Blocking in the Spatial Domain

Journal of Experimental Psychology: Animal Behavior Processes 1997, Vol. 23, No. I , 110-118

Copyright 1997 by the American Psychological Association, Inc. 0097-7403/9743.00

Blocking in the Spatial Domain T. Rodrigo and V. D. Chamizo University of Barcelona

I. P. L. McLaren and N. J. Mackintosh University of Cambridge

An initial series of experiments with rats in a swimming pool established that they could find a hidden platform the location of which was defined in terms of 3 or 4 landmarks and that, when trained with all 4, any subset of 3 (or even, after a sufficient number of swimming trials, 2) landmarks was sufficient to produce accurate performance. When only one landmark was present during testing, however, performance fell to chance. Two additional experiments demonstrated a significant blocking effect: If rats were first trained to locate the platform with 3 landmarks, they did not learn to use a 4th landmark added to their initial set of 3.

How do animals solve spatial discriminations or find their way to a goal in a particular location? O'Keefe and Nadel (1978) drew a distinction between three possible mechanisms of spatial navigation that has had a profound impact on subsequent thinking about this question. According to an orientation hypothesis, animals can find their way to a goal by learning a series of orienting movements (e.g., go straight ahead for 10 paces; turn left or right at some specific cue). Alternatively, they may learn to approach a particular stimulus or set of stimuli associated with their final goal; when using this guidance system, a hungry rat trained to find food in a maze will associate stimuli at the goal box (or immediately adjacent to it) with food and, by a process of backward chaining, will associate earlier stimuli along the correct path with these goal stimuli and, thus, approach each in turn (see Deutsch, 1960, for an implementation of a guidance theory). These first two possibilities were sharply contrasted with true spatial or locale learning, which was said to involve animals using the entire configuration of distal landmarks to establish a spatial representation or map of the environment in which the goal was located. There is ample evidence that, in the laboratory, rats can solve maze problems by learning to turn left or right at a choice point (Scharlock, 1955) and that, in the field, a variety of animals can find their way to a goal by a system of dead reckoning that simply involves moving a given

distance in a given direction (Gallistel, 1994). The distinction between guidance and locale learning, however, is not quite so clear. There seems little doubt that, just as the locale theory requires, rats and other animals can use complex configurations of landmarks to locate their goal. In Morris's (1981) swimming pool, they can swim straight toward their goal, the submerged platform, even though it is quite invisible. That the processes involved in such navigation differ from those required to find a visible, raised platform is suggested by the devastating effects of hippocarnpal lesions on the former but not the latter ability (e.g., Morris, Garrud, Rawlins, & O'Keefe, 1982; Sutherland, Whishaw, & Kolb, 1982). However, only a few studies have investigated the way in which external landmarks are used in this apparatus. Several studies of maze learning, on the other hand, have shown that, where a number of landmarks have been available, a rat's performance is disrupted if their spatial configuration is changed (Suzuki, Augerinos, & Black, 1980) but not by the complete removal of a certain proportion, even of those most immediately adjacent to their goal (Barnes, Nadel, & Honig, 1980; Chamizo, Sterio, & Mackintosh, 1985). Thus, even where it would be perfectly possible for rats to solve a spatial discrimination by learning to approach a specific stimulus or small set of stimuli adjacent to their goal, they do not do so: They appear to rely on a much wider configuration of landmarks, not one of which is necessary. As Leonard and McNaughton (1990), among others, argued, this is hardly sufficient to establish that rats possess a truly maplike representation of their environment. We could imagine a sophisticated guidance system that relied on configurations of several cues or landmarks. A truly maplike representation would presumably be one from which new spatial information could be derived: Given the distance and direction from A to B and from A to C, it would allow derivation of the distance and direction from B to C, even though C was invisible from B. A guidance system, such as that proposed by Deutsch (1960), incorporates no such knowledge. An additional question, however, is whether there is any distinction in the nature of the learning processes underlying

T. Rodrigo and V. D. Chamizo, Departament de Psicologia Basica, Universitat de Barcelona, Barcelona, Spain; I. P. L. McLaren and N. J. Mackintosh, Department of Experimental Psychology, University of Cambridge, Cambridge, United Kingdom. This research was supported by grants from the Comissionat per a Universitats i Recerca de la Generalitat de Catalunya, the Spanish Ministerio de Education y Ciencia (Direccion General de Investigation Cientifica y Tecnica Grant PB93-0739), and the United Kingdom Biotechnology and Biological Sciences Research Council. Correspondence concerning this article should be addressed to N. J. Mackintosh, Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, United Kingdom.

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SPATIAL BLOCKING guidance and locale learning. Guidance learning was characterized by O'Keefe and Nadel (1978) as being incremental or associative and presumably, a matter of Pavlovian conditioning of approach or avoidance to particular stimuli (or configurations of stimuli). Locale learning, on the other hand, was said to involve a different process, perhaps occurring in an all-or-none manner and involving the building and updating of a complete representation of the environment in response to novelty. How might one set about testing this distinction? Morris (1981) suggested that if spatial navigation depends on learning processes common to those studied in Pavlovian conditioning experiments, then phenomena routinely found in such experiments, for example, latent inhibition and blocking, should be readily demonstrable in spatial learning as well. Chamizo et al. (1985) and March, Chamizo, and Mackintosh (1992) provided some evidence relevant to this issue. They trained rats in a maze either on a nonspatial discrimination between intramaze cues (rubber- or sandpaper-covered arms, with one cue correct regardless of its spatial location), or on a spatial discrimination between extramaze cues (where the correct arm was always in a fixed location), or on the combined problem where both sets of cues were relevant. They found reliable evidence of overshadowing and blocking between the two sets of cues. Thus, rats that had learned to find food by going down the rubber-covered arm, regardless of its location, did not learn, in the second stage of the experiment, that the rewarded, rubber-covered arm now always pointed to the north corner of the room. Although this evidence is suggestive, a more critical question is whether such a blocking effect would occur entirely within the spatial domain. Thus, if rats learned to navigate toward a goal defined by reference to a particular set of landmarks (A, B, and C) would they fail to learn to use a new landmark (X) when it was added to the original set? If maps are automatically updated whenever there is a mismatch between the actual and predicted input to a place representation, as O'Keefe and Nadel (1978) stated, then X should be incorporated into the rats' map and become available to guide navigation in the same way as A, B, or C. A simple-minded guidance theory, on the other hand, would presumably predict that A, B, and C should block the acquisition of control by X. As we have already seen, however, there is good reason to suspect that a simpleminded guidance theory provides an inadequate account of spatial navigation. If rats find their way to a goal by using configurations of several different landmarks, none of which is necessary, their representation of the goal is, so to say, overdetermined because it relies on a redundant set of cues. However, the occurrence of blocking and overshadowing suggests that the conditioning process pays little attention to redundant predictors of reinforcement. Thus, it is an open question whether we should expect to see blocking within the spatial domain. The experiments reported below addressed two questions concerning the way in which rats learn to find a submerged platform in a circular swimming pool. Experiment 1 asked whether they can learn to use a specified set of external

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landmarks to locate the platform and whether they can continue to do so successfully when one or more of the landmarks is removed. Experiments 2 and 3 asked whether blocking of one landmark by others can be observed. Having learned to locate the platform by landmarks A, B, and C, would rats learn to use a new landmark (X) when it was added? The apparatus consisted of a circular pool located in a larger, circular black enclosure with a number of objects placed around its walls. These objects or landmarks defined the location of the platform, which was situated between landmarks B and C, as is shown in Figure 1. To ensure that the rats used these landmarks, rather than any inadvertently remaining static room cues, to locate the platform, the landmarks and platform were rotated, with respect to the room, between each trial. Several pilot experiments were run to establish reliable procedures. In our initial attempts, static cues such as an air-conditioning inlet or the draught from the room door opening and shutting appeared to interfere with any control by our designated landmarks. It was also important to find landmarks that were of approximately equal salience such that performance was equally well maintained when any one of them was removed. Finally, we also wished to develop a procedure that permitted good spatial learning when rats were simply allowed to observe the relevant landmarks.

Method We describe first the apparatus and general procedures common to all experiments.

Apparatus The apparatus was a circular swimming pool, made of plastic and fiberglass, modeled after that used by Morris (1981). It mea-

© C

Figure 1. A schematic representation of the pool and four cues, A, B, C, and X, as well as the platform P.

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sured 1.58 m in diameter and 0.65 m in depth, and was filled to a depth of 0.49 m with water rendered opaque by the addition of 1 cl/1 of latex. The water temperature was maintained at 22 ± 1°C. The pool was situated in the middle of a large room, mounted on a wooden platform 0.43 m above the floor. The pool was surrounded by thick black curtains reaching from ceiling to floor and forming a circular enclosure 2.40 m in diameter. A closed circuit video camera with a wide-angle lens was mounted 1.75 m above the center of the pool in a black false ceiling, and its picture was relayed to recording equipment in an adjacent room. A circular platform, 0.11 m in diameter and made of transparent Perspex, was mounted on a rod and base and could be placed in one quadrant of the pool, 0.38 m from the side, with its top 1 cm below the surface of the water. The four landmarks used in Experiments 1 and 2 were as follows: A, a 40-W light placed inside a white plastic inverted cone 11 cm in height and 13 cm in diameter at the base; B, a plastic beach ball 30 cm in diameter with alternate blue-white, yellowwhite, and orange-white vertical segments; C, an intermittent 1-W light flashing on and off at a frequency of 60-80 times per minute; X, a green plastic plant approximately 35 cm in diameter and 30 cm in height. These could be suspended from the ceiling 35 cm above the surface of the water and with their mid-line directly above the wall of the pool. Their location, relative to the platform, was as shown in Figure 1. The entire false ceiling, with these landmarks suspended, could be rotated from trial to trial with the platform always rotating with them. Experiment 3 called for an additional three landmarks. These were A', a string of colored Christmas tree lights, consisting of eight 2.75-W bulbs flashing on and off 15 times per minute; B', a white cardboard cone 16 cm in diameter and 59 cm in height, with 1-cm thick black horizontal stripes spaced 3.5 cm apart; C', a white cardboard cube, 20 cm on edge, with a black circle 9.5-cm in diameter painted on each side.

A test trial consisted of placing the rat in the pool, with landmarks present but without the platform, and leaving it there for 120 s. For purposes of recording the rat's behavior, the pool was divided into four quadrants, A-B, B-C (the platform quadrant), C-X, and X-A, and the amount of time that the rat spent in each quadrant was recorded.

Experiment 1 The aim of Experiment 1 was to establish that, when trained with three or four landmarks that, along with the platform, were rotated from trial to trial, the rats would learn to locate the platform—the test of such learning being that they would spend more time in the platform quadrant than in any of the other three quadrants on a test trial. A second aim was to establish that the various landmarks were equally effective by showing that the rats performed equally above chance with any subset of the four. The third was to show that they could not locate the platform when only a single landmark was present. Finally, although our initial pilot work used standard escape trials, on which rats were free to swim to the platform from any start position, we also wished to develop a placement training procedure

(see

Whishaw, 1991). This was intended as a control against uninteresting explanations of any blocking effects observed in later experiments. One design of a blocking experiment would involve initial training of one group with three landmarks, A, B, and C, followed by a second stage in which this blocking group and a control group were trained with A, B, C, and X. Any difference between the two groups in control by X might be due to the fact that the control group spent more time swimming round the pool during this

General Procedure There were four types of trial: pretraining, escape training, placement, and test. Pretraining consisted of placing the rat into the pool, without landmarks but (except in Experiments 1A and IB) with the platform present. The rat was given 180 s to find the platform, where it was allowed to stay for 30 s. If it had not found the platform within 180 s, it was picked up, placed on it, and left there for 30 s. Rats were given between two and five such pretraining trials, at a rate of one per day. The platform was moved from one trial to the next, and the rat was placed in the pool in a different location on each trial (at one of the four points, A, B, C, X, in Figure 1) equally often as far as possible on the same or opposite side of the pool from the platform and with the platform to the right and to the left of where the rat was placed. The procedure for escape training was exactly the same as for pretraining, except that some landmarks were always present and, in Experiments 1A and IB only, the rats were given only 120 s to find the platform. There were usually four escape trials per day, with an average intertrial interval (IT1) of 10—20 min. The landmarks and platform were rotated 90° anticlockwise between trials. On each block of four trials, the rat was placed in the pool once at A, once at B, once at C, and once at X. Placement trials involved placing the rat directly onto the platform and leaving it there for 30 s. Landmarks were always present. There were usually eight placement trials per day with an average ITI of 7-8 min. The platform and landmarks were rotated 90" anticlockwise between each trial.

second stage and, thus, had more opportunity to learn the relationship between X and the other landmarks and the platform. If it were possible to give most training as placement trials, this would tend to control the animals' exposure to the various landmarks in this second stage of the experiment. It proved to be impossible, however, to obtain good performance

on the

test

without giving

some

escape

training. Experiment 1 consisted of three subexperiments. In Experiment 1A, we gave extensive escape training with four landmarks, followed by test trials with all possible combinations of two or three of them. In Experiment IB, we gave a moderate amount of escape training with four landmarks, followed by test trials with all possible pairs and with each landmark in isolation. Finally, in Experiment 1C, we used placement trials with three landmarks followed by test trials with either one, two, or three landmarks.

Experiment 1A Method Animals. Ten hooded Long Evans rats, four males and six females, were used. They were maintained on ad-lib food and water, in a colony room maintained on a 12:12-hr light-dark cycle, and were tested within the first 4 hr of the light cycle. Procedure. After two 180-s pretraining trials with no platform,

SPATIAL BLOCKING all rats received 14 days of escape training, 8 trials per day (a total of 112 trials) with all four landmarks. A, B, C, and X. On each of the next 2 days, they received 4 escape trials with all four landmarks, followed by a single test trial. On the first test trial, two rats were tested with the combination A, B, and C, and two with A, C, and X; two with A and B; two with A and X, and two with B and X. On the second day, the remaining five combinations of landmarks were used for two rats each: B, C, and X; A, B, and X; C and X; B and C; and A and C.

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Results All animals decreased their latency to find the platform over the 6 days of escape training, averaging 50.12 s on Day

1 and 16.86 s on Day 6. On the two-landmark test, the mean rank of the platform quadrant was 1.5, significantly better than chance ((11) = 3.45, p < .01; but on the one-landmark test, performance was slightly worse than chance, with a mean rank of 2.75. The difference between the two tests was also significant,

r(ll) = 2.92, p < .05. Results All 10 rats decreased their latency to find the platform over the 14 days of escape training. On Day 1, their average latency was 43.85 s; on Day 14, it was 8.93 s, having been below 20 s since Day 4. In this and in all subsequent experiments, we measured test performance by rank ordering the four quadrants of the pool in terms of the amount of time spent in each out of the 120 s of the test trial. Perfect performance on this measure would be a rank of 1.0 for the platform quadrant; chance performance would be a rank of 2.5. We used this measure, rather than the more usual one of actual time spent in each of the four quadrants because we found this latter measure to be very variable. Although rank-order scores are conventionally analyzed by nonparametric statistics, in most cases, we report the result of parametric tests because we were often interested in interactions between certain variables. In practice, this makes essentially no difference to our conclusions: With one exception, all the statistical inferences that we report are confirmed by nonparametric tests. The results of Experiment 1A are described simply. On Day 1, all 10 rats had a rank of 1.0 for the platform quadrant (p < .01, binomial test); on Day 2, 8 had a rank of 1.0 and 2, a rank of 2.0 (p < .01). One of these latter rats had been tested with A, C and the other with B, C, X. Although there are too few data points for each pair of landmarks to justify any strong conclusion, the suggestion is that any subset of two or more of the four landmarks was as effective as any other in enabling rats to find the platform.

Experiment

IB

Method Animals. The animals were 12 Long Evans rats, six males and six females, maintained as before. Procedure. After two pretraining trials as in Experiment 1A, the rats received 6 days of escape training, at eight trials per day, with the four landmarks. A, B, C, and X. On the following day, they received four escape trials with all four landmarks, followed by a single test trial with two landmarks. Two rats were tested with each of the six possible combinations of two landmarks. On the following day, they received an additional four escape trials with all four landmarks before a final test trial on which they were tested with a single landmark, three rats with A, three with B, and

Experiment 1C Method Animals. The animals used were 21 Long Evans rats, 10 males and 11 females, maintained as before. Procedure. After four pretraining trials with the platform present, the rats received 14 days of 30-s placement trials, eight trials per day, with the three landmarks A, B, and C. On the following day, they received a single pretraining trial with the platform, but no landmarks, followed by four placement trials and a single test trial. On this trial, four rats were tested with each of the three possible combinations of two of the three landmarks; three of the remaining nine rats were tested with each of the three single landmarks (except that, by error, only two rats were tested with A and four with B). On the next day, all rats received four escape training trials, followed by a single test trial with all three landmarks; they then received two additional escape trials, followed by a single test trial on which three rats were tested with each of the combinations of two landmarks and four rats, with each of the individual landmarks. Animals tested with two landmarks on the first test trial were tested with one on this trial and vice versa.

Results On Day 1 of preliminary training, only five rats found the platform within the 180-s time limit. By Day 4, all rats found the platform, with a mean latency of 37.14 s. The first test trial, to one or two landmarks, without any prior escape training, gave no evidence of the rats' being able to find the platform. For those tested with two landmarks, the mean rank of the platform quadrant was 2.17; for those tested with only one landmark, it was 2.33. These scores were not significantly better than a chance score of 2.50 (for the two tests combined, t = 1.05, p > .10). Before the next test trial, rats received four escape training trials. Their mean latency to find the platform on the first of these trials was 40.45 s; on the fourth, it was 31.14 s. This decline was not significant. On the second test, with all three landmarks, the mean rank of the platform quadrant was 1.43, with 14 of the 21 rats spending more time in this quadrant than in any other. A t test showed that performance was significantly better than chance, f(20) = 7.25, p < .01. On the two escape trials before the third test, the mean latency to find the platform was 31.40 s, much the same as it had been on the last escape trial the previous day. On this

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test, with one or two landmarks, performance reverted to marginally worse than chance, the mean rank of the platform quadrant being 2.78 with two landmarks and 2.67 with

control group than had been dropped from the blocking group.

Method Discussion Experiment 1A established that extensive escape training with the four landmarks could produce very efficient performance: The latency to find the platform declined to under 10s and, when tested with any combination of two or three of the landmarks, rats spent more time in the platform quadrant than in any other section of the pool. Experiment IB confirmed that rats could locate the platform using any two of the four landmarks, even after substantially fewer escape trials, but that they were unable to locate it with any one of the landmarks on its own. Although the number of placement trials in Experiment 1C was the same as the number of escape trials in Experiment 1A and twice as many as the escape trials in Experiment IB, it seems clear that placement was not as effective in teaching the rats to find the platform (see Whishaw, 1991, for a similar conclusion). When given subsequent escape trials, latencies to find the platform remained about 30 s and there was no evidence of better than chance performance on test trials with two landmarks. However, when tested with the three landmarks used in training, rats performed well above chance, so that we may conclude that placement training (with a small number of additional escape trials) is sufficient to produce significant spatial learning.

Experiment 2 Having established that rats use our landmarks to locate the platform and that any combination of two or more landmarks is sufficient but a single landmark alone is not, we are now in a position to investigate whether spatial landmarks can block one another. Experiment 2 included two groups of rats, one given initial training in Stage 1 with A, B, and C, and both then trained in Stage 2 with A, B, C, and X. Because this training was via placement trials, we assumed, on the basis of the results of Experiment 1C, that rats would perform above chance only when tested with three landmarks. Therefore, control by X was assessed by testing animals with A, C, and X; rats were also tested with A, B, and C to see whether they had learned the basic spatial discrimination. In Experiment 1C and in other pilot work using the placement procedure, it was evident that not all rats would show good location of the platform after such training. Therefore, we adopted a criterion for retaining animals for additional testing. The blocking group was given an initial test with A, B, and C following Stage 1 training, and animals spending less than 35 s of this 120-s test trial in the platform quadrant were excluded. A similar criterion was used to exclude animals in the control group, with the proviso that no more rats should be dropped from the

Animals. Thirty Long Evans rats were used, 15 male and 15 female, maintained as in Experiment 1. They were divided at random into two groups of 15, 7 males and 8 females in one group and 8 males and 7 females in the other. Procedure. After 5 pretraining trials, the blocking group received 10 days of placement trials, 8 trials per day (a total of 80 trials) with the landmarks A, B, and C. On the following 2 days, they received 4 escape trials each day followed by a single test trial, in counterbalanced order, either to A, B, and C or to A andC. In the second stage of the experiment, blocking and control groups received 40 placement trials (8 per day) with A, B, C, and X. (For the control group, these were immediately preceded by 5 pretraining trials.) On each of the following 4 days, all animals received 4 escape trials with all four landmarks, followed by a single test trial, on Days 1 and 2 with A, C, and X and on Days 3 and 4 with A, B, and C.

Results On the first pretraining trial, the mean latency to find the platform was 127.7 s in the blocking group and 113.3 s in the control group. By Day 5, these latencies had declined to 31.9 s and 33.9 s, respectively. Two rats in the blocking group became ill during the course of the experiment, one hi the first stage and the other in the second stage. Both were discarded from the analysis. Of the remaining 13 rats, 5 spent less than 35 s in the platform quadrant on their test with A, B, and C following this stage, leaving 8 rats to continue through to the second phase. On this first A, B, and C test, the mean rank of the platform quadrant for these eight rats was 1.25, t(7) = 7.64, p < .01; but on the A and C test, the mean rank for the platform quadrant was 2.37, not significantly better than chance (r < 1). As in Experiment 1C, therefore, placement training was not sufficient to establish good control by two landmarks. The critical test results are those for A, B, and C and A, C, and X following Stage 2 of the experiment. On each of the two A, B, and C tests, 4 animals from the control group spent less than 35 s in the platform quadrant and were discarded. The results of each of the four test trials for the remaining 11 rats in the control group and 8 rats in the blocking group are shown in Figure 2. On the first A, C, and X test, there was only a slight difference between the two groups, and neither performed much above chance. However, although the control group's performance improved on the second A, C, and X test, that of the blocking group did not. Finally, although both groups performed above chance on both A, B, and C tests, the blocking group now performed better than the control group, particularly on the first day. An analysis of variance (ANOVA) with groups, test type, and days as factors revealed a significant difference between A, C, and X and A, B, and C tests, F(l, 17) = 21.99,

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SPATIAL BLOCKING

Blocking group

Control group

1.0

ACX1

ACX2

ABC1

ABC 2

Test Figure 2. Experiment 2. Mean rank of the platform quadrant on each of the 4 test days for animals in the blocking and control groups. Error bars show the standard error of the mean of each data point.

p < .01, and a significant interaction between groups and tests, F(l, 17) = 11.94, p < .01. The interaction between groups and days was not significant, F(l, 17) = 1.99, p > .10. All other Fs < 1. Analysis of simple effects revealed that the blocking group performed significantly worse than the control group on the A, C, and X test, and significantly worse on this test than on the A, B, and C test (ps < .05); no other difference was significant. Although not strictly justified by the results of the overall ANOVA, it is worth reporting the results of other analyses undertaken separately on Days 1 and 2 of each type of test trial. On the first A, C, and X and A, B, and C test trials, there was a significant difference between A, C, and X and A, B, and C tests, F(l, 17) = 11.14, p < .01, but the interaction between groups and type of test fell short of significance, F(l, 17) = 4.14, p < 0.10. On Day 2, however, both the main effect of test trial and the interaction between test and group were significant, F(l, 17) = 13.37 and 10.49, respectively, p < .01, and analyses of simple effects revealed that the groups differed on the A, C, and X test and that the blocking group performed more accurately on A, B, and C than on A, C, and X. As we noted earlier, one possible interpretation of a blocking effect is that blocking and control groups differ in their opportunity to learn about the added cue. Our use of placement trials for training was designed to equate our two groups' experience with the added landmark, X. However, we found that it was necessary to provide a minimal number of escape trials before each test trial in order to obtain performance above chance. If some learning about the landmarks occurred on these trials, it is possible that even

though both groups had received the same number of placement trials with A, B, C, and X, the blocking group's previous training might have meant that they swam more directly to the platform on these escape trials and, thus, had less opportunity to learn about X. We attempted to measure this in two ways: by latencies on these escape trials and by the extent to which animals deviated from the direct path. Averaged over all eight escape trials to A, B, C, and X given before the A, C, and X tests, there was, indeed, a difference between the two groups in latency to find the platform; for the blocking group, the mean latency was 22.0 s; for the control group, the mean latency was 30.18 s. This difference was significant, F(l, 17) = 5.98, p < .05. However, the difference between the two groups' escape latencies decreased from 12.18 s on Day 1 to only 1.46 s on Day 2, although the difference in their test performance was present only on the second, not on the first A, C, and X trial. Moreover, the fact that the blocking group's test performance did not improve from the first to the second A, C, and X trial suggests that time spent swimming on escape trials was not the critical factor in determining performance. Following Whishaw (1991), we also measured the extent to which rats stayed within a narrow "corridor" between starting point and platform during their escape trials. Over all escape trials preceding the A, C, and X tests, 16% of the blocking group's and 11% of the control group's trajectories were confined to a 0.3-m wide corridor, whereas 25% of the blocking group's and 16% of the control group's trajectories were confined to a 0.5-m wide corridor. Although the blocking group was somewhat more accurate than the control group on both these measures, neither difference approached significance, (F < 1 and 1.24, respectively).

Discussion Experiment 2 provides clear evidence of blocking within the spatial domain: Rats that had already learned to locate the submerged platform in a swimming pool by reference to three landmarks, A, B, and C, learned less about a fourth landmark, X, when it was added than did a control group trained with all four landmarks from the outset. Because both groups had the same opportunity to learn about X during 40 placement trials, the small difference between them in latency on the 4 escape trials before the first test seems barely sufficient to explain why the control group showed as much control by X as by other landmarks and the blocking group showed none.

Experiment 3 Experiment 3 was designed to confirm the blocking effect observed in Experiment 2, using a slightly different design, in which the control group also received placement trials in the first phase of the experiment, but with a different set of landmarks, A', B', C' (see Experimental Method earlier for a description of these landmarks).

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Method Animals. Twenty Long Evans rats were used, 10 males and 10 females, maintained as in previous experiments, divided equally into two groups of 10, and matched for latency to find the platform on pretraining trials. Two rats from the control group were dropped during the course of the experiment: 1 because of illness, the other because of experimental error. Procedure. Following five pretraining trials, all rats received 6 days of placement trials, eight trials per day. The length of each trial was increased from 30 to 60 s so that the total amount of exposure to the platform was greater than in Experiment 2. A second change from previous experiments was that the platform and landmarks were rotated to a random new position from trial to trial rather than always being rotated 90° anticlockwise. For the blocking group, the three landmarks, A, B, and C were present on all placement trials, with the platform between B and C. For the control group, the three landmarks were A', B', and C', with the platform between A' and B'. At the end of this first stage of the experiment, all rats received four escape trials on each of 2 days, followed immediately, on Day 2, by a single test trial; the three landmarks on which they had been trained in Stage 1 were present on all these trials. One rat from the blocking group was eliminated at this point for spending less than 35 s in the platform quadrant on the test trial. In Stage 2, the remaining 17 rats received 4 days of placement trials, eight 60-s trials per day, with the four landmarks, A, B, C, and X (with the platform between B and C). On the next day, they received four escape trials with all four landmarks and, on each of the following 4 days, four escape trials with the four landmarks plus one test trial. On the first and second of these test trials, the landmarks present were A, C, and X; on the third and fourth, they were A, B, and C.

ANOVA, with groups, type of test, and days as factors did not reveal a significant difference between the two groups, F(l, 15) = 3.94, p < .10; but there was a significant difference between type of test, F(l, 15) = 18.93, p < .01; and significant interactions between groups and days, F(l, 15) = 5.03, p < .05, and, most important, among groups, tests, and days, F(l, 15) = 5.33, p < .05. No other effect was significant (largest F = 1.05). The interaction among groups, test trial, and days was evaluated further by analysis of the Group X Test interaction separately for each day of testing. The interaction was not significant on Day 1, but was significant on the second, F(l, 15) = 6.01, p < .05. Tests for simple effects revealed that the groups differed on this second A, C, and X test but not on the second A, B, and C test. Moreover, the blocking group, but not the control group, was more accurate on the A, B, and C tests than on the A, C, and X tests. Discussion In spite of their difference in design, the main results of the two blocking experiments are relatively similar. On their first test trial, with A, C, and X, at the end of Stage 2, neither group showed much evidence of locating the platform. But by the second test trial, after four more escape trials, the control group performed significantly better than chance, whereas the blocking group was still unable to locate the platform. Both groups performed at better than chance levels on both A, B, and C trials. Whether this was because they had by now received more escape trials or because the combination of A, B, and C provided a better set of land-

Results Over the eight escape trials that preceded the test trial at the end of Phase 1, the mean latency to find the platform was 27.99 s for the blocking group and 22.81 s for the control group (F < 1). On this test trial, all animals in the control group and all but one in the blocking group spent more time in the platform quadrant than any other (for this last animal, the rank of the platform quadrant was 2.0). The mean latencies on each of the 5 days of escape training at the end of Stage 2 are shown in Figure 3. As can be seen, both groups showed some decline in latency over the 5 days, but except perhaps on Day 2, there is little suggestion of any difference between the two groups. An ANOVA revealed a significant effect of days, F(4, 60) = 4.10, p < .01, but no difference between groups (F < 1) and no interaction, F(4, 60) = 1.65, p > .10. The results of the four test trials are shown in Figure 4. As can be seen, there was little difference between the blocking and control groups on the first A, C, and X test trial, the performance of neither group being much better than chance; on the second A, C, and X test, however, the control group's performance improved, but that of the blocking group did not. On the A, B, and C test trials, on the other hand, both groups performed above chance on the first test, both improved slightly from Test 1 to Test 2, and there was only a small difference between the two groups. An overall

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Figure 3. Experiment 3. Mean latencies for the blocking and control groups over the 5 days of escape training that preceded testing in Stage 2.

SPATIAL BLOCKING

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ACX1

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Test Figure 4. Experiment 3. Mean rank of the platform quadrant on each of 4 test days for the blocking and control groups.

marks for locating the platform than A, C, and X, cannot be determined from these results alone. Because the platform was located between B and C, it is certainly possible that they would have served as the best landmarks. The results of Experiment 1, however, provided little reason to believe that some combinations of landmarks were better than others (see later for additional documentation of this) and certainly showed the importance of escape training. Although there was some difference between the two groups in latency on escape trials preceding the A, C, and X tests in Experiment 2, there was none in Experiment 3. A plausible explanation is that, in Experiment 3, the control group had also received escape trials at the end of Stage 1. At any rate, the important inference is that the difference in performance on A, C, and X trials cannot reasonably be attributed to any such difference in escape latencies.

General Discussion Experiment 1 established, for the first time so far as we know, that rats could find an unseen platform in a swimming pool when the only way of determining its location was by reference to three or four specific external landmarks and that, when most of their initial training involved learning to swim to the platform, any two of these landmarks were sufficient for its accurate location. When only a single landmark was present, however, they showed no preference for the platform quadrant on test. Although placement trials plus minimal swimming practice were sufficient to allow accurate location of the platform with three landmarks, they provided a notably less efficient method of training (see Whishaw, 1991, for earlier documentation of

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this point), and test performance with only two landmarks was at chance when most training had been by placement. Experiments 2 and 3 provided evidence of blocking within the spatial domain. If rats were trained with three landmarks, A, B, and C, they did not appear to use a fourth landmark, X, when it was added to the array. One potential explanation of blocking in this situation is that if such pretrained animals had swum more rapidly and directly to the platform on A, B, C, and X trials than control animals, they might have been less likely to notice X. We attempted to minimize this possibility by giving most A, B, C, and X training as placement trials, which ought to have allowed both blocking and control groups equal opportunity to view X. We. found, however, that without some swimming trials, the rats' performance on test trials was very inaccurate, so all rats did receive a small number of such trials with A, B, C, and X before their critical test trials. In Experiment 2, rats in the blocking group did find the platform significantly more rapidly on the first few of these escape trials than did rats in the control group. However, a measure of how closely they followed the direct route from start position to platform revealed no significant difference between the groups. In Experiment 3, there was no difference between groups in their latencies to find the platform on these trials (unfortunately, data on how closely they kept to the direct route were not available for this experiment). There is, thus, no evidence to support the conjecture that the blocking group's failure to use landmark X was attributable to any lesser opportunity to see it. It is also true that the added landmark X was always on the opposite side of the pool from the platform and so might have been of lesser salience than the other landmarks. Although any such difference in salience would presumably have applied equally to blocking and control groups, the results of Experiment 1 tend to contradict this possibility. In Experiment 1A, all four landmarks appeared to be equally effective; whereas in Experiment IB, where test performance was slightly less efficient, performance on test trials that included X was actually slightly more accurate (a mean rank of 1.0 for the platform quadrant) than it was on test trials that did not include X (mean rank of 2.00 for the platform quadrant). This difference was not, however, significant. We conclude that Experiments 2 and 3 provide good evidence of blocking of one spatial landmark by others. Such a conclusion has important implications for theories of spatial localization and navigation. According to the version of a cognitive mapping hypothesis, initially espoused by O'Keefe and Nadel (1978) and taken over by other investigators (e.g., Poucet, Chapuis, Durup, & Thinus-Blanc, 1986), animals' maps or representations of their spatial environments are automatically updated to incorporate new or changed landmarks. A mismatch detector notes any discrepancy between observed and expected landmarks and engages exploratory activity that leads to the incorporation of the new landmark into an updated map. On the reasonable assumption that such marginal updating should take less time than constructing a complete map de novo, one would expect the blocking group to learn more about X than

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the control group. Any version of the mapping hypothesis that hopes to accommodate these data must find a more suitable analogy than the rat as cartographer. To the extent that a guidance theory of spatial navigation is based on general principles of associative learning, it may seem a simple matter for such a theory to predict a blocking effect. However, it is clear that animals' ability to locate themselves and their goals in a spatial environment depends on their use of a complex configuration of most, if not all, available spatial landmarks. As we noted in the beginning, this reliance on a number of redundant landmarks is a feature of spatial learning that might make it problematic to predict blocking. An associative theory that uses a standard error-correcting rule, however, can expect that the learning required to incorporate X into new configurations with A, B, and C to define the location of the platform may occur more slowly in pretrained animals than in control animals because the effect of such pretraining with A, B, and C should be to reduce the mismatch between them and the goal position. This is to be contrasted with the apparent prediction of the cognitive mapping hypothesis that the updating required to incorporate X into a partially complete map should occur more rapidly than that required to construct an entirely new one. To this extent, our results point to an associative and configural theory of spatial navigation.

References Barnes, C. A., Nadel, L., & Honig, W. K. (1980). Spatial memory deficit in senescent rats. Canadian Journal of Psychology, 34, 29-39. Chamizo, V. D., Sterio, D., & Mackintosh, N. J. (1985). Blocking and overshadowing between intra-maze and extra-maze cues: A test of the independence of locale and guidance learning. Quarterly Journal of Experimental Psychology, 37B, 235-253. Deutsch, J. A. (1960). The structural basis of behavior. Chicago: University of Chicago Press.

Gallistel, C. R. (1994). Space and time. In N. J. Mackintosh (Ed.), Animal learning and cognition (pp. 221-248). San Diego: Academic Press. Leonard, B., & McNaughton, B. L. (1990). Spatial representation in the rat: Conceptual, behavioral and neurophysiological perspectives. In R. P. Kesner & D. S. Olton (Eds.), Neurobiology of comparative cognition (pp. 363-422). Hillsdale, NJ: Erlbaum. March, J., Chamizo, V. D., & Mackintosh, N. I. (1992). Reciprocal overshadowing between intra-maze and extra-maze cues. Quarterly Journal of Experimental Psychology, 45B, 49-63. Morris, R. G. M. (1981). Spatial localization does not require the presence of local cues. Learning and Motivation, 12, 239-260. Morris, R. G. M., Gamid, P., Rawlins, J. N. P., & O'Keefe, J. (1982). Place navigation impaired in rats with hippocampal lesions. Nature, 297, 681-683. O'Keefe, J., & Nadel, L. (1978). The hippocampus as a cognitive map. Oxford, England: Clarendon Press. Poucet, B., Chapuis, N., Dump, M., & Thinus-Blanc, C. (1986). A study of exploratory behavior as an index of spatial knowledge in hamsters. Animal Learning and Behavior, 14, 93-100. Scharlock, D. P. (1955). The role of extramaze cues in place and response learning. Journal of Experimental Psychology, SO, 249-254. Sutherland, R. J., Whishaw, I. Q., & Kolb, B. (1982). A behavioural analysis of spatial localization following electrolytic, kainate- or colchicine-induced damage to the hippocampal formation in the rat. Behavioural Brain Research, 7, 133-153. Suzuki, S., Augerinos, G., & Black, A. H. (1980). Stimulus control of spatial behavior on the eight-arm maze in rats. Learning and Motivation, 12, 239-260. Whishaw, I. Q. (1991). Latent learning in a swimming pool place task by rats: Evidence for the use of associative and not cognitive mapping processes. Quarterly Journal of Experimental Psychology, 43B, 83-103.

Received February 22, 1996 Revision received July 13, 1996 Accepted July 22, 1996

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