Contents lists available atScienceDirect
Behavioural
Processes
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / b e h a v p r o c
Potentiation
rather
than
distraction
in
a
trace
fear
conditioning
procedure
M.A.
Pezze,
H.J.
Marshall,
H.J.
Cassaday
∗SchoolofPsychology,UniversityofNottingham,UnitedKingdom
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received10December2015 Receivedinrevisedform5April2016 Accepted5April2016
Availableonline6April2016
Keywords: Traceconditioning
Conditionedemotionalresponse Attention
Distractor Potentiation
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Traceconditioningproceduresaredefinedbytheintroductionofatraceintervalbetweenconditioned stimulus(CS,e.g.noiseorlight)offsetandunconditionedstimulus(US,e.g.footshock).Theintroduction ofanadditionalstimulusasadistractorhasbeensuggestedtoincreasetheattentionaldemandsofthe taskandtoextendtheusefulnessofthebehaviouralmodel.InExperiment1,theCSwasnoiseandthe distractorwasprovidedbyanintermittentlight.InExperiment2,theCSwaslightandthedistractorwas providedbyanintermittentnoise.Inbothexperiments,theintroductionofa10straceintervalweakened associativelearningcomparedwiththatseenina0sdelayconditionedgroup.However,therewasno consistentevidenceofdistraction.Onthecontrary,inExperiment1,associativelearningwasstronger (inbothtraceanddelayconditionedgroups)forratsconditionedalsointhepresenceoftheintermittent light.InExperiment2,therewasnosucheffectwhentherolesofthestimuliwerereversed.Theresultsof Experiment2didhoweverconfirmtheparticularsalienceofthenoisestimulus.Thefindingofincreased associativelearningdependentonsalienceisconsistentwitharousal-mediatedeffectsonassociative learning.
©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Traceconditioningproceduresaredefinedbytheintroduction ofatraceintervalbetweenconditionedstimulus(CS,e.g.noise) offsetandunconditionedstimulus(US,e.g.foodorfootshock)onset (Kamin,1965).Thecharacteristicresult−ofreducedconditioning inconsequenceoftemporaldiscontiguity−canbedemonstrated inavarietyofPavlovianconditioningprocedures(bothappetitive andaversive)butaversiveprocedureshavebeenmuchmorewidely adopted,bothbecauseacquisitionisrapidandtheneuralcircuitry necessarytobasicfearconditioningiswelldocumented.
Theabilitytobridgetimedelaystoshowassociativelearningina traceconditioningprocedureallowsanimalstoassociatewhatgoes withwhat,whenpotentiallycausally-relatedeventsareseparated intime.Thus,asameasureofworkingmemory,trace condition-ingholdspromiseasabehaviouralassayforage-relatedmemory decline:itisimpairedinagedrabbits(GravesandSolomon,1985), rats(McEchronetal., 2004;Moyerand Brown,2006)andmice (Galvezetal.,2011;Kishimotoetal.,2001),aswellasinamouse modelofsenescence(Lopez-Ramosetal.,2012).
∗Correspondingauthor.
E-mailaddress:helen.cassaday@nottingham.ac.uk(H.J.Cassaday).
Inyoungeradultanimals,traceconditioninghasbeenshown to requirean intact hippocampusto process thetemporal gap betweentheCSandUS(McEchronetal.,1998;Weissetal.,1999; McEchronetal.,2000;Beylinetal.,2001;Quinnetal.,2002;Rogers andKesner,2006)and−asisthecasefortaskswhichmeasure declarative memory− seemstodependupon awareness(Clark andSquire,1998).Consistentwithknownprojectionsfrom hip-pocampus,medialprefrontalcortex(mPFC)hasalsobeenshown tobepartofthetraceconditioningnetwork.Comparingacrossa varietyoftraceconditioningpreparations,theemergingpattern seemstobearolefortheprelimbic(PL)sub-regionwhen mem-oryprocessesaredirectlyengaged,forexamplewhenretentionis tested(Runyanetal.,2004;Oswaldetal.,2008,2010),when neu-ronalactivityisexaminedduringarelativelylongtraceinterval (GilmartinandMcEchron,2005)orwhenlongerCSdurations com-poundthememoryload(McLaughlinetal.,2002).Incontrast,there isevidencetosuggestthattheanteriorcingulate(AC)sub-region isimportantforearlieracquisition-relatedprocesses (Kronforst-CollinsandDisterhoft,1998;Weibleetal.,2003,2000;Kalmbach etal.,2009;Hattorietal.,2014).Thisdistinctionmayrelatetothe roleofACinattentionalprocessesand−consistentwiththis inter-pretation−excitoxiclesionsoftheACsub-regionofmPFCwere reportedtoreducetraceconditioninginamousefearconditioning
http://dx.doi.org/10.1016/j.beproc.2016.04.003
procedurewhichwassensitivetotheeffectsofanexperimental distractorstimulus(Hanetal.,2003).
Ineye-blinktraceconditioningprocedures,humanparticipants’ abilitytoreportontheCS-USrelationshipissimilarlyimpairedby concurrentdistraction,and thisfindinghasalsobeenconfirmed usingatracefearconditioningprocedure,inthiscasewithafinger shockUSCarteretal.(2003).Thislatterstudywasdesignedtobe analogoustotheHanetal.rodentstudy,thoughthenatureofthe experimentaldistractionwasdifferent.Carteretal.(2003)useda concurrentn-backtask,whichrequiredparticipantstotrack pre-viouslypresenteddigitsinalistofnumbers,bywayofadistractor intendedtocompeteforworkingmemorycapacity.Aswasthecase intheHanetal.(2003)mouseconditioningstudy,distractorstimuli weresimilarlyfoundtointerferewithtracefearconditioning,delay conditioningbeingmuchmoreresilienttotheeffectsofdistraction (Carteretal.,2003).
Thus, it has been argued that the use of a distractoris an importantprocedural modificationin order tomodel the puta-tiveattentionalroleofACinataskwithdemonstratedsensitivity toattentionalparametersandhightranslationalrelevancetoour understandingofnormalhumanageing.Moreover,itfollowsthat increasedattentionalloadmaybeacontributingfactorintheevent traceconditioningdeficitsaredemonstratedinrodentmodels,at leasttotheextentthatthesedependonattentionalprocesses medi-atedbytheAC(Pezzeetal.,2016).
In a series of trace conditioningexperiments using rat fear conditioning procedures, we have routinely used an extended backgroundstimulus(NormanandCassaday,2003;Horsleyand Cassaday,2007;Grimond-Billa etal., 2008;Nelsonet al.,2011; Pezzeetal.,2016).Thiswasprovidedbyacontinuouslyflashing lightpresentedforthefulldurationoftheconditioningsessionand hasbeenintendedtoprovideanexperimentalcontextratherthan adistractorstimulus.ThedistractorstimulususedbyHanetal. (2003)wasalsoprovidedbyaflashinglight,differentonlyinits temporalproperties.Therefore,sincedistractionisofboth theo-reticalandpracticalimportance−toboththeinterpretationand demonstrationoftraceconditioningimpairments−weadapted ourexistingfearconditioningprocedureinanattempttoestablish areliabledistractorsuitableforuseinrats.
Itmustbenotedthatundersomeexperimentalcircumstances theintroductionofextraneousstimuliisalreadyknowntoresultin potentiationratherthandistraction(DurlachandRescorla,1980; Pearceetal.,1981;Rescorla,1982;HallandHoney,1993).Itwasnot ourobjectivetoaddtothisbodyofknowledge.Ratherthepresent studysoughttoexplorethefeasibilityofadaptingapublished dis-tractorprocedure,inorder(inthelongerterm)tofurtherexamine theroleofACinworkingmemory.Thisbehaviouralworkwasdone inaratratherthanamousemodelandusingadifferentvariantof tracefearconditioning(suppressionoflickingratherthanfreezing), asperanumberofearlierstudiesconductedtoexaminethe neu-ropharmacologicalsubstratesoftraceconditioning(Normanand Cassaday,2003;HorsleyandCassaday,2007;Grimond-Billaetal., 2008;Nelsonetal.,2011;Pezzeetal.,2016).
2. Methods
2.1. Animals
Ineachoftwoexperiments,48experimentallynaïveadultmale Wistarrats(CharlesRiver,UK)werecagedingroupsof4in indi-viduallyventilatedcages(IVCs),ona12:12hlight/darkcyclewith foodandwateradlibitum.Cageswerecleanedouttwiceperweek andcardboardtubesandnestingmaterialswereprovidedas envi-ronmentalenrichment.Theratswerehandledforapproximately 5minperdayfor1weekandthenatmeanweight199g(range
168–224g)inExperiment1and218g(range193–246g)in Exper-iment2wereplacedonwaterdeprivationimmediatelypriorthe conditioningprocedures.Onerat(inExperiment1)washumanely killedforanunrelatedreason,ontheadviceoftheNamed Veteri-narySurgeon.Allprocedureswerecarriedoutinaccordancewith theUnitedKingdom(UK)AnimalsScientificProceduresAct1986, ProjectLicensenumberPPL40/3716,whichensuresfullcompliance withtheEUDirective2010/63/EUforanimalexperiments.
2.2. Behaviouralconditioningapparatus
Four identical fully automated conditioning boxes, housed withinsound-attenuatingcasescontainingventilationfans (Cam-bridgeCognition, Cambridge, UK), were used. Theinner condi-tioningboxwallsconsistedofplainsteel(25cm×25cm×22cm high)withaPlexiglasdoor(27cm×21cmhigh),atthefront.The floorwasashockgridwithsteelbars1cmapartand1cmabove thelipofa7cmdeepsawdusttray.Awaterspoutwasmounted ononewall.Thespoutwas5cmabovethefloorandconnected toalickometersuppliedbya pump.Lickswereregisteredbya breakin thephotobeam withinthespout,which alsotriggered waterdeliveryof0.05mlperlick.Thewaterspoutwasilluminated whenwaterwasavailable.Aloudspeakerforthepresentationof auditorystimuliwassetintheroof.InExperiment1,a5smixed frequencycontinuousnoisesetat80dBservedastheCSandthe distractorwasanintermittentlight providedbythethree wall-mounteddome-shapedstimuluslightsandthehouselightsetto flashintermittently(130mson/off,at8lx,for3sdurationwithan interstimulusintervalrandomlychosenfrom5,10,15or20s).In Experiment2,a5sflashinglightservedastheCS(inthiscase pro-videdbythethreewallmountedstimuluslightsandthehouselight flashing(500mson/off,at8lx)andthedistractorwasan intermit-tentnoise(130mson/offfor3s,setat80dBwithaninterstimulus intervalsequencerandomlychosenfrom5,10,15or20s). Foot-shockof1sdurationand 1mAintensityprovidedtheUCS.This wasdeliveredthroughthegridfloorbyaconstantcurrentshock generator(pulsedvoltage: outputsquarewave 10mson, 80ms off,370Vpeakundernoloadconditions,MISACSystems, New-bury,UK).StimuluscontrolanddatacollectionwasbyanAcorn ArchimedesRISCcomputerprogrammedinBasicwithadditional interfacingusinganArachnidextension(CambridgeCognition).
2.3. Behaviouralconditioningprocedure
Waterdeprivationwasintroduced1daypriortoshapingand allratsreceived1hofadlibitumaccesstowaterintheirhomecage atthesametimeeachday,inadditiontoaccesstowaterinthe conditioningapparatusonalltheexperimentaldaysexcept con-ditioning.Thestagesofthetraceconditioningprocedurewereas follows:
2.3.1. Pre-conditioningtoestablishbaselinelickresponse
2.3.2. Conditioningwithfootshock
Conditioningwasconductedfollowingpre-training.Nowater wasavailablewithintheboxandthewaterspoutwasnot illumi-nated.Therewere2conditioningtrialsinwhichtheUCSfootshock wasdeliveredata0sor10straceintervalfollowingtermination ofthe5sCS.ThefirstpairingofCSandUCSwaspresentedafter 5minhadelapsed,andthesecondpairingwas5minafterthefirst, followedbyafurther5minleftintheapparatus.Intheabsenceof drinking,therewerenobehaviouralmeasurestorecord.Inboth experiments,thedistractorwasprovidedbyanintermittent back-groundstimulus(lightinExperiment1,noiseinExperiment2).For animalsinthedistractorcondition,thepresentationofthe distrac-torstarted1minbeforethefirstCS-UCSpairingandended1min afterthefinalCS-UCSpresentation.
2.3.3. Reshapingafterfootshock
Onthedayfollowingconditioning,animalswerereshaped fol-lowingthesameprocedureasinpre-trainingsessions.Thiswas donein orderto re-establishdrinkingafterconditioning. Addi-tionally,thereshapinglatenciesprovidedameasureofcontextual conditioningasreflectedinsuppressiontothecontextualcues pro-videdbytheexperimentalchambers.
2.3.4. Conditionedsuppressiontests
On thedayfollowingreshaping,theanimals wereplaced in theconditioningboxesandunderwent anextinction testtothe CS.Waterwasavailablethroughoutthetestandthewaterspout wasilluminated.Oncetheanimalshadmade50licks,theCSwas presentedfor15min.Thelatencytomake50licksintheabsence oftheCS (theA period,timedfromthefirstlick madein each box)providedameasureof anyindividualvariation inbaseline lickresponding.Thiswascomparedwiththetimetakento com-plete50licksfollowingCSonset(Bperiod)inasuppressionratio (A/(A+B))toassessthelevelofconditioningtotheCS,adjustedfor anyindividualvariationindrinkrate.Conditionedsuppressionwas alsomeasuredasthenumberoflicksmadeduringthefirst1minof CSpresentation.Asecondextinctiontestmeasuredsuppressionto thedistractorstimulusinthesameway.Theextinctiontestswere runinacounterbalancedorder.
2.4. Experimentaldesignandstatisticalanalysis
Inboth experiments, therewere2 conditioninggroups con-ditioned with or without a concurrently presented distractor stimulusina2×2factorialdesignatlevels0or10strace inter-valandwithorwithoutthedistractor(n=12/group).Statistical analyseswereperformedusinganalysisofvariance(ANOVA).The pre-conditioning and reshaping dependent variables were lick latencies,aswellasthenumberoflicksmadeinthefirst1min ofthesession.Thetestdependentvariableswerethesuppression ratios,aswellasthenumberoflicksmadeduringthefirst1mintest presentation.Thesamevariableswereanalysedinthesamewayfor thetestsofsuppressiontotheCSandthedistractorstimulus. How-ever,examinationoftheeffectsoftraceintervalonsuppressionto thedistractorwasnecessarilyrestrictedtothoseratsinthegroups actuallypresentedwithadistractor.Finally,acombinedanalysis comparedconditioningasafunctionofnoiseversuslighttargetCS identityandlightversusnoisedistractoridentity.Non-significant effectsonbaselinerespondingarenotreported.
3. Results
3.1. Experiment1:noiseCSwithintermittentlightdistractor
Therewasa clear effectof traceonboth suppression ratios tothenoiseCS,F(1,43)=8.573,p=0.005,aswellasonthe
mea-sureoflearningprovidedbythenumberoflicksmadeinthefirst 1minoftest,F(1,43)=8.160,p=0.007.Fig.1panelsAandBshow that,asexpected,ratsconditionedthatthe0straceintervalwere moresuppressedthanthoseconditionedatthe10strace inter-val.Countertoprediction,therewasnoevidencethatthelight distractormoderatedconditioningoverthetraceintervalinthat theinteractionbetweentraceanddistractorwasnotsignificantfor eithermeasure,maximumF(1,43)=0.004,p=0.949(Fig.1A). Con-firmingthatthelightdistractorwasnotwithouteffect,therewas amaineffectonthesuppressionratiomeasureofconditioningto thenoiseCS,F(1,43)=4.786,p=0.034(Fig.1A).Thisarosebecause ratspresentedwiththeflashinglightdistractorduring condition-ingwereoverallmoresuppressedthanthoseconditionedwithout the‘distractor’. Althoughnon-significant,the samepattern was reflectedinthelicksmeasureofconditionedsuppression(Fig.1B). Direct tests of conditioning to the light distractor stimulus showed no evidence of conditioning to this stimulus in that therewasnodifferenceinsuppressionratiosforratsconditioned withorwithouttheintermittentlightpresentedasadistractor, F(1,43)=1.131,p=0.294.Fig.1Cshowsthatthemeanswerevery comparable(overallSR=0.378withand0.340without).Giventhe lackofdifferenceinsuppressionforratsconditionedwithand with-outthedistractor, thefact thattheobservedsuppressionratios werebelow0.5 canbeattributedtounconditionedsuppression andconfirmsthesalienceofthelightdistractorstimulus.Thesame patternwasreflectedinthelicksmeasureofconditioned suppres-sion,onlytheinteractionapproachedsignificance forthemin1 licksmeasure,F(1,43)=3.758,p=0.059(Fig.1D).However,ANOVA restrictedtothoseratsactuallypresentedwiththelight distrac-toratconditioningconfirmedthattherewasalsonoeffectofthe traceintervalatwhichthenoiseCShadbeenpresentedoneither measure of conditioningto thelight, maximum F(1,21)=0.166, p=0.688.
3.2. Experiment2:lightCSwithintermittentnoisedistractor
Theeffectoftraceonthesuppressionratiostothelightwas marginal,F(1,44)=3.824,p=0.057(Fig.2A),buttherewasaclear effectoftraceonthenumberoflicksmadeinthefirst1mintest presentation,F(1,44)=6.116,p=0.017(Fig.2B).Therewasno sig-nificanteffectofthepresenceorabsenceoftheintermittentnoise distractoronconditioningtothelightCS,maximumF(1,44)=3.246, p=0.078,fortheinteractionbetweentraceanddistractorforthe 1mindrinkingattest.
Directtestsofconditioningtothenoisedistractorshowedno effect, F(1,44)=1.836, p=0.182, for the suppression ratio mea-sure(SR=0.123withpriorconditioningandSR=0.171withoutany priorconditioning;Fig.2C).Thefactthattheobservedsuppression ratios were relatively low irrespective of whetherthe distrac-torhadbeenpresentatconditioningmeansthatitspresentation resultedinunconditionedsuppressionandsuggeststhatthe inter-mittentnoisestimuluswaslikelyparticularlysalient.Althoughthe suppressionratiomeasurewasinsensitivetoitseffects,rats con-ditionedinthepresenceofthenoisedistractorshowedreduced drinkingduringthefirst1mintestpresentationofsamethe inter-mittentnoisestimulus,F(1,44)=6.539,p=0.014(Fig.2D).ANOVA restrictedtothoseratsactuallypresentedwiththenoisedistractor atconditioningconfirmedthattherewas,however,noeffectofthe traceintervalatwhichthelightCShadbeenpresentedoneither measureof conditioningtothenoise, maximumF(1,22)=0.343, p=0.564.
3.3. Comparisonofexperiments1and2
0 0.1 0.2 0.3 0.4 0.5
Delay 0s Trace 10s
Suppr
essio
n
r
ao
0 40 80 120 160 200
Delay 0s Trace 10s
Min 1 lick
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0 0.1 0.2 0.3 0.4 0.5
Delay 0s Trace 10s No distractor Distractor
0 40 80 120 160 200
Delay 0s Trace 10s
A
B
C
D
Fig.1. showssuppressionmeasuresofconditioningtothenoiseCSandlightdistractorinExperiment1.Fig.1AshowsthelevelofconditioningtothenoiseCSexpressed asmeansuppressionratio(calculatedasA/(A+B);whereAwasthetimetakentocomplete50lickspriortoCSpresentationandBwasthetimetakentocomplete50licks duringCSpresentation).Fig.1BshowsthelevelofconditioningtothenoiseCSexpressedasthenumberoflicksmadeduringthefirstminuteofCSpresentation.Theshaded histogramsshowhowrats’responsesweremoderatedbythepresence(darkgrey)orabsence(lightgrey)oftheintermittentlightdistractor.Fig.1Cshowsthesuppression ratiosupontestpresentationofthelightdistractor.Fig.1Dshowsthelevelofsuppressiontothelightexpressedasthenumberoflicksmadeduringthefirstminuteof presentation.Theshadedhistogramsshowhowrats’responsesdependedonpriorconditioning(darkgrey)orwereunconditioned(lightgrey).Errorbarsshowtwostandard errorsofthemeanforapproximatebetweengroupscomparisons.
0 0.1 0.2 0.3 0.4 0.5
Delay 0s Trace 10s
Suppr
essio
n
r
ao
0 40 80 120 160 200
Delay 0s Trace 10s
Min 1 lick
s
0 0.1 0.2 0.3 0.4 0.5
Delay 0s Trace 10s
No distractor
Distractor
0 40 80 120 160 200
Delay 0s Trace 10s
A
B
C
D
Fig.2. showssuppressionmeasuresofconditioningtothelightCSandnoisedistractorinExperiment2.Fig.2AshowsthelevelofconditioningtothelightCSexpressed asmeansuppressionratio(calculatedasA/(A+B);whereAwasthetimetakentocomplete50lickspriortoCSpresentationandBwasthetimetakentocomplete50licks duringCSpresentation).Fig.1BshowsthelevelofconditioningtothelightCSexpressedasthenumberoflicksmadeduringthefirstminuteofCSpresentation.Theshaded histogramsshowhowrats’responsesweremoderatedbythepresence(darkgrey)orabsence(lightgrey)oftheintermittentnoisedistractor.Fig.1Cshowsthesuppression ratiosupontestpresentationofthenoisedistractor.Fig.1Dshowsthelevelofsuppressiontothenoiseexpressedasthenumberoflicksmadeduringthefirstminuteof presentation.Theshadedhistogramsshowhowrats’responsesdependedonpriorconditioning(darkgrey)orwereunconditioned(lightgrey).Errorbarsshowtwostandard errorsofthemeanforapproximatebetweengroupscomparisons.
theSRmeasureofconditioningtothetargetCS,F(1,87)=10.927, p<0.001, aswellasfor 1stminlicks,F(1,87)=14.019,p<0.001. Therewasnoeffectofdistractoronthesemeasuresof condition-ingtotarget,eitheroverallorininteractionwithtrace,maximum F(1,87)=1.716,p=0.194.Therewasamaineffect ofCSidentity on the suppression ratio measure of learning, F(1,87)=23.243,
4. Discussion
Aswouldbeexpected,inbothexperiments,theintroductionof a10straceintervalweakenedassociativelearningcomparedwith thatseenina0sconditionedgroup.However,therewasno evi-denceofdistractioninthegroupsconditionedinthepresenceof eitheranintermittentlight(inExperiment1)oranintermittent noise(inExperiment2).Onthecontrary,inExperiment1, associa-tivelearningtothenoisewasstronger(andforbothtraceanddelay groups)forratsconditionedalsointhepresenceofthe intermit-tentlight.Asmightbeexpectedgiventhattheintermittentlight increasedratherthandecreasedconditioningtothenoise,there wasnoevidencetosuggestthatthelightacquiredanyassociative strengthofitsown.
Althoughbasedonaratfearconditioningprocedure,the pro-cedureadoptedinExperiment1wasbroadlysimilartothatused inthestudyofHanetal.(2003).Ofcoursetheremaybespecies differencesbetweenratsandmicewhichcanaccountforthese dis-crepantfindingsand(relatedly)theconditioningparametersused formicearequitedifferent(seebelow).
Onepossibilitywhich isimportanttoconsideris thatofthe modalitiesofthestimuliinuse.FollowingthepublishedHanetal. (2003)design,stimulusidentitywasnotcounterbalancedwithin Experiment1:thenoisewastheCSandtheintermittentlightthe notionaldistractor forall animals. Thus, takenin isolation, the resultsof Experiment1leaveopenthepossibilitythat interfer-encewithtraceconditioningmightdependonstimulusmodality and/orthelevelofarousalgenerated.Inappetitiverat condition-ingprocedures,effectsofstimulusmodalityhavepreviouslybeen foundtooutweighdifferencesduetotheinformationvalueofthe cueinrelationtodrugeffects(HorsleyandCassaday,2008). More-over,infearconditioningprocedures,effectsofstimulusmodality havebeenfoundtodependonstrainofrat(NormanandCassaday, 2004).Therefore,inExperiment2ofthepresentstudy,the stimu-lusroleswerereversed:anintermittentlightwastheCSandthe samenoisestimuluswasnowpresentedintermittentlyinthe back-ground,toprovidetheputativedistractor.InExperiment2,when therolesofthestimuliwerereversed,therewasnoeffectof gen-erallyincreasedconditioningtotheCSinthedistractorcondition. ThislackofeffectinExperiment2isconsistentwiththepotential importanceofstimulusmodality.Moreover,combinedanalysesof Experiments1and2,confirmedthattheidentityofthestimulus selectedasCSwasthemajordeterminantofthelevelofassociative learning,forbothtraceanddelayconditionedgroups.Nonetheless, theresultsofExperiment2dodemonstratetheparticularsalience oftheintermittentnoisestimulus.Thus,theeffectofmodalitycould ineffectbemediatedbysalience.
There was also a potentially important amodal difference betweenthestimuliwhichwaslikelytocontributetodifferences inperceivedsalience.TheauditoryCSusedinExperiment1was continuous(asperHan etal., 2003; and thetraceconditioning proceduresroutinelyusedinourlaboratory).However,continuous lightstimuliseemtobeofrelativelylowsalienceanditisinany casedifficulttomatchonthebasisofphysicalintensity(loudness versusbrightness)acrossmodalities.Wehavetypicallyused inter-mittentratherthancontinuouslightCSsinanattempttomatch thelevelofconditioningproducedbyalternativeauditoryversus lightCSsasfaraspossible,andanintermittentlightCSwasused inExperiment2.Thiswaspresentedwiththeflashingparameters usedpreviouslytoproduceconditioningratherthanthoseadopted whentheflashinglightwasintendedasadistractorinExperiment 1.Wesubmitthattheobserveddifferencesinconditioningwiththe auditoryversuslightCSscouldalsobeattributabletodifferencesin theamodalcharactersticsprovidedbytheirtemporaldifferences. However,thedifferenceintemporalparametersdoesnotseemto providethebestaccountofthepresence(inExperiment1)versus
absenceofpotentiation(in Experiment2)asthereshouldhave beengreaterpotentialforwithincompoundassociationsbasedon similarityinExperiment2(RescorlaandGillan,1980).
Thediscrepancybetweenthepresentresultandthemousefear traceconditioningvariantstillstandsinneedofexplanation.Itmust beacknowledgedthatwedidnotattemptafullreplicationofthe fearconditioningparametersusedbyHanetal.(2003)inmice. Insteadweadaptedaproceduredevelopedspecificallyforratsand whichhasalreadybeenusedinanumberoftracefear condition-ingstudies,inanattempttoestablishareliabledistractorsuitable foruseinrats.InthestudyconductedbyHanetal.(2003),the responsemeasurewasfreezingratherthanconditioned suppres-sionofamotivatedresponse(ashere).Accordingly,differencesin thebasicfearconditioningprocedureinuseincludedtheduration ofthepre-experimentallicktraining,thelengthoftheCS,tone fre-quencyandintensity,thenumberofconditioningtrialsand the shockintensity.
Therewerealsotwodifferencesinthedistractionaspectofthe procedure.First,inordertomakeitstemporalpropertiesmore dis-similarfromourstandardflashinglightCS(500mson/off)which hasprovenaneffectiveconditioningstimulusintherattrace con-ditioningprocedure,weincreasedthefrequencywithwhichthe intermittentdistractorwaspresented(130mson/offfor3s dura-tions,comparedwith250mson/offfor3sdurationsinHanetal., 2003).We cannotexcludethepossibilitythat a distractorwith temporalpropertiesmoresimilartothoseusedintheHanetal. (2003)wouldhavehadthedesiredeffectofdistractingfromrather thanenhancinglearningaboutthetrace-conditionedCS.The sec-onddifferencecanbeseenasastrengthinthat,inExperiment2, wealsocomparedtheeffectsofanintermittentnoisestimulusas a distractorfromthestandard flashing lightCS (500ms on/off) in anotherwise identicalprocedure. Thisis animportant com-parisonbecauseevenwithinspecies,effectsofstimulusmodality candependonstrain(NormanandCassaday,2004).The intermit-tentnoisestimuluswassimilarlyineffectiveasadistractor,though therewasnoevidenceforpotentiationeitherinExperiment2.
4.1. Conclusions
Overalltheresultssuggestthatthepresentationofanadditional cueduringtheconditioningsessioncanparadoxicallyimprove con-ditioningtothetargetCS,dependingontherelativesalienceofthe additionalcue.Inprinciple,thisfindingcouldreflectpotentiation basedon within-compound associations (Durlach and Rescorla, 1980;Pearceetal.,1981).However,potentiationwasseenonly inExperiment1whichusedacontinuousnoiseCSwhichwasmost dissimilartotheintermittentdistractor(RescorlaandGillan,1980). Moreover,therewasnoevidencethateitherthelightornoise dis-tractoracquiredassociativestrength andneitherwasthereany differencebetweentraceanddelayconditionedgroupsin Experi-ment1inwhichthelightdistractorincreasedassociativeleaning tothenoise.Thefindingthatthepresenceofsalientbackground cuescanincreaseassociativelearningisconsistentwith arousal-mediatedinterpretations.However,thenoisedistractorwhichwas moresalientthanthelighthadnosucheffect.Thusthelevelof arousalmaybecritical(YerkesandDodson,1908).
Acknowledgements
Thisresearchwassupportedbya grantfromtheBBSRC(ref. BB/K004980/1).WethankAndySmithfortechnicalsupportand assistancewithprogramming.
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