Guidelines for using human event-related potentials to study - AIP

26 Pages·2000·0.2 MB·English

Psychophysiology,37~2000!,127–152.CambridgeUniversityPress.PrintedintheUSA.
Copyright©2000SocietyforPsychophysiologicalResearch
COMMITTEE REPORT
Guidelines for using human event-related potentials
to study cognition: Recording standards
and publication criteria
T.W. PICTON,a S. BENTIN,b P. BERG,c E. DONCHIN,d S.A. HILLYARD,e R. JOHNSON, JR.,f
G.A. MILLER,g W. RITTER,h D.S. RUCHKIN,i M.D. RUGG,j and M.J. TAYLORk
aRotmanResearchInstitute,BaycrestCentreforGeriatricCare,Toronto,Canada
bDepartmentofPsychology,HebrewUniversityofJerusalem,MountScopus,Jerusalem,Israel
cDepartmentofPsychology,UniversityofKonstanz,Konstanz,Germany
dDepartmentofPsychology,UniversityofIllinois,Champaign,USA
eDepartmentofNeuroscience,UniversityofCaliforniaatSanDiego,LaJolla,USA
fDepartmentofPsychology,QueensCollege,CUNY,Flushing,NewYork,USA
gDepartmentsofPsychologyandPsychiatry,UniversityofIllinois,Champaign,Illinois,USA
hDepartmentofNeuroscience,AlbertEinsteinCollegeofMedicine,Bronx,NewYork,USA
iDepartmentofPhysiology,UniversityofMaryland,Baltimore,USA
jInstituteofCognitiveNeuroscience,UniversityofLondon,England
kCentredeRechercheCerveauetCognition,UniversitéPaulSabatier,Toulouse,France
Abstract
Event-related potentials ~ERPs! recorded from the human scalp can provide important information about how the
humanbrainnormallyprocessesinformationandabouthowthisprocessingmaygoawryinneurologicalorpsychiatric
disorders.ScientistsusingorstudyingERPsmuststrivetoovercomethemanytechnicalproblemsthatcanoccurinthe
recordingandanalysisofthesepotentials.ThemethodsandtheresultsoftheseERPstudiesmustbepublishedinaway
thatallowsotherscientiststounderstandexactlywhatwasdonesothattheycan,ifnecessary,replicatetheexperiments.
Thedatamustthenbeanalyzedandpresentedinawaythatallowsdifferentstudiestobecomparedreadily.Thispaper
presentsguidelinesforrecordingERPsandcriteriaforpublishingtheresults.
Descriptors:Event-relatedpotentials,Methods,Artifacts,Measurement,Statistics
Event-related potentials ~ERPs! are voltage fluctuations that are surements is on the order of milliseconds, ERPs can accurately
associatedintimewithsomephysicalormentaloccurrence.These measurewhenprocessingactivitiestakeplaceinthehumanbrain.
potentials can be recorded from the human scalp and extracted The spatial resolution of ERP measurements is limited both by
from the ongoing electroencephalogram ~EEG! by means of fil- theoryandbyourpresenttechnology,butmultichannelrecordings
tering and signal averaging.Although ERPs can be evaluated in canallowustoestimatetheintracerebrallocationsofthesecere-
both frequency and time domains, these particular guidelines are bralprocesses.Thetemporalandspatialinformationprovidedby
concerned with ERPs recorded in the time domain, that is, as ERPsmaybeusedinmanydifferentresearchprograms,withgoals
waveforms that plot the change in voltage as a function of time. thatrangefromunderstandinghowthebrainimplementsthemind
These waveforms contain components that span a continuum be- tomakingspecificdiagnosesinmedicineorpsychology.
tween the exogenous potentials ~obligatory responses determined Datacannothavescientificvalueunlesstheyarepublishedfor
bythephysicalcharacteristicsoftheelicitingeventintheexternal evaluation and replication by other scientists. These ERP guide-
world!andtheendogenouspotentials~manifestationsofinforma- linesarethereforephrasedprimarilyintermsofpublicationcrite-
tionprocessinginthebrainthatmayormaynotbeinvokedbythe ria.Thescientificendeavorconsistsofthreemainsteps,andthese
eliciting event!.1 Because the temporal resolution of these mea- mapwellontothesectionsofthepublishedpaper.Thefirststepis
themostimportantbuttheleastwellunderstood—thediscoveryof
Addressreprintrequeststo:TerenceW.Picton,RotmanResearchIn-
stitute, Baycrest Centre for Geriatric Care, Toronto, Ontario, M6A2E1, class of potentials that display stable time relationships to a definable
Canada.E-mail:[email protected]. referenceevent”~Vaughan,1969!.Thispaperusestheterm“event-related
1In recent years, there has been a tendency to use the term “event- potentials”toincludebothevokedandemittedpotentials.Evokedpoten-
relatedpotentials”tomeantheendogenouspotentialsandtodifferentiate tialscanbeeitherexogenousorendogenous~orboth!.Emittedpotentials
theevent-relatedpotentialsfromthe~exogenous!“evokedpotentials.”How- ~always endogenous! can be recorded when a cognitive process occurs
ever, this is not what the words mean logically and is certainly not the independentlyofanyspecificevokingevent~e.g.,whenadecisionismade
original meaning of the term “event-related potentials” as “the general oraresponseinitiated!.
127
128 T.W.Pictonetal.
some new way of looking at the world. This step derives from A.FormulationoftheStudy
creativeprocessesthatareprobablysimilartothoseusedtosolve
(i)TheRationalefortheStudyMustBePresentedClearly
problemsinotherdomains~Langley,Simon,Bradshaw,&Zytkow,
The rationale for an experimental study usually derives from a
1987!.Unfortunately,thisstepisoftentheleastdocumentedaspect
reviewoftheliterature,whicheithershowsimportantgapsinour
ofascientificstudy.Whereverpossible,theintroductiontoapaper
knowledgeorleadstoareinterpretationofknownfactsintermsof
should therefore try to describe how the authors arrived at their
a new theory. These two situations require further experiment,
hypothesesaswellassimplystatingthem.Thesecondstepinthe
eithertofillinthegapsortotestthenewtheory.Itisessentialto
scientificprocessinvolvesthedesignofanexperimentorasetof
communicatetherationaleclearlytothereaderssothattheymay
experimentstotestthehypotheses.Settinguptheexperimentsto
seethepurposeandsignificanceofthestudy.Itisnotsufficientto
provide information that convincingly tests the hypotheses and
state that the experiments are intended to clarify something in
rules out other competing hypotheses requires clarity of thought
physiology or psychology without specifying what is to be clari-
andeleganceofdesign.Thethirdstepinvolvesthecarefultesting
fiedandwhysuchclarificationisimportant.BecauseERPstudies
of the hypotheses. Scientific statements are valid as long as they
relatetobothphysiologyandpsychology,termsandconceptsspe-
arenotfalsifiedwhentested~Popper,1968!.Themethodsandthe
cific to one field should be explained ~e.g., linguistic categories,
results of an experimental paper provide the details of how this
chemicalsusedtoevokeolfactoryERPs!.
testingwascarriedoutandwhatresultswereobtained.Becausethe
resultsofanexperimentaltestmaybetheconsequenceofafailure
inthemethodorofnoiseinthemeasurement,theauthorsmustper- (ii)TheHypothesesoftheExperiment(s)Should
suadethereaderthatthemeasurementswerevalid,accurate,andre- BeStatedClearly
liable.Thediscussionsectionofthepaperreturnstothecreativepart Specifichypothesesandpredictionsabouttheexperimentalresults
ofscience.Thenewfindingsmustberelatedtootherpublishedre- mustbederivedfromtherationale.Thesehypothesesandpredic-
sults.Viewsoftheworldthathavebeenclearlyfalsifiedbythenew tionsshouldbestatedinpositivetermseventhoughthestatistical
findingsshouldbesummarized.Newviewsjustifiedbythefind- testswillexaminenullhypotheses.ThefirstchapterofthePubli-
ingsmustbeclearlyworkedoutandformulatedforfuturetesting. cationManualoftheAmericanPsychologicalAssociation~Amer-
ThecompilationofthepresentguidelineswasinitiatedbyJohn ican PsychologicalAssociation, 1994! provides useful advice for
CacioppowhenhewaspresidentoftheSocietyforPsychophysi- settingouttherationaleandhypothesesforanexperimentalstudy.
ological Research in 1993. A complementary set of guidelines Althoughtrueforallareasofresearch,looselymotivated“shots
exists for recording the EEG in research contexts ~Pivik et al., inthedark”areparticularlydangerousinstudiesinwhichdataare
1993!. Draft ERPguidelines were then proposed, discussed, and abundant.TheoverwhelmingamountofERPdataalongthetime
revisedbytheauthorsofthisreport.Thepaperalsobenefitedfrom andscalp-distributiondimensionscaneasilyleadtoincorrectpost
thecommentsandsuggestionsoffouranonymousreviewers.These hocconclusionsbasedontrial-and-erroranalysesofmultipletime
ERPguidelinesupdatethosederivingfromtheInternationalSym- epochs and electrode sites. Huge arrays of data make it easy to
posiumonCerebralEvokedPotentialsinManheldinBrusselsin obtain “significant” results that are not justified in theory or reli-
1974~Donchinetal.,1977!.Sincethen,severalsetsofguidelines able on replication. Hypotheses should therefore describe partic-
havebeendevelopedforrecordingexogenousevokedpotentialsin ular ERPmeasurements ~e.g., that the experimental manipulation
clinicalcontexts~AmericanEncephalographicSociety,1994a;Hal- willincreasethelatencyoftheP300wave!ratherthannonspecific
liday,1983!,butnoneofthesehasspecificallyconsideredERPsin ERPchanges~e.g.,thattheexperimentalmanipulationwillchange
relation to normal and abnormal human cognition.Although put theERPinsomeway!.
together under the aegis of the Society for Psychophysiological
Research,theseERPguidelinesshouldapplytopaperspublished (iii)AsaGeneralRule,TasksShouldBeDesignedSpecifically
anywhere.Itisthescientist’sresponsibilitytoselectapublication toElicittheCognitiveProcessesBeingStudied
venuethatcancommunicatehisorherfindingstotheappropriate If relating attributes of the ERPto cognition is desired, the ERP
audienceandtoensurethattherationale,method,results,analysis, shouldberecordedinanexperimentalparadigmthatcanbeinter-
andconclusionsofthestudyarepresentedproperly. preted in terms of the information processing invoked and exer-
The guidelines or recommendations are stated in the titles to cised in the paradigm. To demonstrate ERP concomitants of
each subsection of this paper. The paragraph or paragraphs fol- particularcognitiveprocesses,theERPsshouldberecordedwhen
lowingthesetitlesexplainthecommittee’sreasonsfortheguide- theseprocessesareactive~andtheiractivitycanbeshownthrough
lines and provide advice and suggestions about ERP procedures behavioral measurements!. It is unlikely ~although possible! that
that can be used to follow them. Although mainly addressed to anERPmeasurementrecordedwhenasubjectperformsapartic-
scientistswhoarebeginningtouseERPstostudycognition,these ular task will turn out to be a specific marker for a cognitive
guidelinesshouldhelpallwhoworkwithERPstorecordtheirdata process that does not occur during the task. This result would
andcommunicatetheirresultsmoreeffectively.Theguidelinesuse require that whatever affects the cognitive process independently
the following codes to indicate committee agreement: “must” in- affectstheERPmeasurement.
dicates that the committee agreed unanimously that the guideline Experimental paradigms that have been well studied, and for
applies in all cases, and “should” indicates that the committee which well-developed cognitive models are available, provide a
agreed unanimously that the guideline applies in most situations goodframeworkforthestudyofERPs.Standardparadigmsused
~andthattheinvestigatorshouldbeabletojustifywhytheguide- by investigators of memory, attention, or decision making will
lineisnotfollowed!.Guidelinesaboutspecifictechniquesclearly more likely lead to useful mappings of ERP data on cognitive
applyonlyifthisparticulartechniqueisused.Someoftheguide- modelsthannewparadigms.However,novelparadigmscanyield
lines,suchasthoseconcerningtherationaleforthestudyandthe exciting and useful results, provided the investigators can also
discussionoftheresults,arenotlimitedtoERPstudies,although presentacarefullydevelopedmodeloftheparadigmininformation-
theyareparticularlyimportantinthisfield. processingterms.
ERPguidelines 129
Historically, the most frequently used ERP paradigm has in- if the investigators want to consider processing resources they
volvedthedetectionofanimprobabletargetstimulusinatrainof shouldobtaindataforareceiver-operatingcurve,andiftheywant
standardstimuli.This“oddball”paradigmelicitslargeERPcom- toaddressspeedandaccuracy,theyshouldhaveclearbehavioral
ponents,andprovidesusefulinformationabouthowthebraindis- datashowingtheeffectsofchangingresponsespeedonperformance.
criminatesstimuliandevaluatesprobability.Thisparadigmcanbe Insomeexperiments,ERPsareusedasarelativelyunobtrusive
adaptedtothestudyofothercognitiveprocessessuchasmemory monitorofcerebralprocesseswithouttheneedforrecordingovert
and language. However, it is often better to use paradigms more responses.AclassicexampleismeasuringtheERPstounattended
specifictotheseprocessesthantoforcetheoddballparadigmtofit stimuli.Thismeasurementcanindicatehowthesestimuliarepro-
the processes. Nevertheless, many other paradigms share charac- cessed without the need to ask for overt responses to the un-
teristicsoftheoddballtask,anditisessentialtoconsiderwhether attendedstimuli,whichcouldclearlydisruptthefocusofattention.
the ERPs recorded in these paradigms can be interpreted more Instudiesofautomaticprocesses,ERPscanbeusedtoassessthe
parsimoniously in terms of oddball parameters ~e.g., probability brain’s responses to stimuli without these stimuli evoking ~either
anddiscriminability!thanintermsofotherprocesses.Toprevent perceptually or electrically! controlled cognitive responses. For
confounding the effects of probability with other experimental example, the mismatch negativity is best recorded when the sub-
variables, the investigator should therefore keep the probabilities ject is not attending to the auditory stimuli. When the subject
ofstimulus0responsecategoriesconstantwithinandacrossrecord- attends to the stimuli, the mismatch negativity is difficult to rec-
ingconditions. ognizeduetothesuperimpositionofotherERPcomponentssuch
Afinalaspectofthisrecommendationisthatthetasksshould as the N2b or P300. When the subject does not attend to the
be adapted to the subjects studied. When studying language in stimuli,adescriptionofwhatthesubjectisdoing~e.g.,readinga
children,forexample,researchersmusttakeintoconsiderationthe book!mustbeprovided,andwherepossiblethisactivityshouldbe
languagelevelofthesubjects,andnotusevocabularythatwould monitored. It is usually better to have the subject perform some
betooadvancedfortheyoungerchildren.Whenstudyingsubjects taskratherthanjustlistenpassively.IncaseswhereintheERPsare
withdisorderedcognition,itisprobablyworthwhiletoadjustthe recorded without any attention to the stimuli or behavioral re-
difficultyofthetasktotheircognitivelevel.Ifthesubjectscannot sponses,additionalstudiesrecordingonlybehavioralresponses~or
perform a task, it is difficult to determine if the absence of par- both behavioral and electrical responses! can be helpful in deter-
ticular ERPs are associated with the cause of their cognitive dis- miningthetimingandthedifficultyofsensorydiscrimination.For
order or simply the result of the task not being performed. The example, the investigator must demonstrate that the stimuli are
tasksneedtobeofshorterdurationforclinicalanddevelopmental equally difficult to discriminate before concluding that particular
studies than for ERPstudies in normal young adults, because at- typesofdevianceelicitmismatchnegativitieswithdifferentlaten-
tention span is generally shorter in clinical patients or children. ciesoramplitudes~Deoull&Bentin,1998!.
When studying clinical groups, the experimenter can decide to ERP studies of language ~Kutas, 1997; Kutas & Van Petten,
keepthetaskthesameortoadjustthetasksothattheperformance 1994! provide a clear example where recording behavioral re-
isequivalentbetweentheclinicalpatientsandthenormalcontrol sponsesatthesametimeastheERPsmaybecounterproductive.
subjects ~e.g., Holcomb et al., 1995!. When the stimuli are the Many language processing activities occur without explicit rela-
same,theresultsbearmoreondifferencesinsensoryprocessing; tiontoanyassignedtask,andmanystudiesofsemanticprocessing
when the difficulty is the same, the results are more related to havebeenperformedinthecontextofgeneralinstructionsto“read
cognitiveprocesses.Arelatedproblemconcernswhethertocom- silently”or“listen”~whichdonotyieldaccuracyorreactiontime
pare ERPs only on trials for which performance is correct. Al- @RT# data!. Indeed, many tasks in behavioral psycholinguistics
though it is probably best to compare ERPs for both correct and ~e.g.,lexicaldecision!arereallysecondarytasksthatdonotoccur
incorrect performance across the subject groupings, this compar- innaturallanguageprocessing.OneclearbenefitoftheERPmethod
ison is often impossible unless the task is adjusted so that the isthatsuchartificialtaskscanbedropped.Adetrimenttoinclud-
accuracy of performance is similar across the groups. These and ingsuchtasksisthattheyelicitdecision-relatedP300s,whichmay
otherissuesofhowtocomparegroupswithdifferentabilitieshave obscureotherERPcomponentssuchastheN400wave~seeKutas
beendiscussedextensivelybyChapmanandChapman~1973!. & Hillyard, 1989; Kutas & Van Petten, 1994!. However, even
when no overt responses are being made, it is still important to
(iv)TheSubject’sBehaviorintheExperimental specify as much as possible what the subject is doing during the
ParadigmShouldBeAssessed ERPrecordings.Becauseitisoftenimportanttoacquireaccuracy
When using ERPs to evaluate the cerebral processes that occur andRTdatainordertocompareERPresultswiththebehavioral
duringcognition,theexperimentershouldusuallymonitorbehav- literatureusingtaskssuchaslexicaldecisionandnaming~whichis
ioralresponsesatthesametimeasthephysiologicalresponsesare incompatiblewithERPrecordingduetoartifactscausedbytongue
recorded, provided that this comonitoring can be done without movements and muscle activity!, a useful strategy has been to
excessive artifactual contamination of the recordings. In many conduct a behavioral study first, followed by an ERPstudy with
perceptual tasks, a simple motor response to a detected target the same stimuli. In other cases, it has been of some interest to
provides a measure of the speed and accuracy of perceptual per- compare ERP data obtained under general “read” or “listen” in-
formance. In memory tasks, simple yes-no recognition perfor- structionswiththoseobtainedwithanoverttaskthatforcesatten-
mancemeasuresarehelpfulnotonlyinmonitoringthatencoding tion to some aspect of the stimuli. Such comparisons can reveal
andretrievalareoccurring,butalsoinaveragingERPsatencoding whichaspectsofstimuliareprocessedautomaticallyversusthose
onthebasisoflaterretrieval.Ingeneral,themorebehavioraldata thatareoptional.Forinstance,thesecomparisonshaveshownthat
thatareavailable,themorereadilythepsychophysiologicalmea- sentence semantic congruity effects occur independently of the
sures can be evaluated within the context of an information- assignedtask~Connolly,Stewart,&Phillips,1990!,butthatrhym-
processingmodel.Thetypeofbehavioraldatacollectedwilldepend ing effects for visually presented word pairs occur only when
onthetypeofcorrelationsthatmaybehypothesized.Forexample, rhymemonitoringistheassignedtask~Rugg&Barrett,1987!.
130 T.W.Pictonetal.
(v)SubjectStrategiesShouldBeControlledbyInstructionand clinicalconditionsthatmightimpedeinformedconsent,theexper-
ExperimentalDesign,andShouldBeEvaluatedbyDebriefing imentershouldconsiderpublishedguidelinesforobtainingsubsti-
Perhaps the most difficult variables to bring under experimental tute consent from family or caretakers ~e.g., Keyserlingk, Glass,
controlarethecognitivestrategiesandmentalprocessesunderly- Kogan,&Gauthier,1995!.Whenthesubjectsareunder18years
ing the performance of the subject. It is therefore essential to old,theinvestigatorshouldobtaininformedconsentfromthechild’s
describeindetailhowthesubjectsareinstructedabouttheexper- guardians and provide information to the child at a level that the
imental situation and task. In situations in which subjects are re- child might understand ~Van Eys, 1978!. Academic and clinical
spondingactivelytothestimuli,thereportshouldclarifywhether institutionsspecifyhowtherightsofhumansubjectsareprotected
thesubjectshavebeentoldtoemphasizeresponsespeedoraccu- andhavecommitteestoapproveresearchprotocolsandtomonitor
racy, and which motivating instructions and0or tangible rewards theresearchasitproceeds.Investigatorsmustfollowtheinstruc-
were used. In conditions in which subjects are asked to ignore tionsofthesecommittees.
auditoryorsomatosensorystimuli,itisgenerallydesirabletogive
thematasktoperform~e.g.,readabook,solveapuzzle!inorder (ii)TheNumberofSubjectsinEachExperiment
tohavesomecontroloverwhatthesubjectactuallydoes.When- MustBeGiven
everpossibleitisadvisabletouseataskwithmeasurableconse- The number of subjects in an experiment must be sufficient to
quencessothatthedegreetowhichthesubjectsactuallyundertake allow statistical tests to demonstrate the experimental effects and
theassignedtaskcanbeassessed.Ageneraldescriptionofthetask to support generalization of the results. The number of subjects
situationsuchas“passivelistening”or“reading”isnotadequatein requiredtodemonstrateaparticularsizeofeffectcanbeestimated
experimentsinwhichstatevariablescouldaffecttheERPs. using evaluations of statistical power. In addition to being suffi-
In general, explicit and consistent instructions to subjects can cienttodemonstrateanexperimentaleffect,thesamplesizemust
minimize the “subject option” ~Sutton, 1969! to react to the situ- also be large enough to represent the population over which the
ationinanidiosyncraticanduncontrolledfashion.Debriefingthe results are to be generalized. Because ERPdata can vary consid-
subjects after the experiment can provide information about how erably from one subject to the next, it is often advisable when
theyviewedthetaskandwhatcognitivestrategiestheyused.De- using small numbers of subjects to sample from a population as
briefing can be done by simply asking subjects how they per- homogenous as possible, for example, in terms of age, gender,
formedthetaskorbyusingaformalquestionnairethatdescribes educational level, and handedness. This method can, of course,
thepossiblestrategiesthatmighthavebeenused.Nottoaskone’s limitthegeneralizabilityoftheresults.
subjectswhattheyweredoinginanexperimentindicatesafaithin Thetotalnumberofsubjectsrecruitedandthereasonsfornot
one’s experimental paradigm that may not be justified. Relations beingabletoincludealloftheminthefinalresults~e.g.,artifacts,
amongtheERPmeasurements,thebehavioraldata,andthesesub- incompleterecordings!shouldbedescribed.Comparedwithstud-
jectivereportscanhelptheinvestigatorinterpretwhatwasgoing ies of normal young adults, developmental and clinical studies
on during the task and to test specific hypotheses about how the often have a higher number of subjects who cannot be tested
subjectsinterpretedthetask. successfully.Inthesestudies,itisparticularlyimportanttodocu-
mentthereasons~e.g.,lackofcooperation,inabilitytounderstand
(vi)TheOrderingofExperimentalConditions orcompletethetask!,becausethesereasonsmayhavesomebear-
MustBeControlledandSpecified ingonwhatcanbegeneralizedfromtheresults.
Thewayinwhichthetrialsforeachofthedifferentexperimental
conditionsareputtogetherintoblocksmustbedescribedclearly. (iii)TheAgeRangesofSubjectsParticipating
Differentexperimentalconditionscanoccurinseparateblocksor inERPExperimentsMustBeProvided
can be combined within blocks. For example, attention can be Because many ERPs change with age, the mean and range of
studiedbyhavingsubjectsattendtostimuliinoneblockoftrials subject ages must be provided. The normal adult age range for
andignoretheminaseparateblockoftrials~blockdesign!,orby mostERPstudiescanbeconsideredas18–40years.2Whencom-
having subjects attend to some of the stimuli in one block while paring ERPs across groups of subjects, ages should be balanced
ignoringothersinthesameblock~mixeddesign!.Theamountof across the groups ~unless, of course, age is one of the variables
time required for each block of trials and the sequence in which under study!. Subjects older than the age of 40 years should be
theblocksaredeliveredmustbespecified.Manyaspectsofbehav- stratifiedintodecades.
iorandmanycomponentsoftheERPchangeovertime,andsuch In subjects younger than the age of 18 years, significant ERP
changesmustnotbeconfoundedwiththeexperimentalmanipula- changescanoccurovershorttimeperiods~Friedman,1991;Stauder,
tions.Itisthereforeadvisabletobalanceexperimentalconditions Molenaar,&vanderMolen,1993;Taylor,1988,1995!.Theyoun-
overtimeeitherwithineachsubjectoracrosssubjects.Timeisbut ger the children, the more marked are these age-related changes.
oneofmanyfactorsthatmustbecontrolled.Cognitivebehavioris Thus,itisimportanttousenarroweragerangesthanforadults.In
veryflexibleandheavilyinfluencedbycontext.Becausethegen- infants and young children ~,24 months! researchers should use
eralworkinghypothesisisthatdifferentcognitiveprocessesareas- 1-monthranges,recordingatseveralpointsintime~e.g.,6months,
sociatedwithdifferentERPs,cognitiveelectrophysiologicalstudies 12months,and18months!ratherthanaveragingacrossevenafew
shouldexertthesamescrupulouscontrolofexperimentaldesignas months.Inolderchildren,1-yearagegroupingsarerecommended,
requiredinexperimentalpsychologywhenstudyingcognition. although 2-year groupings are acceptable over the age of 8 years
and3-yeargroupingsareacceptableamongteenagers.
B.Subjects
2Significantdifferencescanoccurevenwithintheagerangeof18–40
(i)InformedConsentMustBeDocumented
years.Ingroupstudiesitissometimeshelpfultouseageasacovariateto
Informedconsentisessentialforanyresearchwithhumansubjects decreasethenoiselevelsacrossgroups~providedthereisnocorrelation
~Faden, Beauchamp, & King, 1986!. In the case of patients with betweenageandtheexperimentalgroups!.
ERPguidelines 131
(iv)TheGenderoftheSubjectsMustBeReported (vii)ClinicalSubjectsShouldBeSelectedAccordingtoClear
Becausegenderaffectsmanyelectrophysiologicalmeasurements, DiagnosticCriteriaandtheClinicalSamplesShouldBeMade
theinvestigatormustreporthowmanyofthesubjectsweremale asHomogeneousasPossible
and female, and must ensure that any group effects are not con- Theselectioncriteriaforclinicalsubjectsshouldbeexplicitlystated.
founded by differences in the female0male ratios across groups. TheDiagnosticandStatisticalManualoftheAmericanPsychiat-
Whenstudyingnormalsubjects,theinvestigatorshouldgenerally ricAssociation~AmericanPsychiatricAssociation,1994!provides
useeitherasimilarnumberoffemaleandmalesubjectsorsubjects criteriaformostpsychiatricdisorders.Diagnosticcriteriaforneuro-
ofonegenderonly.Itisoftenworthwhiletoincludegenderasan logicaldisorderscanbefoundintherelevantliterature.Whenthe
experimentalvariable.Iftheexperimentcomparesnormalsubjects clinical disorder is heterogeneous ~e.g., schizophrenia, attention
withsubjectswithaclinicaldisorderthatismorecommoninone deficit disorders!, the experimenter should attempt to limit the
gender,themale0femaleratiosshouldbeapproximatelyequivalent subjectstooneofthevarioussubtypesofthedisorderortostratify
acrossthetwogroups. the patient sample according to the subtypes.The sample should
also be made as homogeneous as possible in terms of both the
durationandtheseverityofthediseaseprocess.Itisneverpossible
(v)SensoryandMotorAbilitiesShouldBeDescribed
todevisepurepatientgroupings.Nevertheless,someattemptshould
fortheStimuliBeingPresentedandthe
bemadetolimitheterogeneityandanyresidualsourcesofhetero-
ResponsesBeingRecorded
geneity should be described. In addition, the sample should be
This recommendation is to ensure that subjects can perceive the
characterized carefully with respect to demographic and psycho-
stimulinormally.Formoststudiesofnormalyoungsubjects,itis
metricvariables.Forexample,inastudyofpatientswithdementia
sufficienttodocumentthatallsubjectsreportednormalhearingor
of Alzheimer type, the investigator should include information
vision ~with correction!. Such self-report is usually correct about
about the age and gender of each subject, along with data on
normal sensory ability. However, the accuracy of self-report will
current mental status ~e.g., Mini-Mental State Examination!, pre-
depend on the type of questions asked. The answer to “Do you
morbidintelligence~e.g.,NationalAdultReadingTest!,andmem-
havenormalhearing?”ismuchlessinformativethananswerstoa
oryfunction~e.g.,selectedsubtestsoftheWechslerMemoryScale!.
setofquestionsabouthearingunderdifferentsituations~Coren&
Forpatientswithfocalbrainlesions,suchdatashouldalsoinclude
Hakstian,1992!.
detailed information about the location and nature of the lesions.
In experiments designed specifically to evaluate perceptual
function, particularly in studies of disordered perception, more
(viii)MedicationsUsedbySubjectsShouldBeDocumented
intensive evaluations should be used to clarify what is normal or
In ERP studies of normal subjects, the investigator should make
to categorize levels of abnormality. For auditory stimuli, sub-
surethatthesubjectsarenottakingprescriptionmedicationsthat
jects should be screened for normal hearing at 20 dB HL at the
mayaffectcognitiveprocesses.Itisprobablyalsoworthwhilenot
frequencies tested. For visual stimuli, acuity should be measured
tousesubjectswhohavetakenalcoholorotherrecreationaldrugs
~withrefractivecorrection!atadistanceappropriateforthestim-
withinthepreceding24hours.Becauseclinicalpatientsarecom-
uli used. Because most visual stimuli are presented at close dis-
monlytreatedwithmedications,itisoftendifficulttodisentangle
tances, acuity normally would be checked using Jaeger rather
theeffectsoftheclinicaldisorderfromtheeffectsofthetreatment.
than Snellen charts. If stimulus color is manipulated during the
Wherever possible, some control for medication should be at-
experiment, color vision should be checked ~e.g., using one or
tempted.Insomecasesunmedicatedpatientscanbestudied.Ifthe
several Ishihara plates!. Unfortunately, there are no widely ac-
patientshavevariousdosagesofmedication,thelevelofmedica-
cepted quantitative screening tests for normal somatic, taste, or
tionshouldbeconsideredinthestatisticalanalysis,orpreferably
smell sensations.
in the experimental design ~e.g., selecting different subgroups of
Whensubjectsaremakingmotorresponsesduringtheexperi-
patientswithdifferentmedicationlevels!.Unfortunately,itisnot
mentalparadigms,theinvestigatorshouldprovidesomebasicde-
possibletouseananalysisofcovariancetoremovetheeffectsof
scription of the subjects’ability to perform the task. It is usually
different medication levels ~or other variables! from other group
sufficienttoensurethatthesubjectsreportnohistoryofweakness.
differences~Chapman&Chapman,1973,pp.82–83;Miller,Chap-
Inallstudiesusingmotorresponses,thehandednessofthesubject
man,&Isaacks,submitted!.Ananalysisofcovariancecanbeused
should be reported and preferably measured using a validated
toreducethevariabilityofmeasurementsingroupsthatvaryran-
questionnaire.
domly on the variable used as a covariate. However, if groups
differ on each of two variables, covarying out the effects of one
(vi)TheSubjects’CognitiveAbilitiesRelevant variablewilldistortanymeasurementoftheeffectoftheother.
totheTasksBeingStudiedShouldBeDescribed
The experimenter should provide some basic assessments of the (ix)InClinicalStudies,ControlSubjectsShouldBeChosen
subjects’ability to perform the tasks being evaluated. In normal soThatTheyDifferFromtheExperimentalSubjects
subjects, the educational level is a reliable indicator of general OnlyontheParametersBeingInvestigated
cognitive abilities, and descriptions of the subjects such as “un- The selection criteria for the control subjects should be stated
dergraduate students” is sufficient. However, this approach is in- clearly,asshouldthevariablesonwhichthecontrolsubjectsand
adequateinthecontextofclinicalpatients,children,andtheelderly, patients have been matched. In general, the groups should be
for whom more specific evaluations should be provided. For ex- matched for age, gender, socioeconomic status, and intelligence.
ample, mental status tests should be used when evaluating the Thepremorbidintelligenceofthepatientgroupmaybecompared
ERPsofdementedpatients,standardizedreadingassessmentswhen withtheactualintelligenceofthecontrolgroupusingeducational
ERPparadigmsthatrequirereadingareusedinchildren,andneuro- level or some more formal psychological assessment such as the
psychologicaltestsofmemorywhenERPsareusedtostudymem- NationalAdultReadingTest.Bothcontrolandexperimentalsub-
orydisordersintheelderly. jects should be evaluated on standardized behavioral, psycholog-
132 T.W.Pictonetal.
ical,orneuropsychologicaltests.Thesetestsshoulddocumenthow of a series of discrete pulses as the raster process activates the
thepatientsareequivalenttothecontrolsubjectsinsomeareasbut region of the screen beneath the receptor during each screen re-
notinothers.Exclusioncriteriamustalsobestatedexplicitlyand fresh.Theconversionofthisstimulusintoasustainedvisualsen-
appliedtobothclinicalandcontrolgroups.Inmanycases,ahealthy sationisdescribedinBuseyandLoftus~1994,particularlyAppendix
control group may not be sufficient. Clinical patients with disor- D!.Becausethestimulusiscomposedofdiscretepulses,thereare
ders different from those of the patients being studied are often oftendiscrepanciesbetweentheprogrammedonsetanddurationof
better controls than completely normal subjects. For example, in thestimulusandtheactualstimulusparameters.
studies of the effects of a specific focal brain lesion, a helpful
control group will consist of patients with lesions of a similar (iii)AspectsoftheStimuliRelevanttotheCognitive
etiologybutoutsidethebrainregionofinterest. ProcessesBeingExaminedShouldBeDescribed
When words or other complex stimuli are used, they should be
selected keeping in mind which of their properties might affect
C.StimuliandResponses
theirprocessing.Becausethenumberoftrialsnecessarytorecord
(i)TheStimuliUsedintheExperimentsMustBeSpecified ERPsisusuallylargerthanthenumberoftrialsneededforbehav-
inSufficientDetailThatTheyCanBeReplicated ioral measurements,3 extensive manipulation of stimulus param-
byOtherScientists etersduringanERPparadigmisusuallynotpossibleandextracare
Thestimulimustbedescribedaccuratelyintermsoftheirintensity, during stimulus selection is required. Factors such as familiarity,
duration,andlocation.Theguidelinesforclinicalevokedpotential wordfrequency,andmeaningareofparamountimportancewhen
studies ~American Electroencephalographic Society, 1994a! pro- studyingtheERPstowords.Ifnotmanipulatedintheexperiment,
videcleardescriptionsofthesimplestimuliusedinsuchstudies. thesefactorsshouldbecontrolledrigidlyandkeptconstantacross
Where possible, similar descriptions should be provided for the conditions.Wheneverpossible,thestimulishouldberotatedacross
stimuli used in all ERP studies. An extensive description of the conditions to prevent any inadvertent confounding of some stim-
differentstimulithathavebeenusedinERPstudiesandthewayin ulus parameter with the experimental manipulation.All the rele-
whichthesestimuliaredescribedandcalibratedisgiveninRegan vantstimulusselectioncriteriaandcharacteristicsshouldbereported
~1989,pp.134–155!.Investigatorsusingvideodisplaystopresent ~such as the mean and range of the number of letters, phonemes
visual stimuli can consult Poynton ~1996!.All stimuli should be and syllables composing the words, word frequency, and, where
calibratedintermsoftheirintensityandtimingusingappropriate relevant, the degree of semantic relatedness of the words!. If im-
instrumentation~e.g.,aphotoreceptorforvisualstimuliandami- agesorpicturesareused,theinvestigatorshouldspecifywhether
crophone for auditory stimuli!. It is important to realize that the they are drawings or photographs, black and white or color. A
presentationofastimulusinonemodalitymaybeassociatedwith figureshowingasampleimageorimagesisworthmorethanmany
stimulationinanothermodalityandtheeffectsofthisotherstim- wordsofdescription.Forauditorystimuli,particularlywhenwords
ulusshouldbemasked.Forexample,airpufforstrobeflashstimuli are used, provide the duration ~the range, mean, and standard
are often associated with simultaneous acoustic artifacts. If deci- deviation!andtheobviousmeasuressuchasintensity~root-mean-
bels are used to describe intensity, it is essential to provide the square@RMS#!,frequency,andmaleorfemalevoice.
reference level because decibels are meaningless without the ref-
erence.Commonreferencesintheauditorysystemaresoundpres- (iv)ResponsesMadebytheSubjectsShouldBeDescribed
surelevel~aphysicalreference!,hearinglevel~relativetonormal InmanyERPparadigms,subjectsmakeovertresponseswhiletheir
hearing!andsensationlevel~relativetotheindividual’sthreshold!. ERPs are being recorded. In some paradigms, the ERPs are re-
cordedinreferencetotheseresponsesinsteadoforinadditionto
(ii)TheTimingoftheStimuliMustBeDescribed. the sensory stimuli. The investigator must clarify the stimulus-
The minimum temporal parameters that should be described are responsemappingrequiredduringtheparadigm~e.g.,whichbut-
stimulus duration and the intervals between the stimuli. If the ton was pressed by which finger in response to which kind of
experimentinvolvestrialscontainingmorethanonestimulus,the stimulus! and how this response was manipulated. The nature of
interval between the trials must also be given. The experimenter theresponseshouldbedescribedintermsofthelimbusedtomake
shouldclarifywhethertheintervalsarefromonsettoonset~stim- theresponseandthetypeofmovementmade.Whentheresearch
ulusonsetasynchrony!orfromtheoffsetoftheprecedingstimulus focusesonmotor-relatedresponses,theforce,speed,andextentof
~ortrial!totheonsetofthenext~interstimulusorintertrialinter- themovementsshouldalsobemeasuredandreported.
val!.Ifthesubjectsareexpectedtoexecuteamotorresponseorto
provide a verbal response, the timing of these responses with re-
specttothestimulishouldbespecified.Thestructureofthestim- D.Electrodes
ulus sequences is also an important attribute of the experimental
(i)TheTypeofElectrodeShouldBeSpecified
design.Thus,investigatorsshouldspecifywhethertrialsareiniti-
Becauseelectrodesactasfilters,theyshouldbechosensoasnot
atedbythesubjectorbytheexperimenter.Theyshouldalsospec-
to distort the ERP signals being measured. Nonpolarizable Ag0
ifytherulesbywhichthestimulussequencesaregenerated~e.g.,
AgCl electrodes can accurately record very slow changes in po-
completelyrandomstimuliaccordingtosetprobabilities,orrandom
tential ~e.g., Kutas, 1997; Rösler, Heil, & Hennighausen, 1995!,
stimuli with the proviso that no two targets occur in succession!.
Becausehumansubjectsarecapable~consciouslyorunconsciously!
of picking up regularities and rules of stimulus sequences, subtle 3Clearbehavioralmeasurementscanbeobtainedsometimesonsingle
changesinthesecanleadtoERPeffects. trials~e.g.,yes-nodecisionsaboutwhetherastimuluswasperceived!but
thisisusuallynotpossiblewithERPs.Inbehavioralstudies,usingmore
Timingisaparticularproblemwhenusingavideodisplay.The
subjects often compensates for the smaller number of trials per subject.
investigatorshouldcheckthetimingofthesestimuliusingaphoto- ThismethodisnotcarriedoutinERPstudiesbecauseofthetimeinvolved
receptor.Anapparentlycontinuousstimulusisactuallycomposed inpreparingthesubjectfortherecording.
ERPguidelines 133
althoughprecautionsmustbetakentoeliminatedriftwhenultra- needle or lancet is usually less painful than abrasion and leaves
slow ~less than 0.1 Hz! potentials are recorded ~Tassinary, Geen, visible marks less frequently. The investigator must balance the
Cacioppo,&Edelberg,1990!.Suchslowdriftsinthepolarization needforreducingskinpotentialswiththenecessityofpreventing
of the electrodes can be estimated using linear regression tech- any possibility of infection. Impedances of less than 2 kV occur
niques and then subtracted from the recordings ~Hennighausen, only if the skin layer is effectively breached, which clearly in-
Heil,&Rösler,1993;Simons,Miller,Weerts,&Lang,1982!.For creasestheriskofinfection.Specialcaremustbetakentoprevent
potentialsofhigherfrequency,avarietyofdifferentelectrodema- the transmission of infective agents via the instruments used to
terials ~e.g., gold, tin! may be used. Depending on the electrode reducetheimpedanceorbytheelectrodes.Disposableinstruments
material,thesurfaceareaoftheelectrodeandtheinput-impedance mustbeusedtoabradeorpuncturetheskin,andelectrodesmust
of the amplifier, many electrodes will attenuate the low frequen- be disinfected properly between subjects. Previously published
ciesintherecordedsignal~Picton,Lins,&Scherg,1995!.Because guidelines for reducing the risk of disease transmission in the
manymodernEEGamplifierswithhighinputimpedanceusevery psychophysiology laboratory ~Putnam, Johnson, & Roth, 1992!
lowelectrodecurrents,eventhesepolarizableelectrodescanoften mustbefollowedscrupulously.
beusedtorecordslowpotentialswithoutdistortion.Unfortunately,
itisdifficulttocalibratethefrequencyresponseoftheelectrode– (iii)TheLocationsoftheRecordingElectrodes
skininterfaceandforfrequencieslessthan0.1Hz,nonpolarizable ontheScalpMustBeDescribedClearly
electrodes are recommended. The low-frequency response of an Whenever possible standard electrode positions should be used.
electrode can be estimated in situ by observing the signals re- The most helpful standard nomenclature is the revision of the
cordedduringsustainedeyemovements~Polich&Lawson,1985!. original10-20systemtoa10-10systemasproposedbytheAmer-
The investigator could also estimate the transfer function of the ican Electroencephalographic Society ~1994b!. Electrodes should
electrodesbymeasuringthepotentialswhentheeyesfollowpen- be affixed to the scalp with an accuracy of within 5 mm. Unfor-
dularmovementswiththesameamplitudebutdifferentfrequencies. tunately there is no standardized placement system for electrode
arrays having large numbers of electrodes.The 10-20 system de-
(ii)InterelectrodeImpedancesMustBeReported scribes 75 electrode locations but does not state which of these
The recording electrodes are affixed to the surface of the scalp. shouldbeusedinamontagecontainingasmallernumberofchan-
Subcutaneous needle electrodes should not be used for ERPs be- nelsorhowtolocateelectrodesifmorethan75channelsaretobe
causeoftheriskofinfection.Theconnectivityoftheelectrodeto used. In general, we recommend using approximately equal dis-
the scalp is measured by passing very low currents through the tancesbetweenadjacentelectrodes,andplacingelectrodesbelow
electrodes and measuring the impedance to the flow of current. aswellasabovetheFpz-T7-Oz-T8equator.
These measurements tell the experimenter four things: how ac- Theexactlocationsoftheelectrodescanbedeterminedrelative
curately the amplifier will record the potentials, the liability of tosomefiducialpoints~suchasthenasion,inion,andpreauricular
theelectrodetopickupelectromagneticartifacts,theabilityofthe points defined in the 10-20 system! using a three-dimensional
differential amplifiers to reject common-mode signals, and the digitizer~Echallier,Perrin,&Pernier,1992!.Thesepositionscan
intactnessoftheskinunderlyingtheelectrode.Fortheamplifierto thenbecomparedwiththelocationsofthe10-20systembypro-
recordaccurately,theelectrodeimpedanceshouldbelessthanthe jectingtheselocationsontoasphere~Lütkenhöner,Pantev,&Hoke,
input impedance of the amplifier by a factor of at least 100.The 1990!. This projection onto a sphere is necessary for spherical
higher the impedance of an electrode the greater the effect of spline interpolations and for source analysis using spherical head
electromagneticfields~e.g.,linenoise,noisefromelectricmotors, models.Variousrelationsbetweenthe10-20electrodesystemand
videodisplaysystems!ontherecording.Theseeffectsarecaused theunderlyingbrainhavebeenevaluated~Lagerlundetal.,1993;
mainlybycurrentsinducedintheelectrodecircuits.Thesecurrents Towleetal.,1993!.
vary with the area surrounded by the circuit ~and hence can be Thenewlyemergingdense-arraysystemsthatallowplacement
reduced by braiding the electrode wires together!. Inequalities in of 128 or 256 electrodes present challenges for specifying elec-
the electrode impedance between the two inputs to a differential trode placement as the number of electrodes clearly exceed the
amplifierwillreducetheabilityoftheamplifiertorejectcommon capacityofthe10-20system.Whatevernomenclatureisused,itis
mode signals ~Legatt, 1995!. Finally, electrode impedance mea- important to identify within the dense array landmark electrodes
surestheintactnessoftheskinandthusitsabilitytogenerateskin thatcorrespondtothestandardsiteswithinthe10-20system.
potentials. Cephalic skin potentials are large, slow potentials that
occurwhentheautonomicnervesandsweatglandsintheskinare (iv)ERPsShouldBeRecordedSimultaneouslyFrom
activated by heat or arousal ~Picton & Hillyard, 1972!. They are MultipleScalpElectrodes
mostprominentlyrecordedfromtheforehead,temples,neck,and Insomecases,simpleevokedpotentials~e.g.,thebrainstemauditory-
mastoidregions. evoked potentials! can be adequately examined for clinical pur-
Theinterelectrodeimpedancemeasuredatsomefrequencywithin posesusingasinglerecordingchannel.However,formostERPs,
theERPrange~e.g.,10Hz!shouldbereducedtolessthan10kV simultaneous recording from multiple electrode locations is nec-
by abrading the skin. Electrode–scalp interfaces with higher im- essarytodisentangleoverlappingERPcomponentsonthebasisof
pedancesmayyieldadequaterecordingswhenamplifierswithhigh- theirtopographies,torecognizethecontributionofartifactualpo-
inputimpedancesareusedandwhengoodcommonmoderejection tentials to the ERP waveform, and to measure different compo-
isavailable~Taheri,Knight,&Smith,1994!.Thesesystemscanbe nentsintheERPthatmaybeoptimallyrecordedatdifferentscalp
usedtorecordERPs,butgreatcaremustbetakenininterpreting sites.As examples, recording from parietal electrodes in addition
slow potentials, because skin potential artifacts can occur easily. tofrontocentralelectrodescanhelpdistinguishbetweenmotorand
Toeliminateskinpotentials,theimpedanceatthescalp–electrode re-afferentsomatosensorypotentials;time-lockedblinksareeasily
junctionwillneedtobereduced~byabrasionorskinpuncture!to distinguished from the late positive wave by being maximally
less than 2 kV. Puncturing the skin with a fine sterile disposable recordeddirectlyabovetheeyes;andthemismatchnegativitycan
134 T.W.Pictonetal.
usuallybedistinguishedfromtheN2wavebyitspolarityreversal addunnecessarynoisetograndaverages.Ontheotherhand,ifthe
inearormastoidelectrodes.Manyearlystudiesoftheendogenous channel is omitted, data averaged across conditions ~or across
ERPsusedmidsagittalelectrodes~Fz,Cz,Pz!tomakesomeim- subjects!wouldthenbeavailableonlyforthosechannelsthatwere
portant distinctions among ERP components. However, such lo- recordedinallconditions~orallsubjects!.
cationsarenotappropriateforthevisual-evokedpotentialsorfor Oneusefulsolutiontothisproblemistoestimatethemissing
anylateralizedERPs.Anydevelopmentalstudiesshoulduseboth data, either using linear or spherical spline ~Perrin, Pernier, Ber-
lateral and midline recording electrodes. Midline electrodes are trand,&Echallier,1989!interpolation.Althoughlinearinterpola-
important ~for comparison with both older papers and older sub- tion is mathematically simpler, it has the disadvantages that ~a!
jects!,butindevelopmentalstudiesthelargestage-relatedchanges electrodes at the edge of the array cannot be estimated, and ~b!
areoftenseeninlateralelectrodes~e.g.,Taylor&Smith,1995!. onlyafewadjacentelectrodesareusedtoestimatetheinterpola-
The optimal number of recording channels is not yet known. tion. Using spherical splines, an estimate of the signal at one
Thisnumberwilldependonthespatialfrequenciesthatarepresent missingelectrodelocationismadefromthesignalsatalltheother
inthescalprecordings~Srinivasan,Tucker,&Murias,1998!,pro- electrodes, leading to less sensitivity to noise at individual elec-
videdthatsuchfrequenciesaredeterminedbythegeometryofthe trodes.Missingdataattheedgeoftheelectrodearraymayalsobe
intracerebralgeneratorsandnotbyerrorsinpositioningtheelec- estimated, because the splines assume continuity over the whole
trodesormodelingtheimpedancesofthehead.Theproperuseof ~spherical!head.
high-density electrode arrays requires techniques for accurately Themethodofsphericalsplineshasotherusefulapplications,
measuringthelocationoftheelectrodesandforhandlingtheloss apart from mapping, for which it was originally intended. Using
of one or several recording channels through poor contact. Vari- thesameinterpolationmethod,asetofdatarecordedatdigitized
ance in the placement of the electrodes ~or the measurement of locationscanbe“normalized”togeneratedataatasetofstandard
suchplacements!actsasnoiseinanyanalysisoftopographiesor 10-10 or 10-20 locations. Grand averages can then be generated
intracerebralsources. fromthenormalizeddata.Anotherpossibleapplicationistheau-
tomaticdetectionofbadelectrodes.Datafromeachelectrodeare
(v)TheWayinWhichtheElectrodesAreAffixed comparedwiththeestimatecomputedfromtheotherelectrodes.A
totheScalpShouldBeDescribed badelectrode0signalisdetectedwhenthedifferencesbetweenthe
Thehairpresentsthemajorprobleminkeepingelectrodesingood realandestimateddatabecomelargerthanagiventhreshold.
contact with the scalp. Ordinary metal electrodes can be affixed
with adhesive paste, which serves both to hold the electrode in (vii)ReferentialRecordingsShouldBeUsed
placeandtoconnectitelectricallytothescalp,orwithcollodion andtheReferenceShouldBeSpecified
~eitherdirectlyoringauze!.Collodioncanberemovedwithace- Almost all ERPrecordings are made using differential amplifiers
toneor~preferably!ethylalcohol.Innonhairyregionsofthehead, sothatelectricalnoiseinphaseatthetwoinputscanbecanceled.
theelectrodescanbeaffixedusingstickytapeortwo-sidedadhe- Thesedifferentialrecordingscanbemadeusingeitherreferential
sive collars.Ag0AgCl electrodes in plastic housings do not work montages~whereinthesecondinputtoallchannelsisacommon
wellwitheitheradhesivepasteorcollodion.Theycanbeaffixed reference!orbipolarmontages~thatlinkelectrodesinchainswith
tothescalpbyusingcollodion~aloneoringauze!tomatthehair thesecondinputtoonechannelbecomingthefirstinputtothenext
around the site and then using two-sided adhesive decals. When channel!. By providing the slope of the potential field, bipolar
using large numbers of electrodes, an elastic cap ~Blom &Ann- recordings help localize a maximum or minimum at the point at
eveldt,1982!ornet~Tucker,1993!ishelpfultoholdelectrodesin which the recording inverts in polarity. However, they are often
position.Caremustbetakentoensurethatthecapornetfitswell very difficult to interpret in ERP studies. Because bipolar mon-
andthattheelectrodesarelocatedproperly.Arangeofcapsizesto tagescanalwaysberecalculatedfromreferentialmontagesbutnot
coverthedifferentheadsizesisclearlynecessary.Inchildren,an viceversa,referentialrecordingsarerecommendedforERPstudies.
electrode cap is definitely preferable to applying electrodes indi- Theexperimentermustspecifythereference.Avarietyofref-
vidually. Although electrodes can be placed individually, the in- erenceelectrodescanbeuseddependingonthetypeofERPand
tersubject variability would be greater in children due to placing the recording system. Offline calculations can allow the sub-
the electrodes on moving targets. Infants and young children do sequent rereferencing to any site or set of sites desired ~Dien,
not always like having a cap on, but they often do not care for 1998a; Picton et al., 1995!. The physical linking of electrodes
electrodes either, and at least when the electrode cap is placed togethertoformareferenceisnotrecommendedbecausetheshunt-
successfully there is greater chance that the electrodes will be in ingofcurrentsbetweenelectrodesitesmaydistortthedistribution
thecorrectlocations. ofthescalpvoltages~Miller,Lutzenberger,&Elbert,1991!.Most
recordingsystemswillallowsuchalinked-electrodereferenceto
(vi)TheWayinWhichArtifact-ContaminatedSingle berecalculatedlaterifeachelectrodeinthereferenceisrecorded
ChannelsAreTreatedShouldBeDescribed separately.Iftherecordingsareobtainedusingasinglereference,
In high-density multichannel recordings, one or more channels an average reference calculated as the sum of the activity in all
frequentlycontainlargeartifactsduetoapoorcontactbetweenthe recorded channels divided by the number of channels plus one
electrodeandthescalporsomeamplifiermalfunction.Thenumber ~i.e., the number of electrodes! is perhaps the least biased of the
ofsuchchannelsshouldbereported.Thenumberofbadchannels possiblereferences~Dien,1998a!.Thisapproachallowsactivityto
inanyonerecordingshouldnotexceed5%ofthetotal.Evenifthe bedisplayedattheoriginalreferencesite~equivalenttozerominus
numberofchannelsissmall,however,itisdifficulttodecidewhat the value of the average reference!. If the activity at the original
should be done to integrate these data with other data from the referencesiteisnottobeevaluated,thecalculationoftheaverage
sameorothersubjects.Whengeneratingaverages,itmakeslittle reference is determined by dividing by the number of recording
sensetoincludethebadchannelinanyrejectionprotocol~because channels.Thiscalculationmightbedone,forexample,ifdatatobe
allepochsmightberejected!,butinclusionofthebaddatawould usedinsourceanalysiswererecordedusingalinked-earreference
ERPguidelines 135
~because the location of such a reference cannot be specified ac- Themostcommontechniqueforcalibratingtheamplifiersuses
curately!.Average-reference recordings are particularly appropri- asquarewavelastingbetweenonefifthandonehalftherecording
ate for topographic comparisons because they are not biased by sweep and having an amplitude typical of the largest ERP mea-
a single reference site, for source analyses that usually convert surements to be made. Optimally the amplifier is calibrated in
the data to average-reference format prior to modeling and for serieswiththeA0Dconverterandaveragingcomputersothatthe
correlation-based analyses, because the correlations are not in- wholerecordingsystemisevaluated.Anothertechniqueusessine-
flatedbytheactivityatasinglereferencesite.Theinterpretationof wavesignalsatanamplitudeandfrequencytypicaloftheEEG~or
theaveragereferencehasbeenthesubjectofsomecontroversyand ERP! to calibrate the amplifier and A0D converter. With multi-
manyoftheassumptionsunderlyingtheuseofaveragereference channelrecordingsystemsitisessentialtomeasureseparategains
are not satisfied in actual recordings. However, if recordings are for each channel ~and to use these channel-specific gains in the
obtainedfromareasonablesampleofheadlocations~i.e.,includ- amplitude measurements!. These gains should be within 10% of
ing electrodes below the Fpz-T7-Oz-T8 equator!, the signals rel- themeangain.
ative to an average reference will approximate the true voltages
over the head, which must average to zero ~Bertrand, Perrin, & (ii)TheFilteringCharacteristicsoftheRecording
Pernier,1985;Dien,1998a!.Whencomparingwaveformsandmaps SystemMustBeSpecified
tothoseintheliterature,itisessentialtoconsiderdifferencesinthe Analog filtering is usually performed at the same time as ampli-
reference. For example, the classic adult P300 or P3b wave is fication.Thebandpassoftheamplifiermustbeprovidedinterms
usually recorded at Fpz as a negative deflection when using an of the low and high cut-off frequencies ~23 dB points!. We rec-
averagereferencebutasapositivedeflectionwhenusinganearor ommenddescribingthecut-offsintermsoffrequenciesratherthan
mastoidreference.Itisoftenhelpfulwhencomparingwaveforms timeconstants,althoughthemeasurementsaretheoreticallyequiv-
with those in the literature that use another reference to plot the alent. In cases for which the filter cut-offs are close to the fre-
waveforms using both references or, if one is using the average quencies in the ERPs being measured, the slope of the filters ~in
reference, to include the waveform for the other reference elec- dB0octave! should also be described, because analog filters with
trodeinthefigure. steepslopescandistorttheERPwaveformsignificantly.
Analog filtering should be limited at the high end to what is
necessarytopreventaliasingintheA0Dconverter~i.e.,lessthan
E.AmplificationandAnalog-to-Digital(A/D)Conversion onehalfthefrequencyofA0Dconversion!andatthelowendto
whatisnecessarytopreventblockingtheconverterbyslowchanges
(i)TheGainorResolutionoftheRecordingSystem in baseline. Aliasing occurs when signals at frequencies greater
MustBeSpecified thantwiceA0Dconversionratearereflectedbackintothesampled
The recording system consists of the amplifiers that bring the dataatfrequenciesequaltosubharmonicsoftheoriginalfrequen-
microvolt signals into some range where they can be digitized cies~andatotherfrequenciesthatdependontherelationbetween
accuratelyandtheconvertersthatchangethesesignalsfromana- theoriginalsignalandtheA0Drate!.Roughrulesofthumbareto
log to digital form. The amplifier gain is the ratio of the output setthehighcut-offtoapproximatelyonequarteroftheA0Drate
signaltotheinputsignal.TheresolutionoftheA0Dconverteris and the low cut-off to approximately the reciprocal of four times
thenumberoflevelsthatarediscriminatedoveraparticularrange, the sweep duration ~Picton & Hink, 1974!. When recording 1-s
usuallyexpressedasapowerof2~bits!.FormostERPpurposes sweepsusinga200HzA0Dconversionrate,theserulesofthumb
anA0D converter using 12 bits ~4,096 values! is sufficient, pro- would lead to bandpass of 0.25–50 Hz. Further filtering can be
videdthattheincomingsignaltypicallyrangesoveratleast8bits doneofflineusingdigitalfilteringtechniques.Filtersdonotcom-
ofthisconverterrangeanddoesnotleadtoblocking.Converters pletelyremovefrequenciesbeyondthecut-offfrequency.Forex-
with greater precision are necessary if large DC shifts are being ample, if a simple ~6dB0octave! high-pass filter with a cut-off
monitoredwithoutbaselinecompensationsothattheresolutionis ~23dBpoint!atonequarterofthedigitizationfrequencyisused,
sufficientevenwhenthesignalcoversonlyaportionoftherange. theattenuationofasignalathalfthedigitizationfrequencyisonly
Thegainoftherecordingsystemcanbespecifiedintermsof 9dB~i.e.,theamplitudeis35.5%ofwhatitwasbeforefiltering!,
resolution,thatis,asthenumberofmicrovoltsperleastsignificant andstrongsignalswellabovethefilterfrequencymaystillleadto
bit ~smallest level discriminated by the A0D converter! or, in- aliasing.Thehigh-frequencynoisefromavideodisplaymaybea
versely,asthenumberofbitspermicrovolt.Thiscalculationcom- particularproblembecausethenoiseislockedtothestimulus.For
binesboththeamplifiergainandtheresolutionoftheA0Dconverter. example, a 90-Hz video refresh rate may alias into the ERPat a
For example, if the amplifier increases the recorded EEG by a frequencyof5.625Hz.Notch-filterstoexcludethelinefrequency
factorof20,0003andthe12bitA0Dconverterblocksat65V,the range~50–60Hz!maysignificantlydistorttherecordingandare
range of the A0D conversion in terms of the input signal is thereforenotrecommended.
6250mV,andthesystemresolutionis0.122mV0bit~calculatedas
100@20,00034,096#!. (iii)TheRateofA/DConversionMustBeSpecified
Amplifiers should have a sufficient common-mode rejection A0Dconversionshouldbecarriedoutataratethatissufficiently
ratio ~at least 100 dB! so that noise signals occurring equally at rapidtoallowtheadequateregistrationofthosefrequenciesinthe
each of the electrodes can be eliminated. Subjects should be signal that determines the measurements. The minimum rate is
grounded to prevent charge accumulation and the ground should twicethehighestfrequencyinthesignaltobemeasured.Frequen-
beprotectedfromleakagecurrents.Undercertainclinicalcircum- cies in the recording higher than one half theA0D rate must be
stances, full electrical isolation of the inputs ~e.g., using optical attenuatedbyanalogfilteringtopreventaliasing.
transmission! may be needed. These and other considerations of The multiplexing of the different recording channels to the
electricalsafetyarereviewedmorefullyelsewhere~e.g.,Cadwell A0Dconvertersshouldbesetupsothatthedelayintervalbetween
&Villarreal,1999;Tyner,Knott,&Mayer,1983!. themeasurementsofdifferentchannelsdoesnotsignificantlydis-
136 T.W.Pictonetal.
tort any between-channel latency measurements ~Miller, 1990!. roughly the same, then the standard deviation of the difference
Themostusualformofmultiplexingswitchesamongthechannels ERP will be larger than the standard deviations of the original
usingarapidratethatisindependentoftheintervaltoswitchtothe ERPsbyafactorof1.41.
next sample time. Provided this multiplexing rate is much faster Source analysis is particularly susceptible to residual back-
than theA0D rate used for ERPstudies, there will not be signif- groundnoisebecausetheanalysisprocedureswillattempttomodel
icant latency distortion. Optimal sampling would use a separate boththenoiseandthesignal.Forsourceanalysis,thenoisevari-
A0D converter for every channel, so that all channels could be ance~assessedindependentlyofthesourceanalysis!shouldbeless
sampledsimultaneously.Alternatively,asingle,multiplexedA0D than 5% of the signal variance. If the analysis is highly con-
convertercouldbeprecededbyseparatesample-and-holdcircuits strained, the signal-to-noise requirements for source analysis can
foreachchannel.Thesimplestwaytocheckthatthemultiplexing be less stringent.This might occur, for example, if one bases the
is not causing signal distortion is to record calibration sine-wave analysesofindividualERPwaveformsontheanalysisofthegrand
signalssimultaneouslyinallchannelsandtoensurethatthephase mean data by maintaining the source locations and just allowing
ofthedigitalsignalisequivalentineachchannel.Thismethodwill thesourcestochangetheirorientations.
alsocheckforbetween-channeldifferencesintheanalogfilters.
(ii)TheWayinWhichERPsareTimeLockedtothe
StimuliortheResponsesShouldBeDescribed
F.SignalAnalysis
The averaging process is locked to some triggering mechanism
(i)AveragingMustBeSufficienttoMaketheMeasurements thatensuresthattheERPsarereliablytimelockedtotheeventsto
DistinguishableFromNoise whichtheyarerelated.ForERPsevokedbyexternalstimuli,this
Thenumberofresponsesthatneedtobeaveragedwilldependon is usually done by recording a trigger at the same time as the
the measurements being taken and the level of background noise stimulus.There are two sources of variability in this timing.The
presentinsingle-trialrecordings.Thenoiseshouldbeassessedin firstconcernstherelationshipbetweenthetriggerandthestimulus.
thefrequencybandinwhichthecomponentismeasured.Thus,it Ifthestimulusispresentedonavideodisplay,theremaybesome
oftentakesfewertrialstorecordarecognizablecontingentnega- lag between the trigger and the occurrence of the stimulus when
tivevariationthanarecognizableN100ofsimilaramplitudeinan the raster scanning reaches the location of the screen where the
eyes-closedconditioninwhichtheEEGnoisenear10Hzishigh. stimulus is located. If the trigger is locked to the screen refresh
Manydifferenttechniquescanassessthenoiselevelsofaveraged rate, this lag will be a constant fraction of the refresh rate. The
recordings~reviewedinPictonetal.,1995!.Mostofthesemeasure second source of variability derives from the way in which the
thevarianceofindividualtrialsorsubaveragesoftheresponse.A triggersareregisteredintherecordingdevice.Theaccuracyofthis
simplewaytodemonstratethenoiselevelinarecordingistosuper- registrationoftendependsonthespeedofA0Dconversion.
imposereplicatetracingsofsubaveragesoftheresponse.Unfortu- When the ERPs are locked to responses, it is essential to de-
nately,inrecentyearstheincidenceofsuchreplicateERPfigures scribewhatresponsemeasurementisused~Deecke,Grözinger,&
hasdeclined. Kornhuber,1976;Shibasaki,Barrett,Halliday,&Halliday,1980!.
ThefirstquestionthatmightbeaskediswhetherornotanERP Twomaintriggersignalsarepossible:amechanicalsignalsuchas
is present. This question is important when using the ERPs to buttonpressorsomemeasurementoftheelectromyogram~EMG!.
estimatethethresholdfordetectingastimulusordiscriminatinga EMGmeasurementsrequirerecordingsfromelectrodesplacedover
differencebetweenstimuli.Theanswertothisquestionwillneed themainmuscleusedtomaketheresponse.Therecordedsignalis
somedemonstrationthattheaveragedERPisorisnotsignificantly rectifiedandathresholdlevelisselectedforinitiatingthetrigger.
different from the level of activity that would be present if the Thelocationsoftheelectrodesandthetriggeringlevelshouldbe
averaging had been performed on the recorded EEG without any describedclearly.Evenwhentriggeringonamechanicalresponse,
ERP being present. This assessment must, of course, take into itishelpfultorecordtherectifiedEMG.Thisrecordingwillallow
accountthenumberoftestsbeingperformed.Ifeveryoneof200 some estimate of the time between the EMG and the mechanical
points in an ERP waveform is tested automatically to determine signalandthevariabilityofthistime.
whether it is significantly different from noise, approximately 10
of these tests will be significant at p , .05 by chance alone. (iii)WhenLatency-CompensationProceduresAreUsed,
Techniques are available to determine how many such “signifi- TheyShouldBeDefinedClearlyandtheAmount
cant”resultsarenecessarytoindicateatrulysignificantdifference ofCompensationShouldBeSpecified
~Blair & Karniski, 1993; Guthrie & Buchwald, 1991!. Several OneoftheassumptionsofaveragingisthattheERPistimelocked
other techniques are available to demonstrate whether a recorded totheelicitingevent.Thisstatementmeansthatthelatencyofeach
waveform is significantly different from what might be expected ERPcomponent should remain constant across the trials that are
bychance~e.g.,Achim,1995;Ponton,Don,Eggermont,&Kwong, usedtocomputethesignalaverage.Any“latencyjitter”thatoccurs
1997!. whenthetimingofacomponentvariesacrosstrialscansubstan-
A second question is whether ERPs recorded under different tiallyreducethepeakamplitudeoftheaverageERP.Latencyjitter
conditions are significantly different. In general, if one wishes to isparticularlycommonwhentheERPcomponentofinterestisa
demonstratesignificantdifferencesbetweenERPs,thenoiselevel manifestationofaprocessingactivitythatisinvokedatvariabletimes
for each averaged ERP waveform should be reduced below the followingtheexternalstimulus.Insuchcases,usingtheexternal
leveloftheexpecteddifference.DifferencesbetweenpairsofERPs stimulustodefinethezerotimeforaveragingcancreatesubstantial
recordedunderdifferentconditionscanbeevaluatedanddepicted latencyjitterinthedataandtheresultscanbemisleading.Thein-
bycomputingthedifferencebetweenthetwoERPs.Thevariance vestigatormustbeparticularlycarefulwhencomparingERPam-
ofthisdifferencewaveformwillequalthesumofthevariancesof plitudesacrossconditionsthatvaryinlatencyjitter.Areductionin
theindividualERPs~providedthatthenoisesofthetwoERPsare theamplitudeofanaveragedERPmaybecausedbygreaterlatency
notcorrelated!.Forexample,ifthevariancesofthetwoERPsare jitterratherthanachangeinamplitudeoftheindividualERPs.