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\ PERGAMON Neuropsychologia 26 "0888# 196Ð106 9917Ð2821:87:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII]S9917Ð2821"87#99984Ð4 Transcranial magnetic stimulation and neuroplasticity Alvaro Pascual!Leone a\b\ \ Francisco Tarazona b \ Julian Keenan a \ Jose M[ Tormos b \ Roy Hamilton a \ Maria D[ Catala b a Laboratory for Ma`netic Brain Stimulation\ Beth Israel Deaconess Medical Center\ Harvard Medical School\ 229 Brookline Ave\ Boston\ MA\ USA b Unidad de Neurobiolo`ia\ Departamento de Fisiolo`ia\ Universidad de Valencia and Instituto Cajal\ Consejo Superior de Investi`aciones Cienti_ca\ Spain Received 7 July 0887^ accepted 09 July 0887 Abstract We review past results and present novel data to illustrate di}erent ways in which TMS can be used to study neural plasticity[ Procedural learning during the serial reaction time task "SRTT# is used as a model of neural plasticity to illustrate the applications of TMS[ These di}erent applications of TMS represent principles of use that we believe are applicable to studies of cognitive neuroscience in general and exemplify the great potential of TMS in the study of brain and behavior[ We review the use of TMS for "0# cortical output mapping using focal\ single!pulse TMS^ "1# identi_cation of the mechanisms underlying neuroplasticity using paired!pulse TMS techniques^ "2# enhancement of the information of other neuroimaging techniques by transient disruption of cortical function using repetitive TMS^ and _nally "3# modulation of cortical function with repetitive TMS to in~uence behavior and guide plasticity[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ 0[ Introduction A growing body of evidence from animal models and neurophysiologic and neuroimaging studies in humans\ supports the notion that the central nervous system is capable of change and adaptation throughout life "for recent reviews see ð3\ 05L[ While the developing nervous system seems more capable of modi_cation\ dynamic\ plastic changes can be documented in the adult nervous system as well[ Unmasking of existing connections\ shift! ing synaptic weighting\ even sprouting of new dendritic connections and formation of new synapses seem possible ð05L[ The central nervous system is a rapidly adapting\ dynamically changing system in which modi_cation is driven by a}erent input\ e}erent demand\ environmental and behavioral in~uences\ and functional signi_cance[ Plastic changes seem to underlay the acquisition of new skills\ the adaptation to new contexts and the recovery of function after injury[ However\ if plasticity is indeed a fundamental property of the central nervous system throughout life\ then plastic changes may not necessarily represent a behavioral bene_t for a given subject and our challenge is to modulate neural plasticity for the optimal Corresponding author[ Tel[] 990 506 556 91 92^ fax] 990 506 864 42 11^ e!mail] apleoneÝbidmc[harvard[edu[ 0 Translation by the _rst author[ behavioral gain[ The picture of the nervous system that is emerging is rather close to the intuitions of Santiago Ramo n y Cajal who in 0893\ in the {Textura del sistema nervioso del hombre y de los vertebrados| wrote] {{[ [ [ the work of a pianist [ [ [ is inaccessible for the untrained human\ as the acquisition of new abilities requires many years of mental and physical practice[ In order to fully understand this complicated phenom! enon it is necessary to admit\ in addition to the strengthening of pre!established organic pathways\ the establishment of new ones\ through rami_cation and progressive growth of dendritic arborizations and ner! vous terminals [ [ [ Such a development takes place in response to exercise\ while it stops and may be reversed in brain spheres that are not cultivated[|| 0 Transcranial magnetic stimulation "TMS# can be used in di}erent ways for studies of neuroplasticity ð3\ 00Ð 02\ 10\ 29L[ These di}erent applications relate to four principal types of studies] "0# demonstration of plastic changes^ "1# elucidation of mechanisms underlying plas! ticity^ "2# providing functional information to _ndings of neuroplasticity with other neuroimaging techniques^ and "3# modulating neuroplasticity to enhance it or reduce it in order to in~uence behavioral consequences[ TMS can be applied in single\ focal pulses to di}erent
Transcript

\PERGAMON Neuropsychologia 26 "0888# 196Ð106

9917Ð2821:87:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[PII] S 9 9 1 7 Ð 2 8 2 1 " 8 7 # 9 9 9 8 4 Ð 4

Transcranial magnetic stimulation and neuroplasticityAlvaro Pascual!Leonea\b\�\ Francisco Tarazonab\ Julian Keenana\ Jose M[ Tormosb\

Roy Hamiltona\ Maria D[ Catalab

a Laboratory for Ma`netic Brain Stimulation\ Beth Israel Deaconess Medical Center\ Harvard Medical School\ 229 Brookline Ave\Boston\ MA\ USA

b Unidad de Neurobiolo`ia\ Departamento de Fisiolo`ia\ Universidad de Valencia and Instituto Cajal\Consejo Superior de Investi`aciones Cienti_ca\ Spain

Received 7 July 0887^ accepted 09 July 0887

Abstract

We review past results and present novel data to illustrate di}erent ways in which TMS can be used to study neural plasticity[Procedural learning during the serial reaction time task "SRTT# is used as a model of neural plasticity to illustrate the applicationsof TMS[ These di}erent applications of TMS represent principles of use that we believe are applicable to studies of cognitiveneuroscience in general and exemplify the great potential of TMS in the study of brain and behavior[ We review the use of TMS for"0# cortical output mapping using focal\ single!pulse TMS^ "1# identi_cation of the mechanisms underlying neuroplasticity usingpaired!pulse TMS techniques^ "2# enhancement of the information of other neuroimaging techniques by transient disruption ofcortical function using repetitive TMS^ and _nally "3# modulation of cortical function with repetitive TMS to in~uence behavior andguide plasticity[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction

A growing body of evidence from animal models andneurophysiologic and neuroimaging studies in humans\supports the notion that the central nervous system iscapable of change and adaptation throughout life "forrecent reviews see ð3\ 05Ł[ While the developing nervoussystem seems more capable of modi_cation\ dynamic\plastic changes can be documented in the adult nervoussystem as well[ Unmasking of existing connections\ shift!ing synaptic weighting\ even sprouting of new dendriticconnections and formation of new synapses seem possibleð05Ł[ The central nervous system is a rapidly adapting\dynamically changing system in which modi_cation isdriven by a}erent input\ e}erent demand\ environmentaland behavioral in~uences\ and functional signi_cance[Plastic changes seem to underlay the acquisition of newskills\ the adaptation to new contexts and the recovery offunction after injury[ However\ if plasticity is indeed afundamental property of the central nervous systemthroughout life\ then plastic changes may not necessarilyrepresent a behavioral bene_t for a given subject and ourchallenge is to modulate neural plasticity for the optimal

� Corresponding author[ Tel[] 990 506 556 91 92^ fax] 990 506 864 4211^ e!mail] apleoneÝbidmc[harvard[edu[

0 Translation by the _rst author[

behavioral gain[ The picture of the nervous system thatis emerging is rather close to the intuitions of SantiagoRamo�n y Cajal who in 0893\ in the {Textura del sistemanervioso del hombre y de los vertebrados| wrote]

{{[ [ [ the work of a pianist [ [ [ is inaccessible for theuntrained human\ as the acquisition of new abilitiesrequires many years of mental and physical practice[In order to fully understand this complicated phenom!enon it is necessary to admit\ in addition to thestrengthening of pre!established organic pathways\ theestablishment of new ones\ through rami_cation andprogressive growth of dendritic arborizations and ner!vous terminals [ [ [ Such a development takes place inresponse to exercise\ while it stops and may be reversedin brain spheres that are not cultivated[||0

Transcranial magnetic stimulation "TMS# can be usedin di}erent ways for studies of neuroplasticity ð3\ 00Ð02\ 10\ 29Ł[ These di}erent applications relate to fourprincipal types of studies] "0# demonstration of plasticchanges^ "1# elucidation of mechanisms underlying plas!ticity^ "2# providing functional information to _ndings ofneuroplasticity with other neuroimaging techniques^ and"3# modulating neuroplasticity to enhance it or reduce itin order to in~uence behavioral consequences[

TMS can be applied in single\ focal pulses to di}erent

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scalp positions over the motor cortex while recordingmotor evoked potentials or force pulses ð00\ 28Ł[ Thismethodology allows the generation of cortical outputmaps serially in the same subject and the correlation withmeasures of functional capacity[ This can be used todemonstrate the reorganization of cortical motor outputsfollowing transient immobilization\ acquisition of newmotor skills\ amputation\ or recovery from CNS injuryð3\ 07\ 11\ 15\ 16Ł[

Short trains of repetitive TMS "rTMS# at frequenciesof up to 14 Hz can be used to disrupt naming or speechoutput\ generate maps of language function and deter!mine hemispheric language dominance ð5\ 19Ł[ Appliedto stroke patients\ this technique might be useful to dem!onstrate patterns of recovery from aphasia[ Similarly\rTMS can be used to study plastic reorganization in othercortical areas following injury\ such as the functional re!organization of the occipital cortex following peripheralblindness ð03Ł[

Paired!pulse TMS techniques ð06Ł can be used to studyintracortical excitability and the level of activity ofdi}erent cortico!cortical connections and neuro!transmitter systems[ Such studies can illuminate themechanisms of modulation of motor cortical rep!resentation during the acquisition of new skills or tran!sient dea}erentation ð33Ł[

Repetitive TMS can be used to transiently disrupt areasof activation on neuroimaging studies in order to estab!lish their functional signi_cance[ For example\ early blindsubjects show activation of the occipital cortex in PETand fMRI during tactile Braille reading ð23Ł[ This _ndingsuggests cross!modal plasticity[ Transient disruption ofthe occipital cortex with rTMS results in profoundworsening of the Braille reading skill\ thus providing atrue functional insight to the neuroimaging _ndings ð4Ł[This combination of TMS with other neuroimagingmodalities promises to enhance the information fromPET\ fMRI\ or EEG mapping studies as it may providecausal information between a pattern of brain activationand a given behavior "see Paus in this issue#[

Finally\ rTMS can enhance or decrease cortical excit!ability and thus potentiate or reduce neuroplastic pro!cesses ð18Ł[ This application of rTMS might be capableof speeding up recovery from stroke\ reducing the conse!quences of immobilization\ or enhancing acquisition ofnew skills[

In the present paper we will use studies on the neuralsubstrates of implicit motor learning in the serial reactiontime task "SRTT# to illustrate these di}erent applicationsof TMS in the study of neuroplasticity[

1[ Serial reaction time task "SRTT#

The SRTT "Fig[ 0# is a test of procedural learning inwhich both implicit and explicit learning strategies can

be explored[ We have used a variation of the SRTToriginally introduced by Nissen\ Bullemer and Wil!lingham ð08\ 30Ł[ The subject sits in front of a computerscreen and a keyboard with four clearly marked responsekeys[ The subject is asked to rest the index\ middle\ ring\and little _ngers of the hand to use on the appropriateresponse keys in preparation for the task[ An asteriskappears in one of four positions that are horizontallyspaced on the screen and aligned above the response keys[The subject has to push with one _nger\ as fast as possible\the key aligned with the asterisk that appears[ The aster!isk does not disappear until the correct button has beenpushed\ upon which the next stimulus appears[

The test is ordered in blocks of trials[ First\ the subjectcompletes a series of practice blocks that are discardedfrom further analysis but serve to familiarize the subjectwith the task[ Then\ the subject completes one or moreblocks\ in which the visual cues appear in pseudo!randomorder\ and performance is recorded as baseline[ There!after\ depending on the speci_c experiment\ the subjectcompletes a series of additional blocks in which the cuesare presented in a repeating sequence "Fig[ 0#[ The lengthof the repeating sequence may vary depending on theexperiment[ In each block the sequence is generallyrepeated 09 times[ The subject is not told about therepeating character of the sequence and does in fact notrecognize it until having completed a number of blocks[Nevertheless\ despite the lack of conscious recognition ofthe repeating character of the trials\ the subject|s responsetimes show a progressive shortening[ This shortening inresponse time is an indirect measure of implicit\ pro!cedural learning[ Eventually the subject becomes awareof the repeating sequence of the trials and continues toimprove the response times\ though now presumablydriven by explicit learning strategies[ Finally\ the subjectcompletes one _nal block in which the visual stimuli areagain presented in pseudo!random order[ The di}erencein response time between the last repeating block andthis _nal random block provides a second measure ofprocedural learning[

2[ Mapping plastic changes

Maps of motor cortical output to di}erent hand andforearm muscles can be obtained using single pulse\ focalTMS serially during the performance of the SRTT ð11Ł[Performance in the task can then be compared with themodulation of the cortical output maps to musclesinvolved in the task and to uninvolved\ neighboring mus!cles[

Motor cortical output maps to the forearm ~exor mus!cles of the "right# hand were generated using a small 7!shaped coil "each wing 3 cm in diameter# and a Cadwellmagnetic stimulator "Cadwell Inc[\ Kennewick\ WA\U[S[A[#[ Stimuli were applied to a 4×4 grid of scalp

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106 198

Fig[ 0[ Schematic summary of the serial reaction time task "SRTT#[ A visual stimulus is presented on the computer screen and the subject has torespond as fast as possible by pushing the appropriate response key using the appropriate digit "A#[ Upon correct response the visual stimulusdisappears and the next visual stimulus appears\ often after a prede_ned delay "B#[ If the subject pushes the incorrect response key\ the visual stimulusdoes not disappear and the subject has to self!correct[ Unknowingly to the subject the visual stimuli are presented in a repeating sequence "C#[ Despitelack of awareness of this repeating sequence\ the subject|s response times shorten providing an indirect measure of implicit procedure learning[

positions 0 cm apart over the left sensorimotor cortex[The coil orientation was held constant\ the coil wasapplied to the di}erent scalp positions\ and each scalpposition was stimulated _ve times[ Stimulation intensitywas kept at 09) above the subject|s motor thresholdintensity[ Motor evoked potentials "MEPs# induced byTMS were recorded using surface EMG electrodes tapedto the skin over the muscle[ The mean amplitude for theMEPs induced by TMS from each scalp position wascalculated and plotted against the scalp position as acontour map[ Shown in Fig[ 1 is a thresholded bubblemap with the scalp positions projected onto the subject|s2!dimensional reconstructed MRI brain image "Fig[ 1#[

This TMS mapping study ð11Ł demonstrates anenlargement of the motor cortical output map to thecontralateral muscles involved in the task during thephase of implicit learning[ The enlargement of the motorcortical output to the involved muscles cannot be dem!onstrated for uninvolved muscles[ For example\ the fore!arm _nger ~exors "Fig[ 1# or the _rst dorsal interosseusmuscle that participates in the movement of the index

_nger and are required for the SRTT task reveal themodulation of the output maps[ However\ the motorcortical output map to the abductor muscle of the thumb\which is not used for the SRTT\ task does not change[

As mentioned\ when the subject becomes aware of therepeating nature of the stimuli an explicit search strategyis likely to be engaged[ During this time in which both\implicit and explicit learning is probably taking place\the response times continue to shorten but the corticaloutput maps tend to plateau[ Eventually\ as the subjectlearns the full repeating sequence of stimuli\ the per!formance becomes primarily driven by the explicit knowl!edge[ At this point\ there is a rapid reduction of the motorcortical output maps towards the baseline topography"Fig[ 1#[ This return of the maps towards their baselinetopography suggests that as a motor sequence is explicitlylearned\ the contribution of the motor cortex is atten!uated and other brain structures assume more active rolesin the execution of the task[ It seems that ~exible short!term modulation of cortical outputs takes place duringskill acquisition that might in fact be critical in the event!

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106109

Fig[ 1[ Modulation of the cortical motor output maps in the course of the serial reaction time task "modi_ed from ð12Ł#[ The subject completes 09blocks of the task[ During blocks of repeating presentation of the visual stimuli "blocks 1Ð09#\ the subject shows a progressive reduction in responsetime "learning#[ Initially the subject is unaware of the repeating nature of the stimuli "blocks 1Ð3\ {implicit learning|#[ During this time there is amarked increase in the cortical motor output map for the forearm _nger ~exors on the right hand used in the task[ In blocks 3Ð5 the subject knowsthat there is a sequence but does not know what[ Presumably the subject uses both implicit and explicit learning strategies at that point[ After block5\ the subject the entire sequence "{explicit knowledge|#\ performance is likely driven by explicit learning\ but there is continued performanceimprovement[ However\ at this point\ the cortical output maps show a rapid return to baseline[

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106 100

ual development of more permanent structural changesin the intracortical and subcortical networks as the skillbecomes more and more overlearned and automatic[These _ndings with TMS are in agreement with similarmotor learning studies using other brain imaging tech!niques such as fMRI or PET ð7Ð09\ 04\ 24\ 25\ 31Ł[

This type of TMS mapping study can be applied to theidenti_cation of neuroplasticity also in the context ofother forms of motor learning ð15Ł\ adjustment to blind!ness and acquisition of the Braille reading skill ð03Ł\ orrecovery from peripheral or central nervous system injuryð3Ł[

3[ Studying the physiology underlying plastic changes

Rapid modulation of motor cortical outputs in thecontext of skill acquisition is likely the result of unmask!ing of existing connections ð1\ 2\ 05Ł[ Decreased inhibitionor increased synaptic e.cacy of existing neural circuitsmight be considered as possible mechanisms for this rapidplasticity[ In either case\ it is hypothesized\ that this kindof rapid cortical plasticity ought to result in changes inintracortical excitability that might be demonstrableusing the paired!pulse TMS technique ð06Ł[

A _rst\ conditioning stimulus is applied\ followed at avariable interval\ by a second\ test stimulus[ The intensityof both stimuli in~uences the e}ects as di}erent circuitsare recruited by di}erent intensities of stimulation[ Theinterstimulus interval "ISI# in~uences the results as thetime constant of each activated circuit may di}er[ At veryshort ISIs "³ 0 ms# it is possible to study neural timeconstants of the stimulated elements^ at ISIs of 0Ð3 msit is possible to investigate interactions between I!waveinputs to corticoÐspinal neurons\ and at ISIs of 0Ð19 msit is possible to investigate corticoÐcortical inhibitory andfacilitatory circuits[ All these e}ects appear to be corti!cally mediated ð06\ 27Ł and intracortical inhibition andfacilitation appear dependent on the activation of sep!arate circuits ð35Ł[ Medications that enhance GABAergicactivity have been shown to markedly decrease the degreeof corticoÐcortical facilitation evoked by paired TMSstimuli at ISIs of approximately 7Ð01 ms ð34Ł[ In Par!kinson|s disease\ the dopamine de_ciency is associatedwith reduced corticoÐcortical inhibition at short ISIs"³ 4 ms# ð0\ 22Ł\ while dopaminergic drugs enhance cort!icoÐcortical inhibition ð0\ 21\ 22\ 32Ł[

Ziemann et al[ ð33Ł have recently elegantly dem!onstrated the utility of the paired!pulse TMS techniquein the study of the mechanisms of short!term corticalplasticity in a dea}erentation paradigm ð1\ 2Ł[ In addition\their study illustrates the possibility of modulating cort!ical excitability and thus neuroplasticity with rTMS[Transient forearm dea}erentation was induced byischemic nerve block in healthy volunteers[ Plastic changesin the motor cortex contralateral to the dea}erented fore!

arm were probed with paired!pulse TMS to the bicepsbrachii muscle proximal to the level of dea}erentation[Ischaemic nerve block alone induced a moderate increasein the size of the motor evoked potentials in the biceps\but no changes in intracortical inhibition or facilitation[However\ rTMS at 9[0 Hz to the motor cortex con!tralateral to the ischaemic nerve block reduced intra!cortical inhibition and increased intracortical facilitationmarkedly potentiating the plastic changes induced by theischemic nerve block alone[ These _ndings indicate thatthe dea}erented motor cortex becomes modi_able byinputs that are normally subthreshold for inducing chan!ges in excitability[ The dea}erentation!induced plasticchanges can be up!regulated by direct stimulation of the{plastic| cortex and down!regulated by stimulation of theopposite cortex\ probably through inhibitory trans!callosal connections ð33Ł[

Following the example of Ziemann et al[ ð33Ł\ in thesetting of the SRTT\ repeated studies of intracorticalexcitability with the paired!pulse TMS technique mayenhance our understanding about the intracortical mech!anisms responsible for the modulation of motor corticaloutputs described above[

4[ Adding function signi_cance to neuroimaging studies

Functional neuroimaging studies do not de_ne the roleof a given structure for a speci_c behavior\ they simplyestablish an association between activity in a given neuralstructure or network and the performance of a task[Repetitive TMS can transiently block the function of aspeci_c cortical structure and thereby allows the de_!nition of a causal link between behavior and regionalbrain function ð13Ł[ This form of TMS application gen!erates {virtual lesion patients|[ The study of subjects withsuch transient and reversible {lesions| has advantages overthe study of patients with brain injuries[ First\ the studycan be repeated and its subject can therefore be retestedand serve as its own control[ Second\ reversible\ transientlesions limit the in~uence of adaptative changes and func!tional readjustments that take place following structuralbrain injuries[

In this form of application\ rTMS might be viewed asa noninvasive counterpart for human studies of corticalfaradization or of local cortical cooling in animals[ Cohenet al[ ð4Ł have recently illustrated this form of applyingrTMS in cognitive neuroscience in the study of cross!modal plasticity in early blind Braille readers "Fig[ 2A#[Studies with short trains of rTMS can be viewed as _rststeps in the exploration of causal links between corticalactivation and behavior addressing topographical ques!tions and eliminating questions regarding temporal vari!ables[ Follow!up studies can then explore the question ofthe timing of the contribution of a given cortical area toa given behavior[ Hamilton and Pascual!Leone ð03Ł have

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106101

Fig[ 2[ E}ects of TMS on tactile Braille reading ability in sighted control and early blind subjects[ E}ects of trains of repetitive TMS inducing errorsin tactile Braille reading depending on cortical target expand the information derived from PET studies showing activation of sensorimotor andoccipital cortex during Braille reading "A\ modi_ed from ð4\ 24Ł#[ E}ects of single TMS stimuli to occipital or sensorimotor cortex on tactile Braillesymbol discrimination depending on the interval between the peripheral Braille stimulus to the right index _nger pad and the cortical stimulus "B\modi_ed from ð03Ł#[

illustrated this possibility in the study of the occipitalcortical contribution to tactile Braille reading in earlyblind subjects "Fig[ 2B#[

Using this approach of {induction of virtual patients|

with rTMS\ we have studied the role of the dorsolateralprefrontal cortex in the SRTT ð20Ł[ Functional neu!roimaging studies of the pattern of neural activation dur!ing studies of procedural learning suggest that among

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106 102

other cortical regions\ the dorsolateral prefrontal cortexis critical ð7Ð09\ 04\ 24\ 25Ł[ However\ such studies cannotresolve the speci_c role of the dorsolateral prefrontalcortex[ For example\ activity of the dorsolateral pre!frontal cortex might be related to the acquisition of theexplicit knowledge or participate in implicit componentsof procedural learning[

Normal subjects completed several blocks of the SRTTusing only one hand with or without concurrent rTMS[In this version of the SRTT subjects were not speci_callyasked whether the cues were presented in a random orrepeating order at any point during the task[ Blocks con!sisted of 019 trials\ 09 repetitions of a 01!item repeatingsequence[ In order to transiently disrupt their function\rTMS was applied over the supplementary motor area orover the dorsolateral prefrontal contralateral or ipsi!lateral to the hand used for the test[ TMS was deliveredwith a Cadwell High Frequency Magnetic Stimulator"Cadwell Inc[\ Kennewick\ WA\ U[S[A[# equipped witha water!cooled\ eight!shaped coil[ Each loop of the coilmeasures approximately 6[4 cm inner diameter and theintersection of the two loops measures 2[4×0[4 cm[ Forstimulation of the dorsolateral prefrontal cortex the coilwas centered on the lateral convexity\ 4 cm rostral to theoptimal scalp position for the abductor pollicis brevismuscle[ The optimal scalp position was taken to representthe localization of the primary motor cortex ð39Ł[ Forstimulation of the supplementary motor cortex the stimu!lation coil was centered\ along the mid!sagittal line\ 4 cmrostral to the optimal scalp position for activation of theanterior tibialis muscles[ For stimulation of the dor!solateral prefrontal cortex\ the stimulation coil was heldtangentially to the scalp with the current following par!allel to the sagittal axis[ For stimulation of the sup!plementary motor area\ the coil was oriented so thatcurrent ~ow was perpendicular to the head|s sagittal axis[Stimulation was delivered in trains of 4 Hz frequencythat started at the beginning of each block of trials andcontinued for a maximum of 59 s according to the safetyrecommendation ð14\ 27Ł[ In all cases\ this was su.cientto assure stimulation from the beginning until completionof the block[ In order to guarantee that this was thecase\ we varied the SRTT eliminating the delay betweensubject|s response and appearance of the subsequentstimulus[

In the no!TMS condition all subjects showed a pro!gressive decrease in response time during the four blockswith a repeating sequence and a signi_cant increase inresponse time from the last block with a repeatingsequence to the block with randomly presented cues[ Bothof these changes are measures of procedural learning ð08\30Ł[ During the di}erent rTMS conditions\ we found nosigni_cant di}erences in response time and error rate inthe last block of each set in which visual cues were ran!domly ordered[ This ruled out a rTMS e}ect on responseexecution regardless of stimulation site[ However\ rTMS

had profound\ position speci_c e}ects on task per!formance during the blocks in which visual cues werepresented in a repeating sequence[ Stimulation to thecontralateral dorsolateral prefrontal cortex markedlyimpaired procedural\ implicit learning\ as documented bythe lack of signi_cant change in response times duringthe task "Fig[ 3#[ Stimulation over the other areas did notinterfere with learning "Fig[ 3#[

These results support the notion of a critical role ofdorsolateral prefrontal structures in learning of motorsequences and are in agreement with results of patientswith traumatic or cerebrovascular lesions of the dor!solateral prefrontal cortex ð12Ł[ Of note\ is the fact thatblocking of the contralateral prefrontal cortex a}ectedthe reduction in response time during blocks of repeatingvisual stimuli while the subjects were unaware of therepeating nature of the trials[ Therefore\ the inference isthat the dorsolateral prefrontal cortex is needed forimplicit procedural learning[

5[ Modulating neuroplasticity and behavior with rTMS

During the implicit\ procedural learning phase\ corticalexcitability increases in the motor cortex for the motoroutputs to muscles involved in the SRTT task[ Honda etal[ "unpublished data# found in a recent PET study ofimplicit learning during the SRTT a correlation betweenresponse time shortening and motor cortical activity[Results of electroencephalographic coherence studies ð31Łand of the TMS mapping study presented above ð11Łsupport the same notion[ This increase in cortical excit!ability might be necessary for skill acquisition[ If thisis so\ external modulation of motor cortical excitabilitymight in~uence the rate of procedural learning[ In thepresent\ previously unpublished experiment\ we inves!tigated whether modulation of motor cortical excitabilitywith rTMS prior to performance of the SRTT can in~u!ence implicit motor learning[1

rTMS can increase or decrease cortical excitabilitydepending on the stimulation parameters ð18Ł[ Thesemodulatory e}ects of cortical excitability can be docu!mented by combining TMS with a variety of neu!roimaging and neurophysiologic techniques[ There seemsto be substantial interindividual variability on the e}ectsof di}erent rTMS parameters\ such that the same rTMSsettings might result in opposite modulation of corticalexcitability in di}erent subjects[ On the other handthough\ there seems to be a fair amount of intraindividualstability of the e}ects[

Subjects completed three SRTT blocks in each of whicha 01!item sequence was repeated 09 times[ Then\ they

1 These data are part of the doctoral thesis work of Francisco Tara!zona\ M[D[ presented during June 0887 at the University of Valencia\Spain[

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Fig[ 3[ E}ects of repetitive TMS to di}erent cortical targets on the shortening of response time in the serial reaction time task "modi_ed from ð21Ł#[Graph displays the average response time in all subjects during 3 blocks of a repeating sequence "blocks 0Ð3# and a _nal block of random presentationof visual stimuli "block 4#[ Response times are expressed as percentage change from a previous\ baseline block of the task in which stimuli werepresented in random order[ Note the lack of change in response time during rTMS to the contralateral dorsolateral prefrontal cortex "c DLPFC#[Targets of rTMS {sham| � coil angulated away from the head at 89>^ {SMA| � supplementary motor area^ {c DLPFC| � contralateral dorsolateralprefrontal cortex^ {iDLPFC| � ipsilateral dorsolateral prefrontal cortex[

underwent either sham\ 0 Hz\ or 09 Hz rTMS at sub!threshold intensity[ Finally\ they completed three moreSRTT blocks with a di}erent repeating sequence[ Both01!item sequences were equivocal and their order "beforeor after rTMS# was random and counterbalanced acrosssubjects[ In one of the SRTT versions the stimuli werenumbers "0\ 1\ 2\ or 3# presented in the middle of thescreen[ In the second SRTT version the sequence wasdi}erent and in addition stimuli were circles presented inone of four horizontally spaced positions on the computerscreen[ We elected to use two di}erent types of stimuli inorder to minimize transfer of learning from one SRTTversion to the next[

We studied 10 normal\ right!handed subjects "01 menand 8 women\ mean age 15[2 years# randomly assignedto receive either 0 Hz\ 09 Hz\ or sham rTMS[ The e}ectsof rTMS on cortical excitability were tested in all subjectsprior to their participation in the study by applying rTMSto the motor cortex and measuring cortical excitabilitybefore and after the rTMS trains[ In all subjects recruitedfor the study\ sham rTMS did not a}ect motor corticalexcitability\ while 0 Hz rTMS reduced it and 09 Hz rTMsenhanced it ð18Ł[ We do not know if rTMS to the dor!

solateral prefrontal cortex exerts the same e}ects on cort!ical excitability as in motor cortex[ Nevertheless\ weassume a correlation between the rTMS e}ects on corticalexcitability of di}erent cortical areas[

TMS was applied with a Dantec Magpro magneticstimulator "Dantec Medical Inc[\ Denmark# and a focal\7!shaped coil[ All subjects were studied on two di}erentdays separated at least by one week[ On one of the days\rTMS was applied to the motor cortex while on the otherday it was applied to the dorsolateral prefrontal cortex[The position of the stimulation coil on the scalp wasmarked in all subjects with a vitamin A capsule andthereafter all underwent an anatomical MRI study inorder to localize the site of stimulation in their brain[ Themotor cortex position did indeed target the central sulcuswith a maximal error of less than 0 cm[ The dorsolateralprefrontal cortical position targeted the dorsolateral pre!frontal cortex in all subjects\ being centered over theborder between areas 8 and 35 and a}ecting both[

Figure 4 summarizes the results[ RT in Block 0 wasanalyzed to determine if there was a baseline di}erencebetween the groups[ A two!way factorial "rTMS con!dition "2# by region of stimulation "1## was performed on

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106 104

Fig[ 4[ E}ects of modulation of excitability of motor or contralateral dorsolateral prefrontal cortex by repetitive TMS on procedural learning in theserial reaction time task[ Graph A shows the average response times for all subjects during the three block before "0\ 1\ and 2# and after "0?\ 1?\ and2?# rTMS "gray bar#[ The visual stimuli in all blocks were presented in a repeating sequence unknowingly to the subjects\ none of which became awareof the sequential nature of the task[ Repetitive TMS was applied either at 0 Hz\ 09 Hz\ or with the coil angulated away from the head "89>\ sham#[Graph B shows the change in response time from block 0 "or 0?# to 2 "or 2?# as an index of procedural implicit learning[ Solid bars display the mean"2standard deviation# change in response time before rTMS[ Pattern bars express the change in response time "mean 2s[d[# following rTMS "0Hz\ 09 Hz or sham# to motor or dorsolateral prefrontal cortex[

A[ Pascual!Leone et al[ : Neuropsycholo`ia 26 "0888# 196Ð106105

the RT scores[ We found no signi_cant interaction "F1\ 09#�9[35\P× 9[94# or main e}ects for rTMS con!dition "F "1\ 29#�9[42\P× 9[94# or region of stimu!lation "F "1\ 29#�9[07\P× 9[94#[ A similar analysis wasperformed for the error rates and no signi_cant inter!action or main e}ects were found[ The rate of learningprior to rTMS exposure was assessed by examining thechange scores from Block 0 to Block 2[ We found thatthese scores did not di}er "F "4#�9[32\P× 9[94#\ thusruling out baseline di}erences in task performance acrossstudy groups[ A further analysis was performed for the_rst block pre!rTMS and the _rst block post!rTMS"Blocks 0 and 0?# in order to rule out e}ects of rTMSon response time independent of implicit learning[ Nosigni_cant di}erence was found between these two Blocks"t "24#�−9[51\P× 9[94#[

Changes in RT across blocks 0 to 2 and 0? to 2? providea measure of implicit learning pre! and post!rTMS respec!tively[ To determine the e}ects of rTMS on changes inRT\ we tested for overall di}erences in RT across allblocks with a three!way factorial analysis of rTMS fre!quency\ stimulation site\ and pre vs post condition[We found a signi_cant interaction "F"1\ 09#�4[49\P³ 9[994#[ A series of planned comparisons were thenperformed to determine the various e}ects of rTMS onRT[

First\ the change scores in RT across blocks 0 "0?# to 2"2?# were compared pre! and post!rTMS to the motorcortex[ A one!way ANOVA comparing pre! and post!rTMS scores depending on rTMS condition "sham\ 0 Hzor 09 Hz# revealed an overall signi_cant interaction"F"4\ 29#�2[44\ P³ 9[90#[ Corrected post!hoc Bonferronitests demonstrated that in the 09 Hz group the changesin response time were signi_cantly greater post!rTMSthan pre!rTMS "t "09#�1[88\P³ 9[90#[ The other tworTMS conditions "sham and 0 Hz rTMS# did not yieldsigni_cant results[

Second\ similar comparisons were carried out for thedorsolateral prefrontal rTMS group[ While we foundno overall signi_cant di}erence\ the planned post!hocanalysis revealed that the 09 Hz condition had a sig!ni_cantly smaller reduction in reaction time across thethree SRTT blocks post!rTMS than pre "t"09#�−1[25\P³ 9[94#[ The other two rTMS conditions were foundto induce no signi_cant di}erences[

A _nal planned comparison was then carried out com!paring the motor and dorsolateral prefrontal rTMSgroups and we found a signi_cant overall interaction"F"4\ 29#�2[51\ P³ 9[90#[ A post!hoc comparisonrevealed that the 09 Hz group di}ered signi_cantly asa function of stimulation site "t "09#�3[91\P³ 9[994#\while the other rTMS conditions did not yield signi_cantdi}erences[

Error rates were analyzed in a likewise manner[ All ofthe planned comparisons and interactions described foranalysis of the RT results were completed also for the

error rates and no signi_cant di}erences were found ineither the overall comparisons or the planned speci_ctests[

This experiment demonstrates that modulation of cort!ical excitability with rTMS can indeed in~uence behavior[Enhancement of excitability of the motor cortex seemedto speed up procedural learning[ On the other hand\reduction of motor cortical excitability slowed down pro!cedural learning "though this e}ect did not reach stat!istical signi_cance#[ Contrary to the e}ects over the motorcortex\ enhancement of excitability of the dorsolateralprefrontal cortex actually interfered with procedurallearning suggesting that a speci_c level of activation isneeded to obtain maximal behavioral bene_t[

This experiment suggests the possibility of using rTMSin conjunction with physical\ occupational\ behavioral orother rehabilitative therapies in order to enhance theirbene_cial e}ects for patients recovering from braininjury[ {Preactivation| of a given cortical region priorto more traditional therapeutic interventions might helpenhance their e}ect[ Similarly\ modulation of corticalexcitability as an adjunct to medication treatment mighto}er therapeutic advantages in neuropsychiatric illnessesð6\ 17\ 29Ł[

Acknowledgements

We thank Drs Mark Hallett\ Leonardo Cohen\ JordanGrafman\ and Eric Wassermann for their critical con!tributions to some of the experiments reviewed in thepresent article[ Parts of this work were supported bygrants from the Milton Fund\ the National Eye Institute\the National Institute of Mental Health\ the Stanley VadaFoundation\ and the Direccio�n General de Ciencia yTecnologia "Spain#[

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