Reversal from blocking in humans as a result of posttraining...

Copyright 2001 Psychonomic Society Inc 354

Animal Learning amp Behavior2001 29 (4) 354-366

Responding to a stimulus that predicts an outcome canbe weakened if other stimuli are presented in compoundwith this stimulus during training Accounting for such stim-ulus competition has been central to the development ofmost contemporary theories of learning (eg Dickinson ampBurke 1996 Mackintosh 1975 Miller amp Matzel 1988Pearce amp Hall 1980 Rescorla amp Wagner 1972 Van Hammeamp Wasserman 1994 Wagner 1981) A frequently cited ex-ample of stimulus competition is the so-called blocking ef-fect Kamin (1968) showed that conditioned responding toa conditioned stimulus (CS) was diminished (blocked) ifthis CS (hereafter identified as X) was presented duringtraining in compound with another CS (hereafter identi-fied as A) that had already been established as a good pre-

dictor of the occurrence of the unconditioned stimulus(US ie AndashUS pairings followed by AXndashUS pairings) Thecapacity to explain blocking and other stimulus competi-tion effects separated modern formulations of the mecha-nisms of Pavlovian learning (eg Rescorla amp Wagner)from previous formulations (eg Atkinson amp Estes 1963Bush amp Mosteller 1951 Konorski 1948 Pavlov 1927)

The better-known modern theories of associative learn-ing (eg Mackintosh 1975 Pearce 1987 Pearce amp Hall1980 Rescorla amp Wagner 1972 Wagner 1981 hereafteridentified as traditional associative theories) all assume thatblocking occurs because the initial learning that a CS (A)predicts the US prevents the formation of an association be-tween the added CS (X) and the US That is these theoriesassume that the blocking effect is a deficit in the acquisi-tion of an association between the added CS and the USMoreover these theories assume that following blockingtreatment only the actual pairings of the blocked CS withthe US can counteract the blocking deficit (in theoreticalterms increase the strength of the XndashUS association)Other stimulus competition effects (eg overshadowing[Pavlov 1927] and the relative stimulus validity effect[Wagner Logan Haberlandt amp Price 1968]) are explainedin a similar way by traditional associative theories Thatis they all assume that stimulus competition occurs dur-ing acquisition and that the competing CS impedes the de-velopment of an association between the added CS (X)and the US

Contrary to the assumptions of traditional associativetheories several recently developed theories suggest thatstimulus competition effects can be reversed by posttrain-ing extinction of the competing stimulus (ie conditionedresponding to the added CS [X] can be increased if thecompeting CS [A] is extinguished) There are two major

The experimental preparation and Experiments 1 and 2 were part ofa dissertation submitted by F A to the University of Deusto in partialfulfillment of the doctoral degree Support for this research was pro-vided by Grants PI96006 from Departamento de Educacioacuten Universi-dades e Investigacioacuten (Basque Government) and PB95-0440 from Di-reccioacuten General de Ensentildeanza Superior (Spanish Ministry of Education)and by NIH Grant 33881 F A was supported by Fellowship BFI97013from Programa de Formacioacuten de Investigadores del Departamento deEducacioacuten Universidades e Investigacioacuten (Basque Government) Wethank Mirko Gerolin Jennifer Kelschenabch Kerstin Kindling OscarLandeta Nuria Ortega Oskar Pinentildeo and Sonia Vegas for their assis-tance in data collection We also thank Ralph R Miller for enlighteningdiscussions and for his encouragement and insightful comments on anearly version of the manuscript We are especially grateful to Ana Martiacutenez-Pampliega for her help in recruiting participants Finally F A thanks hisdissertation committee members Imanol Amayra Victoria ChamizoAntonio Maldonado Ana Martiacutenez-Pampliega and Ricardo Pelloacuten fortheir insights and advice relating to this project F A and M E are nowat the State University of New York at Binghamton Correspondenc econcerning this article should be addressed to F Arcediano PsychologyDepartment Binghamton University Binghamton NY 13902-6000 (e-mail arcedianbinghamtonedu)

Reversal from blocking in humans as a result of posttraining extinction

of the blocking stimulus

FRANCISCO ARCEDIANO MARTHA ESCOBAR and HELENA MATUTEUniversidad de Deusto Bilbao Spain

In a blocking procedure conditioned stimulus (CS) A is paired with the unconditioned stimulus (US)in Phase 1 and a compound of CSs A and X is then paired with the US in Phase 2 The usual result ofsuch a treatment is that X elicits less conditioned responding than if the AndashUS pairings of Phase 1 hadnot occurred Obtaining blocking with human participants has proven difficult especially if a behav-ioral task is used or if the control group experiences reinforcement of a CS different from the blockingCS in Phase 1 In the present series in which human participants and a behavioral measure of learningwere used we provide evidence of blocking using the above described control condition Most impor-tant we demonstrate that extinction of the blocking CS (A) following blocking treatment reverses theblocking deficit (ie increases responding to X) These results are at odds with traditional associativetheories of learning but they support current associative theories that predict that posttraining ma-nipulations of the competing stimulus can result in a reversal of stimulus competition phenomena

REVERSAL FROM BLOCKING 355

families of models that predict this effect On the onehand there are models based on a response rule (eg Den-niston Savastano amp Miller 2001 Gallistel amp Gibbon2000 Miller amp Matzel 1988) and on the other hand thereare models based on new learning about absent stimuliduring posttraining extinction of CS (A) (eg Dickinsonamp Burke 1996 Markman 1989 Tassoni 1995 Van Hammeamp Wasserman 1994) Both families of theories predictthat if the blocking stimulus is extinguished followingblocking treatment conditioned responding to the blockedstimulus will be increased

Models based on a response rule differentiate betweenthe process of acquisition and the production of a responseFor example Miller and Matzel (1988) assume that CSndashUS associations are learned through a pure contiguitymechanism and that at (each) opportunity to respond (iepresentation of CS X) AndashUS and XndashUS associationscompete with each other to determine whether CS X willelicit a conditioned response In general terms these mod-els assume that conditioned responding to a CS dependson recurring comparisons between the strength of theCSndashUS association and the associative strength to the USof other stimuli (discrete or contextual) that were presentduring training with the CS For example in a blockingprocedure the previously trained stimulus (A) blocks con-ditioned responding to the added CS (X) because A has astronger association to the US than does X However if theassociation between A and the US is weakened (eg viaextinction of A) the conditioned response to X should beenhanced thereby revealing an otherwise latent associa-tion between X and the US

Models based on new learning about absent stimuli(eg Dickinson amp Burke 1996 Markman 1989 Tassoni1995 Van Hamme amp Wasserman 1994) were derived fromtraditional associative models (specifically Rescorla amp Wag-ner 1972 and Wagner 1981) and share with their prede-cessors the general assumption that competition betweenstimuli reflects a failure to acquire an association betweenthe added stimulus (X) and the US That is stimulus com-petition is regarded as an acquisition deficit These newmodels differ from the traditional models in that they positthat the associative strength of an absent stimulus may bemodified if a stimulus that was previously trained in com-pound with the absent stimulus is subsequently presentedon a learning trial Specifically the associative strength ofa CS will increase not only when it is paired with the USbut also when both the CS and the US are expected tooccur and neither of them is actually presented Accord-ing to these models following blocking treatment theblocked stimulus will have a weak association with theUS Subsequent extinction of the blocking CS should re-sult in an increase in the strength of the blocked CSndashUSassociation because both the blocked stimulus and the USare expected when the blocking CS is presented during theextinction trials (because both have previously been pairedwith the blocking CS) but the two of them are absent

In summary both the models based on a response ruleand the models based on the acquisition of new learning

about absent stimuli contrary to traditional associative the-ories predict that extinction of the blocking stimulus fol-lowing blocking treatment should produce an increment inconditioned responding to a previously blocked stimulusHowever there have been problems in experimentally con-firming this prediction which contrasts with the success-ful demonstrations of increases in conditioned respondingafter extinction of the competing stimulus in overshadowing(Kaufman amp Bolles 1981 Larkin Aitken amp Dickinson1998 Matzel Schachtman amp Miller 1985 Matzel Shus-ter amp Miller 1987 Wasserman amp Berglan 1998) and therelative stimulus validity effect (Cole Barnet amp Miller1995) Only recently has reversal from blocking been re-ported in an animal study (Blaisdell Gunther amp Miller1999) The primary purpose of the present paper was to as-sess the generality of this effect and specifically to deter-mine whether it could be obtained with human participants

The main difficulty in investigating reversal from block-ing with human participants is that the blocking effect it-self has proven to be quite elusive with humans (eg Daveyamp Singh 1988 Lovibond Siddle amp Bond 1988 PelloacutenGarciacutea amp Saacutenchez 1995) and the seemingly successfulblocking studies have generally lacked adequate controlgroups thereby being open to alternative interpretations(see Arcediano Matute amp Miller 1997 for a review) Forexample we previously developed a behavioral preparationthat proved effective in obtaining simple Pavlovian condi-tioning in humans (Arcediano Ortega amp Matute 1996)which seemingly yielded blocking (Arcediano et al 1997)but in subsequent unpublished research with that prepara-tion we had problems in obtaining blocking relative to acontrol condition in which participants were trained witha stimulus different from the blocking stimulus during thefirst phase of the blocking treatment (ie CndashUS trials fol-lowed by AXndashUS trials with X being the target CS) Arce-diano et al (1997) used a control condition in which theblocking CS was uncorrelated with the US during the firstphase of the blocking procedure The uncorrelated controlcondition has the advantage that subjects in both the con-trol and the experimental conditions are exposed to thesame number of presentations of the blocking CS theblocked CS and the US However some studies indicatethat a stimulus presented uncorrelated with the US mightdevelop inhibitory associations (eg Baker 1977 Durlach1986 Hearst amp Franklin 1977 Kaplan 1984 Rescorlaamp LoLordo 1965 Suiter amp LoLordo 1971 Weisman ampLitner 1969 Williams 1986 Williams Dyck amp Tait1986 Williams amp Overmier 1988) excitatory associa-tions (eg Pelloacuten et al 1995) or simultaneous inhibitoryand excitatory associations (eg Droungas amp Lolordo1994 Matzel Gladstein amp Miller 1988) to the US If inthe uncorrelated control condition the blocking CS ac-quires inhibitory properties during the first phase thistreatment could facilitate acquisition of excitatory prop-erties by the target CS X during Phase 2 (ie X could be-come superconditioned eg Rescorla 1971) whichcould explain the apparent blocking effect in the experi-mental condition

356 ARCEDIANO ESCOBAR AND MATUTE

Most other studies previously reporting blocking in hu-mans used either a control group in which the blocking CSwas presented alone (ie without US presentations) duringPhase 1 (eg Chapman 1991 Chapman amp Robbins 1990)or a control group in which neither the CS nor the US waspresented during Phase 1 (eg Pelloacuten et al 1995 Shanks1985) Both of these control groups raise problems for anambiguous demonstration of blocking In the former case(ie A-alone trials in Phase 1) the possibility exists thatduring Phase 2 the target CS becomes superconditioned inthe control group (if some inhibition developed to A be-cause on the basis of the instructions to the participantsthe US is expected but does not occur in the presence ofA) In the latter case (no CSs or USs in Phase 1) the weakresponding to the target CS observed in the blockinggroup could reflect habituation to the US (eg Pelloacutenet al 1995) in this group as compared with the controlgroup owing to its more numerous exposures to the US

In the present series of experiments we examined block-ing in human participants relative to a control group thatreceived CndashUS pairings in Phase 1 in order to minimizethe aforementioned possible confounds in the present re-search This treatment is generally considered a more ad-equate control condition for blocking than is no treatmentor uncorrelated treatment in Phase 1 Most important wetried to determine whether extinction of the blocking stim-ulus after completion of the blocking treatment could re-sult in an increment in conditioned responding to the pre-viously blocked stimulus Experiment 1 demonstratedbehavioral blocking in humans with our preparation withcontrol participants receiving CndashUS treatment in Phase 1Experiment 2 provided evidence of increases in condi-tioned responding to a previously blocked stimulus as aresult of posttraining extinction of its blocking stimulusFinally Experiment 3 demonstrated that reversal fromblocking is specific to extinction of the blocking stimulusas opposed to extinction of any other conditioned excitor

EXPERIMENT 1

Experiment 1 was performed to investigate whether it waspossible to obtain behavioral blocking in humans using acontrol condition in which a stimulus different from theblocking stimulus is reinforced in Phase 1 (see Table 1) Weconsider this task behavioral because although there wereverbal instructions explaining the task and the US wasgiven motivational value through instructions our depen-dent variable was suppression of an ongoing nonverbal be-havior (ie barpresses on a computerrsquos keyboard)

MethodParticipants

Twenty-three undergraduate students from Deusto University(Spain) volunteered for the study The participants were randomlyassigned to either group blocking (n = 11) or group control (n = 12)

ApparatusAll the participants performed the experiment simultaneously

using personal computers in a large room with many computers pres-

ent The participants were at least 15 m apart from each other and theparticipants at adjacent computers received different group andstimuli assignments Owing to the number of participants and thesize of the room three experimenters were present

The task was a computer video game and had many features incommon with the behavioral preparation described by Arcedianoet al (1996) Associative learning was evidenced in the suppressio nratio of ongoing behavior (playing of a video game during which thestimuli of a Pavlovian procedure were presented) Instead of the USrsquosbeing inherently fear inducing (as with a footshock US) the US wasgiven negative motivational value through instructions 1

A graphic representation of the task is displayed in Figure 1 Onthe computer screen the participants saw a jail room with two wallsan Exit door composed of vertical bars a refuge a circular figure (witha face and arms) which the participants could make move with thecursor (arrow) keys on the computerrsquos keyboard and a square-shape dresponse panel The direction of the figurersquos movement which wascontrolled by the arrow keys was rotated 45ordm from perpendicular tothe participant rsquos visual axis in order to permit the room to be shownin perspective for this reason a drawing showing the four cursorkeys and the direction in which each key made the figure move wereshown in the lower left corner of the screen at all moments The onlyexit from the room was a door initially closed composed of 15 ver-tical bars A message indicating whether the Exit door was closed oropened was displayed on the upper right corner of the screen Thegoal of the participants was to escape from the room through theExit door as many times as they could within the experimental ses-sion They gained one point each time they escaped from the roomand the total number of points they had gained was displayed at alltimes during the game on the upper left and lower right corners ofthe screen In order to escape from the room the participants had tocompletely open the Exit door which could be done only by makingthe figure press the response panel To press the response panel theparticipants had to move the f igure with the cursor keys until ittouched the response panel and then the participants had to press thespace bar on the keyboard to make the arms of the figure go downthereby pressing the response panel on the monitor screen Thismade the vertical bars that closed the door go up one at a time Thedoor was ldquoopenedrdquo when all 15 bars were lifted Then the figurecould go through the door and escape from the room Upon so doingthey earned one point and the figure was again returned to the jailroom

The Pavlovian CSs were colors displayed in two light panels thatwere placed on one of the walls of the jail room When only one CSwas presented only one of the panels (which was randomly selectedfor each trial) displayed this CS and the other light panel remainedwhite When a compound of two stimuli was presented each panel(randomly selected on each trial) displayed a different color All CSswere 6 sec in duration The US consisted of the floor of the roomflashing in black and white (except for the area corresponding to therefuge) cycling two times in a second The flashing continued for

Table 1 Design of Experiment 1

Group Phase 1 Phase 2 Test

Blocking AndashUS AXndashUS XDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

Control CndashUS AXndashUS XDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

NotemdashThe letters A C D E F and X represent different CSs US in-dicates the occurrence of the unconditioned stimulus and noUS indi-cates its absence The letters representing the filler stimuli are shown initalics See text for a description of the stimuli


Figure 1 Screenshots of the task during training Words on the screen werein Spanish for Experiments 1 and 2 and in English for Experiment 3 All pan-els show the exit door to the right the refuge to the left the control panel in themiddle of the room the light panels where the CSs were presented on the wallsthe figure that the participants moved with the cursor keys the score in theupper left and lower right corners a message indicating whether the exit doorwas opened or closed in the upper right corner and the directions of the cur-sor keys during the task in the lower left corner The top panel presents the fig-ure pressing the control panel to lift the bars and open the exit door The mid-dle panel presents the effects of US presentation while the figure is out of therefuge The bottom panel presents successful avoidance of the US


2 sec on each occasion If the flashing light occurred and the figurewas out of the refuge the figure was shown flashing in black andwhite simultaneously with the floor and all the vertical bars that hadbeen lifted prior to that moment came down one by one closing theExit door completely Thus the participants had to start lifting thevertical bars all over again when the flashing stopped The partici-pants could avoid this presumably aversive consequence by goinginto the refuge before the flashing light occurred that is they couldavoid the closing of the partially open door if they learned to antic-ipate when the flashing floor (surrogate US) would occur Thus weexpected that the participants would learn to stop pressing the spacebar and go to the refuge in the presence of an effective CS (ie onethat predicted the US) We also assumed that the participants wouldcontinue pressing the space bar in the presence of an ineffective CS(ie one that did not predict the US) Thereby the suppression of theparticipants rsquo pressing the space bar in order to go to the refuge wasviewed as an avoidance response which was incompatible with theongoing response of lifting bars by making the figure press the re-sponse panel

In our procedure the intertrial interval was variable because it de-pended in part on the participants rsquo behavior A trial began only im-mediately after the participant pressed the space bar continuouslyfor at least 1 sec Then the computer recorded barpress responsesduring the subsequent 6 sec with these 6 sec constituting the base-line preCS interval to be used in calculating suppression ratios Im-mediately after the preCS interval had elapsed and the number ofbarpresses during these 6 sec had been recorded the CSs were dis-played on the light panels and the number of barpresses during the6 sec of CS presentation was recorded The first second was in-cluded to increase the likelihood that the participants were pressingthe space bar consistently during the 6 sec previous to the CS pre-sentation A further restriction was imposed on initiating trials sothat if the participants had already opened 10 of the 15 bars consti-tuting the Exit door no CSs were displayed until the participant hadescaped from the room and started a new round with the Exit com-pletely closed The reason for this restriction was that if only 5 barsremained down the door could be opened in less than 12 sec (thesum of a preCS period and a CS period) and the participants couldhave stopped pressing the space bar (ie the response panel) duringCS presentation to escape from the room which would result in anapparent suppression of barpressing in the presence of the CS thatwas unrelated to the experimental design

We used as our dependent variable a suppression ratio which is aconventional measure in animal conditioned suppression studies(Annau amp Kamin 1961 Estes amp Skinner 1941) It was computedas a(a + b) where a was the number of barpresses during the 6 secof CS presentation and b was the number of barpresses recorded dur-ing the immediately prior 6-sec preCS interval A suppression ratioof zero indicates that response suppression to the CS was completeIf the CS presentation does not affect the ongoing behavior the num-ber of responses during the CS and during the period of time imme-diately preceding the CS will be similar and therefore the resultantsuppression ratio will approach 5

DesignTable 1 summarizes the design of Experiment 1 The participant s

were exposed to two phases of treatment During Phase 1 CS A pre-dicted the occurrence of the US in Group Blocking whereas CS Cpredicted the occurrence of the US in Group Control During Phase 2the AX compound predicted the occurrence of the US for bothgroups Finally CS X was presented alone as a test During Phases1 and 2 of treatment the participants in both groups were also pre-sented with the nonreinforced filler Stimuli D and EF The presen-tation of these filler stimuli in addition to the critical stimuli understudy is a common practice in human research because with humanparticipants some nonreinforced stimuli need to be included inorder to prevent indiscriminate suppression to all the stimuli pre-

sented on the screen The addition of these f iller stimuli trials forboth groups did not affect the relevant prediction of a between-groups difference indicative of blocking

ProcedureInstructions The instructions of the video game were displayed

in four successive screens An approximate Spanish-to-Engli shtranslation of these instructions reads as follows

Screen 1 In this experiment you will control a person that is locked inside aroom Inside that room you will see an Exit door composed of verti-cal bars a refuge and a response panel that you can use to lift the barsof the Exit door You will begin the game with 0 points Each timeyou get out of the room by going through the Exit door your score willincrease by one point Your goal is to obtain as many points as you can

Screen 2In order to go out through the Exit door you must raise all the bars inthe door To do this you have to move the person until it is in contactwith the response panel and press it with the figurersquos arms To makethe figure press the response panel with the arms you have to pressthe space bar on your computerrsquos keyboard To move the figure aroundthe room you have to use the four cursor (arrow) keys on the key-board The direction of motion for each key will always be shown onthe screen If you hold down the space bar the figurersquos arms will pressthe response panel constantly and the bars will go up faster than if youintermittently press and release the space bar You will not be able togo through the Exit door unless all the bars are up and the messageover the door ldquoExit door closedrdquo has changed to ldquoExit door openrdquo

Screen 3If you feel threatened at any moment you should go to the refugewhich is the only safe place in the room A message displayed on thescreen will inform you how many points you have Before you beginwith the experiment you will have one practice round so that you canbecome comfortable with using the keyboard to move the figurearound the room After you complete this practice round the experi-ment will begin

Screen 4If you have problems or questions please do not hesitate to ask oneof the experimenters Try to get as many points as you can Good luckPress the space bar when you are ready to start

Once all the participants had read the instructions on their screensone of the experimenters emphasized the most important instruc-tions aloud in order to make sure that they were understood At thesame time one of the experimenters performed the pretraining phaseon a large screen that could be seen by all the participants Next theparticipants performed the pretraining phase on their screens

Pretraining phase The participants were presented with ascreen displaying the room and the figure inside the refuge Theyhad to move the figure around the room with the arrow keys pressdown the response panel to lift the bars and open the Exit door andescape from the room During this phase the light panels were notpresent and no CSs or USs were displayed Once the participants es-caped from the room a message on the screen informed them thatthe experiment was about to begin To ensure that all the participant sbegan the treatment phases at the same time the participants couldnot advance to the next screen until the experimenters asked them totype a ldquopasswordrdquo During this waiting time all questions asked bythe participants were answered aloud by one of the experimenters so that all the participants received the same information The mainpurpose of the pretraining phase was to familiarize the participant swith the experimental environment and teach them the basic skillsrequired for a good performance in the video game

Pavlovian conditioning Immediately after completion of thepretraining phase the participants were presented with the blockingtreatment and testing The two light panels were always displayed aswhite except when CSs were presented on them CSs and USs weresuperimposed on the operant task The trial order was pseudoran -domized and was the same for all the participants The order of tri-


als was AndashUS AndashUS DndashnoUS AndashUS EFndashnoUS AndashUS AndashUSAndashUS AndashUS DndashnoUS AndashUS EFndashnoUS AndashUS AndashUS AXndashUSEFndashnoUS AXndashUS DndashnoUS and AXndashUS for Group Experimental The same sequence applied to Group Control except that the AndashUStrials were replaced by CndashUS trials CS duration and intertrial inter-vals were as described above CSs A and C were blue brown or pinkcounterbalanced (Three colors were counterbalanced because inthe subsequent experiments of the series counterbalancing of threeCSs was required) CSs D E and F were red light blue and purplerespectively CS X was green Testing consisted of one presentationof CS X The training and the testing phases occurred without in-terruption

We normally use a data selection criterion in order to ensure thatparticipants have understood the instructions and are paying atten-tion to the experiment According to this criterion the data from par-ticipants who do not discriminate between reinforced and nonrein-forced stimuli are eliminated from the analyses This criterion wasused in Experiments 2 and 3 However because in the present ex-periment we gave only two nonreinforced DndashnoUS filler trials (asagainst the 10 AndashUS or CndashUS presentations) it seemed inappropri -ate to take into account the data from the DndashnoUS trials Thus in thisexperiment we just made sure that a good suppression of ongoingbehavior was taking place when CSs A or C were presented at theend of Phase 1 (note that if the blocking CS did not produce strongconditioned suppression at the end of Phase 1 we would not expectthat this CS would later block conditioning to the target CS X)Thus the data of participants whose suppression ratio on the last re-inforced trial of Phase 1 (ie the last AndashUS trial for Group Block-ing or the last CndashUS trial for Group Control) was greater than 3were eliminated Data from 2 participants of Group Blocking wereeliminated following this criterion

Results and DiscussionFigure 2 shows the suppression ratio for the target CS

(X) recorded during testing As can be seen in this figure

when CS X was presented at test the participants in GroupBlocking suppressed their barpressing behavior less thandid the participants in Group Control [t(19) = 259 p lt 05]That is conditioned responding to CS X was blocked inGroup Blocking

An analysis of the number of responses recorded dur-ing the 6 sec prior to the presentation of the target CS (preCSscores) revealed no differences between Groups Blockingand Control [t(19) = 060 p gt 1] Thus the observed dif-ference between groups in suppression to CS X was dueto differences in suppression to CS X rather than to theirbaseline responding In addition there were no differencesin suppression ratios on the last trial of the AX compoundin Phase 2 [t(19) = 073 p gt 1] Thus Phase 1 treatment ap-pears to have been the major source of the observed dif-ference in conditioned responding to CS X at testing

As was previously stated we used a data selection cri-terion with human participants because it allowed us to elim-inate data from participants that failed to exhibit suppres-sion to a consistently reinforced stimulus for reasonsunrelated to the experimental training (eg they did notunderstand the instructions were color-blind or did nottake the experiment seriously) This criterion gave greatersensitivity to our measures although it did not changesignificantly the results Without the elimination criterionthe average suppression ratios were 14 and 33 for the con-trol and blocking groups respectively a difference thatstill reflects the blocking deficit [t(21) = 275 p lt 05]

The results of Experiment 1 replicated the behavioralblocking effect in humans previously reported by Arcedi-ano et al (1997) but with a different task and with a con-trol group trained during Phase 1 with pairings of a stimu-lus different from the blocking stimulus and the US (ieCndashUS) instead of presentations of the blocking CS (A)uncorrelated with the US The control group used in thisexperiment avoids many of the interpretative problems ofthe other control conditions mentioned in the introduction

EXPERIMENT 2

Both the models that focus on a response rule and themodels that focus on new learning about absent stimuli pre-dict that conditioned responding to a blocked stimulus canbe increased by posttraining extinction of its blockingstimulus Thus the prior difficulty in obtaining this effectis problematic for both families of models

MethodIn Experiment 2 two groups of participants received conventiona l

blocking treatment similar to that in Experiment 1 Then one ofthese groups received posttraining extinction of the blocking CS(Group Reversal) whereas the other group received nonreinforce dpresentations of a novel stimulus (Group Blocking see Table 2) Athird group of participants did not receive blocking treatment (GroupControl) Blocking would be evident if conditioned responding tothe target CS was greater in Group Control than in Group BlockingReversal from blocking would be evident if conditioned respondingto the target CS was greater in the group that received extinctiontreatment of the blocking CS after blocking treatment (ie Group

Figure 2 Experiment 1 Mean suppression ratios to the targetstimulus X recorded during testing for both groups Error barsrepresent 6 SE


Reversal) than in the blocking group that did not receive the extinc-tion treatment (ie Group Blocking)

In this experiment we introduced some modifications into thetask and the procedure relative to Experiment 1 in an attempt to in-crease its sensitivity These changes were introduced because previ-ous attempts to reverse blocking using the same preparation as thatdescribed in Experiment 1 had proven unreliable (we were able tofind reversal from blocking but we failed to replicate the result with-out these changes see Arcediano 1998) Our expectation was thatan increase in task sensitivity would yield consistent replicable re-sults which would speak to whether reversal of behavioral blockingcan be obtained with human participants There were three majorchanges made on the basis of several pilot studies (Arcediano 1998) First the number of filler trials was increased In Experi-ment 1 the participants were exposed to 13 reinforced trials (ie 10A[or C]ndashUS trials in Phase 1 and 3 AXndashUS trials in Phase 2) andonly 6 nonreinforced filler trials (ie DndashnoUS and EFndashnoUS [4 tri-als in Phase 1 and 2 trials in Phase 2]) Thus it is possible that therewas a great amount of generalized suppression toward all the pre-sented stimuli In order to reduce this possibility the participants inExperiments 2 were exposed during Phases 1 and 2 to 11 reinforcedtrials (8 trials in Phase 1 and 3 trials in Phase 2) and 14 nonrein-forced filler trials (12 trials in Phase 1 and 2 trials in Phase 2)

The second change consisted in ldquopunishingrdquo all movements of thefigure around the room that were not directly related to escapingfrom the room Thus unless the Exit door was completely open anymovement of the figure around the room would make the verticalbars go down slowly one at a time In previous experiments it wasnot unusual that perhaps by excessive prudence or ldquofearrdquo some par-ticipants went to the refuge when any stimulus was displayed al-though some stimuli (ie D and EF) consistently predicted the ab-sence of the US We expected that as a result of this ldquopunishment rdquothe participants would be more selective in deciding when to go tothe refuge enhancing the discrimination between reinforced andnonreinforced stimuli

The third change consisted in giving the participants more explicitinstructions The participants were instructed that (1) they would seeduring the task two light panels on the walls of the room (2) somecolored lights would be displayed in these light panels on some oc-casions (3) these colored lights might or might not provide infor-mation about the occurrence of a possible threat and (4) if this threatoccurred and the figure was out of the refuge all the vertical barsthat they had already lifted to open the Exit door would go down andthey would lose all the effort they made toward opening the door onthat round These last changes in the instructions were expected to

facilitate the participants rsquo discrimination of the stimuli Arcedianoet al (1996) reported that participants who received explicit in-structions about the presence of stimuli that might predict the oc-currence of the US during a video-game-like operant task showedfaster and better discrimination of the stimuli than did participant swho did not receive explicit instructions

ParticipantsSixty-four students from Deusto University (Spain) volunteere d

for the study The participants were randomly assigned to Group Con-trol (n = 20) Group Blocking (n = 19) or Group Reversal (n = 25)Variability in group size was due to the vicissitudes of scheduling

ApparatusThe computers were identical to those used in Experiment 1 ex-

cept that the computers were located in a small room with the con-sequence that a small rather than a large number of participant sperformed the experiment at the same time The experimental prepa-ration was identical to that described in Experiment 1 except for thethree changes described above Furthermore the two light panels onwhich the stimuli were displayed were smaller and located on dif-ferent walls of the room and at different heights with the purpose ofdiscouraging configural learning of compound stimuli In additionthe probability of any scheduled stimulusrsquos being displayed duringtreatment on the left (or right) light panel was 50 and the order ofthe location of each stimulus on each light panel was identical for allthe participants

ProcedureThe experimental task instructions and pretraining phase were

similar to those in Experiment 1 However we presented the addi-tional new instructions when the task was explained aloud to all theparticipants Once the experimenter explained the task the partici-pants were informed that they would see during the game two lightpanels in which sometimes color lights were going to be displayed Some of these colors indicated the occurrence close in time of anaversive event Moreover if this event occurred and the figure wasout of the refuge all the verticals bars lifted until that moment wouldgo down one bar at a time closing the Exit door completely The ex-perimenter emphasized that the refuge was the only safe place in theroom when this aversive event occurred and that if the participan tthought this event was going to occur the best decision was to movethe figure toward the refuge to be safe In addition the experimente rshowed on a computer screen (which could be seen by all the par-ticipants) that unless the Exit door was completely opened anymovement of the figure around the room would make the verticalbars go down The speed at which these bars went down while thefigure was moving around the room was half of the speed at whichthey were lifted when the figure pressed the response panel with itsarms

Phases 1 and 2 were identical to Experiment 1 with the exceptionof the number of trials In Phase 1 the participants in Groups Block-ing and Reversal were presented with eight AndashUS pairings eightDndashnoUS and four EFndashnoUS presentations The sequence of trialswas DndashnoUS AndashUS DndashnoUS AndashUS EFndashnoUS DndashnoUS AndashUSDndashnoUS EFndashnoUS DndashnoUS AndashUS DndashnoUS AndashUS EFndashnoUSDndashnoUS AndashUS EFndashnoUS AndashUS DndashnoUS and AndashUS The se-quence of trials was identical for Group Control except that theAndashUS trials were replaced by CndashUS trials In Phase 2 all the par-ticipants were presented with three AXndashUS one DndashnoUS and oneEFndashnoUS trials The sequence of trials was AXndashUS DndashnoUSAXndashUS EFndashnoUS and AXndashUS In Phase 3 the participants inGroup Reversal received five AndashnoUS presentations whereas theparticipants in Groups Blocking and Control received five BndashnoUSpresentations Testing occurred as described in Experiment 1 Stim-uli A B and C were blue brown and yellow counterbalance dwithin groups Stimuli D E and F were pink light blue and orangerespectively The target stimulus (X) was green


Group Phase 1 Phase 2 Phase 3 Test

Control CndashUS AXndashUS BndashnoUS XDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

Blocking AndashUS AXndashUS BndashnoUS XDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

Reversal AndashUS AXndashUS AndashnoUS XDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

NotemdashGroup Control did not receive blocking treatment Group Block-ing received blocking treatment without posttraining extinction of theblocking CS and Group Reversal received blocking treatment with post-training extinction of the blocking CS The letters A B C D E F andX represent different CSs US indicates the occurrence of the uncondi -tioned stimulus and noUS indicates its absence The letters represent-ing the filler stimuli are shown in italics See text for a description of thestimuli


In Experiment 2 because we presented during Phase 1 the samenumber of trials of a stimulus that predicted the occurrence of the US(AndashUS in the two blocking groups and CndashUS in the control group)and of a filler stimulus that did not predict the occurrence of the US(DndashnoUS) we were able to use the discrimination between thesestimuli (one consistently reinforced and one consistently nonrein-forced) as a participant selection criterion The data from those par-ticipants that had less than a 11 difference between the suppressionratio to the last filler trial of Phase 1 (eighth DndashnoUS trial) and thesuppression ratio to the last reinforced trial of this Phase (eighthAndashUS or CndashUS trial) were eliminated from the analysis We as-sumed that participants who did not selectively suppress their bar-pressing behavior to a stimulus that predicted the US relative to astimulus that did not predict the US after eight trials of training witheach stimulus probably did not understand the video game did notpay sufficient attention to the experimental preparation or had prob-lems in discriminating between colors A discrimination criterionappeared inappropriate in Experiment 1 because of the differentnumber of reinforced and nonreinforced Phase 1 trials The criterionused in Experiment 1 accepted data from all participants whose sup-pression ratios to the last reinforced trial of the first Phase (AndashUS inGroup Blocking or CndashUS in Group Control) was less than 3 how-ever this criterion did not allow us to determine whether the partic-ipants suppressed their behavior selectively to stimuli that predictedthe occurrence of the US or to all stimuli indiscriminately In con-trast the criterion of Experiment 2 allowed us to eliminate data fromparticipants who did not discriminate appropriately by the end ofPhase 1 With this criterion the data from 2 participants from GroupControl 1 from Group Blocking and 7 from Group Reversal werenot included in the analyses of Experiment 2

Results and DiscussionThe participants in Group Blocking suppressed their

barpressing responses during the presentation at test of the

target stimulus (X) less than did participants in GroupsControl and Reversal Therefore the results of this exper-iment replicated the behavioral blocking reported in Ex-periment 1 and showed that conditioned responding to apreviously blocked stimulus could be incremented whenthe blocking stimulus was extinguished following block-ing treatment Figure 3 shows the suppression ratio for thetarget CS (X) recorded during testing

A one-way analysis of variance (ANOVA) performed onthe suppression ratios recorded during the presentation attest of the target CS X yielded a main effect of group[F(251) = 447 MSe = 003 p lt 05] Planned compar-isons using the error term from this ANOVA revealed thatGroups Blocking and Control differed [F(151) = 782 p lt01] That is conditioned suppression to CS (X) wasblocked in Group Blocking Further planned comparisonsrevealed that Groups Blocking and Reversal differed[F(151) = 535 p lt 05] That is conditioned respondingto the target CS (X) which had been subjected to identi-cal blocking treatment in these two groups was increasedowing to posttraining extinction of the blocking CS (A)Moreover there was no difference between Groups Re-versal and Control [F(151) lt 1] That is there was no dif-ference between suppression to the target stimulus afterblocking treatment plus posttraining extinction of theblocking stimulus (Group Reversal) and suppression tothe target stimulus when it did not undergo blockingtreatment (Group Control) Indeed as Figure 3 shows themean suppression ratios to the target stimulus at test inboth groups were very similar

To test whether the differences among groups were dueexclusively to the association between the target CS andthe US and not to nonassociative factors such as differ-ential base rate behavior we performed a one-way ANOVAon the responses recorded during the 6 sec previous to thepresentation of the target stimulus at test this ANOVA re-vealed no effect of group [F(251) lt 1] Therefore the differ-ences between groups apparently were not due to differentresponse rates immediately prior to presentations of thetarget stimulus at test In addition a similar ANOVA re-vealed no differences in the suppression ratios on the lasttrial of the compound AX in Phase 2 [F(251) lt 1] Thusthe differential treatments during Phase 1 appear to be theonly plausible source of the observed difference in re-sponding to CS X at test between Groups Blocking andControl Similarly extinction of the blocking stimulusduring Phase 3 seemed to be the only plausible source ofthe observed difference between Groups Blocking andReversal

The data from all the participants (ie without apply-ing the data selection criterion) were analyzed to determinewhether the results reported above were an artifact of dataselection Without the use of a data selection criterion themean suppression ratios at test were 32 45 and 33 forthe Control Blocking and Reversal groups respectivelyResponding in Group Control differed significantly fromthat in Group Blocking [F(161) = 559 MSe = 003 p lt05] and responding in Group Blocking differed signifi-

Figure 3 Experiment 2 Mean suppression ratios to the targetstimulus X recorded during testing for each of the three groupsError bars represent 6 SE


cantly from that in Group Reversal [F(161) = 529 MSe =003 p lt 05] In addition we analyzed the number ofpoints earned by participants during the experimentwhich can be considered an index of stimulus discrimina-tion and performance during the experiment The meannumber of points by participants that were and were noteliminated from the analyses were 380 and 651 respec-tively This difference was statistically significant [t(62) =447 p lt 001]

EXPERIMENT 3

Experiment 2 demonstrated that posttraining extinctionof the blocking stimulus results in an increase in condi-tioned responding to the previously blocked stimulusHowever one could argue that Groups Control and Block-ing were exposed during Phase 3 to a neutral novel stim-ulus (ie a stimulus without a previous history of rein-forcement) whereas Group Reversal received extinctiontreatment with a previously reinforced stimulus Thus itis possible to argue that responding to the blocked stimu-lus (X) would be increased if a previously reinforced stim-ulus was presented without reinforcement in Phase 3 butnot if a novel neutral stimulus was presented duringPhase 3 That is extinction of any excitor (not necessarilythe blocking stimulus) might result in increased respond-ing to X In Experiment 3 we assessed the stimulus speci-ficity of the reversal effect observed in Experiment 2 andwe did so by extinguishing during Phase 3 a previouslyreinforced blocking stimulus in all the groups

MethodIn Experiment 3 two stimuli A and B were established as block-

ing stimuli for two targets X and Y After completion of blockingtreatment either A or B was extinguished and responding to X was

assessed If the reversal effect observed in Experiment 2 is a spe-cific result of extinguishing Xrsquos blocking stimulus responding to Xshould be increased only in the group in which A was extinguishedwhereas blocking should be evident in the group in which B was ex-tinguished If in contrast the reversal effect reported in Experi-ment 2 was the result of extinguishing any excitor after completionof the blocking treatment blocking of X should be equally attenu-ated in both the group receiving extinction of A and the group re-ceiving extinction of B (see Table 3)

Participants and ApparatusFifty-nine students from State University of New York Bing-

hamton volunteered for the study The participants were randomlyassigned to Group Control (n = 24) Group Blocking (n = 17) orGroup Reversal (n = 18) The computers were similar to those usedin Experiments 1 and 2 and they were located in small rooms withone or two computers per room

ProcedureThe experimental task and the instructions were similar to those

in Experiment 2 However we introduced several changes All textwas in English all instructions were presented on successive screenson the computer (ie no oral instructions were given) the pretrain-ing phase was eliminated and the preCS interval was reduced from6 to 3 sec Because the number of trials was increased from 31 trialsin Experiment 2 to 49 trials in Experiment 3 we decided to reducethe preCS interval in order to avoid significantly increasing the du-ration of the experiment In addition CSs were presented only if theparticipants were constantly pressing the space bar during the fullpreCS interval Consequently there were necessarily no difference sin responding between groups (or individual participants) during thepreCS interval and all the participants were pressing the space barwhen the CSs were presented Suppression ratio was computed asa(a + 2b) where a was the number of barpresses during the 6 secof CSpresentation and b was the number of barpresses recorded duringthe immediately prior 3-sec preCS interval

In Phase 1 the sequence of eight AndashUS eight BndashUS eightDndashnoUS and eight EFndashnoUS trials was DndashnoUS AndashUS EFndashnoUSBndashUS EFndashnoUS DndashnoUS EFndashnoUS AndashUS EFndashnoUS BndashUSDndashnoUS AndashUS EFndashnoUS BndashUS DndashnoUS DndashnoUS AUSAndashUS EFndashnoUS BndashUS DndashnoUS AndashUS EFndashnoUS BndashUSBndashUS DndashnoUS AndashUS BndashUS DndashnoUS EFndashnoUS BndashUS andAndashUS for Groups Blocking and Reversal The sequence of trials wasidentical for Group Control except that the AndashUS trials were re-placed by CndashUS trials In Phase 2 the sequence of three AXndashUSthree BYndashUS two DndashnoUS and two EFndashnoUS trials was AXndashUSBYndashUS DndashnoUS EFndashnoUS AXndashUS EFndashnoUS BYndashUS DndashnoUSBYndashUS and AXndashUS for all groups In Phase 3 the participants inGroup Reversal received five AndashnoUS presentations whereas theparticipants in Groups Blocking and Control received five B-noUSpresentations Testing on X occurred as described in Experiment 1Stimuli A B and C were blue brown and yellow counterbalance dwithin groups Stimuli D E and F were pink light blue and grayrespectively Stimuli X and Y were green and orange counterbal -anced within groups

As in Experiment 2 we used the discrimination between consis-tently nonreinforced stimuli and consistently reinforced stimuli atthe end of Phase 1 as a criterion for retaining participants Becausethere were now two consistently nonreinforced stimuli (D and thecompound EF) and two consistently reinforced stimuli (A or C and B)with an identical number of trials during Phase 1 our criterion wasadapted accordingly That is data from those participants that hadless than a +1 difference between the average suppression ratio tothe two last filler trials of Phase 1 ([eighth DndashnoUS trial + eighthEFndashnoUS trial] 2) and the average suppression ratio to the last re-inforced trials of this phase ([eighth AndashUS (or CndashUS) trial + eighth



Control CndashUS AXndashUS BndashnoUS XBndashUS BY-USDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

Blocking AndashUS AXndashUS BndashnoUS XBndashUS BYndashUSDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

Reversal AndashUS AXndashUS AndashnoUS XBndashUS BYndashUSDndashnoUS DndashnoUSEFndashnoUS EFndashnoUS

NotemdashGroup Control did not receive blocking treatment Group Block-ing received blocking treatment without posttraining extinction of theblocking CS and Group Reversal received blocking treatment with post-training extinction of the blocking CS The letters A B C D E F Xand Y represent different CSs US indicates the occurrence of the un-conditioned stimulus and noUS indicates absence of the US The lettersrepresenting the filler stimuli are shown in italics See text for a de-scription of the stimuli


BndashUS trial] 2) were eliminated from the analysis The data from 12participants from Group Control 5 participants from Group Block-ing and 6 participants from Group Reversal were excluded from theanalyses of Experiment 3

Results and DiscussionFigure 4 depicts the suppression ratio for the target CS

X recorded during testing As can be seen Experiment 3replicated the results reported in Experiments 1 (block-ing) and 2 (reversal from blocking) A one-way ANOVAperformed on the suppression ratios recorded during thetest presentation of the target stimulus (X) yielded a maineffect of group [F(233) = 815 MSe = 002 p lt 01]Planned comparisons using the error term from thisANOVA revealed that the blocking treatment was effec-tive as evidenced by the difference in responding to X be-tween Groups Control and Blocking [F(133) = 1375 p lt001] In addition there was a difference in suppression toCS X between Groups Blocking and Reversal [F(133) =1047 p lt 01] That is conditioned suppression to thetarget stimulus (X) which had received a blocking treat-ment in both groups was increased when the blockingstimulus (A) but not when another excitor (B) was extin-guished Finally there was no difference between GroupsReversal and Control [F(133) lt 1] That is there was nosignificant difference between the conditioned responseelicited by CS X after blocking treatment plus posttrain-ing extinction of the blocking stimulus (Group Reversal)and CS X when it had not been subject to blocking treat-ment (Group Control) Indeed as Figure 4 shows the

mean suppression ratios to the target stimulus at test inboth groups were very similar

Statistical analysis on the number of responses recordedduring the 3 sec prior to the presentation of X were notperformed because they were necessarily identical be-tween groups owing to the experimental preparation (ieCSs were presented only if the participants were continu-ously pressing the space bar during preCS interval) In ad-dition there were no differences in suppression on the lasttrial of the compound AX in Phase 2 [F(233) lt 1] Thusthe blocking effect (ie differences between Groups Block-ing and Control) could plausibly be attributed only to thedifferential treatment in Phase 1 and the increment on theconditioned response to the previously blocked stimulusonce the blocking stimulus was extinguished (ie differ-ences between Groups Reversal and Blocking) could plau-sibly be attributed only to extinction of the blocking stim-ulus in Phase 3

As in the previous experiments the data from all the par-ticipants (ie without applying the data selection criterion)were also analyzed The mean suppression ratios withoutapplication of a criterion of elimination were 22 34 and23 for the Control Blocking and Reversal groups re-spectively Responding in Group Control differed signifi-cantly from that in Group Blocking [F(156) = 611 MSe =002 p lt 05] and responding in Group Blocking differedsignificantly from that in Group Reversal [F(156) = 463MSe = 002 p lt 05] In addition as in Experiment 2 weanalyzed the average number of points obtained by thoseparticipants whose data were and were not eliminatedfrom the analysis The average number of points obtainedwas 343 and 608 respectively The comparison was sta-tistically significant [t(57) = 427 p lt 001]

In conclusion Experiment 3 replicated the behavioralblocking observed in Experiments 1 and 2 as well as thereversal from blocking observed in Experiment 2 More-over Experiment 3 showed that increased responding tothe previously blocked stimulus was specific to posttrain-ing extinction of the blocking stimulus this reversal ofblocking was not observed when an excitor different fromthe blocking stimulus was subject to extinction treatment

GENERAL DISCUSSION

The present series of experiments was intended to in-vestigate whether conditioned responding to a previouslyblocked stimulus can be increased after posttraining ex-tinction of the blocking CS when human participants wereused in a novel behavioral task Traditional associativetheories (eg Mackintosh 1975 Pearce 1987 Pearce ampHall 1980 Rescorla amp Wagner 1972 Wagner 1981) arguethat stimulus competition is the result of a deficit in ac-quiring an association between the target CS and the USMoreover these theories posit that this deficit can be re-versed only if the target CS is paired with the US in the ab-sence of the competing CS However in recent years newassociative theories of acquired behavior have been de-



veloped to explain some stimulus competition effects thatcannot be explained by traditional theories One of themajor predictions of these new theories is that conditionedresponding to a stimulus subject to competition (eg over-shadowing [Pavlov 1927] relative stimulus validity [Wag-ner et al 1968] or blocking [Kamin 1968]) may be in-creased even if it does not receive further pairings with theUS Two families of models make this prediction On theone hand there are models that emphasize a response ruleand on the other hand there are models that emphasize ac-quisition processes including learning about absent stimuli

Models based on a response rule (eg Denniston et al2001 Gallistel amp Gibbon 2000 Miller amp Matzel 1988)assume that the relationships between the CSs and the USare learned through a contiguity mechanism In generalaccording to these models blocking can be reversed be-cause there is an association between X and the US that re-mains latent and does not elicit responding because theAndashUS association is stronger than the XndashUS association(owing to the Phase 1 training) Thus if the associationbetween A and the US is weakened (as a result of post-training extinction of A) conditioned responding to Xwill be observed Models based on new learning about ab-sent stimuli (eg Dickinson amp Burke 1996 Markman1989 Tassoni 1995 Van Hamme amp Wasserman 1994)assume as do traditional associative models (eg Mack-intosh 1975 Pearce 1987 Pearce amp Hall 1980 Rescorlaamp Wagner 1972 Wagner 1981) that competition be-tween stimuli reflects a failure to acquire an associationbetween the target stimulus and the US However they as-sume that the strength of a CSndashUS association will in-crease not only when the CS is followed by the occurrenceof the US but also when both the CS and the US are ex-pected but not presented on a trial Thus during extinctionof A the strength of the XndashUS association is assumed toincrease and responding to X should be observed

These two groups of theories have been successful inexplaining increases in conditioned responding to stimuliunder competition after posttraining extinction of thecompeting stimulus in paradigms such as overshadowing(Kaufman amp Bolles 1981 Larkin et al 1998 Matzel et al1985 Matzel et al 1987 Wasserman amp Berglan 1998)and the relative stimulus validity effect (Cole et al 1995)However their prediction of reversal of conditioned re-sponding to a previously blocked stimulus has proven dif-ficult to confirm Only recently did Blaisdell et al (1999)report this effect under select conditions using rats as sub-jects In order to provide a human analogue of this elusiveeffect we developed a new behavioral procedure to mea-sure learning of associations between stimuli using con-ditioned suppression of ongoing behavior as dependentvariable Our experimental preparation shared many basicideas with the behavioral preparation developed by Arce-diano et al (1996) but attempted to provide greater ex-perimental sensitivity which seemed necessary to detectrelatively small changes in responding that may testify tothe reversal of stimulus competition Indeed the blocking

effect itself has been quite elusive in human research (seethe introduction) and it seemed quite clear that only bydeveloping a more sensitive preparation could we investi-gate both blocking and reversal from blocking in humansIn Experiment 1 we replicated the blocking first reportedwith a nonverbal behavioral measure in humans by Arce-diano et al (1997) but here we used an improved controlcondition for blocking (ie CndashUS followed by AXndashUS) InExperiment 2 some aspects of the behavioral preparationwere modified to provide greater task sensitivity in orderto observe the impact on blocking of posttraining manipu-lations of the blocking stimulus Experiment 2 replicatedthe blocking effect obtained in Experiment 1 and reversed(ie increased) conditioned responding to a previouslyblocked stimulus as a result of posttraining extinction ofits blocking stimulus Finally Experiment 3 demonstratedthat the reversal effect observed in Experiment 2 was spe-cific to extinction of its blocking stimulus extinction ofanother previously reinforced stimulus had no effect onresponding to the blocked target stimulus

The present experimental series demonstrates that in-creases in responding to a blocked stimulus after post-training extinction of the blocking stimulus can be gener-alized to human participants To our knowledge this is thefirst replication of the reversal from blocking owing to ex-tinction of the blocking stimulus that was reported byBlaisdell et al (1999) in rats It is unclear for us why rever-sal of blocking in both humans and animals has been soelusive After repeated failures to obtain increased respond-ing to the blocked stimulus by posttraining extinction ofthe blocking stimulus (eg Miller Schachtman amp Matzel1987) Blaisdell et al reported this effect when a single-phase blocking procedure (AndashUS and AXndashUS trials in-terspersed) was followed by 200 extinction trials of theblocking CS (Experiments 2 and 4) and when a conven-tional two-phase blocking procedure was followed by 800extinction trials of the blocking CS (Experiments 3 and 4)After many modifications of the experimental preparationand several parametric studies (Arcediano 1998) wewere able to provide a human analogue to the results re-ported by Blaisdell et al Interestingly Blaisdell et al (Ex-periments 3 and 4) needed 800 extinction trials of the block-ing stimulus to obtain increased responding to the blockedstimulus whereas we (with a similar two-phase blockingprocedure) required only 5 extinction trials of the block-ing stimulus (Experiments 2 and 3) As Blaisdell et al sug-gested a possible account of why they needed such a largenumber of extinction trials would be based on the conceptof biological significance (ie behavioral control egDenniston Miller amp Matute 1996 Miller amp Matute 1996)The US used by Blaisdell et al was a footshock a US thathas proven to unconditionally control behavior in rat ex-periments (eg Denniston et al 1996) In contrast theUS in the present experiments was not a biologically sig-nificant event the participants lost and gained only pointsevents that are significant enough to produce respondingbut sufficiently nonaversive to be ethically acceptable in


human research We can speculate that more extinctiontrials are required in a preparation that uses a biologicallysignificant US than in a preparation in which the US is oflow biological significance because reversal of stimuluscompetition is observed only if both responding and bio-logical significance are extinguished to the competingstimulus2

The present data support the view that posttraining ma-nipulations of the competing stimulus (in the present casethe blocking stimulus) can result in reversal of stimuluscompetition phenomena Whether such reversal reflectsexpression of a previously latent association (as is sup-ported by models that emphasize a response rule) or anew-learning effect (as is suggested by theories that allownew learning about absent stimuli) is still unclear Furtherexperiments will have to be performed in order to deter-mine which one of these families of models is more accu-rate and to illuminate the true nature of stimulus compe-tition

REFERENCES

Annau Z amp Kamin L J (1961) The conditioned emotional responseas a function of intensity of the US Journal of Comparative amp Phys-iological Psychology 54 428-432

Arcediano F (1998) Recuperacioacuten de la respuesta a un estiacutemulo blo-queado extinguiendo el estiacutemulo bloqueador [Recovery of respondingto a blocked stimulus following extinction of the blocking stimulus]Unpublished doctoral dissertation Bilbao Spain Universidad deDeusto

Arcediano F Matute H amp Miller R R (1997) Blocking ofPavlovian conditioning in humans Learning amp Motivation 28 188-199

Arcediano F Ortega N amp Matute H (1996) A behavioura lpreparation for the study of human Pavlovian conditioning QuarterlyJournal of Experimental Psychology 49B 270-283

Atkinson R C amp Estes W K (1963) Stimulus sampling theory InR D Luce R B Bush amp E Galanter (Eds) Handbook of mathe-matical psychology (Vol 3 pp 121-268) New York Wiley

Baker A G (1977) Conditioned inhibition arising from a between-sessions negative correlation Journal of Experimental PsychologyAnimal Behavior Processes 3 144-155

Blaisdell A P Gunther L M amp Miller R R (1999) Recoveryfrom blocking achieved by extinguishing the blocking CS AnimalLearning amp Behavior 27 63-76

Bush R R amp Mosteller F (1951) A mathematical model for sim-ple learning Psychological Review 58 313-323

Chapman G B (1991) Trial order affects cue interaction in contin-gency judgment Journal of Experimental Psychology LearningMemory amp Cognition 17 837-854

Chapman G B amp Robbins S J (1990) Cue interaction in humancontingency judgment Memory amp Cognition 18 537-545

Cole R P Barnet R C amp Miller R R (1995) Effect of relativestimulus validity Learning or performance deficit Journal of Ex-perimental Psychology Animal Behavior Processes 21 293-303

Davey G C L amp Singh J (1988) The Kamin ldquoblockingrdquo effect andelectrodermal conditioning in humans Journal of Psychophysiolog y2 17-25

Denniston J C Miller R R amp Matute H (1996) Biological sig-nificance as a determinant of cue competition Psychological Science7 325-331

Denniston J C Savastano H I amp Miller R R (2001) The ex-tended comparator hypothesis Learning by contiguity responding byrelative strength In R R Mowrer amp S B Klein (Eds) Handbook ofcontemporary learning (pp 65-117) Mahwah NJ Erlbaum

Dickinson A amp Burke J (1996) Within-compound associations me-

diate the retrospective revaluation of causality judgments QuarterlyJournal of Experimental Psychology 49B 60-80

Droungas A amp Lolordo V M (1994) Evidence for simultaneousexcitatory and inhibitory associations in the explicitly unpaired pro-cedure Learning amp Motivation 25 1-25

Durlach P J (1986) Explicitly unpaired procedure as a response elim-ination technique in autoshaping Journal of Experimental Psychol-ogy Animal Behavior Processes 12 172-185

Estes W K amp Skinner B F (1941) Some quantitative properties ofanxiety Journal of Experimental Psychology 29 390-400

Gallistel C R amp Gibbon J (2000) Time rate and conditioningPsychological Review 107 289-344

Hearst E amp Franklin S R (1977) Positive and negative relationsbetween a signal and food Approachndashwithdrawal behavior to the sig-nal Journal of Experimental Psychology Animal Behavior Processes3 37-52

Kamin L J (1968) ldquoAttention-likerdquo processes in classical conditioningIn M R Jones (Ed) Miami Symposium on the Prediction of Behav-ior Aversive stimulation (pp 9-31) Miami University of MiamiPress

Kaplan P (1984) The importance of relative temporal parameters intrace autoshaping From excitation to inhibition Journal of Experi-mental Psychology Animal Behavior Processes 10 113-126

Kaufman M A amp Bolles R C (1981) A nonassociative aspect ofovershadowing Bulletin of the Psychonomic Society 18 318-320

Konorski J (1948) Conditioned reflexes and neuron organizationCambridge Cambridge University Press

Larkin M J W Aitken M R F amp Dickinson A (1998) Retro-spective revaluation of causal judgments under positive and negativecontingencies Journal of Experimental Psychology Learning Mem-ory amp Cognition 24 1331-1352

Lovibond P F Siddle D A T amp Bond N (1988) Insensitivity tostimulus validity in human Pavlovian conditioning Quarterly Jour-nal of Experimental Psychology 40B 377-410

Mackintosh N J (1975) A theory of attention Variations in the as-sociability of stimuli with reinforcement Psychological Review 82276-298

Markman A B (1989) LMS rules and the inverse base-rate effectComment on Gluck and Bower (1988) Journal of Experimental Psy-chology General 118 417-421

Matzel L D Gladstein L amp Miller R R (1988) Conditionedexcitation and conditioned inhibition are not mutually exclusiveLearning amp Motivation 19 99-121

Matzel L D Schachtman T R amp Miller R R (1985) Recov-ery of an overshadowed association achieved by extinction of the over-shadowing stimulus Learning amp Motivation 16 398-412

Matzel L D Shuster K amp Miller R R (1987) Covariation inconditioned response strength between stimuli trained in compound Animal Learning amp Behavior 15 439-447

Miller R R amp Matute H (1996) Biological significance in for-ward and backward blocking Resolution of a discrepancy betweenanimal conditioning and human causal judgment Journal of Experi-mental Psychology General 125 370-386

Miller R R amp Matzel L D (1988) The comparator hypothesis Aresponse rule for the expression of associations In G H Bower (Ed)The psychology of learning and motivation (Vol 22 pp 51-92) SanDiego Academic Press

Miller R R Schachtman T R amp Matzel L D (1987) [Failureto attenuate blocking by posttraining extinction of a blocking stimu-lus] Unpublished raw data

Pavlov I (1927) Conditioned reflexes (GV Anrep Trans) LondonOxford University Press

Pearce J M (1987) A model of stimulus generalization for Pavlovianconditioning Psychological Review 94 61-73

Pearce J M amp Hall G (1980) A model for Pavlovian learningVariations in the effectiveness of conditioned but not unconditionedstimuli Psychological Review 87 532-552

Pelloacuten R Garciacutea J M amp Saacutenchez P (1995) Blocking and elec-trodermal conditioning in humans Psicoloacutegica 16 321-329

Rescorla R A (1971) Variations in the effectiveness of reinforcement


and nonreinforcement following prior inhibitory conditioning Learn-ing amp Motivation 2 113-123

Rescorla R A amp LoLordo V M (1965) Inhibition of avoidancebehavior Journal of Comparative amp Physiological Psychology 59406-412

Rescorla R A amp Wagner A R (1972) A theory of Pavlovian con-ditioning Variations in the effectiveness of reinforcement and non-reinforcement In A H Black amp W F Prokasy (Eds) Classical con-ditioning II Current research and theory (pp 64-99) New YorkAppleton-Century-Crofts

Shanks D R (1985) Forward and backward blocking in human con-tingency judgment Quarterly Journal of Experimental Psychology37B 1-21

Suiter R D amp LoLordo V M (1971) Blocking of inhibitory Pavlov-ian conditioning in the conditional emotional response procedureJournal of Comparative amp Physiological Psychology 76 137-144

Tassoni C J (1995) The least mean squares network with informationcoding A model of cue learning Journal of Experimental Psychol-ogy Learning Memory amp Cognition 21 193-204

Van Hamme L J amp Wasserman E A (1994) Cue competition incausality judgments The role of nonpresentation of compound stim-ulus elements Learning amp Motivation 25 127-151

Wagner A R (1981) SOP A model of automatic memory processingin animal behavior In E Spear amp R R Miller (Eds) Information pro-cessing in animals Memory mechanisms (pp 5-47) Hillsdale NJErlbaum

Wagner A R Logan F A Haberlandt K amp Price T (1968)Stimulus selection and a ldquomodified continuity theoryrdquo Journal of Ex-perimental Psychology 76 171-180

Wasserman E A amp Berglan L R (1998) Backward blocking andrecovery from overshadowing in human causal judgment The role ofwithin-compound associations Quarterly Journal of ExperimentalPsychology 51B 121-138

Weisman R G amp Litner J S (1969) The course of Pavlovian exci-tation and inhibition of fear in rats Journal of Comparative amp Physi-ological Psychology 69 667-672

Williams D A (1986) On extinction of inhibition Do explicitly un-

paired conditioned inhibitors extinguish American Journal of Psy-chology 99 515-525

Williams D A Dyck D G amp Tait R W (1986) Excitatory back-ward conditioning in conditioned punishment and conditioned sup-pression in rats American Journal of Psychology 99 367-384

Williams D A amp Overmier J B (1988) Some types of conditionedinhibitors carry collateral excitatory associations Learning amp Moti-vation 19 345-368

NOTES

1 A demonstration version of this program can be downloaded fromhttpsiriodeustoesmatutesoftwarehtml

2 The concept of biological significance could also account for thegreater difficulty in demonstrating reversal of blocking in animals rela-tive to reversal of overshadowing and the relative stimulus validity effectIn the latter paradigms the target stimulus (ie the stimulus under com-petition) is presented with the competing stimulus during the firstphase of conditioning and paired with a US that is normally food or foot-shock (ie stimuli of high biological significance) therefore the targetstimulus could acquire greater biological significance (ie behavioralcontrol) than if the target stimulus is paired with the US only in a secondphase of conditioning when the competing stimulus has already ac-quired strong behavioral control (eg blocking) In the case of blockingthe competing (blocking) stimulus should be maximally effective in pre-venting the target (blocked) stimulus from acquiring biological signifi-cance In supportive of this assumption Blaisdell et al (1999) requiredmore posttraining extinction trials to observe reversal of blocking fol-lowing a two-phase blocking procedure than following a single-phase blocking procedure (800 extinction trials and 200 extinction trialsrespectively) Apparently if the competing (ie blocking) stimulus hasnot already acquired complete behavioral control when the target (ieblocked) stimulus is first paired with the US fewer extinction trials withthe blocking CS are needed to reverse the blocking deficit

(Manuscript received December 15 2000 revision accepted for publication June 4 2001)


families of models that predict this effect On the onehand there are models based on a response rule (eg Den-niston Savastano amp Miller 2001 Gallistel amp Gibbon2000 Miller amp Matzel 1988) and on the other hand thereare models based on new learning about absent stimuliduring posttraining extinction of CS (A) (eg Dickinsonamp Burke 1996 Markman 1989 Tassoni 1995 Van Hammeamp Wasserman 1994) Both families of theories predictthat if the blocking stimulus is extinguished followingblocking treatment conditioned responding to the blockedstimulus will be increased

Models based on a response rule differentiate betweenthe process of acquisition and the production of a responseFor example Miller and Matzel (1988) assume that CSndashUS associations are learned through a pure contiguitymechanism and that at (each) opportunity to respond (iepresentation of CS X) AndashUS and XndashUS associationscompete with each other to determine whether CS X willelicit a conditioned response In general terms these mod-els assume that conditioned responding to a CS dependson recurring comparisons between the strength of theCSndashUS association and the associative strength to the USof other stimuli (discrete or contextual) that were presentduring training with the CS For example in a blockingprocedure the previously trained stimulus (A) blocks con-ditioned responding to the added CS (X) because A has astronger association to the US than does X However if theassociation between A and the US is weakened (eg viaextinction of A) the conditioned response to X should beenhanced thereby revealing an otherwise latent associa-tion between X and the US

Models based on new learning about absent stimuli(eg Dickinson amp Burke 1996 Markman 1989 Tassoni1995 Van Hamme amp Wasserman 1994) were derived fromtraditional associative models (specifically Rescorla amp Wag-ner 1972 and Wagner 1981) and share with their prede-cessors the general assumption that competition betweenstimuli reflects a failure to acquire an association betweenthe added stimulus (X) and the US That is stimulus com-petition is regarded as an acquisition deficit These newmodels differ from the traditional models in that they positthat the associative strength of an absent stimulus may bemodified if a stimulus that was previously trained in com-pound with the absent stimulus is subsequently presentedon a learning trial Specifically the associative strength ofa CS will increase not only when it is paired with the USbut also when both the CS and the US are expected tooccur and neither of them is actually presented Accord-ing to these models following blocking treatment theblocked stimulus will have a weak association with theUS Subsequent extinction of the blocking CS should re-sult in an increase in the strength of the blocked CSndashUSassociation because both the blocked stimulus and the USare expected when the blocking CS is presented during theextinction trials (because both have previously been pairedwith the blocking CS) but the two of them are absent

In summary both the models based on a response ruleand the models based on the acquisition of new learning

about absent stimuli contrary to traditional associative the-ories predict that extinction of the blocking stimulus fol-lowing blocking treatment should produce an increment inconditioned responding to a previously blocked stimulusHowever there have been problems in experimentally con-firming this prediction which contrasts with the success-ful demonstrations of increases in conditioned respondingafter extinction of the competing stimulus in overshadowing(Kaufman amp Bolles 1981 Larkin Aitken amp Dickinson1998 Matzel Schachtman amp Miller 1985 Matzel Shus-ter amp Miller 1987 Wasserman amp Berglan 1998) and therelative stimulus validity effect (Cole Barnet amp Miller1995) Only recently has reversal from blocking been re-ported in an animal study (Blaisdell Gunther amp Miller1999) The primary purpose of the present paper was to as-sess the generality of this effect and specifically to deter-mine whether it could be obtained with human participants

The main difficulty in investigating reversal from block-ing with human participants is that the blocking effect it-self has proven to be quite elusive with humans (eg Daveyamp Singh 1988 Lovibond Siddle amp Bond 1988 PelloacutenGarciacutea amp Saacutenchez 1995) and the seemingly successfulblocking studies have generally lacked adequate controlgroups thereby being open to alternative interpretations(see Arcediano Matute amp Miller 1997 for a review) Forexample we previously developed a behavioral preparationthat proved effective in obtaining simple Pavlovian condi-tioning in humans (Arcediano Ortega amp Matute 1996)which seemingly yielded blocking (Arcediano et al 1997)but in subsequent unpublished research with that prepara-tion we had problems in obtaining blocking relative to acontrol condition in which participants were trained witha stimulus different from the blocking stimulus during thefirst phase of the blocking treatment (ie CndashUS trials fol-lowed by AXndashUS trials with X being the target CS) Arce-diano et al (1997) used a control condition in which theblocking CS was uncorrelated with the US during the firstphase of the blocking procedure The uncorrelated controlcondition has the advantage that subjects in both the con-trol and the experimental conditions are exposed to thesame number of presentations of the blocking CS theblocked CS and the US However some studies indicatethat a stimulus presented uncorrelated with the US mightdevelop inhibitory associations (eg Baker 1977 Durlach1986 Hearst amp Franklin 1977 Kaplan 1984 Rescorlaamp LoLordo 1965 Suiter amp LoLordo 1971 Weisman ampLitner 1969 Williams 1986 Williams Dyck amp Tait1986 Williams amp Overmier 1988) excitatory associa-tions (eg Pelloacuten et al 1995) or simultaneous inhibitoryand excitatory associations (eg Droungas amp Lolordo1994 Matzel Gladstein amp Miller 1988) to the US If inthe uncorrelated control condition the blocking CS ac-quires inhibitory properties during the first phase thistreatment could facilitate acquisition of excitatory prop-erties by the target CS X during Phase 2 (ie X could be-come superconditioned eg Rescorla 1971) whichcould explain the apparent blocking effect in the experi-mental condition




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Reversal from blocking in humans as a result of posttraining...

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