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NeuroImage 23 (2004) 849–859

Short-term modulation of regional excitability and blood

flow in human motor cortex following rapid-rate

transcranial magnetic stimulation

Beatrice Takano,a,b Alexander Drzezga,c Martin Peller,c,d,e Iris Sax,a Markus Schwaiger,c

Lucy Lee,f and Hartwig Roman Siebnerd,e,*

aDepartment of Neurology, Munich University of Technology, Munich, GermanybDepartment of Neurology, Stadtisches Krankenhaus Munchen Bogenhausen, Munich, GermanycDepartment of Nuclear Medicine, Munich University of Technology, Munich, GermanydDepartment of Neurology, Christian-Albrechts-University, Kiel, GermanyeNeuroImageNord Kiel-Hamburg-Lubeck at Hamburg University Hospital, Hamburg, GermanyfWellcome Department of Imaging Neuroscience, Institute of Neurology, University College London, London, UK

Received 3 December 2003; revised 23 March 2004; accepted 22 June 2004

Available online 12 October 2004

Repetitive transcranial magnetic stimulation (rTMS) of the human

primarymotor cortex (M1) provides ameans of inducing lasting changes

in cortical excitability and synaptic activity. Here we combined rTMS

with positron emission tomography of regional cerebral blood flow

(rCBF) to examine how an rTMS-induced change in intracortical

excitability of inhibitory circuits affects regional synaptic activity. In a

first set of experiments, we gave 150 biphasic pulses of 5 Hz rTMS at

90% of active motor threshold to left M1 and used single- and paired-

pulse TMS to assess the conditioning effects of rTMS on motor cortical

excitability at rest. rTMS conditioning led to a selective decrease in

short-latency intracortical inhibition (SICI) without affecting short-

latency intracortical facilitation or corticospinal excitability. The

decrease in SICI lasted for approximately 8min. In a second experiment,

we used the same rTMS protocol and measured changes in regional

synaptic activity (as indexed by rCBF) during and for up to 14 min after

the end of rTMS. Subthreshold 5 Hz rTMS induced a region-specific

increase in resting rCBF in the stimulated M1 lasting approximately 8

min. These results suggest that in the stimulated M1, temporary

attenuation of SICI is paralleled by an increase in synaptic activity,

consistent with reduced efficacy of intracortical GABAA-ergic synapses.

The present findings demonstrate that short trains of low-intensity 5 Hz

rTMS can be used to induce a transient change in function within a

distinct cortical area. This opens up new possibilities for studying acute

reorganization at the systems level in the intact human brain.

D 2004 Elsevier Inc. All rights reserved.

Keywords: Functional imaging; Inhibition; Motor cortex; Regional cerebral

blood flow; Synaptic activity; Transcranial magnetic stimulation

1053-8119/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.neuroimage.2004.06.032

* Corresponding author. Department of Neurology, Christian-

Albrechts-University, Schittenhelmstrasse 10, 24105 Kiel, Germany. Fax:

+49 431 5978502.

E-mail address: h.siebner@neurologie.uni-kiel.de (H.R. Siebner).

Available online on ScienceDirect (www.sciencedirect.com.)

Introduction

In recent years, repetitive transcranial magnetic stimulation

(rTMS) has become a well-established tool with which to induce

acute reorganization in the human cortex (Siebner and Rothwell,

2003). Several studies have demonstrated that rTMS to the primary

motor hand area (M1) can induce changes in the excitability of

corticospinal neurons (Chen et al., 1997; Pascual-Leone et al.,

1994). In these studies, corticospinal excitability was assessed at

rest by measuring the motor evoked potentials (MEPs) elicited by a

single pulse in relaxed contralateral hand muscles.

The frequency of rTMS plays an important role in determining

if the lasting modulatory effects on corticospinal excitability are

predominantly facilitatory or inhibitory (Chen et al., 1997; Maeda

et al., 2000; Pascual-Leone et al., 1994).

At intensities above motor threshold, short trains of high-

frequency rTMS (at frequencies of about 5 Hz or more) can

increase corticospinal excitability for several minutes after cessa-

tion of rTMS (Berardelli et al., 1998; Di Lazzaro et al., 2002;

Inghillieri et al., 2004; Pascual-Leone et al., 1994), whereas low-

frequency rTMS (at frequencies around 1 Hz) causes a decrease in

corticospinal excitability (Chen et al., 1997; Muellbacher et al.,

2000; Touge et al., 2001). Similar bidirectional effects have also

been observed after rTMS at intensities below resting motor

threshold (Gangitano et al., 2002; Maeda et al., 2000), but

subthreshold rTMS usually requires a greater number of stimuli

per rTMS session to induce consistent changes in corticospinal

excitability.

Lasting changes in motor cortex excitability can also be studied

using the conditioning test paradigm described by Kujirai et al.

(1993). This technique uses pairs of TMS pulses given through the

same coil. A conditioning (subthreshold) stimulus over M1

modulates the amplitude of a test MEP evoked by a suprathreshold

B. Takano et al. / NeuroImage 23 (2004) 849–859850

stimulus given a few milliseconds later. The test MEP is

consistently inhibited at conditioning test intervals of 1–5 ms, this

inhibitory interaction is called short-latency intracortical inhibition

(SICI). A facilitatory effect on the test MEP is seen with

conditioning test intervals of 8–15 ms (called short-latency

intracortical facilitation). There is converging evidence that these

effects are mediated by separate subpopulations of inhibitory

(presumably GABAA-ergic) and excitatory (presumably glutama-

tergic) cortical interneurons (Di Lazzaro et al., 1998; Kujirai et al.,

1993; Ziemann, 1999).

Using the paired-pulse paradigm of Kujirai et al. (1993), it has

been shown that subthreshold high-frequency rTMS can reduce the

strength of SICI (Di Lazzaro et al., 2002; Pascual-Leone et al.,

1998; Peinemann et al., 2000; Wu et al., 2000). Di Lazzaro et al.

(2002) reported that 50 stimuli of 5 Hz rTMS at active motor

threshold decreased the magnitude of SICI for several minutes.

There was no effect on motor threshold, MEP amplitude, short-

latency intracortical facilitation (SICF), or the duration of the

cortical silent period, suggesting a specific effect on SICI. Epidural

recordings of descending corticospinal volleys confirmed that this

effect of 5 Hz rTMS on SICI occurred at a cortical level.

In this study, we combined rTMS with positron emission

tomography (PET) of regional cerebral blood flow (rCBF) to

examine how a short train of subthreshold 5 Hz rTMS modulates

synaptic activity in the stimulated M1. We first established an

rTMS protocol that induced a transient reduction in SICI lasting

several minutes. We then applied this rTMS protocol to the left M1

in the PET scanner to investigate the immediate and enduring

effects of rTMS on regional synaptic activity (as indexed by

rCBF). In a previous study, we have shown that bon-lineQ PET

imaging during low-intensity rTMS at 90% of active motor

threshold can be used to map rate-dependent activation of the left

M1 (Siebner et al., 2001). Here we demonstrate that boff-lineQ PET

Fig. 1. Experimental approach to assess the time course of changes in excitability

transcranial magnetic stimulation (rTMS): We performed two experiments on tw

biphasic pulses of 5 Hz rTMS to the left primary motor hand area (M1) at 90% of

pulse transcranial magnetic stimulation (TMS) at an interstimulus interval of 2 ms

inhibition (SICI). Paired-pulse TMS to left M1 (hatched bar) was given for 12 min

H215O positron emission tomography (PET) was used to measure the rCBF (an

subthreshold 5 Hz rTMS to M1. In each participants, three PET scans were acquire

5 s before PET scanning. In the bpost-rTMSQ condition, a single rTMS train was

rTMS and PET scanning was systematically varied across scans. In each particip

rTMS, respectively.

imaging after low-intensity rTMS over the left M1 is capable of

mapping changes in regional synaptic activity in the context of

altered intracortical excitability.

Methods

All experiments were carried out on healthy volunteers with no

history of neurological or psychiatric disorders. Written informed

consent for the procedures was given by all subjects before the

studies. The experimental procedures were approved by the Ethics

Committee of the Faculty of Medicine of the Technische

Universit7t Mqnchen.All participants received a short train of subthreshold 5 Hz

rTMS to the left M1 to induce a short-lasting change in function in

the left M1 (Fig. 1). In the first set of experiments (Experiments 1a,

b, and c), we used single- and paired-pulse TMS to investigate the

effects of 5 Hz rTMS on motor cortex excitability. In the second

experiment (Experiment 2), we repeatedly measured the rCBF to

assess changes in synaptic activity during and after 5 Hz rTMS to

left M1 (Fig. 1).

Repetitive transcranial magnetic stimulation

A Mag-Pro stimulator was used for TMS (Medtronic-neuro-

muscular, Skovlunde, Denmark). The conditioning rTMS protocol

consisted of a single 30-s train of subthreshold 5 Hz rTMS (Fig. 1).

Stimulus intensity was 90% of active motor threshold (MT) of the

right first dorsal interosseus (FDI) muscle. Active MT was defined

as the minimum stimulus intensity required to elicit an MEP of

approximately 200 AV in 5 out of 10 consecutive trials during

isometric contraction (approximately 20% maximum force level).

At this intensity, magnetic stimuli cause excitation of intracortical

and net neuronal activity induced by a single train of subthreshold repetitive

o separate groups of healthy young subjects. All participants received 150

active motor threshold (MT). In Experiments 1b and c, we applied paired-

to investigate the effects of rTMS conditioning on short-latency intracortical

at a rate of 0.1 Hz, starting about 30 s after the rTMS train. In Experiment 2,

index of net neuronal activity) during and after a conditioning train of

d during rTMS (white squares). In this condition, a single rTMS train started

given 1–12 min before PET scanning (black squares). The interval between

ant, three PET measurements were performed 1–4, 5–8, or 9–12 min after

B. Takano et al. / NeuroImage 23 (2004) 849–859 851

inhibitory and facilitatory interneurons without evoking descend-

ing corticospinal volleys (Di Lazzaro et al., 1998).

The site for stimulating the left M1 was defined functionally as

the optimum position for eliciting a maximal MEP in the right FDI

muscle. This was determined by moving the coil in 0.5 cm steps

around the presumed M1. The site at which minimally supra-

threshold stimuli consistently produced the largest MEP in the

target muscle was marked with a red wax pen as the bmotor hot

spotQ by drawing a line following the anterior bifurcation of the

coil and a straight line indicating the orientation of the coil handle.

The handle of the coil was attached to an integrated fixation unit of

the head and the coil. Correct coil position was continuously

monitored throughout the experiment.

A specially designed figure-of-eight coil was used for rTMS of

the left M1 (Medtronic-neuromuscular). The coil had an 8-m cable

and an outer winding diameter of approximately 10 cm consisting

of two 10-turn copper windings. The handle of the coil was oriented

perpendicular to the plane of the coil. The coil was placed

tangentially to the scalp with the junction region pointing back-

wards and laterally at 458 to the sagittal plane, approximately

perpendicular to the line of the central sulcus. This orientation was

chosen because motor threshold is at a minimum when the electrical

current induced in the brain flows approximately perpendicular to

the line of the central sulcus (Brasil-Neto et al., 1992). The stimuli

had a biphasic waveform with a pulse width of 200 As and a rise

time of 50 As. The first phase of the biphasic stimulus induced an

anterior to posterior current in the stimulated M1.

Experiment 1: Effects on motor cortex excitability

In this experiment, we assessed the effects of a 30-s train of 5

Hz rTMS on corticospinal and corticocortical excitability by

measuring the amplitude of the MEP in the relaxed right FDI

muscle. The amplitude of the MEP elicited by a single pulse

depends on the excitability of M1 and the spinal cord. Therefore,

the MEP size provides a measure of overall corticospinal

excitability (Rothwell et al., 1999). By contrast, the MEP response

to pairs of magnetic stimuli provides specific information about the

excitability of intracortical inibitory or facilitatory circuits when a

conditioning test approach is used (Kujirai et al., 1993; Rothwell,

1999; Ziemann, 1999).

Participants were comfortably seated in an armchair. The back of

the head was supported by a headrest and the head was fixed to the

headrest by a headband. To ensure constant stimulation conditions

the headrest was connected to a metal frame upon which a flexible

coil holder was mounted. Correct coil position was continuously

monitored by the experimenter throughout the experiment.

Experiment 1a: specificity of cortical excitability changes

The first experiment was designed to test whether rTMS

conditioning caused selective attenuation of SICI in the stimulated

M1. Ten healthy volunteers (two females) aged between 24 and 38

years (mean age: 27 years) participated in the first experiment.

Nine volunteers were consistently right-handed according to the

Edinburgh Handedness Inventory (Oldfield, 1971). One participant

was a converted left-hander.

We gave 150 pulses of 5 Hz rTMS to left M1 at 90% active MT.

Using the method of Kujirai et al. (1993), the excitability of left

M1 was assessed with single and paired-pulse TMS before

(baseline) and up to 3 min after rTMS. Paired magnetic stimuli

at an interstimulus interval (ISI) of 2 ms were used to assess SICI.

SICF was tested with paired stimuli at an ISI of 10 ms. We selected

an ISI of 2 ms to test SICI because at this interval any change in

SICI is not confounded by concomitant effects on bfacilitatory I-

wave interactionQ (Rothwell, 1999). Moreover a recent study

reported that a short train of 5 Hz rTMS at active MT produced a

similar attenuation of SICI at ISIs of 2 and 3 ms (Di Lazzaro et al.,

2002), indicating that an ISI of 2 ms was suitable for probing

changes in SICI.

The intensity of the conditioning stimulus was 90% of active

MT. The intensity of the test stimulus was adjusted to elicit MEPs of

about 1 mV peak-to-peak amplitude when given alone. The intensity

of the test stimulus corresponded to approximately 110–120% of

resting MT. In addition to paired-pulse TMS, the test stimulus was

also given alone to assess changes in overall corticospinal

excitability. Stimulus intensities were determined at the beginning

of each experiment, and the individual stimulus intensities for

single- and paired-pulse TMS were kept constant throughout the

experiment. Magnetic stimuli were given every 6 s to avoid any

modulation of motor cortical excitability by the measurements.

For single- and paired-pulse TMS, the same figure-of-eight coil

which was used for rTMS of the left M1 was attached to a Maglite

stimulator with Twintop option for single- and paired-pulse TMS

(Medtronic-neuromuscular). This enabled us to assess motor

cortical excitability within 30 s of the end of the rTMS train while

maintaining constant stimulation conditions. This is in contrast to

previous studies (Di Lazzaro et al., 2002) where baseline and post-

rTMS measurements were not performed with the same stimulating

coil and pulse configuration as rTMS. With the coil configuration

used in the current study, we ensured that the neuronal circuits

conditioned with rTMS closely matched those assessed with single

and paired-pulse TMS.

Changes in corticospinal excitability (assessed with single-

pulse TMS), SICI (assessed with paired-pulse TMS at an ISI of 2

ms), and SICF (assessed with paired-pulse TMS at an ISI of 10 ms)

were measured in separate trials. A trial consisted of an rTMS train

plus TMS measurements of cortical excitability before and after the

rTMS train. To determine baseline corticospinal excitability, 10

stimuli were applied to the left M1 before rTMS. A conditioning

train of rTMS was then given to the left M1. Thirty seconds after

completion of rTMS, 30 further MEP measurements were

obtained. A single trial (i.e., baseline measurements, rTMS, and

post-rTMS measurements) lasted approximately 6 min. After

completion of each trial, the experiment was paused for 4 min.

A previous study by Di Lazzaro et al. (2002) suggested a transient

change in SICI for less than 10 min following a short train of 5 Hz

rTMS at active MT. Therefore, an intertrial interval of 10 min was

chosen to avoid carry-over effects between trials. In each

participant, five trials were performed for each measure of cortical

excitability. The order of trials was pseudo-randomized and

counterbalanced among subjects. The entire experiment lasted for

approximately 2 1/2 h.

MEPs were recorded from Ag-AgCl surface electrodes over the

right FDI muscle using a belly-tendon montage. EMG signals were

amplified (Toennies Myograph II, Wuerzburg, Germany), filtered

(20 Hz to 2 kHz), digitized at 5 kHz (CED 1401 Laboratory

Interface, Cambridge Electronic Design, Cambridge, UK), and

stored for further analysis (NuCursor, Sobell Department of Motor

Neuroscience and Movement Disorders, Institute of Neurology,

University College of London, London, UK).

Throughout the experiment, the target muscle was completely

relaxed. Absence of background EMG activity was monitored with

B. Takano et al. / NeuroImage 23 (2004) 849–859852

visual (oscilloscope) and auditory (speakers) feedback, and trials

with background EMG activity were rejected.

Peak-to-peak amplitude was measured for individual MEPs.

For each trial, the 30 MEPs recorded after rTMS were divided in

blocks of 10 consecutive MEPs elicited during the 1st, 2nd, and

3rd minute after rTMS. For each measure of cortical excitability,

the mean MEP amplitude for each block was calculated and

expressed as a percentage of the mean amplitude of the 10 baseline

MEPs.

The effects of rTMS on motor cortex excitability were

evaluated using repeated measures analyses of variance (ANOVA).

Using the normalized MEP amplitude as a dependent variable, we

computed a two-way repeated-measures ANOVA with btime binQ(three levels: 1st, 2nd, and 3rd minute after rTMS) and bmeasure of

excitabilityQ (three levels: single-pulse excitability, SICI, and SICF)as within-subject factors.

In a second ANOVA, we tested for time-dependent changes in

MEP amplitude at baseline during the course of the experiment.

Our concern was that there might be carry-over effects from trial to

trial resulting in a gradual shift in cortical excitability. The two-

factorial ANOVA model treated borderQ of trials (six levels: 1st,

2nd, 3rd, 4th, 5th, and 6th trial) and bmeasure of excitability Q(three levels: single-pulse excitability, SICI, and SICF) as within-

subject factors. The dependent variable was the mean MEP

amplitude at baseline before rTMS conditioning. The Green-

house–Geisser method was used to correct for nonsphericity. P

values of b0.05 were considered significant.

Experiments 1b and c: Time course of excitability changes

In Experiments 1b and c, we investigated the time course of

changes in SICI using the same rTMS protocol as Experiment 1a.

Since Experiment 1a suggested a selective effect of subthreshold 5

Hz rTMS on intracortical inhibition, Experiments 1b and c focused

exclusively on SICI. Paired-pulse TMS at an ISI of 2 ms was used

to determine the time course of rTMS-induced changes in SICI.

The procedures for measuring SICI at an ISI of 2 ms were identical

to those described in Experiment 1a. Data acquisition was identical

to Experiment 1a.

Experiment 1b

Twelve healthy volunteers (one female) aged between 23 and 41

years (mean age: 31 years) were studied. Eleven participants were

consistent right-handers and one person was a consistent left-hander

according to the Edinburgh Handedness Inventory (Oldfield, 1971).

In Experiment 1b, we measured SICI at baseline (i.e., immediately

before rTMS) and up to 12 min after a single conditioning train of 5

Hz rTMS.Measurements of SICI were carried out in separate blocks

immediately before 5 Hz rTMS (baseline), as well as three times

after 5 Hz rTMS. Each block consisted of 40 consecutive measure-

ments to reduce the variability of the MEP data. Using an ISI of 2

ms, paired-pulse TMS was given over left M1 every 6 s. Each bock

of measurements lasted 4 min, resulting in four time bins (baseline,

1–4, 5–8, and 9–12 min after rTMS). Peak-to-peak amplitude was

measured for individual MEPs recorded from the relaxed right FDI

muscle. In each participant, the meanMEP amplitude for each block

was calculated and entered as dependent variable into a one-way

ANOVA. The factor was btime binQ (four levels).

Experiment 1c

Six healthy male volunteers aged between 26 and 38 years

(mean age: 29 years) participated in Experiment 1c. All subjects

were consistent right-handers according to the Edinburgh Handed-

ness Inventory (Oldfield, 1971). Experiment 1c also explored the

time course of changes in SICI after a single rTMS train (Fig. 1). In

contrast to Experiment 1b, the rTMS trains were repeated at

approximately 13- to 15-min intervals and the time course of SICI

changes was measured after each train. This experiment was

necessary to demonstrate that the excitability changes observed

after a single conditioning train could be reliably provoked by 5 Hz

rTMS, even when rTMS was repeated several times during a single

session.

The experiment consisted of six consecutive trials. Each trial

consisted of a train of 5 Hz rTMS followed by paired-pulse TMS.

Paired-pulse TMS started about 30 s after the rTMS train and was

applied to left M1 for 12 min at a rate of 0.1 Hz. For each trial, a

total of 72 MEPs were elicited with paired-pulse TMS. MEPs were

grouped in blocks of 24 consecutive measurements to reduce the

variability of the MEP data. Each bock of measurements lasted 4

min, resulting in three time bins (1–4, 5–8, and 9–12 min after

rTMS). Peak-to-peak amplitude was measured for individual

MEPs recorded from the relaxed right FDI muscle. In each

participant, the mean MEP amplitude for each block was calculated

and normalized to the mean MEP amplitude over all blocks.

The time course of SICI was evaluated with repeated measures

analyses of variance (ANOVA). Using the mean MEP amplitude of

each block as a dependent variable, we computed a one-way

repeated measures ANOVAwith btime binQ as within-subject factors(three levels: 1–4, 5–8, and 9–12 min after rTMS). Conditional on a

significant F value, we performed post hoc t tests to directly

compare the magnitude of changes in SICI between epochs. The

Greenhouse–Geisser method was used when necessary to correct

for nonsphericity. P values of b0.05 were considered significant.

Experiment 2: Effects on regional blood flow in the motor cortex

Based on the results of Experiment 1, a PET study (Experiment

2) was performed to (i) compare changes in rCBF during

subthreshold 5 Hz rTMS with changes in rCBF seen after rTMS,

and (ii) to assess the time course of rCBF changes. We studied six

healthy volunteers (one female) aged between 26 and 40 years

(mean age: 31 years). All participants were consistently right-

handed according to the Edinburgh Handedness Inventory (Old-

field, 1971). Permission to administer radioactivity was obtained

from the German radiation protection authorities.

Experimental design

Each participant had 12 consecutive PET measurements of

normalized rCBF (Fig. 1). Three PET scans were acquired during 5

Hz rTMS to the left M1 (bon-lineQ PET imaging). In this condition,

the rTMS train started approximately 5 s before PET scanning.

Since the rTMS train lasted for 30 s, rTMS was only given during

the first half of the PET scan. Nine PET scans were performed up to

12 min after rTMS (boff lineQ PET imaging). To assess the time

course of lasting rTMS effects on rCBF, the interval between PET

scanning and rTMS was systematically varied across scans. In each

participant, three PET measurements were performed 1–4, 5–8, or

9–12 min after rTMS, respectively.

Participants lay supine in the scanner with their eyes closed and

were required to keep still during PET scanning. We recorded a

surface EMG from the right FDI muscle during PET and rTMS

(sample rate: 2000 Hz; band-pass filter: 10–1000 Hz).

B. Takano et al. / NeuroImage 23 (2004) 849–859 853

In this experiment, we did not measure motor cortex

excitability during PET acquisition because this would have

involved repeated administration of single and paired-pulse

TMS. Since suprathreshold TMS pulses induce a consistent

activation of the stimulated M1 (not least because of reafferent

activation caused by the TMS-induced muscle twitch) (Baudewig

et al., 2001), such measurements might confound the interpretation

of rCBF changes as reflecting changes in neuronal activity induced

by rTMS.

PET measurements were conducted using a Siemens ECAT

HR+ scanner (Siemens-CTI Inc., Knoxville, TN, USA) in three-

dimensional mode with interdetector collimating septa removed.

The axial field of view was 15.5 cm covering the whole brain with

no interplane dead space. A well-grounded cylindrical insert

consisting of four layers of 0.5-mm-thick mu-metal was placed

in the patient port of the PET scanner to protect the photo-

multipliers from the effects of the magnetic field generated by the

TMS coil (Paus et al., 1997; Siebner et al., 2001).

Normalized rCBF was measured using 15O-labelled water. For

each measurement of rCBF, 250 mBq of H215O was administered in

the left cubital vein using an infusion pump. Background activity

was calculated over 30 s before each image. Image acquisition

began with the appearance of the tracer bolus in the brain,

approximately 30 s after the start of the infusion and continued for

50 s. This procedure was repeated for each PET scan with

approximately 10 min between scans to allow for adequate decay

of radioactivity. A 10-min head-holder transmission scan with an

external 68Ga/68Ge ring source was acquired before the first rCBF

measurement to allow for a correction of radiation attenuation by

the subject’s head and the objects in the scanner: TMS coil, coil

mount, and metal sheets. A blank transmission scan was performed

with the magnetic shielding in place after the subject was scanned

to correct for the radiation attenuation due to the metal shielding

alone.

All calculations and image transformations were performed on

Sun SPARC 2 workstations (Sun Computers Europe, Inc., Surrey,

UK). PET data were analyzed using statistical parametric mapping

software (SPM99, Wellcome Department of Imaging Neuro-

science, Institute of Neurology, University College of London,

UK; http://www.fil.ion.ucl.ac.uk/spm) implemented in the PRO

Matlab environment (Mathworks Inc., Natic, MA, USA). For each

participant, images were realigned to the first image by rigid body

transformation using a least squares approach on a voxel-by-voxel

basis (Friston et al., 1995a). All images were then normalized using

linear and nonlinear three-dimensional transformations to match

each scan to a template image conforming to the standard

stereotaxic space of the Montreal Neurological Institute (MNI

template) (Friston et al., 1995a). The normalized images were

smoothed using an isotropic Gaussian kernel of 12 mm full-width

at half-maximum to accommodate intersubject differences in

anatomy and enable the application of Gaussian field corrections

during inference. Each voxel of normalized and smoothed images

had a size of 2 � 2 � 4 mm.

Statistical analysis used a general linear model and Gaussian

random field theory to correct for multiple comparisons (Friston et

al., 1990, 1995b). Based on the electrophysiological findings

(Experiment 1), the model included four conditions depending on

the temporal relationship between rTMS conditioning and PET

scanning: (1) rTMS during PET scanning, (2) rTMS given 1–4

min, (3) rTMS given 5–8 min, or (4) rTMS given 9–14 min before

PET scanning. The effect of global differences in cerebral blood

flow among scans was removed by treating global activity as a

confound and scaling to a nominal grand mean global activity of

50 ml/100 ml/min (Friston et al., 1995b). Using appropriately

weighted linear contrasts, this statistical model enabled the

characterization of time-dependent changes in neuronal activity

during and after rTMS. Contrasts were specified to address two

questions. The first set of contrasts tested for differences in rCBF

between scans acquired during rTMS and those acquired after

rTMS (rTMS during PET, rTMS given 1–4, 5–8, or 9–14 min

before PET: 3, �1, �1, �1, and �3, 1, 1, 1). The second set of

contrasts was motivated by the time course of changes in

intracortical inhibition seen after rTMS conditioning. These

contrasts were specified to test for changes in rCBF that mirrored

the changes in strength of SICI in the stimulated M1 after rTMS

conditioning (rTMS during PET, rTMS given 1–4 min before PET,

5–8 min before PET, rTMS given 9–14 min before PET: 0, 1, 1,

�2 and 0, �1, �1, 2).

Statistical inference was prospectively restricted to a region of

interest covering the stimulated left M1, defined by a 20-mm radius

sphere centered on the MNI coordinates: x =�32, y =�22, and z =

56. This site corresponds to the probabilistic location of the left

primary motor hand area as revealed by a meta-analysis of motor

activation studies (Paus et al., 1998). For voxels within the

predefined search volume, a small volume correction was used to

correct for multiple comparisons. For other areas, corrections for

multiple nonindependent comparisons where made over the whole

brain volume. In both cases, P values of b0.05 were considered

statistically significant.

Results

None of the subjects reported any adverse side effects during

the study. The intensities used for 5 Hz rTMS were 36 F 3%

(Experiment 1a), 34 F 3% (Experiment 1b), 35F 4% (Experiment

1c), and 35 F 3% (Experiment 2) of maximum stimulator output.

No EMG responses were evoked in the relaxed right FDI muscles

by the 30-s trains of 5 Hz rTMS.

Experiment 1: Lasting effects on motor cortex excitability

In Experiment 1a, trains of subthreshold 5 Hz rTMS resulted in

a relative increase in the amplitude of MEPs evoked by paired-

pulse TMS at an ISI of 2 ms (Fig. 2). No changes were seen in

MEP amplitudes using single-pulse TMS or paired-pulse TMS at

an ISI of 10 ms. This suggests that 5 Hz rTMS resulted in a

selective reduction of SICI without concomitant changes in

corticospinal excitability or SICF. The selective reduction in SICI

was seen as a significant main effect of measure of excitability in

an ANOVA [F1.4;13.0 = 7.6; P = 0.01]. An interaction between the

factors time bin and measure of excitability [F2.5;22.9 = 3.3; P =

0.045] was also observed. This interaction was due to a relative

increase in amplitude of MEPs evoked by paired-pulse TMS at 2

ms ISI during the 2nd and 3rd min after rTMS compared to the 1st

min after rTMS (Fig. 2). MEP amplitudes elicited by single-pulse

TMS and paired-pulse TMS at 10 ms ISI were stable after rTMS

(Fig. 2).

A second ANOVA tested for time-dependent shifts in excit-

ability at baseline. There was a main effect of measure of

excitability [F1.5;50.1 = 20,2; P b 0.001] but no main effect of

order and no interaction between the factors (P N 0.1). These

Fig. 2. Experiment 1a: Changes in motor cortical excitability after 150

stimuli of 5 Hz rTMS to the left M1 at 90% of active MT. Peak-to-peak

MEP amplitudes were recorded from the relaxed right FDI muscle,

normalized to baseline values (100%), and plotted for each block of

measurement. Motor cortical excitability was assessed with single-pulse

TMS (triangles) and paired-pulse TMS at an interstimulus interval of 2 ms

(squares) or 10 ms (circles). Data are shown as mean F SEM. SICI =

short-latency intracortical inhibition; SICF = short-latency intracortical

facilitation.

B. Takano et al. / NeuroImage 23 (2004) 849–859854

results suggest that cortical excitability had returned to baseline

levels at the end of each trial excluding a carry-over effect from

trial to trial.

Experiment 1b examined the time course of changes in SICI

after a single train of 5 Hz rTMS. An ANOVA showed a significant

main effect of time bin [F1.5;16.7 = 6.41; P = 0.013]. Post hoc

comparisons revealed that the main effect of time was caused by a

transient increase in MEP amplitudes during the first block [t11 =

�3.5; P = 0.005] and second block [t11 = �3.8; P = 0.003] after 5

Hz rTMS relative to baseline. MEP amplitudes returned to baseline

levels 9–12 min after rTMS conditioning (Fig. 3). Together with

the results of Experiment 1a, these results suggest that a single train

Fig. 3. Experiment 1b: Relative changes in intracortical inhibition in left

M1 after a single train of 5 Hz rTMS. Mean peak-to-peak MEP amplitudes

were recorded from the relaxed right FDI muscle and plotted for each block

of measurement. SICI was assessed with paired-pulse TMS over the left M1

at an interstimulus interval of 2 ms. Error bars represent 1 SEM. Compared

to baseline before rTMS (black column), rTMS induced a relative increase

in MEP amplitude 1–4 and 5–8 min after rTMS. Amplitudes returned to the

baseline level 9–12 min after rTMS conditioning. The P values indicate

significant differences as revealed by a pair wise comparison of baseline

and post-rTMS measurements.

of 5 Hz rTMS caused a selective attenuation of SICI which

returned to baseline levels approximately 8 min after rTMS

conditioning.

Experiment 1c examined whether 5 Hz rTMS can reproducibly

induce a transient attenuation of SICI, if rTMS trains are repeated

approximately every 12 min. ANOVA showed a significant main

effect of time [F(1.3,6.4) = 10,81; P = 0.013]. Fig. 4 illustrates that

this time-dependent change in SICI was caused by a relative

decrease in MEP amplitudes during the third block of measure-

ments (9–12 min after rTMS). Post hoc comparisons showed a

difference in mean MEP amplitudes between the first and third

block [t(5) = 3.3; P = 0.01] and the second and third block of

measurements [t(5) = 4.0; P = 0.02]. In conjunction with the results

of Experiments 1a and b, these data indicate that a 5 Hz rTMS train

can reproducibly produce a temporary reduction in SICI, when

pauses between two rTMS trains are longer than 8 min.

Experiment 2: Effects on regional blood flow in the motor cortex

A direct comparison of PET scans during rTMS and after rTMS

showed a bilateral increase in normalized rCBF in temporal

auditory cortex and insular cortex during rTMS (Fig. 5; Table 1).

Medial and lateral occipital areas and the mesial prefrontal cortex

showed a relative decrease in normalized rCBF during rTMS

compared with post-rTMS conditions (Fig. 5; Table 1). No

difference in rCBF in the stimulated left M1 was seen between

the two conditions (during and after rTMS) even at a reduced

statistical threshold (P b 0.01 uncorrected).

A cluster of voxels in rostral left M1 exhibited a pattern of

changes in synaptic activity that closely followed the time course

of changes in intracortical inhibition (SICI) in M1 (Fig. 6). The

regional maximum of the cluster was located in a superficial part

of M1 (MNI coordinates: x = �24, y = �20, and z = 68; t =

Fig. 4. Experiment 1c: Time course of changes in intracortical inhibition

after 5 Hz rTMS to left M1. Changes in mean peak-to-peak MEP amplitude

of the relaxed right FDI muscle were measured six times after the

administration of a 150-pulse rTMS train. MEPs were plotted for each

block of measurement and normalized to the global mean. SICI was

assessed with paired-pulse TMS over left M1 at an interstimulus interval of

2 ms. Error bars represent 1 SEM. The MEP amplitudes recorded 1–4 min

(black column) and 5–9 min after rTMS (hatched column) showed a

relative increase compared with those recorded 10–14 min after rTMS

(white column). P values indicate significant differences as revealed by pair

wise comparisons among post-rTMS measurements.

Fig. 5. Areas of increased (red) or decreased (green) normalized regional

cerebral blood flow (rCBF) during rTMS conditioning (150 pulses of 5 Hz

rTMS at 90% of active MT) relative to rCBF after rTMS. Statistical

parametric maps are rendered on the cerebral surface using the SPM-96

template and a statistical threshold of P b 0.05 (corrected for multiple

comparisons over the whole brain).

B. Takano et al. / NeuroImage 23 (2004) 849–859 855

4.36; P = 0.011). A plot of parameter estimates at this site shows

a relative increase in rCBF during the 5 Hz rTMS train (Fig. 6).

rCBF was increased for approximately 8 min and showed a

relative decrease at 9–14 min after the end of rTMS (Fig. 6).

Outside the predefined region of interest, there were no

significant changes in synaptic activity that followed the pattern

of changes in intracortical inhibition in the stimulated left M1.

Table 1

Brain regions where regional cerebral blood flow (rCBF) during repetitive transcran

after rTMS

Brain regions Side t Va

Transient increase in synaptic activity (only) during rTMS: rCBF during 5 Hz rT

1. Superior temporal gyrus left 5.

Transverse temporal gyrus left 5.

Transverse temporal gyrus left 5.

2. Superior temporal gyrus right 5.

Inferior parietal lobule right 4.

Transverse temporal gyrus right 4.

3. Anterior insular cortex left 4.

Transient decrease in synaptic activity (only) during rTMS: rCBF during 5 Hz rT

1. Medial frontal gyrus medial �4.

2. Inferior occipital gyrus left �4.

3. Cuneus/medial occipital gyrus medial �4.

�4.

4. Medial occipital gyrus right �4.

Discussion

rTMS (5 Hz) to left M1 (150 pulses at 90% active motor

threshold) decreased the strength of SICI (Experiment 1) and

increased rCBF (Experiment 2) without affecting the excitability of

corticospinal output neurons (Experiment 1). The attenuation of

SICI and the increase in rCBF persisted for approximately 8 min

after the end of rTMS. The lasting increase in rCBF was confined

to a superficial site in the stimulated left M1 and did not spread to

other motor areas. These results suggest that the altered profile of

SICI induced by a short train of low-intensity 5 Hz rTMS is

associated with a transient increase in synaptic activity at the site of

stimulation. We discuss the implications of these data in terms of

our understanding of rTMS effects in three sections: (1) the effects

of rTMS on SICI, (2) the temporal profile of rTMS effects, and (3)

the spatial profile of rTMS effects.

Effects of rTMS on SICI

In agreement with a recent study (Di Lazzaro et al., 2002), a

short train of 5 Hz rTMS at 90% of active motor threshold induced

a selective reduction in SICI, lasting approximately 8 min. The

time course of SICI and its augmentation by GABAA-ergic drugs

suggests that SICI is mediated by GABAAergic circuits in M1 (Di

Lazzaro et al., 2000; Nakamura et al., 1995; Ziemann, 1999;

Ziemann et al., 1996). Several pharmacological TMS studies have

shown that dopaminergic and glutamatergic drugs can alter the

efficacy of SICI, indicating that the SICI circuit is influenced by

several neurotransmitter systems (review in: Ziemann, 1999). The

effects of rTMS on SICI may reflect a direct attenuation of the

efficacy of intracortical GABAA-ergic synapses subserving SICI

(Di Lazzaro et al., 2002). Alternatively, rTMS may have reduced

the efficacy of excitatory synaptic inputs to the intracortical

GABAA-ergic neurones mediating SICI. The exact mechanism

by which rTMS selectively attenuated SICI is currently unknown,

the reduction in SICI suggests a shift in the balance between

intracortical inhibition and facilitation mediated by a decrease in

the efficacy of inhibitory synapses.

The H215O-PET experiment explored the relationship between

changes in SICI and regional synaptic activity (as indexed by

ial magnetic stimulation (rTMS) differs significantly from normalized rCBF

lue of peak differences Coordinates of peak differences

x y z

MS N rCBF after 5 Hz rTMS

35 �46 �38 22

14 �32 �28 16

10 �20 �24 14

16 42 2 �4

99 54 �44 26

88 38 �28 14

45 �40 8 8

MS b rCBF after 5 Hz rTMS

95 �4 62 12

88 �42 �92 �6

80 �4 �100 20

51 8 �104 4

64 44 �90 10

Fig. 6. Lasting effects of rTMS conditioning (150 pulses of 5 Hz rTMS at 90% of active MT) on regional cerebral blood flow (rCBF) in the stimulated left M1.

The sagittal, coronal, and axial statistical parametric maps (left panel) show an area in rostral left M1 with increased rCBF lasting approximately 8 min after 5

Hz rTMS. The color-coded Z-score maps are superimposed on corresponding sagittal, coronal, and axial sections of stereotactically normalized T1-weighted

MRI images using the T1-weighted MR template implemented in SPM 99 ( P b 0.001; uncorrected). The cross-hair indicates the local maximum (MNI

coordinates: x = �24, y = �20, and z = 68). The bar chart (right panel) illustrates the time course of changes in normalized rCBF at this site. Each bar

represents the mean percentage change in rCBF (FSE). The rCBF values given on the ordinate are adjusted to the mean.

B. Takano et al. / NeuroImage 23 (2004) 849–859856

rCBF). Using the same protocol as in Experiment 1, low-

intensity 5 Hz rTMS induced a transient increase in rCBF in the

stimulated M1 for approximately 8 min after the end of

stimulation. Since the rTMS-induced changes in rCBF and SICI

shared the same time course, we infer that the reduction in SICI

induced by rTMS was associated with an increase in regional

synaptic activity. Taken together, these two findings may suggest a

possible neuronal mechanism underlying the conditioning effects

of 5 Hz rTMS.

We propose a simple model that can account for a simultaneous

reduction of SICI and an increase in resting rCBF. This model

assumes that excitatory glutamatergic inputs to M1 are subject to

inhibitory feedback via GABAA-ergic inhibitory interneurons

subserving SICI. We suggest that 5 Hz rTMS at submotor

threshold intensities decreases the efficacy of GABAA-ergic

synapses mediating inhibitory gain control (Abbott et al., 1997).

Thus, any inputs to M1 from intrinsic excitatory connections,

corticocortical connections, or extrinsic inputs such as TMS will

evoke a less potent inhibitory ddampeningT response, resulting in

disinhibition of synaptic activity in M1 in response to any given

input. This will be associated with increased rCBF even under

resting conditions. These findings lead us to predict that the

suppressive effects of 5 Hz rTMS on SICI will be reduced by the

action of GABAA-ergic agonists.

It is possible that the effect of 5 Hz rTMS on SICI might be

mediated by a reduced synaptic efficacy of the excitatory inputs to

intracortical GABAA-ergic neurones mediating SICI. However,

this is unlikely given that the activity of inhibitory interneurons is

metabolically demanding (Ackermann et al., 1984; Nudo and

Masterton, 1986) and a recent study suggesting that SICI is itself a

metabolically demanding process (Strafella and Paus, 2001). In this

study, Strafella and Paus (2001) showed that during SICI at an ISI

of 3 ms, there is a positive correlation between the magnitude of

SICI and rCBF increases in the stimulated M1. If 5 Hz rTMS

reduces the magnitude of SICI by reducing the activity of

inhibitory interneurons (by reducing the efficacy of inputs to these

neurones), one would predict a decrease in rCBF after 5 Hz rTMS

because a given excitatory input would result in less activation of

inhibitory interneurones involved in SICI.

Simultaneous recordings of neuronal activity and BOLD

responses in monkeys strongly suggest that the blood oxygen

level-dependent (BOLD) signal primarily measures the input and

processing of neuronal information within a region rather than the

output signal to other brain regions (Logothetis et al., 2001). The

results from combined TMS-PET studies point to a similar

relationship between neuronal activity and rCBF in humans. Motor

responses to single-pulse TMS mainly reflect the excitability of

corticospinal projections (i.e., cortical output) (Rothwell et al.,

1999), and two recent studies failed to demonstrate a correlation

between changes in corticospinal (single-pulse) excitability and

changes in rCBF (Chouinard et al., 2003) or BOLD signal in the

stimulated M1 (Waldvogel et al., 2000). By contrast, SICI probes

the excitability of intracortical inhibitory interneurons (i.e., intra-

cortical processing) (Kujirai et al., 1993), and the present results,

together with the study by Strafella and Paus (2001), suggest that

modulating the efficacy of SICI has a significant effect on rCBF in

M1.

Temporal profile of rTMS effects

The duration of the effects on synaptic activity and intracortical

excitability in these experiments was shorter than previously

reported results (Lee et al., 2003; Siebner et al., 2003). This can be

best explained by the small number of conditioning stimuli given

in the present study. It has been shown that the duration of the

effect of 1 Hz rTMS to M1 on corticospinal excitability can be

prolonged by increasing the number of stimuli per session (Touge

et al., 2001). Therefore, it may be possible to prolong the effects of

low-intensity 5 Hz rTMS on rCBF in M1 by increasing the number

of conditioning stimuli. This would be desirable if the primary goal

is to induce changes that remain stable for longer periods.

However, short rTMS protocols that induce more transient effects

B. Takano et al. / NeuroImage 23 (2004) 849–859 857

make it possible to monitor rCBF at rest or during a task until the

rTMS-induced changes in cortical excitability return to baseline.

The 30-s trains of 5 Hz rTMS caused an increase in rCBF that

persisted for several minutes after rTMS. Physiological auditory

stimulation (caused by the discharge of the TMS coil) increased

synaptic activity in auditory cortex only during the administration

of rTMS. This explains why a subtraction analysis contrasting

rCBF measurements during and after rTMS showed a relative

increase in rCBF in the auditory cortex, but not in the stimulated

M1. The difference in the temporal profile of rCBF changes as a

direct result of rTMS and rCBF changes due to repetitive

auditory stimulation confirm that rTMS has persistent effects that

differ from those caused by repetitive physiological stimuli

(Siebner et al., 2000).

Spatial profile of rTMS effects

In this study, the most prominent change of rCBF in the

stimulated M1 was seen at MNI coordinates x = �24, y = �20, and

z = 68. This is in good agreement with two recent rTMS-PET

studies that have also used the TMS-evoked motor response to

localize the left M1. In these studies, maximal changes induced by

rTMS over M1 were found at MNI coordinates x = �30, y = �27,

and z = 66 (Siebner et al., 2001) and x = �30, y = �26, and z = 62

(Lee et al., 2003), suggesting that the bhot spotQ approach for

functional localization of M1 is reliable and reproducible across

studies. In all three studies, the peak activation in left M1 was in

rostral M1 close to the hemispheric surface. The site of stimulation

was consistently found 1–1.5 cm superior to the peak M1

activation during voluntary finger movements reported in previous

studies (Lee et al., 2003; Paus et al., 1998). The electrical field

induced by TMS attenuates with increasing distance from the

center of the figure-of-eight-coil (Barker, 1999). Therefore, it is not

surprising that the lasting effects of low-intensity rTMS occur

mainly in the superficial portion of M1, which is closer to the

transducing coil.

There is converging evidence to suggest that M1 in primates

can be divided into two subregions on the basis of quantitative

cytoarchitecture and quantitative distributions of transmitter bind-

ing sites (Geyer et al., 1996): The caudal part of M1 (Brodmann

area 4 posterior = BA 4 p) is located deep in the central sulcus and

the rostral part of M1 (Brodmann area 4 anterior = BA 4a) is

located more superficially in the central sulcus adjacent to the

caudal portion of the dorsal premotor cortex. Since low-intensity

TMS to M1 causes preferential activation of the superficial part of

M1 (Siebner et al., 2001), this implies that low-intensity rTMS can

be used to preferentially target BA4a. This is in accordance with a

recent PET study, which demonstrated (compensatory) changes in

effective connectivity between BA4p and frontal motor areas after

subthreshold 1 Hz rTMS to the superficial part of M1 (BA4a) (Lee

et al., 2003).

At an inhibitory ISI of 3 ms, slow (0.33 Hz) paired-pulse rTMS

to left M1 increased the rCBF in the superficial part of the

stimulated M1 depending on the magnitude of SICI (Strafella and

Paus, 2001). The maximum increase in rCBF occurred at MNI

coordinates x = �27, y = �19, and z = 61. This location closely

corresponds to the regional maxima observed in the present study

and confirms a tight coupling between SICI and overall neuronal

activity (as indexed by rCBF) in the rostral M1.

Previous TMS-PET studies on lasting effects of rTMS to M1

used higher stimulus intensities (e.g., z90% of resting motor

threshold). These studies have consistently shown that rTMS to

M1 can induce a lasting modulation of synaptic activity not only

in the stimulated cortex but also in connected brain areas

(Chouinard et al., 2003; Fox et al., 1997; Lee et al., 2003; Paus

et al., 1998; Siebner et al., 2000, 2003a; Strafella et al., 2003).

Extending previous work, this study demonstrates that lasting

effects are restricted to the site of stimulation if short trains of

low-intensity stimuli are used. Two recent studies provided a

possible explanation for this finding. (i) Siebner et al. (2001)

investigated changes in rCBF during a single 50-s train of rTMS

at 90% of active motor threshold over a range of stimulus

frequencies. While rTMS induced a rate-dependent increase in

rCBF in the stimulated M1, no changes in rCBF were seen in

connected brain regions. (ii) In two subjects with high cervical

epidural electrodes, Di Lazzaro et al. (1998) showed that

magnetic pulses to M1 at 90% of active motor threshold evoked

no recognizable descending activity in the corticospinal tract. The

limited spatial extent of the rTMS-induced effect in the current

study is likely to reflect the fact that the stimulus intensity (90%

of active MT) was too low to depolarize corticocortical projection

neurones and cortico-subcortical neurones, preventing the spread

of effects to connected brain regions.

Methodological considerations

We only assessed paired-pulse excitability of the M1at two ISIs

(2 and 10 ms). It is important to recall that the circuits causing SICI

at 2 ms seem to differ from inhibitory circuits that subserve SICI at

other intervals (Fisher et al., 2002; Hanajima et al., 2003). The

same may apply for neuronal circuits that mediate SICF (Munchau

et al., 2002; Rizzo et al., 2004). Moreover, additional measures

such as the cortical silent period, long-latency intracortical

inhibition, and facilitatory I-waves interaction are available to

probe changes in excitability in M1 (Chen, 2000). Therefore, the

paired-pulse measurements performed in this study provide a

limited characterization of motor cortical excitability.

A limitation of the present study is that we did not measure

SICI during PET measurements of rCBF. This would have allowed

for direct correlation of changes in SICI and rCBF. However, a

reliable assessment of SICI would have required at least 15

consecutive measurements during each PET scan. Using the same

paired-pulse paradigm (Kujirai et al., 1993), Strafella and Paus

(2001) observed changes in rCBF in M1 and connected areas when

20 trials of paired-pulse TMS at an ISI of 3 ms were applied to M1

during a 60-s PET scan. Given these findings, we decided not to

collect data on intracortical inhibition during PET acquisition

because the rCBF changes induced by the measurement of SICI

would have confounded the interpretation of rCBF changes

following rTMS. In the current study, rCBF was measured while

participants were at rest; therefore, we are unable to make

inferences about rTMS-induced changes in the efficacy of cortical

inputs to the stimulated M1 from other motor areas. In order to

make such inferences, it is necessary to perturb the motor system

during functional neuroimaging (Lee et al., 2003). Indirect

evidence from a previous study suggests that short trains of low-

intensity 5 Hz rTMS modulate the efficacy of connections between

the stimulated M1 and other motor areas. Oliviero et al. (2003)

gave 50 stimuli of 5 Hz rTMS to the left M1 at active MT and

found a decrease in intrahemispheric EEG-EEG coherence

between M1 and lateral premotor cortex that persisted for several

minutes after stimulation. This decrease in coherence was restricted

B. Takano et al. / NeuroImage 23 (2004) 849–859858

to the upper alpha band and occurred only during voluntary muscle

contraction (Oliviero et al., 2003).

Conclusion

The present data suggest that a short session of low-intensity

rTMS sufficient to induce a persistent change in motor cortical

excitability can cause an increase in synaptic activity (rCBF) that is

confined to the site of stimulation. This may reflect a direct

attenuation of the efficacy of intracortical GABAA-ergic synapses

subserving SICI. In contrast to previous work using prolonged

stimulation at intensities of z90% of resting motor threshold to

induce lasting alterations in synaptic activity in M1 and connected

brain areas (Chouinard et al., 2003; Lee et al., 2003; Siebner et al.,

2000, 2003), short trains of low-intensity stimulation provide a

means of selectively modulating cortical excitability at the site of

stimulation alone. The ability to induce a transient change in

function within a distinct area of a functional network opens up

new possibilities for studying acute reorganization at the systems

level in the human brain.

Acknowledgments

The authors would like to express their gratitude to Ms. S.

Ziegler and Mr. H. Riescher for their technical assistance, to Ms.

C. Kruschke and Ms. G. Dzewas for their assistance during PET

acquisition, and to Prof. J.C. Rothwell and Prof. K.J. Friston for

their helpful comments on the manuscript. H.R. Siebner was

supported by the Deutsche Forschungsgemeinschaft (DFG

Si738/1) and the BMBF (01GO 0206).

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