EVIDENCE FOR RESPONSE MEMBERSHIP IN STIMULUS CLASSES BY PIGEONS

EVIDENCE FOR RESPONSE MEMBERSHIP IN STIMULUS CLASSES BY PIGEONS

PETER J. URCUIOLI1, B. MAX JONES

2, AND KAREN M. LIONELLO-DENOLF3

1PURDUE UNIVERSITY2CURTIN UNIVERSITY

3UNIVERSITY OF MASSACHUSETTS EUNICE KENNEDY SHRIVER CENTER

Response membership in pigeons’ stimulus-class formation was evaluated using associative symmetry andclass expansion tests. In Experiment 1, pigeons learned hue–hue (AA) and form–form (BB) successivematching plus a modified hue–form (AB) task in which reinforcement was contingent upon a left versusright side-key response after the positive AB sequences.On subsequent BA (symmetry) probe trials, pigeonsresponded more often to the comparisons on the reverse of the positive than negative AB sequences and,more importantly, preferentially pecked the side key consistent with symmetry after the reversed positivesequences. In Experiment 2, the original three baseline tasks were supplemented by dot–white (CC)successive matching in which reinforcement was contingent upon a left versus right side-key response afterthe positive CC sequences. Class expansion was then tested by presenting nonreinforced CA and CBsuccessivematching probes. Comparison response rates weremostly nondifferential onCAprobes but wereuniformly higher on CB probes that consisted of the C samples and B comparisons from the same,hypothesized class. Together, these results provide evidence that responses can become members ofstimulus classes, as predicted by Urcuioli’s (2008) theory of pigeons’ stimulus-class formation and Sidman’s(2000) theory of equivalence.Key words: stimulus classes, responsemembership, associative symmetry, successivematching, conditional

discrimination, simple discrimination, pigeons, key peck

In his theoretical analysis of equivalenceclasses, Sidman (1994, 2000) proposed thatresponses and reinforcers can become classmembers if the reinforcement contingenciesduring training do not create a conflict betweenestablishing the n-term analytical units ofbehavior (Sidman, 1986) and class inclusion.In other words, as long as the baselinecontingencies support discrimination learning,responses and reinforcers can enter intoequivalence relations with the stimuli with whichthey have been associated. Sidman (1994, 2000)claims that equivalence relations consist of allpairs of events—conditional stimuli, discrimina-tive stimuli, the experimenter-defined re-sponses, and the experimenter-definedreinforcers—specified by the reinforcementcontingencies.

Consider, for example, conditional discrimi-nation training consisting of two conditional (a.k.a. sample) stimuli (A1 and A2), two discrimi-native (a.k.a. comparison) stimuli (B1 and B2),two defined responses (Resp1 and Resp2), andtwo reinforcing outcomes (O1 andO2). Assumethat O1 is presented only when the subjectmakes Resp1 to the B1 comparison stimulus inthe context of the A1 sample, and that O2 ispresented only when the subject makes Resp2 tothe B2 comparison in the context of the A2sample. According to Sidman (2000), thesecontingencies could generate two 4-memberequivalence classes— [A1, B1, Resp1, O1] and[A2, B2, Resp2, O2] —and, with them, a varietyof new or emergent relations. For example,subjects could preferentially select the A1comparison over the A2 comparison if nowpresented with B1 as the sample and, conversely,select the A2 comparison over the A1 compari-son if presented with B2 as the sample (associa-tive symmetry). Furthermore, presentation ofone of the reinforcing outcomes (O1 or O2) oremission of one of the responses (Resp1 orResp2) might now occasion selection of thesample or comparison stimulus associated withthat reinforcer or response in training.The predicted inclusion of reinforcers in

equivalence classes was confirmed by Dube,McIlvane, Mackay, and Stoddard (1987,

This research was supported by NICHD Grant R01HD061322. Karen Lionello-DeNolf and Max Jones acknowl-edge support for research activity and manuscript prepara-tion from the Commonwealth Medicine Division, Universityof Massachusetts Medical School. The authors thank GregErhardt, Lisa Macklin, Blake Polak, and Melissa Swisher fortheir assistance in conducting this research.Correspondence should be addressed to Peter J. Urcuioli,

Department of Psychological Sciences, 703 Third Street,West Lafayette, IN 47907-2081 (e-mail: [email protected]).doi: 10.1002/jeab.17

JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR 2013, 99, 129–149 NUMBER 2 (MARCH)

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Experiment 1). They trained 2 intellectuallydisabled adults on symbolic and identity match-ing-to-sample (MTS) tasks in which differentfoods (O1 and O2) served as reinforcers forcorrectly selecting different comparisons (B1and B2) following different sample stimuli (A1and A2). After baseline training to high levels ofaccuracy, Dube et al. conducted nonreinforcedprobe trials in which the samples were replacedby their associated foods, and found thatboth participants always selected the compari-son stimulus that, in theory, belonged tothe same class as the O1 or O2 reinforcer(see Dube & McIlvane, 1995; Hall, Mitchell,Graham,&Lavis, 2003; and Joseph,Overmier, &Thompson, 1997 for similar findings).

Demonstrating emergent relations betweenthe samples and comparisons of different MTStasks that involve the same set of outcomesprovides additional, indirect evidence for rein-forcer class membership. For example, Maki,Overmier, Delos, and Gutmann (1995, Experi-ment 3) reported that young children trainedon AB and CD MTS, each involving blue versusred poker chips (later exchangeable for food vs.small toys) as reinforcers for correct choice,subsequently matched each D comparison tothe A sample associated with same coloredpoker chip (i.e., AD matching). Presumably,stimulus classes containing these matchingstimuli developed via the common reinforcersthey shared during training (see also Dube,McIlvane, Maguire, Mackay, & Stoddard, 1989;Schenk, 1994). Indeed, Maki et al. showed thatthe emergent AD effect did not occur in theabsence of differential outcomes training.

Data obtained from nonhuman animals alsoindicate that stimuli associated with the samereinforcing outcome belong to the same stimu-lus class (Urcuioli, 2005; 2013). For instance,sample stimuli that share a commondifferential-outcome association during baseline trainingare interchangeable with one another in newcontexts (e.g., Astley & Wasserman, 2001; Ed-wards, Jagielo, Zentall, & Hogan, 1982; Honig,Matheson, & Dodd, 1984; Meehan, 1999; seealso Urcuioli & Zentall, 1992), and the same istrue for comparison stimuli as well (Urcuioli &DeMarse, 1997).

In contrast to the reinforcer–membershipfindings, compelling evidence for responsemembership in stimulus classes has beenrelatively sparse. This is due partly to methodo-logical difficulties in obtaining directly relevant

data (e.g., how can Resp1 and Resp2 be“presented” as sample stimuli in testing?) andto the interpretive ambiguities of other findings.An example of the latter is the emergentdifferential sample responding effect reportedby Manabe, Kawashima, and Staddon (1995,Experiment 3). They initially trained budger-igars on hue–form MTS in which presentationof the form comparisons was contingent uponhigh- versus low-frequency vocal calls (Resp1 vs.Resp2) to the hue samples. Later, identity MTStrials in which the form stimuli appeared as bothsamples and comparisons were added to thehue–form trials in each session. On the form–form trials, either a high or a low vocal call toeach form sample produced the comparisons.Despite these nondifferential sample–responsecontingencies, the budgerigars began to vocal-ize differentially to the form samples as theirform–form MTS accuracy increased. Specifical-ly, they made a high-frequency call (Resp1) tothe form sample that occasioned the sameform–comparison choice as the hue sample towhich a high-frequency call was required, andthey made a low-frequency call (Resp2) to theform sample that occasioned the same form–comparison choice as the hue sample to which alow-frequency call was required.

Manabe et al. (1995) interpreted their resultsas evidence that the explicitly trained [vocal call! form comparison] relations on the hue–formtrials were symmetrical. In other words, thattraining purportedly yielded two stimulus classes,each containing a specific hue, call, and form.Consequently, each form sample eventuallyoccasioned the high- or low-frequency call in itsrespectiveclass(seeSidman,1994).SaundersandWilliams (1998), however, argued that adventi-tiousreinforcementofhigh- versus low-frequencycalling on the added form–formMTS trials couldequally well account for the findings. Moreover,Urcuioli and Vasconcelos (2008b) adaptedManabe et al.’s procedure for use with pigeons(cf. Urcuioli, Pierce, Lionello-DeNolf, Friedrich,Fetterman, & Green, 2002) and found thatdifferential responding to the added samplesemerged only if those samples occasioned thesame comparison selections as the originallytrained samples. Nominal identity between theadded samples and the comparisons from initialtraining was insufficient to yield the emergenteffect, contrary to a symmetry account.

Urcuioli, Lionello-DeNolf, Michalek, andVasconcelos (2006) sought direct evidence for

130 PETER J. URCUIOLI et al.

response membership in pigeons using a trans-fer-of-control assessment following trainingon many-to-one MTS, a conditional discrimina-tion known to reliably produce acquired sampleequivalence(e.g.,Lowe,Horne,&Hughes, 2005;Urcuioli, Zentall, & DeMarse, 1995; Wasserman,DeVolder, & Coppage, 1992). During many-to-one training, two patterns of center-key pecking(viz., rapid vs. slow pecking generated by fixed-ratio vs. differential-reinforcement-of-low-rates-of-responding schedules, respectively) served as“samples” that occasioned the same reinforcedcomparison selections as twoother, visual samplestimuli. Pigeons then learned tomatcheither thevisual or the response-pattern samples to newcomparison stimuli (reassignment training),and this was followed by a transfer test whichassessed their ability to match the remainingsamples to the new comparisons. If acquiredequivalence between the response and visualsamples had been established in many-to-onetraining (Urcuioli, DeMarse, & Zentall, 1994),then the new comparison choices learned to oneset of samples during reassignment trainingshould be immediately occasioned by the otherset in testing. Three separate experiments(Urcuioli et al., 2006, Experiments 1, 2, and 4),however, failed to confirm this acquired equiva-lence prediction (although see Urcuioli &Vasconcelos, 2008a for another interpretationof the negative findings).To date, Shimizu (2006) has reported the

most convincing demonstration of responsemembership in stimulus classes. Normal adultslearned two 3-alternative MTS tasks (AB andCD) in which reinforcement was contingent notonly on correct comparison selections but alsoon the correct execution of one of three 4-partcomputer-mouse movements (e.g., left, right,up, down). Thus, baseline training was actuallyABR and CDR, where R refers to the differentcomparison–response movements. Subsequenttesting revealed a large number of emergentrelations (e.g., AC, BC, AD, and BD matching)consistent with the idea that the commonresponses across baseline tasks brought theirrespective samples and comparisons togetherinto the same classes. In addition, on some testtrials, different mouse-movement sequenceswere “presented” as samples. These trials beganwith the appearance of a solid white square andsubjects were asked to make one of thepreviously learned mouse movements; thecorrect movement was then followed by the

comparisons. Shimizu found that on these RAand RC test trials, subjects also made class-consistent comparison selections.Lionello-DeNolf and Braga-Kenyon (in press)

have also reported evidence for responsemembership in adult humans. Subjects weretrained to make two different response patterns(pressing a computer touch screen 10 times inrapid succession or two times spaced severalseconds apart) to red and green stimuli. Next,they were trained on form–hue MTS in whichthe required response to each stimulus was asingle touch. Finally, using a procedure similarto Urcuioli et al. (2006), the different responsepatterns served as samples in MTS and thecomparisons were either the hue or form stimuli(in separate test sessions). Two participantsmade class-consistent comparison selections ontheir first exposure to the tests, and a 3rd did soafter repeated tests and additional training.Thepositive results reported by Shimizu (2006)

and by Lionello-DeNolf and Braga-Kenyon (inpress) are theoretically important especially inview of the previous negative or ambiguousfindings from the nonhuman animal literature.Encouraged by their results, the present studysought to demonstrate response membership instimulus classes by pigeons, albeit with a differentapproach to the definition of responses. Giventhe likely difficulties of establishing differenttemporal patterns of comparison pecking andarranging valid emergent-relations tests (see, forexample, Lionello & Urcuioli, 1998), we definedthe responses as moving toward and pecking keysin different locations; namely, pecking a left keyversus pecking a right key. These spatial responseswere incorporated into successive MTS (e.g.,Wasserman, 1976, cf. Konorski, 1959), a condi-tional discrimination in which samples andcomparisons are presented sequentially on asingle manipulandum and comparison respond-ing is reinforced only when the comparisonmatches (physically or symbolically) the preced-ing sample. In our procedure, reinforcementon the positive sequences in symbolic (AB)successive matching was contingent upon differ-ent side-key responses following the comparisonstimuli on those trials. In other words, a simplediscrimination between the comparison stimuli(e.g., peck left after B1 and peck right after B2)was added to a conditional discriminationbetween those stimuli (i.e., peck the comparisonson A1B1 and A2B2 trials but not on A1B2 andA2B1 trials).

RESPONSE MEMBERSHIP IN STIMULUS CLASSES 131

Given that pigeons exhibit a variety ofemergent relations like associative symmetry(Frank & Wasserman, 2005; Urcuioli, 2008,Experiment 3) after successive matchingtraining (see also Strasser, Ehrlinger, &Bingman, 2004; Sweeney & Urcuioli, 2010;Urcuioli, 2011; Urcuioli & Swisher, 2012), Exper-iment 1 asked whether combining this type ofconditional discrimination training with simple(left- vs. right-response) discrimination trainingwould provide evidence for response member-ship in stimulus classes. Specifically, would BARsymmetry, where R refers to different side-keyresponses, emerge from ABR training?

Experiment 1

The rationale for, and the design of, Experi-ment 1 were also based on Urcuioli’s (2008)theory of stimulus-class formation in pigeons.The important theoretical assumptions fromwhich predictions regarding response member-ship (see below) were derived are as follows.First, stimulus-class formation is facilitated bycontinual juxtaposition of nonreinforcementfor certain sample–comparison combinationswith reinforcement for other sample–compari-son combinations. SuccessiveMTS (cf. Nelson &Wasserman, 1978; Wasserman, 1976), in whichboth samples and comparisons are presentedindividually, is ideal in this regard because halfof all sample–comparison combinations end innonreinforcement independently of the level ofconditional discrimination accuracy. Second,the functional stimuli in successivematching arecompounds consisting of the nominal matchingstimuli plus their ordinal positions within a trial.Thus, a red sample stimulus is functionally red-in-the-first-ordinal position (viz., R1), whereas ared comparison stimulus is functionally red-in-

the-second-ordinal position (viz., R2). Third,class members are the elements of the rein-forced sample–comparison combinations. Forexample, reinforced responding to a trianglecomparison following a red sample will yield a[red sample, triangle comparison] class. Fourth,elements common to more than one stimulusclass produce class union (e.g., Mackay, Wilkin-son, Farrell, & Serna, 2011; Maki et al., 1995; seealso Sidman, 1994). For example, if traininginvolves multiple successive matching tasks—forexample, AB and BB matching—in which onereinforced AB combination consists of a redsample and a triangle comparison (R1 and T2)and one reinforced BB combination consists ofa triangle sample and a triangle comparison (T1andT2), the commonT2 element shouldmergethe [R1, T2] and [T1, T2] classes into one [R1,T1, T2] class.

For this experiment, pigeons were concur-rently trained on three tasks: hue–form (AB),hue–hue (AA) and form–form (BB) successivematching using red and greenhues, and triangleand horizontal-lines forms (see Table 1). AA andBB were identity tasks: Reinforcement occurredonly aftermatching sample–comparison sequen-ces—namely, red–red (R1–R2), green–green(G1–G2), triangle–triangle (T1–T2), and hori-zontal–horizontal (H1–H2). In the symbolic(AB) task, reinforcement also occurred onlyafter certain sample–comparison sequences—for example, red–triangle (R1–T2) and green–horizontal (G1–H2)—and only if pigeons madean experimenter-designated correct side-keyresponse (viz., pecking a lit left key or peckinga lit right key) after comparison offset. Anincorrect side-key response on these otherwise“positive” trials ended these trials withoutreinforcement. The remaining symbolicsequences—for example, red–horizontal (R1–H2)

Table 1

Baseline Successive Matching Contingencies in Experiment 1

Hue–Form Symbolic Hue–Hue Identity Form–Form Identity

R ! T - FI 5 s – Leftþ R ! R - FI 5 sþ T ! T - FI 5 sþR ! H - EXT R ! G - EXT T ! H - EXTG ! T - EXT G ! R - EXT H ! T - EXTG ! H - FI 5 s – Rightþ G ! G - FI 5 sþ H ! H - FI 5 sþNote. R ¼ red, G ¼ green, T ¼ triangle, H ¼ horizontal, FI ¼ fixed interval schedule, EXT ¼ nonreinforced, þ ¼

reinforced. The first and second center-key stimuli in a trial sequence (sample and comparison, respectively) are shown to theleft and to the right of the arrows, respectively. Positive (i.e., potentially reinforced) hue–form matching trials ended withfood only if pigeons made the designated (left or right) side-key choice response after the comparison stimulus; otherwise,the trial ended without food. Counterbalancing of the hue–form contingencies has been omitted.


and green–triangle (G1–T2) —were defined as“negative”: They involved no lighting of the sidekeys and no reinforcer delivery, ending automati-cally with comparison offset after a fixed time.Figure 1 depicts the six stimulus classes

hypothesized to result from baseline training(cf. Urcuioli, 2008). The bottom two rows showthe classes arising from hue identity (AA) andform identity (BB) successive matching. Notethat the identity classes contain the matchingsample and comparison of each positive (re-inforced) sequence (e.g., R1 and R2, and G1and G2). The top row shows the correspondingclasses arising from hue–form (AB) symbolicmatching. These include not only the sampleand comparison stimulus of each reinforcedsequence (for example, R1 and T2) but also thecorrect side-key responses for those sequences(for example, the left response or Left3). The“3” denotes the third ordinal position becausethe reinforced side-key response on a positivesymbolic matching trial follows the sample(ordinal position 1) and comparison (ordinalposition 2).The bolded italics are used to highlight

elements common to more than one class—forexample, R1 (red sample) in both the symbolicand thehue identity classes. If commonelementscause their respective classes to merge, as

Urcuioli (2008) hypothesizes, then baselinetraining should yield the two 5-member stimulusclasses shown in Figure 2—namely, [R1, R2, T1,T2, Left3] and [G1, G2, H1, H2, Right3]. Theelements of these classes predict associa-tive symmetry (Frank & Wasserman, 2005;Urcuioli, 2008) in pigeons’ performances onnovel BA test trials. Specifically, their compari-son–response rates should be higher on T1–R2and H1–G2 test trials (viz., the reverse of thepositive baseline trials) than on T1–G2 andH1–-R2 test trials (viz., the reverse of the negativebaseline trials). More importantly, when given aleft versus right choice on the positive BA testtrials, pigeons should make more left responses(Left3) than right responses on the novel T1–R2test sequence andmore right responses (Right3)than left responses on the novel H1–G2 testsequence. Such side-key preferences on the testsequences would constitute evidence for re-sponse membership in stimulus classes bypigeons.

MethodSubjects. Six White Carneau retired

breeders obtained from the Palmetto PigeonPlant (Sumter, SC) began the experiment. Onepigeon’s participation was discontinued forhealth reasons and another’s because it didnot achieve the required baseline matchingaccuracies after 85 training sessions; their dataare not reported. Of the remaining 4 pigeons, 1(CHC1) was experimentally naïve and 3 (CHC3,CHC4, and CHC6) had previously participatedin an unrelated experiment and in a successivematching experiment with differential-reinforcement-of-other-behavior contingencies(Urcuioli, 2010). All were maintained at 80% oftheir free-feeding body weights by restrictingfeeding to the experimental sessions and

Fig. 1. The six stimulus classes hypothesized to resultfrom hue–form symbolic, hue identity, and form identitysuccessive matching training in Experiment 1. Bolded italicsdenote elements that appear in more than one stimulusclass.

Fig. 2. The two 5-member stimulus classes hypothesizedto result from the merging of individual classes containingcommon elements (cf. Figure 1) in Experiment 1.


providing a reduced amount of Purina Pro-Grains in the home cages on the days they werenot run. Water and grit were freely available intheir stainless steel, wire-mesh home cages. Allwere housed in a colony room that was on a 14h-10h light-dark cycle (lights on at 07:00).

Apparatus. Pigeons were run in BRS/LVE(Laurel, MD) pigeon chambers (Model PIP-016three-key panel inside a Model SEC-002 enclo-sure). The three response keys were 2.5 cm indiameter, 5.7 cm apart center to center, andaligned horizontally in a row 7.5 cm from thetop of the panel.Mounted behind each key was aBRS/LVE Model IC-901-IDD stimulus projectorthat was equipped with films and filters fordisplaying red (R), green (G), and white (W)homogeneous fields, and three white horizontallines (H), a solid white inverted triangle (T),and three small white circular dots (D) oriented45o counter-clockwise from vertical, all on blackbackgrounds (BRS/LVE Pattern No. 692). Therear-mounted food hopper could be accessedvia a 5.8-cm-square opening located 13 cmbelow the center key. When the hopper wasraised, it was illuminated by a small miniaturebulb (ESB-28) in the surrounding metal hous-ing. A GE #1829 bulb located 7.6 cm above thecenter key provided general chamber illumina-tion. The opening in the metal shield surround-ing the bulb directed its light toward the ceiling.Ventilation andmasking noise was provided by aconstantly running blower fan attached to thechambers. All experimental events were con-trolled by customized programs written in GW-Basic Version 3.20 and running on an IBM-compatible computer.

ProcedurePreliminary training. All pigeons were ini-

tially trained to peck a key displaying thestimuli that would later appear in the succes-sive matching tasks. In the first and secondsessions, 30 presentations each of the triangleand horizontal lines, and red and green,respectively, appeared in randomized orderon the center key, and a single key peck to thelit key was followed immediately by food. Inthe third and fourth sessions, there were 10center-key presentations of triangle and hori-zontal, and red and green, respectively, plus20 presentations each of white on the left andright side keys, again in random order andwith single key pecks to the lit keys reinforced.Successive stimulus presentations were sepa-

rated by a 15-s intertrial interval (ITI) with thehouse light remaining on throughout eachsession.

Next, pecking the center-key stimuli wasreinforced on fixed-interval (FI) schedules.The first seven FI sessions contained 30randomized presentations each of red andgreen, and the following seven sessions con-tained 30 randomized presentations each of thetriangle and horizontal lines. Within a seven-session block, the FI value was increased from 2to 5 s over the initial four sessions. For the lastthree sessions, completion of a FI 5-s require-ment ended with food on a random 50% ofoccasions. Successive stimulus presentations(trials) within a session were again separatedby a 15-s ITI, the first 14 s of which were spent indarkness. The house light came on for the last1 s of the ITI and remained on until the end of atrial. The duration of food access per deliverywas constant within a session but variedindividually from 1.8–6.0 s across sessions tomaintain a pigeon’s 80% body weight.

Successive matching acquisition. Table 1 sum-marizes the reinforcement contingencies forthe baseline successive matching tasks. Allpigeons were concurrently trained on hue–form symbolic, hue–hue identity, and form–form identity matching. The hue–hue andform–form identity tasks involved typical succes-sive matching contingencies (Nelson &Wasserman, 1978; Wasserman, 1976). Here, a5-s sample stimulus presented on the center keywas followed, after a 1-s dark interval, by a 5-scomparison stimulus presented on the samekey. The trial ended in reinforcement if thecomparison physically matched the precedingsample (positive trial), but without reinforce-ment if the comparison did not match thepreceding sample (negative trial). More specifi-cally, the first key peck to the sample on a trialinitiated a fixed-interval (FI) 5-s schedule thatended with the sample turned off, a 1-s darkinterval and the onset of a comparison stimulus.On positive trials, the first comparison key peckafter a 5-s interval timed from the first peckturned off the comparison and produced food.On negative trials, the comparison stimulus andthe house light went off after 5 s independent ofany key pecking.

The sequence of events on negative hue–formsymbolic matching trials (viz., R1–H2 and G1–T2) was the same as the sequence describedabove for negative identity matching trials.


However, on positive hue–form trials (viz., R1–T2 and G1–H2), comparison offset was accom-panied by the simultaneous illumination ofwhite on the left and right side keys. A singlepeck to either side key within 3 s turned off bothside-key stimuli and produced either food if theside-key response was designated “correct” or ashort blackout if the response was designated“incorrect”. For 3 pigeons (CHC1, CHC4, andCHC6), a left response was correct after the R1–T2 sequence and a right response was correctafter the G1–H2 sequence. For the remainingpigeon (CHC3), a right response was correctafter the R1–T2 sequence and a left responsewas correct after the G1–H2 sequence. For allpigeons, incorrect side-key responses turned offthe house light for a period equal to thereinforcement duration for that session, as didthe absence of any side-key peck within 3 s. Dueto experimenter error, the left- and right-response contingencies were also in effect onthe positive identity trials for the first 10 sessionsfor 2 pigeons (CHC4 and CHC6), but wereremoved thereafter.Each training session consisted of an equal

number (32) of hue–form, hue–hue and form–formmatching trials presented in random orderexcept that no sample–comparison sequencecould occur onmore than two consecutive trials.Successive trials were separated by a 15-s ITI, thefirst 14 s of which were spent in darkness. Thehouse light came on for the last 1 s of the ITIand remained on until the end of the food-hopper cycle on reinforced trials or comparisonoffset on nonreinforced trials. Reinforcementduration (1.8–6.0 s) was again adjusted asneeded on a session-by-session basis for eachpigeon in order to maintain its 80% body weightas closely as possible.Conditional discrimination learning was as-

sessed by calculating a discrimination ratio (DR)in which the total number of comparison peckson the positive trials of each baseline task wasdivided by the total number of comparisonpecks on both positive and negative trials of thattask. Only pecks occurring within the first 5 s ofcomparison–stimulus onset entered into thesecomputations. Simple discrimination learningon the hue–form trials was assessed by calculat-ing the percentage of all side-key responsesdesignated as “correct”. Baseline training foreach pigeon continued until it achieved a DR of0.80 or higher on each successive matching task,and at least 13 correct side-key responses on the

16 positive hue–form trials (81.3% correct), forfive of six consecutive sessions (“criterion”).This was followed by a minimum of 10 over-training sessions, the last six of which had tomeet the aforementioned performance levels.Symmetry testing. Following overtraining, eight

test sessions, run in four 2-session blocksseparated by at least five baseline sessions atcriterion, were conducted to determine wheth-er the hue–form baseline relations weresymmetrical. Each test session contained 96baseline trials divided equally among the threebaseline tasks (hue–form, hue–hue and form–form) plus eight nonreinforced probe trials inwhich the sample was either the triangle orhorizontal lines and the comparison was eitherred or green. The four possible form–hueprobes (T1–R2, T1–G2, H1–R2, and H1–G2)were each presented twice in each test session.The first probe trial did not occur until at leastone of each possible baseline trial was pre-sented, and successive probes were separated byat least six baseline trials. Probe trials that werethe reverse of the negative baseline trials (viz.,T1–G2 and H1–R2) ended automatically (com-parison stimulus and the house light off) 5 safter comparison onset. For probe trials thatwere the reverse of the positive baseline trials(viz., T1–R2 and H1–G2), comparison offsetafter 5 s was accompanied by the simultaneousillumination of white on the left and right sidekeys. A single peck to either side key turnedboth off without food delivery, although thehouse light remained on for a period equal tothe session’s reinforcement duration. All otherprocedural details were identical to thosepreviously described. These probe trials per-mitted two assessments of associative symmetry:1) a DR based on the number of comparisonpecks on “positive” probe trials (viz., on thereverse of the positive baseline trials) divided bythe number of comparison pecks on both“positive” and “negative” probe trials (viz., onthe reverse of the positive and negative baselinetrials), and 2) the percentage of side-keyresponses to the same (left or right) side keyas that designated correct on the positive hue–form baseline trials.Because all 4 pigeons continued to respond at

appreciable rates to the comparisons on thenonreinforced probe trials after eight testsessions, eight additional test sessions were runafter baseline recovery to criterion performancelevels. These latter test sessions were also run in


four blocks of two sessions with a minimum offive baseline sessions at criterion separatingeach block.

For all statistical analyses reported in thispaper, Type I error rate was set at 0.05 using thetabled F values reported by Rodger (1975) forcontrolling error rates on a per decision basis.

ResultsSuccessive matching acquisition. Figure 3

plots discrimination ratios (DRs) for each pigeonon each baseline task (solid symbols) for the first40 training sessions (in blocks of two sessions) andthe percentages of correct choice on the symbolic(hue–form) task (open triangles). Discriminationratios (DRs) reached criterion levels most rapidlyon hue identity matching for 3 of the 4 pigeons(CHC1, CHC3, and CHC6), but most slowly forthe remaining pigeon (CHC4). Nevertheless, allpigeons were performing at criterion levels (DRat or above .80) on all tasks within 40 trainingsessions. Choice accuracy on the hue–form(symbolic) trials was around chance (50%correct) at the beginning of training, but quicklyrose to its criterion level (81.2% or above) within

8-10 sessions, stabilizing at nearly 100%by the endof training.

The average hue–identity DR over the last fiveovertraining sessions preceding testing was 0.94(range: 0.91–0.97). The corresponding DRs forform identity and hue–formmatching were 0.94(range: 0.91–0.95) and 0.93 (range: 0.86–0.98),respectively. The average percentage of correctleft versus right responses on the hue–form(symbolic) task over these five sessions was98.5% (range: 96.3–100%).

Symmetry testing. The panels shown in thetop two rows of Figure 4 plot the number ofcomparison pecks/s averaged over the firsteight test sessions on the hue–form baselinetrials (open circles) and on the nonreinforcedform–hue symmetry probe trials (filled circles).The corresponding panels of Figure 5 plot thecorresponding data averaged over the secondset of eight test sessions. For the baseline trials,“positive” and “negative” refer to the (potential-ly) reinforced (R1–T2 and G1–H2) and thenonreinforced (R1–H2 and G1–T2) sample–comparison sequences, respectively. For thesymmetry probes, “positive” and “negative” refer

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Fig. 3. Acquisition of the three concurrently trained successive matching tasks over the first 20 two-session blocks oftraining for each pigeon in Experiment 1. Discrimination ratios (filled symbols) are the proportions of all comparisonresponses occurring on the positive sample–comparison trials of each task. The percentage of correct symbolic choices (opentriangles) is the percentage of correct side-key responses on positive hue–form matching trials.


to the reverse of the potentially reinforced andthe nonreinforced baseline sequences (viz., T1–R2 and H1–G2, and H1–R2 and T1–G2),respectively. Each positive baseline datum pointrepresents the average number of pecks on fourrandomly selected positive trials (two of each

positive hue–form combination) from each testsession within an eight-session block. Similarly,each negative baseline datum point is theaverage number of pecks on four randomlyselected negative trials (two of each negativehue–form combination) from each of these test

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Fig. 4. Top panels: Comparison–response rates in pecks/s (�1 SEM) on the symbolic matching baseline trials (opencircles) and the nonreinforced symmetry probe trials (filled circles) averaged over the first eight test sessions for each pigeonin Experiment 1. Positive ¼ potentially reinforced symbolic baseline trials and test trials in which the samples andcomparisons of the potentially reinforced baseline trials were presented in reverse order. Negative ¼ nonreinforcedsymbolic baseline trials and test trials on which the samples and comparisons of the nonreinforced baseline trials werepresented in reverse order. Bottom panel: The percentage of symmetry-consistent side-key responses for each pigeon on thepositive symmetry probe trials averaged over its first eight test sessions in Experiment 1. Error bars ¼ one SEM; stars denotesignificant deviation from chance (50%).


sessions. Each probe-trial datum point is theaverage of the four positive, or four negative, testtrials from each test session across an eight-session block.

Throughout testing, pigeons pecked thecomparisons at much higher rates on positivethan on negative baseline trials, just as they did

at the end of the acquisition phase. In short, thehue–form baseline conditional discriminationsremained intact during testing. More impor-tantly, probe-trial comparison-response rateswere numerically higher on the symmetricalversions of the positive than on the negativebaseline trials for all pigeons. For the first eight

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Fig. 5. Top panels: Comparison-response rates in pecks/s (�1 SEM) on the symbolic matching baseline trials (opencircles) and the nonreinforced symmetry probe trials (filled circles) averaged over the second eight test sessions for eachpigeon in Experiment 1. Positive ¼ potentially reinforced symbolic baseline trials and test trials in which the samples andcomparisons of the potentially reinforced baseline trials were presented in reverse order. Negative ¼ nonreinforcedsymbolic baseline trials and test trials on which the samples and comparisons of the nonreinforced baseline trials werepresented in reverse order. Bottom panel: The percentage of symmetry-consistent side-key responses for each pigeon on thepositive symmetry probe trials averaged over its second eight test sessions in Experiment 1. Error bars ¼ one SEM; starsdenote significant deviation from chance (50%).


test sessions, this difference in probe-trialresponse rates was statistically significant inanalysis of variance (ANOVA) only for pigeonsCHC1 and CHC3, Fs(1, 62) ¼ 6.14 and 18.96,respectively. For the second set of eight testsessions, the difference was statistically signifi-cant for all 4 pigeons, Fs(1, 62) ¼ 3.95, 15.80,4.44, and 3.90 for pigeons CHC1, CHC3, CHC4,and CHC6, respectively.The bar graphs shown in the bottom panels of

Figures 4 and 5 show the average percentages ofsymmetry-consistent (“symmetrical”) left versusright side-key responses on the form–hue probetrials for the first and second set of eight testsessions, respectively. For the first eight sessions,these percentages were around 50% correct forthe 2 pigeons (CHC1 and CHC3) whose probe-trial comparison response rates showed evi-dence of symmetry, whereas they were signifi-cantly above chance for the 2 pigeons (CHC4and CHC6) whose corresponding responserates did not show evidence of symmetry, x2

(1) ¼ 0.80, 0.12, 5.83, and 7.26 for pigeonsCHC1, CHC3, CHC4, and CHC6, respectively.For the second set of test sessions, however, thepercentages of symmetrical side-key responseswere significantly above that expected by chancefor all pigeons, x2(1) ¼ 6.37, 3.86, 5.76, and8.60, for pigeons CHC1, CHC3, CHC4, andCHC6, respectively.

DiscussionThe results of this experiment are noteworthy

for two reasons. First, they replicate Urcuioli’s(2008, Experiment 3) and Frank and Wasser-man’s (2005, Experiment 1) findings of associa-tive symmetry in pigeons. Specifically, afterconcurrently training AB (hue–form), AA(hue–hue) and BB (form–form) successivematching, pigeons responded more frequentlyto the comparisons on nonreinforced BA(form–hue) probe trials when those trialsreversed the order of the positive (reinforced)AB baseline sequences than when they reversedthe order of the negative (nonreinforced) ABbaseline sequences. This occurred despite thefact that, unlike those previous experiments,reinforcement on the positive AB baseline trialswas also contingent upon a correct left versusright side-key response following the compar-isons on those trials. This meant that not allpositive AB trials ended in food reinforcement,especially early in training. Nevertheless, rein-forcement occurred only after certain sample–

comparison sequences in the hue–form task.The remaining sequences never ended inreinforcement and these negative sequencesoccurred as frequently as the positive sequences,a condition Urcuioli (2008) hypothesized maybe crucial for pigeons’ stimulus-class formation.The present experiment shows that requiring anadditional, simple discrimination between com-parisons on the potentially reinforced (positive)AB trials did not preclude the emergence of BAsymmetry.Second, the test results show that associative

symmetry was evident in the pigeons’ side-keypreferences as well as in the rate at which theypecked the comparisons on the probe test trials.Specifically, after learning to peck the left key onR1–T2 baseline trials, pigeons then pecked theleft key more often than the right key on thesymmetrical T1–R2 probe trials. Likewise, afterlearning to peck the right key on G1–H2baseline trials, pigeons pecked the right keymore often than the left key on the symmetricalH1–G2 probe trials. For 2 of the 4 pigeons, thiseffect did not materialize during the first eighttest sessions, but it was clearly apparent in thesecond set of eight test sessions. It is important tonote that this delayed emergence of symmetry-consistent side-key preferences cannot bedismissed as further learning on these testtrials because all test-trial responses werenonreinforced.It is curious that the pigeons that did not show

symmetrical side-key preferences in the first setof test sessions nonetheless showed clearevidence of symmetry in their probe-trialcomparison–response rates during these ses-sions. Likewise, the other 2 pigeons exhibitingsignificant symmetry-consistent side-key prefer-ences in the first set of test sessions did not showa significant symmetry effect in their compari-son–response rates. We have no explanation forthis dissociation. In any event, the results fromthe second set of eight test sessions showed thatall pigeons performed in a manner consistentwith associative symmetry, both in the rates atwhich they pecked the comparisons on thepositive versus negative BA probe trials and intheir left versus right side-key preferences on thepositive BA probes.The choice results from BA testing suggest

strongly that the left versus right responses weremembers of the same class as the samples andcomparisons preceding those responses. None-theless, we need to consider another possible


explanation of the pigeons’ side-key preferenceson the BA probes. Note that on the baseline ABtrials (see Table 1), a left side-key response wasreinforced only after a red sample and only aftera triangle comparison (viz., only on R1–T2trials). Likewise, a right side-key response wasreinforced only after a green sample and onlyafter a horizontal comparison (viz., only on G1–H2 trials). On the probe trials during testing,pigeons had the opportunity to choose betweenleft and right responses after these samenominal stimuli, albeit in reversed order (e.g.,on T1–R2 and H1–G2 probes). Perhaps, then,pigeons preferentially responded left after thetriangle sample and the red comparison be-cause the triangle and red stimuli, individuallyor in combination, had become discriminativefor a left-key response during baseline training.Similarly, perhaps pigeons preferentially re-sponded right after the horizontal-lines sampleand the green comparison because both hori-zontal and green, individually or in combina-tion, had become discriminative for a right-keyresponse during training1.

Obviously, this simple discrimination expla-nation ignores the potentially influential ordi-nal positions of these stimuli in testing vis-á-vistraining. Nevertheless, it is a plausible alterna-tive to an account which claims that thereinforced left versus right side-key responses(Left3 and Right3) are members of the samestimulus classes as the sample and compa-rison stimuli preceding them in AB baselinetraining (cf. Figure 2). The purpose ofExperiment 2, then, was to conduct a follow-up response membership test whose resultswould not be amenable to this alternativeexplanation.

Experiment 2

In Experiment 2, pigeons learned a fourthsuccessive matching task using three small whitedots (D) and a homogeneous white (W) field asstimuli. Reinforcement was arranged only ontrials in which the comparison physicallymatched the preceding sample and only ifpigeons made a “correct” left or right side-key

response on these positive trials (see Table 2).Nonmatching (viz., negative) trials were alwaysnonreinforced. Trials of this CC identity match-ing task were later intermixed with those of thethree previously learned tasks, and trainingcontinued until criterion levels of performancewere achieved on all four tasks.

Figure 6 shows the stimulus classes hypothe-sized to arise from all four baseline tasks (cf.Urcuioli, 2008). The stimulus and responsenotations are the same as before and, onceagain, elements common tomore than one classare bolded and italicized. The two new stimulusclasses associated with DW identity matching areshown at the bottom. The two elements in theDW identity classes common to other classes(specifically, to the symbolic classes) are the leftand right side-key responses (Left3 and Right3).If these and other common, across-class ele-ments cause their respective classes to merge,the net result will be the two 7-member classesdepicted in Figure 7. Given these enlargedclasses, new sample–comparison relations in-volving the D and W samples should emerge intesting. Specifically, pigeons should respondmore to the comparisons on novel D1–R2 andW1–G2 trials than to the comparisons on novelD1–G2 and W1–R2 trials because only theformer trials consist of elements from thesame stimulus class. Likewise, they shouldrespond more to the comparisons on novelD1–T2 and W1–H2 trials than to the compar-isons on novel D1–H2 and W1–T2 trials for thesame reason. Experiment 2 tested thesepredictions.

MethodSubjects andApparatus. The 4 pigeons that

completed Experiment 1 initially participated inthis experiment. One pigeon (CHC6) wasdropped prior to testing because it failed tomaintain its baseline performances.

The apparatus was the same as those used inExperiment 1. However, two new center-keystimuli were introduced: three small whitecircular dots (D) oriented 45o counter-clockwisefrom vertical on black background and a white(W) homogeneous field (BRS/LVE Pattern No.692).

Procedure. This experiment consisted offour phases of baseline training and recovery,which began approximately 2-3months after thecompletion of Experiment 1, and two testphases. Each is briefly described below.

1These points are particularly apropos for pigeons CHC4and CHC6 because they were erroneously run with side-keychoice (viz., simple discrimination) contingencies on thematching hue–hue and form–form trials during their first 10acquisition sessions.


Baseline recovery. Each pigeon (CHC1,CHC3, and CHC4) was re-trained on the threebaseline successive matching tasks used inExperiment 1 (see Table 1). Baseline re-training

for each pigeon continued until its DRwas .80 orhigher on all three tasks, and it made aminimum of 13 (of 16) correct side-keyresponses (81.3% correct) on the hue–formtask, for five of six consecutive sessions. This wasfollowed by at least 10 overtraining sessions withperformances at or above these criteria for fiveof the last six sessions.Successive matching acquisition with D and W

stimuli. After baseline recovery, each pigeonwas trained on successive matching using Dand W as center-key (sample and comparison)stimuli. Identity matching contingencies were ineffect for all pigeons (see Table 2): Foodreinforcement was scheduled on trials in whichthe sample and comparison were nominallyidentical (viz., D1–D2 and W1–W2); trials onwhich they differed (viz., D1–W2 and W1–D2)ended without food. In addition, reinforcement

Table 2

Expanded Baseline Successive Matching and Choice Contingencies in Experiment 2

Hue–Form Symbolic Hue–Hue Identity Form–Form Identity

R ! T - FI 5 s – Leftþ R ! R - FI 5 sþ T ! T - FI 5 sþR ! H - EXT R ! G - EXT T ! H - EXTG ! T - EXT G ! R - EXT H ! T - EXTG ! H - FI 5 s – Rightþ G ! G - FI 5 sþ H ! H - FI sþDot–White Identity

D ! D - FI 5 s – LeftþD ! W - EXTW ! D - EXTW ! W - FI 5 s – RightþNote. R ¼ red, G ¼ green, T ¼ triangle, H ¼ horizontal, D ¼ dots, W ¼ white, FI ¼ fixed interval schedule, EXT ¼

nonreinforced, þ ¼ reinforced. The first and second center-key stimuli in a trial sequence (sample and comparison,respectively) are shown to the left and to the right of the arrows, respectively. Positive (i.e., potentially reinforced) hue–formand dots–white matching trials ended with food only if pigeons made the designated (left or right) side-key choice responseafter the comparison stimulus; otherwise, the trial endedwithout food. Counterbalancing of the hue–form contingencies andof the dots–white identity choice contingencies have been omitted.

Fig. 6. The eight stimulus classes hypothesized to resultfrom hue–form symbolic, hue identity, form identity, anddot–white (DW) identity successive matching training inExperiment 2. Bolded italics denote elements that appear inmore than one stimulus class.

Fig. 7. The two 7-member stimulus classes hypothesizedto result from the merging of individual classes containingcommon elements (cf. Figure 6) in Experiment 2.


on matching (positive) trials was contingentupon a “correct” left versus right side-keyresponse. In other words, immediately aftercomparison offset on the D1–D2 and W1–W2sequences, the left and right side keys were eachlit by a white stimulus, and a single peck to eitherside key within 3 s immediately turned both offand yielded food (if the choice response wasdesignated “correct”) or turned off the houselight (if the choice response was designated“incorrect”). If neither side key was peckedwithin 3 s (a very infrequent event), the side keysand house light went off automatically. Forpigeons CHC1 and CHC4, a left response wasreinforced after the D1–D2 sample–comparisonsequence and a right response was reinforcedafter the W1–W2 sequence. For pigeon CHC3,the opposite contingencies were in effect.

Each DW successive matching session con-sisted of 60 trials divided equally among the fourpossible trial types. All other procedural detailswere identical to those described for successivematching acquisition in Experiment 1. Trainingfor each pigeon continued until it achieved aDR(total number of comparison pecks on positivetrials � total number of comparison pecks onboth positive and negative trials) of at least .80and made at least 25 out of 30 correct side-keyresponses (83.3%) for five of six consecutivesessions. At this point, sessions of DW identitymatching were alternated across days withtraining on the three concurrent successivematching tasks from Experiment 1, and thiscontinued until criterion-level performanceswere met for both session types for five sessionseach.

Combined training with all baseline tasks. Priorto testing, pigeons received additional trainingwith trials from all four baseline tasks inter-mixed. These 96-trial sessions consisted of 24trials of each baseline task, hue–form symbolic(AB), hue identity (AA), form identity (BB), anddot–white identity (CC), divided equally amongthe four sample–comparison sequences com-prising each task. Combined training continuedfor each pigeon until its DR was at least .80 for allfour tasks and it made at least 10 out of 12correct side-key responses (83.3%) on bothhue–form symbolic and dot–white identitymatching for five of six consecutive sessions.At least 10 overtraining sessions then followed.Testing commenced when criterion levels ofperformances were met for the last five of sixovertraining sessions.

Class expansion testing. These 104-trial testsessions consisted of 96 baseline trials, dividedequally among the four baseline tasks (cf.Table 2) plus eight nonreinforced probe trials.As in Experiment 1, test sessions were run inblocks of two separated by five baseline sessionsat criterion levels of performance. There weretwo types of test sessions, one in which thenonreinforced probe trials consisted of D or Wsamples followed by R or G comparisons (viz.,D1–R2, D1–G2, W1–R2, and W1–G2) and theother in which the probe trials consisted of Dor W samples followed by T or H comparisons(viz., D1–T2, D1–H2, W1–T2, and W1–H2).Each test block was always of one type (CA orCB, respectively), and two blocks of one typewere alternated with two blocks of the othertype until a total of eight test sessions of eachtype were completed. For pigeon CHC1,the order of testing was T/H comparisons(CB) – R/G comparisons (CA) – T/H compar-isons (CB) – R/G comparisons (CA); forpigeons CHC3 and CHC4, testing occurred inthe reverse order. There was no left–rightchoice component on any test trial in thisexperiment. Otherwise, all other details wereidentical to the corresponding test sessions inExperiment 1.

ResultsBaseline performances. Pigeons learned the

DWidentity task quickly, reaching criterion levelsof performance within 6-10 sessions, and theseaccuracies were maintained when this task waslater combined with the other three baselinetasks. The average DRs for the five baselinesessions prior to the first R/G comparison testsession were .94, .96, .96, and .92 for hue–formsymbolic, hue identity, form identity, and DWidentity matching, respectively, and the percen-tages of correct side-key responses for hue–formsymbolic and DW identity matching were 99.4%and 99.4%, respectively. The average DRs for thefive baseline sessions prior to the first T/Hcomparison test session were .97, .95, .95, and.94, for hue–form symbolic, hue identity, formidentity, and DW identity matching, respectively,and the corresponding percentages of correctside-key responses for hue–form symbolic andDW identity matching were 99.4% and 98.9%,respectively.

Performances on the baseline trials duringtesting with each set of comparisons were wellmaintained, with only scattered instances in


which a DR fell below .80. The average DRs forhue–form symbolic, hue identity, form identity,and DW identity were .94, .94, .94, and .92,respectively, across the eight test sessions withthe R and G comparisons, and .95, .94, .97, and.95, respectively, across the eight test sessionswith the T andH comparisons. The correspond-ing percentages of correct side-key responses onthe baseline hue–form and DW identity taskswere 98.6% and 84.8%, respectively, for theformer tests, and 99.3% and 84.5%, respectively,for the latter tests. These accuracies never fellbelow 80% correct on the baseline hue–formsymbolic task during either type of test, but didoccasionally on the baseline DW identity task(although never below 70% correct). The loweroverall accuracy on DW identity than on hue–form symbolic matching is probably attributableto less training on the former than on the latter

task (which had been extensively trained inExperiment 1).Testperformances. The top panel of Figure 8

shows the results from each pigeon averagedover the eight test sessions in which probe trialsconsisted of D and W samples followed by R andG comparisons (viz., CA tests). The bottompanel of the figure shows the correspondingdata from the test sessions in which probe trialsconsisted of D and W samples followed by T andH comparisons (viz., CB tests). “Positive” and“negative” baseline trials in both panels refer tothe potentially reinforced versus nonreinforcedsymbolic training sequences (R1–T2 andG1–H2versus R1–H2 and G1–T2), respectively. (Eachbaseline datum point was computed in the samefashion as that described in Experiment 1.) Forthe probe trials, “positive” and “negative” referto test-trial sequences in which the sample (D1


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Fig. 8. Top panels: Comparison-response rates in pecks/s (�1 SEM) on the symbolic matching baseline trials (opencircles) and the class expansion probe trials (filled circles) using red and green comparisons averaged over eight test sessionsfor each pigeon in Experiment 2. Bottom panels: Comparison-response rates in pecks/s (�1 SEM) on the symbolicmatchingbaseline trials (open circles) and the class expansion probe trials (filled circles) using triangle and horizontal-linescomparisons averaged over eight test sessions for each pigeon in Experiment 2.


orW1) and comparison (R2 or G2, or T2 or H2)were drawn from the same hypothesized stimu-lus class or different stimulus classes, respective-ly (see Figure 7). Thus, the D1–R2 and W1–G2sample-comparison sequences are “positive”,and the D1–G2 and W1–R2 sequences are“negative”, for the R/G comparison tests.Likewise, the D1–T2 and W1–H2 sample–comparison sequences are “positive”, and theD1–H2 and W1–T2 sequences are “negative”,for the T/H comparison tests.

Figure 8 shows that all pigeons maintainedtheir baseline hue–form symbolic performancesin both tests as evidenced by much highercomparison response rates on positive than onnegative baseline trials. Probe-trial response ratesaveraged across the eight R/G comparison tests(top panel), on the other hand, were mostlynondifferential: The slightly higher rates on thepositive thanon thenegativeprobe trials on thesetests were not significantly different for pigeonsCHC1, CHC3, andCHC4, Fs(1, 62) ¼ 1.05, 0.48,and 0.30, respectively. Moreover, overall probe-trial response rates were considerably lower thanon the baseline trials, and were considerablylower than the overall probe-trial rates with thesesame(RandG)comparisonsonthefirst eight testsessions in Experiment 1 (see Figure 4). Thesedifferencesmight reflect thecumulative effectsofextinction on the probe trials (they were alwaysnonreinforced) which, in turn, could havediminished the chances of obtaining a significantdifference between positive and negative probe-trial response rates with these comparisons.Interestingly, CHC3’s positive versus negativeprobe-trial rates over just the first two R–G testsessions (1.38 versus 0.12 pecks/s, respectively)were significantly different, F(1, 14) ¼ 5.68.

By contrast, the results from eight test sessionsinvolving the T/H comparisons (bottom panelof Figure 8) showed significantly higher com-parison-response rates on positive than onnegative probe trials for all 3 pigeons, Fs(1,62) ¼ 166.24, 17.72, and 77.86 for pigeonsCHC1, CHC3, and CHC4, respectively. Indeed,these response-rate differentials were compara-ble to those observed on the hue–form baselinetrials.

DiscussionExperiment 2 tested the prediction that new

stimulus relations involving D and W samplesand R, G, T and H comparisons would emergeif 1) the side-key responses reinforced on the

hue–form (AB) symbolic and dot–white identity(CC) baseline tasks were members of thesame hypothetical classes containing the sam-ples and comparisons of those tasks, and 2)classes containing common response elements(see Figure 6) merged in the same manner asclasses containing a common sample or com-parison element (see Figure 7). Put simply, weassessed whether association with the sameresponse would render two stimuli equivalent.

The probe-trial data from Experiment 2generally confirmed this prediction. Althoughthe differences between comparison-responserates on positive versus negative trials involvingR and G comparisons were not statisticallysignificant when those rates were averagedover all eight tests, 1 of the 3 pigeons showedsignificantly higher rates on the positive probetrials during its initial two R/G test sessions.Moreover, all 3 pigeons showed significantlyhigher comparison-response rates on positivethan on negative probe trials averaged over alltest sessions in which the D and W samples werefollowed by the T and H comparisons. Theseresults are consistent with the notion thatresponses can become members of stimulusclasses (Sidman, 2000) given that “positive”could only be defined by assuming that commonleft and right side-key responses merged smallerclasses into larger ones that contained the Dand W samples and particular hue and formcomparisons (see Figure 7).

Because comparison response rates on theprobe trials served as the sole dependentvariable in Experiment 2, this experimenteffectively avoided the interpretation issue thatwe faced in Experiment 1. Recall that inExperiment 1, pigeons showed symmetry-con-sistent side-key preferences on BA (form–hue)probe trials that were the reverse of the explicitlytrained AB (hue–form) trials. One interpreta-tion of this result is that the left and rightresponses were also members of stimulus classesinvolving the A and B elements of the reinforcedcombinations of the symbolic (AB) and identity(AA and BB) tasks (cf. Figure 2). However,because the nominal stimuli on the BA probetrials were the same as those on the AB baselinetrials, it is possible that symmetry-consistentleft versus right responding occurred becauseeach side-key response had been explicitlyreinforced following a specific A stimulus anda specific B stimulus on the positive AB trials(e.g., left after red and after triangle—cf.


Table 1). Consequently, if these nominal stimulihad acquired discriminative control over leftversus right responding, our BA probe trialswould not have constituted a test for derived(emergent) relations.In Experiment 2, no such interpretive prob-

lem arises because the CA and CB test trials didnot include a leftversus right-key choice phase.Instead, response membership was indirectlyassessed by asking whether two stimuli associat-ed with the same side-key response generatesuccessive matching behavior consistent withstimulus class formation. Specifically, highercomparison-response rates on positive CA andCB probes than on negative CA and CB probesimplies that the left and right responsesrequired during symbolic and DW identitybaseline training produced class merger and,with it, enlarged classes whose elements com-prised the positive test sequences. The patternof results we obtained was largely consistent withthe pattern predicted by response membershipin those classes.

General Discussion

The present study was motivated in part totest Sidman’s (1994, 2000) proposal that thereinforcement contingency itself generatesequivalence classes and, more specifically, thatunder certain baseline conditions, responseswill become members of those classes. Shimizu(2006) and Lionello-DeNolf and Braga-Kenyon(in press) studied n-alternative MTS perform-ances in humans and reported data consistentwith response membership in stimulus classes.Here, we sought similar evidence in pigeonsperforming successive matching tasks becausethey have previously demonstrated emergentstimulus–stimulus relations indicative of classformation in these tasks (e.g., Sweeney &Urcuioli, 2010; Urcuioli, 2008; Urcuioli &Swisher, 2012). Another motivation for thepresent study, then, was to assess whethertraining with successive matching contingenciesthat include a differential response componentwould generate classes containing those differ-ent responses (cf. Urcuioli, 2008). Thus, ourexperiments are theoretically important for tworeasons. First, they bear directly on Sidman’s(1994, 2000) position regarding the origins ofequivalence. Indeed, if his prediction of re-sponse membership was disconfirmed “…thetheory that equivalence arises directly from the

reinforcement contingency becomes untena-ble.” (Sidman, 2000, p. 133). Second, theyaddress the processes that Urcuioli (2008)hypothesized are crucial for pigeons’ stimulus-class formation.The two experiments reported here tested

predictions regarding response membership bytraining pigeons in a modified successivematching paradigm (Wasserman, 1976; cf.Konorski, 1959): Rather than simply reinforcingresponses to the comparison stimuli on thepositive symbolic (AB) baseline sequences, leftand right side-key pecks were differentiallyreinforced following these sequences. By sup-plementing the conditional discrimination con-tingencies of successive matching with a simplediscrimination involving two side-key responsesfollowing the stimuli comprising those contin-gencies, we were able to obtain independentmeasures of conditional and simple discrimina-tion accuracies with which to assess emergentrelations between the stimuli on positivesequences and the side-key responses theyoccasioned (cf. Figures 1 and 6). This permitteda straightforward test for emergent relationsthat involved those responses (cf. Figures 2and 7).In our estimation, the data we report here

provide good evidence that pigeons’ stimulusclasses can include their differential responses.This evidence took two forms. First, Experiment1 showed that symmetrical BAR relationsemerged from ABR baseline training (viz.,symbolic – AB – matching with an addedchoice-response – R – component) combinedwith AA and BB identity training. In otherwords, after learning which hue–form (AB)sample–comparison sequences were potentiallyreinforced and which side-key response (R) tomake after each positive AB combination,pigeons made that same left or right responsemore often than the alternative response on testtrials that reversed the positive baseline sequen-ces. Thus, if one component of ABR traininginvolved positive trials consisting of a redsample, a triangle comparison, and a leftresponse, then in testing, pigeons pecked leftmore often than right on trials beginning with atriangle sample and a red comparison. Thisassociative symmetry effect was complementedby the additional finding that pigeons peckedmore to the comparison stimuli themselves onpositive test trials (viz., the reverse of thereinforced AB baseline trials) than on negative


test trials (viz., the reverse of the nonreinforcedAB baseline trials). In short, conditional dis-crimination performance on the BA probe trialscoincided with simple discrimination (left keypeck vs. right key peck) performance on thosesame trials.

Second, Experiment 2 showed that afterCCR training (viz., dot–white identity match-ing with a side-key response requirement) wasadded to the ABR, AA, and BB baseline tasks,pigeons responded in a class-consistent fashionon CB and, to a lesser extent, on CA test trials.It is important to note that the samples andcomparisons of the CCR task did not overlapwith the samples and comparisons of the ABR,AA, and BB tasks; only the left and rightresponses in the CCR and ABR matching tasksdid. Without reference to those commonresponses, it would not be possible to classifythe dot and white stimuli in the CCR task asbelonging to, or not belonging to, the samestimulus class as the red, green, triangle andhorizontal stimuli appearing on the otherbaseline tasks. In other words, “class consis-tent” has no meaning on the CA and CB probetrials unless an expanded class can be derivedvia overlapping elements in the individualstimulus classes proposed to arise from thefour baseline tasks. As Figure 6 shows, two ofthe required overlapping elements were theleft and right responses (Left3 and Right3)that were components of the positive symbolic(AB) and positive dot–white identity (CC)training trials.

Emergent relations involving left and rightside-key responses have also been reported inpigeons by García and Benjumea (2006, Experi-ment 1). They devised a novel conditionaldiscrimination training procedure in whichfive consecutive pecks to either the left or rightkey (the “sample”) of a white two-key displayreplaced that display with red and greencomparison stimuli randomized across loca-tions. Pecking the red comparison was rein-forced after one side-key-response sample andpecking the green comparison was reinforcedafter the other side-key-response sample. Afterthese [side-key-response – color comparison]baseline relations were learned to high levels ofaccuracy, the possibility that those relations weresymmetrical was tested by occasionally present-ing nonreinforced probe trials where both keyswere lit either red or green. García andBenjumea found that pigeons preferentially

pecked the left or right key on the red–redand green–green test trials in accordance withwhich side-key response was associated witheach color comparison in training (see alsoVasconcelos & Urcuioli, 2011). For example, if ared comparison choice had been reinforcedfollowing a left-response sample in training,then pigeons pecked the left key more oftenthan the right key on test trials where redappeared on both keys.

This symmetry effect suggests that baselinetraining had produced two classes, each ofwhich contained a response member—namely,[left response, red] and [right response, green].García and Benjumea (2006, Experiment 3) alsoshowed that pigeons did not exhibit side-key-response preferences in testing if they did notpeck the left- and right-key samples duringbaseline training. In other words, simply pre-senting (without the requirement to peck) a litleft key or a lit right key prior to the colorcomparisons during training was not sufficientto produce a preference for pecking left versuspecking right on subsequent red–red andgreen–green test trials. These findings suggestthat the differential proprioceptive stimulationgenerated by emitting leftversus right-keyresponses, rather than the differential extero-ceptive stimulation provided by different lit-keylocations, was responsible for the reportedsymmetrical relations.

Although our results support Sidman’s (2000)response membership proposal, they suggest analternative to his definition of responses inconditional discriminations. Sidman (1986,1994, 2000) assumed that the subject’s re-sponses in these procedures were the actionsrequired to operate whichever manipulandumdisplayed a specific stimulus. Thus, he assumedthat conventional, n-alternative MTS tasksinvolve a single response such as a button press(see Table 3 in Sidman, 1986, and Figure 2 inSidman, 2000). In contrast, we defined re-sponses in terms of which specific manipula-ndum was operated (e.g., left-key pecks andright-key pecks). Although the two side-keyresponses featured in only simple discrimina-tions (i.e., two 3-term contingencies) in ourexperiments, evidence for these responsesbecoming members of stimulus classes suggeststhat the responses in conventional MTS mightalso be usefully defined in terms of their spatiallocation (see Jones, 2003). MTS can still beconceptualized as four-term contingencies


providing that the various comparison–stimulusarrays are viewed as the discriminative stimuli(see Figure 9 in Jones, 2003). Future researchshould explore the implications of this ap-proach to response definition for researchinto equivalence relations.If the assumptions of Urcuioli’s (2008) theory

that prompted the present experiments aresound, theymake other predictions that can andshould be tested in future research. First, theostensible class-expansion effect reported inExperiment 2 should occur only if baseline DWidentity training requires differential side-keyresponding on the two positive sample–compar-ison sequences. In other words, pigeons trainedin the same manner as those in Experiment 2but without the left–right choice phase in theDW identity task should respond nondifferen-tially on subsequent CA and CB probe trials.Nevertheless, such training (viz. concurrentABR, AA, BB, and CC successive matching)would still be expected to yield symmetrical BAcomparison-response effects and symmetricalBAR spatial-response effects, like those observedin Experiment 1.Second, Urcuioli’s (2008) theoretical as-

sumptions predict that concurrent trainingon AB, AA, and BBR successive matching willalso yield the same two five-member classesshown in Figure 2. Consequently, if later givena left versus right choice following the positiveAA trials, pigeons trained in this fashionshould respond differentially to the side keys.Specifically, they should preferentially peck theleft key after a R1–R2 probe trial andpreferentially peck the right key after a G1–G2 probe trial. If obtained, this result couldnot be explained in terms of explicit trainingto peck left versus peck right after the redversus green (A) nominal stimuli, respectively,because no such training is provided in any ofthe AB, AA, and BBR baseline tasks.Third, new successive matching relations

should emerge following just hue–form sym-bolic and DW identity training so long as bothbaseline tasks include a left versus right side-key response requirement (i.e., ABR and CCR,respectively). Thus, if a red sample – trianglecomparison – left response (R1–T2–Left3)sequence is reinforced as part of hue–formsymbolic matching, and a dot sample – dotcomparison – left response (D1–D2–Left3)sequence is reinforced as part of DW identitymatching, Urcuioli’s (2008) theory predicts

the emergence of red sample – dot compari-son (R1–D2) and dot sample – trianglecomparison (D1–T2) symbolic relations. Inother words, these novel sequences shouldyield relatively high comparison response ratesin testing.Ideally, evidence for response membership in

stimulus classes obtained from tests like thosejust described would be complemented byresults from tests of the sort reported by Shimizu(2006) in which the differential responsesthemselves serve as probe-trial samples andare found to control class-consistent comparisonresponding. For instance, after training pigeonson tasks similar to those used in Experiment 1(viz., ABR, AA, and BB), perhaps they wouldrespond more often to certain comparisonsthan to others on test trials that begin with eithera left- or a right-key response (the “sample”) toproduce the center-key comparison (i.e., on RAand RB successive matching probes). Such anoutcome would provide especially compellingsupport for the view that stimulus-class forma-tion in pigeons can, as in humans, incorporatetheir different responses.

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Received: June 28, 2012Final Acceptance: January 4, 2013


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EVIDENCE FOR RESPONSE MEMBERSHIP IN STIMULUS CLASSES BY PIGEONS