Misconceptions about Mirror-Induced Motor Cortex Activation.

0.3 MB·English

by  Praamstra, Peter

CerebralCortexAugust2011;21:1935--1940
doi:10.1093/cercor/bhq270
AdvanceAccesspublicationJanuary12,2011
Misconceptions about Mirror-Induced Motor Cortex Activation
Peter Praamstra1,2,Laura Torney1,Christian J.Rawle1and R. Chris Miall1
1Behavioural Brain SciencesCentre, School ofPsychology, University of Birmingham, Birmingham B152TT,UK and 2Department
ofNeurology, Queen Elizabeth Hospital, University of Birmingham, Birmingham B15 2TT,UK
AddresscorrespondencetoPeterPraamstra,BehaviouralBrainSciencesCentre,UniversityofBirmingham,BirminghamB152TT,UK.
Email:[email protected].
Observation of self-produced hand movements through a mirror, cortex contralateral and ipsilateral to the side of movement.
creating an illusion of the opposite hand moving, was recently Thisdifferencepotentialbetweenleftandrightmotorcortexis
reportedtoinduceipsilateralmotorcortexactivation,thatis,motor normally measured separately for left and right hand
cortex activation for the hand in rest. The reported work goes far movements and then combined. The authors derived the LRP
beyond earlier work on motor cortex activation induced by action for the direct view condition in a standard manner, illustrated
observation, by implying a complete reversal of contralateral and in Figure 1A. The LRP for the mirror view condition was
ipsilateral motor cortex activation under mirror view conditions. derivedonthebasisofdatafromrighthandmovementsunder
Suchareversalwouldrepresentanunprecedenteddegreeofneural direct view (same data as direct view LRP) and right hand
plasticity. We considered such a reversal physiologically implau- movements viewed through a mirror (Fig. 1B). Since the
sible and conducted a study with an improved design. The results authors obtained a mirror view LRP of identical amplitude as
refute the reversal of contralateral and ipsilateral motor cortex the direct view LRP, this implies that the balance of left and
activation under mirrored viewing conditions as methodologically right motor cortex activation, during mirror view of the right
unsound.Theinvestigationconfirmed,however,moresubtleexpres- handmoving,wasidenticaltothebalanceinactivationwithleft
sions of motor cortical activity induced by self-produced move- hand movements under direct view (Touzalin-Chretien and
ments observedthrough amirror. Dufour2008;Touzalin-Chretien etal. 2010).
The implied reversal in balance of ipsilateral and contralat-
Keywords:electroencephalography, lateralizedreadiness potential, mirror eral motor cortex activation, when viewing one’s own hand
visualfeedback, motor cortex, movement-related potentials, visuomotor movinginamirror,wouldrepresentanunprecedenteddegree
adaptation ofneuralplasticity.Wesuspectedthatthereportedresultsare
due to methodological problems, as explained in the Supple-
mentary Material. In view of the promising role claimed for
mirror visual feedback in restoring neurological function
Introduction (Ramachandran and Altschuler 2009), we thought it is impor-
In3recentpublications,Touzalin-ChretienandDufour(2008) tanttopresentamorerealisticestimateofmirrorview-induced
and Touzalin-Chretien et al. (2009, 2010) used the lateralized motor cortex activation and conducted a similar experiment
readinesspotential(LRP)toinvestigatemotorcortexactivation withimproved design.
inducedbymirrorvisualfeedback.Usingamirrorplacedinthe
midsagittal plane in front of the participant, the investigators
Materials and Methods
had participants look at movements of their right hand as if it
wastheirlefthand.Basedonanalysesofthemovement-related Participants
LRP, the authors inferred the presence of motor cortex Participants were 9 right-handed adults (6 males; age 32 ± 11 years)
activationcontralateral tothe resting lefthand. with normal or corrected-to-normal vision. Data of one further par-
Motorcortexactivationinducedbymovementobservationis ticipantwereexcludedbecauseofexcessiveartifacts.Allparticipants
providedtheirinformedconsent,andthestudyhadbeenapprovedby
wellestablishedandexistingevidenceincludesinvolvementof
thelocalresearchethicscommittee.
thecontralateralprimarymotorcortex(Fadigaetal.1995;Hari
et al. 1998; Van Schie et al. 2008), the presumed origin of the
LRP (Praamstra et al. 1999). However, the results reported by ProcedureandStimuli
Touzalin-Chretienandcoworkersimplyacompletereversalof We constructed a mirror box pictured in Figure 2A. The box was
contralateral and ipsilateral motor cortex activation under designed to avoid the forced and asymmetric posture imposed by
mirrored viewing conditions. Such a reversal is incompatible experimentalsetupswithamirrorplacedinthemidsagittalplane.The
boxhadanopeningatthetop.Inthedirectviewcondition,subjects
with existing knowledge of sensorimotor physiology. Even
wereinstructedtolookthroughtheopeningtotheirmovinghand,just
afterprolongedvisuomotoradaptationwithleft--rightreversing
leftorrightfromthemidlineofthebox.Inthemirrorviewcondition
goggles, observed changes in cerebral activation patterns do
(Fig.2B),theylookedawayfromthemovinghandtoitsreflectionjust
not include the ipsilateral primary motor cortex (Sekiyama oppositefromthemidline.Therestinghandonthatsidewashidden
etal. 2000). from view by a cardboard cover, while the moving hand was also
How do the reported results by Touzalin-Chretien and invisible,beingpositionedfurtherawayfromthemidlineclosertothe
coworkers imply a reversal of contralateral and ipsilateral side of the box. It should be noted that there was a small remaining
asymmetryinposture,sinceparticipantshadtolookleftorrightfrom
motor cortex activation under mirrored viewing conditions?
the midline. Hence, gaze direction effects on motor cortical activity
TheauthorsusedtheLRP,representinglateralizedmovement-
(Baker et al. 1999) cannot be ruled out entirely, although we have
related motor cortex activation derived by measuring the previously not been able to detect such effects in movement-related
difference in voltage between electrodes overlying the motor electroencephalography(EEG)potentials(Hesseetal.2004).
(cid:1)TheAuthors2011.PublishedbyOxfordUniversityPress.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionNon-CommercialLicense(http://creativecommons.org/licenses/by-nc/2.5),whichpermits
unrestrictednon-commercialuse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Subjectsmadeabriskindexfingerextensionfollowedbyimmediate
flexiontoreturnthefingertothesurfaceonwhichthehandrested.
ThemovementwasmadeinresponsetoanLEDlightmountedinthe
rearwallofthebox.TheLEDwasfittedindifferentpositionsforleft
andrighthandmovementconditions,sothatitwasalwaysalignedto
themovingfingeratadistanceof~2to3cm.TheLEDlitupfor100ms
at random intervals between 2 and 3.5 s, in order to prevent
anticipatoryactivation.Subjectswereinstructedtolookattheirfinger
whileattendingtheLEDsignal.
Theexperimentcomprised8blocksof100trials,thatis,2blocksfor
eachof4viewingconditions:directright,directleft,mirrorright,mirror
left.Mirrorrightreferstotheconditionwheretherighthandmovesbut
isseenthroughthemirrorboxasalefthand.Likewise,mirrorleftrefers
to the condition where the left hand moves but is seen through the
mirrorboxasarighthand.The2blocksforthesameconditionwere
alwaysrunconsecutively.Allparticipantsstartedwiththerighthand,but
theorderofmirroranddirectviewconditionswascounterbalanced.
DataAcquisition
EEG was recorded continuously with Ag/AgCl electrodes from 130
scalpelectrodesrelativetocommonmodesenseanddrivenrightleg
electrodes (http://www.biosemi.com/faq/cms&drl.htm) placed adja-
centtothevertexelectrodelocationCz.Theelectrodeswereplaced
accordingto the10-5extensionof theInternational10--20 electrode
systemusinganelasticcap,carefullypositionedrelativetolandmarks
nasion, inion, and preauricular points. Vertical eye movements were
monitoredusingelectrodesFp1andFp2positionedabovetheleftand
righteye.Horizontaleyemovementsweremonitoredbyinspectionof
Figure 1. (A) Touzalin-Chretien et al. derived a ‘‘normal LRP,’’ based on
electrodesFFT9handFFT10hpositionedclosetothelateralcanthusof
measurements during direct view movements of the right hand (first element of
equation)anddirectviewmovementsofthelefthand(secondelementofequation). theleftandrighteye.Electromyographicactivity(EMG)wasrecorded
(B)Forthe‘‘mirrorLRP,’’theyusedthemeasurementsobtainedduringdirectview from left and right musculus extensor indicis by 2 bipolar electrode
movements of the right hand (first element of equation) and subtracted pairswithelectrodesplacedat~2 cmdistancefromeachother.EEG
measurementsduringrighthandmovementsseenthroughamirror(secondelement and EMG signals were amplified with a band-pass of 0--128 Hz by
of equation). Since the normal and mirror LRPs were found to be of identical BioSemiActiveTwoamplifiersandsampledat512Hz.Whilethisdata
amplitude, the data imply that mirror viewed movements reverse the balance of acquisitionrateundersamplestheEMGsignalandrequiredalow-pass
ipsilateralandcontralateralmotorcortexactivation. filtersettingthatattenuatesEMGamplitude,thiswasnotcriticaltothe
analysesthatweperformed.
DataProcessingandAnalysis
For off-line analysis, the EEG data were recalculated to an average
reference montage. To prevent confounds due to differences in
response latency between conditions, lateralized movement-related
EEG activity was analyzed relative to the movement instead of time
lockedtothereactionsignal.Tothispurpose,theEMGdatawerehigh-
passfiltered(10Hz,12dBslope)andrectified.EMGtimemarkerswere
then placed by a statistical algorithm, defining an EMG amplitude
threshold of 4 times the standard deviation of the EMG signal in the
timewindow–200to0msrelativetothereactionsignal(forreviewof
EMGonsetdetectionmethods,seeVanBoxteletal.1993).EMGonset
detectionwasnotsettodetecttheveryearliestactivitybecausethis
levelofprecisionwasnotnecessaryandtendedtodispersetheEMG
peaklatencyinourdata.BasedontheEMGmarkers,theEEGdatawere
segmentedinepochsfrom–300to500msrelativetotheEMGmarker
inordertocreateresponse-lockedaveragespersubjectandcondition.
Individual trials containing eye movements and other artifacts (on
average15±7%)wereremovedbeforeaveraging,basedonindividually
tailored artifact rejection thresholds and visual inspection. Rejection
thresholds varied between ±40 and ±80 lV. Prior to the removal of
artifact contaminated trials, an artifact correction based on principal
component analysis (Ille et al. 2002) was applied to the data of 3
participantswithfrequenteyeblinks.Inaverageddata,thebaselinewas
definedasthetimeperiodfrom–300to–150msrelativetotheEMG
marker. Grand-average waveforms displayed in the figures were low-
Figure2. (A)Mirrorboxusedforthepresentinvestigation.Thehandsarerestingon passfilteredat12Hz.EMGwasanalyzedbycomputingthearea-under-
amirrorsurface,andmirrorsareplacedatanangleof90(cid:2),asindicatedbythelines the-curve of the rectified EMG signal in subject averages. The area
onthebox.Picturedisthepositioningofhandsforthemirrorrightcondition,inwhich measurewascomputedinthewindow0--200msforthemovinghand
therightindexfingerismoved,viewedthroughthemirrorsasthelefthandindex EMG and a window of 0--100 ms for the resting hand EMG, as
finger.Thelefthandisscreened-offfromview.(B)Lookingthroughtheopeningatthe determinedonthebasisofthegrandmeanEMGsignals.
topofthebox,themirrorview(forthiscondition)requirestheparticipanttolookto ThedirectviewandmirrorviewLRPswerederivedasdescribedin
theleftofthemidline. Figure 3. The LRP quantifies motor cortical activity in terms of the
1936 Mirror-InducedMotorCortexActivation d Praamstraetal.
voltage difference measured between electrodes located over the Wefirstinspectedthescalpdistributionofmovement-related
ipsilateralandcontralateralmotorcortex.Consequently,anamplitude lateralized cortical activity by subtracting left and right hand
differenceofmirrorviewcomparedwithdirectviewLRPcanbedueto movementconditions,separatelyforthedirectviewandmirror
anactivationchangeinthemotorcortexcontralateraltothe‘‘viewed’’
view conditions. The resulting plots of the scalp voltage
or the ‘‘acting’’ hand (or both), making the LRP a less than ideal
distributions (see Fig. 4A) thus represent left and right motor
instrument for evaluating mirror-induced motor cortex activation.
Against this background, LRP analyses were complemented with an cortexactivityofoppositepolarity.Thescalpdistributionsare
analysisoftheamplitudevaluesatelectrodesC3andC4thatareused helpfulindemonstratingthatthescalpmaximaofleftandright
tocomputetheLRP,inordertoevaluatetherespectivecontributions motorcorticalactivityarefoundaroundelectrodesC3andC4,
ofipsilateralandcontralateralmotorcortextotheLRP.Tosimplifythe respectively, for both the direct view and the mirror view
presentation of this analysis, left and right hand conditions were
conditions.Thesearetheelectrodesclosesttothemotorcortex
collapsed,separatelyfordirectviewandmirrorviewconditions.This
and generally chosen for computation of the LRP. The scalp
wasdonebytransposingleftandrighthemispheredatainthelefthand
conditiontopermitaveragingwiththerighthandcondition. distributionsalsoestablishthattherearenodifferentialeffectsof
residualeyemovementsdistortingthescalpdistribution.
Figure4Bshowstheresponse-lockedLRP,asrecordedfrom
Results electrodesC3andC4,inthetoppanel.Thesecondpanelshows
the grand-average EMG signals of the moving hand and the
Wehypothesizedthatviewingyourownhandthroughamirror
would not or only minimally influence the lateralization of
primary motor cortex activity, measured by the LRP. That is,
theactivationofthemotorcortexcontralateraltothemoving
hand was not expected to be subject to change. However,
observation of the self-produced movement could induce
additional activity contralateral to the mirror viewed hand.
Thiswouldreducethenetdifferenceinactivationbetweenleft
andrightmotorcortexandthusresultinaloweramplitudeof
themirrorviewLRPcomparedwiththedirectviewLRP.Note
that, in our design, results equivalent to those reported by
Touzalin-Chretien and colleagues would mean not just a
reduction in amplitude of the mirror view LRP but a reversal
inpolarity(see Fig.3).
Figure3. ComputationofthedirectviewandmirrorviewLRPsinthecurrentstudy. Figure 4. (A) Scalp distribution of direct view and mirror view movement-related
ThemethodforcomputingtheLRPdiffersfromthatinFigure1,toclarifythelogicof activity. The scalp distributions are based on a subtraction of left and right hand
thederivation(seeSupplementaryMaterials).PanelBdepictsopposingpredictions movementdata(leftminusright).(B)DirectviewandmirrorviewLRPsmeasuredat
forthemirrorviewLRP:motorcortexactivationuninfluencedbytheviewofthehand C3/C4.EMGactivityofmusculusextensorindicis,combinedacrossleftandrightside
(continuousline)versusmotorcortexactivationfullydeterminedbytheviewofthe movements,forthemovingandtherestinghand(scalebarforrestinghandEMGis
hand.Themirrorinthemidsagittalplanerepresentsthemirrorbox. 5lV).Time0iswheretheEMGsignalcrossedthedefinedamplitudethreshold.
CerebralCortexAugust2011,V21N8 1937
resting hand. There was no difference between direct and
mirrorviewconditionsforeitherthemovinghand(t =0.42,P=
8
0.69)ortherestinghand(t =0.001,P=0.99).TheidenticalEMG
8
signalsforthemovinghand,inthedirectviewandmirrorview
conditions, ensure that any difference in cerebral activation is
not due to differences in force, amplitude, or duration of the
movements. Identical EMG signals for the resting hand ensure
that the comparison of the LRP in the direct view and in the
mirrorviewconditionsisnotinfluencedbyagreatertendency
to synkinesia of the resting hand in the latter condition. Fol-
lowinginspectionoftheEMGsignals,theLRPwasstatistically
evaluated in terms of its mean amplitude around the peak
latency(40--60ms).Althoughasmalldifferenceinpeakampli-
tude between conditions is visible in Figure 4B, this was not
statisticallysignificant(t =0.76,P=0.47).
8
Note that later in their time course, the direct view and
mirror view LRPs diverge more robustly. We therefore also
compared their amplitude in a time window of 190--210 ms
coincidingwithasecondpeakinthedirectviewLRP(seeFig.
4B). As supported by the scalp distribution (not shown), the
second phase probably represents motor cortex activation
related to the flexion of the index finger, returning it to a
resting position following brisk extension. The lower ampli-
tude of this second phase of the LRP may be explained by
temporal jitter, by the movement being more automatic than
intentional, and by the movement being less forceful than the
preceding extension. That is, participants were instructed to
elevatetheindexfingerbrisklyandthenletitdropbackorat
least avoid an active tapping movement. The amplitude dif-
ferencebetweendirectviewandmirrorviewLRPs,duringthis
secondphase, was significant (t =3.29, P <0.05).
8 Figure5. (A)Dataaveragedacrossrightandlefthandmovements,afterleft--right
Whether the second phase of the mirror view LRP was
transpositionofthedataforthelefthandcondition.Contralateralwaveformsshow2
attenuated compared with the direct view LRP as a result of phases corresponding to motor cortex activation for index finger extension and
mirror-inducedmotorcortexactivationcannotbedetermined flexion, respectively. At the peak of the first phase, focal activation of the motor
from the LRP data alone. We therefore reviewed the direct cortex can be distinguished in the scalp voltage distribution, shown for the mirror
view condition (B). Contralateral and ipsilateral waveforms show higher amplitude
and mirror view conditions in the (combined) right and left
movement-relatedactivityforthemirrorviewcondition.Subtractingmirrorviewand
hand data sets on which the LRP was based. As illustrated in directviewconditionsrevealstheexcessactivityinthemirrorviewconditiontobe
Figure 5A, the waveforms recorded contralateral to the side maximal over the ipsilateral hemisphere (C), indicatingmirror-induced activation of
of movement show 2 separate peaks, coincident with the 2 ipsilateral motor cortex. Time 0 is where the EMG signal crossed the defined
amplitude threshold. Electrodes marked in white are C3 (contralateral) and C4
phases of the LRP and representing movement-related activity
(ipsilateral).
associated with the index finger extension and flexion move-
ments, respectively. Comparing the mirror and direct view phase. Accordingly, the view by hemisphere interaction was
conditionsreveals asustainedhigheramplitude forthemirror notsignificantinthistimewindow(40--60ms)(F =1.63,P=
1,8
viewcondition.Inaddition, atthelatencyofthesecondpeak, 0.273), while the effect of view only approached significance
theamplitudesatcontralateralandipsilateralelectrodesitesC3 (F =4.73,P =0.061).
1,8
andC4arealmostidenticalforthemirrorviewcondition,while TheaboveanalysiscomplementstheanalysisoftheLRPin2
there is a marked asymmetry for the direct view condition. important ways. First, the distinct enhancement of sensorimo-
Amplitude values at these electrodes were analyzed with a 2- torcortexactivityinthemirrorviewconditionimpliesthatany
way analysis of variance with factors view (direct vs. mirror) null effects in the analysis of the LRP cannot be attributed to
and hemisphere (contralateral vs. ipsilateral). At the latency the paradigm being ineffective. Secondly, the enhancement
coinciding with the second LRP peak (190--210 ms), this favored the ipsilateral hemisphere in the time window of the
analysis revealed a significant effect of view (F1,8 = 17.11, P = flexion movement, thus decreasing the amplitude difference
0.01) due tothehigher amplitude activity forthe mirrorview between contralateral and ipsilateral motor cortex activation.
condition.Atthesametime,therewasasignificantinteraction It therefore explains the attenuated amplitude of the mirror
of view by hemisphere (F1,8 = 10.82, P = 0.011) due to the viewLRPinthistimewindow,relativetothedirectviewLRP.
amplitude difference being larger over the ipsilateral than the The attenuation of the ‘‘late’’ mirror view LRP is thus revealed
contralateral hemisphere. This is illustrated in the scalp aslikely being due tomirror-induced motor cortexactivation.
topography of the subtraction mirror view minus direct view
datainFigure5C.Notethattheamplitudedifferencebetween
Discussion
direct and mirror view conditions already starts in the time
window of the first LRP phase and accounts for the small In recent publications, Touzalin-Chretien and coworkers
(nonsignificant) amplitude difference found in this first LRP reported movement-related EEG potential data suggesting a
1938 Mirror-InducedMotorCortexActivation d Praamstraetal.
complete reversal of ipsilateral and contralateral motor cortex self-produced. Hence, in our experiment, a stronger sense of
activation when self-produced hand movements are observed agency for the extension than for the flexion movement may
throughamirror.Theresultsofthepresentinvestigationrefute haveinhibitedtheipsilateralmotorcortexmoreeffectivelyfor
such a reversal. During the performance of brisk index finger extension thanfor flexion movements.
extensionsinresponsetoavisualsignal,thebalanceofipsilateral Against the background of established action observation
and contralateral motor cortex activation was not significantly effects on motor cortical activity, motor cortex activation
affected by whether the movement was viewed directly or induced by self-produced movements observed through
throughamirror.Weproposethattheopposingresultsaredueto a mirror is not lacking credibility. The phenomenon is
themethodologicalimprovementsofourapproach,butwewill interestinginitsownrightandhaspossibleclinicalapplication
brieflyconsiderwhetherdifferencesintaskmayberesponsible. (RamachandranandAltschuler2009),albeitnotunambiguously
Touzalin-Chretienetal.usedachoiceresponsetask,whereas supported(Moseleyetal.2008;Ezendametal.2009).Wehave
we used a simple response task. Although the use of a choice shown here, however, that recent neurophysiological demon-
response(betweenfingersofthesamehand)wasnotexplicitly strations of mirror-induced motor cortex activation cannot be
motivated,itcouldberelevantthatthistaskmayinvolvestronger taken at face value. If not artifactual, the sheer magnitude of
recruitmentofnonprimarymotorcortexthanasimpleresponse the contested effects implies extreme flexibility of motor
task. In combination with the fact that premotor rather than corticalactivity.Whilesucheffectswouldseemgoodnewsfor
primary motor cortex is found activated in imaging studies of neurological rehabilitation, the implied degree of flexibility
actionobservation (e.g.,Buccino et al.2001),one couldargue might well beincompatible with stable function.
thatachoiceresponseismoreconducivetoelicitingobserva-
tion-inducedipsilateralmotorcortexactivation.Inresponseto
Supplementary Material
this argument, we should point out that a premotor cortex
Supplementary material can be found at: http://www.cercor.
contribution to the LRP is hypothetical. Premotor cortex
oxfordjournals.org/
activation is also less strongly lateralized than primary motor
cortexactivity(Tanjietal.1987;Horensteinetal.2009),hence
unlikelytodominatethelateralizationdirectionandmagnitude Funding
oftheLRP.Acceptingthattheprimarymotorcortexismainly RCMissupported bythe Wellcome Trust.
responsiblefortheLRP,thepremotorcortexcouldstillinfluence
the LRP through facilitatory and inhibitory ipsilaterally and Notes
contralaterally directed premotor--motor cortex interactions
We thank Dagmar Fraser and Michael Hanson for technical support.
(Koch et al. 2006). Such interactions are typically invoked in
ConflictofInterest:Nonedeclared.
between-hand choice, however, which does not apply here.
Takentogether,theuseofachoice—insteadofsimpleresponse
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