Time-course of "off-line" prefrontal rTMS effects--a PET study.

Abstract

Low-frequency "off-line" repetitive transcranial magnetic stimulation (rTMS) over the course of several minutes has attained considerable attention as a research tool in cognitive neuroscience due to its ability to induce functional disruptions of brain areas. This disruptive rTMS effect is highly valuable for revealing a causal relationship between brain and behavior. However, its influence on remote interconnected areas and, more importantly, the duration of the induced neurophysiological effects, remain unknown. These aspects are critical for a study design in the context of cognitive neuroscience. In order to investigate these issues, 12 healthy male subjects underwent 8 H(2)(15)O positron emission tomography (PET) scans after application of long-train low-frequency rTMS to the right dorsolateral prefrontal cortex (DLPFC). Immediately after the stimulation train, regional cerebral blood flow (rCBF) increases were present under the stimulation site as well as in other prefrontal cortical areas, including the ventrolateral prefrontal cortex (VLPFC) ipsilateral to the stimulation site. The mean increases in rCBF returned to baseline within 9 min. The duration of this unilateral prefrontal rTMS effect on rCBF is of particular interest to those who aim to influence behavior in cognitive paradigms that use an "off-line" approach.

Full text

Page 1

Time-course of “off-line” prefrontal rTMS effects — a PET study☆
C. Eisenegger,a,⁎V. Treyer,bE. Fehr,a,cand D. Knocha,c,d,⁎
aCenter for the Study of Social and Neural Systems, Institute for Empirical Research in Economics, University of Zurich, Switzerland
bPET Center, Nuclear Medicine, Department of Radiology, University Hospital Zurich, Switzerland
cCollegium Helveticum, Schmelzbergstrasse 25, 8092 Zurich, Switzerland
dDepartment of Neurology, University Hospital Zurich, Switzerland
Received 25 February 2008; revised 9 April 2008; accepted 11 April 2008
Available online 20 April 2008
Low-frequency “off-line” repetitive transcranial magnetic stimulation
(rTMS) over the course of several minutes has attained considerable
attention as a research tool in cognitive neuroscience due to its ability to
induce functional disruptions of brain areas. This disruptive rTMS
effect is highly valuable for revealing a causal relationship between
brain and behavior. However, its influence on remote interconnected
areas and, more importantly, the duration of the induced neurophy-
siological effects, remain unknown. These aspects are critical for a
study design in the context of cognitive neuroscience.
Inordertoinvestigatetheseissues,12healthymalesubjectsunderwent
8 H2
long-trainlow-frequencyrTMStotherightdorsolateralprefrontalcortex
(DLPFC).Immediatelyafterthestimulationtrain,regionalcerebralblood
flow (rCBF) increases were presentunder thestimulation siteas wellas in
other prefrontal cortical areas, including the ventrolateral prefrontal
cortex (VLPFC) ipsilateral to the stimulation site. The mean increases in
rCBF returned to baseline within 9 min. The duration of this unilateral
prefrontalrTMSeffectonrCBFisofparticularinteresttothosewhoaimto
influencebehaviorincognitiveparadigmsthatusean“off-line”approach.
© 2008 Elsevier Inc. All rights reserved.
15O positron emission tomography (PET) scans after application of
Keywords: After effects; Cerebral blood flow; Low frequency; Prefrontal
cortex; Transcranial magnetic stimulation
Introduction
TMS has become an indispensable investigation tool in cog-
nitive neuroscience. Its ability to transiently disrupt normal func-
tion of human brain regions makes it a unique tool for studying the
causal contribution of different brain areas to behavior. A very
promising approach is the application of low-frequency repetitive
TMS (rTMS) for several minutes before performing a given task in
order to partially elude the unspecific effects of concurrent rTMS
stimulation (Abler et al., 2005; Pascual-Leone et al., 1998;
Robertson et al., 2003). This approach, also referred to as “off-
line” rTMS, has been used extensively in recent years. It has been
applied, inter alia, to study parietal contributions to spatial attention
(Hilgetag et al., 2001) and spatial hearing (Lewald et al., 2002),
or prefrontal contributions to visual working memory (Mottaghy
et al., 2002), affective processes (d'Alfonso et al., 2000) and de-
cision making (Knoch et al., 2006a,b).
Despite considerable advances in this field of research, cognitive
neurosciences still rely primarily on the use of functional imaging.
Imaging experiments, however, do not allow drawing firm con-
clusions about the nature of neural network nodes: activation could
bespuriouslycorrelatedwithtaskperformanceandnotnecessaryfor
proper task execution (Sack et al., 2005, 2007; Sack and Linden,
2003; Walsh and Cowey, 2000). rTMS seems to be a method well
suited for studying the functional relevance of a cortical region,
which has been previously identified by a functional imaging ex-
periment, for a specific cognitive function (Hallett, 2007).
When designing an experiment using “off-line” low-frequency
rTMS, several aspects should be considered. First, the duration of the
“off-line“ disruptive effect restricts the number of trials in an ex-
periment and, hence, the data collection. Previous studies (d'Alfonso
et al., 2000; Mottaghy et al., 2002; Robertson et al., 2001) have
provided some information about the duration of behavioral effects
after low-frequency “off-line” long-train rTMS applied to the
dorsolateral prefrontal cortex (DLPFC). The findings of these studies
led to the formulation of a rule of thumb: the duration of the after
effectsisgenerallyhalfthedurationofthestimulationtrain(Robertson
et al., 2003). However, these studies only report behavioral measures
and provide no information about the duration of the neurophysio-
logical effect of “off-line” low-frequency rTMS applied to the pre-
frontal cortex. Second, stimulation of the homologue contralateral
region as a control stimulation site has proven to be an expedient
www.elsevier.com/locate/ynimg
NeuroImage 42 (2008) 379–384
☆This paper is part of the research priority program at the University of
Zurich on the “Foundations of Human Social Behavior — Altruism versus
Egoism”. www.socialbehavior.uzh.ch.
⁎Corresponding authors. Center for the Study of Social and Neural Systems,
Institute for Empirical Research in Economics, University of Zurich, Switzerland.
E-mail addresses: eisenegger@iew.uzh.ch (C. Eisenegger),
dknoch@iew.uzh.ch (D. Knoch).
Available online on ScienceDirect (www.sciencedirect.com).
1053-8119/$ - see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.neuroimage.2008.04.172

Page 2

strategy for the control of the side effects of rTMS intervention.
Likewise, the side effects of rTMS intervention that ultimately
influence the outcome variable are similar. rTMS induces muscle-
twitches and tickling sensations at the stimulation site. Especially
when rTMS is applied to frontal regions, it causes discomfort, con-
stituting a possible confounding effect (Abler et al., 2005). As these
side effects typically are equally present when stimulating the homo-
loguecontralateralregiononecancontrolforthesesideeffectsbyalso
stimulatingthisregion.However,acontralateralcontrolstimulationis
only reasonable if a unilateral stimulation leads to unilateral, and not
bilateralneurophysiological“off-line“effects.Itisthereforeimportant
toknowwhether“off-line”rTMSleadstounilateralorbilaterallasting
effects. Third, related to the second point, one should consider that
applying rTMS to a given brain region not only affects the target site
itself,butalsobrainareaseffectivelyconnectedtothetargetarea.Such
remote effects evoked by low-frequency rTMS over prefrontal areas
havebeenshowninseveralstudies(Kimbrelletal.,2002;Knochetal.,
2006c; Nahas et al., 2001; Ohnishi et al., 2004; Speer et al., 2003).
These studies, however, differed from the usual “off-line” protocol in
that either regional cerebral blood flow (rCBF) (Nahas et al., 2001;
Speer et al., 2003) or glucose uptake (Kimbrell et al., 2002) was
measured “on-line” (i.e., during rTMS application), or in that the
duration of the stimulation trains were shorter than usual in the “off-
line” approach (Knoch et al., 2006c; Ohnishi et al., 2004).
Despite the wide-spread use of “off-line” rTMS in cognitive
tasks, and more recently, social cognitive neuroscience, no experi-
menthaseverinvestigatedthetime-courseoftheneurophysiological
lasting effects after a single train of low-frequency rTMS applied to
the prefrontal cortex. We specifically designed our study to assess
the issues mentioned above. We were particularly interested in the
after effects of right sided prefrontal stimulation as we previously
found lateralized behavioral effects using this stimulation protocol
(Knoch et al., 2006a,b). We used H2
rTMSonregionalcerebralbloodflow.Anadvantageofthisimaging
technique is the possibility to compare the relative size of regional
blood flow across different brain regions and over time. We applied
one single train of low-frequency rTMS to the right DLPFC of 12
subjects and subsequently performed 8 sequential H2
15O PET to assess the effects of
15O PETscans.
Materials and methods
Subjects
12 healthy right-handed male subjects (mean age 25.3 years, SD
4.0 years) participated in the study that the local ethics committee
approved. All subjects provided written informed consent. Subjects
had no history of psychiatric illness or neurological disorder and
were naive to TMS.
PET procedures
We used H2
Eight sequential scans were performed in each subject (1 baseline
scan, 7 post stimulation scans). PETscans were acquired on a whole-
bodyscanner(DLSGEMedicalSystems,Waukesha,WI,USA)in3D
mode with a 15-cm axial field of view. In each scan, 300–400 MBq
H2
injection device. Injection of the H2
automatic preparation and pump system 30 s before the start of the
actual H2
60 s were then taken as measure for cerebral blood flow. Data were
15O PET as a measure of regional synaptic activity.
15O was administered as a bolus using a remotely controlled
15O bolus was initiated with an
15O PET scan. The accumulated radioactivity counts over
reconstructed into 35 image planes with a resolution of 7 mm full-
width at half-maximum (FWHM). Subject preparation included the
insertion of an indwelling cubital vein catheter for injection of the
H2
subject was confined in a vacuum cushion. The scan room was
darkenedandsubjectsworefoamearplugsforhearingprotection.One
low-dose CT scan for attenuation correction was performed at the
beginning of the experiment, immediately followed by the baseline
H2
remaining seven scans were divided into two blocks. Each block
consisted of three and four scans, respectively (see Fig. 1). The order
of the two blocks was pseudorandomized across subjects.
15O. In order to ensure complete head fixation, the head of the
15O scan. Subjects kept their eyes closed during all scans. The
Location of the target region
For the exact localization of the stimulation site, a T1-weighted
MRI was acquired for all subjects prior to the PET-experiment. The
right DLPFC was located based on fixed coordinates: x=39, y=37,
z=22; radius=6 in Talairach space. These coordinates were then
transformed to each subject's native brain space using Brainvoya-
ger QX 1.6 software (Brain Innovation BV, Maastricht, NL). The
real-time neuronavigation option for Brainvoyager QX 1.6 with the
Zebris CMS20S measuring system for real-time motion analysis
(Zebris Medical GmbH, Isny, DE) allowed correct placement of the
TMS coil in space.
rTMS procedure
rTMS was applied to the right DLPFC for 15 min before each
PET scanning block (see Fig. 1). A Magstim Rapid 2 Stimulator
(Magstim, Winchester, USA) and a commercially available figure-
of-eight-coil (70 mm diameter double circle, air-cooled) was used.
Intensity of stimulation was set to 54% of maximum stimulator
output. The coil was held tangentially to the subject's head with the
handle pointing rostrally. Each subject received one train of 15 min
duration at 1 Hz (900 pulses) before each block of PET scanning,
amounting to a total of 1800 pulses per session and subject. The
time interval between the two stimulation blocks was 30 min. The
rTMS parameters employed were well within recommended safety
guidelines (Wassermann et al., 1998).
Image analysis
All images were processed using Statistical Parametric Mapping
software SPM99 (Wellcome Department of Imaging Neuroscience,
UCL, UK; http://www.fil.ion.ucl.uk/spm). Image processing was
Fig. 1. Experimental design. To achieve a virtual time resolution of 4 min
between H2
session,each scanningsequencewasprecededbya baselinescan,whichwas
followed by 15 min of 1 Hz rTMS at a fixed intensity of 54% of maximal
stimulator output and either scanning block A or B. Time points (in min after
rTMS train) of H2
15O scans, we set up an interleaved scanning procedure. In every
15O scans are indicated within the grey arrow. t, time.
380 C. Eisenegger et al. / NeuroImage 42 (2008) 379–384

Page 3

performedasfollows:headmovementswerecorrectedusingtheleast-
squares method and images were then normalized into stereotaxic
space[MontrealNeurologicalInstitutecoordinates(MNI)asprovided
by SPM99] by bilinear interpolation using a PET template [SPM,
1999 (Friston et al., 1995)]. Scans were smoothed using a Gaussian
filter of 15 mm in-plane and axial FWHM in order to improve the
signal to noise ratio. To remove the effect of global differences in
cerebral blood flow between scans, a subject-specific ANCOVA
scaling of global activity to a mean of 50 ml/100 g/min was applied.
Region of interest analysis
We performed whole-brain analyses after 1, 5, 9, 13, 17, 21 and
25 min to determine the contrast with the largest rCBF response
to low-frequency rTMS. Based on these analyses, the contrast
“1 minNbaseline” was used to determine regions of interest (ROIs)
in the prefrontal cortex (see Table 1; for rCBF increases or decreases
in other than prefrontal cortical regions please see Supplementary
data). Analysis of later time points did not reveal additional clusters
of either significantly increased or decreased rCBF. The ROIs were
defined as follows: for the stimulation site, a spheric ROI was
defined (MNI coordinates, radius and volume of ROI: x=50, y=48,
z=16; radius=7 mm, volume=339 mm3). A second spheric ROI
was created in the right ventrolateral prefrontal cortex (VLPFC;
x=42, y=42, z=−8; radius=7 mm, volume=339 mm3) and a third
spheric ROI was created in the right inferior frontal sulcus (x=32,
y=36, z=20; radius=7 mm, volume=339 mm3). Three homotopic
contralateral ROIs were defined by mirroring the ROIs in the right
hemisphereusingMARSBAR(Brettetal.,2002).Inordertocontrol
for unspecific arousal effects (Critchley et al., 2000, please see
Discussion section), two additional control regions were defined in
the cerebellum (ipsilaterally: x=32, y=−70, z=−41 and contral-
aterally: x=−32, y=−70, z=−41; both 16936 mm3). ROIs were
then normalized to baseline and analyzed using SPSS version 13.0
(SPSS, Inc., Chicago, IL, USA). A two-way repeated measures
ANOVA was calculated for each region with the factors: “hemi-
sphere” (left/right) and “time” (1, 5, 9, 13, 17, 21 and 25 min).
Masked correlation analysis
Subject-specific time-courseswere derived from the right DLPFC
andthecontrolregion(left/rightcerebellum)usingMARSBAR(Brett
etal., 2002). These individualtime-courses were thenincorporatedas
covariates of interest (correlation analysis). The linear association
between rCBF at every time point and each value derived from the
ROI inthe rightDLPFC and the control regionwas testedon a voxel-
by-voxel basis using the SPM covariates option. To focus only on
those regions within the prefrontal cortex that showed significant
rCBF changes in response to “off-line” low-frequency rTMS, the
correlation analysis of rCBF in the right DLPFC was subsequently
masked by the subtraction contrast “1 minNbaseline” (pb0.001,
correspondingtoasingle-voxelthresholdoftN3.22).Thresholdinthe
correlation analysis was set at pb0.001, corresponding to a single-
voxel threshold of tN3.22.
Results
Time-course of “off-line” prefrontal rTMS effects on rCBF
The contrast “1 minNbaseline” revealed a significant rCBF
increase at the stimulation site (pb0.05, corrected for multiple
comparisons over the whole brain). Fig. 2 illustrates the anatomical
coordinates of the stimulation site chosen for navigation and the
corresponding region of increased rCBF. The contrasts at other
time points (i.e. 5, 9, 13, 17, 21, and 25 min after rTMS) showed
no significant rCBF effects compared to baseline (pb0.05,
corrected for multiple comparisons over the whole brain). After
having identified the time point where rTMS effects were most
prominent, we used this contrast to define ROIs for further
analysis. Besides the activation cluster at the stimulation site,
increased rCBF was present in two other prefrontal areas, i.e. in the
VLPFC (BA47) and in the inferior frontal sulcus (BA46/48) which
were all located ipsilateral to the stimulation site (pb0.001,
uncorrected; see Table 1). These three regions were then defined as
ROIs as described in Materials and methods section.
The two-way repeated measures ANOVA revealed a significant
maineffectof“time”(F=2.98,df=6,66;p=0.01)and“hemisphere”
(F=6.62, df=1,11; p=0.03) in the stimulated region. Importantly,
the “hemisphere”דtime” interaction was significant (F=6.40,
Table 1
Prefrontal brain regions that show significantly increased rCBF 1 min after
rTMS
Brain regionBAHemisphereMNI
coordinates
T value
xyz
Dorsolateral
prefrontal cortexa
Ventrolateral
prefrontal cortexb
Inferior frontal sulcusb
45/46R 50 48165.31
47R 4242-84.39
46/48R3236 203.77
BA, Brodmann area; R, right.
aMaxima of regional increases in normalized rCBF based on the contrast
“1 minNbaseline” (pb0.05, corrected for multiple comparisons over the
whole brain).
bMaxima of regional increases in normalized rCBF based on the contrast
“1 minNbaseline” (pb0.001, uncorrected).
Fig. 2. Three-dimensional illustration depicts the chosen stimulation co-
ordinates (x=39, y=37, z=22; radius=6) and the locus of significant in-
crease in rCBF at the stimulation site. Blue spot, stimulation coordinates;
orange spot, significantly increased rCBF.
381 C. Eisenegger et al. / NeuroImage 42 (2008) 379–384

Page 4

df=6,66; pb0.001), indicating that the stimulated region shows a
time effect of rCBF that is specific to one hemisphere (Fig. 3A). In
the VLPFC, a significant main effect of “hemisphere” (F=15.09,
df=1,11;p=0.003), butnot“time”(F=1.44, df=6,66;p=0.21)was
identified. As in the DLPFC, the “hemisphere”דtime” interaction
was also significant in the VLPFC (F=2.68, df=6,66; p=0.02)
(Fig. 3B). In the inferior frontal sulcus, a significant main effect of
“hemisphere”(F=5.72,df=1,11;p=0.04)wasidentified.However,
neitherthemaineffectof“time”(F=1.38,df=6,66;p=0.24)northe
interaction term (“hemisphere”דtime”) was significant (F=0.49,
df=6,66; p=0.81). Thus, this region shows a hemispheric rCBF
effect that is not time specific (see Supplementary data). In the
controlregion(cerebellum),themaineffectof“hemisphere”wasnot
significant (F=3.275, df=1,11; p=0.098), while the main effect of
“time” was significant (F=3.28, df=6,66; p=0.007). The interac-
tion effect “hemisphere”דtime” was not significant (F=1.39,
df=6,66; p=0.23), suggesting that there is no time effect of rCBF
specific to one hemisphere in the control region (see Supplementary
data). In summary, these findings suggest a spatially and temporally
welldescribed specific“off-line”effectofprefrontalrTMSonblood
flow in the DLPFC (BA45/46) and the VLPFC (BA47).
Fig. 4 shows the results of the masked correlation analysis. A
significant positive correlation of the time-course of activity in the
stimulated region with the time-course of activity in the VLPFC
(BA47) was revealed (T=8.31). We found no regions that showed
significant negative correlations in the prefrontal cortex (see
Supplementary data). Moreover, we found no significant correla-
tion between the time-course of rCBF in the control region (left/
right cerebellum) with the time-course of rCBF in the DLPFC
(BA45/46).
Insummary,wefoundincreasedrCBFinthestimulatedarea,i.e.,
DLPFC (BA45/46) and in two other prefrontal regions ipsilateral to
the stimulated site: the VLPFC (BA47) and the inferior frontal
sulcus.TheseregionsshowedastrongincreaseinrCBFimmediately
after rTMS that returned to baseline within 9 min. Moreover, we
found that the time-course of rCBF in the VLPFC(BA47) is
correlated with the time-course of rCBF in the stimulated area.
Discussion
This study examined the neurophysiological effect of one long-
durationtrainoflow-frequencyrTMSappliedtotherightDLPFC.By
sequentially measuring rCBF after prefrontal stimulation we were
able to investigate three issues in this study: (1) the duration of the
“off-line” neurophysiological effect, (2) whether unilateral prefrontal
stimulationleads tounilateral or bilateral“off-line“effectsand (3)the
effects of low-frequency rTMS on remotely connected areas.
The most prominent increase in rCBF was found 1 min after the
rTMS train in the right DLPFC (BA45/46) and in the right VLPFC
(BA47). While other studies found effects with a delay of several
minutes after cessation of the rTMS train (Chouinard et al., 2003;
Fig. 3. The time-course (1 to 25 min) of mean normalized activity (±SE) (A) in the stimulated area (x=50, y=48, z=16; BA45/46) and (B) in the VLPFC (x=42,
y=42, z=−8; BA47). BL, baseline.
Fig. 4. The time-course of rCBF in the VLPFC (BA47) correlates positively
with the time-course of rCBF in the stimulated area (BA45/46). Results are
displayed on a coronal section (y=47 mm) of averaged anatomical MRI
scans (correlation analysis displayed at pb0.001, uncorrected; masked with
the contrast “1 minNbaseline” at pb0.001, uncorrected). L, left; R, right.
382 C. Eisenegger et al. / NeuroImage 42 (2008) 379–384

Page 5

Johnson et al., 2007), we did not find significantly changed rCBF at
later time points, as revealed by whole-brain analyses. One possible
explanation is that in the former study a different brain region was
stimulated (the left primary and premotor cortex) while in the latter
study behavioral effects and not rCBF were reported after low-
frequency rTMS of the left DLPFC. In our study, ROI analysis
revealed that increased rCBF in the right DLPFC (BA45/46) and
rightVLPFC(BA47)showedasimilardecayovertime andreturned
tobaselinevalueswithin9min.ItispossiblethattheobservedrCBF
change is the result of unspecific arousal or pain due to rTMS. The
cerebellum has previously been reported to positively co-vary with
experimentally induced cardiovascular arousal (Critchley et al.,
2000). Additionally, it has been reported to be activated during
capsaicine induced pain (May et al., 1998). To exclude that no such
unspecific effects confounded our results, we additionally analyzed
rCBF in both cerebellar hemispheres and found no “hemisphere”×
“time” interaction. Moreover, we found no significant correlation of
thetime-course ofrCBFinthecerebellum asour controlregionwith
the time-course of rCBF at the stimulation site (BA45/46). It thus
seems that low-frequency rTMS indeed induced a temporally well
circumscribed “off-line”-rCBF effect on the DLPFC (BA45/46) and
the VLPFC (BA47) in our experiment. Thus, as a first conclusion,
wefindthe general ruleofthumb confirmed:the effects of15min of
low-frequency rTMSapplied tothe rightDLPFC lastapproximately
half the duration of the stimulation train. One has to keep in mind,
however,thatwithrCBFwemeasuredindirectlyneuronalactivity.It
might be that rTMS affected neuronal activity for a longer duration
and in other regions. Moreover, rCBF was measured while subjects
wereinarestingstate.Theeffectsof“off-line”rTMSonrCBFmight
be different depending on whether the targeted brain region is
involved in the execution of a task or not.
OurresultsshowthatstimulatingtherightDLPFC“off-line”leads
to unilateral prefrontal but not bilateral effects on blood flow. Most
studies that applied rTMS either “off-line” or“on-line” tothe DLPFC
found no effect under the site of stimulation (Kimbrell et al., 2002;
Ohnishi et al., 2004; Speer et al., 2003). In one study however, low-
frequency rTMS was applied to the left DLPFC and a bilateral
activation in the DLPFC was found during stimulation (Nahas et al.,
2001). In contrast to this, we measured rCBF “off-line” and found
onlyipsilateralprefrontalactivationclusters.Asasecondconclusion,
we therefore argue that experimental paradigms in cognitive
neuroscience which plan to use “off-line” low-frequency rTMS to
affect the functional integrity of an area in the prefrontal cortex could
employ a contralateral control stimulation toavoid a side-effects bias.
By measuring rCBF sequentially, we were able to show that the
time-course of rCBF in the right VLPFC (BA47) is positively
correlated with the time-course of rCBF at the stimulation site
(BA45/46). Within the prefrontal cortex, the region in BA47 is the
only one that shows a time-course that is correlated with the one in
the stimulated area. This finding corroborates the hypothesis of
connectivity between dorsal (BA45/46) and ventral aspects (BA47)
of the frontal lobes (Petrides and Pandya, 1999). The ventrolateral
prefrontal cortex plays an important role in cognitive neuroscience,
such as in decision making (Sakagami and Pan, 2007) or emotion
regulation (Ochsner et al., 2004). A direct stimulation of this brain
area involves great discomfort for the volunteer. As a third con-
clusion, we thus propose that “off-line” long-train low-frequency
rTMS applied to the right DLPFC can be readily used to target
remoteconnectedareas,such astheVLPFCvia indirectstimulation.
Interestingly, we only found lasting rCBF increases (and not
decreases) in the prefrontal cortex. At first sight, this seems
difficult to reconcile with studies showing a decrease in cortical
excitability after long-train low-frequency rTMS (Chen et al.,
1997; Gerschlager et al., 2001; Kosslyn et al., 1999; Maeda et al.,
2000; Muellbacher et al., 2000; Wassermann et al., 1998). It is
important to note, however, that these findings were exclusively
based on studies in primary motor or primary visual cortex.
It is assumed that rTMS as employed in our study induces long
term depression (LTD) (Hallett, 2000; Thickbroom, 2007). Since it
has been suggested that rCBF changes reflect (pre)synaptic activity
changes (Jueptner and Weiller, 1995), one could expect a decrease in
rCBFafterLTDinductionbylow-frequencyrTMS.Whilethechosen
TMS protocolmightindeedhave induceda decreaseinexcitability in
the stimulated region, it also might have evoked compensatory
regulatory responses in order to maintain normal brain function,
which in turn could account for the observed increase.
On the other hand, the observed increased rCBF might also
reflect an active inhibition process requiring energy (Ackermann
et al., 1984). This interpretation, however, implies that low-
frequency rTMS as used in the current study increases the activity
of inhibitory interneurons. As several studies suggested, low-fre-
quency rTMSof theprimary motor cortex doesnot lead toincreased
intracortical inhibition as revealed by subsequent paired-pulse TMS
(Brighinaetal.,2005;Daskalakisetal.,2006;Fitzgeraldetal.,2002;
Romero et al., 2002).Based on thisevidence it seems less likely that
the observed increased rCBF results from an active inhibition
process induced by low-frequency rTMS. Irrespective of the
underlying physiological mechanisms, one can assume that rTMS
application induces a “virtual lesion” simply by adding neural noise
to the system (Husain et al., 2002; Walsh and Cowey, 2000; Walsh
and Rushworth, 1999). The observed increase in rCBF could then
again be interpreted as a regulatory response with a time-course
reflecting the duration of the “virtual lesion” effect.
In summary, there are three main results which are critical
aspects for the design of a study using brain stimulation in the
context of cognitive neuroscience: first, the duration of rCBF
changes in the prefrontal cortex is approximately half the duration
of the stimulation train itself. Second, right dorsolateral prefrontal
stimulation leads to ipsilateral prefrontal rCBF effects. Third,
15 min of “off-line” low-frequency rTMS of the dorsal aspects of
the frontal lobe (BA45/46) leads to rCBF changes in areas assumed
to be effectively connected to the stimulation site, such as the
VLPFC (BA47).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.neuroimage.2008.04.172.
References
Abler, B., Walter, H., Wunderlich, A., Grothe, J., Schonfeldt-Lecuona, C.,
Spitzer, M., Herwig, U., 2005. Side effects of transcranial magnetic
stimulation biased task performance in a cognitive neuroscience study.
Brain Topogr. 17, 193–196.
Ackermann, R.F., Finch, D.M., Babb, T.L., Engel Jr., J., 1984. Increased
glucose metabolism during long-duration recurrent inhibition of
hippocampal pyramidal cells. J. Neurosci. 4, 251–264.
Brett, M., Anton, J.-L., Valabregue, R., Poline, J.-B., 2002. Region of
interest analysis using an SPM toolbox. NeuroImage 16, 497.
Brighina, F., Giglia, G., Scalia, S., Francolini, M., Palermo, A., Fierro, B.,
2005. Facilitatory effects of 1 Hz rTMS in motor cortex of patients
affected by migraine with aura. Exp. Brain Res. 161, 34–38.
383 C. Eisenegger et al. / NeuroImage 42 (2008) 379–384