Memorization of serial items by Japanese monkeys, a chimpanzee, and humans 1

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1 Japanese Psychological Research 1997, Volume 39, No. 3, Special Issue: Cognition and behavior of chimpanzees Memorization of serial items by Japanese monkeys, a chimpanzee, and humans 1 NOBUAKI OHSHIBA 2 Faculty of Human Sciences, Osaka University, Yamadaoka, Suita 565, Japan Abstract: The memorization of serial items was investigated with three Japanese monkeys, a chimpanzee, and six humans. The subjects were trained to touch two to four different-sized circles in an order determined by the experimenter. Two types of list were presented: monotonic and nonmonotonic. The subjects performed better with monotonic than with nonmonotonic lists, both in terms of the percentage of correct trials and reaction times. The subjects search strategies were classified as either a serial search or a collective search. The chimpanzee more often used a collective search than a serial search, compared with the Japanese monkeys. This tendency was also prevalent with the human subjects. Therefore, a collective search might be an evolutionarily new cognitive function. A serial search model based on evaluation and motion is proposed, and this could well explain the subjects reaction times. Key words: list learning, simultaneous chain, monotonicity, serial search, species differences. Simultaneous chaining, which was developed by Terrace (1983), is a commonly used experimental paradigm for investigating the seriallearning capacity of animals and humans. For example, Terrace (1987, 1991) and Terrace and Chen (1991a, 1991b) used simultaneous chaining to determine that pigeons may form chunks. In a study by D Amato and Colombo (1988), five tufted capuchins learned either a four-item list (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-item subset lists extracted from the learned lists. The subjects tended to select the items in these subset lists in the order of A B C D or A B C D E. An analysis of their reaction times revealed that the later the position in the original list that the initial item of the subset list occupied, the longer it took to respond to that item, and that as the number of items increased that were between the initial item and the second item in the subset lists, the reaction time to that second item also increased. D Amato and Colombo (1989) then found that the subjects performance remained stable when one or two items were replaced with wild card(s), suggesting that the subjects did not select items based on any association between adjacent items, but did form representations as to what items occupied the positions in the lists. 1 This research was supported by the Cooperation Research Program of the Primate Research Institute, Kyoto University, 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 faculty and staff of the Primate Research Institute, Kyoto University, for their generous help. I thank Professors Naosuke Itoigawa and Tetsuhiro Minami of the Faculty of Human Sciences, Osaka University, for their constructive instruction. Thanks are also due to anonymous referees Japanese Psychological Association. Published by Blackwell Publishers Ltd, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.

2 Memorization of serial items by primates 237 Still photographs as well as geometric forms have been used as stimuli. In a study by Swartz, Chen, and Terrace (1991), two rhesus monkeys learned eight 4-item lists consisting of pictures of birds, flowers, frogs, and so on. The reaction times to the subset lists implied that the subjects formed linear representations of the lists. Tomonaga, Matsuzawa, and Itakura (1993) used a modified version of a simultaneous chain, where the selected items disappeared, to have a chimpanzee learn a list consisting of numerals. An analysis of the reaction times revealed a symbolic distance effect: the greater the difference between the two numerals presented, the shorter the reaction time the subject needed to judge which was smaller than the other. The simultaneous chain paradigm has also been applied to human subjects. In a study by Terrace and McGonigle (1994), 5- and 7-yearold children learned three types of simultaneous chain. One was an arbitrary list consisting of five colored rectangles (white red green yellow pink). The remaining two lists consisted of five different-sized rectangles (labeled, from small to large, 1, 2, 3, 4, and 5); one was a monotonic list ( ), and the other was a nonmonotonic list ( ). Both the 5- and 7-year-old children showed poorer performance for the nonmonotonic list than for either the arbitrary list or a monotonic list. The 7-year-old children made significantly fewer errors with the monotonic list than with the arbitrary list. However, there was no significant difference in the performance of the 5-year-old children between the number of errors for the monotonic list and that for the arbitrary list. Based on these findings, the authors concluded that humans need to be 7 or more years old to use a selection strategy exploiting a rule, which in this case was the monotonicity among the sizes of the stimulus items. As stated above, there have been various studies using different species (pigeons, monkeys, chimpanzees, humans, etc.) and different types of lists (e.g., monotonic and nonmonotonic). However, an integrated investigation focusing on these factors has not yet been performed. Therefore, three comparisons were made in the present study. The first involved the difference in performance between the original simultaneous chain invented by Terrace (1983) and the modified version used by Tomonaga et al. (1993). In the original version of the simultaneous chain, the selected stimuli remain throughout a trial. In the version of Tomonaga et al., the selected stimuli are removed, or disappear. The present study examined how this difference in procedure affected subjects performance. The second comparison involved the difference in performance between a monotonic list and a nonmonotonic list. Hulse (1978) found for rats and Terrace et al. (1994) found for 7-year-old human children that performance with monotonic lists is better than that with nonmonotonic lists. The present study investigated whether or not this finding also applies to Japanese monkeys and chimpanzees. The third comparison involved inter-species differences in list-learning capacity among Japanese monkeys, a chimpanzee, and humans. Although there have been separate studies of different species, there is no previous integrated study directly comparing the listlearning capacities of several primate species. Clarifying the differences among these closely related species might contribute to our understanding of cognitive evolution. Method Subjects Three Japanese monkeys (Macaca fuscata), Atsushi, Kyoko, and Yusaku, a female chimpanzee (Pan troglodytes), Pan, and six humans (Homo sapiens) served as subjects. All the monkey subjects were laboratoryborn, and had had no experience of learning sequences experimentally. They were housed in individual cages with ad-libitum access to water. Daily food (biscuits and sweet potatoes) was delivered after each experimental session. The daily rations were sufficient to maintain the subjects at more than 90% of their freefeeding body weights. Atsushi and Yusaku were 4-year-old males at the start of the experiment, and Kyoko was a 6-year-old female.

3 238 N. Ohshiba The chimpanzee subject, Pan, had had extensive experience with experiments on perceptual and cognitive abilities before the present experiment (e.g., Kojima, 1989; Tanaka, 1995, 1996). She lived in an open enclosure with other chimpanzees. Daily food (biscuits, apples, bananas, sweet potatoes, etc.) was delivered at noon and in the evening. No restrictions on food or water were imposed. She was 11 years old at the start of the experiment. The monkeys and the chimpanzee were handled strictly following the guidelines of the Primate Research Institute, Kyoto University. All the human subjects were graduate students at the Primate Research Institute of Kyoto University. Five were males and one was a female. They ranged in age from 24 to 29 years. They were divided into two groups of three, and labeled A1, A2, A3, B1, B2, and B3. The author was not one of the subjects. Apparatus For the monkeys. The chamber in which the monkeys worked measured cm. The front panel housed a 14-inch color monitor ( pixels) on which the stimuli appeared. The monitor supported a touch-screen system. Small pieces of an apple or sweet potato served as reinforcers, and were delivered to a food cup located on the left of the panel. The experimental room was illuminated by a white bulb. Extraneous sounds were masked by white noise. Displaying of stimuli, delivery of food rewards, illumination, and recording of data were all controlled by a personal computer (NEC PC9801RA). For the chimpanzee and the humans. The room in which the chimpanzee and the humans worked measured cm. The front panel housed a 20-inch color monitor ( pixels) on which stimuli appeared. The monitor again had a touch-screen system. Small pieces of an apple served as reinforcers and were delivered to a food tray beneath the monitor. The experimental room was illuminated by fluorescent lights. Displaying of stimuli, delivery of food rewards, and recording of data were all controlled by a personal computer (NEC PC9801 Xn). Stimuli The same stimuli were used for all species. The stimuli were four different-sized circles labeled 1, 2, 3, and 4, and were 18, 36, 54, and 72 pixels in diameter, respectively. The colors of the circles varied according to the subjects and the tasks. For the disappearing-type simultaneous chain, the monotonic list was , and the nonmonotonic list was For the remaining-type simultaneous chain, the monotonic list was , and the nonmonotonic list was For half the human subjects, the order of the items was reversed. The lists were designated A B C D, respectively. Procedure General. Almost identical procedures were used for all the species. The disappearing-type simultaneous chain was investigated first, and the remaining-type simultaneous chain was examined next. For either type of simultaneous chain, the subjects were to respond to the stimuli in the order requested by the experimenter. There were two orders for each subject and each task. These two orders were cued by the color of the stimulus item. If the stimuli were all one color (say, red), then the subjects were required to select the items in the order or (monotonic lists). If the stimuli were another color (say, cyan), then the subjects were required to follow the order or (nonmonotonic lists). When a trial began, a start key (a black rectangle) appeared at the center of the monitor. If the subject touched the start key, it disappeared and two to four same-colored and different-sized stimuli appeared. There were four positions on the screen (upper right, upper left, lower right, and lower left) at which the stimuli could appear, and the configurations of the stimuli were randomized and counterbalanced across trials. The subjects had to select the stimuli by touching them in the order determined by the experimenter. If the subject successfully selected all the stimuli, an electric chime was sounded and a food reward was provided. If the subject made an error by selecting a wrong item or failed to respond to

4 Memorization of serial items by primates 239 any of the items after 10 s, then all the stimuli disappeared, the room light was turned off for 5 s (blackout, for monkey subjects only), and the same trial was repeated (correction). Intertrial intervals (ITI) were 5 s. Each session consisted of 192 trials: 96 trials with the monotonic list and 96 trials with the nonmonotonic list. Training. Training was conducted with the monkeys and the chimpanzee, but not with the humans. The subjects were first trained with 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 of A B, and the subjects were trained with a three-item list, A B C. After the subjects performance reached the criterion level of 85% correct trials, item D was added to the tail of A B C. The criterion for the four-item list, A B C D, was 80% correct trials. Even if the performance for one list reached the criterion, training was continued until the performance for the other list satisfied the criterion. Hence, the subjects had the same amount of experience with the monotonic and nonmonotonic lists. Kyoko started with a four-item list for the disappearing-type simultaneous chain. Reaction times to subset lists. All the possible two-, three-, and four-item subset lists were presented in each session. Each subset list was tested 120 times. The procedures were the same as in the training. Reaction times were measured in units of 10 ms. For human subjects, only the four-item lists were presented. Paired t-tests and two-factor ANOVA were used for statistical analyses. A p value less than 0.05 was considered significant. Results Training General. Acquisition functions for the monkeys and the chimpanzee are shown in Figure 1. It took more sessions to acquire the remainingtype simultaneous chain than the disappearingtype chain. Paired t-tests revealed that the percentage of correct trials for the monotonic lists was significantly better than that for the nonmonotonic lists for all the subjects for the Figure 1a. For caption see next page.

5 240 N. Ohshiba Figure 1b. (a) Percentage of correct trials as a function of the number of sessions for disappearingtype 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-type simultaneous chains in training. M = monotonic; NM = nonmonotonic.

6 Memorization of serial items by primates 241 Table 1. Significance of difference in the percentage of correct trials between monotonic lists and for nonmonotonic lists during training, by the paired t-test Subjects Type of task Atsushi Kyoko Yusaku Pan Disappearing t(14) = t(12) = t(16) = t(17) = p.01 p.01 p.01 ns Remaining t(11) =.498 t(57) = t(54) = t(29) = ns ns p.05 p.01 disappearing-type simultaneous chain, but only for Yusaku for the remaining-type chain (Table 1). Disappearing-type simultaneous chain. When the lists were increased in length, performance with the nonmonotonic lists deteriorated, whereas that with the monotonic lists did not. These results imply that the subjects regarded the monotonicity of the list items as a rule (in this case, from smaller to larger). Hence, the subjects could be expected to make a correct response regardless of the length of the monotonic lists. Remaining-type simultaneous chain. The difference in performance between the monotonic lists and the nonmonotonic lists was not as remarkable as it was for the disappearingtype simultaneous chain. However, the performance with the four-item monotonic list was better than that with its nonmonotonic counterpart. Reaction times to subset lists General. Figures 2 and 3 present the reaction times to each item of all the possible subset lists. Two-factor analyses of variance type of list (monotonic/nonmonotonic) item were conducted for each subject and each task (Table 2). The reaction times with the nonmonotonic lists were significantly longer than those with the monotonic lists. This implies that the nonmonotonic lists were more difficult to perform than the monotonic lists. The interaction between type of list and item was not tested since the factor item branched under the factor type of list. Disappearing-type simultaneous chain. Let us examine the performance with the two-item subset lists for the disappearing-type simultaneous chain (Figure 2). The reaction times to the second or final items were almost uniform. In these trials, the stimulus items disappeared as soon as the subjects touched them. When the final item was to be selected, no other item was available. Hence, the subjects did not have to consider which item to select. Naturally, this was also true in the three- and four-item subset lists. We found that the later the position of an item in the original list that the initial item of the subset list occupied, the longer was the reaction time to that item. For example, in the case of the first item in the two-item subset lists, the reaction times when B was the first item were longer than when A was the first item, and the reaction times when C was the first item were even longer. This was presumably because the subjects searched for the items in the order A B C. These results suggest that the subjects performed serial searches. The subjects first tried to locate item A; if they could locate it, then they selected it, otherwise they tried to locate item B, and so on. This assumption explains the prolonged reaction time to item C in the subset list A C D and that to item B in the subset list B C D. D Amato and Colombo (1988) found that the reaction times to the first items in the subset lists increased as the items occupied later positions in the original list, and that the reaction times increased substantially as the number of missing items which separated the current item from the previous item increased. Remaining-type simultaneous chain. Individual differences were found with this type of

7 242 N. Ohshiba Figure 2a. Reaction times to the subset lists of disappearing-type simultaneous chains: (a) the monkeys data.

8 Memorization of serial items by primates 243 Figure 2b. The chimpanzee and human data. M = monotonic; NM = nonmonotonic. simultaneous chain (Figure 3). Atsushi responded uniquely, whereas the other animals showed similar tendencies. Atsushi s performance was similar to that for the disappearingtype simultaneous chain. This implies that he was also performing a serial search for the remaining-type simultaneous chain. For Kyoko, Yusaku, and Pan, the reaction times for the first items of the lists were much longer than those for the other items. This suggests that they touched the stimulus items after they had identified all the target items. This strategy was referred to as a collective search. Serial search model. As stated above, the reaction times suggested that the subjects performed serial searches with the disappearingtype simultaneous chain. The reaction times could be composed of two independent processes: evaluation and motion (Sternberg, 1967, 1969a, 1969b). Evaluation is the process whereby the subject decides whether the item concerned is the current target. The duration of evaluation processes depends on the difficulty of the stimuli to which the subjects should respond. If the stimuli are easy to evaluate, then it should

9 244 N. Ohshiba Figure 3a. Reaction times to the subset lists of remaining-type simultaneous chains. (a) The monkeys data.

10 Memorization of serial items by primates 245 Figure 3b. The chimpanzee and human data. M = monotonic; NM = nonmonotonic. 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) Items (54,6664) Remaining Types of lists (1,6664) Items (54,6664) The values of F are displayed. The values of p were all less than The interactions between type of task and item were not tested since the factor item branched under the factor of type of task.

11 246 N. Ohshiba take a short time to evaluate them, and vice versa. Motion literally refers to moving the hand to touch the target. Motion processes mainly reflect physical factors such as muscle movement. Therefore, the time necessary for a single motion process should be uniform for a given subject. Considering both evaluation and motion processes, the author has proposed a serial search model. This model assumes that a subject makes zero or more evaluations before it makes one motion. The subject s reaction times can be estimated based on the time necessary for a single evaluation, α, and the time necessary for a single motion, β. Let us consider a three-item list A C D as an example to explain how to calculate the expected reaction times. When the trial begins, there are three stimulus items displayed on the monitor, labeled I1, I2, and I3. The first target item is A. The subject evaluates whether or not I1 is equivalent to A. If I1 is A, then the subject will touch it. In this case, the subject makes one evaluation and one motion. If I1 is not A, then the subject evaluates whether or not I2 is A. If I2 is A, then the subject will touch it. In this case, the subject makes two evaluations and one motion. If I2 is also not A, then the subject evaluates whether or not I3 is A. If neither I1 nor I2 is A, then I3 should be A, and the subject will touch I3. In this case, the subject makes three evaluations and one motion. While it is certain that I1, I2, or I3 is A, exactly which item is A is probabilistic. Since the probabilities that I1, I2, or I3 is A are all 1 3, the expected reaction time to select A in A C D is: 1 3 {(α + β) + (2α + β) + (3α + β)} = 2.0α + β When the subject selects A, it disappears. Now there are two stimulus items displayed on the monitor, labeled J1 and J2. The target item is now B. The subject then evaluates whether either J1 or J2 is B, but in this example neither item is B. Therefore, the subject shifts the target item from B to C. The subject evaluates whether or not J1 is C. If J1 is C, then the subject will touch it. In this case, the subject makes three evaluations and one motion. If J1 is not C, then the subject evaluates whether or not J2 is C. If J1 is not C, then J2 should be C, and the subject will touch J2. In this case, the subject makes four evaluations and one motion. While it is certain that either J1 or J2 is C, exactly which item is C is probabilistic. Since the probabilities that J1 or J2 is C are both 1 2, the expected reaction time to select C in A C D is 1 2 {(3α + β) + (4α + β)} = 3.5α + β When the subject selects C, it disappears. Now there is only one stimulus item displayed on the monitor, and the subject has only to touch it. In this case, the subject makes no evaluation and one motion. The expected reaction time to select D in A C D is β. The expected reaction times for any items in any lists can be calculated by the same algorithm, as summarized in Table 3. The coefficients of α vary, whereas the coefficients of β are constantly 1. Therefore, the reaction times can be regarded as a linear function of the number of evaluations. Conformity of the serial search model to experimental results. Linear regression analyses were performed between the number of evaluations expected from the serial search model and the actual reaction time (Figure 4). The values of the coefficients and the equations of the regression lines are summarized in Table 4. For the monkeys, the agreement between the results of the serial search model and the actual reaction times was higher for the disappearing-type simultaneous chain than for the remaining-type chain. For the chimpanzee, this conformity was not as high as that for the monkeys for either type of simultaneous chain. The slope of the regression line reflects how much the reaction times would increase if the number of evaluations was increased by one, which is the time necessary for a single evaluation, α. The intercept of the regression line denotes the reaction time when the subject makes no evaluation, that is, the time necessary for a single motion, β. Table 4 shows that the values of the slopes varied remarkably while

12 Memorization of serial items by primates 247 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 β A C 1.5α + β β A D 1.5α + β β B C 3.5α + β β B D 3.5α + β β C D 5.5α + β β A B C 2.0α + β 1.5α + β β A B D 2.0α + β 1.5α + β β A C D 2.0α + β 3.5α + β β B C D 5.0α + β 1.5α + β β A 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. those of the intercepts did not. This supports the assumption that the time necessary for a single evaluation may vary while the time necessary for a single motion should be constant. Discussion Comparison of disappearing- and remaining-type simultaneous chains The disappearing-type simultaneous chain is logically easier to perform than the remainingtype chain. The percentage of correct trials expected when a subject randomly selects the stimulus items is 16.7%, 8.3%, 4.2%, and 0.9% for three-item lists of the disappearing-type simultaneous chain, three-item lists of the remaining type, four-item lists of the disappearing type, and four-item lists of the remaining type, respectively. Hence, the better performance observed for the disappearing-type simultaneous chain than for the remaining-type chain was expected by chance. Comparison of monotonic and nonmonotonic lists The better performance with the monotonic lists than with the nonmonotonic lists suggests that the monotonic lists were easier to acquire than the nonmonotonic lists. Also, shorter reaction times with the monotonic lists suggests that they were easier to memorize than the nonmonotonic lists. Monotonicity among the list items may have promoted acquisition and improved the performance. Hulse (1978), using rats, also found better performance for monotonic lists than for nonmonotonic lists. With monotonic lists, the subject can resort to possible rules (e.g., from smaller to larger). This rule learning might make it easier to acquire monotonic lists. In contrast, nonmonotonic lists do not offer such rules. Therefore, the subject must learn which item to select before/after the other items. Comparison of Japanese monkeys, a chimpanzee, and humans A major difference among the subject species was found regarding the reaction times for the disappearing-type simultaneous chain. Two types of search strategies were inferred: a serial search and a collective search. A serial search is a strategy whereby the subject decides which item to select at each stage. A collective search, on the other hand, is a strategy whereby the subject identifies all the target items before selecting them. The types of search used by the subjects are summarized in Table 5. The monkeys used a serial search for both the

13 248 N. Ohshiba Figure 4a. Linear correlation between the number of evaluations expected from the serial search model and the actual reaction time for (a) disappearingtype simultaneous chains.

14 Memorization of serial items by primates 249 Figure 4b. Remaining-type simultaneous chains. M = monotonic; NM = non-monotonic.

15 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 ( ) r 2 = 0.948y = 180x r 2 = 0.813y = 224x r 2 = 0.703y = 108x r 2 = 0.422y = 69x Nonmonotonic ( ) r 2 = 0.915y = 334x r 2 = 0.877y = 346x r 2 = 0.770y = 206x r 2 = 0.601y = 173x Remaining Monotonic ( ) r 2 = 0.916y = 196x r 2 = 0.641y = 132x r 2 = 0.512y = 51x r 2 = 0.376y = 60x Nonmonotonic ( ) r 2 = 0.888y = 281x r 2 = 0.822y = 228x r 2 = 0.762y = 137x r 2 = 0.532y = 179x N. Ohshiba Upper = coefficient of determination; lower = equation of the regression line.

16 Memorization of serial items by primates 251 Table 5. Summary of types of searches Species Type of task Type of list Japanese monkey Chimpanzee Human Disappearing Monotonic Serial Collective Collective Nonmonotonic Serial Serial Collective Remaining Monotonic Collective Collective Collective Nonmonotonic Collective Collective Collective monotonic and nonmonotonic lists. The chimpanzee also used a serial search for nonmonotonic lists, but used a collective search for monotonic lists. The humans used a collective search for both the monotonic and nonmonotonic lists. These results indicate that species with higher cognitive abilities tended to use a collective search more often than a serial search. Hence, a collective search is presumed to be an evolutionarily new cognitive function. With the remaining-type simultaneous chain, all the subjects used a collective search for both the monotonic and nonmonotonic lists. Serial search model This paper has also described a serial search model based on two processes: evaluation and motion. According to this model, reaction times can be regarded as linear functions of the number of evaluations. Linear regression analyses of the number of evaluations and the actual reaction time confirmed the feasibility of this model, especially for disappearing-type simultaneous chains for the monkeys and the chimpanzee. The greatest merit of the serial search model is that it is possible to represent the difficulty of a list in terms of the time for a single evaluation. If the time required for a single evaluation is long, then the list is difficult, and vice versa. Therefore, the serial search model may be helpful for investigating how difficult lists are compared with one another. References D Amato, M. R., & Colombo, M. (1988). Representation of serial order in monkeys (Cebus apella). Journal of Experimental Psychology: Animal Behavior Processes, 14, D Amato, M. R., & Colombo, M. (1989). Serial learning with wild card items by monkeys (Cebus apella): Implications for knowledge of ordinal positions. Journal of Comparative Psychology, 103, Hulse, S. H. (1978). Cognitive structure and serial pattern learning by animals. In S. H. Hulse, H. Fowler, & W. K. Honig (Eds.), Cognitive processes in animal behavior (pp ). Hillsdale, NJ: Lawrence Erlbaum Associates. Kojima, S. (1989). Vocal-auditory functions in the chimpanzee: Vowel perception. International Journal of Primatology, 10, Sternberg, S. (1967). Two operations in character recognition: Some evidence from reaction-time measurements. Perception and Psychophysics, 2, Sternberg, S. (1969a). The discovery of processing stages: Extensions of Donders method. Acta Psychologica, 30, Sternberg, S. (1969b). Memory-scanning: Mental processes revealed by reaction-time experiments. American Scientist, 57, Swartz, K. B., Chen, S., & Terrace, H. S. (1991). Serial learning by rhesus monkeys: I. Acquisition and retention of multiple four-item lists. Journal of Experimental Psychology: Animal Behavior Processes, 17, Tanaka, M. (1995). Object sorting in chimpanzees (Pan troglodytes): Classification based on physical identity, complementarity, and familiarity. Journal of Comparative Psychology, 109,

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