Neuroscience, Graduate School of Medicine, The University of Tokyo,
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8655, Japan
Accepted 11 March 2007
Available online 23 April 2007
Abstract
Objective: Repetitive paired-pulse TMS (rPPS) given at an interstimulus interval (ISI) of 1.5 ms has been reported to induce a lasting
motor evoked potential (MEP) facilitation. This after-effect was considered to be a cortical event because F-waves were not affected
by the same rPPS. To confirm its cortical facilitation, we compared the after-effects of rPPS on MEPs to single pulse TMS over the motor
cortex (motor cortical MEPs) with those to brainstem stimulation (brainstem MEPs).
Methods: Subjects were 10 healthy volunteers. Suprathreshold paired-pulse TMS at an ISI of 1.5 ms was applied to the motor cortex for
30 min at a rate of 0.2 Hz. After intervention, we measured motor cortical MEPs for 30 min. We also studied brainstem MEPs in five
subjects.Origin of facilitation in repe
pulse transcranial magneti
human m
*1388-2457/$32.00  2007 International Federation of Clinical Neurophysiolo
doi:10.1016/j.clinph.2007.03.009tive, 1.5 ms interval, paired
stimulation (rPPS) of the
or cortex
www.elsevier.com/locate/clinph
18 (2007) 1596–1601gy. Published by Elsevier Ireland Ltd. All rights reserved.

rophitive, monophasic, paired pulses TMS delivered at an
I-wave periodicity (the rPPS protocol). rPPS with TMS
pulses paired at 1.5 ms for 30 min facilitated motor evoked
potentials (MEPs) to a single TMS delivered over the
motor cortex (motor cortical MEPs), which lasted up to
10 min after the cessation of the intervention. This facilita-
tion was proposed to occur at the cortical level because
F-waves were not affected by the same protocol. However,
since F-waves can reflect the activity of only a small portion
of spinal motor neurons, we cannot be sure whether the
whole spinal cord excitability was indeed unchanged. In
contrast, MEPs to brainstem stimulation (brainstem MEPs)
are considered to reflect the excitability of a larger popula-
tion of spinal motor neurons than do F-waves, because it
can activate most of the motor neurons in the spinal cord.
In this communication, we aimed to differentiate between
the cortical and subcortical loci of facilitation by comparing
the after-effects of rPPS between motor cortical and brain-
stem MEPs. Determination of the site of action would have
an important implication for the future application of this
novel protocol for treating neurological disorders.
2. Subjects and methods
2.1. Subjects
Subjects were 10 healthy volunteers (2 women, 8 men,
mean age ± SD: 38.0 ± 5.3 years) who gave their informed
consent to participate in the experiments. None of the sub-
jects had neurological, psychiatric or other medical prob-
lems or had any contraindication to TMS (Wassermann,
1998). The protocol was approved by the Ethics Committee
of the University of Tokyo and was carried out in accor-
dance with the ethical standards of the Declaration of
Helsinki.
2.2. Stimulation and recordings
Subjects were seated on a reclining chair and motor
evoked potentials (MEPs) were recorded from the right
first dorsal interosseus muscle (FDI) (dominant hand in
all subjects). Pairs of Ag/AgCl surface cup electrodes
(9 mm in diameter) were placed over the muscle belly
(active electrode) and the metacarpophalangeal joint of
the index finger (reference electrode). Responses were
amplified with an amplifier (Biotop, GE Marquette Medi-
cal Systems, Japan) through filters set at 100 Hz and
3 kHz, digitized with A/D converter at a sampling rate of
20 kHz and stored in a computer (TMS bistim tester, Med-
ical Try System, Japan) for later offline analysis.
Transcranial magnetic stimulation (TMS) was given
over the hand area of the left primary motor cortex
(M1) at a position optimal for eliciting MEPs in the
right FDI with a figure of eight coil (external diameter
at each wing 9 cm, Magstim Co., Whitland, Dyfed,
M. Hamada et al. / Clinical NeuUK). The coil was held tangential to the scalp with the
handle pointing backwards at about 45 laterally, thatis perpendicular to the central sulcus. According to the
previous study (Sakai et al., 1997), this is the optimal
orientation for activating the corticospinal system trans-
synaptically via horizontal cortical connections. To deter-
mine the optimal site for FDI, we stimulated several
positions separated by 1 cm with the same intensity
and defined the hot spot as the site where the largest
responses were elicited. The position was marked with
a red pen on the scalp for repositioning the coil at the
same site throughout the experiments.
Single pulse TMS was delivered by a Magstim 200 mag-
netic stimulator (Magstim Co., Whitland, Dyfed, UK), and
paired pulse stimuli were delivered by two Magstim 200
stimulators connected with a Bistim module (Magstim
Co., Whitland, Dyfed, UK).
For sham stimulation (see below), surface cup electrodes
were placed over the vertex (Cz of international 10–20 sys-
tem) and the left-hand motor area in each subject. An elec-
tric pulse of 0.2 ms duration was delivered through those
electrodes with a conventional electric stimulator. The
intensity was fixed at two times the sensory threshold for
skin sensation.
Brainstem (BST) electrical stimulation was performed to
evaluate spinal motoneuronal excitability changes because
MEPs to this stimulation must reflect activity of most of
spinal motor neurons (Ugawa et al., 1991). Anode (right)
and cathode (left) were attached over the mastoid pro-
cesses. Stimulation was given with a high voltage electric
stimulator (D180A: 0.1 ms duration, maximal output
1.2 A, 1200 V; Digitimer, Welwyn Garden City, UK).
2.3. Interventions
2.3.1. rPPS intervention
For rPPS intervention (rPPS 30 in Fig. 1), we used the
same protocol as that reported by Thickbroom et al.
(2006). Paired stimuli of equal strength were delivered at
an interval of 1.5 ms. The stimulus intensity was set to elicit
MEPs of 0.3–0.5 mV when delivered as a pair. Paired stim-
uli were applied over the left hand motor area every 5 s,
and 360 paired stimuli were administered in total for
30 min. During intervention, 360 MEPs (each one MEP
elicited by one paired TMS) were obtained and a series
of 60 MEP amplitudes were averaged to obtain the mean
MEP amplitudes at 5 min intervals. Even in real rPPS,
two electrodes were fixed at the same positions over the
scalp as those in the sham stimulation (see the next para-
graph). No currents were in real rPPS.
2.3.2. Realistic sham intervention
In sham intervention (sham 30 in Fig. 1), we performed
realistic sham stimulation described previously (Okabe
et al., 2003). Paired electric stimuli were given every 5 s
for 30 min. The coil was not connected to a stimulator
and placed over left M1. Another coil was placed near
ysiology 118 (2007) 1596–1601 1597the subject. This coil was connected to Bistim module com-
bining two Magstim 200 stimulators. These stimulators