Memorization of Serial Items by Japanese Monkeys, a Chimpanzee, and Humans

© Japanese Psychological Association 1997.

Simultaneous chaining, which was developedby Terrace (1983), is a commonly used experi-mental paradigm for investigating the serial-learning capacity of animals and humans. Forexample, Terrace (1987, 1991) and Terrace andChen (1991a, 1991b) used simultaneous chain-ing to determine that pigeons may form chunks.In a study by D’Amato and Colombo (1988),five tufted capuchins learned either a four-itemlist (A→B→C→D) or a five-item list (A→B→C→D→E). After the training was complete,the subjects were presented two- or three-itemsubset lists extracted from the learned lists. The subjects tended to select the items in thesesubset lists in the order of A→B→C→D or

A→B→C→D→E. An analysis of their reactiontimes revealed that the later the position in theoriginal list that the initial item of the subsetlist occupied, the longer it took to respond tothat item, and that as the number of items in-creased that were between the initial item andthe second item in the subset lists, the reactiontime to that second item also increased. D’Amatoand Colombo (1989) then found that the sub-jects’ performance remained stable when oneor two items were replaced with wild card(s),suggesting that the subjects did not select itemsbased on any association between adjacentitems, but did form representations as to whatitems occupied the positions in the lists.

Japanese Psychological Research1997, Volume 39, No. 3, 236–252Special Issue: Cognition and behavior of chimpanzees

Memorization of serial items by Japanesemonkeys, a chimpanzee, and humans1

NOBUAKI OHSHIBA2

Faculty of Human Sciences, Osaka University, Yamadaoka, Suita 565, Japan

Abstract: The memorization of serial items was investigated with three Japanese monkeys, achimpanzee, and six humans. The subjects were trained to touch two to four different-sizedcircles in an order determined by the experimenter. Two types of list were presented: monotonicand nonmonotonic. The subjects performed better with monotonic than with nonmonotoniclists, both in terms of the percentage of correct trials and reaction times. The subjects’ searchstrategies were classified as either a serial search or a collective search. The chimpanzee moreoften used a collective search than a serial search, compared with the Japanese monkeys. Thistendency was also prevalent with the human subjects. Therefore, a collective search might bean evolutionarily new cognitive function. A serial search model based on evaluation andmotion is proposed, and this could well explain the subjects’ reaction times.

Key words: list learning, simultaneous chain, monotonicity, serial search, species differences.

1 This research was supported by the Cooperation Research Program of the Primate Research Institute, KyotoUniversity, from 1994 to 1995, and was conducted at the Primate Research Institute, Kyoto University.2 I am grateful to Professor Tetsuro Matsuzawa, Associate Professor Kazuo Fujita, and other members of the facultyand staff of the Primate Research Institute, Kyoto University, for their generous help. I thank Professors NaosukeItoigawa and Tetsuhiro Minami of the Faculty of Human Sciences, Osaka University, for their constructive instruction.Thanks are also due to anonymous referees.

© 1997 Japanese Psychological Association. Published by Blackwell Publishers Ltd, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.

Still photographs as well as geometric formshave been used as stimuli. In a study by Swartz,Chen, and Terrace (1991), two rhesus monkeyslearned eight 4-item lists consisting of picturesof birds, flowers, frogs, and so on. The reactiontimes to the subset lists implied that the sub-jects formed linear representations of the lists.Tomonaga, Matsuzawa, and Itakura (1993)used a modified version of a simultaneouschain, where the selected items disappeared, tohave a chimpanzee learn a list consisting ofnumerals. An analysis of the reaction times re-vealed a symbolic distance effect: the greaterthe difference between the two numerals pre-sented, the shorter the reaction time the sub-ject needed to judge which was smaller thanthe other.

The simultaneous chain paradigm has alsobeen applied to human subjects. In a study byTerrace and McGonigle (1994), 5- and 7-year-old children learned three types of simultan-eous chain. One was an arbitrary list consistingof five colored rectangles (white→red→green→yellow→pink). The remaining two lists con-sisted of five different-sized rectangles (labeled,from small to large, 1, 2, 3, 4, and 5); one was amonotonic list (5→4→3→2→1), and the otherwas a nonmonotonic list (3→2→5→1→4). Boththe 5- and 7-year-old children showed poorerperformance for the nonmonotonic list than foreither the arbitrary list or a monotonic list. The7-year-old children made significantly fewererrors with the monotonic list than with thearbitrary list. However, there was no significantdifference in the performance of the 5-year-oldchildren between the number of errors for themonotonic list and that for the arbitrary list.Based on these findings, the authors concludedthat humans need to be 7 or more years old touse a selection strategy exploiting a “rule,”which in this case was the monotonicity amongthe sizes of the stimulus items.

As stated above, there have been variousstudies using different species (pigeons, monkeys,chimpanzees, humans, etc.) and different typesof lists (e.g., monotonic and nonmono-tonic). However, an integrated investigationfocusing on these factors has not yet beenperformed.

Therefore, three comparisons were made in the present study. The first involved thedifference in performance between the originalsimultaneous chain invented by Terrace (1983)and the modified version used by Tomonaga et al. (1993). In the original version of thesimultaneous chain, the selected stimuli remainthroughout a trial. In the version of Tomonagaet al., the selected stimuli are removed, ordisappear. The present study examined howthis difference in procedure affected subjects’performance. The second comparison involvedthe difference in performance between a mono-tonic list and a nonmonotonic list. Hulse (1978)found for rats and Terrace et al. (1994) foundfor 7-year-old human children that perform-ance with monotonic lists is better than thatwith nonmonotonic lists. The present studyinvestigated whether or not this finding alsoapplies to Japanese monkeys and chimpanzees.The third comparison involved inter-speciesdifferences in list-learning capacity amongJapanese monkeys, a chimpanzee, and humans.Although there have been separate studies of different species, there is no previousintegrated study directly comparing the list-learning capacities of several primate species.Clarifying the differences among these closelyrelated species might contribute to our under-standing of cognitive evolution.

Method

SubjectsThree Japanese monkeys (Macaca fuscata),Atsushi, Kyoko, and Yusaku, a female chim-panzee (Pan troglodytes), Pan, and six humans(Homo sapiens) served as subjects.

All the monkey subjects were laboratory-born, and had had no experience of learningsequences experimentally. They were housedin individual cages with ad-libitum access towater. Daily food (biscuits and sweet potatoes)was delivered after each experimental session.The daily rations were sufficient to maintainthe subjects at more than 90% of their free-feeding body weights. Atsushi and Yusaku were4-year-old males at the start of the experiment,and Kyoko was a 6-year-old female.

Memorization of serial items by primates 237


The chimpanzee subject, Pan, had had extens-ive experience with experiments on perceptualand cognitive abilities before the present experi-ment (e.g., Kojima, 1989; Tanaka, 1995, 1996).She lived in an open enclosure with other chim-panzees. Daily food (biscuits, apples, bananas,sweet potatoes, etc.) was delivered at noon andin the evening. No restrictions on food or waterwere imposed. She was 11 years old at the startof the experiment.

The monkeys and the chimpanzee werehandled strictly following the guidelines of thePrimate Research Institute, Kyoto University.

All the human subjects were graduate stu-dents at the Primate Research Institute ofKyoto University. Five were males and one wasa female. They ranged in age from 24 to 29years. They were divided into two groups ofthree, and labeled A1, A2, A3, B1, B2, and B3.The author was not one of the subjects.

ApparatusFor the monkeys. The chamber in which themonkeys worked measured 75 × 75 × 75 cm.The front panel housed a 14-inch color monitor(640 × 400 pixels) on which the stimuli ap-peared. The monitor supported a touch-screensystem. Small pieces of an apple or sweet potatoserved as reinforcers, and were delivered to afood cup located on the left of the panel. Theexperimental room was illuminated by a whitebulb. Extraneous sounds were masked by whitenoise. Displaying of stimuli, delivery of foodrewards, illumination, and recording of datawere all controlled by a personal computer(NEC PC9801RA).

For the chimpanzee and the humans. Theroom in which the chimpanzee and the humansworked measured 167 × 210 × 180 cm. Thefront panel housed a 20-inch color monitor(640 × 400 pixels) on which stimuli appeared.The monitor again had a touch-screen system.Small pieces of an apple served as reinforcersand were delivered to a food tray beneath themonitor. The experimental room was illumin-ated by fluorescent lights. Displaying of stimuli,delivery of food rewards, and recording of datawere all controlled by a personal computer(NEC PC9801 Xn).

StimuliThe same stimuli were used for all species. Thestimuli were four different-sized circles labeled1, 2, 3, and 4, and were 18, 36, 54, and 72 pixelsin diameter, respectively. The colors of thecircles varied according to the subjects and thetasks. For the disappearing-type simultaneouschain, the monotonic list was 1→2→3→4, andthe nonmonotonic list was 3→1→4→2. For theremaining-type simultaneous chain, the mono-tonic list was 4→3→2→1, and the nonmono-tonic list was 2→4→1→3. For half the humansubjects, the order of the items was reversed.The lists were designated A→B→C→D,respectively.

ProcedureGeneral. Almost identical procedures wereused for all the species. The disappearing-typesimultaneous chain was investigated first, andthe remaining-type simultaneous chain wasexamined next. For either type of simultaneouschain, the subjects were to respond to the stimuliin the order requested by the experimenter.There were two orders for each subject andeach task. These two orders were cued by the color of the stimulus item. If the stimuliwere all one color (say, red), then the subjectswere required to select the items in the order1→2→3→4 or 4→3→2→1 (monotonic lists). Ifthe stimuli were another color (say, cyan), thenthe subjects were required to follow the order3→1→4→2 or 2→4→1→3 (nonmonotonic lists).

When a trial began, a start key (a blackrectangle) appeared at the center of the mon-itor. If the subject touched the start key, itdisappeared and two to four same-colored anddifferent-sized stimuli appeared. There werefour positions on the screen (upper right, upperleft, lower right, and lower left) at which thestimuli could appear, and the configurations ofthe stimuli were randomized and counterbal-anced across trials. The subjects had to selectthe stimuli by touching them in the orderdetermined by the experimenter. If the subjectsuccessfully selected all the stimuli, an electricchime was sounded and a food reward wasprovided. If the subject made an error byselecting a wrong item or failed to respond to

238 N. Ohshiba


any of the items after 10 s, then all the stimulidisappeared, the room light was turned off for5 s (blackout, for monkey subjects only), andthe same trial was repeated (correction).Intertrial intervals (ITI) were 5 s.

Each session consisted of 192 trials: 96 trialswith the monotonic list and 96 trials with thenonmonotonic list.

Training. Training was conducted with themonkeys and the chimpanzee, but not with the humans. The subjects were first trainedwith a two-item list, A→B. After the subjects’performance reached the criterion level of 90%correct trials, item C was added to the tail ofA→B, and the subjects were trained with athree-item list, A→B→C. After the subjects’performance reached the criterion level of 85%correct trials, item D was added to the tail ofA→B→C. The criterion for the four-item list,A→B→C→D, was 80% correct trials. Even ifthe performance for one list reached the cri-terion, training was continued until the perform-ance for the other list satisfied the criterion.Hence, the subjects had the same amount ofexperience with the monotonic and nonmono-tonic lists. Kyoko started with a four-item listfor the disappearing-type simultaneous chain.

Reaction times to subset lists. All the possibletwo-, three-, and four-item subset lists werepresented in each session. Each subset list was tested 120 times. The procedures were thesame as in the training. Reaction times weremeasured in units of 10 ms. For human subjects,only the four-item lists were presented.

Paired t-tests and two-factor ANOVA wereused for statistical analyses. A p value less than0.05 was considered significant.

Results

TrainingGeneral. Acquisition functions for the monkeysand the chimpanzee are shown in Figure 1. Ittook more sessions to acquire the remaining-type simultaneous chain than the disappearing-type chain. Paired t-tests revealed that thepercentage of correct trials for the monotoniclists was significantly better than that for thenonmonotonic lists for all the subjects for the



Figure 1a. For caption see next page.

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Figure 1b. (a) Percentage of correct trials as a function of the number of sessions for disappearing-type simultaneous chains in training. Kyoko was not trained with either two- or three-item lists.(b) Percentage of correct trials as a function of the number of sessions for remaining-typesimultaneous chains in training. M = monotonic; NM = nonmonotonic.

disappearing-type simultaneous chain, but only for Yusaku for the remaining-type chain(Table 1).

Disappearing-type simultaneous chain. Whenthe lists were increased in length, performancewith the nonmonotonic lists deteriorated, where-as that with the monotonic lists did not. Theseresults imply that the subjects regarded themonotonicity of the list items as a “rule” (inthis case, from smaller to larger). Hence, thesubjects could be expected to make a correctresponse regardless of the length of themonotonic lists.

Remaining-type simultaneous chain. The dif-ference in performance between the mono-tonic lists and the nonmonotonic lists was notas remarkable as it was for the disappearing-type simultaneous chain. However, the perform-ance with the four-item monotonic list was betterthan that with its nonmonotonic counterpart.

Reaction times to subset listsGeneral. Figures 2 and 3 present the reactiontimes to each item of all the possible subsetlists. Two-factor analyses of variance – type oflist (monotonic/nonmonotonic) × item – wereconducted for each subject and each task(Table 2). The reaction times with the non-monotonic lists were significantly longer thanthose with the monotonic lists. This implies thatthe nonmonotonic lists were more difficult toperform than the monotonic lists. The inter-action between type of list and item was nottested since the factor item branched under thefactor type of list.

Disappearing-type simultaneous chain. Letus examine the performance with the two-item

subset lists for the disappearing-type simul-taneous chain (Figure 2). The reaction times tothe second or final items were almost uniform.In these trials, the stimulus items disappearedas soon as the subjects touched them. When thefinal item was to be selected, no other item wasavailable. Hence, the subjects did not have toconsider which item to select. Naturally, thiswas also true in the three- and four-item subsetlists. We found that the later the position of anitem in the original list that the initial item ofthe subset list occupied, the longer was thereaction time to that item. For example, in thecase of the first item in the two-item subset lists,the reaction times when B was the first itemwere longer than when A was the first item,and the reaction times when C was the firstitem were even longer. This was presumablybecause the subjects searched for the items inthe order A→B→C.

These results suggest that the subjectsperformed serial searches. The subjects firsttried to locate item A; if they could locate it,then they selected it, otherwise they tried tolocate item B, and so on. This assumptionexplains the prolonged reaction time to item C in the subset list A→C→D and that to itemB in the subset list B→C→D. D’Amato andColombo (1988) found that the reaction timesto the first items in the subset lists increased asthe items occupied later positions in the ori-ginal list, and that the reaction times increasedsubstantially as the number of missing itemswhich separated the current item from theprevious item increased.

Remaining-type simultaneous chain. Indivi-dual differences were found with this type of



Table 1. Significance of difference in the percentage of correct trials between monotonic lists and fornonmonotonic lists during training, by the paired t-test

Subjects

Type of task Atsushi Kyoko Yusaku Pan

Disappearing t(14) = 4.047 t(12) = 3.997 t(16) = 4.675 t(17) = 0.813p , .01 p , .01 p , .01 ns

Remaining t(11) = –.498 t(57) = 1.981 t(54) = 2.633 t(29) = 3.261ns ns p , .05 p , .01

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Figure 2a. Reaction times to the subset lists of disappearing-type simultaneous chains: (a) the monkeys’data.

simultaneous chain (Figure 3). Atsushi re-sponded uniquely, whereas the other animalsshowed similar tendencies. Atsushi’s perform-ance was similar to that for the disappearing-type simultaneous chain. This implies that hewas also performing a serial search for theremaining-type simultaneous chain. For Kyoko,Yusaku, and Pan, the reaction times for thefirst items of the lists were much longer thanthose for the other items. This suggests thatthey touched the stimulus items after they hadidentified all the target items. This strategy wasreferred to as a collective search.

Serial search model. As stated above, thereaction times suggested that the subjects per-formed serial searches with the disappearing-type simultaneous chain. The reaction timescould be composed of two independent pro-cesses: evaluation and motion (Sternberg,1967, 1969a, 1969b).

Evaluation is the process whereby thesubject decides whether the item concerned isthe current target. The duration of evaluationprocesses depends on the difficulty of thestimuli to which the subjects should respond. Ifthe stimuli are easy to evaluate, then it should



Figure 2b. The chimpanzee and human data. M = monotonic; NM = nonmonotonic.

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Figure 3a. Reaction times to the subset lists of remaining-type simultaneous chains. (a) The monkeys’data.



Table 2. Results of two-factor analysis of variance (type of list × item) of the reaction time to the subset lists

Subjects

Type of task Factors df Atsushi Kyoko Yusaku Pan

Disappearing Types of lists (1,6664) 817.062 257.667 489.240 238.481Items (54,6664) 142.784 145.630 149.521 18.510

Remaining Types of lists (1,6664) 177.686 326.335 593.140 738.178Items (54,6664) 147.500 157.589 72.527 105.709

The values of F are displayed. The values of p were all less than .0001. The interactions between type of task and itemwere not tested since the factor item branched under the factor of type of task.

Figure 3b. The chimpanzee and human data. M = monotonic; NM = nonmonotonic.

take a short time to evaluate them, and viceversa. Motion literally refers to moving thehand to touch the target. Motion processesmainly reflect physical factors such as musclemovement. Therefore, the time necessary for asingle motion process should be uniform for agiven subject.

Considering both evaluation and motion pro-cesses, the author has proposed a serial searchmodel. This model assumes that a subject makeszero or more evaluations before it makes onemotion. The subject’s reaction times can beestimated based on the time necessary for asingle evaluation, α, and the time necessary fora single motion, β.

Let us consider a three-item list A→C→D as an example to explain how to calculate theexpected reaction times. When the trial begins,there are three stimulus items displayed on themonitor, labeled I1, I2, and I3. The first targetitem is A. The subject evaluates whether or not I1 is equivalent to A. If I1 is A, then thesubject will touch it. In this case, the subjectmakes one evaluation and one motion. If I1 isnot A, then the subject evaluates whether ornot I2 is A. If I2 is A, then the subject willtouch it. In this case, the subject makes twoevaluations and one motion. If I2 is also not A,then the subject evaluates whether or not I3 isA. If neither I1 nor I2 is A, then I3 should beA, and the subject will touch I3. In this case, thesubject makes three evaluations and onemotion. While it is certain that I1, I2, or I3 is A,exactly which item is A is probabilistic. Sincethe probabilities that I1, I2, or I3 is A are all 1⁄3,the expected reaction time to select A inA→C→D is:

1⁄3 {(α + β) + (2α + β) + (3α + β)} = 2.0α + β

When the subject selects A, it disappears. Nowthere are two stimulus items displayed on themonitor, labeled J1 and J2. The target item isnow B. The subject then evaluates whethereither J1 or J2 is B, but in this example neitheritem is B. Therefore, the subject shifts thetarget item from B to C. The subject evaluateswhether or not J1 is C. If J1 is C, then thesubject will touch it. In this case, the subject

makes three evaluations and one motion. If J1is not C, then the subject evaluates whether ornot J2 is C. If J1 is not C, then J2 should be C,and the subject will touch J2. In this case, thesubject makes four evaluations and one mo-tion. While it is certain that either J1 or J2 is C,exactly which item is C is probabilistic. Sincethe probabilities that J1 or J2 is C are both 1⁄2,the expected reaction time to select C inA→C→D is

1⁄2 {(3α + β) + (4α + β)} = 3.5α + β

When the subject selects C, it disappears. Nowthere is only one stimulus item displayed on themonitor, and the subject has only to touch it. Inthis case, the subject makes no evaluation andone motion. The expected reaction time toselect D in A→C→D is β.

The expected reaction times for any items inany lists can be calculated by the same algo-rithm, as summarized in Table 3. The coeffi-cients of α vary, whereas the coefficients of βare constantly 1. Therefore, the reaction timescan be regarded as a linear function of thenumber of evaluations.

Conformity of the serial search model toexperimental results. Linear regression ana-lyses were performed between the number ofevaluations expected from the serial searchmodel and the actual reaction time (Figure 4).The values of the coefficients and the equationsof the regression lines are summarized in Table 4. For the monkeys, the agreementbetween the results of the serial search modeland the actual reaction times was higher for thedisappearing-type simultaneous chain than forthe remaining-type chain. For the chimpanzee,this conformity was not as high as that for themonkeys for either type of simultaneous chain.

The slope of the regression line reflects how much the reaction times would increase ifthe number of evaluations was increased byone, which is the time necessary for a singleevaluation, α. The intercept of the regressionline denotes the reaction time when the subjectmakes no evaluation, that is, the time necessaryfor a single motion, β. Table 4 shows that thevalues of the slopes varied remarkably while

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those of the intercepts did not. This supportsthe assumption that the time necessary for asingle evaluation may vary while the time ne-cessary for a single motion should be constant.

Discussion

Comparison of disappearing- andremaining-type simultaneous chainsThe disappearing-type simultaneous chain islogically easier to perform than the remaining-type chain. The percentage of correct trialsexpected when a subject randomly selects thestimulus items is 16.7%, 8.3%, 4.2%, and 0.9%for three-item lists of the disappearing-typesimultaneous chain, three-item lists of the re-maining type, four-item lists of the disappear-ing type, and four-item lists of the remainingtype, respectively. Hence, the better perform-ance observed for the disappearing-type sim-ultaneous chain than for the remaining-typechain was expected by chance.

Comparison of monotonic andnonmonotonic listsThe better performance with the monotoniclists than with the nonmonotonic lists suggeststhat the monotonic lists were easier to acquirethan the nonmonotonic lists. Also, shorter

reaction times with the monotonic lists suggeststhat they were easier to memorize than thenonmonotonic lists. Monotonicity among thelist items may have promoted acquisition andimproved the performance. Hulse (1978), usingrats, also found better performance for mono-tonic lists than for nonmonotonic lists. Withmonotonic lists, the subject can resort to pos-sible rules (e.g., from smaller to larger). Thisrule learning might make it easier to acquiremonotonic lists. In contrast, nonmonotonic listsdo not offer such rules. Therefore, the subjectmust learn which item to select before/after theother items.

Comparison of Japanese monkeys, a chimpanzee, and humansA major difference among the subject specieswas found regarding the reaction times for the disappearing-type simultaneous chain. Twotypes of search strategies were inferred: a serialsearch and a collective search. A serial searchis a strategy whereby the subject decides which item to select at each stage. A collectivesearch, on the other hand, is a strategy wherebythe subject identifies all the target items beforeselecting them. The types of search used by the subjects are summarized in Table 5. Themonkeys used a serial search for both the



Table 3. Reaction times expected from the serial search model

Item position in the list

Lists First Second Third Fourth

A→B 1.5α + β βA→C 1.5α + β βA→D 1.5α + β βB→C 3.5α + β βB→D 3.5α + β βC→D 5.5α + β βA→B→C 2.0α + β 1.5α + β βA→B→D 2.0α + β 1.5α + β βA→C→D 2.0α + β 3.5α + β βB→C→D 5.0α + β 1.5α + β βA→B→C→D 2.5α + β 2.0α + β 1.5α + β β

α = time necessary for an evaluation; β = time necessary for a motion.

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©Japanese Psychological A

ssociation 1997.

Figure 4a. Linear correlation between the number of evaluations expected from the serial search model and the actual reaction time for (a) disappearing-type simultaneous chains.

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Figure 4b. Remaining-type simultaneous chains. M = monotonic; NM = non-monotonic.

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ssociation 1997. Table 4. Results of linear regression analyses between the number of evaluations expected from the serial search model and the actual reaction time

Subjects

Type of task Type of list Atsushi Kyoko Yusaku Pan

Disappearing Monotonic (1→2→3→4) r 2 = 0.948y = 180x + 317 r2 = 0.813y = 224x + 467 r2 = 0.703y = 108x + 268 r2 = 0.422y = 69x + 770Nonmonotonic (3→1→4→2) r 2 = 0.915y = 334x + 373 r2 = 0.877y = 346x + 461 r2 = 0.770y = 206x + 268 r2 = 0.601y = 173x + 877

Remaining Monotonic (4→3→2→1) r 2 = 0.916y = 196x + 279 r2 = 0.641y = 132x + 387 r2 = 0.512y = 51x + 321 r2 = 0.376y = 60x + 404Nonmonotonic (2→4→1→3) r 2 = 0.888y = 281x + 251 r2 = 0.822y = 228x + 372 r2 = 0.762y = 137x + 337 r2 = 0.532y = 179x + 431

Upper = coefficient of determination; lower = equation of the regression line.

monotonic and nonmonotonic lists. The chim-panzee also used a serial search for nonmono-tonic lists, but used a collective search formonotonic lists. The humans used a collectivesearch for both the monotonic and nonmono-tonic lists. These results indicate that specieswith higher cognitive abilities tended to use acollective search more often than a serialsearch. Hence, a collective search is presumedto be an evolutionarily new cognitive function.

With the remaining-type simultaneous chain,all the subjects used a collective search for boththe monotonic and nonmonotonic lists.

Serial search modelThis paper has also described a serial searchmodel based on two processes: evaluation andmotion. According to this model, reactiontimes can be regarded as linear functions of thenumber of evaluations. Linear regression ana-lyses of the number of evaluations and theactual reaction time confirmed the feasibility ofthis model, especially for disappearing-typesimultaneous chains for the monkeys and thechimpanzee. The greatest merit of the serialsearch model is that it is possible to representthe difficulty of a list in terms of the time for a single evaluation. If the time required for asingle evaluation is long, then the list is difficult,and vice versa. Therefore, the serial searchmodel may be helpful for investigating howdifficult lists are compared with one another.

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Table 5. Summary of types of searches

Species

Type of task Type of list Japanese monkey Chimpanzee Human

Disappearing Monotonic Serial Collective CollectiveNonmonotonic Serial Serial Collective

Remaining Monotonic Collective Collective CollectiveNonmonotonic Collective Collective Collective

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chunk. Journal of Experimental Psychology:Animal Behavior Processes, 17, 94–106.

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(Received Oct. 17, 1996; accepted March 15, 1997)

252 N. Ohshiba


Memorization of Serial Items by Japanese Monkeys, a Chimpanzee, and Humans

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