Threat visibility modulates the defensive brain circuit underlying fear and anxiety

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Rigoli, F., Ewbank, M., Dalgleish, T. & Calder, A. (2016). Threat visibility

modulates the defensive brain circuit underlying fear and anxiety. Neuroscience Letters, 612,

pp. 7-13. doi: 10.1016/j.neulet.2015.11.026

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NeuroscienceLetters612(2016)7–13

ContentslistsavailableatScienceDirect

Neuroscience

Letters

j ou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / n e u l e t

Research

paper

Threat

visibility

modulates

the

defensive

brain

circuit

underlying

fear

and

anxiety

Francesco

Rigoli

_,

_Michael

_Ewbank

_,

_Tim

_Dalgleish

_,

_Andrew

_Calder

a_{WellcomeTrustCentreforNeuroimagingatUCL,London,UK} b_{MRCCognitionandBrainSciencesUnit,Cambridge,UK}

h

i

g

h

l

i

g

h

t

s

•WeusedhumanfMRIandanovelcomputer-basedtasktostudytheeffectsofvisualthreatuncertaintyonbrainactivity.

•Lackofvisualthreatinformationincreasedactivityinhippocampus,ventromedialprefrontalcortexandamygdala(regionsinvolvedinanxiety). •Presenceofvisualthreatinformationincreasedactivityinperiaqueductalgray(involvedinfear).

•Hightrait-anxietyparticipantsanticipatedhippocampalactivationwhenvisualthreatinformationwasnotprovided.

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t

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l

e

i

n

f

o

Articlehistory: Received25August2015

Receivedinrevisedform24October2015 Accepted16November2015

Availableonline2December2015

Keywords: Fear Anxiety Uncertainty Hippocampus PAG Amygdala

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Recenttheoriesdistinguishanxietyfromfearinthebrain.Anxietyisassociatedwithactivationin ven-tromedialprefrontalcortexandhippocampus,whilefearisassociatedwithactivationinperiaqueductal gray,withamygdalainvolvedinprocessingaspectsofbothemotionalresponses.Thesetheoriespropose thattheamountofinformationavailableaboutthreatdetermineswhichofthetwodefensiveresponsesis elicited,withfearandanxietyassociatedwithwell-definedanduncertainthreatsrespectively.However, adirecttestofthishypothesisislacking.HereweprovidesuchatestusingfMRItorecordparticipants’ brainactivitywhiletheyperformedacomputer-basedtaskwhichrequiredtopressabuttontomove anartificialagenttoatargetpositionwhileanartificialpredatorchasedtheagent.Inonecondition (associatedwithfear)thepredatorwasvisible,whileinanothercondition(associatedwithanxiety)the predatorwasinvisible.Ventromedialprefrontalcortex,hippocampus,andamygdalashowedincreased activitywhenthepredatorwasinvisiblecomparedtovisible,whiletheoppositeeffectwasobservedin periaqueductalgray.Wealsoobservedthatparticipantswithhighbutnotlowtrait-anxietyshowedan hippocampalactivationwithinvisiblethreatatanearliertimestageduringthetrial.Thesefindingshelp clarifytheneuralmechanismsthatunderliedifferentdefensiveemotionsandshedlightonhowthese mechanismsmaycontributetoexaggeratedanxiety.

©2015TheAuthors.PublishedbyElsevierIrelandLtd.ThisisanopenaccessarticleundertheCCBY license(http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Complex organisms are equipped with a vast repertoire of defensiveresponsesthathaveevolvedtoadapttoaconsiderable varietyofaversiveconditions.Research investigatingtheneural substrates underlying defensive behavior suggests that defen-siveresponsesare supportedbya braincircuitextendingfrom ventromedialprefrontalcortex(vmPFC),hippocampusand amyg-dalatoperiaqueductal gray(PAG;[11,23]).Centraltothisbrain

∗ Correspondingauthorat:WellcomeTrustCentreforNeuroimagingatUCL, QueenSquare,LondonWC1N3BG,UK.

E-mailaddress:[email protected](F.Rigoli).

systemistheamygdala,aregioninvolvedinlearningand coordi-natingconditionedresponses[8,10]andregulatedbybidirectional connections withvmPFC [31].The key role of hippocampusin defensivebehaviorissupportedbyseveralfindings[1,2,17,33]such astheevidencethatanxiolyticeffectsofbenzodiazepinesare medi-atedbyanimpactonhippocampus[2].Anotherimportantregionof thedefensivenetworkisPAGwhichplaysacentralroleinguiding freezingandfight/flightreactions[12,15,26,27].

Althoughtheareascomprisingthebrain’s defensivenetwork appearwell-established,itremainsunclearhowactivationinthese areasismodulatedbydifferentaversivecontexts.Contemporary theoriesproposethatevolutionhasfavoredthedifferentiationof twokindsofdefensiveresponsesthatcanbetracedbacktofear andanxiety,eachrecruitingdistinctneuralregions.Fearhasbeen

http://dx.doi.org/10.1016/j.neulet.2015.11.026

8 F.Rigolietal./NeuroscienceLetters612(2016)7–13

Fig.1. (A)Taskdescription.ParticipantsstartedeachtrialbyindicatingonaVASscaletheirexpectancyofbeingcapturedbytheartificialpredatorinthenexttrial.Atthe sametime,participantswereinformedaboutthecondition(HIDorVIS)ofthenexttrialbyapaneldisplayedonthebottomofthescreenreproducingthecondition.After, arectangularpathwasdisplayedtogetherwithablueballrepresentingtheagentpositionedinthemiddleofthepathplus,inVIStrialsonly(presentedintheexample showninthisfigure),aredballrepresentingapredatorappearingontheleftextremesideofthepath.After1–3s,theblueballturnedgreenandparticipantshadtopressa buttonandkeepitpressedtomovethegreenball/agenttowardthetargetpositionrepresentedbyagraysquareatthefarrightsideofthepath.Atthesametime,thered ball/predatormovedclosertotheagent.On50%oftrialscaptureoccurred(50%ofthetimeattargetposition,asintheexample,50%alongthepath),whileon50%oftrials theagentreachedthetargetwithoutbeingcaughtandasafetysignal(twoyellowhorizontalarrows)wasdisplayeduponthetarget.(B)Relationshipbetweentrait-anxiety andaverageVASscoreindicatingthesubjectiveprobabilityofbeingcapturedbythepredator(r(22)=0.498,p=0.018).(Forinterpretationofthereferencestocolorinthis figurelegend,thereaderisreferredtothewebversionofthisarticle.)

associatedwithactivationinPAG,andanxietywithactivationin hippocampusandvmPFC,withtheamygdalainvolvedin process-ingaspectsofbothemotionalresponses[9,11,23,28,29].Thelevel ofuncertaintyregardingdangeristhoughttobeoneofthekey dimensionsthateliciteitherofthetwodefensiveresponses,with fearandanxietybeingevokedbywell-definedandundetermined threatsrespectively[9,17,19,25,32].Threatuncertaintyisaffected bytheamountofvisualinformation,anaspectimportantinseveral ecologicalcircumstances.Forinstance,thenightpreventsviewing apredator,inducingaresponsedifferentfromthatexhibitedinthe daylight[18].Howeveradirectinvestigationoftheimpactofthreat uncertainty,andmorespecificallyofthreatvisibility,onactivityin thedefensivebrainsystemislacking.

Inordertostudytheimpactofuncertaintyonthedefensive brain network, we used a paradigm which manipulated visual informationaboutthreat.Weusedfunctionalmagneticresonance imaging(fMRI)torecordtheneuralresponseofhealthy individu-alswhiletheyperformedacomputer-basedtask(Fig.1A)inwhich theyhadtopressabuttontomoveanartificialagent,displayed onthe screen, toa targetposition, while an artificialpredator waschasingtheagent.Inaconditionassociatedwithlowthreat uncertainty, visualfeedback onthe predatorposition was pro-videdthroughoutthetrial(visiblethreat:VIS),whileinanother condition,associatedwithhighthreatuncertainty,visualfeedback onthepredator’spositionwasnotprovided(hiddenthreat:HID). Onhalfofthetrialstheagentreachedthetargetwithoutbeing caughtandontheotherhalfthepredatorcapturedtheagentand aloudscreamnoisewasdeliveredaspunishment.Consistentwith recentproposals[9,11,23],wepredictedthatVIScomparedtoHID wouldactivatePAG,whichguidesfight/flightreactionsassociated withfear, whereasHID comparedtoVISwould activatevmPFC andhippocampuswhicharethoughttounderliethecognitive pro-cessescharacterizinganxiety.Wealsopredictedtheinvolvement

oftheamygdala,althoughgivenitsroleinbothfearandanxiety responses,wedidnothaveapriorihypothesesregardingthisregion

[11,23].

We were also interested in investigating the relationship betweenindividualdifferencesinemotionalrespondingandthe functionofthedefensivebraincircuit.Toaddressthis,westudied theimpactoftrait-anxiety[36]onneuralresponsetoHID com-paredtoVIS.Ithasbeensuggestedthatakeydifferencebetween anxiousandnon-anxious individualsis thattheformer tendto anticipateintimeananxietyresponsetodanger[22].Basedonthis, wepredictedthat,inhightrait-anxietybutnotinlowtrait-anxiety individuals,theneuralresponseinhippocampusandvmPFCforHID comparedtoVISwouldemergeatanearliertimepointduringthe trial,reflectingananticipatedanxietyreactioninhightrait-anxiety participants.

2. Methods

2.1. Participants

Cam-F.Rigolietal./NeuroscienceLetters612(2016)7–13 9

bridgeLocalResearchEthicsCommittee.Allparticipantsprovided informedwrittenconsentandwerepaidforparticipating.

2.2. Experimentalparadigm

Attrialstart,arectangularpathmadeof23squareswas pre-sented on the screen together with a blue ball representing a virtualagent(participantswereinstructedthatthevirtualagent representedthemselves)thatappearedinthemiddleofthepath (Fig.1A).After1–3stheballturnedgreenandparticipantshadto pressabuttontomovetheball/agenttoatargetposition(agray squarelocatedattherightendofthepath).Participantshadto keepthebuttonpressedtomaketheagentadvanceonesquareper secondresultingin12storeachthetarget.Overall56trialswere playedsplitacrosstwoconditions(VISandHID)orderedrandomly. InVIStrials,aredballrepresentinganartificialpredatorwas pre-sentedontheleftsideofthepathandonceparticipantspressedthe buttonthepredatorstartedmovingrandomly1–3squaresper sec-ondtowardtheagent(thepredatorneverpassedtheagent).On50% ofVIStrials,theagentreachedthetargetwithoutbeingcaptured andontheother50%thepredatorcapturedtheagent,50%ofthe timealongthepath(thepositionwasrandomlyselected)and50% ofthetimeattargetposition(i.e.,atthefarrightend).InHIDtrials, theredball/predatorwasinvisiblebutparticipantswereinstructed thatthepredatorwaspresentandbehavedasintheVIScondition. AspertheVIScondition,on50%ofHIDtrialstheagentwascaptured, 50%ofthetimealongthepathand50%ofthetimeattargetposition. ForbothVISandHID,whencaptureoccurred,theredball/predator appearedupontheagentandaloudscream(103dB)wasdelivered for1sviaheadphonesaspunishment.Whenthetargetwasreached withoutcapture,asafetysymbol(twoyellowhorizontalarrows) appearedonthetargetpositionfor2s.Trialswereseparatedby 6–12s.Oneachtrial,ifthebuttonwasnotpressedwithin2s,or wasreleasedbeforereachingthetarget,theagentwascaptured andthetrialimmediatelyrepeated.

Beforeeachtrial,participantswereaskedtoestimatethe proba-bilityofbeingcapturedonavisualanalogscale(VAS).Participants wereinformedofthenexttrialconditionduringtheVAS presen-tationbyareproductionoftheconditiondisplayedatthebottom ofthescreen.TrialsstartedimmediatelyafterVASchoiceswere finalized.

2.3. fMRIdataacquisitionandanalysis

MRIscanningwasconductedattheMedicalResearchCouncil CognitionandBrainSciencesUnit.Visualstimuliwerepresented with E-prime 2. Echo-planar T2*-weighted (EPI) images were acquiredwithSiemensTimTrio3TMRsystemwitha12 chan-nelheadcoilandeachimagevolumeconsistedof32interleaved sliceswith3×3×3resolution and2srepetitiontime. Thefirst five volumes acquired were discarded to allow for equilibra-tion effects. T1-weighted structural images were acquired at a 1×1×1mmresolution.Imagingdatawereanalyzedusing Statisti-calParametricMapping(SPM)version8(WellcomeTrustCentrefor Neuroimaging).Preprocessingincludedspatialrealignmenttothe meanvolume,slicetimecorrection,co-registration,normalization tothestandard MontrealNeurologicalInstitute(MNI)template witha3×3×3voxelsize,andsmoothingusing8mmGaussian kernel.High-passtemporalfilterwithacutoffof128sandAR-1 modelwereapplied.

Neuralactivationwasmodeledwithacanonicalhemodynamic responsefunctionandaGeneralLinearModel(GLM)including6 movementregressorsofnointerestplusboxcarfunctionregressors atVASpresentation(oneforHIDandoneforVIS),attrialstart(i.e., whenparticipantspressedthebuttonaftertheblueball/agentturns green;oneforHIDandoneforVIS),attrialend(i.e.,athalfofthe

Table1

Brainareassignificantlyactivatedacrossallparticipantsattrialendforeachofthe differentcontrasts(nosignificantactivationwasobservedattrialstart).Areaswith asterisksindicateROIsinwhichstatisticsweresmall-volumecorrected,whereas forotherareasawhole-braincorrectionwasused.Inbothcases,p<0.05FWEwas appliedassignificancecriterion.(A)HIDminusVIS;(B)VISminusHID.

(a)HIDminusVIS

Area Peakcoordinates Z p

Lefthippocampus* −28,−22,−16 2.64 0.042

Righthippocampus* 28,−22,−16 3.06 0.020

Leftamygdala* −26,−8,−20 2.82 0.016

Rightamygdala* 24,−4,−22 2.78 0.030

vmPFC* −6,58,4 4.33 0.006

Posteriorcingulate(BA29) −14,−44,10 5.09 0.001

Cuneus(BA18) 12,−86,20 4.99 0.016

Lingualgyrus(BA18) 14,−72,−2 4.74 0.044

(B)VISminusHID

Area Peakcoordinates Z p

PAG* −3,−27,−6 2.81 0.020

Leftinferiortemporalgyrus −50,−68,4 5.94 0.000

Rightprecuneus(BA31) 28,−74,30 5.79 0.000

Rightmiddlefrontalgyrus(BA6) 36,−2,44 4.88 0.020

Fusiformgyrus(BA19) 40,−80,−8 6.63 0.000

Inferiorfrontalgyrus(BA44) 54,12,16 5.56 0.001

Precuneus(BA7) 22,−56,54 5.07 0.012

Middlefrontalgyrus(BA6) 24,2,64 4.98 0.016

Leftcerebellum −6,−76,−30 4.75 0.042

pathcorrespondingto6saftertrialstart;oneforHIDandonefor VIS)andtwostickfunctionregressorsatoutcomedelivery(onefor captureandoneforsafety).Trialsinwhichthepredatorcaptured theagentalongthepathweremodeledwithseparatedregressors attrialstart(oneforHIDandoneforVIS).

Wepredictedthattheparticipants’emotionalresponseelicited bytheexperimentalconditionswasevidenttoalargerdegreeat trialend,whenthepredatorwascloserintimeandspacetothe agent.Therefore,toincreasethestatisticalpower,wefocusedon thistimepointanalyzingthecontrastbetweenHIDandVISforall subjects(withoutconsideringthiscontrastattrialstart).In addi-tion,wealsopredictedthatparticipantswithhighlevelsoftrait anxiety(accordingtoamediansplit)wouldexhibitanemotional responseearlierduringthetrial,henceforthissubgroupof par-ticipantsweanalyzedthecontrastbetweenHIDand VISattrial start.

Analyses focused onthefollowing regions ofinterest (ROIs) extractedfrompreviousstudiesthatusedasimilarparadigmto investigateactivityintheseregions[26,27]:bilateralhippocampus (inMontrealNeurologicalInstitute(MNI)coordinates,left:−28,

−10,−22;right:28,−10,−22),bilateralamygdala(left:−28,−6,

−27;right:28,−6,−27),vmPFC(−1,51,−1)andPAG(−2,−28,−8). ROIsweredefinedas6mmspherescenteredonpriorcoordinates

[26,27].Forhypothesistesting,statisticsweresmall-volume cor-rected(SVC)foreachROIseparatelyandwhole-braincorrectedfor otherbrainareas.Inbothcases,ap<0.05familywiseerror(FWE) wasusedassignificancecriterion.Areaswithsignificantactivation accordingtothesecriteriaarereportedinTable1.

10 F.Rigolietal./NeuroscienceLetters612(2016)7–13

Fig.2.(A)ActivityattrialendforHIDminusVIS(red/orangeactivation)andVISminusHID(green/blueactivation)inROIsindicatedbyyellowcircles:bilateralhippocampus, bilateralamygdala,vmPFC,andPAG(thresholdforactivationmapsisp=0.005).(B)Top,contrastcoefficientattrialstart(graybars)andtrialend(whitebars)forHIDminus VIS(inpeak-activationvoxelinlefthippocampus,leftamygdalaandleftvmPFC)andbottom,forVISminusHID(relativetothepeak-activationvoxelinPAG).Significant effectsaremarkedwithasterisks.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

3. Results

3.1. Behavior

In a post-scan questionnaire, participantsshowedno differ-enceinratings(ona0-10VAS)ofemotionalintensityforHIDand VISconditions(HD:mean4.59,SD1.89;VIS:mean4.45,SD1.90;

t(21)=0.603,p=0.55;twotailedp<0.05isusedassignificance cri-terionforbehavioralanalyses)indicatingthatthesetwoconditions werematched.Howeverreactiontimesforbuttonpressingafter theblueball/agentturns greenwerefasterintheVIScondition comparedtoHIDcondition(t(21)=3.860,p=0.001),indicatingthat VISwasassociatedwithincreasedmotorreactivity,consistentwith theideathatthisconditioninducedafight/flightreaction charac-teristicoffear.

Acrossparticipants,averageVASscoresoftheestimateof cap-tureprobabilitydidnotdiffersignificantlyfrom0.5(i.e.,thetrue captureprobability),thoughatrendtowardpessimisticestimates wasevidentt(21)=1.764,p=0.09;withnodifferencebetweenHID and VIS,(t(21)=0.860,p=0.4). Thismight suggest that partici-pantswereslightlypessimistic,thoughcautionshouldbetaken becauseofthenostatisticallysignificantdata.Trait-anxiety[36]

correlatedwithaverageVASscore(r(22)=0.498,p=0.018)bothfor HID(Fig.1B;r(22)=0.425,p=0.048)andVIS(Fig1B;r(22)=0.518,

p=0.012)indicatingthathightrait-anxietyindividualsweremore pessimistic(Fig.1B).

3.2. Brainimaging

ToinvestigatetheneuralactivationinresponsetoVIScompared toHID,werestrictedouranalysistotrialend(i.e.,thesecondhalf

ofthetrialstarting6safterbuttonpressingandlastinguntiltrial end)becauseweexpectedtheneuraleffecttoemergeespeciallyat thistimepointwhenthepredatorwascloserintimeandspaceto theagentthusenhancingparticipants’emotionalresponse.Across allparticipants(Fig.2),attrialendHIDminusVISwasassociated withincreasedactivationinbilateralhippocampus(left:,−28,−20,

−16;Z=2.64,p=0.042SVC;right:28,−22,−16;Z=3.06,p=0.02 SVC),bilateralamygdala(left:−26,−8,−20;Z=2.82,p=0.016SVC; right:24,−4,−22;Z(21)=2.78,p=0.03SVC),andvmPFC(−6,58,4;

Z=4.33,p=0.006SVC),butnotPAG(p>0.05SVC).ThecontrastVIS minusHIDwasassociatedwithincreasedactivationinPAG(−3,

−27,−6;Z=2.81,p=0.02SVC)butnothippocampus,vmPFC,or amygdala(p>0.05SVC).

Consistentwithpreviousmodelsofanxiety[22],wepredicted that highbut not low trait-anxiety individuals would showan increasedresponseinhippocampusandvmPFCforHIDminusVIS alsoatanearlier timepoint duringthetrial.Based onthis, we investigatedthebrainresponseattrialstart(i.e.,buttonpressing) and,acrossallparticipants,wefoundnodifferenceinactivation betweenHIDandVISinanyregion(p>0.05SVC).However,attrial start,wefoundthatthecoefficientofthecontrastHIDminusVIS (relativetothepeak-activationvoxelintheROIs)showeda pos-itivecorrelationwithtrait-anxietyinlefthippocampus(Fig.3A;

F.Rigolietal./NeuroscienceLetters612(2016)7–13 11

Fig.3.Top,relationshipbetweentrait-anxietyandlefthippocampalactivationforHIDminusVIS(inarbitraryunits)attrialstart(A)andtrialend(B).(C)Contrastcoefficient attrialstart(graybars)andtrialend(whitebars)forHIDminusVISinpeak-activationvoxelinlefthippocampus,forhighandlowtrait-anxietyparticipantsaccordingto amediansplit.(D)RelationshipbetweenVASscoreandlefthippocampalactivationforHIDminusVIS(inarbitraryunits)attrialstart.Significanteffectsaremarkedwith asterisks.

Finally,wedeterminedwhethertherelationshipbetween trait-anxietyand hippocampalactivityforHIDminusVISwaslinked to participants’VAS scores relating tocapture expectancy. We observedthattheaverageVASscorecorrelatedwithactivityinleft hippocampusforHIDminusVISattrialstart(Fig.3D;r(22)=0.507;

p=0.016)butnottrialend(p>0.05).Giventhattrait-anxiety,VAS scoresand hippocampalresponse forHID minusVISare corre-latedwitheach other,weinvestigated amediationmodel with the hypothesis that the anticipatory hippocampal response for HID minus VIS influences theVAS score which in turn affects trait-anxiety.Thismediationanalysiscanbeperformedwith hier-archicalregressionwhichshowedanon-significantresultforthe hippocampalresponse(t(22)=1.22,p=0.237)butalsofortheVAS score(t(22)=1.62, p=0.122),thusnotsupportingthemediation model.

4. Discussion

Influential theories propose that evolution has shaped two differentkinds of defensiveemotions,namely fear and anxiety

[9,11,32,23],andthateachisassociatedwithspecificcognitiveand neuralmechanisms.Theamountofinformation availableabout dangeristhoughttounderlietheelicitationofeitherofthetwo emotions,with fear elicited bywell-defined dangers and anxi-etyelicitedbyundeterminedthreats[9,19,16,25].Herewetested aspectsofthismodelbymanipulatingtheamountofvisual infor-mation about danger and investigating its impact on behavior andactivationinthedefensivesystemofthebrain.Importantly, punishmentwasmatchedinquantity, location,andtime across conditions,andparticipantsevaluatedHIDandVISasequally neg-ative,indicating that emotionalintensitywasequivalent across conditions.However,RTswerefasterinVIScomparetoHID, sug-gestingthatVISwasassociatedwithincreasedmotorreactivity. Althoughalternativeexplanationscannotberuledoutcompletely, thisobservationisconsistentwiththepossibilitythatthetwo con-ditionseliciteddifferentkindsofemotionalresponses,withVIS beingassociatedwithagreaterfight/flightreactioncharacteristic offear.

Inlinewithourpredictions,attrialend(whendanger, repre-sentedbytheartificialpredator,wascloserintimeandspace)HID wasassociatedwithgreateractivationinvmPFCandhippocampus, regionsimplicatedinthecognitiveprocessesunderlyinganxiety.A similareffectwasobservedintheamygdala.Bycontrast,VISwas associatedwithincreasedactivationofPAG,whichislinkedwith thecontroloffight/flightreactionsconnectedtofear.

Thekeyroleofamygdalainlearningandcoordinatingdefensive responsesiswellestablished[8,10].Despiteevidenceindicating thatamygdalaisinvolvedinprocessingaspectsofbothfearand anxiety[11,23],we observedincreasedactivationinthis region duringHIDcomparedtoVIS.Thisresultcanbeexplainedbythe factthatamygdalaisparticularlyrecruitedduringtheprocessing of threateninginformation underconditions ofuncertainty and ambiguity[39],characteristicofHID.

Amygdala is widely connected with regions of the vmPFC thatprocesshigh-ordercontextualinformationsuchasthevalue ofexpectedoutcomes,andareimportantinemotionregulation

[30,31].Extensiveevidencefromstudiesofhumanandnon-human animals has shown that activation in hippocampus, and espe-ciallyinitsventralportion,elicitsabehavioralinhibitionresponse thatcharacterizesanxiety[1,2,17,33,35].Inaddition,recentdata indicatesthatthebehavioral inhibitionelicited byhippocampal activationisaccompaniedbysuppressionofconditionedresponses whicharesignaturesoffear,suchasfight/flightreactions[35].PAG isthoughttoplayacentralroleinregulatingfearresponsesbased onsubstantialevidencethatactivityinthisareaaffectsthe per-formanceof freezingand fight/flight reaction[12,15,26,27].Our findings buildonpreviousworkonthedefensivebrain system byprovidingevidencethatdifferentamountsofvisual informa-tionaboutthreatareassociatedwithactivityinspecializedbrain regions.

12 F.Rigolietal./NeuroscienceLetters612(2016)7–13

intermsoffearoranxietyandinfluenceactivationofthedefensive brainnetwork.

OfnoteisthattheregionsactivatedforHIDcomparedtoVIS correspondtothedefaultmodenetwork,asetofbrainstructures recruitedduringrestingstatecomparedtotaskperformance[5,21]. Thoughatfirstthismightappeartosuggestareducedengagement inourtaskduringHIDcomparedtoVIS,thisisunlikelygiventhat participantsattributedequalemotionalintensitytothetwo condi-tions.Onepossibilityisthatthedefaultmodenetworkisinvolved inthecognitiveprocessescharacteristicofanxietysuchasworry, whichmightbeinhibitedduringfearfulfight/flightresponses.This isalsosupportedbydatainanxietypatientsshowing,during rest-ingstate,anenhancedactivityinvmPFC,akeyregionofthedefault modenetwork[40].

Participants’estimateofcaptureprobabilitywashigherthan 50%(i.e.,thetrueunderlyingprobability),indicatingapessimistic bias.Though thisemergedasa significancetrendand therefore shouldbetakenwithcaution,itreplicatesapreviousstudy[16]. Theseresultsindicateapessimisticbiasinaversivecontexts, espe-ciallyunderambiguityanduncertainty.Givensubstantialevidence indicatinganoptimismbiasinappetitivecontexts[34],ingeneral theseresultsmightsuggestthathumansoverestimatethe occur-renceofsalientoutcomescomparedtonullevents,independentof whetherthepredictedoutcomeisrewardorpunishment.

Enhanced pessimistic biases have been reported in normal subjectswithhightraitanxiety[7,24,37]andinpatientswith gen-eralizedanxiety[4,6],socialanxiety[13,14],andposttraumatic stressdisorder[38].Thisfitswiththeideathatexaggeratedanxiety ischaracterizedbypessimisticbiasesthatmightunderlieenhanced worryandrelentlessthinkingaboutpossibledangers[3,17].We replicatethese findings showing a correlationbetween partici-pants’trait-anxietyandsubjectiveestimatesoftheprobabilityof beingcaptured bythe predatorin thetask. To ourknowledge, thisisfirstvalidatedparadigmwherearelationshipbetween pes-simismandtraitanxietyemergeswhichalsoallowssimultaneous brainrecording,andthereforerepresentsapromisingoptionfor thestudyofbrainprocessesunderlyingpessimisticjudgementsin psychopathology.

Animalstudieshaverevealedarelationshipbetweenenhanced hippocampalactivityand anxiety[2,17,35]which haverecently beensupportedbyhumanfMRI studies[1,20,33].Aninfluential modelproposesthatthecentralcharacteristicofexaggerated anx-ietyisananticipatedresponsetodangerbasedonthepossibility thatanxiousindividualsworrymorethannon-anxiousindividuals withregardtodistalthreats,butequallyinthecontextofproximal threats[22].Inaccordancewiththismodelandliteraturelinking anxietywithhippocampalactivity,wepredictedthathigh com-paredtolowtrait-anxietyindividualswouldshowgreateractivity inhippocampusforHIDcomparedtoVISattrialstart,whenthe threatwasdistantintimeandspace.Theresultssupportedour prediction,andappearconsistentwiththehypothesisthathigh trait-anxietyindividualsanticipateananxietyreaction[22]which isassociatedwithanticipatoryhippocampalactivation.However,it isimportanttostressthatthecurrentstudyonlyincludedhealthy individuals,andthereforefurtherinvestigationisrequiredto estab-lishwhetherourresultsgeneralizetoclinicalpopulations.

Insummary,weshowthattheamountofvisualinformation aboutthreatmodulatesactivityinthedefensivebraincircuit,with lackofinformation activatingareasunderlyinganxiety,suchas hippocampusand vmPFC but also amygdala, a regioninvolved inprocessing aspectsof bothfear and anxiety.By contrast,the presence of visual information activates areas underlying fear suchasPAG.Hightrait-anxietyindividualsshowedan anticipa-toryhippocampalresponsewhenvisualinformationwasabsent. Altogether,thesefindingshelpclarifyingtheneuralandcognitive

mechanismsunderlyingdefensivebehaviorandexaggerated anx-iety.

Acknowledgment

Thisworkwasfunded bytheU.K.Medical ResearchCouncil (GrantMC US A0605PQ50).

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