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absence of overshadowing and blocking between landmarks and the geometric cues provided by the shape of a test arena

absence of overshadowing and blocking between landmarks and the geometric cues provided by the shape of a test arena
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  Absence of overshadowing and blockingbetween landmarks and the geometric cuesprovided by the shape of a test arena Andrew Hayward, Anthony McGregor, Mark A. Good, and John M. Pearce Cardiff University, Cardiff, UK  In three experiments rats were required to escape from a pool of water by swimming to a sub-merged platform. The position of the platform was determined by the shape of the pool, whichwas either rectangular or triangular. A landmark that was located on the surface of the pool nearthe platform failed to overshadow (Experiment 1) or block (Experiment 2) learning about theposition of the platform with reference to the shape of the pool. Experiment 3 revealed a similaroutcome with cues outside the pool, which could be used, in addition to the shape of the pool, toidentify the location of the platform. These findings imply that theories of learning that assumethat stimuli must compete with each other for the control that they acquire may not apply tospatial learning based on the shape of the environment. Demonstrations of blocking and overshadowing are regarded as important for at least tworeasons.First,theyindicatethatstimuliareincompetitionwitheachotherforthecontrolthattheyacquireoverbehaviour,and,assuch,theseeffectssupporttheoriesoflearningthatincor-porateanerrorcorrectionprinciple(e.g.,Pearce,1994;Rescorla&Wagner,1972).Accordingtothisprinciple,learningaboutastimuluswillceasewhentheoutcomeitsignalsisaccuratelypredicted by the stimulus itself together with the stimuli that accompany it. Second, demon-strationsofovershadowingandblockinginavarietyofsettingshavebeentakenasevidenceforthe generality of the laws of learning. Hence, as Mackintosh (1974) points out, the claim thattaste aversion conditioning is different to conditioning with exteroceptive stimuli was calledintoquestionwhenitwasfoundthatbothblockingandovershadowingcouldbedemonstratedwith flavours that were paired with illness (Revusky, 1971).Experimentsshowingcuecompetitioneffectsinspatialtaskshavelikewiseledtoagrowingacceptance that spatial learning is governed by the same theoretical principles that operate in Requests for reprints should be sent to John M. Pearce, School of Psychology, Cardiff University, Cardiff CF13YG, UK. Email: Research Council. We are grateful to Peter Jones for his assistance with Experiment 3, and to VictoriaChamizo for her suggestions concerning the interpretation of the results. © 2003 The Experimental Psychology Society DOI:10.1080/02724990244000214THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2003,  56B  (1), 114–126 Q0667—QJEP(B)SI B9/Jan 13, 03 (Mon)/ [13 pages – 0 Tables – 3 Figures – 0 Footnotes – 0 Appendices]. .Centre single caption.  shortcut keys . READ AS KEYED  the conditioning laboratory, despite initial arguments to the contrary (O’Keefe & Nadel,1978). Spatial learning refers to the ability to find a hidden goal by reference to cues that liesome distance from it, and there are a number of demonstrations that this type of learning isaffected by both overshadowing and blocking (for a review, see Chamizo, 2003). It would,however,beprematuretoconcludethatallaspectsofspatiallearningaregovernedbythesameprinciplesthatapplytoconditioningandassociativelearning.Severalexperimentshaveindi-catedthatifthelocationofahiddengoalcanbeidentifiedbyreferencetotheshapeofthetestenvironment, then the presence of landmarks at or near the goal may not restrict learningbasedontheshapeoftheenvironment.Thesefindingshavepromptedtheclaimthatlearningbasedontheshapeofatestenvironmentisgovernedbyratherdifferentprinciplestothosethatgovern learning based on other cues (Cheng, 1986; Gallistel, 1990). The purpose of thepresent article is to examine this claim in some detail.Thefirstdemonstrationthatlearningbasedontheshapeofatestenvironmentmaynotbeinfluenced by the presence of other landmarks was provided by Cheng (1986). Rats wererequired to find food hidden in one corner of a rectangular chamber, which had a distinctivelandmark in each corner. Rather than relying solely on these landmarks, rats made consider-ableuseoftheinformationprovidedbytheshapeofthearena.Duringtheearlystagesoftheirtrainingtheymadeerrorsbysearchinginthecornerthatwasdiametricallyoppositetothecor-rectcorner.Moreover,whenatesttrialwasconductedinwhichthelandmarknearestfoodwasmoved to an adjacent corner, they were no more likely to search near the landmark than in acornerthatcorrespondedgeometricallywiththecornerwherethefoodwassrcinallyhidden.On the test trial, therefore, subjects did not prefer a stimulus that provided unambiguousinformation about the location of food to one that provided ambiguous information. Thistraining can be likened to a feature-positive discrimination, and, for both empirical (e.g.,Wagner,1969)andtheoretical(e.g.,Rescorla&Wagner,1972)reasons,itshouldhaveresultedin the landmark gaining more control over searching for the food than did the geometricpropertiesofthetestarena.ForrelatedfindingsseeKelly,Spetch,andHeth(1998),Tommasiand Vallortigara (2000), and Vallortigara, Zanforlin, and Pasti (1990).TheexperimentbyCheng(1986)didnotincludeacontrolgroup,whichmakesitdifficulttobecertainthatthelandmarknearthegoalhadnoovershadowinginfluenceonspatiallearn-ingbasedonthegeometriccuesprovidedbytherectangulararena.Theresultsfromtwootherstudies, using a similar design to that of Cheng (1986), did include a control condition, andboth of these failed to find any evidence of overshadowing (Kelly et al., 1998; Pearce, Ward-Robinson,Good,Fussell,&Aydin,2001).AplanoftheapparatususedbyPearceetal.(2001)is shown in the left-hand side of Figure 1. The triangular test arena was created by loweringtwo boards into a circular pool of water, which was enclosed by curtains suspended from theceiling.Duringthetrainingstageoftheexperiment,groupnonewasrequiredtoescapefromthepoolbyswimmingtoasubmergedplatformthatwasalwayslocatednearaspecificcorneronthebaseofthetriangle.Thistrainingwasfollowedbyatesttrial,withouttheplatform,dur-ing which the time spent searching in the correct and incorrect quadrants at the base of thetriangle was recorded (see Figure 1). Group none exhibited a substantial preference for thecorrectquadrantonthistrial,whichconfirmsthattheshapeofthepoolhadbeenusedforfind-ingtheplatform.Groupbeaconwastrainedinthesamewayasgroupnone,exceptthataverti-cal rod was attached to the platform throughout the training stage. In terms of associativelearningtheory,therodabovetheplatformshouldovershadowlearningaboutthepositionof  ABSENCE OF OVERSHADOWING AND BLOCKING  115  theplatformwithreferencetotheshapeofthepool.Thetesttrialrevealed,however,thatthepreference expressed by group beacon for the correct over the incorrect quadrant was nodifferent to that shown by group none.One purpose of the present experiments was to extend the generality of the findingsreportedbyPearceetal.(2001).Inthefirstexperiment,theextenttowhichalandmarknearahiddengoalcaninfluencespatiallearningbasedonarectangle,ratherthanashape,wasexam-ined. Moreover, instead of using a rod over the submerged platform, the landmark that wasusedtointeractwithlearningbasedontheshapeoftheenvironmentwasanobjectinthepoolthat was some distance from the platform in the first two experiments and cues outside thepoolinthethirdexperiment.Ifnoevidenceofcuecompetitioncanbefoundwithanyofthesechanges,thenitwouldindicatequiteforcefullythattheremaywellbesomethingspecialaboutspatial learning based on the shape of the environment.Iftheexperimentsshouldrevealthatspatiallearningbasedontheshapeofthepoolisunaf-fected by the presence of other landmarks, then one way in which associative learning theorymightexplainthefindingsisbyreferringtotherelativesalienceofthestimulithatwereused.Theories proposed by Rescorla and Wagner (1972), or Mackintosh (1975), for example, pre-dict that if a stimulus is of relatively low salience then it might exert rather little influence onconditioningwithastimulusofgreatersalience.Onthisbasis,therefore,thefailureofaland-mark to restrict spatial learning based on the shape of the environment could be due to thelowersalienceofthelandmarkthanoftheshape.Anadditionalconcernoftheexperimentswasto evaluate this possibility. EXPERIMENT 1 A rectangular pool was created for the first experiment by lowering panels that were either1.8mor0.9mlongintoacircularpoolwithadiameterof2m.Thelandmarkinthepoolwasaspherewithadiameterof12cm.Thereweretwogroupsintheexperiment.Grouplandmarkreceived12sessionsoftraininginwhichthesubmergedplatform(diameterof10cm)andthe 116  HAYWARD ET AL. Figure1.  Plansofthetriangularandrectangulararenasthatwerecreatedinacircularpool.Thedashedcircleindi-cates the location of the submerged platform, and the filled circle indicates the location of the landmark for grouplandmarkinExperiment1.Thedottedlinesindicatehowthearenasweredividedintoquadrantsforthetesttrials.  sphere were on a line that bisected one of the corners of the pool. The centre of the platformwas 30 cm from the corner, and the centre of the sphere was 60 cm from the corner (see theright-handsideofFigure1).Thesamecornerwasusedthroughoutthisstage.Theinfluenceofdistalcueswasminimizedbysurroundingthepoolwithcurtainsandbyrotatingtheorien-tationoftherectangulararenaby90°fromtrialtotrial.Ratswerereleasedfromthecentresof thefourwallsinarandomsequenceandwererequiredtoescapefromthepoolbyclimbingonthe platform.Therewasamarkedreductioninthelatencytoescapefromthepoolastrainingprogressed,whichimpliesthatratswereusingthetwodifferenttypesofcuethatwereavailabletothemtofindtheplatform.Inordertoassesstheextenttowhichtheshapeofthepoolwasusedforthispurpose, the experiment concluded with a test trial in which subjects were placed in the poolfor60sintheabsenceoftheplatformorthelandmark.Duringthistrial,subjectsspent66%of theirtimesearchinginthegeometricallycorrectcorners(thosecontainingcornersAandCinFigure 1). Although this preference for the correct over the incorrect quadrants implies thatthegeometricpropertiesoftheshapeofthepoolacquiredsomecontroloversearchingfortheplatform,thiscontrolmayhavebeenpartiallyrestrictedbythelandmark.Theresultsfromthesecond group in the experiment indicate, however, that this was not the case. Group nonereceived training during the first stage, which was designed to ensure that maximum controlwasacquiredbytheshapeofthepooloversearchingfortheplatform.Therewasasubmergedplatform located in both of the geometrically correct corners, and no landmarks were used.Despite this method of training, the test trial revealed only a modest preference for the geo-metricallycorrectcorners,withthegroupspendingameanof57%ofthetimeduringthetesttrialinthecorrectquadrantsofthepool.Thefactthatthispreferenceisweaker,butnotsignif-icantly so, than the preference exhibited by group landmark suggests that the presence of thelandmark in group landmark had little impact on learning about the position of the platformwith reference to the rectangular shape of the pool.Oneexplanationfortheresultsfromgrouplandmarkisthatverylittleattentionwaspaidtothelandmarksothatitwouldnotbeexpectedtointerferewithlearningbasedontheshapeof the pool. Fortunately, performance during the training stage of the experiment allows thisexplanationtoberejected.Ifsubjectspaidlittleattentiontothelandmarkduringtrainingthenthey would fail to discriminate between the correct corner and its geometric equivalent—thediametrically opposite corner. In fact, as training progressed, subjects developed a markedpreference for swimming to the corner containing the platform and the landmark. By way of example, in the final training session, group landmark swam directly to the correct corner on90% of the trials and to the diametrically opposite corner on 7% of the trials. These findingsmakeitclearthatratsattendedtothelandmarkinthepoolandthattheyhadlearnedaboutitssignificance for finding the platform.Afurtherexplanationfortheresultsoftheexperimentisthatthelandmarkisnotasuitablecueforrestrictingthecontrolacquiredbyothercues.Althoughitisnotpossibletorejectthisaccount on the basis of the present findings, the results from another study indicate that it isunlikely. Two groups of rats were required to swim to a platform that was always 20 cm duewest of the spherical landmark. Training took place in a circular pool in full view of the cuesprovided by the room, and, from trial to trial, the platform and landmark were moved as onethroughoutthepool.Tofindtheplatform,therefore,itwasnecessarytomakereferencetothelandmark and the room cues, with the latter providing information about the direction of  ABSENCE OF OVERSHADOWING AND BLOCKING  117  the platform from the former (see Roberts & Pearce, 1998). Evidence that this training wassuccessfulwasrevealedbyamarkedreductioninescapelatenciesfromabout80sonthefirsttraining session to about 10 s on the final session of training. For the subsequent test session,bothgroupsreceivedfourtrialswiththelandmarklocatedforthefirsttimeinthecentreofthepool. The platform for group block was in its normal position due west of the landmark,whereas for group control the landmark was its usual distance from the landmark but unex-pectedlydueeast,ratherthanduewest.Finally,bothgroupsreceivedatesttrialintheabsenceof the landmark and the platform to assess the degree to which the room cues by themselvescame to control searching for the platform in its new location. As far as group block is con-cerned, the platform during the final four training trials could be readily found by relying onthe same strategy that had been effective during the previous sessions. This strategy wastherefore expected to block learning about the position of the platform with reference to theroomcuesalone.Inconfirmationofthisexpectation,only24%ofthe60-stesttrialwasspentsearching in the quadrant where the platform had been located during the last four trainingtrials—thisoutcomeisnodifferenttothatexpectedonthebasisofchance.Incontrast,groupcontrol spent 47% of the trial searching in the quadrant where the platform had just beenlocated. The results from group control show that the final four training trials, for which theplatformremainedinthesameplace,weresufficienttoenablesubjectstoidentifyitspositionwith reference solely to the cues outside the pool. The results from group block indicate thatthe presence of the landmark was sufficient to block such learning.Insummary,therefore,thefirstexperimenthasshownthatthepresenceofalandmarkneara platform does not overshadow spatial learning based on the shape of the pool. It is unlikelythat this outcome is a consequence of subjects failing to attend to the landmark, and it isunlikelythatitisaconsequenceofthelandmarkbeinginherentlyineffectiveasacueforover-shadowing and blocking. EXPERIMENT 2 The second experiment was designed to demonstrate the generality of the previous findingsby assessing the extent to which the landmark used in Experiment 1 could influence spatiallearningbasedonatriangularratherthanarectangularpool.Moreover,toenhancethelikeli-hood of finding such an influence a blocking rather than an overshadowing design was used.Theentireexperimentwasconductedwithcurtainsdrawnaroundthepoolandwiththeappa-ratus rotated by 90° from one trial to the next. For the first stage of the experiment, groupblock was required to swim in a rectangular pool (1.8 m × 0.9 m) to a platform, the centre of which was 30 cm from a corner on the line that bisected the corner. The spherical landmarkusedinthepreviousstudywaslocatedonthesamelinewithitscentre60cmfromthecornerof thepool.Thecornerwasselectedrandomlyfromtrialtotrialtoensurethatthelandmarkpro-videdtheonlyreliablecueforfindingtheplatform.Thegroupwasthentrainedtoescapefromatriangularpool,whichwascreatedbyinsertingtwopanelsoflength1.8mintoacircularpool,whichwas2mindiameter(seeFigure1).Throughoutthisstagetherewasasubmergedplat-formnearonecornerwiththelandmarkneartheplatform.Thesamecornerwasusedforeachtrial,andthedetailsoftheplacementoftheplatformandthelandmarkwerethesameasthoseforthepreviousstage.Ifspatiallearningbasedontheshapeofanenvironmentisgovernedbyerror correction (e.g., Rescorla & Wagner, 1972), then the presence of the landmark in the 118  HAYWARD ET AL.
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