PSYCHOLOGICAL SCIENCE

Research ArticleKNOWLEDGE OF THE ORDINAL POSITION OF LIST ITEMS

IN RHESUS MONKEYS

Shaofu Chen/ Karyl B. Swartz/ and H.S. Terrace^'Columbia University and -Lehman CoUege, City University of New York

Abstraci—What is learned during mastery of a serial task: associa-tions between adjacent and remote items, associations between anitem and its ordinal position, or both? A dear answer to thisquestion is lacking in the literature on human serial memory be-cause it is difficult to control for a "naive" subject's linguisticcompetence and extensive experience with serial tasks. In this arti-cle, we present evidence that rhesus monkeys encode the ordinalpositions of items of an arbitrary list when there is no requirementto do so. First, monkeys learned four nonverbal lists (1-4). eachcontaining four novel items (photographs of natural objects). Themonkeys then learned four 4-item lists that were derived exclusivelyand exhaustively from Lists 1 through 4, one item from each list.On two derived lists, each item's original ordinal position wasmaintained. Those lists were acquired with virtually no errors. Thetwo remaining derived lists, on which the original ordinal positionof each item was changed, were as difficult to learn as novel lists.The immediate acquisition of lists on which ordinal position wasmaintained shows that knowledge of ordinal position can developwithout the benefit of language, extensive list-learning experience,or explicit instruction to encode ordinal information.

More than 100 years of research on serial human memory hasyet to provide a clear answer to a critical question posed byEbbinghaus (1885/1964). While mastering a list, what knowledgeof list items does a subject acquire above and beyond that repre-sented by associations between adjacent and remote items? Speci-ficity theorists assume that subjects master a list by learning associ-ations between successive items (Ebbinghaus, 1885/1964) and.to a lesser degree, between nonadjacent list items (Young. 1961.1968). Ordinal-position theorists assume that subjects learn item-position associations, for example, an association between Item1 and the first position and between Item 2 and the secondposition (Ebenholtz, 1972; Ladd & Woodworth, 1911). Othertheorists have advanced hybrid hypotheses about what a subjectlearns, for example, knowledge of the ordinal positions of itemsin the middle of a list but not of the extreme items (Young,Patterson. & Benson. 1963) and knowledge of ordinal positions ofthe extreme items but not of the middle items (Ebenholtz. 1963).

All theorists of serial learning have drawn upon what is argua-bly the most extensive empirical literature in experimental psy-chology. The size of that literature should not, however, obscurethe fact that virtually every experiment on serial learning hasbeen performed on highly verbal college students. Verbal subjectshave obvious advantages. They can readily grasp spoken andwritten instructions, they can use verbal symbols to encode list

Address correspondence to H.S. Terrace. Department of Psychology,Columbia University. 406 Schermerhorn Hall, New York, NY 10027;e-mail: terrace@columbia.edu.

items, and they can describe the experience of learning a list. Lessobvious are some serious limitations of data on serial learningobtained from verbal subjects. There is no known way to controlfor the influence of two types of serial knowledge that subjectsacquired prior to serving in a formal experiment on list learning;linguistic knowledge (e.g.. a lexicon and a grammar) and abstractknowledge about lists obtained from mastering rote tasks (e.g.,the alphabet, sequences of numbers, and days of the week). Theuse of verbal list items also assumes that subjects acquire verbalassociations, either between adjacent and nonadjacent itemsthemselves, between items and their ordinal positions (e.g.,"first," "fourth,'" "'middle," "second-before-middle"), or both.

Recent research on serial learning by pigeons and monkeysshows that it is feasible to train nonverbal organisms to produceand comprehend "lists" composed of arbitrary items, such ascolors, forms, and photographs of natural objects (D'Amato &Colombo, 1988, 1989, 1990; Straub & Terrace, 1981; Swartz,Chen, & Terrace. 1991; Terrace. 1993). Lists were trained by thesimultaneous chaining paradigm (Terrace, 1984), in which all listitems are displayed simultaneously on each trial (typically ona touch-sensitive video monitor), and subjects are required torespond in a particular order to list items, regardless of theirspatial configuration. The configuration of list items is variedfrom trial to trial to prevent subjects from using a physicallydefined chain of responses to execute the required sequence (aswhen making a telephone call or entering a personal identificationcode at a cash machine). No differential feedback (eitber extero-ceptive or proprioceptive) is provided as the subject responds tosuccessive items. Reward occurs if. and only if. the subject exe-cutes the sequence correctly.

After learning a few lists by the successive-phase method,naive monkeys develop list-learning skills that enable them tobypass the early phases of training, in which items are added oneat a time, and to start with all items present from the beginningof training (Chen. 1993; Swartz et al., 1991). The availability oflist-sophisticated monkeys allowed us to perform a unique testof knowledge of the ordinal positions of list items. This test wasmodeled after one used with human subjects (Ebenholtz, 1963).Groups I and II in Ebenholtz's experiment learned two lists ofnonsense syllables. The first was a 10-item list of novel items;A - * B ^ C ^ D - * E ^ F ^ G ^ H ^ I - ^ J ; t h e second, a10-item list on which half of the items were derived from thefirst list and the remaining items were new. Items derived fromthe first list occupied every other position. For Group I, theordinal positions of those items were maintained on the secondlist (ni -> B -> /i2 -^ D ^ n, ^ E ^ n4 ̂ H ^ n; -> J). FoiGroup II, they were changed (F —> n4 ^ H ^ n, ̂ J ^ n, -iB ^ nj -» D ^ «3).

The derived lists should have been equally difficult if a subject's knowledge of the original list was limited to item-iten

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asociations: Each derived list required the same number of newitem-item associations. However, Group I mastered its derivedlist more rapidly than Group II. Indeed, Group II required asmanj' trials to learn its derived list as a control group needed tolearn a single list. The positive transfer shown by the subjects ofGroup I provides compelling evidence that they acquired knowl-edge of the ordinal positions of list items while learning theirfirst list.

Two features of the derived lists used in the present experi-ment simplify the paradigm that Ebenholtz used to assess knowl-edge of ordinal position. Each derived list was composed exclu-sively of items drawn from previously learned lists. Because allof those lists were trained to the same accuracy criterion, all ofthe items on the derived lists were equally familiar. Also, becauseeach derived list contained only one item from each of the pre-viously learned lists, all previously acquired item-item associa-tions were irrelevant.

Subjects were trained to produce four derived lists. On twoof the derived lists, each item's original ordinal position wasmaintained. On the other two, ordinal positions were changed.Maintained lists could be executed correctly from the start oftraining by using each item's original ordinal position as a basisfor ordering responses. The only way to determine the correctsequence on changed lists was to respond to each item by trialand error. The changed and the maintained lists should havebeen equally difficult to learn if the subjects' knowledge of thefour original lists was limited to immediate (or remote) item-itemassociations. To the extent that our subjects had acquired anyknowledge of each item's original ordinal position, the main-tained lists should have been easier to acquire than thechanged lists.

METHOD

Subjects

Two male rhesus monkeys (Macaca mulatta), Rutherford andFranklin, sers'ed as subjects. They were born in the New YorkState Psychiatric Institute Primate Facility and housed in accor-dance with current guidelines of the National Institutes of Health.Rutherford and Franklin, who were 10 and 9 years old, respec-tively, at the start of the experiment, had participated in twoprevious studies of serial learning (Chen, Swartz, & Terrace,1996; Swartz et al.. 1991). The relevant features of their list-learning histories are described later in this section. Each subject'sweight was monitored daily. In addition to the food they obtainedduring experimental sessions, subjects were fed a mixed diet ofnrimate chow (Prolab 26), high-protein bread, and various fruits.

Apparatus

Subjects were tested in an experimental chamber that wasoused in a BRS/LVE sound-isolated booth (see Swartz et al..^91, for details). The front wall of the experimental chamber

': xommodated the picture tube of a color video monitor and a'< ;ht-sensitive touch frame. A transparent lexan template was1 aced in front of the touch frame to minimize the likelihood of' rors caused by swipes across the screen as subjects moved their

hands from one stimulus to another. The template contained nine4- X 3.4-cm cutouts (in a 3 x 3 matrix) that corresponded to thenine positions at which hst items could appear. Banana-flavoredpellets (190 mg. P.J. Noyes improved formula L) were deliveredfollowing correctly completed trials by a Gerbrands pellet dis-penser.

List items were digitized images of color photographs of natu-ral objects that were obtained from high-quality magazines,books, calendars, and travel slides. Each digitized image was 150X 120 pixels (7.1 cm wide x 5.6 cm high). Photographs (ratherthan colors or geometric forms) were used as list items to providea large set of discriminable stimuli for generating numerous listsof novel items. No assumptions were made as to what the subjectsperceived in the photographs (i.e., natural objects or discrimina-ble collages of colored pixels; Herrnstein, 1985; Wasserman &Bhatt, 1992).

Previous List Learning

In earlier experiments. Rutherford learned to produce 15 four-item lists; Franklin, 20 four-item lists (Chen et al., 1996; Swartzet al.. 1991). The successive-phase method was used to train allbut the two most recently trained lists (Straub & Terrace. 1981).Subjects were initially trained to respond correctly to A, then toA and B, then to A and B and C. and finally to A and B and Cand D. An accuracy criterion was used to determine when subjectswere advanced from one phase to the next: correct completionof at least 75% of the trials during two consecutive sessions. Aftersubjects satisfied the accuracy criterion on the four-item phaseof one list, they began training on the single-item phase of thenext list, and so on. The successive-phase method was ehminatedgradually starting with each subject's ninth list (Chen. 1993; Chenet al., 1996). The present experiment commenced after both sub-jects acquired two novel four-item lists on which all four itemswere present from the start of training.

Overall Procedure

At the start of this experiment, both subjects were tested ontheir retention of the first four lists on which they had beentrained (Swartz et al., 1991). The top panel of Table 1 shows thecontents of those lists. The bottom panel shows the four 4-itemlists that were derived from Lists 1 through 4 by selecting oneitem from each list. Two of the derived hsts maintained theordinal position of each item (Maintained Lists 1 and 2). On theother two. the ordinal position of each item was changed{Changed Lists 1 and 2).

Training parametersSubjects were trained daily in 60-trial sessions during all phases

of this experiment. Each trial was signaled by a 2-s "ready'' signal,a brief flash of the houselight. The duration of each trial was 20s. Trials were separated by an intertrial interval whose durationranged from 5 to 15 s. Each session began with 2 warm-up trials.Performance on those trials was excluded from data analyses.

Subjects could make forward or backward errors as they pro-gressed through a sequence. A forward error is skipping one ormore items of the sequence, for example, responding first to B

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Table 1. Composition of lists

List Composition'

Original lists1 Al -^ Bl -̂ . Cl ^ DI bird -^ flower -* frog -^ shells2 A2 -^ B2 -• C2 -* D2 tree -• weasel -^ dragonfly -^ water3 A3 ^ B3 ^ C3 -» D3 elk -^ rocks -^ leaves -^ person4 A4 -> B4 ^ C4 —» D4 mountain —> fish -* monkey —» tomato

Derived lists'"Maintained 1 A2 ^^ B4 -^ Cl -^ D3 tree -^ fish —> frog -+ personChanged 1 B3 -^ Al -^ D4 -̂ - C2 rocks -^ bird -^ tomato -^ dragonflyChanged 2 DI —» C3 -^ B2 -^ A4 shells -^ leaves -^ weasel —> mountainMaintained 2 A3 —» Bl —» C4 —» D2 elk —> flower —» monkey -^ water

"Alphanumeric symbols define each item's position on its original list. The letter refers to the item's position;the number, to the list on which the item appeared.''Rutherford was trained to produce derived lists in the order Maintained 1. Changed 1. Changed 2. andMaintained 2: Franklin, in the order Maintained 2. Changed 2. Changed 1. and Maintained 1.

when the required sequence is A -^ B, responding to A and thento C when the required sequence is A ^ B ^ C, or respondingto A. to B. and then to D when the required sequence is A —»B —» C ^ D. A backward error is returning to an item the subjectresponded to previously, for example, responding in the sequenceA —» B ^ A. Repetitive responses were considered correct ifthey occurred in the correct order, for example. A ̂ A —> A ^B ^ B ^ B - * B - » C ^ C ^ D . On each trial, brief visual andauditory feedback followed the first correct response to eachphotograph (a 0.3-s presentation of a 0.5-cm-wide red borderthat surrounded the item in question and a 0.3-s presentation of a1200-Hz tone). No feedback was provided after repeat responses.Two banana-flavored pellets were delivered following the correctcompletion of each sequence. An error, or the failure to completea trial within 20 s. terminated the trial and was followed by a 10-to 20-s time-out during which the houselight of the experimentalchamber was extinguished.

List items were presented simultaneously in any of the nineavailable positions. The configuration of list items differed ran-domly from trial to trial to ensure that the list could not beexecuted as a rote motor sequence. There were 3.024 [9!/(9 -4)!] possible configurations of list items on each trial.

Reacquisition of Lists 1 through 4Both subjects were retrained on Lists 1 through 4 until they

satisfied our standard accuracy criterion: executing the requiredsequence correctly on 15% of the trials during each of two succes-sive sessions (Swartz et al.. 1991). Because there were 60 trialsper session, a minimum of 120 trials was needed to satisfy theaccuracy criterion. The sessions in which the criterion was satis-fied are referred to as the criterial sessions.

Training on derived listsAfter the reacquisition of Lists 1 through 4, both subjects were

trained to produce the four derived lists in an ABBA order (cf.Table 1). The first and last derived lists were Maintained 1 or 2;

the intervening lists. Changed 1 or 2. So that possible differencesin list difficulty would be counterbalanced, the order used forFranklin was the reverse of that for Rutherford. Subjects ad-vanced from one derived list to the next after they satisfied theaccuracy criterion.

RESULTS

Acquisition of the two types of derived list differed dramati-cally. Lists on which ordinal position was maintained were ac-quired rapidly and with virtually no errors. Lists on which ordinalposition was changed required extensive training, as much asnovel lists. Also, on changed lists, an item's original ordinal posi-tion was a good predictor of initial errors.

Figure 1 shows the number of trials that the subjects neededto satisfy the accuracy criterion on each derived list and on twonovel lists they mastered prior to the start of this experiment.Both subjects satisfied the accuracy criterion for their first main-tained list in the minimum possible number of trials (120). Ruther-ford satisfied the accuracy criterion for his second maintainedlist in 180 trials: Franklin, in 120 trials.

Not shown in Figure 1 is the virtually flawless performanceat the start of training on maintained lists. Items on those listshad never been juxtaposed. During the first 30 trials of trainingon their first maintained lists. Rutherford responded correctly on28 trials (93%) and Franklin responded correctly on 29 trials(97%). That level of accuracy is not distinguishable from the-highest levels of accuracy obtained during the criterial session •for the reacquisition of the highly familiar original lists (Listf-4; range: .82-.98), t(l) = 0.262,/j > .8.

Both subjects had difficulty learning the changed lists. Rutherford and Franklin needed 599 and 358 trials, respectively, t)satisfy the accuracy criterion on Changed List 1. They neede i1,655 and 938 trials, respectively, on Changed List 2. The mostlikely explanation of the difficulty of Changed List 2 (DI -» C3 - <B2 -* A4), as compared with Changed List 1 (B3 ̂ Al -^ D4 - >

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1800

NOVEL LISTS DERIVED LISTS

RutherfordFranklinAccuracy Criterion

Novel 1 Novel 2 Maintained 1 Changed 1

LISTChanged 2 Maintained 2

Fig. 1. Number of trials needed to satisfy the accuracy criterion on four derived lists and on two novel lists. The novel lists werethe last lists learned by Rutherford and Franklin prior to this experiment (Swartz et al.. 1991). See Table 1 for the order in whichthe derived lists were trained.

C2). is the reversal of the ordinal positions of the first and thelast items (with respect to the original lists). On Changed List 1.the items that were originally first and last occupied the mid-dle positions.

Both changed lists were as difficult as novel lists on which allitems were new (left-hand panel of Fig. 1). A one-way analysisof variance(ANOVA) of the number of trials needed to satisfy'the accuracy criterion on each derived list was significant, f ( l .)̂ = 11.38, p < .05. Post hoc pair-wise comparisons (Tukey

'lonestly significant difference) showed that Changed List 2 washe most difficult list to learn and that Changed List 1 was moreifficult than Maintained Lists 1 and 2. A two-way ANOVA:5ubjects x Lists) of the number of trials needed to satisfy the•curacy criterion on the changed and the novel lists yielded nognificant effects: for subjects. f( l , 1) = 4.38, p > .1; for lists,1. 1) = 0.47, p > .5; interaction f( l , 1) = 1.82. p > .1.The difficulty of Changed List 2 (DI -^ C3 -^ B2 ^ A4)

<• 'Uld have resulted from either or both of the following factors.le ordinal positions of the extreme items of Changed List 2

were reversed with respect to their original positions, and themagnitude of the shift in ordinal position was twice as great onChanged List 2 as on Changed List f. On Changed List 1. thesum of the absolute values of tbe number of steps that eachitem was shifted is 4. On Changed List 2, it is 8. The relativecontributions of the two factors could not be evaluated in this ex-periment.

The first item selected on both maintained lists and on thefirst changed list was readily predicted from the original ordinalpositions. Figure 2 (top row) shows the probability with whicheach subject's first response was to Items A, B. C, and D duringthe first 30 trials of training on each maintained list. On theirfirst maintained lists, Rutherford and Franklin chose A on 93%and 100% of the first 30 trials, respectively. Because the chanceprobability of selecting a particular item first is .25, Rutherford'sand Franklin's initial choices on their first changed lists werehighly significant. f(l) = 35.7,/) < .001. On their second main-tained lists, they chose A on 100% and 97% of the first 30 tri-als, respectively.

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QQ<QQOCCQ.

MAINTAINED LISTS

SECOND

I RUTHERFORD

^ FRANKLIN

.„ ChancePerformance

NOVEL LISTS

C D

POSITION OF ITEM ON DERIVED LIST

Fig. 2. Initial responses on each derived list. The graphs show the probability with which each item, as defined by its ordinal positionon each derived list, was selected as the initial response for that list. The top panels show the probabilities for each subject on hisfirst and second maintained lists. The bottom left and middle panels show the probabilities for each subject on his first and secondchanged lists. (See Table 1 for the composition of the changed lists and the order in which they were trained.) Responses to Alare counted as responses to Item B on Changed List 1 because the required sequence on Changed List 1 was B3 ^ Al -* D4 -•C2. On Changed List 2, responses to A4 are counted as responses to D because the required sequence was DI -^ C3 —^ B2 —» A4.For comparison purposes, performance on two novel lists, which were trained prior to this experiment, is shown in the bottomright-hand panel. All data are from the first 30 trials on each list. See the text for additional detaOs.

The first item selected by each subject on his first changedlist was also readily predicted from the original ordinal positions.That result is not surprising with respect to each subject's initialresponse on his first changed list. Logically, he should have re-sponded first to the item that was the first item on the relevantoriginal list (cf. Table 2). What was surprising was the persistencewith which each subject continued to rely on the original ordinalposition of the initial item on the first changed list. On his firstchanged list (B3 -^ Al -* D4 -• C2), Rutherford responded firstto Al (rather than to B3) on 80% of the first 30 trials (see Fig.2, bottom row). Franklin responded to A4 (rather than to DI)on 77% of the first 30 trials on his first changed list (DI -^ C3 ^B2 —> A4). Indeed, neither subject responded correctly to thefirst item of his first changed list on any of the first 30 trials oftraining (Fig. 2, lower left-hand panel).

Both subjects relied less on the first item's original ordinal

position on their second changed lists. As can be seen in Figure2 (bottom middle panel), Rutherford responded first to A4(rather than DI) on only 37% of the first 30 trials of his secondchanged list (DI ^ C3 ^ B2 ^ A4): Franklin responded firstto Al (rather than to B3) on only 23% of the first 30 trials of hissecond changed list (B3 -* Al -^ D4 -̂ > C2). These choices didnot differ significantly from those predicted by chance, f(l) =0.7, p > .6. Similarly, each subject's initial choices on novel listdid not differ from those predicted by chance (Fig. 2. lower righthand panel). On novel lists, however, the relative frequency o'initially selecting any item also did not differ from that predicted iby chance, ^•'(7, A' = 170) = 5.5, p > .8.

Not shown in Figure 2 is the poor predictive value of an item >original ordinal position with respect to accuracy of respondin:to the second, third, and fourth items on changed lists. In th:few instances in which a subject responded correctly to the fir; t

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item on a changed list, the choice of the next item varied randomlyamong the remaining three items. By contrast, Rutherford andFranklin relied almost exclusively on an item's original ordinalposition when responding to the second, third, and fourth itemson their first maintained lists. During the first 30 trials, the rangeof the probability of responding correctly to those items was .93to 1.0.

The pattern of errors on derived lists suggests that subjectsadopted a hierarchy of strategies to execute those lists. The domi-nant strategy was to rely on knowledge of each item's originalordinal position (Fig. 2, top row). When that strategy failed, asit did on Changed Lists 1 and 2 (Fig. 2, bottom row), subjectsreverted to the tdal-and-error strategy they used to determinean item's ordinal position on novel lists on which all items werepresent from the start of training (Chen, 1993; Chen et al., 1996).This shift in strategy occurred more rapidly on the secondchanged list than on the first (Fig. 2, middle panel of bottomrow). However, both subjects relied on their knowledge of anitem's original ordinal position at the start of training on theirsecond maintained lists. Early confirmation that such knowledgewas relevant resulted in the nearly perfect selection of the firstitem (Fig. 2, middle panel of top row).

DISCUSSION

This experiment provides clear evidence that monkeys acquireknowledge of the ordinal positions of list items without the benefitof language, extensive experience with serial tasks, or any explicitrequirement to encode ordinal information. Knowledge of theordinal positions of list items is also significant because chainingtheory cannot account for that knowledge.

Implications for Chaining Theory

None of the items on the derived lists on which our monkeyswere trained were associated with each other previously. Accord-ingly, chaining theory cannot predict that maintained andchanged lists would differ in difficulty, that maintained lists wouldbe acquired with virtually no errors, and that changed lists wouldbe as difficult to master as novel lists.

Chaining theory can predict positive transfer with respect tothe first item of a maintained list by postulating a forward associa-tion between the start of a trial and the first item. To predicttransfer with respect to the last item, a chaining theorist wouldhave to assume a backward association between the last itemand the end of the trial. However, the most generous estimatea chaining theorist can make of the chance probability of re-sponding correctly to both interior items is ,25 (,5 X ,5), Theessentially errorless selection of the interior items at the start ofraining on maintained lists indicates that both subjects relied on:nowledge of the original ordinal positions of those items.

Representation of Ordinal Position

In the present study, knowledge of ordinal position presup-;>ses a between-hst reference system because each derived listJntained only one item from each of the original lists. Thus,ter learning lists Al ^ Bl ^ Cl -^ DI, A2 ^ B2 -^ C2 ^

D2, and so on, a subject had no basis other than a commonreference system to determine the relative positions of, for exam-ple. C2 and Bl. or Cl and B2,

How a nonverbal organism encodes knowledge of an item'sordinal position has yet to be determined. In the absence ofany evidence that monkeys use numerals to represent ordinalposition, a spatial representation of ordinality is the most likelycandidate. The traditional method of loci (Yates, 1966). whichassumes that list items are coded with respect to a spatial map,provides a plausible match between an animal's nonverbal cogni-tive abilities and its ability to represent knowledge of an item'sordinal position (Bower, 1971), The spatial abilities of animals(Gallistel, 1990: Menzel, 1991; Olton, 1978) and evidence thatmonkeys rely on linear representations of lists they execute(D'Amato & Colombo, 1988; Terrace, 1993) also support thehypothesis that monkeys may use a spatial code to define anitem's ordinal position.

Earlier Stndies of Knowledge of Ordinal Position

The present evidence of an animal's knowledge of ordinalposition differs from that obtained in earlier experiments byvirtue of the length of the list that subjects were required tolearn. With but one exception (D'Amato & Colombo, 1989). listlength in the previous studies did not exceed three items (Roit-blat, Bever, Helweg. & Harley, 1991; Roitblat, Scopatz, & Bever.1987; Terrace, 1986a, 1986b), On a three-item list, knowledge ofthe ordinal position of the salient start and end items could beencoded by associations with trial onset and trial offset, respec-tively. The ordinal position of the single interior item could thenbe determined by default (Terrace, Chen, & Newman. 1995),Accordingly, evidence of knowledge of ordinal position of listitems obtained from performance on three-item lists is equivocal.

D'Amato and Colombo (1989) assessed knowledge of ordinalposition by a wild-card test that was administered to monkeysafter they mastered a five-item list. On approximately half thetrials, one of the five original items was replaced by a wild card(W), Subjects could earn a reward by responding to the wild cardin the position in which the original item was omitted (W —» B ^C^D-*E, A^W^C^D-^E, A^B-H.W^D^E,A-^B^C-^W^E, A^B-»C^D^W), Monkeysrapidly learned to execute each of the five types of wild-card listsat a uniformly high level of accuracy that substantially exceededthe level predicted by chance.

Chaining theory cannot explain such performance because itwould have to make the implausible assumption that a monkeyrapidly formed seven new associations between the old items andthe wild card (W-B, A-W, W-C, B-W, W-D. C-W, and W-E),Chaining theory would also predict that trials on which the wildcard replaces a start or an end item (e.g., W - ^ B - ^ C ^ D ^E) should be easier than trials on which the wild card replacesan interior item (e,g,, A -* W -^ C —» D ^ E), On the lattertype of trial, subjects would have to master two new associations(rather than one). D'Amato and Colombo concluded that theirmonkeys relied on knowledge of the ordinal position of the miss-ing item because they responded at a uniformly high level ofaccuracy on each type of wild-card trial, A simpler explanationis that D'Amato and Colombo's monkeys recognized gaps in awell-learned sequence and that they filled those gaps with the

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wild card. No evidence of knowledge of the ordinal position ofgaps was provided.

The Comparative Psychology of Serial Leaming

The ease with which our subjects acquired knowledge of theordinal positions of list items suggests that the monkeys' perfor-mance on maintained lists underestimates, to a considerable de-gree, a monkey's potential for encoding ordinal information. Inthe absence of similar research on human infants, there is nobasis for comparing Rutherford and Franklin's serial skills tothose of human subjects. The practical difficulties of conductingextensive experiments on infants pose a formidable obstacle toinvestigating the development of seriation in human subjects. Bycontrast, it is feasible to investigate the development of cognitiveskills that monkeys use to learn arbitrary sequences. Those skillsare clearly necessary for learning any language. As shown by thepresent experiment, however, they are phylogenetically mucholder than language.

Monkeys who can learn arbitrary hsts without special trainingalso provide a basis for the comparative investigation of otherimportant questions about serially organized behavior, for exam-ple, the development of serial order in free recall (Mandler &Dean, 1969), the temporal structure of serial information (Broad-bent. 1975; Wickelgren, 1964), the serial position effect (Lewan-dowsky & Murdock. 1989), and the neural control of seriallyorganized behavior (e,g,, Nissen. Willingham. & Hartman. 1989).As shown by research in other areas of animal cognition (e.g,,Blough. 1984: Sands & Wright, 1980; Wright, Santiago, Sands, &Urcuioli, 1984), investigators of human cognition have much togain by studying cognitive processes under the well-controlledconditions of the animal laboratory.

Ackoowledgments—This research was supported by grants from theNational Institute of Mental Health (MH 40462), the National Insti-tutes of Health (NIGMS 8022.5), and the Whitehall Foundation, Thedata reported in this article are from Experiment 2 of a doctoraldissertation (Chen, 1993). We thank M, Colombo. J. Hochberg, G,Musen. and W, Wickelgren tor constructive comments about a previ-ous draft of this article.

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