Grouping, Chunking, Memory, and Learning

Transcription

1 The Quarterly Journal of Experimental Psychology (1986) 38B, 53-8 Grouping, Chunking, Memory, and Learning E. J. Capaldi, Timothy M. Nawrocki, Daniel J. Miller, and Donna R. Verry Department of Psychological Sciences, Purdue University, West Lafayette, Indiana, U.S.A. An example of grouping is dividing a long series of digits into two or more smaller units by, for example, pausing for a short time between items within groups but for a longer time between groups. Grouping, virtually neglected in animal learning, was examined in each of five rat investigations reported here in which a series of two or more runway trials was either grouped together with or apart from a terminal large reward trial. It was found that running speed on early small reward trials in the series was greater when prior trials were grouped together with rather than apart from the terminal large reward trial and this when the intertrial interval was short, 2 sec, or long, 2 to 4 min. Three possible explanations of the present findings were examined: a reward schedule view, a rule-learning view, and an anticipation view based on memory. The most feasible of these explanations, it was suggested, was the anticipation view. According to this view, when prior trials are grouped together with a terminal large reward, there is a tendency for the rat to anticipate the terminal large reward well before its scheduled occurrence, elevating running speed on earlier small reward trials. Such anticipation occurs, it was suggested, because when trials are grouped together the memory of each reward event in the series is retrieved on each subsequent trial, including the remote terminal large reward trial, where it becomes a signal for large reward. Thus the present results implicate not merely adjacent associationsassociations between adjacent events-but also the highly controversial remote associations-associations between two events separated not only in time but by one or more intervening events as well. GENERAL INTRODUCTION Chunking, combining separate items into larger order units, as when the individual letters c, a, and t are combined into the word cat, has been seen as involved in a variety of activities, among them perception (e.g. Requests for reprints should be sent to E. J. Capaldi, Department of Psychological Sciences, Purdue University, West Lafayette, In 4797, U.S.A. Copyright 1986 The Experimental Psychology Society

2 54 E. J. Capaldi et al. Koftka, 1935), immediate memory span (e.g. Neisser, 1967), serial learning (e.g. Restle, 1972), and clearly cognitive activities such as playing chess (e.g. Chase and Simon, 1973). Chunking may occur as a result of a strategy employed by the organism, or, as in the five investigations reported here, it may be facilitated by presenting items in groups. For example, to induce a person to chunk a 1-digit number into units of three and four items, we would group it in such units, i.e., , slashes indicating grouping of items by any number of means, stresses, pauses, rhythms, and so on (Bower and Winzenz, 1969). While it has been known for quite some time that grouping profoundly affects learning and memory in people (see Katona, 194, for a review of early work), its effects on animals have only recently come under direct investigation (Capaldi, Verry, Nawrocki and Miller, 1984; Fountain, Henne and Hulse, 1984). These few recent investigations indicate that the effects of grouping on learning may be as extensive in animals as in people. In an attempt to examine further the effects of grouping on learning and performance here, rats were given fixed series of smaller and larger rewards (.45-g food pellets) in the goalbox of a runway. Studies employing such regular reward schedules have recently come to be called serial learning investigations, the concern being in the development of anticipatory responding-i.e., slower running on smaller than on larger reward trials. In each of the five investigations reported here, a series of two or more small reward and nonreward runway trials preceded a terminal large reward trial. The prior trials were either grouped together with or apart from the terminal large reward trial. Prior and terminal trials were grouped together by presenting them in a runway of the same brightness (Experiments 1 to 3) or in the same spatial location (Experiments 4 and 5). They were grouped apart by presenting them in runways of different brightness or in different spatial locations. Neither brightness nor spatial location was itself a discriminative cue, however. If on Day 1, for example, prior trials were presented in a black runway and the terminal large reward trial in a white runway, on Day 2 prior trials would occur in the white runway and the terminal large reward trial in the black runway. Thus the brightness and spatial cues were employed as grouping cues not as discriminative cues. An empirical generalization consistent with human grouping data is that part of the group has the tendency to reproduce all of the subsequent items of the group (Katona, 194). Is this generalization true for rats? If it is, then responding should be more vigorous on prior small reward and nonreward trials when these are grouped together with rather than apart from a terminal large reward trial. That is, if the above generalization is true for rats, then when prior small reward and

3 Grouping and Chunking 55 nonreward trials are grouped together with a subsequent large reward trial, presenting early small reward trials should tend to reproduce the remote large reward event, which should elevate vigour of responding on the small reward trials. This hypothesis was tested employing a short, 2-sec, intertrial interval (ITI) in Experiments 1 and 3, and a long 2- min and 4-min ITI, respectively, in Experiments 4 and 5. The effects of grouping, it appears, have not previously been investigated at long ITIs, using either people or animals. EXPERIMENT 1 The hypothesis that speed of running will be greater on small reward trials when prior trials are grouped together with rather than apart from a terminal large reward trial was tested in Experiment 1 employing three groups, each of which received the same six-item series each day, Group 6- received all six items in a black runway on odd-numbered days and in a white runway on even-numbered days. In Group 6-, all six trials were presented as a single group, and so the initial five trials of the series were part of a group that included the terminal 18-pellet item of the series. In Group 3-3 runway brightness was changed from Trial 3 to Trial 4, from black to white (odd days) or white to black (even days). In Group 3-3, then, items were grouped each day and -1-18, respectively. In the grouping of Group 3-3, prior trials are part of a group that includes a terminal large reward. Group 5-1 was treated like Group 3-3, except that runway brightness changed from Trial 5 to Trial 6. In Group 5-1, then, items were grouped and 18. In Experiment 1, only in Group 5-1 were prior trials not part of a group that included a terminal large reward. Subjects Method The subjects were 18 naive male albino rats, 77 days old on arrival at the laboratory, purchased from the Holtzman Co., Madison, Wisconsin. The rats were divided into three groups of six each. Apparatus The apparatus consisted of two adjacent runways, identical except for brightness, one black, one white. The startboxes, also black and white, were 21.6 cm long, the goalboxes being cm long. Each alley was 19.8 cm long, 8.57 cm wide, and enclosed by cm sides covered by a top wire mesh on a hinged frame. Lowering the brass startbox door started a completely silent.1-sec digital clock, which was stopped when a photobeam located cm beyond

4 56 E. J. Capaldi et al. the startbox door was broken by the rat. Pellets (.45-g Noyes) could be placed in a goal cup (3.8 x 3.8 x.95 cm) at the end of the runway. The pellets were not visible prior to breaking the photobeam. A brass door confined the rat to the goalbox. Pretraining On arrival at the laboratory, the rats were caged individually and allowed ad-lib food and water for 6 days. On Days 1 to 6 of pretraining, each rat was handled for 1 min and then fed the daily ration, which consisted of 14-g Wayne Rodent Blox, in the home cage. On Day 7 the animals were fed 18 pellets in the home cage. On Days 8 to 1 each rat was exposed to each unbaited runway for 2 min, during which the startbox and goalbox doors were lowered briefly and raised at the 1-min mark, and then returned to the home cage, where it was fed 18 pellets followed by the daily ration. Throughout the experiment all pellets eaten by the rat were subtracted from the 14-g daily ration. On Day 11 the rats were given a single trial in the black runway and on Day 12 in the white runway, reward being 18 pellets. Experimental Training Experimental training began on Day 13 and lasted for 18 days. On each day of experimental training each rat received six runway trials in succession, the IT1 being about 2 sec. The six trials terminated in 18, 1,,, 1 and 18 pellets reward, respectively, an series. In Group 6-, the six trials occurred in a black runway on odd-numbered days and in a white runway on even-numbered days. In Groups 3-3, runway brightness changed from black to white (odd days) or white to black (even days) between Trials 3 and 4. Group 5-1 was treated identically to Group 3-3, except that runway brightness changed from Trial 5 to 6. Each of two experimenters ran half the rats from each group. All rats were brought into the runway room on a small cage rack in their home cages. The rats were run in rotation such that a rat received all six of its trials before the next rat was run. Rotation orders were randomized daily but were the same for each experimenter. To begin a trial, the rat was placed in the startbox. Approximately 3 sec later the startbox door was lowered, and the rat was allowed to traverse the runway. The maximum time allotted per run was 9 sec. If the rat did not reach the goalbox in 9 sec it was removed from the runway and placed in the goaibox. The rat was removed from the goalbox after all food pellets had been eaten on rewarded trials, or after 15 sec on nonrewarded trials. The rats were fed the reduced daily ration about 1 min after dunning had been completed. As a control for odour, an open canister of food pellets was placed near the goalbox. Pellets were poured into the food cup from a cloth-lined cup. Results Speed of running on each of the six trials of the series is shown in Figure 1 for each of the three groups in blocks of two days. Two trends are apparent in Figure 1. (1) Group 5-1 ran more slowly than Groups 6- and 3-3, which differed little. (2) Group 5-1 tended to

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6 58 E. J. Capaldi et al. run more rapidly on its large reward trials, Trials 1 and 6, than on its small reward trials, Trials 2, 3, 4 and 5, an anticipatory tendency apparent neither in Group 6- nor in Group 3-3 by the termination of training. An analysis of variance over the data shown in Figure 1 indicated each of the following to be significant at the.1 level or better: Groups, F(2, 12) = 1.7; Trials, F(5, 6) = 9.93; Groups x Trials, F(1,6) = 9.4; Group x Trials x Blocks, F(4,48) = The major statistical findings obtained in Experiment 1 can be communicated most efficiently by reporting the results of a breakdown of the significant Groups x Trials x Blocks interaction using Newman-Keuls tests. On Block 1 no difference was significant either within or between groups. On each of the last four blocks, however, a simple and consistent series of differences emerged. Consider first within-groups differences. On each of the last four blocks, Group 5-1 ran significantly more slowly on each of its small reward trials, Trials 2,3,4 and 5, than on either of its large reward trials (ps <.5 or better). In addition, Group 5-1 was slower on Trial 6 than on Trial 1 on Blocks 6 and 9 (ps <.5). In Groups 6- and 3-3 no within-groups difference was significant on Blocks 6, 7, 8 or 9. Consider now between-groups differences on each of the last four blocks of trials. On none of these blocks did Groups 6- and 3-3 differ on any trial. On each of the Blocks 6, 7,8, and 9, however, Group 5-1 ran more slowly than either Group 6- or 3-3 on each of Trials 2,3,4,5 and 6 (ps <.5 or better), no difference being significant on Trial 1. Discussion In Experiment 1, Groups 6 4 and 3-3 ran more rapidly overall than Group 5-1. Further, while Groups 6- and 3-3 showed no tendency to exhibit serial learning, slower running on nonrewarded and small rewarded trials than on large rewarded trials, Group 5-1 exhibited a strong tendency toward serial learning early in training. In Groups 64 and 3-3 prior trials were grouped together with a terminal large rewarded trial, which was not the case with Group 5-1. Thus the results obtained in Experiment 1 are consistent with the hypothesis that speed of running on prior small reward trials will be greater when they are grouped together with rather than apart from a terminal large reward trial. In Group 3-3, trials were presumably formed into two groups of three trials, an initial group and a subsequent group. The initial grouping did not terminate in large reward, yet running was rapid on Trials 2 and 3 of the initial grouping. This matter will be considered in the General Discussion section.

7 Grouping and Chunking 59 EXPERIMENT 2 The results obtained in Experiment 1 indicated that vigour of responding on small reward trials would be elevated when prior trials were grouped together with rather than apart from a terminal large reward trial. This hypothesis was tested in Experiment 2, employing a wider set of experimental conditions than were employed in Experiment 1. Experiment 1 employed a single series grouped in three different ways. In Experiment 2, two different groups received three-item series, a decreasing series 18-14) for one group and an increasing series for the other; each series was presented three times each day for each group. Employing brightness cues as in Experiment 1, each series was grouped in two different ways, as a single group of nine trials or in three groups of three trials. In the nine-trial condition, all trials occurred in a black runway on odd-numbered days and in a white runway on evennumbered days. In the three-trial condition, runway brightness was changed each day between Trials 3 and 4 and 6 and 7 (B-W-B on odd days, W-B-W on even days). Following acquisition, the groups were extinguished. In extinction, of course, only nonrewarded trials occur, providing another measure of the tendency of each group to run rapidly on zero or small reward trials (e.g. Capaldi, 1966). In the increasing condition prior trials are grouped together with subsequent large reward trials both when trials are grouped in threes and when they are presented as a single group (-1-18/-1-18/-1-18). In the decreasing condition, however, prior trials are grouped together with subsequent large reward trials when trials are presented as a single group of nine, but not when they are presented in three groups of three (18-1-/18-1-/18-1-). Thus, if there is a greater elevation in speed on small reward and nonreward trials when they are grouped together with rather than apart from a terminal large reward trial, then presenting trials in groups of threes should reduce running speed more in the decreasing condition than in the increasing condition. Subjects Method The subjects were 32 rats of the same description as employed in Experiment 1. They were divided into four groups of eight each. Apparatus The apparatus was the same as that employed in Experiment 1.

8 6 E. 1. CaDaldi et al. Pretraining Pretraining was identical to that employed in Experiment 1. Experimental Training Experimental Training was identical to that employed in Experiment 1, except for the following: There were four groups, two of which, Groups 1-D and 3-D, received the nine trial series each day, , and two of which, Groups 1-1 and 3-1, received the nine trial series, -1-lgQ-1-18-&1-18, each day. Group 1-D and 1-1 received all trials in a black runway on odd days and in a white runway on even days. For groups 3-D and 3-1 runway brightness changed from Trial 3 to 4 and from Trial 6 to 7, from black to white to black (on odd days) and from white to black to white (on even days). If the rat did not reach the goalbox within 6 sec, it was picked up and placed directly in the goalbox. Each of two experimenters ran half the rats from each group. The rats were brought into the experimental room in squads of four, one from each group. The order of running the rats within a squad was changed daily. After the last rat in a squad had been run, the four rats were returned to the home cage and fed the daily ration about 1 min later. Following 18 days of acquisition training, the rats were given 2 days of extinction training at six trials each day. Procedures in extinction were as in acquisition, except that all trials were nonreinforced, there were only six trials each day rather than nine and all trials occurred in a black runway on Day 1 of extinction and in a white runway on Day 2. Results Running speed in acquisition in blocks of 3 days is shown for the series in Groups 1-D and 3-D and for the series in Groups 1-1 and 3-1 in Figure 2. The following observations are relevant to the predictions outlined earlier: Large differences in running speed on both 1-pellet and -pellet trials appeared between Groups 1-D and 3-D as early as Block 3. But only by Block 5 did Group 3-1 show slower running that Group 1-1, and that only on -pellet trials, the two groups running equally rapidly on 1-pellet trials. Thus only one group, Group 3-D, showed complete serial learning, running more slowly on both 1- pellet and -pellet trials than on 18-pellet trials. That the effects of grouping were greater in the decreasing condition than in the increasing condition is supported statistically. In what follows, all differences said to be significant are at the.5 level or better. An analysis of variance was performed on each of the blocks shown in Figure 2. Of greatest relevance to the prediction that the effects of grouping should be greater in the decreasing condition than in the increasing condition was the finding that from Block 3 on, the triple interaction of Grouping x Series type x Trials was significant, Fs(2, 56) = 4.3, 5.94, and on Blocks, 3, 4, 5 and 6, respectively.

9 Grouping and Chunking in \ I1 Y 8 n 6 W L a 4 z a 2 W I - \ I 1 b. - 1-D I 3-D I I I I I I I I I I 1 I I I I A breakdown of this interaction employing subsequent Newman-Keuls tests revealed the following: We consider first the tendency of each group to show appropriate serial learning, faster running on 18-pellet trials than on 1-pellet or -pellet trials. Group 3-D ran significantly more slowly on 1-pellet and -pellet than on 18-pellet trials on each of the Blocks 3, 4, 5 and 6; on no block did Group 1-D run significantly more slowly on either 1-pellet or -pellet trials than on 18-pellet trials. Groups 1-1 and 3-1 ran more slowly on -pellet trials than on 18-pellet trials only on Blocks 5 and 6; in no case was running slower on 1-pellet than on 18-pellet trials in these groups. Consider now between-groups differences. Group 3-D ran significantly more slowly than Group 1-D on 1-pellet and -pellet trials on each of the Blocks, 3,4,5 and 6, the two groups not differing on 18-pellet trials. Group 3-D ran more slowly on 1-pellet trials than did either Group 3-1 or 1-1 on Blocks 3,4,5 and 6. Group 3-D ran more slowly on -pellet trials than Groups 3-1 and 1-D on Blocks 3 and 4, differing from Group 1-D on -pellet trials on Block 5 as well. On 18-pellet trials, Groups 3-D and 1-D differed significantly from Groups 1-1 and 3-1 on Block 3. Figure 3 shows running speed in extinction for each group on each of the six trials collapsed over the 2 days of extinction. If anything, the major trend shown in acquisition is even more obvious in extinction;

10 62 E. J. Capaldi et al. h v) 12 \ n 6 W P cn 4 * 2 a W s o TRIALS Figure 3. Running speed in extinction for each group on each of the six trials collapsed over the 2 days of extinction. grouping produced greater differences in running speed on small reward trials in the decreasing condition than in the increasing condition. Figure 3 shows that on the first extinction trial of the day each of the groups performed as in acquisition: the increasing groups ran slowly, the decreasing groups ran rapidly. Over the subsequent trials, however, the tendency was for each of the groups, except Group 3-D, to run rapidly. An analysis of variance applied to the data shown in Figure 3 revealed, importantly for present purposes, that the triple interaction of Groups X Series type x Trials was highly significant, F(5, 14) = A breakdown of this interaction revealed that on each of the Trials 2 to 6 Group 3-D ran significantly more slowly than each of the remaining groups, only one other difference being significant, slower running by Group I-D than Group 3-1 on Trial 6. On Trial 1, each increasing group ran more slowly than each decreasing group, no other differences being significant. Discussion The results obtained in Experiment 2 are consistent with the hypothesis that responding should be more vigorous on small reward trials when they are grouped together with rather than apart from a terminal large reward trial. Of the four groups employed in Experiment 2, only in Group 3-D were prior trials not grouped together with terminal large

11 Grouping and Chunking 63 reward, and Group 3-D ran more slowly on small reward and nonreward trials than any of the remaining three groups in acquisition and in extinction. Moreover, Group 3-D showed better serial learning than any of the other remaining three groups. Thus differences in running speed on small reward trials were greater in the decreasing condition, Group 3-D vs. Group 1-D, than in the increasing condition, Group 3-1 vs EXPERIMENT 3 The hypothesis tested and confirmed in Experiments 1 and 2 on the basis of various between-group and within-group comparisons is consistent with a variety of earlier reported within-group findings from this laboratory. As one example, in Experiment 1 of a report by Capaldi, Nawrocki and Verry (1983), rats trained in a gray runway received two series each day, each series presented as a single group using temporal grouping, the two series being separated by a longer interval, 2 min, than the items of each series, about 2 sec. One series was 1--1, the other 1--, a given rat always receiving the series in a particular order each day always first, or 1O-O-O always first. All events were anticipated, running being faster on all 1-pellet trials than on all - pellet trials. However, running speed to the second item, pellets in both series, was greater in the 1--1 series than in the 14- series. Thus running speed on Trial 2, a -pellet trial in both series, was faster when that trial was grouped with a subsequent large reward, the 1--1 series, than when it was not grouped with a subsequent large reward, the 1-- series. If the hypothesis of concern in this report is correct, then it should be possible to reduce running speed to Item 2 of the 14-1 series by presenting that series not as a single group (1--1) but as a group of two trials (1-) and a group of one trial (1). Experiment 3 employed two groups, both of which received the two series 1--1 and For both groups the 1O-O-O series was presented as a single group. The difference between the two groups was that in one group the 1--1 series was presented as a single group of three trials, whereas in the other group the 1--1 series was presented as a group of two trials (1-) and one trial (1). This was accomplished as follows: Group 1--1 received all of its trials of both series in a black runway on odd-numbered days and in white runway on evennumbered days. Group 1-jlO was treated identically with but one exception: Trial 3 of the 14-1 series occurred in a white runway on odd-numbered days and in a black runway on even-numbered days.

12 64 E. J. Capaldi et al. Subjects Method The subjects were 8 rats of the same description as employed in Experiments 1 and 2. They were divided into two groups of four each. Apparatus The apparatus was the same as that employed in Experiments 1 and 2. Pretraining Pretraining was identical to that employed in Experiments 1 and 2. Experimental Training Experimental training was identical to that employed in Experiment 2, except for the following: Experimental training lasted 28 days. There were two groups of four rats each. All eight rats were placed into holding cages, water being freely available, and brought into the experimental room. Each rat received two series of three trials each day, terminating in either pellets or 1 pellets, the series being 1--1 and 1O-O-O. For half the rats in each group, randomly selected, the 1-- series occurred initially each day, the 1--1 series subsequently. The remaining rats always received the series in the opposite order. The rats were run individually, and the order of running the rats was varied daily but was the same within a day for both series. Ail rats were given the three trials of the first series before any rat received the three trials of the second series. The trials of a series were separated by about a 2- to 3-sec interval, and the series were separated by about a 15- to 25- min interval. After receiving the three trials of the first series, the rats were placed in the holding cage to await the second series, min later. Group 14-1 received both series in a black runway on odd-numbered days and in a white runway on even-numbered days. Group 14/1 was treated identically, with but one exception: On even days, Trial 3 of the 14-1 series occurred in the black runway and on odd days in the white runway. Results Figure 4 shows running speed for each group on Trial 1, on Trial 2, and on Trial 3 in each series, 1--1 and 1O-O-O in blocks of 2 days. By the end of training, the rats in both groups correctly anticipated every reinforced and every nonreinforced trial in each series by running faster to the former than to the latter. While no differences between the groups on Trial 1 even approached significance, there were very substantial differences between the groups on Trials 2 and 3. On Trial 2, nonreinforced in both series, the groups differed in two major respects: (1) Group 1-/1 anticipated nonreward much sooner in training, and much more effectively overall, than did Group 1&-1.

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14 66 E. J. Capaldi et al. This difference between the groups on Trial 2 was significant over the first half of training, Blocks 1 to 7 [F(l, 6) = 53.47, p <.1]. (2) The groups also differed in that on trial 2, Group l-/1 ran as slowly to pellets in the 1--1 series as in the 1-- series, but Group 1--1 ran considerably faster to pellets in the 1--1 series than in the 1-- series beginning about Block 7. The Groups x Series interaction was not significant over Blocks 1 to 7 (F < 1) but was significant over Blocks 8 to 14 [F(1, 6) = 18.62, p <.11. Subsequent Newman- Keuls tests revealed that while Group 1--1 ran faster on Trial 2 of its 1--1 series than on Trial 2 of its 1-- series (p < O.Ol), this difference was not significant in Group 1-/1. On Trial 3, which was rewarded in one series (1-4-1) and not rewarded in the other (l--), Group l-/1 ran more slowly than Group 1--1 over both the first and second half of training [Fs(l, 6) = and 18.6, ps <.1 and.1, respectively]. The Groups x Series interaction was likewise significant over both halves of training [Fs(l, 6) = and ps <.1 and.1, respectively], indicating that the difference between reinforced and nonreinforced speeds on Trial 3 was greater in Group 1-/1 than in Group 1--1, as may be seen in Figure 4. Subsequent Newman-Keuls tests indicated that on Blocks 1 to 7, running was significantly slower to nonreward than to reward in Group lo-jlo (p <.1) but not in Group Further, on Blocks 1 to 7, Group 1-OjlO ran slower than Group 1--1 to both reward (p <.1) and nonreward (p <.1). Over Blocks 8 to 14, Group 1--1, like Grbup lo-o/lo, ran more slowly to nonreward than to reward (p <.1). And while the speeds of the two groups did not differ to reward, Group lo-jlo ran more slowly to nonreward than did Group 1--1 (p <.1). Discussion In Experiment 3, each of two groups was trained on two series, 14-1 and 1--. The difference between the two groups was this: The 1--1 series was presented either as a single group of three trials, Group 1--1, or grouped as two trials and one trial, Group lo-jlo. This seemingly small difference produced what can only be called an enormous difference in serial learning, Group 1-/1 anticipating all - pellet trials more effectively and much earlier in training than Group Moreover, while Group 1--1 ran more rapidly on Trial 2 in the 1--1 series than on Trial 2 in the 1-- series, Group lo-o/ 1 ran equally slowly on both these trials. The findings obtained in Experiment 3 are consistent with the hypothesis that vigour of respond-

15 Grouping and Chunking 67 ing on nonrewarded trials will be greater when they are grouped together with rather than apart from a terminal large reward trial. A person required to learn two lists of items simultaneously might be provided with any one of several means of discriminating between them: each list might begin with a different item, the items of each list might be printed in different colours, and so on. It is abundantly clear that the rats in Group 14-1, when trained under the two lists, 14-1 and 1--, simultaneously, must have had some means of discriminating between them, or they could not have behaved differentially on Trials 2 and 3 of the two series. Whatever the basis of this list discrimination, it did not rest on externally present cues, the two series being identical until Trial 3 (1-). Memory must lie at the basis of list discrimination in Experiment 3, but memory of what? Recall that a given animal always received the two series in a particular order each day, 1--1 always first or 1-- always first. One might conjecture that the animal determined which series was to occur by remembering whether or not it had received a series or, at least, one or more trials earlier that day. EXPERIMENT 4 Grouping is generally thought to be involved in many activities from the perceptual to the cognitive and can occur on many bases, spatial or temporal separation of items, brightness cues, employing rhythms, stresses, and so on. In one respect, however, information with respect to grouping is limited: We do not know whether items separated by long ITIs can be grouped. We know, of course, that when a series of items is separated by both long and short temporal intervals, the items at the short intervals can be grouped, an example of this being provided in Experiment 3. Experiment 4 was concerned with determining whether items separated by long intervals can be grouped. Whereas Experiments 1 to 3 tested the hypothesis that running speed on small-reward trials would be greater when prior trials were grouped together with rather than apart from a terminal large-reward trial employing an IT1 of 2 sec, Experiment 4 tested that hypothesis using a minimum IT1 of 2 min. Two groups of rats were employed in Experiment 4, each of which received each day the single series The series was presented as a single group of four trials for one group (Group 8---8) and a group of three trials (8--3) and one trial (8) for another group (Group 8--/ 8), a minimum IT1 of 2 min elapsing between each trial. In each of the previous experiments, items were grouped using brightness cues; in Experiment 4 grouping was accomplished by presenting items in different spatial locations. To our knowledge, Experiment 4 is the first to

16 68 E. J. Capaldi et al. attempt to determine whether grouping is effective in connection with items separated by more than a few seconds. Subjects Method The subjects were 14 female Long Evans rats bred at Purdue University, about 215 days old at the start of the experiment. The rats had been used in a previous unrelated study concerned with saccharin consumption in the home cage. The rats were divided into two groups of seven each. Apparatus The apparatus consisted of a circular central platform, 52. cm in diameter, and two identical arms, each cm in length and 8.89 cm in width. The arms extended out from the central platform and were separated by 18" angles. The arms were painted gloss black and stood 1.33 cm above the floor on wooden sawhorses. Each arm had Plexiglas side rails nailed to each side that were 5.4 cm in height, with the initial cm on the right side of each arm (looking toward the goalbox) having cm-high Plexiglas sides. The final cm of each arm was closed off by a Plexiglas guillotine door to form the goalbox (31.75 x 8.89 x 12.7 cm). Wooden blocks, 4.9 x 8.89 x 3.8 cm, located at the end of each arm, contained circular food cups, 3.81 cm in diameter and 2.54 cm in depth, in which food pellets were placed on rewarded trials. A rectangular Plexiglas startbox, x 8.89 x 12.7 cm, on the central platform, could be positioned flush with either arm, the animal having access only to that arm. Raising the Plexiglas startbox door triggered a microswitch, which started an electronic.1-sec clock. The clock stopped when the rat broke a photobeam of light 18.9 cm down an arm. A second clock started simultaneously with the stopping of the first clock, which timed nonreward confinement duration. Lowering the Plexiglas goalbox door confined the rat to the goalbox. An overhead string pulley system allowed the experimenter to raise and lower the startbox and goalbox doors manually. Pretraining During the previous saccharin-consumption experiment, the rats had been water deprived (for about 3 days), being allowed to consume water only during a 3-min period each day. Following completion of the experiment, the rats were maintained on ad-lib water (and food) for about 4 days. On Day 1 of pretraining of the present experiment, the rats were weighed and placed on food deprivation consisting of 1 g of Wayne Rodent Blox. This daily ration was decreased to 9 g on Day 13, remained as such for eight days, and was then increased to 1 g for the remainder of the experiment. The rats were weighed daily throughout the duration of the experiment. Additionally, on Day 1 to 6 each rat was handled for 1 min and then fed the daily ration in the home cage. On Day 7 each rat was handled and fed eight.45-g Noyes food pellets in the home cage, followed by the daily ration. Throughout the experiment, all pellets eaten by the rat were subtracted from the daily ration.

17 Grouping and Chunking 69 On Days 8 to 1, all of the rats were taken into the experimental room, and each rat was placed on the central platform, with the startbox removed and the goalhox doors opened, and allowed to explore the entire unbaited maze for 4 min. When all the rats had explored the maze, they were returned to the adjacent colony room and each fed 8 pellets, followed by the daily ration. On Days 11 to 14, each rat received one run for 8 pellets, with the run occurring in the left arm on Days 11 and 13 and in the right arm on Days 12 and 14. The rat was placed in the startbox, the doors were open approximately 3 sec later and the rat was allowed to run to the goalbox for the 8 pellets (placed in the goalcup), upon which the doors were closed. When all rats had received the day s run, they were returned to the colony room and fed the daily ration. Experimental Training This began on Day 15, lasting a total of 1 days. The four-trial series was received each day by each rat. For Group all trials occurred in the left arm on odd-numbered days and in the right arm on even-numbered days. Group 8--/8 was treated identically, except for Trial 4: it occurred in the right arm on odd days, in the left arm on even days. All 14 rats were brought into the experimental room in cages (with water available), and each rat received Trial 1 before any rat received Trial 2, and so on. This generally produced an IT1 of about 2 min; the experimenter waited if the IT1 fell below 2 min. The running order of the rats was randomized daily. A trial began with the removal of the rat from the holding cage and its placement in the startbox. Approximately 3 sec later the startbox door was raised by the experimenter. The rat was given a maximum of 6 sec to traverse the arm. If this time elapsed without the rat s reaching the goal, the animal was picked up and placed gently in the goalbox. The rat was removed when it had eaten all of the pellets on rewarded runs, or after 15 sec on nonrewarded runs. A cloth-lined cup was used to bait the goalcup and an open canister of pellets was placed near the apparatus to control for noise and odour cues, respectively. Results Running speed in experimental training on each trial of the series is shown in Figure 5 for each of the groups in blocks of 2 days. The results are entirely clear. First, the tendency to run more slowly on -pellet than on 8-pellet trials appeared early in training in Group 84/8 and still had not appeared in Group by the time training was terminated. Moreover, by Block 3, Group 8--/8 ran faster on 8-pellet trials and slower on -pellet trials than did Group An analysis of variance applied to the data shown in Figure 5 revealed, importantly for present purposes, that the triple interaction of Groups x Trials x Blocks was highly significant, F(12, 144) = 6.25, p <.1. A subsequent breakdown of this interaction using Newman- Keul s tests indicated the following: While Group failed to run more slowly on either -pellet trial than on either 8-pellet trial on any

18 7 E. J. Capaldi et al * TRIALS IN BLOCKS OF TWO DAYS Figure 5. Running speed in acquisition on each trial of the each of the groups in blocks of 2 days. series for block, these differences were significant in Group 8--/8 on Blocks 3, 4, and 5 (ps <.1). Group 8-O-e/8 ran more slowly than Group 8-8 on each of the -pellet trials on Block 4 and 5 (p <.1). Only on Block 4 did Group 84-/8 run significantly more rapidly than Group 8--8 on Trial 1, while on Trial 4 this difference was significant on each of the blocks (ps <.5). Discussion Experiment 1, 2, and 3 of this report employed a 2-sec IT1 and supported the hypothesis that speed of running would be greater on small reward trials when prior trials are grouped together with rather than apart from a terminal large reward trial. Experiment 4 supported the same hypothesis employing a 2-min ITI. Thus Experiment 4 demonstrated that grouping is effective when items are separated by an IT1 of 2 min, that the effects of grouping are not and need not be limited to situations in which items are presented rapidly. EXPERIMENT 5 Experiment 4 indicated that grouping prior trials either together with or apart from a terminal large reward trial was effective when the minimum IT1 was 2 min. Experiment 5 was similar to Experiment 4 except that

19 Grouping and Chunking 7 1 the minimum IT1 was now 4 min and the series employed was 8--8 rather than 8---8, it not being practical for us to employ a four-item series with a 4-min ITI. The 8--8 series was presented as a single group of three trials for one group, Group 8--8, and as a group of two trials and one trial for another group, Group 8-/8. Subjects Method The subjects were 12 rats of the same description as those employed in Experiments 1, 2, and 3. The rats were divided into two groups of six each. Apparatus Experiment 5 employed a second apparatus, which was identical in every respect to that employed in Experiment 4. Pretraining Upon arrival at the laboratory the rats were individually caged and given unlimited access to food and water for 29 days. On Day 1 of pretraining, all food was removed from the home cages and the rats were placed on food deprivation, which consisted of 14 g of Wayne Rodent Blox, with any food pellets received by the rat being subtracted from this daily ration. Water remained available in the home cages at all times. Each rat was handled for 1 rnin on Days 1 to 7, then fed the daily ration of Rodent Blox. On Days 8 to 1 each rat was given 5 min of unbaited maze exploration, with the startbox removed from the central platform and both goalbox doors open. In addition, on Days 7 to 1, each rat received eight.45-g Noyes Food Pellets in metal canning lids in the home cages and a reduced ration of Rodent Blox. On Days 11 and 12, each rat ran one trial per day, which terminated in eight pellets. On Day 1 1 the trial was in one arm and on Day 12 the trial was in the other arm. Experimental Training Experimental training began on Day 13, lasting a total of 2 days. Experimental training in Experiment 5 was identical to that in Experiment 4, except for the following: The series was Group 8--8 received all three trials in the left arm on odd days, in the right arm on even days. Group 84/8 was trained similarly, except for Trial 3; it occurred in the right arm on odd days, in the left arm on even days. The IT1 was 4 min. Since it took only about 15 min to administer a trial to each of the 12 rats, the experimenter waited about 25 rnin between each trial. Results Running speed on each trial of the series on each day is shown in Figure 6 for each of the groups. Clearly, Group 8-18 ran more slowly on - pellet trials than on either 8-pellet trial, while Group 8--8 did not. A

20 72 E. J. Capaldi et al. 5; 6 \ I 5 u - 4 n W 3 W ; 2 z 1 U W I TRIALS IN BLOCKS OF TWO DAYS Figure 6. Running speed in acquisition on each trial of the 8--8 series for each of the groups in blocks of 2 days. breakdown of the significant Groups x Trials x Blocks interaction, F(18,18) = 8.64,~ <.1, revealed the following in connection with the last six blocks of trials: On Blocks 8 and 1, Group 8--8 was slower on Trials 1 and 2 than on Trial 3, was slower on Trial 1 than on Trials 2 and 3 on Block 9 (p <.5), no other within-groups difference being significant. On each of the Blocks 5 to 1, Group 8--/8 was slower on Trial 2 than on Trials 1 and 3 (ps <.1). On each day of the Blocks 6 to 1, Group 84/8 ran more slowly than did Group 8--8 on Trial 2 (ps < O.Ol), no other between-groups difference being significant. Discussion It was found in Experiment 5 that running speed on small-reward trials was greater when prior trials were grouped together with rather than apart from a terminal large-reward trial. Thus, in Experiment 5 the grouping variable was effective with items separated by a minimum 4- min ITI. GENERAL DISCUSSION In each of five investigations reported here the performance of rats given a series of smaller and larger rewarded trials was shown to vary considerably, depending upon whether the smaller reward trials were grouped together with or apart from subsequent large reward trials.

21 Grouping and Chunking 73 Such grouping affected responding in extinction as well as acquisition and both when the IT1 was short, about 2 sec, and when it was long, as long as 4 min. It may be concluded on the basis of the present results that responding will be more vigorous on smaller reward trials in acquisition and in extinction both at long and at short ITIs when such trials have been grouped together with rather than apart from subsequent large reward trials. As indicated, the effects of grouping at long ITIs do not seem to have been directly investigated previously. Investigations employing people have allowed no more than a few seconds to elapse between the items to be grouped, and the few previous animal studies of grouping have employed short ITIs of about 2 see (see, e.g., Bower and Winzenz 1969; Capaldi et al., 1984; Fountain et al., 1984; Katona, 194). A major theoretical approach to grouping, one emphasized in Gestalt Psychology (see, e.g., Kof ka, 1935; Neisser, 1967) is that it depends upon noting relationships among the items. This raises the question whether relationships easily detected at short ITIs are equally detectable at long ITIs? For example, it has been suggested that when items are presented very rapidly, as in a digit-span study, grouping may occur as a result of either imposing some sort of rhythmic pattern upon items or recognizing an existing one (Neisser, 1967). Are rhythmic patterns that may be easily detectable at short ITIs equally detectable at long ITIs? Consider another relationship, one said to be more detectable at short than at long ITIs. It has been suggested that the effects of grouping depend upon noting rule relationships among the items. In a series such as , items decrease monotonically, and these rule relationships may be more easily detectable at short than at long ITIs, an IT1 of about 5 min being considered long (Hulse 1978; 198). While the effects of IT1 on grouping were not directly examined here, rats do not seem to have encountered what might be called major difficulty, if any at all, in chunking items at ITIs of 2 min or even 4 min. In any case, even were it to be shown ultimately that the effects of grouping decline as IT1 increases, the results of Experiment 4 and 5 indicate that grouping is nevertheless a very powerful factor at an IT1 as long as 4 min. Furthermore, a single grouping variable, whether prior trials were grouped together with or apart from a subsequent large reward, was found to be effective over an IT1 range of 2 sec to 4 min. The effects of grouping have been woefully neglected in animal learning. Inevitably, however, trials are grouped in all learning situations, sometimes in this fashion, sometimes in that fashion. If the effects of grouping are general and powerful, a not unlikely hypothesis on the basis of the results reported here, we have no choice but to attempt to understand them. In some learning situations, the runways employed

22 74 E. 1. Caoaldi et al. here, for example, we have the option of presenting a series of trials as a single group or in a variety of smaller groupings. In still other learning situations trials are by definition grouped in a particular way, as in the paired-associates task. In some other situations, discrimination learning being an example, trials can only be presented in smaller groupings, not as a single group, the resultant grouping being dependent upon how trials are distributed over the S+ and S- alternatives. One might conjecture, reasonably in view of the present results, that whatever else might be the function of differentially reinforced S + and S - cues in discrimination tasks, they may act as well, as did the brightness and spatial cues employed here, as grouping cues. This is more than mere speculation, since there is considerable evidence, consistent with a reward-cue-memory analysis, that how trials are grouped in discrimation tasks has a considerable effect upon performance (e.g. Capaldi, Berg and Morris, 1975; Capaldi, Nawrocki and Verry, 1984; Grosslight and Radlow, 1956; Haggbloom, 198a, 198b, 1982). These few comments may serve to indicate that grouping may be a factor determining learning and performance in all animal learning situations. It remains to consider how the grouping observed in the present experiments should be explained. One explanation of grouping was supplied by Fountain et al. (1984), who, employing five repetitions daily of the series , examined what they called good grupin.g ( / , etc.), bad grouping (14-7-3/ / , etc.), or presenting items as a single group of 25. Better anticipation of pellets occurred with good grouping than either bad grouping or presenting 25 items as a single group. Capaldi et al. (1984) obtained similar results for the series and, in addition, found that bad grouping produced greater resistance to extinction than good grouping. Good grouping facilitated serial learning according to Fountain et al. (1984) because it allowed the rats more effectively to isolate the periodic structure of the series (repetitions of the series) than either bad grouping or single grouping. However, as may be seen, the Fountain et al. (1984) findings are consistent with the hypothesis tested in this report since under good) grouping, prior trials were grouped apart from the large 14-pellet reward, while under either bad grouping or single grouping prior trials were grouped together with the large 14-pellet reward. Thus, not surprisingly according to the present hypothesis, rats ran faster on -pellet trials in the single grouping and bad grouping conditions than in the good grouping condition. The Fountain et al. (1984) results are unfortunately ambiguous and cannot be employed to determine which is the more potent variable, allowing the periodic structure of the series to become more or less obvious, or grouping prior trials together with or apart from a subsequent large

23 Grouping and Chunking 75 reward. However, a similar ambiguity does not seem to be associated with Experiments 1 and 2 here, which were designed in part with the objective of differentiating between the two hypotheses. Thus it was found in Experiments 1 and 2 that in groups in which prior trials were grouped together with large reward, performance was poor despite the periodic structure of the series being obvious. Thus in Experiment 1 the six-item series was grouped for Group 3-3 so as to make the periodic structure of the series obvious, an initial decreasing threeitem series (18-la), followed by a subsequent increasing three-item series (-1-18) while simultaneously allowing prior trials to be grouped together with large reward (-1-18). On the other hand, the six-item series was grouped for Group 5-1 ( /18) so as to obscure or at least make less obvious the periodic structure of the series, while not allowing prior trials to be grouped together with large reward. However, Group 5-1, which received so-called relatively bad grouping, showed much better serial learning than Group 3-3, which received so-called relatively good grouping, suggesting that making the periodic structure of a series obvious is of less consequence than grouping prior trials together with or apart from subsequent large reward. In Experiment 2 presenting trials in three groups of three rather than a single group of nine should have made the periodic structure of the series more obvious for the increasing series as well as the decreasing series. Yet, in Experiment 2, presenting trials in groups of three facilitated serial learning relative to the single grouping much more in connection with the series than the series. The reason, as previously indicated, is that in the series prior trials are grouped together with a terminal large reward trial when trials are presented as either three groups of three or as a single group of nine. But in the series, prior trials are grouped together with terminal large reward only when trials are presented as a single group of nine. Thus the findings of Experiment 2 also suggest that making the periodic structure of a series obvious is of less consequence than whether prior trials are grouped together with rather than apart from a terminal large reward. The present findings may be explained by assuming that rats can discriminate between trials in the same runway or spatial alternative and in a changed runway or spatial alternative. Applying this view to Experiment 4, for example, it would be suggested that rats in Group learned that repeated trials in the same arm of the maze received partial reinforcement, whereas the rats in Group 8--/8 learned that repeated trials in the same arm were nonreinforced. A difficulty with this view is that it cannot explain prior findings obtained with the series both by Capaldi et al. (1984) and by Fountain et al. (1984). In both of those investigations, as indicated, the -pellet item

24 76 E. J. Capaldi et al. was more rapidly anticipated when rats were trained / / , etc., than when they were trained / / , etc. Under either method of training, repeated trials in the same runway were rewarded. Despite this, the group for which small reward and nonreward were grouped apart from the large 14-pellet reward showed superior anticipation of pellets. Thus, considering all available evidence, the critical factor regulating performance seems to be whether small reward and nonreward trials are grouped together with or apart from a subsequent large reward. As indicated, Katona (194) suggested the empirical generalization that presenting part of a group tends to reproduce the entire group. We inferred from this that if small reward trials are grouped together with a subsequent large reward trial, presenting a small reward trial will lead to an anticipation of large reward and thus to vigorous performance. The present results are in accord with this reasoning. A question is: What is the mechanism of such anticipation? We suggest it to be memory. In informal terms, if the memory of, say, an early -pellet trial is present when subsequently a large reward occurs, that memory will become a signal for large reward. Accordingly, when the memory of pellets is subsequently active, it will give rise to an anticipation of large reward and thus to vigorous performance. Let us express this view more formally. We are suggesting that the memory of one or more prior items in a series becomes a signal for the current item by being retrieved when the current item is presented. This view suggests that items form a chunk, a new unit or whole, when each item in the series is signaled by the memory of each prior item in the series. As an example, in the three-item series A-B-C we would have a single chunk if the memory of A signaled B and the memories of A and B signaled C. A chunk terminates when its final item fails to signal a subsequent item. Thus if A signaled B but B did not signal C, we would have two chunks, the two-item chunk AB and the single item chunk C. There are two general reasons why the memories A and B might fail to become a signal for C. First, the memories A and B might not be retrieved when C is presented. We shall elaborate on this possibility below. Second, the memories A and B might be retrieved when C is presented, but they might be overshadowed by the grouping cue. Consider this possibility in connection with Group 5-1 of Experiment 1, which received the events in an alley of one brightness and the final event 18 in an alley of another brightness. It is possible that memories of earlier reward events were retrieved on the terminal 18-pellet trial but were overshadowed as a signal for 18 pellets by the change in alley brightness. In any case, both the retrieval view and the overshadowing view yield a similar conclu-

25 Grouping and Chunking 77 sion: The grouping cue reduces the capacity of item memories to become signals for subsequent items. Another way of expressing this is that at group boundaries associations between successive items are weak or nonexistent. It is suggested that grouping affects memory retrieval and thus chunk formation as follows: Memories are always stored in a particular context, and a memory will be better retrieved to the extent that cues that accompanied storage are subsequently presented. When items are presented as a single group, as for example in the same spatial location, each item is presented in the same context as the preceding ones, which facilitates memory retrieval. If items are presented in two groups, say A and B in a different spatial location than C, it is likely that the memory of A will be retrieved when B is presented (same context), but the memories of A and B may be retrieved less strongly, if at all, when C is presented, terminating the chunk at B (different context). Only in one instance in this report was rapid running associated with small reward trials that were not grouped together with a large reward. Group 3-3 of Experiment 1, which received the grouping 18-1-/ -1-18, ran rapidly on 1- and -pellet trials of the grouping. Notice, however, that the memory of 1-pellet reward occurs subsequently on both -pellet trials (18-1-) and 18-pellet trials (-1-18). Accordingly, one may assume that when given the initial 18-pellet event, the rat anticipated the subsequent 1-pellet event, the memory of which was associated with both pellets and 18 pellets. To the extent that the terminal 18-pellet reward is anticipated, running should be rapid. Whatever interpretation one may favour as an explanation of grouping, the present results suggest that memory must be involved in at least the following manner: The grouping operation as employed here involves either changing or not changing a brightness or spatial cue from one trial to the next. In order for this operation to be successful, it would seem, the animal must remember the brightness or spatial cue from the immediately previous trials, comparing it to the current external available brightness or spatial cue. If the animal forgot which brightness or spatial cue occurred on the immediately previous trials, it would be difficult to understand how the grouping operation employed in any of the investigations reported here could be successful. Since the grouping operation was successful here, the implication is that the animal remembered the brightness or spatial cue from the previous trials. In the case of the spatial cue, comparing the previous spatial cue to the current one appeared to offer little difficulty even when 4 min separated the two events to be compared. This is consistent with other evidence indicating that rats are able to remember spatial locations over long intervals (Beatty and Shavalia, 198). As indicated previously, the effects

26 78 E. J. Capaldi et al. obtained in this report depend upon memory for other events as well, remembering whether smaller or larger reward occurred on prior trials. That rats were able to remember reward events at the 4-min IT1 employed in Experiment 5 is consistent with other evidence indicating such memory even at a 24-hr IT1 (e.g., Capaldi and Spivey, 1964; Jobe, Mellgren, Feinberg, Littlejohn and Rigby, 1977). Represent the memory of 8 pellets as 8, the memory of pellets as, and so on. We are suggesting that when a current item is presented, the animal may remember not merely the immediately prior item, as 8 on Trial 2 of the 8--8 series (Experiment 5), but each of two or more prior items, and 8 and (8 + ) on Trial 3 of the 8--8 series. This view is consistent not merely with the data presented here, but with a variety of previously presented data (Capaldi et al., 1983; Capaldi and Verry, 1981; Capaldi, Verry and Nawrocki, 1982; Capaldi et al., 1984). When 8 is retrieved on Trial 3 of the 8--8 series, we have what has been called a remote association, an association between two events separated in time and by one or more intervening events. On the basis of human serial learning data, some have stoutly denied that remote associations are formed (e.g. Bower, 1981; Slamecka, 1964), some have suggested the evidence to be ambiguous (e.g. Crowder, 1976) and some have vigorously defended the idea of remote associations (e.g. Bugelski, 1965; Shebilske and Ebenholtz, 1971). Whatever the status of the human serial learning data, the animal data cited above supported the notion that remote associations occur or, as we would prefer to express it, that items can become signals for remote items as well as for adjacent ones. And this may occur when, as in Group 8--8 of Experiment 5, the initial 8- pellet item occurs as much as 8 min prior to the terminal 8-pellet item (see Capaldi, 1985). REFERENCES Beatty, W. A. and Shavalia, D. A. (198). Spatial memory in rats: Time course of working memory and effect of anesthetics. Behavioral and Neural Biology, 28, Bower, G. (1981). Theories of learning. Englewood Cliffs, N.J.: Prentice Hall. Bower, G. H. and Winzenz, D. (1969). Group structure, coding, and memory for serial digits. Journal of Experimental Psychology Monographs, 8 (2, Pt. 2). Bugelski, B. R. (1965). In defense of remote associations. Psychological Review, 72, Capaldi, E. J. (1966). Partial reinforcement: A hypothesis of sequential effects. Psychological Review, 73, Capaldi, E. J. (1985). Anticipations and remote associations: A configural approach. Journal of Experimental Psychology : Learning, Memory, and Cognition, 11,

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