exposure learning in young and adult laboratory rats

Anim. Behav., 1979,27,586-591

EXPOSURE LEARNING IN YOUNG AND ADULT LABORATORY RATS

BY GEOFFREY HALL Department of Psychology, University of York, Heslington, York

Abstract. Hooded rats born in the laboratory were exposed in their home cages to objects (cut-out triangles and circles) which were to be used as the stimuli in a subsequent visual simultaneous discrimination task. Control subjects received no prior exposure. In confirmation of the results of Gibson & Walk (1956) it was found that rats given exposure to the stimuli when still immature showed positive transfer to discrimination learning in adulthood. Facilitation of learning was also found in subjects exposed to the stimuli in adulthood. The relevance of these findings to theories of the ‘special’ effects of early experience and to current theories of the effects of stimulus-exposure is discussed.

Gibson & Walk (1956) discovered the phenomenon which will be referred to as ‘exposure learning’. They raised albino rats from birth in cages containing black metal cut-out shapes, two triangles and two circles. At 90 days old these rats and control subjects which had not been exposed to the shapes were required to learn a visual discrimination between triangle and circle. Experimental subjects learned much more readily than controls.

This finding is of importance for two reasons. First, it is relevant to the discussion concerning the possible ‘special’ effects of early experience. Hebb’s (1949) suggestion, that experience early in life may have profound effects on later behaviour since it is capable of modifying the course of development in a way that later experience is not, has been widely influential. Attempts to test the suggestion, however, have proved difficult to interpret. There have been a large number of experiments, beginning with one by Hebb himself (1947), which have investi- gated the effects of exposure to an ‘enriched’ environment early in life on later performance in learning tasks. It has often been found that subjects given an ‘enriched’ environment show better learning than ‘impoverished’ animals reared in isolation but it is not clear what animals actually learn during their exposure to the enriched or the impoverished environment and how their learning is capable of modifying test performance (e.g. Morgan 1973). We may hope, however, that the much simpler experimental procedures used by Gibson & Walk will prove more amenable to analysis; in particular, it might be possible to integrate the results of experiments on exposure learning with those from experiments within general learning theory on the transfer of learning.

The second point of interest in exposure learning is that the results seem, at first sight,

to run counter to the findings of analogous experiments carried out by learning theorists. In these experiments animals are required to form an association between two events, the first being an external stimulus or some specific pattern of responding and the second a reinforcer such as food or electric shock. Prior exposure to any of these events is usually found to retard the formation of the association. Thus, studies of ‘latent inhibition’ (Lubow 1973) show that when an animal is pre-exposed to a stimulus which is to become the conditional stimulus in subsequent classical conditioning, learning tends to be hindered. Pre-exposure to the electric shock which will subsequently be used to support escape and avoidance learning may totally prevent animals from learning the escape response (Maier & Seligman 1976). Perhaps the best parallel to studies of exposure learning is provided by experiments in which the animals are given pre-exposure both to the stimulus and the reinforcer that are to be paired in the test stage. This procedure, too, produces a marked retardation in subsequent learning; the effect is said to be the result of ‘learned irrelevance’ (Mackintosh 1973; Baker 1976) on the grounds that the animals are presumably learning during exposure that the two events are not related to one another. In most studies of learned irrelevance the reinforcer has been electric shock and the test phase has involved classical conditioning but the effect is not restricted to these procedures. Hall (1976) has been able to demonstrate a retardation of a food-reinforced simultaneous discrimination in animals given prior experience of the stimuli uncorrelated with the presentation of food. The design of this last experiment is formally very similar to that of traditional experiments on exposure learning but the results are quite different. What is the reason for the difference ?

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HALL: EXPOSURE LEARNING IN RATS 587

However similar they are formally, the two classes of experiment, those on exposure learning and those directed at the phenomenon of learned irrelevance, do differ in many procedural details and it is to these that we must look to explain the discrepancy in outcome. There are a number of possibilities. The length of time for which stimulus pre-exposure is given may be important; in exposure learning the animals are given continuous access to the stimuli for weeks or months whereas in the learned irrelevance experiments the stimuli are presented for a few hours at most. The context in which pre- exposure is given may also play an important role. Habituation etfects are often specific to the environment in which the stimuli are presented (Wagner 1976; Tomie 1976) and we might therefore expect no retardation of learning when stimulus pre-exposure is given to the animals in their home cages. (It remains to explain, of course, why such exposure should actually facilitate later learning.) A third possibility concerns the age of the animals at the time of their exposure to the stimuli. Experiments on exposure learning are carried out with young, immature animals whereas those done within learning theory invariably use adults. If we accept those theories which give a special role to early experience it comes as no surprise that animals treated in roughly similar ways at different ages react in quite dissimilar ways. All three possibilities deserve experimental investiga- tion but the third should be dealt with first, for if it proves to be the case that processes are at work in young animals that are different in principle from those found in adults then attempts to explain early experience in terms of the concepts derived from learning theory will be difficult to devise.

Accordingly, the experiment reported here asks the question: Is the exposure learning effect described in Gibson & Walk unique to young animals ? There are already two published reports of experiments in which the animals received exposure to the stimuli at different ages but neither answers this question satis- factorily. Gibson et al. (1959) compared the effects of stimulus exposure from birth to 50 days with the effects of exposure from day 50 to day 90 but since neither procedure resulted in any facilitation of later learning (perhaps be- cause the stimuli used were inappropriate) the results are difficult to interpret. Forgus (1956) found that discrimination learning was im- proved both by early and by late prior exposure

to the stimuli but since late exposure occurred from 4 1 to 66 days of age (as opposed to 25 to 4 1 days for early exposure) his study cannot tell us what the effects of these procedures might be with fully adult subjects. Rats are usually regarded as adult at 90 days of age and thus it is difficult to assert that some subjects in Forgus’s study received ‘early’ experience while another group did not. Certainly, exposure from 41 to 66 days falls well within the range of ages that other workers have thought suitable for investigations of ‘early’ experience (e.g. Baird & Becknell 1962; Ernst et al. 1976; Gibson et al. 1959).

Method Subjects and Stimulus Exposure

The subjects were 30 hooded Lister rats bred in the York laboratory from parental stock (a male and four females) obtained from the colony maintained by the Animal House, University of Sussex. The four litters (which comprised 13 male and 17 female pups) were born over the span of five days and the ages given below are exact only for the oldest animals.

Pups remained with their litter-mates until they were 40 days old at which age they were transferred to the cages in which stimulus exposure (for experimental groups) was given. The cages used (type RCl from North Kent Plastics Ltd.) are made of translucent white plastic. They have a section which holds food and water bottles which effectively forms one (short) wall of the living area. This area is 42 cm long, 35 cm wide and the remaining three walls of white plastic are 16 cm high. The cage roof is of wire mesh. In those cages containing experimental subjects, stimulus-objects were sus- pended from the roof so that they rested against the white walls of the cage. The stimuli were cut from black plastic sheet 0.8 cm thick and were of two types, circles 7.5 cm in diameter and equilateral triangles 9 cm a side. Two stimuli (one upright triangle and one circle) were hung against each long wall of the living area with the lowest point of each stimulus 5 cm from the floor. On each wall the two stimulus-objects were 22 cm apart, centre to centre, and each was 10 cm distant from the nearer short wall of the cage. On one wall the triangle was in the position adjacent to the food container; on the opposite wall the circle occupied this position.

At this stage the rats were assigned to one of four groups. Group experimental-early consisted of four males housed together in one cage equipped with stimulus-objects and four females similarly

588 ANIMAL BEHAVIOUR, 27, 2

housed. Group control-early consisted of three males and four females and for these subjects no stimuli were present in the cages. Group experimental-late consisted of three males and five females again housed with the stimulus- objects; group control-late of three males and four females. Subjects were allocated to groups at random apart from the constraints imposed by the need to have each litter and each sex represented in roughly equal numbers in each of the four groups. The animals were kept in a small room illuminated for 18 h each day and were left undisturbed (apart from weekly main- tenance : cage-cleaning, replenishment of food containers and so on) for 40 days when discrimination training was begun for both early groups. The late groups were left undisturbed until they were 120 days old at which age stimuli were inserted in the cages of the experimental- late subjects. Discrimination training for all late subjects began 40 days later when they were 160 days old.

For both experimental groups, the stimuli were kept in place in the cages during the period of discrimination training. Some previous studies of exposure learning have adopted this procedure while others have removed the stimuli at the start of discrimination training and while not all previous studies have succeeded in demonstra- ting the Gibson-Walk effect, this particular procedural difference is not one that differenti- ates between them. Gibson et al. (1959) manipu- lated this variable in the course of a single experiment but unfortunately none of their experimental groups showed a superiority over control subjects and we can therefore reach no firm conclusion about its effects. The decision to leave the stimuli in the cages was thus a purely arbitrary one.

Discrimination Training: Apparatus and Pro- cedures

After 40 days of stimulus-exposure, each experimental group and its appropriate control group was subjected to a schedule of food deprivation, being allowed access to food for only 2 h each day. On the next day pretraining began with the jumping-stand.

This apparatus, in a separate room adjoining the rats’ living quarters, consisted of a goalbox with two adjacent apertures each 15 cm square in its front wall. A small landing platform 15 cm wide and 8 cm deep was tied in front of each opening. A vertical partition 12 cm deep tied to the goalbox prevented animals from step-

ping from one platform to the other. The rats jumped to the landing platforms from a stand shaped like a small elevated Y-maze; its overall length was 20 cm and its arms, 9 cm long and 7 cm wide, directly faced the goalbox apertures. The goalbox doors, 15 cm square, were them- selves the stimulus objects. In pretraining two identical brown, hardboard doors were used. The doors used in discrimination training were painted white; one bore a cut-out plastic triangle fixed centrally, the other a cut-out circle.

Only seven animals from each group were given discrimination training, one male being omitted from the experimental-early group and one female from the experimental-late group. The animals were given 48 pretraining trials spread over six days. During the first eight trials a single brown goalbox door was locked in place, on the left or on the right according to a pre-determined Gellerman sequence. The subjects could therefore cross from the stand and enter the goalbox against one of the apertures and could eat for 5 s from a foodcup containing wet mash. On the remaining pretraining trials two brown doors were in place, one of them locked. The rats learned to push down the unlocked door and to enter the goalbox. The gap between the stand and the goalbox, which on the first day of pretraining was 2 cm, was gradually increased reaching 17 cm by the end of pretraining. This gap was sufficiently wide that the animals had to jump across it. If an animal jumped to the locked door, it was returned to the stand and allowed to jump again; thus each animal was rewarded each time it was put into the apparatus.

Ten trials of discrimination training were given each day, the interval between trials being about 5 min. After a correct response the rat gained access to the goalbox and to wet mash while after an incorrect response it was detained on the landing platform in front of the locked goalbox door for 10 s. It was then returned to the stand and allowed to jump again. If the rat repeated its incorrect choice it was again put back on the stand and gently pushed toward the correct goalbox door. The latency of the first jump on each trial was recorded to the nearest second. The positive stimulus appeared equally often each day on the left and on the right, its position being determined by a sequence which ensured that a stimulus could not appear in the same position for more than two consecutive trials. Four animals in each group were trained with the triangle as the positive stimulus, the

HALL: JZXPGSURE LEARNING IN RATS 589

remainder with the circle as positive. Twenty days of discrimination training were given.

RC?SUItS The data of primary interest are the errors made by each group during discrimination training. Figure 1 shows the group mean daily error score over four five-day blocks, the score used being errors made on the first choice of each trial. Analysis was also carried out on a total error score (errors on first choice plus any errors made on the second choice allowed on a trial) but the pattern of results followed very closely that seen in Fig. 1 and will not be discussed further.

Figure 1 shows that in all four groups errors tend to be eliminated as training progresses and that this occurs more rapidly in the two experimental groups than in the control groups. There is also some sign, particularly in the control groups, that late animals tend to make fewer errors than early animals. Statistical analysis largely confirms these impressions. An analysis of variance of the scores shown in Fig. 1 reveals a significant reduction in errors over days (I; = 56.18, df = 3, 72, P < O-01), a significant difference between the experimental and control conditions (F = 24.86, df = 1, 24, P < 0.01) but no significant difference between the early and late conditions (F = 2.39, df = 1, 24). There was also a significant interaction between days of training and the experimental versus control factor (F = 9.12, df = 3, 72, P < 0.01). No other effect approached statistical signifi- cance; thus there is no statistical evidence to support the suggestion that late animals learn more quickly than early animals in the control condition.

Fig, 1. Group mean error score5 for discrimination learning.

Pilot work had shown that animals given prior exposure to the stimuli tend to respond rather slowly in the jumping stand and response late&es were therefore analysed in this experiment. They are shown in Fig. 2. The scores plotted are group means based on the daily median latencies of individual subjects. These scores were transformed to reciprocals before statistical analysis. The mean scores for the four groups over the four blocks of discrimination training give some suggestion that subjects in the experimental condition tend to respond more slowly than the control subjects. An analysis of variance, however, showed only a significant change in latency over blocks (F = 7.92, df = 3, 72, P < OeOl), there being no statistically reliable differences among the groups.

The other scores given in Fig. 2 allow a comparison of performance on the last day of pretraining (P on Fig. 2) with that shown on the first day of discrimination acquisition (A on Fig. 2). This comparison is potentially of importance in that a difference between experimental and control groups during pretraining (i.e. in the absence of the stimuli) would be evidence for some relatively general effect of pre-exposure. Figure 2 shows that all groups responded more slowly when the stimuli were introduced, with the increase in latency being more marked in the experimental groups, The mean latencies of the late subjects were shorter than those of the early subjects over both days. Unfortunately, latency scores varied very markedly between individuals within each group and an analysis of variance

Fig. 2. Group mean response latencies for discrimination learning. P: final day of pretraining; A: first day of discrimination acquisition.

590 ANIMAL BEHAVIOUR, 27, 2

including the factors of early or late exposure, experimental or control condition, and pretraining versus acquisition day yielded no significant effects. All Fs < 1 apart from the interaction between age at exposure and day of training (F = 2.51, df = 1, 24, P > O-1).

Discwsion This experiment shows that the exposure learning effect of Gibson & Walk (1956) can be found as readily with adult rats as with young ones. At no stage in the experiment was a reliable difference found between early subjects and the comparable late subjects. The only sizeable difference between early and late animals was shown in response-latencies at the end of pretraining when the early subjects tended to respond more slowly but this difference was not statistically reliable.

It is thus no longer possible to believe that the outcome of an experiment on exposure learning (i.e. whether a facilitation is observed rather than a retardation) depends in some simple fashion upon the age of the animal and we must look for alternative hypotheses. One possibility raised in the introduction was that length of exposure might be important; that the exposure learning effect is found only when the stimuli are presented for many days continuously. However, in a study using rhesus monkeys as the subjects, Bateson & Chantrey (1972) have demonstrated a clear retardation in later learning in animals previously exposed to the test stimuli for up to 50 days. The explanation they put forward (see also Chantrey 1972) is that when monkeys are presented simultaneously with a pair of stimuli they tend to ‘classify them together’, a process which interferes with later discrimination learning in which the stimuli must be classified apart. It is further suggested (Chantrey 1974) that rats, which are not pri- marily ‘visual animals’, are likely to approach and make contact with each stimulus individually when a pair of stimuli are present in the home- cage. For this reason rats might tend to classify the stimuli apart even though both stimuli are present simultaneously in the cage. Discrimina- tion will proceed readily between stimuli which have already been classified apart. This explanation remains, as yet, untested. It is made less plausible by experiments (e.g. Bitterman & Elam 1954; Winefleld 1978) which show that rats exposed to a pair of stimuli in the discrimination test apparatus subsequently show retarded discrimination learning between these

stimuli. It can be argued, however, that the special conditions of stimulus exposure used in these experiments promoted a tendency to classify the stimuli together.

The other hypothesis discussed in the introduction directly concerns the role of the context in which stimulus exposure is given. It was mentioned above that some theories of habituation and latent inhibition imply that the deleteri- ous effects of stimulus exposure should be specific to the original context. Lubow et al. (1976) have taken this idea further and have suggested that familiar, pre-exposed stimuli will be learned about even more readily than novel stimuli when they are experienced in a context different from that in which pre- exposure occurred. They have demonstrated experimentally that rats pre-exposed to two different odours learn a discrimination between them relatively slowly when training is given in the same environment as was used for stimulus exposure but that the same discrimination is learned rapidly when a novel test environment is used. This finding lends support to the suggestion that giving stimulus exposure in the home-cage may be a critical determinant of the Gibson-Walk effect.

Finally, although the experiment reported here rules out any explanation of exposure learning in terms of special early experience effects, there is still one version of the early experience hypothesis which remains tenable. Although the late subjects in the present experiment were chronologically quite old when first exposed to the stimuli, this exposure was still ‘early’ experience in the sense that they had not previously had experience of geometrical objects. Their visual experience up to this time, like that of the early groups, had been limited to that supplied by their cage-mates and the interior of the cage. It is possible that animals of the same chronological age which had received more varied visual experience would have been influenced quite differently by exposure in the home-cage to the triangle and circle.

Acknowkdgoxents This work was supported by a grant from the U.K. Science Research Council. I thank E. M. Macphail and J. M. Pearce for their help.

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HALL: EXPOSURE LEARNING IN RATS 591

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(Received 1 July 1978; revised 17 August 1978; MS. number: 1784)

exposure learning in young and adult laboratory rats

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