一种基于小波的眼伪影校正的脑电图去噪技术毕业论文外文翻译.docx
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一种基于小波的眼伪影校正的脑电图去噪技术毕业论文外文翻译.docx
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一种基于小波的眼伪影校正的脑电图去噪技术毕业论文外文翻译
AWAVELETBASEDDE-NOISINGTECHNIQUEFOR
OCULARARTIFACTCORRECTIONOFTHEELECTROENCEPHALOGRAM
TatjanaZikov,StéphaneBibian,GuyA.Dumont,MihaiHuzmezan
DepartmentofElectricalandComputerEngineering,theUniversityofBritishColumbia,BC,CANADA.
Abstract–Thispaperinvestigatesawaveletbaseddenoisingoftheelectroencephalogram(EEG)signaltocorrectforthepresenceoftheocularartifact(OA).Theproposedtechniqueisbasedonanover-completewaveletexpansionoftheEEGasfollows:
i)astationarywavelettransform(SWT)isappliedtothecorruptedEEG;ii)thethresholdingofthecoefficientsinthelowerfrequencybandsisperformed;iii)thede-noisedsignalisreconstructed.ThispaperdemonstratesthepotentialoftheproposedtechniqueforsuccessfulOAcorrection.Theadvantageoverconventionalmethodsisthatthereisnoneedfortherecordingoftheelectrooculogram(EOG)signalitself.Theapproachworksbothforeyeblinksandeyemovements.Hence,thereisnoneedtodiscriminatebetweendifferentartifacts.Toallowforapropercomparison,thecontaminatedEEGsignalsaswellasthecorrectedsignalsarepresentedtogetherwiththeircorrespondingpowerspectra.
Keywords-stationarywavelettransform(SWT),electroencephalogram(EEG),electrooculogram(EOG),ocularartifact(OA).
I.INTRODUCTION
Theelectroencephalogram(EEG)givesresearchersanon-invasiveinsightintotheintricacyofthehumanbrain.Itisavaluabletoolforcliniciansinnumerousapplications,fromthediagnosisofneurologicaldisorders,totheclinicalmonitoringofdepthofanesthesia.Forawakehealthysubject,normalEEGamplitudeisintheorderof20-50μV.TheEEGisverysusceptibletovariousartifacts,causingproblemsforanalysisandinterpretation.Incurrentdataacquisition,eyemovementandblinkrelatedartifactsareoftendominantoverotherelectrophysiologicalcontaminatingsignals(e.g.heartandmuscleactivity,headandbodymovements),aswellasexternalinterferencesduetopowersources.Eyemovementsandblinksproducealargeelectricalsignalaroundtheeyes(intheorderofmV),knownaselectrooculogram(EOG),whichspreadsacrossthescalpandcontaminatestheEEG.Thesecontaminatingpotentialsarecommonlyreferredtoasocularartifact(OA).TherejectionofepochscontaminatedwithOAusuallyleadstoasubstantiallossofdata.Askingsubjectsnottoblinkormovetheireyesortokeeptheireyesshutandstill,isoftenunrealisticorinadequate.ThefactthatthesubjectisconcentratingonfulfillingtheserequirementsmightitselfinfluencehisEEG.Hence,devisingamethodforsuccessfulremovalofocularartifacts(OA)fromEEGrecordingshasbeenandstillisamajorchallenge.
Widelyusedtime-domainregressionmethodsinvolvethesubtractionofsomeportionoftherecordedEOGfromtheEEG[1,2].Theyassumethatthepropagationofocularpotentialsisvolumeconducted,frequencyindependentandwithoutanytimedelay.However,Gasseretal.in[3]arguedthatthescalpisnotaperfectvolumeconductor,andthus,attenuatessomefrequenciesmorethanothers.Consequently,frequencydomainregressionwasproposed.Inaddition,nosignificanttimedelaywasfound,whichwasinconsistencywiththeEOGbeingvolumeconducted.
In[4]itwasreportedthat,inreality,thefrequencydependencedoesnotseemtobeverypronounced,whiletheassumptionofnomeasurabledelaywasconfirmed.Thus,whilesomeresearcherssupportthefrequencydomainapproachforEOGcorrection[3,5],othersdisputeditsadvantages[4,6,7].However,neithertimenorfrequencyregressiontechniquestakeintoaccountthepropagationofthebrainsignalsintotherecordedEOG.ThusaportionofrelevantEEGsignalisalwayscancelledoutalongwiththeartifact.Further,thesetechniquesmainlyusedifferentcorrectioncoefficientsforeyeblinksversuseyemovements.TheyalsoheavilydependontheregressingEOGchannel.
Inaddition,CroftandBarry[7]demonstratedthatthepropagationoftheEOGacrossthescalpisconstantwithrespecttoocularartifacttypesandfrequencies.Theyproposedamoresophisticatedregressionmethod(thealigned-artifactaveragesolution)thatcorrectsblinksandeyemovementartifactstogether,andmadepossibletheadequatecorrectionforposteriorsites[6].TheyclaimthattheinfluenceoftheEEG-to-EOGpropagationhasbeenminimizedintheirmethod.
InanattempttoovercometheproblemoftheEEG-to-EOGpropagation,amultiplesourceeyecorrectionmethodhasbeenproposedbyBergandScherg[8].Inthismethod,theOAwasestimatedbasedonthesourceeyeactivityratherthantheEOGsignal.Themethodinvolvesobtaininganaccurateestimateofthespatialdistributionoftheeyeactivityfromcalibrationdata,whichisaratherdifficulttask.
Duetoitsdecorrelationefficiency,theprincipalcomponentanalysis(PCA)hasbeenappliedforOAremovalfromthemulti-channelEEGanditoutperformedthepreviouslymentionedmethods.However,ithasbeenshownthatPCAcannotfullyseparateOAfromtheEEGwhencomparableamplitudesareencountered[9].
Recently,independentcomponentanalysis(ICA)hasdemonstratedasuperiorpotentialfortheremovalofawidevarietyofartifactsfromtheEEG[10,11],eveninacaseofcomparableamplitudes.ICAsimultaneouslyandlinearlyunmixedmulti-channelscalprecordingsintoindependentcomponents,thatareoftenphysiologicallyplausible.Also,thereisnoneedforareferencechannelcorrespondingtoeachartifactsource.However,ICAartifactremovalisnotyetfullyautomatedandrequiresvisualinspectionoftheindependentcomponentsinordertodecidetheirremoval.
Otherattemptshavebeenbasedondifferentadaptivesignalprocessingtechniques[12-16].TheperformanceofthesemethodsalsoreliesonacerebralactivitytominimallycontaminatetheEOGreference.
TheEEGmaycontainpathologicalsignals,whichresembleOA.Thus,thesesignalsaremostlikelytoberemovedfromtheEEGrecordings,leadingtoerroneousdiagnosis.Therefore,itisimportanttodistinguishbetweenartifactsandpathologicalEEGsignalspriortoartifactremoval.Artificialintelligencetechniquesprovetobesomehowhelpfulinachievingthisgoal[17].
Ouraimistopresentareal-timeOAremovaltechnique,basedonstationarywavelettransform(SWT)de-noisingofasinglefrontalchannelEEG.Theproposedtechniqueisbasedonanover-completewaveletexpansionoftheEEGasfollows:
i)astationarywavelettransformisappliedtothecorruptedEEG;ii)thethresholdingofthecoefficientsinthelowerfrequencybandsisperformed;iii)thede-noisedsignalisreconstructed.NoreferenceEOGchannelisneededandthesameapproachisusedforboththeblinksandeyemovementartifacts.
ThetimeandfrequencycharacteristicsofOAareaddressedinSectionII,whileSectionIIIdiscussestheproposedmethod.ResultsarefurtherpresentedinSectionIV.
II.OCULARARTIFACTS
Therearetwodifferentoriginatingphenomenaforocularpotentials[1,18,19].Thereisapotentialdifferenceofabout100mVbetweenapositivelychargedcorneaandnegativeretinaofthehumaneye,thusforminganelectricaldipole(i.e.corneo-retinaldipole).Firstly,therotationoftheeyeballresultsinchangesoftheelectricalfieldacrosstheskull.Secondly,eyeblinksareusuallynotassociatedwithocularrotation;however,theeyelidspickupthepositivepotentialastheyslideoverthecornea.Thiscreatesanelectricalfieldthatisalsopropagatedthroughtheskull.Hence,ocularpotentialsspreadacrossthescalpandsuperimposeontheEEG.
Themechanismoforigin(eyemovementsversuseyeblinks)andthedirectionofeyemovementsdeterminetheresultingEOGwaveshape.Vertical,horizontalandroundeyemovementsusuallyresultinsquare-shapedEOGwaveforms,whileblinksarespike-likewaves.
Fig.1UncontaminatedBaselineEEGandVariousArtifacts
(a)UncontaminatedbaselineEEG(b)EEGcontaminatedwithslowblinkartifact(c)EEGcontaminatedwithfastblinkartifact(d)EEGcontaminatedwithverticaleyemovementartifact(e)EEGcontaminatedwithhorizontaleyemovementartifact(f)EEGcontaminatedwithroundeyemovementartifact
Ocularartifactsdecreaserapidlywiththedistancefromtheeyes[18].Therefore,themostsevereinterferenceoccursintheEEGrecordedbytheelectrodesplacedonthepatient'sforehead.Yet,thisisthemostconvenientregionfortheirplacement.Thereto,thefrontalandprefrontallobes,whichareattheoriginofhighercognitivefunctions,arelocateddirectlybehindtheforehead.Therefore,thetaskofEOGcorrectionforfrontalchannelsischallenging.
Forthepurposeofthispaper,wehaveacquiredEEGdatafromanawakehealthymalesubject.Asinglefrontalchannelwasrecorded,correspondingtotheFpzelectrodeplacementinthenomenclatureoftheInternational10-20System.ThebaselineEEGandfivevariousartifactswererecordedinthefollowingfashion.Foreachartifact,thesubjectwasfirstinstructedtokeephiseyesshutandstill.SixtysecondsofpresumablyuncontaminatedbaselineEEGwasthusrecorded.Then,forthenext60seconds,thesubjectwasinstructedtoblinkormovehiseyesinapredeterminedfashion.Finally,anotherrestingperiodof60secondswithnoEOGactivitywasrecorded.Fiveocularartifactswererecordedinthisfashion:
slowblinks(1blinkper2seconds),fastblinks(2blinkspersecond),vertical,horizontalandroundeyemovements.Thesignalwasnotchfilteredat50-60Hzandsampledat128Hz.
Fig.2PowerSpectraofUncontaminatedBaselineEEGandVariousArtifacts
(a)UncontaminatedbaselineEEGandotherrestingperiods(b)EEGcontaminatedwithslowblinkartifact(c)EE
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