Pilley and Reid 2011

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Behavioural Processes 86 (2011) 184195

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Behavioural Processes

j o u rn a l h om epa ge : www.e l sev i e r. com/ loca t e /behavproc

Border collie comprehends object names as verbal referents

John W. Pilley a , , Alliston K. Reid b , a 101 Seal St., Spartanburg, SC 29301, USAb Department of Psychology, Wofford College, 429N. Church St., Spartanburg, SC 29303, USA

a r t i c l e i n f o

Article history:Received 6 August 2010Received in revised form16 September 2010Accepted 30 November 2010

Keywords:Referential understandingInferential reasoning by exclusionExclusion learningBorder collieDogReceptive language

a b s t r a c t

Four experiments investigated the ability of a border collie (Chaser) to acquire receptive language skills.Experiment 1 demonstrated that Chaser learned and retained, over a 3-year period of intensive training,theproper-nounnames of 1022objects.Experiment2 presentedrandom pair-wisecombinationsof threecommands and three names, and demonstrated that she understood the separate meanings of proper-noun names andcommands.Chaserunderstood thatnames refer to objects, independent of thebehaviordirected toward those objects. Experiment 3 demonstrated Chasers ability to learn three commonnouns words that representcategories. Chaser demonstrated one-to-many (commonnoun) andmany-to-one(multiple-name) nameobject mappings. Experiment 4 demonstrated Chasers ability to learn wordsby inferential reasoning by exclusion inferring the name of an object based on its novelty amongfamiliar objects thatalready had names. Together, these studies indicate thatChaseracquired referentialunderstanding of nouns, an ability normally attributed to children, which included: (a) awareness thatwords mayrefer to objects, (b)awareness of verbal cues thatmap words upon theobjectreferent, and(c)awareness thatnames may refer to unique objects or categories of objects, independent of the behaviorsdirected toward those objects.

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1. Introduction

In an article in Science Kaminski et al. (2004) reported thata 9-year-old border collie (Rico) knew the names of more than200 items. Their rst experiment demonstrated that Ricos acqui-sition of the names of toys was indeed genuine not a CleverHansphenomenon, in which the successfulretrieval of toyswouldbe due to subtle cues other than the words. Their second exper-iment demonstrated Ricos exclusion learning inferring thename of an object based on its novelty in the midst of famil-iar objects ( Carey and Bartlett, 1978 ). As an example, Carey andBartlett arranged a scenario during which 3 and 4-year-old chil-dren were shown two trays and asked to retrieve the chromium,not the red tray. Despite the childrens lack of knowledge of theword chromiumas a shade of green,the children correctly inferredthat the teacher wanted the green tray. Carey and Bartlett dubbedthis rapid linking of a proper-noun label upon an object as fastmapping.

Markman and Abelev (2004) f ound the report of Ricos appar-ent exclusion learning to be fascinating. Demonstrations of word

Corresponding author. Tel.: +1 864 597 4642; fax: +1 864 597 4649. Corresponding author. Tel.: +1 864 576 7880.

E-mail addresses: [email protected] (J.W. Pilley), [email protected](A.K. Reid).

learning by exclusion in children have usually led researchers toconclude that the child has learned Baldwins (1993) elements of referential understanding. Baldwins work with children led her toconclude that if learning is limited to associative factors alone,learning would be slow. She identied two elementary factors of referential understandingthatshe believedto be necessaryto expe-dite rapid word learningby children: (a)awareness that words mayrefer to objects, and (b) awareness of social cues that enable themapping of the words upon the referent. Testing her hypotheses,Baldwin tried to eliminate associative factors by pitting temporalcontiguity against her two elements of referential understanding.Anexperimenterpresentedtwo opaque containers to infants.Look-ing inside the rst container, the experimenter exclaimed, Its amodi . Immediately thereafter, the experimenter withdrew the toyfrom the second container and gave it to the child for play. Aftera 10-s delay, the object in the rst container was also given tothe infant for play. Baldwin assumed that if associations based ontemporal contiguity alone were critical for learning, the infantswould identify the object from the second container as modi .However, despite the 10-s delay, the infants chose the object inthe rst container as modi indicating that awareness of thereference cue inuenced choice more than simple temporal con-tiguity. Baldwin concluded that fast word learning is mediated byreferential understanding as opposed to associative mechanisms.Thus, the conclusion by Kaminski et al. (2004) that a border col-lie is able to learn words rapidly by exclusion invites intriguing

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J.W. Pilley, A.K. Reid / Behavioural Processes 86 (2011) 184195 185

questionsaboutthe differences in wordlearningbetweendogsandchildren.

Markmanand Abelev (2004) w ereunabletoacceptRicosdataascompelling evidence for exclusion learning because they identiedtwo potentialdifculties with thestudy: (a)lackof control forbase-line novelty preference; and (b) reward after the exclusion choiceresponse could have mediated the subsequent exclusion learningtest trial. Thus, they questioned the validity of Ricos ( Kaminskiet al., 2004 ) demonstration of exclusion learning.

Bloom(2004) also considered the Rico data to be less thancom-pelling. He acknowledged the possibility that Ricos learning of thenames of objects may be qualitatively similar to that of a child, butmay differ only in degree. However, he questioned the conclusionthat Ricos words actually referred to objects. Did Rico treat thesound sock as a sock or did Rico treat the sound as a commandto fetch a sock, and nothing more? If Rico treated the sound as aone-word proposition fetch-the-sock, then his performance mayhave had little to do with language learning in the human sense.In addition, Bloom argued that words for children become symbolsthat refer to categories of things in theexternalworld. They appre-ciate that a word can refer to a category, and thereby can be usedto request a sock, or point out a sock, or comment on the absenceof one (p. 1605).

We obtainedour border collie, Chaser, soonafter thepublicationof the Kaminski et al. (2004) study. The article led us to focus ourresearch on the questions resulting from their intriguing research(Bloom, 2004; Fischer et al., 2004; Markman and Abelev, 2004 ).Experiment 1 investigated Chasers ability to learn the names of over 1000 proper-noun objects over a 3-year period of intensivedaily training. We wanted to know whether Ricos acquisition of over 200 words represented an upper limit for border collies, orwhether an intensive training program with abundant rehearsalcould teach a more extensive vocabulary. Experiment 2 testedBlooms (2004) concern that words actually refer to objects, inde-pendent in meaningfrom thegiven commandrelativeto theobject.Experiment 3 explored thedegree to whichChaser could learn sev-eral commonnouns names that representcategoriesof objects, in

additionto thepreviously learned proper-noun names. Experiment4 measured Chasers ability to learn nouns by inferential reasoningby exclusion inferring the name of an object based on its nov-elty in the midst of familiar objects, logically excludingthe familiaralternatives. The four studies were designed to allow us to addressconcerns posed by Bloom (2004) and Markman and Abelev (2004)in their critiques of Kaminski et al. (2004) and to evaluate whethera dogcan acquire referential understanding of nouns,as denedbyBaldwin (1993) , when all proper control conditions are included.

2. Experiment 1: investigating the ability of a border collieto learn proper nouns

Kaminski et al. (2004) reported that a 9-year-old border col-lie (Rico) knew the names of more than 200 objects. We wantedto know whether Ricos vocabulary represented an upper limit forborder collies, or whether an intensive training program provid-ing abundant rehearsal could teach a more extensive vocabulary.Experiment 1 provided 45h of daily training over a 3-year periodto teach Chaser the names of more than 1000 proper-noun objects.

2.1. Materials and method

2.1.1. Subject The subject was a registered female border collie, Chaser, born

in May 2004. We acquired Chaser when she was 8 weeks old, andshelivedinourhomeprimarilyasapetaswellasaresearchsubject.

She exhibited the usual characteristics of her breed: intense visual

focus/concentration, instinct to nd, chase, herd, attentive to audi-tory cues even during complex visual stimulation (such as herdingsheep), responsive to soft levelsof praiseand verbalcorrection, andboundless energy.

2.1.2. MaterialsOver a period of 3 years, we obtained 1038 objects for Chaser to

identify and fetch. Objects were toys for children or dogs obtainedmostly from second-hand discount stores, consisting of over 800cloth animals, 116 balls, 26 Frisbees, and over 100 plastic items.There were no duplicates. Objects differed in size, weight, texture,conguration, color, design, and material. Despite some similari-ties,eachobject contained unique featuresenablingdiscrimination.We gave 1022 of the objects a distinctive proper name consist-ing of 12 words (e.g., elephant, lion, tennis, Santa Claus).[We duplicated the names for 16 objects, so these 16 objects werenot used in this experiment, leaving 1022 objects with uniquepropernames].We wrote this name onthe objectwitha permanentmarker to ensure that all trainers used the correct name consis-tently in all training sessions. Because Chaser handled each objectwith her mouth, we washed the objects when necessary to elim-inate acquired odors and to maintain sanitary conditions. Fig. 1shows a photograph of 42 of these objects with their associatednames. Photographsof all 1038objectswiththeir associatednamesare available as online supplemental material .

2.1.3. ProceduresWe initiated simple obedience and socialization training, 45h

daily, as soon as Chaser was brought into our home at 8 weeksof age. Behaviors and discriminations were taught by means of associative procedures, such as classical and operant condition-ing including shaping procedures. Gradually, we began to providetraining for herding, agility training, and tracking behaviors. Thesebehaviors are not relevant to the focus of this paper, so we limitour discussions to the teaching and learning of nouns, with theexception of those commands that were essential to the teachingand testing of nouns. In Chasers fth month, we began to focus

more of our time on word learning. Incentives and rewards usedto encourage desired behaviors were petting, attention, and pro-viding opportunities to engage in enjoyable activities (e.g., tugging,ball chasing, toy shaking, Frisbee play, agility play, walks, searchby exploration, outdoor tracking, and stalking). For Chaser thesetypes of incentives and rewards were more powerful than the tra-ditional use of food. Furthermore, they were less distracting thanfood and more resistantto satiation. We used food to shape behav-ior only when food served as a lure, such as turning around in acircle. Throughout the paper, we use the word novel to refer tothe objects for which Chaser had not learned a unique name. Theword familiar denotes objects forwhich shehad learned a uniquename.

2.1.4. Specic training procedures for proper-noun objectsWe taught Chaser one or two proper-noun names per day. Sev-

eral trainers taught Chaser using the same procedures. All trainerswere consistent in theiruse of the correctproper nounsbecausethename of each objectwas written on the object. Most of the trainingtook place in ourhome and front and back yards.Eachtime we gaveChaser a new name to learn, we held and pointed to the object tobe associated with the name and always said, Chaser, this is .Pop hide. Chaser nd . No other objects were available on theoor for retrieval, so errors were unlikely.A 35min play rehearsalperiod followed retrieval. During trials and play rehearsal periods,we repetitivelyverbalized thename of the object2040 times eachsession in order to facilitate the association of the name andobject.

Following the initial training in the absence of other objects,

the newly learned object was placed on the oor among other


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186 J.W. Pilley, A.K. Reid / Behavioural Processes 86 (2011) 184195

Fig. 1. This photograph displays 42 of the 1022 objects used in Experiment 1, along with their corresponding names. Photographs and names of all objects are available asonline supplementary material .

objects that had been recently learned our working group. Overa period of 24 weeks, we gave Chaser daily rehearsal testingand play with these items, during which we repeatedly pairedthe name with the object, along with reinforcing play. As Chasersvocabulary for proper-nouns grew, ultimately 50 or more newlylearned objects were usually available in open Tupperware tubsfor play and rehearsal testing, which could be initiated by Chaseror a trainer. As new names were learned, they were phased in fordaily rehearsal and the older objects were phased out of the work-ing group until monthly tests were given. When Chaser failed toretrieve an object upon command, we removed the other objects

and gave Chaser additional training trials until she met our learn-ingcriterion(describedbelow).Subsequently,the workinggroup of objects wasreturned,and playwith rehearsal testing continued. Noobjectwas removed from theworking group unless Chaser fullledour learning criteria.

Theprocedureof teachingthe names of humans,dogs, cats, loca-tions,and stationary objects was similar tothatused inteaching thenames of objects, except that we told Chaser to go to the desig-natedtargetbothduringlearningand during testing.Rewards werepraise and the opportunity to engage in enjoyable activities, suchas playing catch with one of her toys.

We adjusted our daily training procedures, the duration of oursessions, andthe amountof rehearsal to adapt to Chasers ability tolearn and retain new words. Each time Chaser made an error with

any nameobject pair, we provided additional training with thatword until Chaser again completed the learning criterion for thatnameobject pair.Therefore, the majority of most training sessionsinvolved rehearsal of previously learned words and many tests of retainedknowledge.This procedureallowedus tomeasure Chaserscumulative knowledge of proper nouns andto ensure that Chasersvocabulary was increasing systematically, rather than new knowl-edge replacingprevious knowledge. Because we adjusted our dailyprocedures to meet Chasers retention and to keep her interestedin the tasks, our training procedures did not allow us to assess themaximum rate of word learningor themaximum number of wordsthat Chaser could learn. We were not concerned with measuringChasers innate abilities independent of training; rather, our pri-mary goal was to discover what language accomplishments Chasermight achieve when given daily intensive training over years.

2.1.5. Design and statistical analysisLanguage training, ongoing assessment, and formal blind tests

of knowledge involve separate procedures that must maintain thesubjectsinterestin participating. Herman et al.(1984) f acedsimilarconstraints as they trained and assessed dolphins acquired lan-guage. They introduced terminology to help distinguish betweenthe many informal tests of knowledge carried out in the highlysocial environment of training, and the rigorous formal tests of knowledge carried out under highly controlled conditions. Theyused the term local to describe the frequent tests of knowledgecarried out during the training sessions, which normally lacked

the rigorous experimental control necessary to rule out the inu-ence of unintended cues. In contrast, the term formal describedthe formal assessments of acquired knowledge using the rigorousexperimental control necessary to rule out the inuence of unin-tended cues by the experimenter. We adopted these two terms todescribe our various tests of wordobject mapping.

2.1.6. Daily local tests during training We rstestablisheda learningcriterion thatwouldprovideclear

evidence of nameobject mapping of each nameobject pair. Thiscriterionwas implemented as a testing procedure, usedthroughouttraining, that required Chaser to select the single correct object outof a varying collection of eight familiar objects (which we called a

local 1-of-8 test, with probability of success = .125) without erroreight times in a row (local 8-of-8 binomial test). The binomialprobability of completing this difcult task due to random chancewas p =5.96E 8 for each word, which we used as our learning cri-terion for each of the 1022 names. If Chaser made an error beforeselecting the object correctly eight times in a row, then the testended, and we provided additional training until she successfullycompletedthe local 8-of-8 test.Duringeach local 1-of-8 test,Chaserwas asked to select each of the remaining objects by name, with-outreplacement, in order to rehearse prior learning. If Chasermadean error selecting any of the seven familiar distracter objects, thenthat object would be put aside for further training at a later time,andthe local 8-of-8test would continue. We often variedthe sevendistracter objects during these tests to provide increased rehearsal

of prior learning.


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Fig. 2. Filled circles depict the total number of proper-noun names learned overthe months of training. Learning of each word was demonstrated by local 8-of-8binomial tests (see text). Open circles display the minimum number of individual1-of-8 word tests completed accuratelyover the months of training. Errors resultedin an increased number of tests.

Fig. 2 shows the minimum number of local 1-of-8 tests thatChaser had to complete as training progressed over the 3-yearperiod, assuming she never made an error. Each nounobject pairwas used in a minimum of eight 1-of-8 tests, yielding more than8000 separate tests over the course of training. Similarly, Fig. 3shows the minimum number of local 8-of-8 binomial tests thatChaser had to complete assuming no errors were made [note thelogarithmic scale]. Of course, Chaser did make errors, so the actualnumber of local tests was much higher than these gures depict.Recall that our learning criterion (8 of 8 correct consecutive selec-tions)was equivalent to a binomial test yielding p = 5.96E 8. Thus,over the course of training, over 1000 different 8-of-8 tests werecompleted; or equivalently, Chaser rejected the null hypothesisover1000 timesin individual binomialstatistical testsduring infor-

mal training. Because the number of tests during training sessionswas so high, we did not record the total number of additional8-of-8 tests required when Chaser made errors during training,nor the individual errors made. Instead, we adjusted our dailytraining sessions to ensure the learning criterion was met foreach nounobject pair to encourage over-learning. We separately

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Formal 20-Item Random Blind Tests per Month

Fig. 3. This gure displays the minimum number of tests completed involving 8 or20 names over the months of training. Note the logarithmic scale on the y-axis. Thecurveswith opensymbolsdepictlocal tests carried outin thepresence of thetrainer.Thecurvewith lled trianglesdepicts formaltestscarriedout when thetrainer was

not present.

assessed nounobject mapping with formal tests outside of thetraining context, described below.

2.1.7. Monthly local tests during training Once Chaser was 5 months old, we gave monthly local tests of

Chasers retention for all the proper nouns that she had previouslylearned. These tests also served to provide additional rehearsal of all nounobject mappings she had learned, distributed through-out her training. Each object was randomly placed upon the oorin sets of 20 items. We asked Chaser to retrieve each item insuccession by its unique name. Objects were not replaced afterretrieval, reducing test time and enabling immediate correction of error responses. As Chaser learned more and more names in eachsucceeding month, it was necessary later in training to space outthe monthly rehearsal and testing over a 2-week period. Inter-spersed between the retrieval of toys, we asked Chaser to ndor go to stationary objects, humans, cats, dogs, and locations. Weinserted interim play between sets of 20 trials in order to main-taininterest. Because these procedures producedselectionwithoutreplacement, we replaced the binomial test with the hypergeo-metric test, designed for this purpose. As with all local tests, these20-item local selectiontestsoccurred in thepresence of thetrainer.Fig.3 depicts the number of 20-item local selectiontests carried outeach month as training progressed. For example, once Chaser hadlearned 1000 nouns, 50 separate 20-item tests were required eachmonth to assess and rehearse her learning.

2.1.8. Formal monthly 20-item random blind testsAllof thedaily and monthly localtestsdescribedabove were car-

ried out during training in the presence of the trainer. Therefore,they maynot have hadthe rigorous experimentalcontrolto ensurethat Chaser selected objects exclusively on the basis of the verbal-ized name of the object. Even though multiple trainers providedtraining, it is possible that Chaser learned to attend to subtle visualor social cues that guided her behavior. Therefore, we carried outformal monthlytests of Chasers long-termretention in a controlledsituationin whichthe trainer and Chaser were outof visualcontact

in separate rooms. Chaser could not see the trainer, and the trainercould not see the location of the objects nor Chaser as she madeher selection, until Chaser returned to the trainer with the selectedobject in her mouth and placed it into the empty Tupperware tub.

Every month,a randomsample of 100 objects wasselectedfromthe total number of objects that Chaser had learned at that date(i.e. successfully met our learning criterion). These objects wererandomly divided into ve groups of 20 items, and ve lists of thenamesof the 20 items were created,eachin randomorder. Each 20-item group was dumped from a Tupperware container in randomorder on the oor of a distal bedroom. The 20 objects were thendispersed by movingthe objects on the topto an outside perimeteruntil two concentric circles were formed, so that all objects werevisible and did not overlap. The order of objects on the oor was

always random and was not correlated with the order of the nameon the list. Each trial began with Chaser standingbeside the trainerin one room (the living room), and Chaser was asked to retrievethe objects on the list from the bedroom, one at a time withoutreplacement. Chaser would enter the bedroom, select the object,and returnto the livingroom to place the objectin the Tupperwaretub. Trials were considered correct if the name of the object (aswritten on the object) matched the name on the list. Trials wereconsidered errorsif the namesdid notmatch. Each of the 20 objectswas retrieved in this fashion without replacement. Once this 20-item test was completed and the results recorded, the remaining20-item tests were carried out using the same procedure. The timebetween these tests varied considerably eachmonth, fromminutestohours, inorderto maintainChasers interest, eatmeals, or engage

in other activities.


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Fig. 4. This gure shows how the theoretical probability of making correct choicesdue to random chance, when selection without replacement is used, depends uponthe total number of objects learned at that point.

Because these 20-item tests were carried out without replace-ment, hypergeometric tests (not binomial tests) were used to

calculate the probability of selecting objects correctly from theavailable options. With hypergeometric distributions the proba-bility of making, say, 18 of 20 correct choices due to randomchance depends upon the total number of objects learned at thatpoint (equivalently, the number of objects from which the 20 wereselected). Fig. 4 shows how this probability changed as the numberof learned objects changed. Over the 3-year duration of this exper-iment, these formal monthly 20-item random blind tests werecompleted145 times, eachyieldingindependent randomized mea-sures of Chasers retention of noun/object mapping.

2.1.9. Formal public double-blind demonstrationAt the endof 3 years of trainingand several months after the for-

mal tests above had terminated, we set up a public demonstration

of Chasers acquired vocabulary in a college auditorium. Althoughit was not as comprehensive as the formal tests described above,it was designed as a formal controlled, double-blind test in frontof about 100 psychology students. Out of sight of the trainer, vestudents randomly selected 10 objects (50 total) from the mass of 1022 objects piled on the auditorium oor. The students wrote thenames of each objecton paper andprovidedeach list to the trainer.The 10 objects in each group were placed upon the oor in randomorder by the students behind and out of sight of the trainer. Chaserwas then requested to retrieve each of the 50 objects in the orderprovided by the students lists, in 5 consecutive sets of 10 objects.The audience determined the accuracy of Chasers selection, andthe entire demonstration was videotaped.

2.2. Results and discussion

2.2.1. Number of words learnedOverthe course of training,Chaser learned all 1022name/object

pairs, correctly completing our local 8-of-8 binomial tests( p =5.96E 8) for every word. Recall that when errors were made,Chaser received additional training until she again completed ourlearning criterion. This over-learningprocedure required Chaser torepeatthe local 8-of-8binomialtests several times forsome namesover the course of training, and it helped ensure that names werenot forgotten as new names were learned. Fig. 2 shows the num-ber of words learned over the months of training. It is importantto recognize that the linear learning rate depicted was producedby our procedure of training only one or two words per day, and it

does not reect innate cognitive abilities or limitations that Chaser

might have. Chaser may have been able to learn at a faster rate, butour procedure did not provide the opportunity. We observed nodecrease inspeedof learningoverthecourse of training, even as shelearned over 1000 words. Therefore, we are not able to extrapolatehow long the linear learning rate would continue, nor the maxi-mum number of words she would be able to learn. We stoppedtraining after 3 years, not because Chaser had reached some cogni-tive limit, but because we could no longer invest the 45h per daytraining her.

2.2.2. Local monthly 20-item test resultsFor 32 consecutive months, Chaser was tested for her reten-

tion of all the names of objects that she had previously learned.As her vocabulary grew, the number of 20-item tests given permonth increased from1 inthe rst month to51 inthe 32ndmonth,resultingin a total of 838independenttests. Recall that each objectwas randomly placed upon the oor in sets of 20 items. We askedChaser to retrieve each item in succession by its unique name,and objects were not replaced after retrieval. Because these 20-item tests were carried out without replacement, hypergeometrictests (not binomial tests) were used to calculate the probability of selecting objects correctly from the available options. With hyper-geometric distributions the probability of making, say, 18 of 20correct choices dueto randomchance depends upon the total num-ber of objects learned at that point (equivalently, the number of objects from which the 20 were selected). Fig. 4 shows how thisprobability changed as the number of learned objects changed.During all 838 independent tests, Chaser successfully recalled thenames of 18, 19, or 20 objects out of the many sets of 20 items that is, in no test did Chaser fail to recall the name of at least 18items correctly in each set of 20 items. The probability of selecting18 of 20 correctly due to random chance asymptotes at a max-imum value of p = .00016; 19 of 20 at p =.000016; and 20 of 20at p = .0000008. Therefore, Chasers exceptional accuracy demon-strates that her overall vocabulary size increased to 1022 nouns,rather than new words replacing previously acquired vocabulary.

2.2.3. Formal monthly 20-item random blind test resultsBecause the local 20-item tests described above were carried

out in the presence of the trainer, the formal20-item random blindtests were designed to ensure that the trainer could not have inad-vertently provided visual or social cues that inuenced Chasersselections. Thus, these formal tests controlled for the presence of,and ability to see, the trainer. If accuracy was lower in these teststhan when the trainer was present, then we would be suspicious of a Clever Hans effect.

Recall that these monthly tests randomly selected 100 objectsfrom the entire set of learned objects, and divided them into ve20-itemrandomblind tests. Thus,145 independenttests werecom-pletedover thecourse of training. Duringall 145independent tests,Chaser successfully recalled the names of 18, 19, or 20 objects out

of the many sets of 20 items that is, in no test did Chaser failto recall the name of at least 18 items correctly in each set of 20items. Therefore, these tests duplicated the results of the local 20-item tests. The presence of the trainer did not increase Chasersexceptional accuracy, so there was no Clever Hans effect.

2.2.4. Formal public double-blind demonstration resultsRecall that the double-blind public demonstration tested

Chasers ability to correctly retrieve 50 objects randomly selectedfrom all 1022 objects she had learned. Thus, this test was not ascomprehensive as the tests described above. Nevertheless, Chasersuccessfully retrieved from the ve sets of 10 objects, the follow-ing numbers of objects: 10, 9, 9, 8, 10, resulting in a total of 46 outof 50 or 92% of the total items (hypergeometric test, p < .0000001).

This impressive accuracy was demonstrated in an auditorium with


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about 100 noisy, distracting students, in a double-blind procedurecarried out months after training was terminated.

In summary, in no case was Chaser unable to distinguishbetween verbal labels given as names for her objects. Her learningand retention of more than 1000 proper nouns revealed clear evi-dence of several capacities necessary for learning receptive humanlanguage: the ability to discriminate many nouns phonetically, theability to discriminate many objects visually, a sizable vocabulary,and a sufcient memory system.

3. Experiment 2: testing independence of meaning of nouns and commands

Recall that in the Rico study ( Kaminski et al., 2004 ), Bloom(2004) w as not convinced that words for Rico conveyed reference thatlabels actually referred to objects, independent fromthe mean-ing of an associated command. For example, when Rico was told tofetch sock, did Rico comprehend that the label sock re f erred toa specic object and separately comprehend that the word fetchmeant that he should produce a specic behavior involving thatspecic object? If Rico actually treated the label sock as a com-mand to fetch sock only, then it would not be evidence thathe understood reference . That is, Rico may not have understoodthat the label sock referred to a specic object, independent of a behavior directed toward the sock. Thus, Bloom argued that Ricomight not understand reference atall and might be limited toasso-ciating the word spoken by the owner with a specic behavior (p.1604). If so, then Ricos word learning may have little to do withlanguage learning as exhibited by humans.

In essence, Blooms concern addresses the question as towhether Rico understood that the phrase fetch sock representedtwo independent morphemes that objects are independent inmeaningfrom theactivity requestedthat involvedthat object.Thus,the primary purpose of Experiment 2 was to test whether Chasertreatsthenameof anobject independentin meaning from the givencommand. A secondary purpose of the study was to investigateChasers ability to combine nouns andcommandsanddemonstrateunderstanding of the combination of the two words, even thoughthe command and noun had never been used in combination priorto the testing condition. In 14 independent trials, we randomlypaired one of three commands with one of three different namesof her objects to see whether Chaser would demonstrate indepen-dence of meaning between the nouns and commands.

3.1. Method

3.1.1. Subject and materialsThesubject used in this study,Chaser, was the same as that used

in Experiment 1. We arbitrarily selected three of Chasers objectsthat had been used in Experiment 1, so Chaser had already learnedtheir proper-noun names. All three objects were similarly sized

cloth toys. The object named Lips resembled human lips. Theobject named ABC was a cloth cube with the letters A, B, andC written on its sides. Lamb was a stuffed toy resembling a lamb.Each object had its assigned name written on its side in permanentink.

3.1.2. ProcedureIn order to demonstrate that commandnoun combinations

were indeed two independent units of meaning, Chaser was askedto produce appropriate behaviors when three different commands(take, paw,and nose)were randomlycombined withthreedifferentobjects (Lips, ABC, Lamb) across 14 independent trials in a double-blind procedure. None of the three objects had previously beenpaired with any of the three commands. To guarantee indepen-

dent and random selection of each object and each command, the

Table 1Sequence of trials in Experiment 2 testing independent meanings of three namesand three commands presented in random pairs.

Instruction given: Trial Cumulative probability of success

Command Name Accuracy

Trial 1 Take Lamb Correct (1/9) 1 0.11111Trial 2 Paw Lamb Correct (1/9) 2 0.01235Trial 3 Take Lips Correct (1/9) 3 0.00137Trial 4 Nose Lamb Correct (1/9) 4 0.00015Trial 5 Paw Lips Correct (1/9) 5 0.000017Trial 6 Paw Lamb Correct (1/9) 6 0.000002Trial 7 Nose Lips Correct (1/9) 7 2.09E 07Trial 8 Nose ABC Correct (1/9) 8 2.32E 08Trial 9 Take ABC Correct (1/9) 9 2.58E 09Trial 10 Nose Lamb Correct (1/9) 10 2.87E 10Trial 11 Paw ABC Correct (1/9) 11 3.19E 11Trial 12 Take Lamb Correct (1/9) 12 3.54E 12Trial 13 Paw Lamb Correct (1/9) 13 3.93E 13Trial 14 Take ABC Correct (1/9) 14 4.37E 14

three objects and three commands were each assigned differentnumbers, and a table of random numbers was used to select eachobject and each command for each of the 14 trials. Table 1 showsthe details of each trial.

Testing was carried out in a large carpeted room at WoffordCollege. The three cloth objects were placed upon a soft pad (1.5-m long) positioned about 38cm apart in a row, 1.5-m in front of and parallel to a vertical cloth barrier. The cloth barrier (1-m highand 1.4-m wide) separated the experimenter from the objects, andthe experimenter knelt behind the barrier before giving Chaserinstructions and until the trial was over. Neither Chaser nor theexperimenter could see the other during execution of the choicetask, andthe experimentercouldnot see whichobject was selectednor the actual command carried out. A confederate, seated 4.3mfrom the objects, was able to observe Chasers behavior and sig-nal to the experimenter (by silently waving her hand) that the trialwas over. Neither the experimenter nor the confederate measured

the accuracy of Chasers behavior or the object selected. The con-federate was used only to indicate when the trial was over, sincethe experimenter could not see Chaser during the trial. When eachtrial ended, the experimenter called out Gooddog! (independentof the accuracy of Chasers behavior), thus terminating the trial forChaser and beginning a brief play session with a rubber ball. Playbetween trials was essential in order to maintain interest in theassigned tasks. Following each trial, objects were returned to theiroriginalordinalposition. Fig.5 shows thetestingscenario, includingthe room, the clothbarrier, theexperimenter, Chaser, placementof the three objects, and the location of the confederate.

Fig. 5. This photograph shows the testing scenario used in Experiment 2, includingtheroom,the clothbarrier, theexperimenter,Chaser, placementof thethreeobjects,and the location of the confederate. A video of this procedure is available as online

supplementary material .


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The entire testing session was videotaped with sound record-ing (available as online supplementalmaterial ). Three independentraters were recruited to measure independently the accuracy of each command and the accuracy of each object selected. We readthem a denition of the actions required for each command, butexplainedthat theirjudgment wasnal. Take wasdenedas pick-ing up the object with the mouth. Paw was dened as an actualtouchingof anobject withthe frontpaw or a close sweep of thepawover the object. Nose was dened as actually touching the objectwith the nose without using the mouth or a nose-oriented move-ment very close to the object. [Note:This leniencein dening pawand nose responses resulted from Chasers previous experiencewith abrasive objects, such as trees and pinecones, which encour-aged Chaser to produce paw and nose responses slightly shortof actual touch.] Each rater rst watched the videotape with thesound turned off (so the rater could not know which instructionswere given to Chaser), and recorded which command was actuallyexecuted towards which object. Once all 14 trials were rated, therater then watched each trial again with the sound turned on inorder to assess whether Chasers behavior accurately matched thecommand and object instructed by the experimenter. Inter-raterreliability was assessed.

3.2. Results and Discussions

Inter-rater reliability for the three raters across the 14 trials forboth commands and objects was 100%. All raters judged Chaser tobe 100% accurate across the 14 trials, performing the correct com-mand to the correct objectas instructed. Table 1 displays the ordersof the pairings of the three commands and three nouns, Chasersaccuracy in each trial, and the obtained cumulative probability of correct responses over the 14 trials in the testing session. With thecombination of three commands and three objects, the probabil-ity of success on a given trial was 1/9 = .111, and the probabilityof failure was 8/9 = .889. As indicated in the table, judgments bythe three raters clearly showed that Chaser successfully demon-strated appropriate responses to the correct objects on all 14 trials

(P =(1/9) 14 =4.37E 14).

3.2.1. Independent meanings of commands and nounsThus,Chaserproduced a unique responseorientedto eachobject

depending upon the meaning of the associated command. Sheresponded as though the commands and the proper-noun nameswere independent entities or morphemes. Thus, in effect, Chasertreated phrases like fetch sock as though the sock was a sockand not fetch sock indicting that her nouns referred to objects.Thus, Blooms (2004) concern that Rico may not have understoodthe difference between sock and fetch the sock is ruled out inthis study. These results clearly support the conclusion that Chaserunderstood reference that the verbal noun of an object referred toa particular object with distinct physical features independent of

actions directed toward that object.

3.2.2. Combinatorial understanding Not only did Chaser demonstrate that she ascribed indepen-

dent meanings to commands and names, she also demonstratedthat she could combine the two meanings accurately withoutexplicit training to do so. None of thethree objects hadbeen pairedpreviously with any of the three commands prior to this experi-ment, and she never received differential reinforcement for correctpairings (even during testing). Chaser hadreceived extensive expe-rience with these three commands directed toward other familiarobjects, but she was able to combine these novel commandnounphrases accurately on the rst trial without additional training(perhaps due to response generalization). Thus, after learning the

meanings of the commands and the nouns, Chaser was able, as

do children, to understand the novel combination of two-wordphrases.

4. Experiment 3: learning common nouns

Bloom (2004) noted that Kaminski et al. (2004) did not pro-vide evidence that words for Rico represented categories, as nounsoften do for children: For a child, words are symbols that referto categories and individuals in the external world (p. 1606). Theimportant distinction here is the difference in reference betweena proper noun and a common noun. A proper noun is a symbolthat refers to a particular object, as demonstrated in Experiment1. A common noun, however, is a symbol that refers to a cate-gory of objects. Categorization in animals has been widely studiedin animal cognition in the last few decades, and a few studieshave demonstrated that dogs can form categories. For example,Range et al. (2007) demonstrated that dogs are able to categorizevisual stimuli, treating pictures of dogs differently from picturesof landscapes, and the dogs successfully extended these cate-gories to novel stimuli. Similarly, Heffner (1975) demonstratedthat dogs could form categories of auditory stimuli, differentiat-ing between dog versus non-dog sounds, and extending these

categories to novel stimuli. To our knowledge, no one has demon-strated that dogs can form categories identied by the names of objects.

This study assessed Chasers ability to learn three commonnouns (toy, ball, Frisbee)that represented differentcategoriesof objects. Chaser had already learned the proper-noun name of allof the objects in each category (see Experiment 1). Because objectsin the categories ball and Frisbee had similar shapes (generallyround or disk-shaped, respectively), we predicted that these phys-ical features would form the basis for categorization. Objects in thetoy category, however, differed widely in their physical features,but they were similar in function: Chaser had been allowed to playwith all of the toys, but she was not allowed to play with the manyavailable non-toys in the house that had similar appearance.


The subject, Chaser, and materials used in this study were thesame as those used in Experiment 1.

4.1.1. CategoriesChaser hadalreadylearned theproper-nounnames of allobjects

used in this study.We selected threecategorylabels that theexper-imenters commonly used (whentalking to each other)to representmany of Chasers objects (toy, ball, Frisbee). Chaser had manytoys, many balls, and many Frisbees.

4.1.1.1. Toy. Toy represented a category for all 1022 objects forwhich Chaser had learned a unique name and was allowed to play.Non-toys were physically similar objects that belonged to familymembers with which play wasforbidden or they were outof reach,such as certain cloth animals, balls, dolls, shoes, socks, and a vari-ety of small personal and household objects. These were locatedthroughout the house on shelves, desks, tables, and closets. Thus,the physicalcharacteristics of toysandnon-toyswerenot discrimi-nativefeatures of thecategory, butthe categories diddiffer reliablyin function: Chaser learned that shecouldplay with toys; shecouldnotplay with non-toys. We selected eight non-toys (see Fig. 6) thathad been located within Chasers sight but out of reach. She hadnever been expressly forbidden to touch these particular non-toys,but she had never been given the opportunity to treat them as her

toys.


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Fig. 6. This photograph displaysthe 16 objects (eighttoysand eight non-toys)usedto test Chasers comprehension of the common noun toy in Experiment 3.

4.1.1.2. Ball. Ball represented a category of 116 round, bouncyobjects, which we identied from all of the objects Chaser hadlearned to identify. Thus, ball was a subset of Chasers toys, andChaser had previously learned proper names for all balls and toysin Experiment 1.

4.1.1.3. Frisbee. Frisbee represented a category of 26 objectsthat had disk-like qualities, which we identied from all of theobjects Chaser had learned to identify. Thus, Frisbee was a subsetof Chasers toys, andChaser hadpreviouslylearned proper namesfor all Frisbees and toys in Experiment 1.

Thus, for each of the three categories, the name of each cate-gory was a second or third label for Chasers objects: (1) they wereidentied by the unique proper-noun label; (2) they were all cate-gorized by the common noun toy; and, (3) Balls and Frisbeeswere further categorized with these common-noun labels, while

still being identied as toy andby the unique proper-noun name.

4.2. Procedures

4.2.1. Generalization and discrimination training We taught the common-noun name for an object representing

a particular category using the same methods as those followed inExperiment 1 in teachingproper nouns. For generalizationtrainingof a common noun, we placedupon the oor eight exemplars of theparticular common noun. We asked Chaser to retrieve each of thecommon-noun items successively in response to the given nameof the category (fetch another toy). We replicated this trainingprocedure at least three times, using different exemplars of thecommon nounin eachof thethree replications.Sinceno other items

were available on the oor, errors in selection were unlikely.Following generalization training, we initiated discriminationtraining during which eight non-exemplars of the category wererandomly placed on the oor among eight exemplars that had notbeen used during generalization training (such as eight balls andeight non-balls). Arrangement of the 16 objects formed an innercircle of 8 objects and an outer circle of 8 objects. In both circles,exemplars of the, say, ball and non-ball categories were randomlyplaced in alternating positions in order to inhibit choice of itemsbased on the objects position. We asked Chaser to retrieve eachof the exemplars representing the category (e.g., fetch a ball orfetch another ball). If she retrieved correctly, she was reinforced;if she chose a non-exemplar, she was gently told, No, that is not(name of common noun). This procedure of correcting her wrong

choices immediately with a soft no was effective in that Chasers

selectionof non-exemplarsdecreased. Discrimination trainingcon-tinued until Chaser successfully retrieved eight exemplars of thecommon nouns upon command without retrieving anyof the eightnon-exemplars. Our criteria for the learning for each of the threecategories required eight consecutive demonstrations of both gen-eralization and discrimination without error.

4.2.2. Testing comprehension of common nounsDuring testing,all objects wereplacedin an adjoining room, out-

side of the vision of the tester, to ensure that the tester could notprovide inadvertent cues inuencing the selection of the objects.As in training, arrangement of the eight exemplar and eight non-exemplars of each category formed an inner circle of eight objectsand outer circle of eight objects. In each circle, exemplars and non-exemplars of the category were randomly placed in alternatingpositions to prevent selection of an object based on its position.

Each trial began with Chaser standing beside the trainer in oneroom (the livingroom). Chaser wasasked eight times to retrieve anexemplar from the bedroom, one at a time without replacement(e.g., fetch a ball or fetch another ball). Chaser would enterthe bedroom, select the object, and return to the living room toplace the objectin the Tupperware tub. Trials were considered cor-rect if the object was a member of the set of objects described bythe common noun (an exemplar). Trials were considered errors,if a non-exemplar was selected. Each of the eight exemplars wasretrievedin thisfashionwithoutreplacement, leavingall eight non-exemplars in the bedroom. Videotaped demonstrations of Chasercompleting this procedure for the three categories are availableonline as supplemental material .

Because the number of exemplars available in the toy and ballcategories was high, we were able to test Chasers comprehen-sion of those two commonnouns without using the exemplars andnon-exemplars that had been used during training. With only 26Frisbees, however, we had to use some of the same Frisbees duringboth training and testing.


4.3.1. ToyThe photograph in Fig. 6 displays the 16 objects (eight toys

and eight non-toys) used to test Chasers comprehension of thecommon noun toy. When Chaser was asked to retrieve a toyeight times in succession out of the 16 objects available, she cor-rectly selected a toy without error in every trial (i.e., withoutselecting a non-toy). Hypergeometric tests were used to calculatethe probability of selecting objects correctly without replacementfrom the available options. The probability of making eight cor-rect choices out of the 16 options due to random chance wasP (toy=8)=7.77E 5. Thus, Chaser comprehended the common

noun toy as a label for the toy category, even though Chaser alsoknew each object by its proper-noun name.

4.3.2. BallThe photograph in Fig. 7 displays the 16 objects used to test

Chasers comprehension of the common noun ball. When Chaserwas asked to retrieve a ball eight times in succession out of the16 objects available, she correctly retrieved a ball without error inevery trial. The hypergeometric probability of making eight cor-rect choices out of the 16 options due to random chance wasP (ball = 8)= 7.77E 5. Thus, Chaser comprehended the commonnoun ball as a label for the ball category, even though Chaser alsoknew each object by its proper-noun name and by the commonnoun toy.


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Fig.7. Thisphotograph displaysthe 16 objects (eight ballsand eightnon-balls)usedto test Chasers comprehension of the common noun ball in Experiment 3.

4.3.3. FrisbeeThe photograph in Fig. 8 displays the 16 objects used to test

Chasers comprehension of the common noun Frisbee. WhenChaser was asked to retrieve a Frisbee eight times in succession outof the16 objects available, shecorrectly retrieveda Frisbee withouterror in everytrial. The hypergeometric probability of making eightcorrect choices out of the 16 options due to random chance wasP (Frisbee = 8)= 7.77E 5. Thus, Chaser comprehendedthe commonnoun Frisbee as a label for the Frisbee category, even thoughChaser also knew each object by its proper-noun name and by thecommon noun toy.

4.3.4. Discrimination of categoriesThus, Chaser learned that names of objects may represent cate-

gorieswith many exemplars foreach of the three common nouns,she mapped one label onto many objects. Membership in two of the categories, ball and Frisbee, could be discriminated basedon common physical properties. For example, all balls had simi-lar round shapes, and all Frisbees were shaped like a disk. The toycategory required a more abstract discrimination. The toy andnon-toy objects did not differ in terms of common physical char-acteristics. Thus, discrimination of whether an objectis a toyor notwas unlikely to be based upon identifying physical toy-like fea-

Fig. 8. This photograph displays the 16 objects (eight Frisbees and eight non-Frisbees) used to test Chasers comprehension of the common noun Frisbee in

Experiment 3.

tures. However, Chaser learned that she could play with toys, butshe could not play with non-toys they differed in functionality.While this study was not designed to assess how Chaser createdcategories, it seems likely that the common noun toy reected amore abstract type of categorization than did ball or Frisbee.

4.3.5. Nounobject mapping By forming categories represented by common nouns, Chaser

mapped one label onto many objects. Chaser also demonstratedthat she could map up to three labels onto the same object withouterror. For example, Experiment 1 demonstrated that Chaser knewthe proper-noun names of all objects used in this study. Chaseralso mapped the common noun toy onto these same objects. Heradditional success withthe twocommonnouns ball andFrisbeedemonstrates that she mapped a third label onto these objects.

Bloom (2004) had commented that Kaminski et al. (2004) didnot provide evidence that words for Rico represented categories.Chaser, however, has demonstrated that words may representcategories. Her demonstrations of one-to-many and many-to-onenoun/object mappings extend our understanding of the referentialnature of words in border collies.

5. Experiment 4: learning words by exclusion

Markman and Abelev (2004) asserted that the most strikingnding of Ricos word learning ( Kaminski et al., 2004 ) was Ricosapparent demonstration of learning new words by exclusion inferring the referent of a novel word by logically excluding otherpotential alternatives. Successfully learning new names by exclu-sion consists of two or more separate abilities. The procedureinvolves a choice trial in whichthe subject is provided a novelnameand expected to choose a novel object located among a group of familiar objects that already have associated names. This choicecondition requires inferential reasoning by exclusion, dened asthe selection of the correct alternative by logically excludingother potential alternatives ( Aust et al., 2008 , p. 587). This choice

response cannot be based on associative learning mechanismsbecause the name andobject referent are not presentedtogetherinthis single trial. Aust et al. (2008) demonstrated inferential reason-ing by exclusion in about half of their dogs in a non-verbal visualdiscriminationtask. Similarly, Erdhegyi etal.(2007) demonstratedthat dogs can nd hidden objects using inferential reasoning byexclusion. Following Kaminski et al. (2004) , the current experimentexplores whether the names of novel objects can be learned byexclusion.

Retention of the novel nameobject mapping maybe a separateability beyond that of the original inferential reasoning that ledto the selection of the object in that rst choice trial. Retentionmay depend uponadditional factors, including associative learningsuch as the reinforcing consequencesof selectingthat novel object.

Thus, it is possible that a dogcouldcorrectly selecta novel objectinresponseto a novelname, butbe less able toretain thisnameobjectpairing over minutes, days, or weeks. Nevertheless, Kaminski et al.(2004) claimed that Rico was able to select the novel object andretain this nameobject knowledge over time, comparable to theperformance of 3-year-old toddlers.

Markman and Abelev were unwilling to accept Ricos data ascompelling evidence of word learning by exclusion because theyperceived potential aws in the design: (a) a lack of control forbaseline novelty preference, and (b) the possibility that positivefeedback after the exclusion trial could have confounded the sub-sequent retention test. Fischer et al. (2004) replied that Rico wasasked to retrieve two familiar objects beforebeing asked to retrievethe novel object, revealing that correct retrieval was not due to

preference for novel objects alone. Rico showed clear impulse con-


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trol by retrieving the requested novel object only after retrievingtwo requested familiar objects. We replicated this procedure withChaser, butwe alsomeasuredChasersbaselineprobabilitiesfor thechoice of objects based on novelty alone, prior to the rst oppor-tunity to learn words by exclusion. We also measured Chasersretention of wordobject mapping after several delay intervals.


Thesubject,Chaser, used in this study wasthe same as in Exper-iment 1. The materials consisted of both novel andfamiliar objects.Each of the familiar objects had been used in Experiment 1. Thenovel objects were64 additional childrens toysand stuffed animalsthat had not been used in the previous experiments.

5.2. Procedures

5.2.1. Measuring baseline novelty preferenceInorderto control forchoice of an objectbased on novelty alone,

we placed two novel and eight familiar objects in a separate room.Chaser was asked successively to retrieve each of the eight familiarobjects by its unique name, one at a time, without replacement.If Chaser had a novelty preference over familiar objects, then sheshould retrieve novel objects. We repeated this procedure sevenmore times using different novel and familiar objects in each trial.Novel objects were not given names during this procedure.

5.2.2. Testing inferential reasoning by exclusionTesting procedures for inferential reasoning by exclusion were

similar to those used with Rico ( Kaminski et al., 2004 ). We placeda single novel object among seven familiar objects in an adjacentroom. On the rst two trials, we asked Chaser to retrieve familiartoys by name. On the third trial, we asked her to retrieve the novelitem by its novel name. Upon successful retrieval of the novel item,Chaser was told Good dog.

5.2.3. Testing immediate and delayed name retention

We immediatelyfollowedthis novel choice trial witha retentiontest. This novel item was placed in an adjoining room among threenovel and four familiar objects. Objects were arranged in a circle,randomly alternating the placement of familiar and novel objects.On the rst two trials, we asked Chaser to retrieve familiar toys byname. On thethirdtrial, we asked her to retrieve thenewlylearnednovel item by its new name. We repeated this retention test afteran additional 10-min delay, and again 24-h later. We completedeight replications of this four-part procedure over a period of 8days, using different novel and familiar items in each replication.

5.2.4. Retesting at 5 yearsTwo years later, when Chaser was 5-year old, we retested

Chasers ability for inferential reasoning by exclusion and name

retention by repeating the procedures above with a new group of novel items.


5.3.1. Baseline rates for choice based on novelty aloneWhenwe measuredthe baselinerates forchoiceof objects based

upon novelty alone, Chaser retrieved each of the eight familiaritems in succession without error, ignoring the two novel items, ineach of theeightseparate tests.Thus, herbaselinerate for choosingobjects based on novelty alone was zero.

5.3.2. Inferential reasoning by exclusionEarly in Chasers training, we corrected her immediately if she

retrieved an incorrect object. As the result of this training, if she

did not remember the name of an object, she would simply standamong all the objects without selecting one. Thus, on the rstexclusion learning trial when she did not return with an object,a quick look revealed that she was simply standing among theobjects. Only with repeated encouragements to fetch the newlynamed novel object, did she nally retrieve the novel object.Special encouragement was also necessary on the second replica-tion. On the remaining six replications, she continued to retrievenovel objects successfully without further encouragement or error.Thus, Chaser was successful in eight successive replications, con-ducted over 8 days, in retrieving the novel objects (binomial test,P ( X =8)=5.96E 8).

Recall that Markman and Abelev (2004) were unwilling toaccept Ricos data as compelling evidence of word learning byexclusion because they were concerned about inadequate controlfor baseline novelty preference. Chasers baseline rate of select-ing novel items was zero. In addition, as with Rico, Chaser rstretrieved two familiar objects upon command before the requestto retrieve the novel object, thereby demonstrating impulse con-trol for retrieving novel objects. Therefore, her demonstration of inferential reasoning by exclusion was not due to preference forselecting novel items.

5.3.3. Immediate and delayed name retentionAftereach of theeight testsof inferential reasoningby exclusion,

we tested Chasers retentionimmediately,after a 10-min delay, andafter 24-h delay. Immediately after correctly mapping the novelname to the novel item, the item was randomly placed amongthree other novel objects and four familiar objects. Chaser wasasked to retrieve two familiar items before the request to retrievethe newly named object. In each of the eight replications, Chaserwas successful in retrieving the newly named object when testedimmediately after the exclusion choice response (binomial testwith PROB(success) = 1/6, P ( X =8)=5.95E 7). When tested after10-min delay, shewassuccessful in retrieving veof theeight novelitems (binomialtest, P ( X = 5)= .0042). With 24-h delays, Chaserdis-played little retention of the names of the novel items, successfully

retrieving only one of the eight items (binomial test, P ( X =1)=.37).

5.3.4. Re-testing at 5-year oldApproximately 2 years later when Chasers was 5-year old,

we retested Chasers ability for inferential reasoning by exclu-sion and name retention by repeating the procedures above witha new group of novel items. Again, in each of the eight repli-cations, Chaser correctly chose the novel objects without error(binomialtest, P ( X =8)=5.95E 7). When testedfor memoryreten-tion immediately after the exclusion choice trial, Chaser selectedthe newly named object in six of the eight tests (binomial test,P ( X = 6)= .0004). When tested at the 10-min delay interval, sheselected thenewly named objectin four of theeight tests (binomialtest, P ( X =4)=.026).

A central issue is whether inferential reasoningby exclusioncanlead to word learning in border collies. Chaser demonstrated accu-rate nameobject mapping when tested immediately followingthe exclusion choice trial. However, although statistically signif-icant, retention was reduced when tested at the 10-min delayand essentially non-existent when tested at the 24-h delay. Infor-mal observations indicated that additional pairings with the name(playing with the object) were necessary for retention intervalsof an hour or more. Markman and Abelev (2004) were concernedthat positive feedback given to Rico after the exclusion trial couldhave affected the follow-up retention test 4 weeks later. Chaseralso received minimal positive feedback following correct choices,so we cannot know what role associative learning mechanismscontributed to Chasers successful, but transient, word learning.

It is likely that rehearsal play with the objects would provide


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additional opportunities for associative learning to produce longerretention, but this retention would no longer reect properties of inferential reasoning by exclusion. Working with toddlers, Horstand Samuelson (2005, 2006) and Horst et al. (2006) reportedthat the exclusion choice trial was not sufcient to produce long-term retention without additional rehearsal. Kaminski et al. (2004)reported Ricos successful word learning by exclusion with reten-tion intervals up to 4 weeks intervals comparable to thosesometimes observed in toddlers. However, Chaser did not showsuch retention unless we provided additional rehearsal with theobject during play periods ( Carey and Bartlett, 1978; Golinkoff et al., 1992; Horst et al., 2006; Horst and Samuelson, 2005, 2006;Markman and Wachtel, 1988 ).

6. General discussion

6.1. Experiment 1: investigating the ability of a border collie tolearn proper nouns

Experiment 1 showed that over a 3-year period, Chaser demon-strated impressive abilities to learn the names of proper nouns,and her extensive vocabulary was tested undercarefullycontrolled

conditions. She met our rigorous learning criteria repeatedly for1022 proper nouns. She accumulated and maintained this knowl-edge of nouns over a lengthy 32-month period. In each month,when we tested Chaser on her entire vocabulary, she was ableto respond correctly to more than 95% of the 1022 proper-nounobjects that she had learned. She revealed no difculty in discrim-inating between the many different sounds of the nouns given toher as names for objects. Likewise, she revealed no problems indistinguishing objects visually, despite their similarity and vary-ing positions (often partially occluded) on the oor. Her ability tolearn and remember more than 1000 proper nouns, each mappedto a unique object, revealed clear evidence of several capacitiesnecessary for learning receptive human language: the ability todiscriminate between 1022 different sounds representing names

of objects, the ability to discriminate many objects visually, a siz-able vocabulary, andan extensivememory systemthat allowed themapping of many auditory stimuli to many visual stimuli.

6.2. Experiment 2: testing independence of meaning of nouns andcommands

In Experiment 2 Chaser demonstrated she understood theunique combinations of three commands (take, paw, nose) withthree proper-noun names (Lips, Lamb, ABC), producing the correctbehavior towards the correct objectin each trial. Chaser respondedto each objectdepending upon the meaning of the associated com-mand such that Chaser responded as though the commandsandthe proper-noun names represented independent morphemes.

Chaser understood that names refer to particular objects, indepen-dent of the activity requested involving that object. Thus, Blooms(2004) concern that Rico may not have understood the differencebetween sock and fetch-the-sock is ruled out in this study.

Chaser did not require special training before producing theappropriate response when the three commands (take, paw, nose)were paired with the three names (Lips, Lamb, ABC). BecauseChaser had previous learned the meaning of the three commandsand the names of the three objects, she was able to respond cor-rectly, even on the rst trial, when each of the three commandsand the three nouns were randomly paired together in verbnounphrases demonstrating combinatorial understanding of two-word phrases. Chaser was able to understand the combination of the two words and behave appropriately in each trial, even though

these commands and nouns had never been used in combination.

6.3. Experiment 3: learning common nouns

Experiment 3 showed that Chaser was successful in learn-ing three common nounswords that represent categories. Chaserlearned the names of two concrete categories, balls and Fris-bees. For each of these concrete categories, exemplars of eachcategory displayed physical characteristics that may have helpeddene the category, such as all balls being relatively round.Chaser also successfully learned the more abstract concept toythat seemed to be based on function, rather than shared physi-cal characteristics. Toys were objects for which she had learnedunique proper-noun names. Non-toys were similar objectsbelonging to family members. The physical characteristics of toys and non-toys were not reliable discriminative features of the category instead, toys and non-toys differed in function-ality. Chaser could play with toys; she could not play withnon-toys.

By forming categories represented by common nouns, Chasermapped one label onto many objects. Conversely, Chaser alsodemonstrated that she could map up to three labels onto the sameobject without error. For example, Experiment 1 demonstrated thatChaser knew the proper-noun names of all objects used in thisstudy. Chaser also mapped the common noun toy onto thesesame objects. Her additional success with the two common nounsball and Frisbee demonstrates that she mapped a third labelonto these objects. Her demonstrations of one-to-many and many-to-one noun/object mappings reveal exibility in the referentialnature of words in border collies.

6.4. Experiment 4: learning words by exclusion

Experiment 4 demonstrated that Chaser was able to use infer-ential reasoning by exclusion to map words upon objects, partiallyreplicating Ricos performance ( Kaminski et al., 2004 ). Chaserdemonstrated strong retention of the nameobject mapping whenshe was tested immediately after the exclusion choice trial. How-ever, as indicated earlier, retention of the name of the object was

reduced at the 10-min delay interval and essentially non-existentat the 24-h delay interval. These results support the conclusion of Horst and Samuelson (2005, 2006) that a single exclusion choicetrial with toddlers may not be sufcient to create durable memorytraces of word learning. Rehearsal or play that provides additionalnameobjectpairingsmay be necessaryfor strong long-termreten-tion, as we provided in Experiment 1.

6.5. Referential understanding of nouns

Recognizing that infants and young children often fall short of skilled referential understanding of words, Baldwin (1993) identi-ed twobasicfactorsthat dene a childs referential understandingof wordsthatenable rapid word learning: (a)awareness that words

mayrefer toobjects,and(b)awareness ofsocial cues that enablethemapping of the words upon the referent. Bloom (2004) expandedthe view of referential understanding by pointing out, They [chil-dren] appreciate that the word may refer to a category, andtherebycan be used to request a sock, or point out a sock, or comment onthe absence of one (p. 1605).

Together, our four experiments provide compelling evidencethat Chaser understood that objects have names. She demon-strated sensitivity to our social cues that that enabled her to mapwords upon the referents. To our knowledge, Chaser learned moreproper-noun names (1022) than any other dog, fullling our strictlearning criteria and satisfying 1000s of statistical tests. Chaserdemonstrated the one-to-many nameobject mappings requiredfor common nouns to represent categories. Conversely, she also

understood that an object may have more than one name, such as


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a unique proper-noun name as well as one or more common-nounnames (ball and toy). By successfully responding to randomcombinations of names and commands, she demonstrated herunderstanding that names refer to objects, independent of thebehaviors directed toward those objects. Thus, she understoodthe difference in meaning between names and commands andtreated their combination the same way humans do. Combined,theseexperiments provide clear evidence thatChaser acquired ref-erential understanding of nouns, an ability normally attributed tochildren.

Acknowledgements

We give special thanks to Wayne West, owner of Fleet HillFarms, Pauline, SC, for his breeding of Chaser, wise advice, andopening hisgates forChaser to fulllher passion for herding sheep.Special thanks to Robin Pilley for videotaping and photographingChaser on so many occasions. We thank P. Bloom for his helpfuldiscussions of this work, and Kara Bopp for comments on an earlierdraft. We thank Sally and Robin Pilley for their daily support, nur-ture, and help in training Chaser. We thank two Wofford Collegestudents (Caroline Reid andKatie Grainger) for their work and playwith Chaser.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.beproc.2010.11.007 .

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http://dx.doi.org/10.1016/j.beproc.2010.11.007http://dx.doi.org/10.1007/s10071-007-0123-2http://dx.doi.org/10.1007/s10071-007-0123-2http://dx.doi.org/10.1007/s10071-007-0123-2http://dx.doi.org/10.1016/j.beproc.2010.11.007

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Pilley and Reid 2011