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Page 1: Bechtel.the Endogenously Active Brain

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TheEndogenouslyActiveBrain:

TheNeedforanAlternativeCognitiveArchitecture

WilliamBechtelDepartmentofPhilosophy,CenterforChronobiology,and

InterdisciplinaryPrograminCognitiveScienceUniversityofCalifornia,SanDiego

 Abstract

Mostproposalsofcognitivearchitecturesincognitivescienceandaccountsofbrainprocessesinneuroscienceconstruethemind/brainasreactive:processingisinitiatedbyastimulusandterminatesinaresponsetoit.Butthereisgrowingevidencethatbrainsareendogenouslyactive:oscillationsinelectrochemicalactivityatmultiplefrequenciesareongoinginthebrainevenintheabsenceofstimuliandstimuliservetomodulatetheseoscillationsratherthaninitiateactivity.Moreover,evidenceisgrowingthatthisendogenousactivityisusedinvariousinformationprocessingactivities.Iappealtoevidencefromsingle-cellrecording,EEG,andrestingstatefMRItosupporttheclaimofongoingoscillatorybehaviorinthebrainandidentifyseveralwaysitmaycontributetocognition.Ifcognitivescienceistounderstandhowweperformcognitivetasksitneedstodevelopcognitivearchitecturesthatincorporatethesortofendogenousdynamicactivityexhibitedbythebrain.

1.TheSearchforCognitiveArchitecturesThecognitivetraditionisdistinguishedfromitsbehavioristpredecessorbyfocusingoninformationprocessingmechanismsthatarethoughttoexistwithinthemind-brainand

hypothesizedtoexplainbehavior.Cognitiveresearchershaveoftenassumed,eitherimplicitlyorexplicitly,thatthebraintakesininformationthroughthesenses,representsit,performsoperationsontherepresentations,andrespondsbyeitherchangingitsinternalstateorplanningandexecutingactions.Intheearlydecadesofcognitivescienceitwasdifficulttoidentifytheneuralprocessesthatservedasrepresentationsforhigh-levelcognitiveprocessessuchasmemory,reasoning,andproblemsolvingandtheneuraloperationsthroughwhichtheywereprocessed.Asaresult,mostcognitivetheoriesofinformationprocessinghadtorelyonindirectmeasuressuchasreactiontimesanderrorpatternstoguideandevaluatehypothesesastotherepresentationsemployedandtheoperationsperformedonthem.Forexample,Sternberg(1966)usedthetimerequiredforsubjectstodeterminewhetheratestitemwasonalistofitemstheyhadmemorizedto

determinethathumansperformexhaustiveserialsearch.Onewaycognitivescientists,especiallythosefocusedoncomputationalmodelingofcognition,furtherconstrainedtheirinquirywastodevelopproposalsastothenatureofthecognitivearchitecture.Acognitivearchitecturespecifiestheprimitiveoperationsthemind/brainisthoughttoperform.ProposedarchitecturessuchasNewell’sSOAR(Laird,Newell,&Rosenbloom,1987)andAnderson’sACT-R(Anderson,1990,2007)weredefendedontheoreticalgroundsthattheypossessedtheappropriateprimitivecapacities

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forcognitivesystemssuchasfoundinhumans.Forthoseskepticaloftheproposalthatthemindperformsoperationsonsymbolstructures,theparalleldistributedprocessingframeworkorconnectionismprovidedanalternativeclassofarchitectures(Rumelhart&McClelland,1986),emphasizinginparticularfeedforwardnetworkstrainedbybackpropagation.(Foranoverviewandtaxonomyofcognitivearchitectures,seeDuch,

Oentaryo,&Pasquier,2008.)Researchersworkingwithaparticulararchitectureconstrainthemselvestoaccountforvariousaspectsofhumancognitiveactivityusingtheprimitiveoperationsprovidedinthearchitecture.Noneofthesearchitectureswasoriginallydevelopedprimarilywithaneyetocharacterizingtherepresentationsandprocessesusedinthebrain(althoughconnectionistssometimesrefertotheirapproachasbrain-stylecomputing),butastechniqueshavebeendevelopedtorelatecognitiveprocessingtoneuralactivity,advocatesofsomecognitivearchitectureshavetriedtoshowthattheirarchitecturefitswithourunderstandingofhowthebrainfunctions.Thesecognitivearchitecturestypicallyadoptareactiveperspectiveonthemind/brain.Cognitiveactivityisassumedtobeginwiththepresentationofataskorstimulus,whichis

representedandtherepresentationisthentransformedviaoperationsspecifiedbythearchitecture.Thisreactiveconceptionofcognition(itoccursinresponsetoastimulus)hasalsobeensharedastheneurosciencesbegantoprovideinsightintotherepresentationsandoperationsperformed.Thefirstsuccessesinidentifyingneuralprocessesthatrepresentinformationresultedfrominvestigationsofsensoryandmotorprocessinginwhichitwaspossibletolinkbrainactivity(typicallyspikingratesofneurons)withsensorystimuliormotoractivities.Withrespecttovisualprocessing,forexample,researchersbeginningwithKuffler(1953)andHubelandWiesel(1962,1968)employedsuchtechniquesassingle-andmulti-cellrecordingtodeterminewhatfeaturesofvisualstimuliwerecorrelatedwithspecificneuronalactivity.Theactivityoftheseneuronswasthenviewedasrepresentingthecorrelatedfeaturesofthevisualstimulus,andresearchers

hypothesizedoperationsthroughwhichtheserepresentationsweresuccessivelytransformedinahierarchyofprocessingareas(vanEssen&Gallant,1994;forananalysisofthishistory,seeBechtel,2008).Inthe1970sresearchesdevelopedtechniquesforrelatingtheelectricalsignalrecordedatthecorticalsurface(electroencephalographyorEEG)tostimuluspresentation(measuringwhatareknownasEvokedResponsePotentialsorERPs).WhileERPstudiesprovidedlittleinformationaboutthespatiallocusofactivity,theyofferedinformationaboutthetemporalorderofprocessing.Forexample,KutasandHillyard(1980)foundthatwhenthelastwordofasentencewasanomaloustheEEGexhitibedanegativedeflectionpeakingabout400millisecondslater(hence,anN400response).TheintroductionofPositronEmissionTomography(PET)inthelate1980sandfunctionalMagneticResonanceImaging(fMRI)inthe1990sallowedresearcherstobegin

tolocatecognitiveoperationsinbrainregionsinhumans.Petersen,Fox,Posner,Mintun,andRaichle(1988),forexample,adaptedthesubtractivemethod,developedbyDondersinthemid-19thcenturyforidentifyingthetimerequiredforacognitiveoperation,toidentifythebrainregionsresponsibleforsuchoperations.Thus,intheirpioneeringPETstudytheysubtractedthebloodflowmeasuredwhensubjectssimplyreadanounaloudfromthatmeasuredwhentheyfirstgeneratedarelatedverbandreaditaloud.Theyfoundincreasedactivityintheleftprefrontalcortex(alsothecerebellumandtheanteriorcingulate),whichtheyarguedtobethelocusofthesemanticprocessingrequiredforthetask.

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Researchersadoptingthereactiveperspectivehaveprovidedagreatdealofinformationaboutinformationprocessinginthebrain,especiallyinareasofsensoryandmotorprocessingandincreasinglywithrespecttomemory,attention,andemotionalresponses.Buttherearereasonstobeskepticalabouttheadequacyofthereactiveperspective.Since

thepioneeringstudiesofLorentedeNó(1938),researchershaverecognizedthatthereareatleastasmany,andlikelymanymore,backwardsandcollateralprojectsthanforwardones.1Thereactiveperspective,however,hasbeenabletoprovidelittleinsightintowhatcontributiontoinformationprocessingtheseoperationsperformsincewhatevercontributiontheymakehasalreadyaffectedhowbrainareasrespondtostimuliandtypicallycannotbeseparatelyidentified.(Oneexceptionhasbeentheinvestigationofhowattentionmodifiesprocessingofperceptualstimuli.SeeCorbetta&Shulman,2002.)Althoughithasreceivedfarlessattention,thereisanalternativetraditioninbrainresearchthathasemphasizedtheendogenousactivityofthebrain,viewingstimuliasperturbingon-goingbrainactivity,notinitiatingactivity.ThistraditiontracesbackatleasttoThomas

GrahamBrown(1911,1914),whostudiedneuralmechanismsformotorbehaviorindecerebratecatsintheLiverpoollaboratoryofCharlesScottSherrington,achiefproponentofadoptingareactiveperspectiveonthebrain.Bothwereinterestedinmotoractivitysuchaswalkingwhichappeartooriginatefrominsidetheorganism.Sherrington(1923)extendedthereactiveframeworktoexplainsuchbehaviorintermsofasequentialreflexmechanism,bywhichperipheralinput(e.g.,tothecat’sfeetwhenplacedonamovingtreadmill)producedasequenceofneuralsignals(tothespine,withinthespine,andouttoflexorandthenextensormuscles).Eachcycleofsteppingresultedinrenewedinput(sensoryfeedback)andhenceongoing,rhythmicsteppingmovements.Browndiscoveredthathecouldobtainsimilarrhythmicsteppingevenafterisolatingthespinalcordfromafferent(peripheral)inputbycuttingthedorsalrootnerves.Heaccountedforitsrhythmic

outputsbyproposingacouplednetworkofspinalneurons—oneforflexionandoneforextension—thateachinhibitedtheother’sbehaviortogenerateoscillations.Brown’sproposalsanticipatedresearchonwhatcametobeknownasthecentralpatterngenerator(Wilson&Wyman,1965).Morerecently,Brown’sperspectiveonendogenousactivityhasbeenadvancedbyresearchersusingtheverytoolsemployedbythereactivetradition—single-cellrecording,EGG,andfMRI.InthispaperIwillarguethattheevidencefortheendogenouslyactiveperspectiveonthemind-brainisextremelycompellingandthatinlightofitcognitiveresearchersshouldfundamentallyreconceivetheirconceptionsofcognitionandcognitivearchitecturestoincorporateandrecognizethesignificanceofendogenousactivity.Ibeginbyconsidering

whatitmeanstoclaimthatabiologicalmechanismexhibitsendogenousactivityandfocusonhowindividualneuronsareendogenouslyactive,thenexplorehowEEGandfMRIare1ForthemostpartinthispaperIwillfocusonconnections,forward,recurrent,andcollateralwithinthecerebralcortexandhippocampalformation.Butthesepresentonlyaportionofthestory.Mostsensoryinformationreachescortexviaregionsinthethalamus,andtherearealmosttentimesasmanyrecurrentprojectionsfromcortexbacktothesethalamicregionsasthereareforwardprojection.

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revealingsynchronousactivityofpopulationsofneuronsandinspiringproposalsastohowtheyfigureinneuralprocessing.Ithenconcludebyemphasizinghowthisbodyofresearchpointstotheneedforfundamentalrevisionsinthecognitivearchitecturesemployedincognitivescience.

2.EndogenouslyActiveMechanismsintheBrainClaimingthatthebrainisendogenouslyactivemaystrikesomeascomparabletoproposingthatitisaperpetualmotionmachine.Thatis,however,farfromwhatisbeingproposed.Alllivingorganisms,andaccordinglythosewithanervoussystemandabrain,areopeninthethermodynamicsensetomatterandenergyfromtheirenvironment.Whatisdistinctiveaboutbiologicalorganismsisthattheyareorganizedsystems—hencenotinequilibriumwiththeirenvironment—andthattheymaintainthemselvesinthisnon-equilibriumstatedespitethetendencyexhibitedbyclosedsystemstowardsequilibrium(highentropy).Theequilibriumtendenciesaremanifestinthecontinualdegradationoforganicstructuresthatrequirelivingsystemsregularlytorepairthemselves(Rosen,1991)orelseceasetoexistas

organizedsystems.Moreover,theymustperformtheoperationsneededtoconstructthemselves—incorporatematterintotheorganizedstructurewithwhichtheyareidentified.MaturanaandVarela(1980)havereferredtothisasautopoiesisandvieweditasthefoundationofcognition(seeLyon,2006,forasystematicdiscussion).Crucialtoautopoiesis,butnotemphasizedbyMaturanaandVarela,isthatorganismsmustcaptureandemployfree-energyfromtheirenvironmentsinautopoiesisandrepair.Ruiz-MirazoandMoreno(2004)havemadethisacentralfeatureintheircharacterizationoflivingorganismsasautonomousandBarandiaranandMoreno(2006)haveextendedtheviewtocognition.Organismsmustinitiatetheseactivitiesofautopoiesisandrepairfromwithin,andthis

entailsthattheybeendogenouslyactive,notreactive.AsGánti(2003)emphasizedinhisproposalofthechemotonastheminimalchemicalsystemcapableofexhibitingthecharacteristicsoflife,cyclicprocessesthatcanregularlyreturntothesameconditionarefundamentaltoautonomoussystems—asaresultofcyclicorganization,anorganismcanregularlyrestoreitselftotheconditionswhereitcanperformtheoperationsnecessarytobuildandrepairitself.Aslongastheorganizationofthecyclicallyorganizedsystemisadequatetorecruitfreeenergyfromtheenvironment,itcancontinuetoiteratethestagesinthecycle.Ifonetracksvariablesrepresentingstatesofthesystemthroughtime,theywilloscillate.Iftherearetimedelayswithinthecyclicsystem,andespeciallyiftheoperationswithinthecyclearenon-linear,theseoscillationscanbesustainedindefinitelyprovidedsufficientmatterandenergyareavailable.Goodwin(1963)pioneeredtheanalysisof

sustainedoscillationsinbiologicalsystems,showinginacomputationalmodelhowafeedbackloopinspiredbyJacobandMonod’s(1961)operonmodelforgeneregulationinbacteriacould,withappropriateparameters,generateasustainedoscillatorysystem.Subsequently,othertheoristssuchasGoldbeter(1996)havepursuedtheapproachandhavedrawnattentiontoasubstantialnumberofendogenousoscillationsinbiologicalsystems.(ForfurtherdiscussionseeBechtel&Abrahamsen,2011.)

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Whilemechanismsthatareendogenouslyactiveclearlyoccurinnatureandarefundamentaltolivingsystems,manyoftheaccountsofmechanismandmechanisticexplanationadvancedbythenewmechanisticphilosophersofscienceareinadequatetocharacterizethem.Followingwhathasbeenthedominantemphasisinmechanisticresearchinbiologyoverthepastseveralcenturies,theseaccountsemphasizethepartsand

operationsofmechanismsandtheprocessesbywhichresearchersdecomposemechanismsintotheirpartsandoperations(Bechtel&Richardson,1993/2010;Bechtel&Abrahamsen,2005;Glennan,1996,2002;Machamer,Darden,&Craver,2000;Thagard,2003;Wimsatt,2007).Theseaccountsofmechanismalsonotetheimportanceofhowcomponentsareorganizedbut,likemanyscientists,theytendtoprivilegesequentialorganization.Thus,Machamer,Darden,andCraverincludeintheirdefinitionofamechanismthattheyare“productiveofregularchangesfromstartorset-uptofinishorterminationconditions”(p.3).Sequentialorganization,however,cannotproduceendogenousactivity—thisrequiresnegativefeedbackorothercyclicdesigns.Moreover,notallnegativefeedbacksystemssustainoscillations;onlyoneswithnon-linearcomponentsandappropriateparametervaluescandoso.Determiningwhethera

particularmechanismwillgeneratecontinuedactivity(e.g.,oscillation)orsettleintoastablestaterequiresemployingthetoolsofcomputationalmodelinganddynamicalsystemsanalysis.BechtelandAbrahamsen(2010;seealsoBechtel,2011)designateexplanationsthatinvokecomputationalmodelingtounderstandpatternsofchangeovertimeinthepropertiesofthepartsandoperationsofamechanismasdynamicmechanisticexplanations.Sustainedoscillatorsarethesimplestendogenouslyactivemechanisms—aslongastheycanrecruitfreeenergyfromtheirenvironmenttheyarecontinuallyactive.Neuronsareexamplesofsuchsustainedoscillators.Theendogenousnatureofneuronalactivity,however,isoftennotappreciated.Sinceearlyinthe20thcentury,whentechniqueswere

developedforrecordingtheelectricalactivityofindividualneurons,neuroscientistshaveoftentreatedthegenerationofactionpotentialsastheprincipalactivityofneurons.Thecommonpicture,bothintextbooksandinphilosophicaldiscussions,isthatneuronsresideattheirrestingpotentialuntiltheyreceivesufficientinputsontheirdendritestoraisethevoltageabovethreshold,afterwhichtheygenerateanactionpotentialontheiraxonand,overtime,returntotheirrestingpotential.Onthisview,neuronsarereactivecomponents.Althoughmostaccountsofneuralprocessingfocusonactionpotentials,theyrepresentonlyasmallpartoftheelectricalactivityinthebrain.Evenbeforeindividualactionpotentialscouldberecorded,researcherssuchasduBois-Reymond(1848-1884)hadrecordedelectricalpotentialsfrommusclecellsandneurons.Bernstein(1912)hadidentifiedagreaterconcentrationofpotassium(K+)ionsinsidethaninsidethecell,and

proposedthattheelectricalcurrentresultingfromdiffusionoutofthecellexplainedthenegativepotentialofthecellatrest.Hefurtherproposedthat,whenexcited,otherionswoulddiffuseacrossthemembrane,eliminatingthediffusionpotentialoftheK+ions.HodgkinandHuxley(1952)determinedthatinadditiontothepotassiumgradient,thereisasodium(N+)gradient(withN+concentrationsgreateroutsidethecellduringtherestingpotentialphase)andthattheconductanceofeachionvariesindependentlyanddependsonthevoltageacrossthemembrane.Exceptwhentherelativeconcentrationofagivenionon

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eachsideofthemembrane(itsmembranepotential)equalsitsreversalorequilibriumpotential,ionswillfloweitherintooroutofthecell.HodgkinandHuxley’sinterestinthesecurrentswasprincipallytoexplaintheactionpotential,whichmostresearcherscontinuedtoviewfromareactiveperspective.Butsome

researchers,workingasHodgkinandHuxleyhadwithinvertebrateneurons,foundspecialized pacemaker neuronsthatgeneratedtheirownrhythmicactionpotentials(Alving,1968).Othersbegantofocusonthevarietyofvoltage-gatedandothercurrentsthatcouldbeobservedacrossneuronalmembranesandexploredthecomplexpatternsofchangeinthesecurrentsnotonlyinaxonsbutalsointhedendritesandcellbodiesofneurons(reviewedbyKandel,1976).Initiallymammalianresearchers,whofollowedinthetraditionofSherringtonandtreatedneuronsasintegratinginputsandfiringwhentheseinputspushedthemabovetheirthreshold,viewedthesestudiesskeptically.Thisbegantochangeinthe1970sand1980swhenLlinásandhiscollaboratorsfoundavarietyoffunctionallyimportantioncurrentsin

neuronsoftheinferioroliveandcerebelluminmammalsandbirds.Mostwerespatiallydistributedandgatedbyvoltageinadifferentmannerthanthesodiumandpotassiumchannelsintheaxon,equippingthemforfunctionsotherthanthedirectgenerationofactionpotentials.Notably,thedendriteswereendowedwithchannelsprovidinghigh-thresholdconductancetocalcium(Ca2+)ions,enablingdynamicallycomplexdendriticexcitationincontrasttoearlierassumptionsofpassivetransmissionofsignalsfromsynapses.2Moreover,thecellbodiesofsomeneuronsintheinferiorolivehadadifferentkindofcalciumchannelwithaseeminglyparadoxicallow-thresholdconductancethat,ininteractionwithsodiumandhigh-thresholdcalciumconductances,enabledtheseneuronstofunctionassingle-celloscillators“capableofself-sustainedrhythmicfiringindependentofsynapticinput”(Llinás,1988,p.1659).3Theysenttheserhythmicactionpotentialsto

targetneuronsinthecerebellumthatwereabletorespondatthesamefrequency,qualifyingthemasresonatorsinthedynamicallexiconchampionedbyLlinás—reacting,butinwaysshapedbytheirinternalproperties.Llinásalsoinvestigatedspontaneousoscillationsinelectricalpotentialselsewhereinthebrain.Llinás’findingsrevealedthattheneuronsareoscillators—someofwhichmaintainoscillationsontheirownwhileothersresonatetooscillationsinitiatedbyothers.Thishasconsequencesnotjustforhowweconceiveofneuronsbuthowweunderstandtheirinteractions.AsHuygenshadobservedinhis1665letterstodeSluse(lettersno.1333of24February1665,no.1335of26February1665,no.1345of6March1665publishedinHuygens,1888),aslongasthereisameanstoconveyasignal(evenaveryweakone)

betweentwopendulumclocks(apendulumclockisanoscillator)withnearlythesame2ThislinkednerveexcitabilitywiththeCa2+-dependentsecondmessengersystemthatisimportantforregulatinggeneralcellularfunctions.3Forfurtherexposition,seeBuzsáki(2006,pp.181-183),whocomments:“Thesefindings...illustratethatnaturewenttoalotoftroublebringingtogetherthesechannelsattherightdensitiesandlocationjusttoserveonepurpose:oscillation.”Forevidenceextendingthefindingstosensoryneuronsinvariousmammalianspecies,seeHuguenard(1996).

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periods,theytendtosynchronizetheiroscillationsandexhibitacommonphaserelation.Whensignalscanbepassedbetweenmultipleoscillators,somewithdifferentnaturalfrequencies,complexpatternsofactivitycandevelop.Agivencomponentmaysynchronizewithsomebutoscillateoutofphasewithothercomponents.

Emphasizingtheoscillatorydynamicsacrossneuralmembranesisnotinconflictwithfocusingonthemasgeneratingactionpotentialsinresponsetostimuli.Inputsreducetheelectricalpotentialoftheneuronandactionpotentialsarisewhentheycrossathreshold.Iftheelectricalpotentialofneuronsisoscillating,therewillbeperiodswhentheyarelesspolarizedandhenceclosertothresholdandotherperiodswhentheyaremorepolarized.Aninputthatmaysufficetopushneuronsacrossthethresholdwhentheyarehypopolarizedmaybeinsufficienttodosowhentheyarehyperpolarized.Moreover,whentwoneuronsarelinkedinacircuitandtheiroscillationsaresynchronized,theperiodswhenthefirstneuronismostlikelytogenerateanactionpotentialwillcorrespondtoperiodswhenthesecondislikelytogenerateanactionpotentialinresponse.Ontheotherhand,whentheyaredesynchronized,thesecondneuronwillbelesslikelytogeneratean

actionpotentialatthetimewhenthefirstneuronismostlikelyto.Iturnnowtoconsideringhowsuchdynamicalbehaviorofneuronsismanifestinpopulationsofneuronsengagedincognitivetasks.3.OscillationsDetectedwithEEGandLFP

Avarietyoftechniqueshavebeendevelopedthatcandetectsynchronizedelectricalpotentialsofpopulationsofneurons.Oneofthefirstinvolvedelectrodes,initiallyinsertedintothescalpandsubsequentlyplacedonthescalp.InhispioneeringresearchinwhichhecoinedthenameElektrenkephalogramm(electroencephalogramorEEGinEnglish)forthetechnique,Berger(1929,1930)reportedthathecouldrecordelectricaloscillationsfrom

leadfoilelectrodesaffixedtotheheadsofhumanbeings.Inparticular,hedetectedlarge-amplitudeoscillationsofapproximately10Hzwhensubjectswereawakewiththeireyesclosedthathecalledalpharhythmsandsmalleramplitude,fasteroscillations(20-30Hz)thatbecameapparentoncealpharhythmsdeclinedwhensubjectsopenedtheireyes,receivedinputfromanothersense,orperformedanattentiondemandingtask(seeFigure1).

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Figure1.Inthiseight-secondextractofarecordingmadebyBerger(1930),theupperlineisasubcutaneousEEG.Itshowsthreesecondsofpredominantlyalphawavesthatwere

blocked0.27secondsafterhestrokedthesubject’shandwithaglassrod(indicatedbyarrow“B”onthe10Hztimingsignalatthebottom).ForatleastthenexttwosecondstheEEGshowslower-amplitude,higher-frequencybetawaves,notalphawaves.Themiddlelineisanelectrocardiogramrecordedsimultaneously.ExtractedfromFigure5inGloor’stranslation(1969,p.82)ofBerger(1930).

BergerandothersinitiallyassumedthattheEEGwasgeneratedfromactionpotentials.Eventuallyresearchersrecognizedthattheyresultednotfromactionpotentialsbutfromsynchronizedpost-synapticpotentialsindendritesdiscussedintheprevioussection(Bremer,1958).Moreover,researchsuchasthatofJahnsenandLlinás(1984)onthethalamusandthalamocorticalrelayneuronshelpedlinkdynamicbehaviorofindividualneuronstothelarge-scaledynamicsseeninEEG.InthedecadesafterBerger’spioneeringwork,researchersidentifiedbothfasteroscillations(greaterthan30Hz)thattheydesignated gammarhythmsaswellasavarietyofsloweroscillations(delta,0-4Hz,andtheta,4-8Hz,rhythms).TypicallyoscillationsatdifferentfrequenciesareallcombinedintheelectricalactivityresearchersrecordandtechniquessuchasfastFourieranalysisarerequiredtodifferentiatecomponentsintheoverallsignal.Moreover,researchershavealsofoundthatthesameactivitycanbedetectedfromelectrodesimplantedintobraintissueaslongastheelectrodesarenottooclosetoanygivenneuron.Whendetectedinthiswaythecurrentsarereferredtoaslocalfieldpotentials.ThemostcommonapplicationofthevariousrhythmsdetectedwithEEGhasbeentodifferentiateoverallcognitivestates—statesofactiveawareness(gamma),quietresting(alpha),andvariousstagesofsleep(delta,theta).FormanyyearslittleprogresswasmadeinlinkingEEGoscillationstomorespecificallycharacterizedcognitivefunctions.Instead,theprimaryuseofEEGincognitiveresearchhasbeeninERPstudiesthatreflectthereactiveconceptionofneuralprocessing.Thedifferenceintheoverallelectricalactivityproducedduetothestimulusisquitesmallinrelationtotheelectricalactivitythat

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originateseitherendogenouslyorinresponsetootherstimuli,soERPresearchersmustaverageovermanytrialsinordertoextractasignal.Ironically,theyaretherebyremovingfromconsiderationtheendogenousbackgroundactivitythatBergerhadfirstidentified.Inrecentdecades,however,someresearchershavebeguntoidentifycorrelatesof

cognitiveactivitydirectlyintheEEGsignalitself.Oneofthefirsthypothesesresultedfromthedetectionofgammaoscillationsinvisualprocessingareasinanesthetizedcatspresentedwithmovingbarswithaparticularorientation(Gray,Konig,Engel,&Singer,1989).Singerandhiscolleaguesdevelopedaninterpretativeframeworkinwhichgammaoscillationsservedtotemporarilybindtogethertheneuralrepresentationsofdifferentfeaturesofaparticularobjectwhicharerepresentedintheactionpotentialsofneuronsindifferentvisualprocessingregions(Singer,1999,characterizedthisasthebinding-by-synchronizatonhypothesis).Subsequentresearchershavedetectedgammaoscillationsinawake,alertcats,monkeys,humans,andotherspecies,inresponsetothepresentationofshapesandsmoothlytransformingshapes(reviewedbyFries,2009).Gammaoscillationshavealsobeenfoundinsomatosensoryandauditoryprocessingareas.Recentstudieshave

shownthatgammasynchronizationrequiresnotjustinputsbuttop-downactivationduetoattention.Thesegammaoscillationsappeartobetheresultofinitialinputstopyramidalcellsthatthenelicitresponsesinbasketcellsthatdistributeaninhibitorysignalthatblocksfurtheractionpotentialsinthepyramidalcellsonwhichtheysynapseuntiltheireffectswearoffinasynchronizedmanner,allowingsynchronizedspikingbehavior.Thepopulationofbasketcellsarethenentrainedtomaintainaregularoscillationwhichleaveswindowsinwhichactionpotentialscanbegeneratedinthepyramidalcells(Hasenstaub,Shu,Haider,Kraushaar,Duque,&McCormick,2005).Theeffectofsynchronizedactionpotentialsallowstheoutputsofmanycellstobereceivedbyadownstreamneuroninacommontemporal

windowsothattheysumsufficientlytoproduceanactionpotentialinit.Binding-by-synchronizationretainsareactiveconceptionofbrainprocessingsinceitrepresentsthefeatureselicitedbyastimulusascomingtobebound.Butotherinvestigationsofgammaoscillationsuggestthattheymayreflectanendogenouscomponentofbrainprocessing.Whittington,Traub,andJefferys(1995)foundthatpharmacologically-isolatedinhibitoryneurons,whenprovidedwithatonicexcitatorydrive(activationofmetabotropicglutamatereceptors),generategammaoscillationswithoutspecificinputs.ResearcherssoonfoundotherconditionsthatwouldgenerategammaoscillationsandWangandBuzsáki(1996)pioneeredtheprojectofconstructingcomputationalmodelstoexamineconditionsunderwhichnetworksofinhibitory

interneuronswouldgenerategammaoscillations.However,theirmodelrequiredconditionsnotfoundintheexperimentalpreparations.Kawaguchi,Katsumaru,Kosaka,Heizmann,andHama’s(1987)discoveryoffast-spikinginhibitoryneuronsthatexpressthecalciumbindingproteinparvalbuminprovidedaclueastohowtheseoscillationsarise.Parvalbuminexpressingneuronsformnetworksofmutuallyconnectedneuronsinwhichgammaoscillationsarecreatedandthentransmittedtopyramidalcellsviatheiroutputconnectionstothesecells(Bartos,Vida,&Jonas,2007).Theparvalbumin-expressingcellsarehighlyactiveduringgammaoscillationsduringwhichtheygenerateactionpotentials

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oncepercycleinaphase-lockedmanner(Gloveli,2005),supportingthehypothesisthattheyplayafundamentalroleingeneratinggammaoscillations.Moreover,theyaffectthepyramidalcellsinadistinctiveway—theydonotcausethemtohyperpolarize,butusechloridechannelstoshuntelectricalactivity.Theresultisamechanismthatisrobustagainsttheheterogeneityofthetonicdrive—withhighexcitationanearlyconductance-

dominatedphasecausessubsequentactionpotentialstooccurlaterwhereaswithlowexcitationthedepolarization-dominatedphasecausessubsequentactionpotentialstobeadvanced.Theresultistocreateregular,clock-likeepochsinwhichactionpotentialscanarise;informationprocessingisthusstructuredintodiscretetemporalwindows.Fries(2009)proposesthatthesegammaoscillationsmaybelocatedwithinslowerthetaoscillationsinsuchafashionthatthetaoscillationsprovidelargertemporalwindowsinwhichgammasynchronymaydevelopbutistheninterruptedsoastoprocessnewinput.AsevidenceforthisproposalhecitesRollenhagenandOlson’s(2005)recordingsfromindividualinferotemporalneuronsafterfirstpresentingapreferredstimulusandthen600mslateraddinganon-preferredtothepreferredstimulus.Presentationofthesecond

stimulusresultedinareductioninfiringrate,followedbyanapproximately5Hz(thetarange)oscillationbetweenperiodsofenhancedandreducedfiring,suggestingathetafrequencyoscillationbetweentheresponsetothepreferredandnon-preferredstimulus.Sincesuchoscillationisalsoobservedinrecordingfromindividualneuronswhenonlythepreferredstimulusispresented,Friesproposedthethetaoscillationsareanendogenousprocessthatallowsaperiodfordevelopingaresponsetoastimulusfollowedbyaperiodthatopenstheprocessingsystemuptoseekingnewinputratherthansimplycontinuingtorespondtothefirststimulus.Itthusinsuresthatanyresponsetoastimulusisonlymetastableandthatprocessingdoesnotstopwithit.Bothgammaandthetaoscillationsarealsoexhibitedinthehippocampuswhere

researchershaveadvancedhypothesesastotheircontributiontocognitiveprocessingofspatialinformation.Thehippocampusisorganizedintoaloopstructure(Figure2)—itreceivesinputfromtheentorhinalcortex(EC)thatistransmittedalongapathwaytothedentategyrus,theCA3fields,theCA1fields,andsubiculumbeforesendinginputsbacktotheentorhinalcortex.ThereisalsoapathwaydirectlyfromtheECtotheCA1fields.

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Figure2.ThelooparchitectureofthehippocampusinwhichinputfromentorhinalcortexiseitherprocessedinthedentategyrusandCA3beforereachingCA1orisdirectlyreceivedbyCA1.

Followingreportsthatratswithhippocampallesionsexhibitdeficitsinthesortsof

navigationtasksthathadledTolman(1948)toproposethattheypossessacognitivemapoftheirenvironment,O’Keefebegandetailedfunctionalanalysisoftheresponsecharacteristicsofhippocampalneurons.RecordingfromindividualcellsintheCA1andCA4regions(CA4isasmallregionnotcommonlydiscussed)ofthehippocampus,O’KeefeandDostrovsky(1971)identifiedanumberofcellsthatgeneratedseveralburstsofactionpotentialswheneveraratwasinaparticularlocationinitsenclosure.O’Keefelatertermedthese placecellsandarguedthattheyconstitutedtherat’scognitivemapofitsenvironment(O'Keefe&Nadel,1978).DuringthesameperiodRanck(1973)identifiedcellsthatgeneratedburstsofactionpotentialsinthethetarangewhentheratwasmoving.Overthefollowingdecaderesearchfocusedprimarilyonplacecells,especiallyonconditionsinvolvingchangesintheenclosurethatwouldcausedifferentcellstoproduceburstsof

spikesdespitetheratbeinginthesamelocation.O’KeefeandRecce(1993)initiatedanewphaseofresearchwhentheyexploredtherelationbetweenplacecellactivityandthetaoscillationsanddeterminedthattheburstsofactionpotentialsfromplacecellsexhibitedasomewhathigherfrequencythantheongoingthetaoscillation.Asshowninfigure3,theinitialburstwouldtypicallyoccurduringthepeakofthethetaoscillationsandsubsequentburstswouldprecessearlierinthethetaoscillationssothatbythetimetheratlefttheplacefield(theregionthatelicitedactivityfromaparticularcell),theburstswouldhaveadvancednearlyafullcycle.Consideringonlytheactivityofasingleplacecell,theamountofprecessionspecifieshowlongtherathasbeenintheplacefieldandthushowfarithasadvancedthroughit.Fromapopulationperspective,however,theinformationisevenricher.Amongcellswhoseplacefieldspartiallyoverlap,thoseplacecellswhoseaction

potentialshaveprecessedthemostrepresentplacestheratenteredearlierwhiletheonesinwhichactionpotentialshaveprecessedtheleastrepresentthosetherathasjustentered.Thus,theactivityacrossthepopulationofplaceinrelationtothetaspecifiestherat’strajectory.

Figure3.Illustrationofthetaprecession.Asratrunsalongthemaze,itcrossestheplacefieldofacell.Theplacecellspikes,showninred,precessagainsttheunderlyingthetaoscillation,firingfirstjustafterthepeakandmovingprogressivelyearlyonsubsequentthetacycles.FromWang2010.

Inadditiontothetaactivity,thehippocampusalsoexhibitsgammaoscillationsattwodifferentfrequencies,oneoriginatinginmedialEC,whichexhibitsfastgammaoscillations(>60Hz),andtheotherintheCA3region,whichgeneratesslowgammaoscillations(<60Hz).AsshowninFigure2,theserepresentthetworegionsfromwhichCA1receivesinput.Inotedabovethatneuronsaremostlikelytofirewhentheyaresynchronizedwiththose

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fromwhichtheyreceiveinput.FromtheseconsiderationsandthefactthatCA1atdifferenttimesexhibitsfastandslowgammaoscillations,Colgin,Denninger,Fyhn,Hafting,Bonnevie,Jensen,Moser,&Moser(2009)havehypothesizedthatthesedifferentgammaoscillationsregulatetheprocessingofinformation—atdifferenttimestheCA1fieldssynchronizewiththefastgammaoftheentorhinalcortexandrespondtoinputfromitand

atothertimessynchronizewiththeslowgammaofCA3andhencerespondtoinputsfromit.TheprocessinginDGandCA3isthoughttofacilitaterecognizingalocationasapreviouslyexperiencedone;whenthatfails,CA1respondsdirectlytotheassumedtobenovelplacerepresentedbytheECactivity.Animportantprocessinthehippocampusislong-termpotentiation(LTP),whichstrengthenssynapticconnectionsonwhichaninputisreceivedwhentherecipientneuronactuallygeneratesanactionpotential.LTPoccursonlywhenaneurongeneratesaspikeduringthethroughofthethetacycle,whichhappenswhenCA1issynchronizedwithentorhinalcortex.ThislimitslearninganewrepresentationtooccasionswhentheDG-CA3loopsfailstorecognizethelocation,asituationinwhichitisappropriatetoacquireanewrepresentation.Whenanoldlocationisrecognized,CA1synchronizeswithCA3andspikesoccuratthepeakofthethetacycle

whenLTPisblocked.AfteraperiodduringwhichtheonlywayresearcherswereabletorelateEEGactivitytocognitionwasthroughERP,whichadoptedareactiveperspective,researchersarebeginningtoformulatehypothesesastohowoscillations,especiallythoseinthethetaandgammabands,thatariseindependentlyofspecificstimulifigureininformationprocessing.TheyhavebeenproposedtocreatetimewindowsinwhichneuralprocessesmaydevelopresponsestoinputsbutthenbreakoutoftheresponsetosampleotherinputsaswellaswaysofgatingtheflowofinformationbetweenbrainregionsandcontrollingprocessessuchasLTP.Theseproposalssuggestthatthebrainisadynamicallyactiveprocessingsysteminwhichtheongoingdynamicsstructuresandregulatesprocessingintime.

4.OscillationsDetectedwithPETandfMRIOneofthelimitationsofEEGforstudyingcognitiveprocessinginthebrainisthatwhileitprovideshigh-resolutioninformationaboutthetimingofelectricalactivity,itisverydifficulttolocalizespatiallythesourceoftheelectricalsignal.ThusalthoughasmallcadreofresearchersemployedEEGandERPtoinvestigatecognitiveprocessinginthebraininthe1970sand1980s,theirinvestigationsfailedtosparkthedevelopmentofcognitiveneuroscienceasaprominentfieldofinquiry.Rather,thishappenedonlyafterthedevelopmentoftechniquessuchaspositronemissiontomography(PET)inthelate1980sandfunctionalmagneticresonanceimaging(fMRI)inthe1990s.Thesetoolsused

tomographicmethodstolocalizethesourceofeitheraradioactiveormagneticsignalthatwasassumedtoberelatedtospecificcognitiveactivities.Asdescribedabove,intheinitialresearchwithPETandfMRIstudiesresearcherssoughttoidentifyregionsinwhichbloodflowwascorrelatedwithcognitiveactivities.ThuslikeERPresearchers,PETandfMRIresearchersinitiallyviewedthecognitivesystemasareactivemechanismandfocusedondetectingincreasedactivitythatcouldbeattributedtothetaskasubjectwasperforming.Accordingly,theypaidlittleattentiontothevaryingbackgroundsignalexceptinsofarasitposedchallengesindetectingthesignalattributedtothestimulusortask.

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Inthemid-1990ssomeresearchers(e.g.,Ghatan,Hsieh,Wirsén-Meurling,Wredling,Eriksson,Stone-Elander,Levander,&Ingvar,1995;Baker,Rogers,Owen,Frith,Dolan,Frackowiak,&Robbins,1996)drewattentiontothefactthatsomebrainregionsseemedregularlytoexhibitreducedactivity(weredeactivated )intaskconditionsthaninaresting

state(aconditioninwhichthesubjectlayquietlyinthescannerwithnoassignedtask).Todeterminehowwidespreadthiseffectwas,Shulman,Corbetta,Buckner,Fiez,Miezin,Raichle,andPetersen(1997)conductedameta-analysisofPETstudiesinwhichataskconditionwascomparedtoanon-taskconditioninwhichthesamestimuluswaspresented(thisproducedresultsverysimilartotherestingconditionwithnostimulus).Theyidentifiedaspecificsetofbrainareasthatwerereliablylessactiveintasksituations:thejunctionofprecuneusandposteriorcingulatecortex(PCC),theinferiorparietalcortex(IPC),theleftdorsolateralprefrontalcortex(leftDLPFC),amedialfrontalstripthatcontinuedthroughtheinferioranteriorcingulatecortex(inferiorACC),theleftinferiorfrontalcortex,theleftinferiorfrontalgyrus,andtheamygdala.

ShortlythereafterRaichleandhiscollaboratorsshiftedtheirperspectivefromviewingtheseareasasdeactivatedintaskconditionstoviewingthemasbeingmoreactiveinnon-taskconditionsandbegantocharacterizethemasconstitutingadefaultmodenetwork —onewhichperformsactualfunctionsbestcarriedoutwhentherearenoexternaltaskdemands.OneclueastotheirfunctionwasprovidedbyAndreasen,O'Leary,Cizadlo,Arndt,Rezai,Watkins,Ponto,andHichwa(1995)whofoundthatepisodicmemorytaskswereonetypeoftaskthatdidnotresultinreducedactivityintheseregions.Someresearchersdrewuponthisfindingtosuggestthatthedefaultmodenetworksupportsundirectedthinkingorwhathadbeenlabeledmind-wandering(Antrobus,Singer,Goldstein,&Fortgang,1970).ThiscoheresthereportsbyAndreasenetal.’ssubjectsthat,whenrequiredtolayinthescannerwithnotaskrequirements,theythoughtabouttheirownpastorplannedfuture

activities.Thesearebothactivitiesthatdrawuponepisodicmemory.Adoptingthemind-wanderinghypothesis,Buckner,Andrews-Hanna,&Schacter(2008,p.2)linkmind-wanderingtotheabilitytocarryout“flexibleself-relevantmentalexplorations—simulations—thatprovideameanstoanticipateandevaluateupcomingeventsbeforetheyhappen”(p.2).IndefendingthisviewtheycitenotonlyAndreasenetal.’sresultsbutalsocorrelationsfoundbyMasonetal.(2007)betweenstimulusindependentthoughtsandactivityinthedefaultnetwork.Gilbert,Dumontheil,Simons,Frith,andBurgess(2007)offeranalternativeviewthatactivityinthedefaultnetworkgenerateslow-levelgeneralizedawarenessorwatchfulness.ThisalternativegainssupportfromHahn,Ross,andStein’s(2007)findingsofincreasedactivityinthedefaultnetworkin

atarget-detectingtaskwhenthetargetcouldappearanywhere,butnotwhenitwasexpectedinaspecificlocation.Apotentialproblemforbothofthesetreatmentsofthefunctionofthedefaultmodenetworkisthatithasbeenfoundtobeactivenotjustwhenindividualsareawakebutalsoduringsleep(Fukunaga,Horovitz,vanGelderen,deZwart,Jansma,Ikonomidou,Chu,Deckers,Leopold,&Duyn,2006;Larson-Prior,Zempel,Nolan,Prior,Snyder,&Raichle,2009)andunderanesthesia(Vincent,Patel,Fox,Snyder,Baker,VanEssen,Zempel,Snyder,Corbetta,&Raichle,2007)—circumstancesinwhichneitherspontaneousconsciousthoughts(suchasAndreasenetal.’ssubjectsreport)nor

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generalizedawarenessandwatchfulnessoccur.Evenifactivityinthedefaultmodenetworkisfoundintheabsenceofconsciousthought,thedefaultmodenetworkmaynonethelessprovideaneurophysiologicalfoundationformind-wanderingorwatchfulnesswhenthesubjectisengagedinconsciousthought.Mind-wanderingandwatchfulnessarefeaturesofourmentallifethathavebeenneglectedbythereactiveframeworkbutas

Buckneretal.suggestmaybeimportantcognitiveactivitiesthatenableustoanticipateandcopewithourenvironments.Theinitialstudiesofthedefaultmodenetworkdidnotfocusonthedynamicsoftheactivityintheseareas,butastudybyBiswal,Yetkin,Haughton,andHyde(1995)showedhowfMRIcouldbeusedtoidentifydynamicprocessesintaskactivatednetworksthatweresoonextendedtothedefaultmodenetwork.TheseresearchersperformedatimeseriesanalysisofafMRIrecordingtakenevery250msandobservedverylow-frequencyoscillations(<.1Hzoronecycleevery10-15seconds)assubjectsperformedasimplemotortask(movingtheirhand).Moreover,theyfoundthattheseoscillationssynchronizedacrosssensoryandmotorregionsofthebraininbothhemispheres,whichtheyinterpretedasindicating

functionalconnectionsbetweentheseregions.Cordes,Haughton,Arfanakis,Wendt,Turski,Moritz,Quigley,&Meyerand(2000)foundsimilaroscillationsinrestingstateBOLDsignalsinnetworksofareaspreviouslyidentifiedasjointlyexhibitingincreasedactivationinsensorimotor,visual,receptivelanguage,orexpressivelanguagetasks.Moreover,their functionalconnectivityMRI (fcMRI)analysis—applyingcorrelationalstatisticstorestingstateBOLDtimeseriesdatatodeterminepatternsofsynchronization—yieldedfunctionalnetworksverysimilartothoseidentifiedfromactivityduringtasks.Thatis,areaswithinthesamenetworkhadcorrelatedpatternsofactivityacrosstime(risingandfallinginsynchrony)regardlessofwhetheroveralllevelofactivitywasrelativelyhigh(e.g.,thesensorimotornetworkwhilemovingahand)orrelativelylow(e.g.,thesamenetworkinarestingstatecondition).

Todeterminewhetherthedefaultmodenetworkalsoexhibitedsynchronizedactivity,Greicius,Krasnow,Reiss,andMenon(2003)employedfcMRIwithtwoseedareasidentifiedwiththedefaultmodenetwork,thePCCandinferiorACC.UsingthePCCastheseed,theyfoundthatitsrestingstateoscillationswerecorrelatedwiththoseinmuchofmedialprefrontalcortex(includinginferiorACCandorbitofrontalcortex),leftDLPFC,IPCbilaterally,leftinferolateraltemporalcortex,andleftparahippocampalgyrus,virtuallythesamerangeofareasasShulmanetal.hadidentifiedintheirmeta-analysis.Theyregardedtheirresultsasproviding“themostcompellingevidencetodatefortheexistenceofacohesive,tonicallyactive,defaultmodenetwork”(p.256).WheninsteadtheventralACCwasemployedastheseedarea,theydemonstratedcorrelatedactivitynotjustinthePCC

butalsointhemedialprefrontalcortex/orbitofrontalcortex,thenucleusaccumbens,andthehypothalamus/midbrainandtheyarguedthattheseprimarilyparalimbicandsubcorticalareascomprisedaseparatenetworkimportantforcalibratingaffectiveandautonomicoperations.TheyarguedfurtherthatthestrongconnectionbetweeninferiorACCandPCCprovidedacruciallinkbetweenthisandthedefaultmodenetwork.SubsequentlyGreiciusandMenon(2004)foundthatthedefaultnetworkincludedthehippocampusandVincentetal.(2006)determinedthatbyseedingananalysiswithahippocampalregiontheycouldfindcorrelatedactivityintherestofthedefaultnetwork.

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Buckner,Andrews-Hanna,andSchacter(2008,pp.4-5)summeduptheperspectiveprovidedbythisresearch:“Thedefaultnetworkisabrainsystemmuchlikethemotorsystemorthevisualsystem.Itcontainsasetofinteractingbrainareasthataretightlyfunctionallyconnectedanddistinctfromothersystemswithinthebrain.”

TheCordesetal.resultsnotedaboveshowedthatevenintherestingstateonecoulddemonstratecorrelatedactivityacrossnetworksthatwouldthenbejointlyactivatedintaskconditions.Foxetal.(2005)foundcorrelatedactivityintherestingstateinanetworkthatwasespeciallyactiveduringattention-demandingtasks(intraparietalsulcus,frontaleyefield,middletemporalregion,supplementarymotorareas,andtheinsula)andshowedthatactivityinthatnetworkandinthedefaultnetworkwereanticorrelated.Thatis,theareasthatwerepositivelycorrelatedwithineachnetworkwerenegativelycorrelatedwithareasintheothernetwork—anoutcomemoreinterestingthanazerocorrelation.Foxetal.emphasizedthat:

anticorrelationsmaybeasimportantascorrelationsinbrainorganization.Littlehasbeensaidpreviouslyintheneuronalsynchronyliteratureregardingtheroleof

anticorrelations.Whilecorrelationsmayserveanintegrativeroleincombiningneuronalactivitysubservingsimilargoalsorrepresentations,anticorrelationsmayserveadifferentiatingrolesegregatingneuronalprocessessubservingoppositegoalsorcompetingrepresentations(p.9677).

TheoscillationsfoundusingfMRIareonetotwoordersofmagnitudeslowerthanthoseobservedwithEEG.Dothey,likethetaandgammaoscillations,playaroleincognition?Sofarnotalotisknownabouttheirfunctionalsignificance,butaveryintriguingfindingbytheRaichlegroupsuggeststhatitmayhavebehavioralconsequences.AsIhavenoted,inreactivestudiesresearchersconstantlyconfrontthefactthatthereisgreatvariabilityintherecordedsignal,whichisoftentreatedasnoise.Fox,Snyder,Zacks,andRaichle(2006)

devisedaninnovativestrategyforshowingthatalargepartofthisvariabilitymaybetheendogenousoscillation.Assubjectsperformedabutton-presstaskwiththeirrighthandwhenevertheyidentifiedaneweventinavideo,amotoractivitythatisdirectedbytheleftsideofthebrain,theresearchersidentifiedtheongoingoscillationinsensory-motorareasonbothsidesofthebrain.TheyfoundthattheoscillationsintherightsideofthebrainaccountedformuchofthevariationfoundinthefMRIsignalontheleftsideofthebrain.Moreover,inasubsequentstudythesamegroupfoundthatthespontaneousfluctuationexhibitedinthecontralateralhemispheretothatcontrollingthebuttonpushaccountedfortheforcewithwhichasubjectpressedthebutton(Fox,Snyder,Vincent,&Raichle,2007).Thisfindingissuggestivethatendogenousactivitymaydirectlyaffectedmentalprocessesandbehavioralactivities.

5.DynamicallyOrganizedNetworksintheBrain

Thepatternofactivitywithincorrelatednetworksandbetweenanticorrelatednetworks,whetherthesenetworksaretaskcharacterizedorthedefaultmodenetwork,duringconditionswheresubjectsarenotrequiredtoperformanytasksrevealsthatthereishighlyorganizedendogenousactivityinthebrain.DuringthesameperiodaswhenfcMRIwasrevealingthesefunctionalnetworks,othertechniques(e.g.,diffusiontensorMRI)were

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beingdevelopedtoanalyzestructuralconnectivityinthebrain.Thestructurallyandfunctionallycharacterizednetworksappeartobehighlycorrelated.Greicius,Supekar,Menon,&Dougherty(2009)showedthattheregionsinthedefaultmodenetworkareanatomicallyconnected;vandenHeuvel,Mandl,Kahn,&Pol(2009)foundthateightoftheninenetworkstheyidentifiedintherestingstatecorrespondtoonesthatcanbe

characterizedanatomicallyasconnectedbyfibertracts.Moreover,thesenetworkshaveasimilartypeofstructure.Whetheranalyzedstructurallyorfunctionally,theyhavebeenfoundtoexhibitasmall-worldstructure(Watts&Strogratz,1998).Small-worldorganizationisaformoforganizationthatliesbetweenregularlatticesandrandomlyorganizednetworks—asinlatticesmostconnectionsarebetweennearbyunitsbutafewconnectbetweendistantlocations.Forinformationprocessingpurposes,latticestructureshavethevirtueofcreatingmodulesofunitsthatfunctiontogethertoperformaparticulartaskwhereasrandomnetworksexhibitashortpathofcommunicationacrossthenetwork.Small-worldnetworkpossessbothpropertiesandmanyreal-worldnetworkshavebeenfoundtoexhibitsmallworldorganization,includingthedefaultmode

networkandtaskdirectednetworks(Sporns,2010).Inthinkingaboutcognitivearchitectures,itiscommontoconstruethemassetdownbeforecognitiveactivityandsimplysettingthepossibilitiesforsuchactivity.Butactivityinthebrainisnowunderstoodascapableofmodifyingthestructuralorganizationofthebrain.ThebrainisplasticandprocessessuchasLTP,discussedabove,canchangetheconnectivityofthebrain.AnintriguingpossibilitythatRaichlehasadvancedisthatthesynchronizedslowoscillationswithinnetworksmayplayaroleinsculptingthebrain.GongandvanLeeuwen(2004)showedinacomputationalmodelthatwhenchangingconnectionstrengthsbetweenunitsoccurredwhenunitsexhibitsynchronizedactivity,small-worldorganizationwoulddevelopinthenetwork.Evenwhenthesenetworksbeginwithrandom

connectivity,theyself-organizeintoclusterslinkedtoeachotherthroughhubs.Rubinov,Sporns,vanLeeuwen,andBreakspear(2009)appealtoGongandvanLeeuwen’smodeltosuggestthataspatternsofsynchronizedactivityoccurinthebrain,thebrainmightself-organizeintoasmallworldnetworkwithhubs.Someofthesynchronybetweenneuronsmightbetheresultofspecificstimuliortasksthebrainisrequiredtoperform.Butifthesynchronizationbetweenbrainregionsisongoing,occurringduringtherestingstateaswellasundertasksconditions,thensomeofthesynchronizedactivitythatshapesthewiringofthebrainwillbearesultofendogenousfunctioning,notjuststimulusinducedactivity.6.DevelopinganEndogenouslyActiveCognitiveArchitecture

IntheprecedingsectionsIhavepresentedevidencefromavarietyofsourcesthatbrainsnotonlyrespondtotasksorstimulibutalsoareendogenouslyactiveandthatthisactivitymaybeimportantforunderstandingcognition.Tobeginwith,neuronsexhibitendogenousoscillationsintheelectricalactivityontheirmembranesandtheyaremorelikelytogenerateactionpotentialswhentheyareinahypopolarizedphasethanwhentheyarehyperpolarized.Inaddition,someneuronsarecapableofgeneratingactionpotentialsindependentlyofanyinput.Further,attheleveloflargepopulationsofneuronsbothEEG

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andfMRIrevealongoingoscillatoryactivitythathastraditionallybeentreatedasnoise.EEGdetectsoscillationsinavarietyoffrequencybandsabove1Hz.Bergeridentifiedactivitydominantlyinthealphabandwhensubjectsaresittingquietlywithoutstimulation,butevenduringthisperiodthereareoscillationsinotherfrequencyranges.Boththetaandgammaoscillationsincortexappeartobeimportantincreatingtemporalwindowswith

gammawindowsappearingtoenableorganizedprocessingofinputsandthetaoscillationsservingtoperiodicallyresettheprocessingsystemtorespondtoalternativeinputs.Inthehippocampusdifferentregionsoscillateatdifferentfrequencies,withsomeregionsabletoentraintoandthenrespondtotheoscillatoryactivityofdifferentregions.ThetaoscillationsprovideatimingwindowinwhichtherelativetimingofactionpotentialscarriesinformationandregulateswhetheractivitysuchasLTPistooccur.Therecentfindingofoscillationsatlessthan.1HzwithfMRIprovidefurtherperspectiveontheroleofendogenousactivityinthebrain,especiallyinrevealingongoingcoordinationofactivityinmultiplebrainnetworksevenduringtherestingstate.Thereissuggestiveevidencethatthisactivityismanifestinbehaviorandunderliesongoingthoughtnottiedtotasks(mind-wandering).Thepossibleroleofthisactivityinsculptingthebrainsuggeststhatitis

importantinestablishingtheprocessingcapacitiesofthebrain—i.e.,thecognitivearchitecture.Theevidenceofendogenousactivityinthebrainandsuggestionsthatitmaybeimportantforcognitionposesachallengetocognitivesciencethatreliesonproposedcognitivearchitecturesthatarepurelyreactive.Moreover,itraisesthequestionofhowcognitiveresearchersmightdevelopaccountsofthecognitivearchitecturethatincorporateendogenousactivity.Withindividualneurons,endogenousactivityappearsintheongoingoscillationsintheelectricalpotentialsoverthemembranes.Onewayofcreatinganendogenouslyactivesystemistoconstructitoutofoscillators.Populationsofoscillatorshavethepotentialtosynchronize,butdependingonthepatternofconnectionsbetween

them,theymayonlyformtemporarycoalitionsofsynchronizedcomponentsandsoexhibitmeta-stability.Accordingly,oscillatorswithprocessesforsynchronizingthemconstitutesystemscapableofthesortsofendogenousactivityfoundinthebrain.Cognitivearchitecturesareinstantiatedincomputationalmodels.Itisrelativelyeasytodevelopcomputationalmodelsthatexhibitongoingoscillations—infieldsofbiologydealingwithoscillatoryphenomena,suchascircadianbiology,researchersemploymodelsofoscillatorycomponents.Someoftheserepresentthecomponentsofthemechanism(involvinginthecircadiancase,thetranscriptionandtranslationofgenes)whileothersabstractfromthesedetailsandemploy,forexample,equationsdescribingavanderPoloscillator(Bechtel&Abrahamsen,2010).Populationsaremodeledbyaddingtermstothe

basicequationsforoscillatorstocharacterizethetransmissionofsignalsbetweenthem.Inthecontextofneuroscience,computationalmodelershaveincreasinglydirectedtheirattentiontomodelingmembranepotentialsandusingthesemodeltounderstandtheroleofmembranepotentialsingeneratingpopulation-levelrhythms(Destexhe&Sejnowski,2003).Thesemodelsprovideresourcesforcreatingcomputationalmodelsofcognitivearchitecturesthatexhibitendogenousdynamicalactivity.Developingandusingsuchalternativecognitivearchitectureswouldprovidecognitiveresearchersabasisforinvestigatingthepossiblecontributionsofendogenousactivitytocognition.Ontheother

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hand,byrelyingonlyoncognitivearchitecturesthatdonotincorporateendogenousactivity,cognitivescientistsmaylacktheresourcesformodelingcognitionthatoccursinbrainsthatdomakeendogenousactivityavailableasacognitiveresource.References

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