Trial and intertrial durations in appetitive conditioning in rats

15 Pages·2010·1.72 MB·English

AnimalLearning& Behavior
2000.28(2).121-135
Trial and intertrial durations
in appetitive conditioning in rats
PETERC.HOLLAND
DukeUniversity,Durham,NorthCarolina
Theeffectsoftrial(T)andintertrial(l)durationswereexaminedintwoPavlovianconditioningex
perimentswithrats, inwhichanoise conditionedstimulus (CS)waspaired withfood delivery.In Ex
periment1,Twaseither10or20sec, and Iranged from 15to 960sec,inseparategroupsofrats.The
acquisitionrateandfmallevel ofconditionedrespondingshowedratio invariance: Theywere better
predictedbythe1/Tratio than byIor Talone.InExperiment2,the1/Tratio was6.0inallthegroups,
and Twas20,40,80,or160sec.Ratioinvariancewasnotobserved: DespitethecommonI1Tratio,the
rate ofacquisition, fmallevelofconditionedresponding, and the abilityofthe CSto block condition
ingofanotherstimulus differedamongthegroups.Atthesametime,thetemporaldistributionofcon
ditioned respondingwithinTwassimilarinallthegroupsthroughoutconditioningandextinctionand
showedsuperpositioningwhennormalizedacrossT.Manybutnotallaspectsofthe datawere consis
tentwithscalartimingtheory.
Timehas long beenassumedtobeakeyvariableinas thetrialduration, T). Forexample,Gibbon,Baldock,Lo
sociative learning. Notonly has temporal contiguitybe curto, Gold, and Terrace (1977) found that the number
tween events been viewedas acritical condition for the oftrials for pigeons to reach an acquisition criterion in
occurrenceoflearning,butalsotemporalinformationhas autoshaped keypecking procedures was best described
been positedto be an importantaspectofthe contentof bythe liTratio across abroadrange ofabsolutetime in
learning(Gallistel, 1990;Gibbon &Church, 1990;Miller tervals. Similarratio effects have been observed in dis
& Barnet, 1993; Savastano & Miller, 1998). Most early criminationtasks. Williams(1998)trainedrats inacon
researchconcentratedonsingletemporalparametersand ditionaldiscriminationinwhichtwocues signaledwhich
absoluteeffectsoftime.Forexample,explorationofnew oftwoleverswouldbethecorrect(reinforced)leverwhen
conditioningpreparationsoften beganwiththe determi they were subsequently inserted into the chamber. Dis
nationofinterstimulusinterval(lSI)functions (Holland, criminationperformancewas betterwhen the delaysbe
1980; Kamin, 1965; Ross, 1961; Schneiderman& Gor tween cuepresentationandlever insertionwereshortand
mezano, 1964)that relatedtheamountorrate oflearning whenI was long, butthe delay effectswere more evident
tothe time betweenthe onsetsofthe conditionedstimu with shortIs. Similarly, Spetchand Rusak(1992) found
lus(CS) andtheunconditionedstimulus(US). Likewise, that variations insampledelays in delayedmatching-to
trial-spacing effects figured prominently in early theo sample(DMTS)procedureshadmoreeffectwith shortIs
ries ofinterferenceand memoryfor eventsincondition than with lengthy Is. Roberts and Kraemer (1982) fur
ing episodes(Capaldi, 1967). ther found that pigeons' DMTS performancewas better
Itisnowclearthatthe effectsofone temporalvariable describedby the ratio ofI and the delay intervalthan by
can be modulatedbyothertemporalvariables. Evidence eitherofthose absolutetime intervalsalone.
from a variety ofconditioning paradigms suggests that Holland(1995)and Hollandand Morell(1996)found
conditionedresponding often involves sensitivityto the that rats' acquisition ofoperant serial feature-positive
ratio oftwo time intervals, the average interval between and serial feature-negative discriminations was affected
trials (usually termed the cycle or intertrialduration, I, both by the interval between the feature and the target
inthisliterature)andsomewithin-trialinterval, suchasthe and byI. ConsistentwithRoberts and Kraemer's(1982)
time between CS and US presentation (usually termed results, the ratio ofI and the feature-target interval was
thebestpredictorofdiscriminationacquisition.However,
inbothHolland's(1995)and Hollandand Morell'sstud
ies, the differences indiscriminationlearningacrossthe
ThisresearchwassupportedinpartbygrantsfromtheNational Sci
trainingconditionsreflecteddifferentiallossesinrespond
ence Foundation and the National Institute ofMental Health. Ithank
lohnChenandlamesWallacefortechnicalassistanceinExperimentI ingon nonreinforcedtrials, ratherthandifferencesinre
andR.ChurchandW.Meekforhelpfulcommentsonanearlierversion spondingonreinforcedtrials. Indeed,eveninsimple con
ofthispaper. lamesWallace's participationwaspart ofamentorship ditioning ofeitherthe feature-etarget compound or the
programattheNorthCarolinaSchoolofScienceandMathematics.Cor
target-alonecue, noeffectsofI were observed.Although
respondenceconcerningthis article shouldbeaddressedtoP.C.Hol
Holland(1995)and Hollandand Morellpointedout that
land,DepartmentofPsychology: Experimental,DukeUniversity,Box
90086,Durham,NC27708-0086(e-mail:[email protected]). theirstudiesmaynothaveprovidedanadequate opportun-
121 Copyright2000 Psychonomic Society, Inc.
122 HOLLAND
itytoexamineinitiallearning-acquisitionwasveryrapid Inthisexperiment, Tsignifiedboththeduration ofthe
inallconditions,andtheratshadbeenpreviouslytrained noise CS and the interval between its onset and the de
to perform the same operant in the presence ofanother livery ofthe food US. Althoughthis confoundingofISI
auditorycue with acommonI value inall the groups andCSdurationmakes itimpossibletoseparatelyassess
they also raisedthepossibilitythat appetitivecondition theeffects ofeachofthose variables, itisconsistentwith
ingofrats is simply not sensitive tothe liTratio. the proceduresofall previousinvestigationsofliTratio
Recent evidencedenies thispossibility. Inseveral ex effectsandwithGibbonandBalsam's(1981) theoretical
perimentsthat used Pavlovian conditioningprocedures, analysisofthose effects.
Lattal(1999)foundthatrats' acquisitionoffoodcupentry Asinmostconditioningstudies,intheacquisitionphase
respondingwas morerapid withlargerliTratios. Never ofExperiment I,theconditionsoflearningitselfandthe
theless, Lattal found that acquisition rates were also af conditions under which that learning is assessed were
fected bythe absolute I and Tvalues; with anygivenliT necessarily confounded (see Lattal, 1999,and Rescorla
ratio, shortervalues ofTgeneratedmore rapid learning. & Holland, 1976,for discussions ofthe implications of
Theexperimentsreportedinthisarticlereplicatedand that confound). Differences in behavior among groups
extendedLattal's (1999) findings in several ways.First, trainedwithdifferentliTratios might reflectdifferences
inExperiment I, theeffects ofarange ofliTratios were intheirlearning,ortheymightreflectdifferencesinhow
examinedsystematicallywithin asingleexperiment.As learning isexpressedatdifferentliTratios.Thislearning
inLattal's studies,theliTratio wasthe best predictorof performancedistinctioniscritical totheoreticalaccounts
acquisitionrate andfinal levelofperformance. Second, for liT. For example, Gibbon and Balsam (1981) sug
in Experiment 2, the generality ofthis ratio effect was gestedthatliTratioeffectsareperformanceeffectsrather
exploredby comparingconditioningover abroad range than acquisition effects. Within their interpretation, the
ofI and Tvalues, while holding the liTratio constant. associative strength ofa CS is determined solely by T,
Third, the contributionsofliTratio to learningand per but performanceofconditionedresponses during Tis a
formance weredisentangledbygivingperformancetests function ofboth that strength and the average strength
with commonliTratios after trainingrats with different duringI. Bycontrast,otherattemptstoaccount forI and
IITratiosandbytestingratsafterextensivenonreinforced liTratioeffects(e.g.,Rescorla& Wagner,1972;Wagner,
exposure to the context. Lattal tested rats with several 1981)haveattributedthemtodifferencesinlearningabout
combinationsofIandTaftertrainingwithasingle com the CSs, appealing, for example, tocompetition between
bination and found test behavior to reflect the training conditioning ofthe context and that ofthe explicit CS.
parameters,regardlessofthevaluesused intesting. Sim TodistinguishbetweentheeffectsofI andtheliTratio
ilarly,LattalfoundthatliTratioeffectsweremostlymain onlearningfrom their effectsonperformance,allofthe
tained after extensive exposure tothe experimentalcon ratsweretestedwithboth60-sec and240-secIsattheend
text in the absence ofthe food US, which reduced the oftraining. Differences in responding observed among
intertrial foodratetozeroinallconditions.Fourth,theef thegroupswhentheyaretestedundercommonconditions
fects ofthese temporal variables onan indirectmeasure are likely to reflect differences in underlying learning
ofassociative strength, the abilityofa CS to block sub amongthose groups.
sequentacquisitionofconditioningto anotherstimulus,
wereexamined.Fifth,theeffectsofthesevariablesonthe Method
temporal distribution ofresponding within T were de Subjects.Ninety-sixexperimentallynaivemale CD-strainrats
scribed. Finally,anumberofmeasuresofconditionedre (CharlesRiverBreeding Laboratories,Raleigh,NC), lOOdaysold
atthebeginningoftheexperiment,servedassubjects.Theratswere
sponding wererecorded, including theduration, rate,and
individuallyhousedinindividual stainlesssteelcagesinavivarium
latency offood cup entries. Otherstudies from this lab
that wasmaintainedat21°C, with the lightsonfrom 6:00 a.m. to
oratory (e.g., Holland, Lamoureux, Han, & Gallagher, 8:00p.m.Alltheexperimentalsessionswerecarriedoutduringthe
1999) suggest that these measures may not always be lightportionofthecycle,between 7:00a.m.and2:00p.m.Therats
wellcorrelated. weremaintainedat85%oftheirad-libweightsbylimitingtheirac
cesstofood;waterwasalwaysavailable.
EXPERIMENT1 Apparatus.The apparatusconsistedofeight individual cham
bers(22.9 X 20.3 X20.3cm)withaluminumfrontandbackwalls,
clearacrylicsidesandtop,andagridfloor(0.48-cm stainlesssteel
In Experiment I, the rats were first given Pavlovian
rods spaced 1.9cmapart). Adimly illuminatedfood cup was re
pairingsofanoiseCSandafoodpelletUS.Forsixgroups
cessedinthe centerofthe front wall, with an infraredphotocell
ofrats,Twas 10sec,andforsixgroups Twas20sec.Pre flushwiththewall.Computerequipmentsampledthephotocellsat
vious studies with long (480-sec)I valuesshowedfaster arate ofapproximately IkHz. A6-Wjeweled lampwasmounted
acquisition with 10-secthan with 20-sec Ts. The values 10cmabovetherecessedfoodcup;this lampwasnotused inEx
ofI used were chosen to produce liTratios of1.5, 3.0, periment 1.Eachexperimentalchamberwasenclosedinasound
resistant shell with an acrylic window for viewing the rats. A
6.0, 12.0,24.0,and48.0foreachvalueofT.Thus,across
speaker,usedtopresenttheauditoryCS,wasmountedontheinside
arange ofI values, the effects ofTcouldbeconsidered
walloftheshell, 10cmabovetheexperimentalchamberandeven
betweengroupstrainedwiththesameIsorbetweengroups withtheendwallthatcontainedthefoodcup.Ventilationfanspro
trainedwith the sameliTratios. vided masking noise(70dB), anda6-W,llO-Vlamp(operatedat
TRIAL AND INTERTRIALDURATIONS INRATS 123
75V)behind aredlens opposite thehouselightprovidedcontinu groupstrainedwithequalIs,respondingwasgreaterwith
ousdimbackgroundillumination.Twolow-lighttelevisioncameras the 10-sec Tthan with the 20-sec T.
weremounted2.1mfromtheexperimentalchamberssoeachcould Figure 1shows the mean percentage oftime the rats
include fourchambersinitsfield ofview.Videocassetterecorders
spent inthe foodcupduring the last 10secofthe CSpe
wereprogrammedtorecord behaviors thatoccurredduringthe 10
riods and the IO-secpre-CS periodsandthemean eleva
secintervals before, during,andafterCSpresentations.
tionscoresoverthecourseoftheconditioningphase.Sep
Procedure.Initially, alltheratsweretrainedtoeatfromthere
cessed food cup.Inasingle 32-min session, there were 16presen arate T X liTratio X sessionblocks analysesofvariance
tations oftheeventusedastheUSthroughoutthisexperiment,two (ANOVAs) for the elevation and during-CS measures
45-mg food pellets (P] Noyes) delivered 0.5 sec apart, onavari (respectively) each showed reliable effects ofliTratio
able-time (VT)2-min, equalprobabilityschedule.Next,alltherats [Fs(5,84) = 26.49and 13.25,ps<.01]andsessionblocks
received eightpairingsofan80-dB(Ascale)white noise CSwith
[Fs(7,588) = 72.96and 184.92,ps<.01]andsignificant
thetwo-pelletfood US ineach of16daily conditioningsessions.
Eachgroup ofratsreceivedasinglecombinationofT(durationof I/Tratio X sessionblocksinteractions[Fs(35,588)= 6.28
thenoiseCStrial)andI(timebetweentrialonsets,equivalenttothe and3.68,ps< .01].Fortheelevationmeasure, there was
interfoodinterval),asspecified(inseconds)bythegroupname,T-I: noreliableeffect ofToranyofits interactions(Fs < 1),
10-15, 10-30, 10-60, 10-120, 10-240, 10-480,20-30,20-60, butthethree-wayinteractionwasreliablefortheduring
20-120,20-240,20-480,and20-960. Ineach group, Twascon CSmeasure [F(35,588) = 1.79,p< .01].
stant.Bycontrast,ineachgrouptheIswerevariable,withthespec
Both measures were then subjected to analyses of
ifiedarithmeticmean; thevalues ofI ranged from 0.5to 1.5times
trend across liTratios, with weights that reflected the
the mean, rectangularly distributed. Note that with this schedule,
theaveragetimetofood delivery isnotconstantthroughoutases equal intervalsamong those ratios whenplottedonalog
sion.Thesession lengthswereeighttimesthemeanI valuesandso scale(as inFigure2).Overall conditionedrespondingin
varied from2to 128minacrossthegroups. the acquisition phase was linearly related to liTratio.
Finally,theratsreceivedtwotestsessions designedtodisentan The lineartrend ofI/Tratio was reliableforboth the el
glelearningandperformanceeffects. Ineachofthese sessions, all
evation andtheduring-CSmeasures [Fs(l,84) = 126.91
thegroupsweretestedwithasingleIvalue.Eachsessioncomprised
and61.17,ps<.01],andnohigherorder trends werere
eight reinforcednoise trials, withtheoriginal trainingvalues ofT
ineach group. Forhalfoftherats ineach group, the first session liable (Fs <2.63,ps ~ .10). Furthermore,the rate ofac
was8minlong,withI = 60sec,andthesecondsessionwas32min quisition ofresponding was also linearly related to liT
long,withI = 240sec.Theremaining ratsreceivedthose twoses ratio. ApartitionoftheliTratio X session blocks inter
sionsintheoppositeorder. action showedthatthe linear increaseinrespondingthat
Datapresentation.Theprimaryresponse measurewastheper occurred over sessions was itselflinearly related toI/T
centage oftimeduringaspecifiedintervalthatthephotocellinthe
intervalwithbothmeasures [Fs(l,84) = 95.64and28.23,
foodcupwasactivated. Thismeasureisreportedforthe 10-secpre
ps< .01]. No other polynomialcontrastsoftheliTratio
CSperiod, the entire periodduring the IO-secnoise CSs, andthe
last IO-secperiod ofthe20-sec noise CSs.Note thatbecauseeach X sessionblocks interactionwere reliable (ps> .10).
intervalwas10sec,thepercentagemeasureiseasilyconvertedtoan Pre-CS respondingalsodifferedamong thegroups; in
absolute measure oftime inthefood cup.The last 10-sec interval general, greaterresponding was observedin the groups
ofthe20-secCSswasselectedtoprovideacomparable-lengthsam with the smallestliTratios. AnANOVAshowedreliable
pling interval with identicalrelation tothetime ofUSdelivery in effects ofliTratio [F(5,84) =4.95,p< .01]and session
bothISIconditionsandtomakecomparisonswiththeresultsofEx
periment2simpler.Thechoiceofmeasurementintervalisdiscussed blocks[F(7,588) = 3.95,p<.OI],butnotofT[F(l,84) =
further intheGeneral Discussionsection. I.31,p =.25].Sessionblocksinteractedsignificantlywith
Inaddition,therateoffoodcupentry andthelatencytothefirst each ofthe otherfactors (Fs > 2.97,ps < .01). Post hoc
foodcupentryafterCSonsetwererecorded.Tofurtherreducewithin comparisonsoftheoverallrespondingofindividualgroups
groupvarianceofthetimeinfoodcupandrateoffoodcupentry,the (Duncan test) showed only that pre-CS responding was
primaryindexofconditioningreportedwasanelevationscore,cal
greater in Group 10-15 than in any other group (ps <
culatedbysubtractingrespondingduringthepre-CSperiodfromthat
.05);noothercomparisonswerereliable(ps> .10).Itis
duringthefinal 10secoftheCSinterval.However,because insome
important to recognize that these differences in pre-CS
casespre-CSrespondingdifferedamongthegroups,respondingdur
ingtheCSandduring thepre-CS intervalsarealsopresented. respondingwerenot crucialtothe observationoftheef
Finally,theseautomatedmeasures weresupplementedbyobser fects ofthe liTratio on conditionedrespondingjustde
vationsfromthevideotapes,madeusingthemethodsdescribedby scribed: those effects werequalitativelysimilarwhether
Holland(1977). Theobservationaldataarenotpresentedindetail, pre-CS responding was compensated for (elevation
becausetheirpatternsweresimilartothoseofthephotocelldatafor
scores) ornot (during-CS responding).
percentageoftimeinthefoodcup.
Figure 2shows asummaryofthe elevation scores for
each group, collapsed across the entire conditioning
Results phase and plottedasafunction ofboth theliTratio (left
Acquisition: Time in food cup measure. Both the panel) and I alone (right panel). Clearly, when compar
rate ofacquisition ofconditioned responding and final ingacross groupstrainedwiththesameliTratio, Talone
performancewere linearly related totheliTratio. There hadnoeffect, overallthegroupsoratanyindividual liT
was no effect ofTalone when responding with the 10 ratio (statisticsgivenpreviously).Indeed, astepwise lin
secand20-sec CSs wascomparedacross groupstrained ear regression analysis ofthese scores, withliTratio, I,
with equal liTratios. However, when compared across andTasindependentvariables,showedtheIITratiotobe
124 HOLLAND
T = 105 T= 205
60 Elevation Elevation
40
20
0
I I
-20
0- 80
o::J During CS
"'C 60
0
0
LL.
.5 40
CD
E
20
i=
~ I I
0
0
80
Pre-CS Pre-CS
60
40
20
I I
0
2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Two.-.S..e.ssion Blocks Two-Session Blocks
10-15 ..... 20-30
"il-V 10-30 "il-V 20-60
____ 10-60 ____ 20-120
0---0 10-120 0---0 20-240
.......... 10-240 .......... 20-480
0-0 10-480 0-0 20-960
Figure1.AcquisitionofconditionedrespondingoversessionsinExperiment1.Theleft panelsshowre
spondingingroupstrainedwith100sectrialdurations(T),andthe rightpanelsshowrespondingingroups
trainedwith T= 20sec.Thetoppanelsshowelevationscores,calculatedbysubtractingrespondingduring
the100secperiodimmediatelypriortoconditionedstimulus(CS)presentation(bottompanels,Pre-CS)from
respondingduringthelast10secoftheCS(middlepanels,DuringCS).Thefirstnumberineachgroupdes
ignationinthelegendsindicatesT,andthesecondnumbersignifiestheintertrialduration,bothinseconds.
Theerrorbarsineachpanelshowtwotimesthepooledstandarderrorofthe between-groupsmeans.
theonlysignificantcontributortoperformance[r = .66; Figure 2).AnI X T X session blocks ANOVAoftheel
F(l,94) = 71.36]. The best two- and three-variable re evation scores ofthe 10 groups for which I could be
gressionmodels (rs = .66)wereno betterthan the one matched (Groups 10-15 and 20-960 were excluded)
variable model. showedareliable effectofT[F(1,70) = 15.16,p< .01],
Nevertheless, T was important when groups trained aswellas ofI [F(4,70) = 14.74,p< .01].The I X Tin
with the same value ofI were compared (right panel of teraction was not reliable (F < 1). Individual post hoc
TRIAL AND INTERTRIAL DURATIONS INRATS 125
40
e-eT=10s
Co
U::J 30 0-0 T =205
"'C
0
0 20
LL
l::
Q) 10 I
E I
i=
0
"#.
-10
1.5 3.0 6.0 12.0 24.0 48.0 30.0 60.0 120.0 240.0 480.0
lIT Ratio I (5)
Figure2.SummaryofacquisitionperformanceinExperiment1.Theleftpanelshowstheperformanceofall12groups,
plottedasafunctionoftheratiooftheintertrialduration(I)andthe trialduration(T),onalogscale. The rightpanel
showsperformanceofthe10groupstrainedwithcommonIs,plottedasafunctionofIonalogscale.Theresponsemea
sureshown isthe elevation score,calculatedbysubtractingrespondingduringthe 10-secperiodimmediatelypriorto
conditionedstimulus(CS)presentationfrom respondingduringthe last10secoftheCS.The errorbarsineach panel
showtwotimesthe pooledstandarderrorofthe between-groupsmeans.
comparisons (Duncan test) showed reliably greater re (Table2)showednoreliable effect ofliTratio foreither
spondingwith the 10-secTthanwiththe 20-sec Tat 30 ofthose measures (Fs < I). By contrast, the inverse re
sec,240-sec, and 480-secIs(ps <.05). lation betweenliTratio andpre-CS food cupentry rates
Acquisition: Food cup entry rate and latency to was maintained; the effect ofliT ratio was reliable
firstcup entry. Each ofthese measures was also sensi [F(5,84) =27.54,p<.01], as wasthe decreasinglinear
tive to the !ITratio, showing both greater conditioning trend across that variable [F(1,84) = 133.47, p < .01].
withlargerIITratios andratioinvariance-thatis,noef Notsurprisingly,therateelevationscores werealso sen
fectofTalone,when compared atequalliTratios. How sitive to liTratio in the last four sessions [lIT ratio,
ever,the acquisitionpatternsofthese measures differed F(5,84) = 17.35; linear trend, F(1,84) = 86.27, ps <
fromthatofthemeasureofthepercentageoftime inthe .01],but given the lack ofliTratio effects on cup entry
foodcup.Unlike the lattermeasure, which showedmax rates during the CS, that sensitivity is more readily at
imum sensitivitytotheliTratio late intraining,cup en tributabletothe differences inpre-CS responding.
try rates and latencies were most sensitive to liTratio
earlyintraining. Tables I and2showcupentryratesand
Table 1
thelatencies tothefirst cupentry after CSpresentations
ElevationinCupEntryRateandLatencytothe
overthefirst foursessionsandoverthelastfoursessions,
FirstFoodCupEntryDuringConditionedStimuli(CSs)in
respectively. theFirstFourSessionsoftheAcquisitionPhaseofExperiment1
First, considerresponding early intraining (Table I). Group Latency Elevation Pre-CSRate During-CSRate
Separate T X liTratio ANOVAsofthe latencies to first 10-15 3.95 -1.3 14.5 13.2
cupentryafterCSonset,rateelevationscores,andduring 10-30 3.84 1.7 12.2 13.9
CS rates each showed a reliable effect of !IT ratio 10-60 3.00 6.9 9.8 16.7
[Fs(5,84) = 13.81,23.57, and 7.46, respectively, ps < 10-120 2.50 9.5 8.6 18.1
10-240 2.57 15.9 8.3 24.2
.0I] butnoeffectofTalone (ps> .10).Aswith themea
10-480 1.36 22.0 5.9 27.9
sure ofthe percentage oftime in the food cup, the in 20-30 4.64 -4.2 16.0 11.8
creasing linear trends across liTratio were reliable for 20-60 2.90 4.2 11.3 15.5
allthreemeasures[Fs(1,84)= 65.76, 116.12,and34.95, 20-120 2.62 3.7 10.4 14.1
20-240 1.75 11.0 8.3 19.3
ps < .01], and no other polynomial contrasts were sig
20-480 1.65 18.5 7.1 25.6
nificant (ps> .10). By contrast, pre-CS cup entry rates
20-960 1.50 18.0 6.7 24.7
wereinversely relatedtoliTratio; the effect ofliTratio SEM 0.32 0.7 1.0 1.2
wasreliable[F(5,84)=6.78,p<.01],aswasthedecreas Note-Thefirst numberineach group designationindicates thetrial
ing linear trend across that variable [F(1,84) = 31.90, duration,andthesecondnumberindicatestheintertrialduration, both
p < .01]. inseconds.Latencyvaluesareinseconds,andratevaluesareincupen
tries/minute.Elevationscoreswerecalculatedbysubtractingtherateof
Neitherthe latency northerateoffood cup entry dur
cupentryinthe10secpriortoeachCSpresentationfromtheentryrate
ingtheCSmaintaineditssensitivitytoliTratio.ANOVAs duringthe final 10secofthe interstimulus interval. SEM=pooled
ofresponding over the last four acquisition sessions standarderrorofthemeans.
126 HOLLAND
Table 2 Gibbon et al.'s. First, Gibbon et al. found liTratio in
ElevationinCupEntryRateandLatencytothe varianceonlyforameasure ofacquisition, thenumberof
FirstFood CupEntryDuringConditionedStimuli(CSs) in trials needed to meet a response criterion or threshold.
the LastFourSessionsoftheAcquisitionPhaseofExperiment1
Unlike in the present study, in their experiments, sus
Group Latency Elevation Pre-CSRate During-CSRate
tained responding didnotshowthissamesensitivity(see
10-15 1.11 -1.0 22.5 21.5 alsoBrown& Hemmes,1997).Second,Gibbonetal.found
10-30 1.11 8.7 18.0 26.7
no effects ofliTratio on another common measure of
10-60 0.88 10.4 18.3 28.7
10-120 0.98 18.8 10.9 29.7 conditioning strength, the latency to respond tothe CS,
10-240 0.78 24.1 6.3 30.4 which showedsensitivitytotheliTratioearlyintraining
10-480 0.53 26.0 4.3 30.3 inthis experiment.
20-30 2.17 6.3 16.1 22.4
Not surprisingly, the groups with the highest rates of
20-60 1.88 8.2 17.7 25.9
food delivery (shortest Is) spent greater percentages of
20-120 1.47 14.5 11.8 26.3
20-240 1.46 15.3 8.1 23.4 the pre-CS intervals inthe food cup. However,with this
20-480 2.40 20.9 4.6 25.5 measure, pre-CS responding was not acritical determi
20-960 1.40 25.0 3.4 28.4 nant oftheliTratioeffect, which wasobservednotonly
SEM 0.45 1.2 0.7 1.5
with the elevation scores but also with responding dur
Note-Thefirst number ineachgroup designation indicatesthetrial
ingtheCSsalone.Furthermore,apart fromthosegroups
duration,andthesecondnumberindicatestheintertrialduration,both
with the shortest Is, the time spent inthe food cup dur
inseconds.Latencyvaluesareinseconds,andratevaluesareincupen
tries/minute.Elevationscoreswerecalculatedbysubtractingtherateof ing the pre-CS period was not systematically related to
cupentryinthe10secpriortoeachCSpresentationfromtheentryrate food delivery rate.
during the final 10secofthe interstimulus interval. SEM= pooled By contrast, throughout acquisition, the rate offood
standarderrorofthemeans.
cup entry duringthepre-CS periodswasdirectly related
to food delivery rate, increasing linearly asI decreased.
Alsounlikethetimeinfoodcupmeasure,foodcupentry
Test: Timein foodcup.Theeffects oftheliTratioon
rateduringtheCSwassensitive totheliTratioonlydur
thepercentagesoftimespentinthefoodcupobservedin
ingearlystagesoftraining. Thesedifferencesinresponse
theacquisitionsessionspersistedduringthetestsessions,
measuresdeserve comment. First, itisworth comparing
in which all the rats were tested with both 60-sec and
theratedatareportedinthepresentexperimentwiththose
240-secIs.Figure3showstheelevationinpercentagetime
ofLattal (1999), whoreportedonly food cup entry rate.
spent in the food cup in the test sessions. Because ofa
As inthepresent experiment, Lattal alsofoundthat pre-
malfunctioninoneofthetestchambers,thedatafrom 14
rats hadtobe discarded(1ineach of10groups and2in
Groups 10-30 and 10-60). A T X training liTratio X
testI ANOVAshowedareliable effectofthetraining liT ~ 10-60
ratio [F(5,70) = 7.30,p<.01]butnoeffectofanyofthe 60 ...... 10-240
other variables ortheir interactions(Fs< 1). a. !:'r---6 20-60
Aswasobservedattheend ofacquisition, the groups <3 0-0 20-240
50
didnotdiffer intheirperformanceontheother response
-g
measuresduring the testsessions (data not shown). 40
o
LL.
Discussion c 30
With the measure oftime inthe food cup, the rate of
~
acquisitionand overallperformancewerebestpredicted 20
bytheliTratio. Thadnoeffectwhencomparedingroups i=
trainedwiththesameliTratios.Although therangeofTs ~ 10 I
o
examined(10 and 20 sec)was limited, those twovalues
OL.------'----~------'--~-~-~-
showed clear T effects when assessed in groups with a
1.5 3.0 6.0 12.0 24.0 48.0
common value ofI. Note that the linear relation oflog
liTratioportrayedintheleftpanel ofFigure 2(respond liT Ratio
ingshowedaconstantincrementforeach doubling ofl)
guarantees theadditiveeffectofTacross differentvalues Figure3.PerformanceinthetestphaseofExperiment1,plot
ofI, an intriguingquantitative aspect ofthese data. ted asafunctionoftheratioofthe intertrialduration(I)andthe
trialduration(T)usedintrainingineachgroup,onalogscale.
Inany event, the liTratio providedmore information
In thelegend,the firstnumberrefersto the T(in seconds) used
than I alone, extending the basic findings of Lattal in boththetrainingandthe testphases,andthe secondnumber
(1999) and Gibbon etal.(1977). Like Gibbonetal.'sre signifiesthe I (in seconds) used in the testphase.The response
sults, but unlike Lattal's, inExperiment 1,conditioning measureshownisthe elevationscore,calculatedbysubtracting
respondingduringthe 10-secperiodimmediatelypriortocondi
wasinvariantacross Tvalues withinliTratio.Indeed,de
tionedstimulus(CS) presentationfrom respondingduring the
spite thelimitedrange ofTsexamined,insomerespects
last10secoftheCS. Theerrorbarshows twotimesthe pooled
these findings may be construed as more robust than standarderrorofthebetween-groupsmeans.
TRIALAND INTERTRIALDURATIONS IN RATS 127
CS food cup entryrate was often animportantcontribu EXPERIMENT2
tor toliTratio effects, declining with longervaluesof1.
Nevertheless,Lattal showeddirecteffectsoftheIITratio Experiment 2testedthe generality ofExperiment 1's
onfood cupentryrate duringthe CSthroughouttraining findings ofratio invariance by examiningthe effects of
and testing. It is notable that Lattal's studies used only variations in Tand I,with a single value ofthe liTratio
four sessions oftraining, and so his data are perhaps (6).The Tswereextendedupwardfrom thoseused inEx
more appropriatelycomparedwith our early-acquisition periment 1; four groups ofrats received training with
data (Table 1). noise CSs with Tsof20, 40, 80, or 160 sec. Ifonly the
Second,observationsofthevideotapesshowedthat in liTratio is important for conditioning, the performance
thepre-CSperiods,the ratstendedtoemit brief-duration ofall four groups should be indistinguishable. Differ
entries into the recessed food cup, whereas during CS encesamongthe groupswould indicateroles forI, T,or
presentations,they tendedtokeep their headsinthefood both individually, beyondthat ofthe liTratio alone.
cup longer, especially as training proceeded. A general TheuseofextendedTsencouragedthedisplayofmore
pointisthat the form ofconditionedbehaviorcontrolled temporallyorganizedbehaviorthan was observedinEx
bythe conditioningcontextalone (or the prioror antici periment 1.Inavarietyofconditioningpreparations,the
pated delivery offood) may differ from that controlled level ofconditioned responding peaks near the normal
byadiscreteCSand thatvarious measuresofcondition time ofUS delivery, indicatingthat associativelearning
ing during such a CS may interact differently with be involvesthe codingoftemporalaswell aspredictivere
haviorcontrolledbyevents otherthanthat CS. Forexam lations(see Savastano&Miller, 1998,forareview). Fur
ple, inautoshaping,keypeckingiscontrolledbydiscrete thermore,thetemporaldistributionofrespondingwithin
visual CSs, but not by contextual cues, which tend to Toften shows scalarproperties-thatis, when respond
controllessdirectedactivity(Balsam& Schwartz, 1981; ing acquiredwith different Tsisnormalizedproportion
Grau &Rescorla, 1984; Killeen, 1975). ally to T,the resultant distributions are nearly identical
Thepatternofpre-CS foodcupentryratesacrossgroups (superpositioning). For example, Brown and Hemmes
observed inthisexperimentmightreflect sustaineddiffer (1997) measured pigeons' proximity to the source ofa
ences inrespondinginthe absence ofCSs throughoutthe food ~S during visual CSs presentedwith Tsof18,24,
sessionsorcyclicincreasesastheaverageexpectedtimeof or 60 sec. When responding in each one-sixthofTwas
the next CS draws near (recall that the Is were variable). expressedas afraction ofthe total responding during T,
Because response timing islikelytobemore accuratethe the distributionswere indistinguishable across the three
shorter theI (Gibbon, 1977),pre-CS respondingmight be values ofT. Similar superpositioning effects have long
especially high with shorter Is. Likewise, groups with been noted in the control ofoperant responding under
shorter Isalsohad shortersessions; thus, anytendencyto fixed-intervalschedules(Catania, 1970).InExperiment2,
explore the food cup when first placed in the chamber thetemporaldistributionofrespondingwasexaminedfor
might contribute more to pre-CS responding in groups evidenceofsuperpositioning.
withshortIs.Unfortunately,respondingwasnotrecorded, InExperiment2,conditionedrespondingwasalso as
exceptduringpre-CS andCSperiods. sessedafterextinctionexposuretotheexperimentalcham
Finally,consistentwith Lattal's(1999) data, theresults bers,intheabsenceofCSsorUSs.Thismanipulationwas
ofthe test sessions indicatethat the liTratio effects ob intendedtoreducethe contributionofany differencesin
served in acquisition reflected differences in learning, contextualconditioningorpre-CSrespondingtoperfor
notjustperformance.Testbehavioroftherats inthevar mance differences across the groups. Furthermore, re
iousgroupswasindicativeoftheI value withwhichthey spondingto the CSs was assessed inthe absenceofany
were trained, regardlessofthe value ofI used intesting. USsinallthegroups, providinganotherwayofdisentang
Thus, theeffects observedinacquisitionwere unlikelyto ling the effects ofI or the liTratio on learningand per
beduetoshort-termfactors, suchasdifferentialsuppres formance. Ifthe magnitude ofconditioned responding
sion ofresponding during a trial as a result offood de reflectsacomparisonoftheassociativestrengthsorrep
liveryontheprecedingtrial. Likewise,thesetest data are resentationsoffoodrateduringTandI(see, e.g., Gibbon
inconsistent with accounts for IlTratio effects that em & Balsam, 1981;Miller& Matzel, 1988), pretestextinc
phasizecomparisonofcurrentfoodratesorexpectancies tion ofthe context shouldproduceequalI valuesfor all
duringthetrial and intertrialperiodsatthetime ofaction the groups, and testing should reveal differences in CS
(e.g., Gibbon& Balsam, 1981),unless itwere addition (T) strengthonly.
allyclaimedthatthecalculationsofintertrialfood rateor Finally, Experiment2also usedablockingprocedure
associativestrengthexhibited substantialinertia.Although toprovideameasureofconditioningstrengththat might
foranumberofreasons,Gibbonand Balsamarguedthat berelativelyindependentofthe form or temporaldistri
these calculationswere adjustedrapidly, the lack ofany butionofrespondingcontrolledbytheCSs inthevarious
obvious adjustment in responding in the test sessions groups(Holland, 1977,1980).Attheendoftraining,a10
when I was altered from the training value is consistent sec visual cue was presented during the final 10sec of
with Spetch and Rusak's (1992) view that reference thenoise CS ineach groupand wasfollowedbyfood de
memory forI is stable. This issue is considered further livery.Conditionedrespondingtothecommonvisual CS
inExperiment2. alone was then assessed in all the groups. The stronger
128 HOLLAND
the conditioning ofthe noise (or the better it predicted
food delivery), the less conditioning ofthe visual CS
wouldbeanticipated.
Method
Subjects.Thirty-twoexperimentallynaivefemale CD-strainrats
(bredin a Duke psychology department vivarium from Charles
River stock),4-5 months old atthe beginningofthe experiment,
I
servedas subjects. They were maintainedin the same way as the
ratsinExperiment I.
Apparatus.The apparatuswasthesame asthat usedinExperi
ment 1.
Procedure.Initially, alltheratswere trainedtoeatfrom there
cessedfoodcup. Inasingle 64-minsession,there were 16deliver
iesofthetwo-pelletUSonaVT2-min schedule.Next, allratsre
ceivedeightpairingsofthe80-dBwhitenoiseCSwiththefoodUS
in each of20 daily conditioning sessions. Eachgroup ofrats re
ceivedasinglecombinationofTandI,asspecified(inseconds)by
the group name, T-I: 20-120,40-240,80-480, and 160-960. In
each group, the noise CS remained onthroughout T.The Iswere
variable, with the specifiedarithmetic mean; the values ranged
from0.5to 1.5timesthemean, rectangularlydistributed,asinEx
periment I.Thesessionlengths wereeighttimesthemeanI values
andsovariedfrom 16to256minacrossthegroups.
Next,theratsreceivedfourcontextextinctionsessions,designed I
toreduce contextconditioningand base rate responding.The first
twoofthese sessionswere 128minlong, and the secondtwowere
16min long, inall the groups. Thenthe rats receivedtwo64-min
testsessions,inwhicheightnoiseCSsbutnofood pelletswerede
livered. Inthose sessions,the Tsineach groupwere those used in
training,butI was480 secinallthegroups.
Afterfour retraining sessions identicalto those ofthe original
conditioningphase,the ratsreceivedfour compoundconditioning .... 20-120
sessions. These sessionswere identicalto the retrainingsessions, "V"-'V 40-240
except that thejeweledlamp above the food cup was illuminated .... 80-480
duringthe final 10sec ofeach noise presentation. Finally, condi <>-0 160-960
tioningtothislightCSwasassessedinasingle I6-mintestsession
ineach group, which includedfour nonreinforced light presenta
tions, withanaverageI of240sec.
Data presentation. In Experiment 2, the only response mea
sures reportedwere the percentageoftime spent in the food cup
duringCSandpre-CSperiodsandthedifferencebetweenthosetwo
I
values (elevationscores).AsinExperiment I,foodcupentryrates
and latencieswere only minimallysensitiveto the temporal vari
ables after the early stagesoftraining. BecauseExperiment2 in
3 4 5 6 7 8
volved even more protractedtrainingthan did Experiment 1,food
cupentry rates and latencies werenotreported. Also, asinExperi Two-Session Blocks
ment I,thefocus wasonrespondingduringthe final 10secofthe
CSineachgroup. However,amore detailedanalysisofthetempo Figure4.Acquisitionofconditionedrespondingoversessions
raldistributionofrespondingisalsopresented. inExperiment2.Thefirstnumberineachgroupdesignationin
the legendindicatesthe trial duration, andthe second number
Results signifiestheintertrialduration,bothinseconds.Thetop panels
showelevationscores,calculatedbysubtractingrespondingdur
Acquisition. Higher levels ofresponding were ac
ing the 10-secperiodimmediatelypriortoconditionedstimulus
quired in groups with shorter values ofI and T,despite
(CS) presentation(bottompanel,Pre-CS)from respondingdur
thecommonliTratio.Figure4showsthepercentagesof ingthelast10secoftheCS(middlepanel).Theerrorbarsineach
timetherats spent inthefoodcupduring thelast 10sec panelshowtwotimesthe pooledstandarderrorofthe between
ofthe CS and in the lO-sec pre-CS periods andthe dif groupsmeans.
ference between those twomeasures (elevation scores).
Group X sessionblockANOVAswereconductedforeach
ofthethreemeasures. Ineachcase,thegroup X session creased morerapidlytheshorter the1(andT). Likewise,
blocks interactionwasreliable [Fs(21,196)~ 1.72,ps < pre-CS responding among the groups divergedastrain
.05].Likewise,contrastsofthelinearvariation(overses ingproceeded, with greater responding theshorter the1
sionblocks) inthelineartrendacross groups (asafunc (and T).
tionof1andT) werereliableinallcases[Fs(l,28)~9.15, Finally,asymptotic responding wasanalyzed bycon
ps < .01]. Thus, conditioned responding to the CS in- ducting separate ANOVAsfor the last halfoftraining
TRIAL AND INTERTRIALDURATIONS INRATS 129
(Sessions 9-16). Those analyses all showed reliable ef Figure6showsthechangeinthetemporaldistribution
fects ofgroup [Fs(3,28) ~ 3.73,ps < .05] and no signif ofelevationscoresoversuccessivequartersoftheacqui
icanteffectsofsessionblocksorgroup X sessionblocks sitiontrainingsessions.Unfortunately,thetimebinsused
interactions(ps> .20).Fortheelevationscores, therewas earlyintrainingpermittedonly normalizingthe data by
adecreasinglinear trend acrossgroups[F(l,28) = 10.34, halves ofTinthisfigure. Priortothedisplayofgreaterre
p < .01]. Post hoc comparisons (Duncan's test) showed spondingduringthe CSthaninthepre-CSperiods(first
onlythatrespondinginGroup 160-960wasreliably(p< quarteroftraining; compare with Figure4), responding
.05)lessthan that inGroups20-120and 40-240.Forre was evenlydistributedacrossT.However,assoon aspo
spondingduringthe last 10secoftheCS, therewasalso sitiveelevationscoresemerged(secondquarteroftraining),
adecreasinglineartrendacrossgroups [F(l,28) = 34.27, that respondingshowed substantialtemporalspecificity.
p < .01]. Duncari's tests showed the differences in re Extinctiontestresponding.Aftertraining,alltherats
spondingbetweeneachpairofgroupstobereliable(p< receivedfourcontextextinctionsessions intheexperimen
.05),exceptforthatbetween Groups20-120and40-240. tal chambers inthe absenceofany explicitevents, inan
Pre-CS responding also showed a decreasing linear attempt toequalizethelevelsofpre-CSrespondingamong
trendacross groups [F(I,28) = 25.69,p<.01]. Duncan's the four groups. Then, responding to the CSs was re
testsshowedthatGroups 20-120and40-240eachdiffered assessed in two extinctiontest sessions. In that test, the
from each ofGroups 80-480and 160-960(ps < .05). highestelevationscores wereobservedinGroup40-240,
Temporaldistributionofrespondingin acquisition. the lowest inGroup 160-960,and intermediatelevels in
By the end oftraining, the rats in all the groups showed the othertwo groups.
graduallyincreasingresponselevelsoverthecourseofT. Figure 7 shows elevation scores over the four half
Theleftpanel ofFigure5showselevationscoresoverthe session blocks oftesting. A group X blocks ANOVA
final foursessions ofacquisition, dividedinto 10-secbins. showeda reliable effect ofgroups [F(3,28) = 3.40, p <
The right panel ofFigure 5 shows responding from the .05]. Bothlinearandquadratictrendsacross groupswere
samesessions,normalizedoverquartersofTineachgroup reliable [Fs(l,28) = 5.56 and 4.19, ps < .05], although
by dividing the time spent in the food cup during each only the comparison ofGroup 40-240 and Group 160
quarterofTbythe totaltime spent inthefood cupduring 960 was reliable(p< .05) by aDuncan'stest. Compara
theentire T.Superpositioningwasevident: The slopes of ble analyses ofresponding during the CSs showed the
the normalized T functions among the groups were in same pattern,but with largerFvalues.The levels ofpre
distinguishable. The extent ofsuperpositioningwaseval CS responding did not differ among the groups (range,
uated byconductingagroup X quarter(ofT) ANOVAof 5.3%-8.8%; F< 1),whichindicatesthat thecontextex
thenormalizedelevationscores. Thelack ofasignificant tinctionphasewas successful.
group X Tquarter interaction [F(9,84) = 1.27,p > .20], Figure 8 shows the normalized temporal distribution
coupled with a reliable effect ofTquarter [F(3,28) = ofrespondingoverthecourseoftheextinctiontestphase.
54.94, p < .01], demonstrates the extent ofthe super A group X quarter (ofT) X block ofextinction trials
positioning. Decompositionofthe Tquartereffectintoor ANOVAofthenormalizedelevationscoresshowedare
thogonalpolynomialsshowedreliablelinearandquadratic liable effectofTquarter [F(3,28) = 25.13, p < .01], but
trends [Fs(l,28) = 73.58 and 15.86,ps< .01]. no other significant effects or interactions (ps> .20).
60 ........ Cr:::l
20-120 '0 30
r:::
a. 50 ~ 40-240 a0.
o:::l ---- <Il
80-480 CIl
0:::
'00 40 <>--<> 160-960 a. 20
U0. o:::l
.5 30 '0
ECIl u00,
i= 20 ]I 10
'#. ~
10 '0 I
~
0 0 0
2 4 6 8 10 12 14 16 2 3 4
10-sInterval QuarterofT
Figure5.Temporaldistributionofrespondinginthe lastfour-sessionblockofacquisitionin Experiment2.Thefirstnum
berineachgroupdesignationinthelegendindicatesthetrialduration(T),andthe secondnumbersignifiesthe intertrialdu
ration,bothinseconds.TheleftpanelshowsrespondingduringpresentationsoftheconditionedstimulusasafunctionoflO-sec
binsofT.Therightpanelshowsthatsameperformance,normalizedacrossquartersofTbydividingthetimespentinthe food
cupduringeachquarterofTbythetotaltimespentin the food cupduringthe entireT.Theerrorbarshows two timesthe
pooledstandarderrorofthe within-groupsmeans.
130 HOLLAND
........
Cl 80 20-120
c:
:.a
~ 40-240
c:
0 70 --- 80-480
a.
<>--0 160-960
Cl)
~
60
a.
~
o::J
50
"'0
0
0
L-L 40
co
-
0 30 I
I-
0
~ 20
0
1 2 1 2 1 2 1 2
HalfofT
1 2 3 4
Four-Session Block
Figure6.Changeintemporaldistributionofrespondinginthe acquisitionphaseofExperiment2.Thefirst
numberineach groupdesignationinthelegendindicatesthetrialduration(T),andthe secondnumbersig
nifiestheintertrialduration,bothinseconds.Ineachfour-sessionblock,performanceineachgroupwasnor
malizedacrosshalvesof Tbydividingthe timespentinthefood cup duringeachhalfofTbythe totaltime
spentin the food cupduringtheentireT.Theerrorbarshows two timesthe pooledstandarderrorofthe
within-groupsmeans.
Thus, thetemporaldistributionofrespondingwasmain andmorerespondingduring both CSandpre-CS periods
tained asits level declined. thandidtheratstrainedwithlongerIsandTs.Afterelim
Blocking (light) test. Figure 9 shows the elevation inationofthedifferentialpre-CSratesbycontextextinc
scores forthe light CSduring the blockingtest.The ele tion,thegreatestelevationscoresandCSrespondingwere
vation scores ofthe four groups ofrats in this test were observed in those rats that received40-sec Ts and 240
inversely relatedto the elevation scores in those groups secIs.Theseintervalsalsogeneratedthemostinterference
inthe extinctiontest with the noise CS (Figure 7).This with conditioning ofthe 10-sec light when it was com
finding isconsistentwiththe claim that variationsinre poundedwiththenoise.Althoughtheabsenceofcontrols
spondingtothe lightintheblockingtest reflectedvaria forconditioningofthe noise CSspriortothecompound
tions intheabilityofthenoise CStoblocktheestablish conditioning phase precluded the identification ofthe
ment ofnew learning. A one-way ANOVA showed a variations in responding tothe light asthe result ofdif
reliable effectofgroups [F(3,28)= 9.40,p< .01];mul ferences in blocking rather than in overshadowing, the
tiplecomparisonsusingDuncan'sprocedureshowedthat implicationsofeitheraresimilar.Allinall,theresultsof
the responding ofGroup 40-240 was lower than that of Experiment2showthat, with verylongintervals,theab
each ofthe other groups and that the responding of solutevaluesofIorTcanbeimportantbeyondtheircon
Group 160-960washigherthan that ofeachoftheother tributiontotheliTratio.
groups (ps < .05). Responding during the CSalone ex Previousstudieswiththisconditioningpreparationalso
hibitedasimilarpattern,and pre-CS respondingdidnot showedthat240-secIs supportedbetterlearningthan did
differ amongthe groups (F < 1).Finally, the correlation shorterIs(e.g., Holland, 1995, 1999;Holland& Morell,
between respondingtothe light in the blockingtest and 1996),althoughthose same studies showedthat480-sec
to the noise in the extinction test among the individual Is supportedeitherequivalent (Holland, 1995)or supe
rats (in allgroupscombined)was -.74. rior(Holland& Morell, 1996)learning. Itispossible that
the relationobservedinExperiment2reflectsatradeoff
Discussion betweenindependentfacilitatoryeffects oflongerIsand
Despite a commonliTratio, the rates ofacquisition shorter Ts. On the otherhand, it might be argued that T
and the final levels ofconditioned responding differed alonedeterminedvariationsinlearningamongthegroups
acrossgroups. Intheacquisitionphase, thoseratstrained and that the greater responding ofGroup 40-240, as
with shorter Is and Ts showed higher elevation scores comparedwithGroup 20-120, inthepostextinctionand