Effects of Light-Emitting Diode Light on Human Electroencephalogram in Comparison with Fluorescent Light

Gwan-Taek Lee1, Chany Lee1, Daeyoung Kim2, HyunTaek Kim3, SungHo Woo3 and Ki-Young Jung1

1Department of Neurology, Korea University Medical Center, Korea University College of Medicine, Seoul, 2Department of Neurology, Samsung Seoul Hospital, Seoul,3Department of Psychology, College of Liberal Arts, Korea University, Seoul, Korea

Received November21,2012Revised December19,2012Accepted December19,2012

Address for correspondenceKi-YoungJung,MDDepartmentofNeurology,KoreaUniversityMedicalCenter,KoreaUniversityCollegeofMedicine,73Inchon-ro,Seongbuk-gu,Seoul136-705,KoreaTel:+82-2-920-6649Fax:+82-2-925-2472E-mail:jungky@korea.ac.kr

Objectives: Selectingsuitableilluminationisanintegralpartofincreasingproductivityintheofficeorfac-tory,becausepoorlightingconditionsmayoftencausedecreasedworkefficiency.Lightemittingdiode(LED)lightisbecomingrecognizedasoneofthemostpromisinggeneralsourcesofillumination.Wecon-ductedspectralpoweranalysisofelectroencephalograms(EEGs)obtainedduringrestingandcognitivetaskactivitiestoidentifytheeffectsonhumanarousalandcognitiveperformanceunderLEDlightcomparedwithconventionalfluorescentlight.Methods: Thirteenhealthy,right-handedstudentsparticipatedinthepresentstudy.Eachsubjecttookpartintwoexperimentalsessions,oneunderfluorescentandoneunderLEDlightingconditions.TheexperimentalmeasurementsconsistedofarestingstateEEG,anevent-relatedpotential(ERP)duringavisualworkingmemory(VWM)task,andaquestionnaireaboutsubjectivefeel-ingsregardingthelightingconditions.TheEEGpowerspectra,theamplitudeandlatencyoftheP300ERPcomponent,thebehavioralresponsesfortheVWMtask,andthequestionnairedatawerecomparedforthetwolightingconditions.Results: TheEEGspectralpowershowednodifferencebetweentheLEDandflu-orescentlightingconditions.TheamplitudeoftheP300componentdecreasedsignificantlywithincreasingnumbersofitems,whiletherewerenodifferencesbetweenthetwolightingconditions.Additionally,be-havioralresponsesandsubjectivefeelingswerethesameunderthetwolightingconditions.Conclusions: OurstudysuggeststhattherearenosignificantdifferencesbetweenLEDlightandfluorescentlightonthehumanarousalstateandVWM. J Korean Sleep Res Soc 2012;9:28-33

Key Words: LED,Fluorescent,EEG,Visualworkingmemory,P300,ERP.

28 Copyright © 2012 Korean Sleep Research Society

ORIGINAL ARTICLEJ Korean Sleep Res Soc 2012;9:28-33 ISSN 1738-608X

Introduction

Wecannotimagineanyworkingenvironmentwithoutil-luminationinmodernsociety,becausehumansgreatlyrelyonopticalinformationfortheirproductivity.Workplaceswithpoorlightingconditionsoftencauselaborerstobest-ressedoreventohaveseriousworkaccidents.Thisoccursbecauselightexertsnotonlyvisualeffectsbutalsononvisualeffectsonnumerousphysiologicalvariables,suchasthehu-mansleep-wakecycleandcognitiveperformance,primarilythroughpropertiessuchasdose,duration,timing,andwave-length.1Thus,selectingsuitableilluminationisanintegralpartofincreasingproductivityintheofficeorfactory.

Althoughfluorescentlampsarestillthedominantsourceofartificialilluminationinmodernlivingenvironments,lightemittingdiodes(LEDs)havebeguntoattractinterestasoneofthemostpromisingcandidatesforsourcesofgeneralillumin-

ationinthenearfuture.Infact,accordingtoamarketresear-chreportfromBCCResearch,theglobalmarketforLEDli-ghtsnoticeablyincreasedbymorethan85%duringthepe-riod2005-2009.2LEDsprovidetechnicaladvantagesoverfluorescentlamps,suchaslowerpowerconsumption,longerlifetime,greatereco-friendlinessduetonotusingmercury,andeasierprocessingduetotheirsmallsizeandadaptableshape.3

Itiswellknownthatlightingcancauseacuteemotional,be-havioral,andcognitivechanges.4-6Additionally,alargebodyofresearchshowsthechroniceffectsoflightingonsleepandneu-rocognitivefunction.1,7,8Theseeffectsoflightmaydependonparameterssuchasintensity,wavelength,andsourceoflight.Thus,priortoashifttonewalightingsource(i.e.,LEDlight)inartificialluminousenvironments,issuesofwhetheritmightaf-fecthumanphysiologyorcognitionshouldbeconsideredtoensurethatworkplaceefficiencyaremaintained.Toourknow-ledge,onlyonestudyhasreportedonthisissue,findingthat

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LEDlighthadpositiveeffectsonworkerbehavioralperfor-manceduringseveralcognitivetasks,comparedwithtradi-tionalfluorescentlamps.9However,sincethestudydidnotev-aluatetheconcurrentelectrophysiologicalresponsesofbrainactivity,informationprocessingatacorticallevelwasnotde-monstrated.

Electroencephalography(EEG)provideasensitivemeansofmeasuringpsychologicalaswellasphysiologicalhumanst-ates.TherestingstateEEGreflectsaparticularlyimportantst-ateofarousal,whichcanbecharacterizedusingfrequencyan-alysis.Inaddition,event-relatedpotentials(ERPs)provideaneurophysiologicalindexofasubject’scognitivefunctioning.Becausethelightingconditionexertsadirectinfluenceonvisualfunction,theERPparadigmusingvisualstimuliisusedtoinvestigatethelights’effects.Specifically,ERPmeasure-ments,suchasexaminingtheP300andbehavioralresponsesduringthevisualworkingmemory(VWM)task,allowobjec-tiveassessmentsofcognitionandbehaviorthroughvisualfunctioning.Thus,thestudyofEEGsandERPsincombina-tionappearstobeusefulforexploringhumanneurophysiol-ogyreflectingarousalstates,cognitions,andtaskperform-ance.Nevertheless,nostudyhasyetaddressedtheelectro-physiologicaldifferencesbetweenLEDandfluorescentlight.

Inthepresentstudy,toidentifytheeffectsofLEDlightonEEGsduringrestandduringperformanceofacognitivetask,comparedwithconventionalfluorescentlight,weanalyzedthespectralpowerofEEGsobtainedduringrestingstatesandERPduringVWMtasks.

Methods

SettingTheexperimentstookplaceinalaboratorywiththewin-

dowcompletelycoveredwithblackcurtainstopreventout-sidelightfromenteringtheroom.Fourtube-straightthree-bandfluorescentlamps(FLU)andfoursimilarlyshapedbi-nary-complementarywhiteLEDlampswerealternatelypl-acedtwometersabovethefloor.ThespectrairradianceofeachlightisshowninFig.1.Eachsubjectunderwenttheexpe-rimentalprocedureswhilesittinginacomfortablechairab-outonemeterfromtheinstalledlighting.

Experimental procedure

Thirteenhealthy,right-handeduniversitystudents(sixma-les,7females;aged23.3±1.6years)participatedinthisstudy.Eachsubjectgavewritteninformedconsenttoparticipate.TheexperimentalprotocolwasapprovedbytheInstitutionalRe-viewBoardofKoreaUniversityMedicalCenter.Participantsvisitedourlaboratoryat19:00.Eachsubjectperformedtwoexperimentalsessions,oneundereachlightingcondition:

fluorescentandLED.OnesessionconsistedofspontaneousEEGfor10minutes,ERPduringaVWMtaskfor15minutes,andaquestionnairetoobtainthevisualanaloguescore(VAS).Aresttimeof5minuteswasgivenbetweenthetwosessions.Atthebeginningofeachsession,theilluminancewasadjust-edsothatthetwolightingconditionswerematched,andthencolortemperaturewasmeasured.TheorderoffluorescentandLEDsessionswascounterbalancedamongtheparticipantstocontroltheordereffect.

The resting state EEG

TheEEGswererecordedusinga64-channeldigitalEEGmachine(GrassNeurodataAcquisitionSystem,GrassTech-nologies,Quincy,MA,USA)withacapelectrode(Quick-Cap,CompumedicsNeuroscan,Charlotte,NC,USA).Therefer-enceelectrodewassettolinkedearlobes,impedancewaskeptbelow10kΩ,andtheband-passfiltersettingwas0.3-70Hzwithasamplingrateof1600Hz.Twoelectrooculographych-annels(placedontheleftandrightoutercanthi)wereaddedtoconfirmeyeballmovements.

TherestingstateEEGofeachsubjectwasreviewedforof-flineanalysis,and10artifact-free2-sepochsintheeye-openstatewereselectedpersubject.EachepochwastransformedintothefrequencydomainusingFastFourierTransforms,andthenthepowerspectraldensityfunctionforeachsubjectwasevaluated.Absolutepowerforthefivefrequencybands(delta:1-3Hz,theta:4-7Hz,alpha:8-12Hz,beta:13-30Hz,andgam-ma:30-45Hz)wasdeterminedatFz,Cz,andPzelectrodes.

Visual working memory task

AVWMtaskwasusedtoestimateparticipants’behavioralandcognitiveresponsesundereachlightingcondition.Allstimuliwerepresentedona17-inchLCDmonitorwithagreybackgroundusingcommercialsoftware(PRESENTATION;Neurobehavioralsystems,Berkeley,CA,USA).Eachpartici-

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pantunderwent300trialsineachcondition.Thetestarraywasshownfor2s,at1safterthememoryarrayonsetineachtrial.Participantswereinstructedtoindicatewhetherornottwoarrayswereidenticalbypushingoneoftwobuttonsasquicklyaspossibleaftertheywerepresentedwiththetestar-ray.Astimulusarrayconsistedof1,2,3,4,8or12itemsthatwerecoloredsquares.Thecolorofeachitemwasrandomlyselectedfromred,blue,violet,green,yellow,black,orwhite.Itempositionswerealsorandomizedineachtrial.Thecolorofonlyoneiteminthetestarraywasdifferentfromthecorre-spondingiteminthememoryarrayin50%ofthetrials.Theywereidenticalintheremainingtrials.

Event-relatedpotentialepochswereextractedatbetween-200and+1200msafterthememoryarrayonsetandsavedforlateroff-lineanalysis.Baselineswerecorrectedbysubtract-ingtherootmeansquareofthepre-stimulusintervalfromthewholeepochlengths.OnlythosetrialswithcorrectresponseswereincludedintheERPanalysis.Independentcomponentanalysiswasappliedtocorrectstereotypedocularandmus-cularartifacts(Jungetal.2000).ERPepochswereaveragedfordifferentnumbersofitemsinthememoryarrayseparate-ly.ERPlatenciesandamplitudesweremeasuredrelativetotheirpre-stimulusbaseline.TheP300componentwasdefinedaspointswithpositivepeakamplitudesbetween300and500msatthePzrecordingsite.TheamplitudesoftheP300com-ponentswereaveragedoverthe±25mstimewindowrelativetothelatencyofeachpeak.

Questionnaire

Subjectivebrightness,visualfatigue,andsatisfaction(from0to10points)wascheckedasvisualanaloguescale(VAS)inthequestionnaireforsubjectstoassesseachlightingconditionimmediatelyaftertheERPtasks.

Statistical analysisElectroencephalograms,ERP,andbehavioralresponses

wereanalyzedbyrepeatedmeasuresanalysisofvariance(AN-OVA).Thewithin-subjectvariablesofthespectralpowerforeachfrequencyband(delta,theta,alpha,beta,andgamma)oftherestingstateEEGwerechannel(threelevels:Fz,Cz,andPz)andlight(twolevels:fluorescentandLED).TheamplitudeandlatencyoftheP300componentduringVWMtaskingwereeachanalyzedatthePzrecordingsite;thewithin-sub-jectvariableswerenumberofitems(sixlevels:1,2,3,4,8,and12)andlight(twolevels:fluorescentandLED).Forthehitrateandthereactiontimeofbehavioralresponses,thewithin-subjectvariableswereidenticaltothosefortheP300analysis.TheGreenhouse-GeissercorrectionwasusedtoevaluateFra-tiosinordertocontrolforType1errorintherepeatedmea-suresdesign.Bonferroniposthoctestswereusedtoidentifythesourcesofsignificantvariance.Thequestionnaireresultswereanalyzedusingpairedt-testsforcomparisonswithun-equalvarianceforthefluorescentandLEDlights.Thedepen-dentvariablesweretheVASscoresofbrightness,visualfa-tigue,andsatisfactionbasedonthequestionnaires.Statisticalsignificancewasdefinedaspvaluesof<0.05.

Results

ThefluorescentandLEDlightinghadcolortemperaturesof8690and6660K,respectively.Lightintensitieswerema-tched,with340luxinbothconditions.

EEG power spectra

ElectroencephalogramsspectralpowersforeachfrequencybandatthethreemidlineelectrodesareshowninFig.2.Foreachfrequencyband,ANOVArevealedthatlightshowedno

Fig. 2. Mean electroencephalogram spectral power within each frequency band for all subjects at Fz, Cz, and Pz electrodes (δ: 1-3 Hz, θ: 4-7 Hz, α: 8-12 Hz, β: 13-30 Hz, γ: 30-45 Hz).

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significantmaineffect,althoughsignificantmaineffectwasobservedwithchannel.Theinteractionbetweenchannelandlightwasnotsignificant(Table1).

ERP analysis

Event-relatedpotentialwaveformsatthePzelectrodesiteareshowninFig.3.TheP300componentswereidentifiedap-proximately350msafterthememoryarrayonset.Thenum-berofitemsshowedasignificantmaineffectontheP300am-plitudes(F2.11,23.26=9.97,p=0.001),whilelightdidnot(F1.0,11.0=0.04,p>0.05),andtheinteractionbetweenthenumberofitemsandthelightdidnot(F3.17,34.92=1.45,p>0.05).Thenum-berofitems(F3.39,37.33=1.32,p>0.05),thelight(F1.0,11.0=3.1,p>0.05),andtheinteractionofnumberofitemsandlight(F3.35,

36.83=0.59,p>0.05)showednosignificanteffectsonlatency.

Behavioral responsesFig.4.showsthechangeinbehavioralresponsesasthe

numberofitemsincreased,withasignificantmaineffectonhitrate(F2.62,28.83=199.17,p<0.001)andreactiontime(F1.8,19.8=

24.73,p<0.001).Thelightshowedneithersignificantmainef-fectonhitrate(F1.0,11.0=3.82,p>0.05)norreactiontime(F1.0,11.0=

0.02,p>0.05),andtheinteractionbetweenlightandnumberofitemsalsoshowednosignificanteffectonbehavioralre-sponses(hitrate:F2.44,26.83=1.52,p>0.05;reactiontime:F2.08,22.85=1.09,p>0.05).

Questionnaire

Theresultsofthesubjectiveassessmentoflightingcondi-tionsareshowninTable2.PairwisecomparisonsoftheVASscoresforbrightness,visualfatigue,andsatisfactionshowednosignificantdifferencesbetweenLEDandfluorescentlight-

Table 1. Summary of analysis of variance of electroencephalogram power spectra

FactorChannel Light Channel×Light

F p F p F pDelta 10.662 0.006 2.121 NS 0.328 NSTheta 9.706 0.003 1.011 NS 6.485 0.020Alpha 10.751 0.001 0.031 NS 3.065 NSBeta 3.356 0.087 0.016 NS 1.216 NSGamma 3.352 NS 0.853 NS 0.633 NS

NS:notsignificant

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ingconditions.

Discussion

Inthisstudy,weanalyzedtheEEGspectralpowermeasuredduringarestingstate,ERPcomponentselicitedbyaVWMtask,andsubjectivedataobtainedwithaquestionnaireinor-dertocomparetheeffectsofLEDandfluorescentlightonhumancognitions,behaviors,andsubjectivefeelings.Inourstudy,theintensityoflightwasmatchedbetweentwolight-ingconditions.Althoughthecolortemperaturecouldnotbematchedperfectlyduetooriginaldifferencesinthespectralirradianceofthetwolightsources,thisisunlikelytoaffecttheresultsbecausethespectralcompositionhasnoeffectontheperformanceofsimplecognitivetasks.10)

TheEEGreflectsaparticularlyimportantphysiologicalstateofarousalwhichcanbecharacterizedusingfrequencyan-alysis.11)Inparticular,alphaoscillationisasensitivemeasureofattentionaldemandsincognitivetasks,12)andfurther,fluore-scentlightwithlowfrequencyballastshassignificantlydecr-easedalphapowercomparedwithhighfrequencyballasts.6)Nevertheless,wefundnosignificantdifferencesinEEGspec-tralpoweralongallfrequencybands,betweentheLEDandfl-uorescentlightconditionsinourstudy.ThisindicatesthatLEDlightingdoesnotdisturbhumanattention,comparedwithconventionallighting.

TheamplitudeandlatencyoftheP300aresensitivetotaskprocessingdemandsandthetimerequiredfordetectingandevaluatingastimulus.Theseindexesvarywithindividualdif-ferencesincognitivecapability.13)OurfindingsindicatethatincreasingthenumberofitemssignificantlydecreasesP300amplitude;however,thelightingconditiondidnotaffectP300

amplitudeorlatency.Similarly,thehitrateandreactiontimewerenotchangedbythelightingcondition,buttheywereaf-fectedbythetaskdifficulty.Inaddition,VASscoresforthetwolightingconditionswerenotsignificantlydifferent,whichmeansthatthesubjectsreportednonoticeabledifferencesinsubjectivefeelingsregardingbrightness,visualfatigue,andsatisfactionbetweenfluorescentandLEDlighting.

Hawesetal.9)recentlyreportedthatLEDlightingmayhavepositiveimplicationsforworkperformance,comparedtotraditionalfluorescentlighting.However,thisfindingcouldleadtomisunderstandingbecausethefluorescentlightingus-edintheirexperimenthadlowerluminanceandcolortem-peraturethantheLEDlighting.Incontrast,ourresultsindi-catethattheLEDlightingdoesnotsignificantlyeffectoncog-nitiveandbehavioralperformance.Inconclusion,ourfind-ingssuggestthattherearenodifferencesintheeffectsonthehumanarousalstateorcognitiveperformancebetweenthelightingconditionsprovidedbyLEDandfluorescentlightsources.Asweinvestigatedonlyshort-termeffectwithrelati-velysmallsample,long-termeffectofLEDlightshouldbestu-diedfurtherinthefuture.

AcknowledgmentsThisworkwassupportedbyIndustrialStrategicTechnologyDevelop-

mentProgram(ProjectNo:10037416,EstablishmentofinfrastructureforLED-marineconvergencetechnologysupportandtechnologydevelop-mentforcommercialization)fundedbytheMinistryofKnowledgeEcon-omy(MKE,Korea).

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