latent learning and latent inhibition in maze discriminations

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Latent learning and latent inhibition in mazediscriminationsV. D. Chamizo a; N. J. Mackintosh ba University of Barcelona, Spainb University of Cambridge, U. K.

Online Publication Date: 01 February 1989To cite this Article: Chamizo, V. D. and Mackintosh, N. J. (1989) 'Latent learning andlatent inhibition in maze discriminations', The Quarterly Journal of ExperimentalPsychology Section B, 41:1, 21 - 31

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THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 1989 418 (1) 21 -31

Latent Learning and Latent Inhibition in Maze Discriminations

V. D. Chamizo University of Barcelona, Spain

N. J.'Mackintosh University of Cambridge, U.K.

Rats were trained on an elevated maze where the rewarded alternative was defined either in terms of intra-maze or in terms of extra-maze cues. Pre- exposure to these cues produced a small latent or perceptual learning effect, i.e. facilitated subsequent learning of both problems, by comparison with animals pre-exposed to the maze with no cues present. Experiment 2 examined whether the effect of pre-exposure on intra-maze discrimination learning varied with the nature of the intra-maze cues. When positive and negative arms were further differentiated by painting the walls white and black, a marginal perceptual learning effect was turned into significant latent inhibition, i.e. a retardation of subsequent learning. Pre-exposure thus reliably facilitated extra-maze discrimination learning, and its beneficial effects on intra-maze discrimination could be reversed by reducing the overlap between the intra-maze cues. Perceptual learning may therefore depend on requiring animals to discriminate between stimuli containing many common elements.

Prior unreinforced exposure to the conditioned stimulus in a Pavlovian conditioning experiment, or to the discriminative stimulus in an operant experiment, typically results in latent inhibition, or a retardation of subsequent conditioning (Halgren, 1974; Lubow, 1973). But two classes of study

Requests for reprints should be sent to Victoria D. Chamizo, Universidad de Barcelona, Facultad de Psicologia, Departamento de Psicologia Basica, Adolf Florensa s/n, 08028- Barcelona, Spain.

This research was supported by grants from the U.K. Science and Engineering Research Council to N. J. Mackintosh, and by grants to both authors from the British Council and the Spanish Ministerio de Asuntos Exteriores.

We thank all the members of the animal learning laboratory, Department of Experimental Psychology, Cambridge, for their constant support, and especially A. Dickinson and I. P. L. McLaren for valuable comments.

0 1989 The Experimental Psychology Society

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22 CHAMIZO AND MACKINTOSH

have produced a quite different outcome. Experiments on perceptual learning (Channel1 & Hall, 1981; Gibson & Walk, 1956) have found that prior exposure to such stimuli as circles and triangles or horizontal and vertical striations may facilitate their subsequent discrimination. And experiments on latent learning (Blodgett, 1929; Tolman & Honzik, 1930) have found that unreinforced exposure to a multiple-unit maze will also facilitate the learning of the true path when reward is subsequently introduced.

There are, of course, numerous differences between those experiments where latent inhibition is observed and those where latent or perceptual learning occurs, and there is no reason to suppose that a single factor is responsible for all differences in outcome. One account of the latent learning observed in maze experiments, that implied by O’Keefe and Nadel’s (1978) theory, would presumably not apply to typical perceptual learning experiments. O’Keefe and Nadel distinguish between two modes of solution for maze problems, taxon and locale learning. Taxon learning is akin to Pavlovian or operant conditioning, with animals learning to associate particular stimuli or responses with reinforcement, whereas locale learning involves the establishment of a spatial map of the animal’s environment in which goals and routes can be located. If rats solved a simple maze problem by taxon learning, one might expect that prior exposure to the maze cues would produce the effect typically observed in conditioning experiments, namely latent inhibition. But if their solution involved locale learning, prior exposure to the maze and its environment could be expected to benefit subsequent performance by ensuring that an accurate map had already been established- hence the latent learning effect observed in some maze experiments.

A test of this supposition requires that it be possible to identify experimental situations that engage taxon or locale learning, or at least those that bias animals towards solving a problem in one or other way. Although the argument is necessarily indirect, it seems reasonable to suggest that when rats are trained on a maze discrimination where the correct alternative is defined by a particular intra-maze cue, regardless of its location, their solution will be based on taxon learning, and that where the correct alternative is defined in terms of a particular location in the room, i.e. by reference to extra-maze cues regardless of any intra-maze cues, their solution will be based on locale learning (Chamizo, Sterio, & Mackintosh, 1985).

We therefore ran a number of experiments in which rats were required to use either intra-maze or extra-maze cues to solve a maze problem, and where experimental animals had received prior exposure to both sets of cues, whereas controls had simply remained in their home cages until the start of discriminative training. To our surprise, prior exposure significantly facilitated the learning of both intra-maze and extra-maze discriminations, and there was no reliable suggestion that this latent learning effect was more

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LATENT LEARNING AND INHIBITION 23

pronounced for one type of problem than the other. One possible explanation of this outcome is that the maze was almost entirely novel for control animals at the start of discriminative training, and being placed on an elevated maze for the first time is a disturbing experience. Latent learning, therefore, may have reflected habituation of this arousal in experimental animals rather than any specific learning about the intra- or extra-maze cues. In order to control for this possibility it would be necessary to expose control animals to the maze, with no cues present, for the same length of time as experimental animals. Experiment 1 reports just such a test of the possibility that pre-exposure may have different effects on intra-maze and extra-maze discrimination learning when all animals are equally habituated to the apparatus.

EXPERIMENT 1

Method

The subjects were 39 male Hooded Lister rats that had previously participated in a lever-press experiment and were approximately five months old at the beginning of the present experiment. They were caged in groups of two and put on food deprivation until they had reduced to 85% of their ad libitum weights. They were kept at this weight by being fed a fixed amount one hour after each day’s trials. Seven animals were dropped from the experiment because they failed to run in the maze on pre-test trials, leaving a total of 32 rats divided into four groups of eight.

Subjects.

Apparatus. The apparatus was an elevated eight-arm radial maze made out of wood, Perspex and aluminium (see Chamizo et al., 1985). Only three of the arms were used on any one trial. They were connected to an octagonal platform 37 cm in diameter, with eight 20-cm high Perspex guillotine doors, which were controlled by a hand-operated device located in the middle of one wall of the experimental room. Each arm was 76 cm long x 9 cm wide and had 2.5-cm high walls made of brown hardboard along the two sides and far end. One arm, used as the start-arm, was plain wood. The other two were used as goal-arms; one was covered with black rubber, the other with dark red sandpaper (aluminium oxide paper, 100 grit). Each goal-arm had a recessed 4.5-cm diameter food-cup located 4cm from the far end, and triangular baffles 12 cm high at the central platform extending 12 cm along the arm from the platform to prevent rats jumping from one goal-arm to another. The maze was placed in the centre of a room, 2.5 x 4.3 x 3.0 m high, illuminated by two fluorescent ceiling lights. The plan of the room and the various extra-maze cues is shown in Figure 1.

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FIG. 1. Plan (to scale) of experimental room, showing all eight possible arms of the maze. 1 = coloured paper (partially covering the door); 2 = waste-paper bin; 3 = big poster; 4 = empty rack (partially covered by a piece of black cloth); 5 = counter; 6 = red fire extinguisher; 7 = washbasin; 8 = set of scales; 9 = rat cage; 10 = grey plastic box; 11 = paper towel container; 12 = radiator; 13 = metal structure; 14 = desk lamp; f 5 = rats' waiting cages; 16 = chair and the experimenter; 17 = ventilator; 18 = controls for the maze doors.

Procedure. On the first day of the experiment, all subjects received two unreinforced pre-test trials in which they were given a choice between sandpaper and rubber arms. They were placed on the start-arm, pointing N on one trial and S on the other, and allowed to run through to the central platform from which the rubber and sandpaper arms ran off E and W. Rats were assigned to groups pre-exposed to the cues or only to the apparatus in such a way as to match their experience and preferences on these two trials.

During pre-exposure all rats received 16 unreinforced trials daily for eight days in squads of eight animals. Only one arm (either rubber or sandpaper for experimental animals, a special goal-arm covered with green plastic for controls) was used on each pre-exposure trial. A trial started by placing a rat at the end of the arm, with all doors closed, facing the central platform. Thirty seconds later, the rat was picked up from the arm by the experimenter and placed into his waiting cage. When all rats in a squad had visited that arm, it was rotated 45" clockwise, and the same procedure was repeated until all eight possible locations with this arm had been visited by all rats. Exactly

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the same procedure was then run through with a second arm (rubber or sandpaper for experimental animals, but the same plastic arm for controls). Each day started with a different location and with a different arm. For control animals extra-maze cues were eliminated (as far as possible) by black curtains that surrounded the maze on all four sides from ceiling to floor. The curtains had not been present on their initial pre-test trials and were removed for the remainder of the experiment.

All subjects then received four more unreinforced trials to re-measure their preferences for the intra-maze cues. On Trials 1 and 2, the same procedure was used as in the original pre-test trials. Trials 3 and 4 were forced, where necessary, to equate animals' experience with the two intra- maze cues. The data from Trials 1 and 2 of this day revealed a slight preference for the rubber arm, and all animals required to learn the intra- maze discrimination were therefore trained with sandpaper positive: 10 rats chose rubber on both trials, and 6 chose sandpaper. The rest of the rats alternated their choices. Rats were assigned to the four experimental groups (pre-exposed and control, trained on intra-maze and extra-maze discriminations), in such a way as to match their experience and preference on these trials.

During the final, discriminative training phase of the experiment, one goal-arm on each trial was always covered with rubber, the other with sandpaper. The start-arm was plain wood. One goal-arm always pointed to the south-east corner of the room, whereas the other could point north-east, north, north-west, west, or south-west, i.e. making an angle of at least 90" with the first. The start-arm also always made an angle of at least 90" with both goal-arms. Two additional constraints were that the goal-arm pointing to the south-east was equally often rubber and sandpaper, and a right turn from the start-arm led equally often either to rubber or to sandpaper and either to the SE corner or to one of the other directions. A total of 16 different configurations of the maze was used. Two groups were trained on an extra-maze cue discrimination, with intra-maze cues irrelevant, i.e. to approach the arm pointing to the south-east, regardless of whether it was covered with rubber or sandpaper; the other two were trained on an intra- maze cue discrimination with extra-maze cues irrelevant, i.e. to approach the arm covered with sandpaper, regardless of its position.

All rats received 112 trials of discrimination training at the rate of eight trials daily with the exception of the first two days, when they received only four trials. The correct arm was baited with four 45-mg sucrose pellets; the incorrect arm was unbaited. After a choice, defined as the rat placing all four feet onto a goal-arm, the door to the central platform was closed behind the rat and no retracing or correction was allowed. The rat was removed from the correct goal-arm after eating the pellets, and from the incorrect arm after 30 sec.

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Results and Discussion

Figure 2 shows the mean percentage of correct choices for all groups throughout discrimination training. Inspection of these results suggests that the extra-maze discrimination may have been learned slightly faster than the intra-maze problem, but that pre-exposure to the intra-maze and extra-maze cues tended to facilitate the learning of both discriminations, with little suggestion that this facilitation was more marked in one case than in the other.

An analysis of variance performed on the data shown in Figure 2 found significant main effects of pre-exposure, F(1,28)= 10.26, p<O.Ol , and of pairs of days, F(6, 168)= 46.69, p < 0.01. No other main effect or interaction was significant-in particular, neither the interaction of pre-exposure and type of cue (F< I ) nor the interaction of these two factors with days [F(6, 168)= 1.59, p>O.lO].

The results of this experiment confirm that pre-exposure can still facilitate the learning of a subsequent maze discrimination, even when control animals have received identical exposure to the maze itself. They imply, therefore, that the latent learning observed in our preliminary experiments could not be entirely attributed to the relative novelty of the apparatus for control animals. It is conceivable that the introduction of the intra- and extra-maze cues at the start of discriminative training may have disrupted habituation to the apparatus in the control animals, but they did have experience of these cues on six pre-test trials. It thus remains the case that we found no stronger evidence of latent learning in extra-maze than in intra-maze groups, and to that extent no support for our tentative hypothesis that one distinction between experiments where pre-exposure results in latent inhibition and

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FIG. 2. Results of Experiment 1 : mean percentage of choices over pairs of days of training (Day Pair 1 includes Days 1-3, since rats received only four trials on Days 1 and 2). Groups trained on the intra-maze discrimination are shown in the left panel, those trained on the extra-maze discrimination in the right panel. Pre-exposed groups were pre-exposed to both intra- and extra-maze cues, while control groups were pre-exposed to the apparatus only.

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those where it produces latent learning is that the latter involve animals in the construction of a spatial map.

EXPERIMENT 2

Experiments on perceptual learning with circles, triangles, and striations have already made it clear that prior exposure can facilitate learning even when there is no question of the discrimination requiring locale learning, or indeed any form of the spatial learning at all. One possible interpretation of this effect suggests that pre-exposure establishes an adequate representation of the relevant stimuli, and that this is necessary for their successful discrimination, because they are more complex than the tones, lights and buzzers used in simple conditioning experiments. The interpretation is not so far removed from OKeefe and Nadel’s analysis of latent learning in terms of the establishment of a cognitive map, which is presumably an adequate representation of the complex spatial relations of the landmarks in a particular environment.

A somewhat more rigorous account of perceptual learning suggests that exposure to two or more stimuli might be particularly likely to enhance their discriminability, i.e. to result in perceptual learning, when the stimuli share many common elements (McLaren, Kaye & Mackintosh, in press). It seems reasonable to suppose that in Experiment 1 both the two intra-maze cues and the various extra-maze cues defining the spatial alternatives shared a number of such common elements or features. For a start, both goal-arms were the same size and shape, with the same brown hardboard walls and baffles. A simple way of reducing the proportion or salience of these common elements, therefore, would be to differentiate these walls by painting one set white and the other black. According to McLaren et al., such a change should reduce the chance of observing a perceptual learning effect and might even result in latent inhibition.

In Experiment 2, therefore, we concentrated on the intra-maze discrimination and compared the effect of pre-exposure when the intra-maze cues were either the same as those used in Experiment 1, or had been further differentiated by painting the walls and baffles black and white. Control groups were pre-exposed to the apparatus only. Rats generally have a preference for black over white, and as they also tended to prefer rubber to sandpaper, we attempted to reduce overall preferences in the two new groups by painting the walls and baffles of the rubber arm white and those of the sandpaper arm black. Preliminary work suggested that this would in fact result in a marked preference for sandpaper + black. In an attempt to reduce this preference, we used, for the new groups, a different sandpaper (brown garnet paper, 100-grit) which preliminary experiments had shown to be rather less liked than the dark red sandpaper.

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Method

The subjects were 37 male Hooded Lister rats that had previously participated in a lever-press and chain-pulling experiment. They were approximately five months old at the beginning of the present experiment, and they were maintained under the same conditions as in Experiment 1. Five animals were dropped from the experiment because they failed to run in the maze on the first pre-test day, and one animal was dropped during discrimination training for refusing to run. There were thus eight rats per group, except for one group where there were seven. Except for the differences noted above, the apparatus and the experimental room were exactly the same as in the previous experiment.

Subjects and Apparatus.

Procedure. Before pre-exposure all subjects received two unreinforced pre-test trials. The procedure was exactly the same as in Experiment 1 , with the exception that half the animals were presented with a choice between dark red sandpaper and black rubber, whereas the other half were presented with a choice between brown sandpaper (+ black walls and baffles) and black rubber (+white walls and baffles). Rats were assigned to the pre-exposed and control groups in such a way as to match their experience and preferences on these two trials.

During pre-exposure all rats received 16 unreinforced trials daily, for eight days. The procedure was exactly the same as in Experiment 1. In the control groups, animals were pre-exposed only to the radial maze, with no intra- or extra-maze cues. In the experimental groups, animals were pre-exposed to both intra-maze and extra-maze cues, with half of these animals being pre- exposed to red sandpaper and black rubber arms, and the other half to brown sandpaper (+ B) and rubber (+ W) arms.

On the day before discrimination training all subjects received four unreinforced pre-test trials. The same configurations of the maze and general procedure were used as in Experiment 1. The data from Trials 1 and 2 on this day were combined with those from the first pre-test day. In the choice between brown sandpaper (+ B) and rubber (+ W) there was a marginal preference for the sandpaper arm: 8 rats chose sandpaper on both trials and 5 the rubber arm. For animals that were given a choice between red sandpaper and rubber, there was a slight preference for the rubber arm: 13 rats chose rubber on both trials and 8 sandpaper. The rest of the rats alternated their choices between both arms. Rats were assigned to groups of eight animals each in such a way as to match their experience and preferences on these trials and were then trained against their preference. For animals that alternated on Trials 1 and 2 on the second pre-test day, preference was defined by their choice on Trial 3. In fact, by good fortune, this meant that in all four groups three animals were trained on the discrimination with sandpaper positive, and five with rubber positive.

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Animals received 80 trials of discrimination training at the rate of eight trials daily, with the exception of the first two days, when they received only four trials. Two groups were trained on the discrimination between rubber (+ W) and brown sandpaper (+ B), and two on the discrimination between rubber and red sandpaper. Within each pair of groups, one had been pre- exposed to the maze without cues present and the other to intra- and extra- maze cues. As mentioned above, one rat in the (BW) group pre-exposed to the maze without cues was dropped from the experiment for failure to run.

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Results

Figure 3 shows the mean percentage of correct choices for each of the four groups throughout discrimination training. Somewhat surprisingly, it can be seen that the two control groups, pre-exposed only to the general apparatus, learned their discriminations at very much the same speed. Although the effect seems smaller than that seen in Experiment I , there was some suggestion that pre-exposure facilitated learning of the intra-maze discrimination between rubber and red sandpaper, but it is equally clear that pre- exposure retarded learning of the intra-maze discrimination between brown sandpaper (+ B) and rubber (+ W).

None of these differences was apparent until relatively late in training, and statistical analysis confirmed that over the first three pairs of days no between-group effect or interaction was significant [maximum F= 1.83, p>O.lO]. Over the last two pairs of days, however, there was a significant difference between groups trained with and without the BW walls, F(1,27)=4.74, p<0.05, and, although there was no main effect of pre- exposure (F= 1.8 I ) , there was a significant interaction between groups and

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FIG. 3. Results of Experiment 2: mean percentage of correct choices over pairs of days of training. Groups trained on the standard intra-maze discrimination are shown in the left panel, those trained on the intra-maze discrimination with the black and white walls (BW) are shown in the right panel. Pre-exposed and control groups were pre-exposed to the cues or to the apparatus only.

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pre-exposure, F( 1,27) = 6.62, p < 0.025. Analysis of simple main effects revealed that there was a significant difference between pre-exposed and control animals trained with BW walls, F(1,27)= 15.24, p<O.Ol; but not in those trained on the standard intra-maze discrimination [F( 1,27) = 1.62,

In summary, animals trained on the standard intra-maze discrimination showed a marginal, but not significant, perceptual learning effect, whereas the addition of the black and white side walls resulted in significant latent inhibition.

p>O.IO].

GENERAL DISCUSSION In Experiment 1 we found that pre-exposure facilitated the learning of both intra-maze and extra-maze discriminations, with no suggestion that the effect was significantly more marked in the latter case, where it would be plausible to argue that the discrimination required animals to establish a spatial map of their environment. Earlier experiments on perceptual learning have made it clear that pre-exposure can facilitate subsequent discrimination learning even when there is no question of a spatial map being involved in the discrimination. Thus it is not really very surprising that pre-exposure should also facilitate discrimination between some intra-maze cues. The results of Experiment 2 suggest that one critical variable is the proportion of common elements between the relevant discriminative stimuli: the differentiation of one feature, the side walls, which had previously been common to both intra- maze cues, was sufficient to turn a marginal perceptual learning effect into significant latent inhibition. One theory of perceptual learning (McLaren, Kaye & Mackintosh, in press) suggests that it is a consequence of better differentiation of stimuli that share many common elements. Pre-exposure will produce more pronounced latent inhibition of common than of unique elements. and may also result in the establishment of inhibitory inter- connections between the unique elements. Either or both of these effects should produce better differentiation of the stimuli, but both depend on the stimuli sharing common elements.

The one surprising result of Experiment 2 is that the addition of the black- and-white walls did not facilitate discrimination learning in the control groups. According to most theories of discrimination, a reduction in common elements should have done so. We have no satisfactory explanation of this anomaly. One possibility is that it is due to sampling error. A comparison of Figures 2 and 3 shows that in spite of virtually identical experimental procedures, the basic intra-maze discrimination was learned very much more rapidly by control animals in the present experiment than it had been in Experiment 1 and that the difference between control and pre- exposed animals trained on this problem was rather smaller in the present experiment. Thus the present results may overestimate the speed at which

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control animals might be expected to learn the basic intra-maze discrimination. Another possibility is related to the fact that, in Experiment 2, all animals were trained against their preference and there were two sources of preference in the BW group, for black walls over white, and for rubber over brown sandpaper. As these were put in opposition to one another, they would not have been detected in group data on pre-test trials, but they might have worked to make the discrimination more difficult than expected. In the absence of further data, further speculation is idle.

One factor that has been shown to affect whether pre-exposure will produce latent learning or latent inhibition is the context in which pre- exposure and conditioning or discrimination learning occur (Channell & Hall, 1981): when they are the same, pre-exposure is more likely to produce latent inhibition; when there is a change of context between pre-exposure and discrimination learning, latent inhibition is attenuated, abolished, or even turned into latent learning. There must have been some change of context between pre-exposure and discrimination training in the present experiments, as only one goal-arm was presented on each pre-exposure trial (and thus only one intra-maze cue), whereas two arms were present on every discrimination trial. This constant change was, however, accompanied both by significant latent learning in Experiment 1 and by significant latent inhibition in Experiment 2. Thus contextual change is not the only variable determining the outcome of pre-exposure.

REFERENCES Blodgett, H. C. (1929). The effect of the introduction of reward upon the maze performance of

rats. University of California Publications in Psychology, 4 , 113-1 34. 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, 378,235-253.

Channell, S . & Hall, G. (1981). Facilitation and retardation of discrimination learning after exposure to the stimuli. Journal of Experimenfal Psychology: Animal Behavior Processes, 7 , 437446.

Gibson, E. J. & Walk, R. D. (1956). The effect of prolonged exposure to visually presented patterns on learning to discriminate them. Journal of Comparative and Physiological Psychology, 49, 239-242.

Halgren, C. R. (1974). Latent inhibition in rats: Associative or nonassociative?. Journal of Comparative and Physiological Psychology, 86, 74-78.

Lubow, R. E. (1973). Latent inhibition. Psychological Bulletin, 79, 398-107. McLaren, I., Kaye, H., & Mackintosh, N. J. (in press). An associative theory of the represen-

tation of stimuli: Applications to perceptual learning and latent inhibition. In R. G. M. Morris (Ed.), Parallel distributed processing-Implications for psychology and neurobiology. London: Oxford University Press.

OKeefe, J. & Nadel, L. (1978). The hippocampus as Q cognitive map. Oxford: Clarendon Press. Tolman, E.C. & Honzik, C.H. (1930). Introduction and removal of reward, and maze

performance in rats. University of California Publications in Psychology, 4 , 257-275.

Revised manuscript received 10 October 1988

latent learning and latent inhibition in maze discriminations

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