Enriched sensory experience in adulthood increases subsequent exploratory behaviour in the rat

Anim. Behav., 1975, 23, 932-940

ENRICHED SENSORY EXPERIENCE IN ADULTHOOD INCREASES SUBSEQUENT EXPLORATORY BEHAVIOUR IN THE RAT BY I . R. INGLIS

Psychology Department, Bristol University, Bristol BSS 1HH Abstract. Hooded rats were reared under identical conditions until 3 months old . They were then exposed for 5 weeks to either a sensorily enriched or an impoverished environment. Each subject was then observed for 15 min in a novel Y-maze . Animals exposed to the enriched environment showed the strongest exploration tendency, particularly early in the trial . The results are discussed in relation to the emphasis placed by earlier workers upon the role of early stimulation in setting adult levels of exploratory behaviour . Numerous experiments have investigated the influence of prolonged sensory experience (i .e . several weeks) upon rat exploratory behaviour . Typically the sensory experience has been imposed upon the subject during or sfiortly after weaning (e.g. Montgomery & Zimbardo 1957 ; Zunbardo & Montgomery 1957 ; Ehrlich 1959, 1961 ; Woods, Ruckelhaus & Bowling 1960 ; DeNelsky & Denenberg 1967a, b ; McCall & Lester 1969 ; Tees 1969 ; Wells et al. 1969). Both the nature of the experience and the methods used to assess exploration have varied widely between experimenters, and general conclusions have proved elusive . Differing types of sensory experience have differing effects upon the subsequent activity levels of the subjects (the measure most commonly used) . Further, the test paradigm used can alter the magnitude of such changes . However, the bulk of this work suggests that rats which have . been handled and/or housed in a sensorily enriched environment during infancy, exhibit a higher level of activity when exploring freely (Welker 1957), than do rats which have not been handled and/or have been housed in sensorily deprived conditions . The influence of prolonged sensory experience imposed at other ages, in particular after the completion of maturation, has seldom been investigated . Possibly as a result of this imbalance in research, some theories concerned with animal exploratory behaviour suggest that the adult animal's behaviour is set by the levels of sensory experience obtained before maturation (e.g . McReynolds 1962). It is true that early sensory restriction of various kinds can produce profound physiological repercussions, especially within the sensory processsing areas of the brain (e.g . Edward, Barry & Wyspianski (1968) found a significant increase

in photic evoked potential latencies in rats which had been raised in an impoverished environment) . Other studies, however, involving prolonged experience have allowed adult sensory Rosenzweig (1966) to conclude that the adult rat brain is in many ways as capable of adaptive growth as is the brain of the juvenile . Are then the effects on exploratory behaviour of early stimulation the consequence specifically of sensory experience gained before maturation, or rather are they general effects produced by prolonged sensory experience at any stage of physiological maturity? In the case of the rat it would seem to be biologically maladaptive for the naturally occurring levels of infantile sensory stimulation to set the adult level of exploratory behaviour . The rat burrow has low and constant levels of light and sound . Further as infant rodents show a marked attachment to the nest site, the natural rearing habitat of the young may be considered to be sensorily impoverished (e .g . Daly 1973) . If the above theoretical proposal was correct then the burrow would seemingly ill equip the adult rat for the high level of exploratory activity necessary for an opportunistic feeder (e .g. Ewer 1971) . In order to clarify this issue experiments investigating the effects of prolonged adult experience upon exploratory behaviour should be conducted . Korn & Moyer (1968) investigated the influence of prolonged adult sensory experience upon subsequent `emotionality'. They found that rats housed alone for 14 weeks had greater reactions to handling, lower activity scores and longer home-cage emergence latencies, than did animals which had been group-housed for the same period . Numerous authors have reported sex differences in exploratory behaviour and emotionality, females having greater activity 932

INGLIS : RAT EXPLORATORY BEHAVIOUR

scores and lower defecation rates than males (e.g . Hughes 1968 ; Lester 1967) . It is interesting therefore that Kom & Moyer (1968) failed to find any sex differences in activity or defecation rates . If we are to accept both earlier work and the latter study then it would seem that the prolonged post-maturation experience differentially affected the sexes so as to reduce the assumed normal differences between them . In order to investigate this further I subjected both male and female rats to prolonged differential sensory experience after they had reached physiological maturity . Met ods The subjects were twelve male and twelve female, experimentally naive, Lister hooded rats . All had been born and reared in the Bristol Psychology Department and at the time of testing were approximately 18 weeks old . Pretrsbing After weaning the animals were placed in opaque plastic cages (0 . 51 x 0 . 38 x 0 .20 m), three littermates of the same sex per cage. When 3 months old the litters were divided so that equal numbers of littermates were assigned to the various experimental groups . Two groups, the enriched groups, one of six males, and one of six females, were placed in a sensorily enriched environment . Each group was housed in a wire cage (1 .22 x 0 .41 x 0 .25 m) having a variety of objects in it (e .g. activity wheels, children's toys) which were changed daily . The cages were in a well-lit room equipped with a loudspeaker providing a variety of low intensity sounds. At night the room was dark apart from randomly chosen hours when it was lit by a flashing light. The rate of flashing was altered each evening. The subjects had food and water ad libitum throughout the entire 5-week pretraining period . Two other groups, the deprived groups, comprised the animals to be subjected to a sensorily restricted environment . These subjects were housed in the original rearing cages, three of the same sex per cage. The cages were placed in a dimly lit, quiet room and disturbed as little as possible throughout the 5-week pretraining period . Again food and water were available ad libitum . Apparatus A Y-maze was chosen as the test apparatus, mazes of a similar type having been used by other workers (e.g. Montgomery & Zimbardo

933

1957 ; Zimbardo & Montgomery 1957 ; Ehrlich 1959 ; Wells et al . 1969). Each arm of the Y-maze measured 0 . 46 m long by 0 . 15 in wide with walls 0. 30 m high. The floor was grey and divided into 0 . 15 m squares . The walls were opaque Perspex . All experiments were done in a darkened soundproof room and the recording apparatus was housed outside this room . Around the maze a butter-muslin `tent' (1 .52, x 1 . 22 x 1 . 22 in) was constructed with a shielded 60-W light bulb that produced a diffuse light within this area . The butter-muslin served two purposes : it provided a homogeneous visual environment outside the maze, and it functioned as an efficient one-way screen . Procedure Each subject was given a 15-min trial . The animal was placed in the axis of the maze ; animals were lowered facing randomly chosen maze arms, and further, the maze was occasionally rotated within the room . Behaviour was then time-sampled every 10 s ; a tone delivered to the experimenter via an earplug signalled the end of each 10-s period and the behaviour occurring at the end of the tone was scored . The acts recorded were defined as (1) Sitting : Crouching or standing on all four paws entirely motionless except possibly for small movements of the head and twitching of the vibrissae . This category therefore includes `freezing' as defined by previous authors, e .g. Bindra & Spinner (1958) . (2) Grooming : Licking or biting the body, wiping the vibrissae or pulling at the fur with the forepaws ; scratching with the hindlimbs . (3) Sniffing : Sniffing the air or maze, while either stationary or moving forward on all four paws . (4) Rearing : Sniffing the air or maze while standing on the hind paws . (5) Miscellaneous : Activities not clearly falling into any of the above categories, e.g . teeth chattering . Activity scores were continuously monitored ; the subject was considered to have entered a square if its head and forepaws were within that square. At the end of each trial the animal was removed, the faecal boluses, if any, were counted, and the maze cleaned . Results The data from the 15-min trial were divided into three consecutive 5-min periods in order to investigate any temporal patterns in exploration . Figure 1 shows the means of the scores for the behavioural acts monitored . The miscellaneous



934

ANIMAL BEHAVIOUR, 23, 4

2 0

N

20

0 Enriched 4

0 Enriched

d

18

• Deprived Q

∎ Deprived

d

18 14

0

WC .D C

- .a

A" ! of

i^b

2 0

I I I I I I 1 2 3 1 2 3

I I I I 1 I 1 2 3 1 2 3

Sitting

Grooming

Sniffing

Rearing

5-min periods Behaviour categories

Fig. 1 . Mean behaviour scores for the three consecutive 5-min periods comprising the 15-min trial . Table I. Table of Variances for Grooming Data Source

SS

df

MS

Between subjects

21 . 876

23

0 . 951

Main effect A : sex

6 . 285

1

6 .285

8 . 531, P < 0 . 01

Main effect C : sensory experience

0 .726

1

0 . 726

0 .985,

Ns

Interaction A x C

0 . 132

1

0 . 132

0 . 179,

Ns

Subjects within groups

14 .734

20

0 . 737

Within subjects

25 . 285

48

0 . 527

Main effect B : time within trial

0 . 919

2

0 . 459

1 .052,

NS

Interaction A x B

0 . 791

2

0 .395

0 . 905, Ns

Interaction B

3 .985

2

1 .992

4 .562, P < 0 . 05

2 . 125

2

1 .063

2 .434,

17 .465

40

0 . 437

X

C

Interaction A x B x C B x subjects within groups

category has been ignored because very few incidences of it occurred . As the standard deviations of the original scores tended to be proportional to the means, these scores were transformed using a log (X + 1) transform (Kirk 1968) . The transformed scores were analysed by a 2 x 2 x 3 analysis of variance with repeated measures on the third level factor . Only the significant effects will be mentioned here, the complete results are presented in the tables of variances .

F

Ns

The types of prior sensory experience seemingly had effects only upon the grooming (Table I) and sitting (Table II) frequencies . With regards to sitting the significant effects of sex, experience and the sex x experience interaction all reflect the large increase of sitting activity by the restricted males (Fig . 1) . The significant experience x time interaction seen in the grooming data reflects an earlier peak in this behaviour shown by the enriched subjects of both sexes . Sniffing (Table III) data reveals a

935

INGLIS : RAT EXPLORATORY BEHAVIOUR Table II. Table of Variances for Sitting Data SS

df

MS

27 . 750

23

0.946

Main effect A : sex

3 .017

1

3 . 017

5 . 858, P < 0 . 05

Main effect C : sensory experience

4 .875

1

4 . 875

9 . 466, P < 0 . 01

Interaction A x C

3 . 556

1

3 . 556

6 . 905, P < 0 . 05

10 . 302

20

0 .515

Within subjects

8 . 348

48

0 . 174

Main effect B : time within trial

1 . 962

2

0 .981

7 .267, P < 0 .01

Interaction A x B

0 . 481

2

0 .240

1 .778,

NS

Interaction B x C

0.467

2

0 . 233

1 . 733,

Ns

Interaction A x B x C

0 . 022

2

0 .011

0 .082,

NS

B x subjects within groups

5 . 416

40

0 . 135

Source Between subjects

Subjects within groups

F

Table III. Table of Variances for Sniffing Data SS

df

MS

Between subjects

6 . 159

23

0 .268

Main effect A : sex

3 . 589

1

3 . 589

Main effect C : sensory experience

0 . 045

1

0-045

0 .400,

NS

Interaction A x C

0 .249

1

0 . 249

2 . 193,

Ns

Subjects within groups

2.274

20

0 . 114

11 . 691

48

0 . 244

Main effect B : time within trial

1 .977

2

0 .988

4 . 714, P < 0 .05

Interaction A x B

0 . 685

2

0 . 342

1 . 633, Ns

Interaction B x C

0 . 368

2

0 . 184

0 . 879, Ns

Interaction A x B x C

0 . 275

2

0 . 137

0 . 653,

B x subjects within groups

8 . 387

40

0 .210

Source

Within subjects

main sex effect, i .e. females sniffing more than males, and a main time effect, i .e. sniffing activity declines throughout the trial (Fig . 1) . Rearing (Table IV) data shows only a main sex effect with females displaying more of this activity than males . Data from a previous experiment (Inglis, unpublished results) had shown that when male and female rats were exploring freely (Welker 1957), sniffing and rearing were behaviours

F

31 . 566, P < 0 . 001

Ns

significantly associated with the animals being in the novel environment while sitting and grooming were associated with the animals being in the familiar environment. As the subjects could move between the two environments at will, I have argued that behaviour occurring in the novel environment is most closely associated with exploration and behaviour in the familiar environment with non-exploratory tendencies . This subdivision of the categories agrees with

936

ANIMAL BEHAVIOUR, 23, 4 Table IV . Table of Variances for Rearing Data Source

SS

df

MS

F

Between subjects

33 . 211

23

1 . 444

Main effect A : sex

22.489

1

22 .489

Main effect C : sensory experience

1 .235

1

1 . 235

2.621,

Ns

Interaction A x C

0.060

1

0 .060

0. 128,

NS

Subjects within groups

9 .428

20

0 .471

Within subjects

9 . 903

48

0.206

Main effect B : time within trial

0.508

2

0 .254

1 . 159,

Ns

Interaction A x B

0 . 004

2

0 .002

0 .009, Ns

Interaction B x C

0 . 597

2

0 .298

1 .363,

Interaction A x B x C

0 .040

2

0 . 020

0 .091, Ns

B x subjects within groups

8 .755

40

0 . 219

47 .710, P < 0 . 001

Ns

Table V . Table of Variances for Exploratory Statistic Data Source

SS

df

MS

Between subjects

7545 . 5

23

328 . 1

Main effect A : sex

5635 . 7

1

5635 . 7

84 .0, P < 0 .001

Main effect C : sensory experience

351 . 2

1

351 . 2

5 . 3, P < 0 .05

Interaction A x C

224 .0

1

224 .0

3 . 3,

Subjects within groups

1334 . 7

20

66 .7

Within subjects

4253 . 3

48

88 . 6

Main effect B : time within trial

1284 . 8

2

642 . 4

Interaction A x B

103 . 5

2

51 . 8

Interaction B x C

305 . 1

2

152 . 6

2 .5,

NS

Interaction A x B x C

105 .2

2

52 . 6

0 .9,

Ns

2454.8

40

61 .4

B x subjects within group

the interpretations of previous workers (e.g . Bindra & Spinner 1958) . By simply summing the subject's grooming and sitting scores and subtracting from them the subject's sniffing and rearing scores, a statistic was obtained which theoretically reflected the relative dominances of exploratory and non-exploratory tendencies . This procedure allows for the possibility that subjects may show individual differences in, for example, their tendency to sniff and rear al-

F

Ns

10 . 5, P < 0 .001 0 . 8, Ns

though previous workers, e.g. Bindra & Spinner (1958), Hughes (1968), have considered both acts to be indicative of exploration . Although the use of a single statistic may oversimplify by suggesting a false unidimensionality to the underlying causes of exploration, nevertheless, the data derived from time-sampling mutually exclusive categories are in any case non-independent. If the frequency of one category decreases it cannot be assumed that any causal



INGLIS : RAT EXPLORATORY BEHAVIOUR

110

1 i

100

0 Enriched 9

• Deprived 9 90

Q Enriched d'

`

80

• Deprived d

i

70

60

1.,

O U 50 T U 40 ID

M 30

937

analysis of variance (Table V) with repeated measures on the third level factor . A highly significant sex difference was found, female rats showing stronger exploratory tendencies than males. Enriched animals exhibited stronger exploratory tendencies than did restricted animals. All groups showed a gradual decline in exploratory tendencies throughout the 15-min trial. There were no significant interactions . The activity scores (Fig . 2) were subjected to a similar analysis of variance (Table VI) . Again a highly significant sex difference was revealed . The females were far more active than the males. All groups showed a decline in activity throughout the trial . Although there were no significant differences in the overall activity levels between enriched and restricted animals, nevertheless, the type of the prior experience did affect the temporal patterns of activity . As can be seen in Fig. 2, the enriched animals showed the greatest decline between periods 1 and 2 while the restricted animals exhibited a relatively constant rate of decline . The sex x trial subdivisions interaction also proved to be significant. The female rats showed a faster rate of decline in activity than did the males . No analysis was performed on the faecal bolus counts because defecation was rare and no group differences were shown .

20

.` 10

0

I 1

I 2

3

5-min periods of 15-min trial

Fig . 2. Mean activity scores for the three consecutive 5-min periods comprising the 15-min trial . Bar length indicates the se about the mean .

factor underlying this category has decreased, for obviously causal factors underlying one or more of the other monitored categories may have been increasing. Therefore as well as comparing differences between groups within any one behavioural category it seems sensible to combine all four scores thereby producing one statistic whose value reflects the level of the subject's exploratory tendencies . For any one 5-min period the exploratory statistic could vary within the range - 30 to 30. The actual scores of this statistic were checked for homogeneity of variance and skewness before being analysed by a 2 x 2 x 3

Discussion Compared with the large sex differences found in this experiment, the effects resulting from the differential adult sensory experience are small but nevertheless present. `Sitting' was apparently the behaviour most affected by the nature of the sensory experience . This behaviour, which includes the category `freezing' as used by, for example Bindra & Spinner (1958), has been used as a measure of emotionality, e .g . Hughes (1968) . However, as already emphasized, the interrelationships of the four categories monitored means that it is invalid to conclude from the present results that adult sensory impoverishment leads to a heightened level of emotionality. The pattern of grooming activity was also apparently affected by the type of the prior sensory experience. Again, interpretation of this result is difficult not only for the above reason but also because there have been many differing views as to the underlying causal factors for grooming (cf. Bindra & Spinner 1958 ; Bolles 1960 ; Fentress 1968) . It would therefore be unwise to draw conclusions concerning underlying causal factor changes from the differences

ANIMAL BEHAVIOUR, 23, 4

938

Table VL Table of Variances for Activity Data SS

df

MS

Between subjects

29463 . 3

23

1281 . 0

Main effects A : sex

23725 . 7

1

23725 .7

316 .7

1

316 .7

1 .2,

Ns

51 . 7

1

51 . 7

02,

NS

5369 . 3

20

268 . 5

Within subjects

39892.0

48

831 . 1

Main effect B : time within trial

25643 .5

2

12821 . 8

76 .3, P < 0 .001

Interaction A x B

4899 .7

2

2449 . 8

14 . 5, P < 0 .001

Interaction B x C

1514 .2

2

757 . 1

4 . 5, P < 0 .05

Interaction A x B x C

1092 . 6

2

546 . 3

3 . 2,

6741 .9

40

168 . 5

Source

Main effect C : sensory experience Interaction A x C Subjects within groups

B x

subjects within groups

in grooming patterns between enriched and restricted subjects . The most obvious finding from these analyses is that sex differences are apparent in all the categories monitored . The emphasis by the females upon sniffing and rearing, behaviours usually considered to be exploratory in nature, is clearly reflected in the results of the exploratory statistic analysis . The latter analysis also revealed that the enriched animals had a higher tendency to show behaviours thought to be indicative of a high level of exploration than did the restricted animals . The prior sensory experience clearly altered the temporal pattern of activity of the two groups of animals . The differences in activity during the first 5-min period (Fig . 2) were the main source of this variation . I made a simple main effect comparison of the data from the period (Kirk 1968) . The scores of the enriched animals were found to be significantly higher than those of the restricted animals (t = 3 .98 ; df = 10 ; P < 0 .01) . Many authors have suggested that a high ambulation score signifies the dominance of an approach tendency (e.g . Zimbardo & Montgomery 1957 ; Bindra & Spinner 1958 ; Halliday 1967, Hughes 1969) . While a certain degree of caution should be used in accepting this interpretation (e.g . see Sheldon 1969), nevertheless the conclusions reached on such a basis are congruent with those derived from the separate exploratory statistic analysis . The more

F

88 .2, P < 0 . 001

NS

rapid initial drop shown by the enriched animals suggests that (a) the shape of their exploration tendency strength against time curve is more peaked than that of the experimental animals or (b) that the shapes of the two curves are the same but that they are displaced in time over the experimental trial with the enriched subjects having the shorter latencies, or (c) a combination of (a) and (b) . Data from further experiments (Inglis, in preparation) favour the second of the above alternatives . Conflicting results have been obtained from experiments involving early sensory experience which used a Y-maze test situation (cf. Montgomery & Zimbardo 1957 ; Zimbardo & Montgomery 1957) . Differences between these experiments in the rigour with which differential handling was eliminated may have contributed to the production of some discrepant results (e .g . Luchins & Forgus 1955) . Ehrlich (1959) compared the effects of juvenile handling and sensory experience upon activity levels in both Y and Dashiell mazes and found that only handling produced a differential effect . In the present experiment however, animals of both groups were not handled after weaning until the actual testing period . In earlier experiments, usually only activity was monitored, and the data from repeated trials was amalgamated for the purpose of analysis . The massing of data from repeated trials would have hidden differing temporal patterns in activity within one trial,



INGLIS : RAT EXPLORATORY BEHAVIOUR

if the differential within trial trends themselves changed with repeated trials. It can at least be concluded that the present experiment has produced effects as strong as the earlier work involving juvenile stimulation . The sex differences found agree with the findings of experiments that investigated such differences per se (e.g. Hughes 1968) . The only evidence that the sexes were differentially affected by the adult sensory experience was within the sitting scores . It is difficult therefore to interpret Korn & Moyer's (1968) failure to detect any sex differences in their measures, particularly within the activity measure . To conclude, the present experiment has shown that an enriched environment during adulthood can produce a higher level of exploration, as measured by activity scores and other behavioural parameters . There is little likelihood that this result is due to maturational changes, but rather that it involves reversible processes (Inglis & Freeman, in press, have evidence demonstrating this) . Any emphasis placed upon the influence of early sensory experience in setting adult optimum exploratory behaviour levels is therefore probably misleading, unless such influences are imposed before weaning and are particularly gross and prolonged, in which case permanent physiological damage may occur . Acknowledgments I am indebted to Dr N . Freeman and Dr M Daly for their constructive criticism of this manuscript. My thanks are also due to Mr P. Wilkins and Mr R . Crawshaw for valuable assistance throughout the rearing of the experimental subjects, and to Dr A. Monk for statistical advice . This research is part of a programme of work supported by an S .R .C . Research Studentship . REFERENCES Bindra, D. & Spinner, N. (1958). Response to different degrees of novelty : the incidence of various activities . J. exp. Analysis Behav.,1, 341-350. Bolles, R . C. (1960). Grooming behaviour in the rat . J. comp . physiol. Psycho!., 53, 306-310. Daly, M . (1973) . Early stimulation of rodents : A critical review of present interpretations . Br . J. Psycho!., 64,435-460 . De Nelsky, G . Y. & Denenberg, V . H . (1967a) . Infantile stimulation and adult exploratory behaviour in the rat : Effects of handling on visual variation seeking. Anim . Behav ., 15, 568-573 .

939

De Nelsky, G. Y . & Denenberg, V . H . (1967b). Infantile stimulation and adult exploratory behaviour : Effects of handling upon tactual variation seeking. J. comp . physiol. Psycho!., 63, 309-312 . Edward, H. P., Barry, W. F. & Wyspianski, J. O. (1968). Early environment effects on rat photic evoked potentials : A preliminary study. Revista Interamericana de Psico 2, 85-92 . Ehrlich, Annette (1959). Effects of past experience on exploratory behaviour in rats . Can. J. Psychol., 13, 248-254 . Ehrlich, Annette (1961). Effects of past experience on the rats response to novelty . Can. J. Psycho!., 15, 15-19. Ewer, R. F . (1971) . The biology and behaviour of a free living population of Black Rats (Rattus rattus) . Anim. Behav. Monogr., 4, 127-174 . Fentress, J . C. (1968). Interrupted ongoing behaviour in two species of vole (Microtus agrestis and Clethrionomys britannicus) . II . Extended analysis of variables underlying fleeing and grooming behaviour. Anim. Behav .,16,154-168 . Halliday, M . S . (1967). Exploratory behaviour in elevated and enclosed mazes . Q . Jl. exp . Psycho!., 19, 254-263 . Hughes, R. N. (1968). Behaviour of male and female rats with free choice of two environments differing in novelty. Anim. Behav ., 16, 92-96. Hughes, R . N. (1969). Social facilitation of locomotion and exploration in rats. Brit. J. Psycho!., 60, 385-388. Inglis, I. R . & Freeman, N (in press) . Reversible effects of housing conditions upon stimulation-seeking behaviour in rats . Q . Ji. Exp. Psycho!. Kirk, R . E . (1968). Experimental Design : Procedures for the Behavioural Sciences. California : Brooks/Cole . Korn, J. H . & Moyer, K. E. (1968) . The behavioural effects of isolation in the rat : The role of sex and time of isolation . J. genet. Psycho!., 113, 263-273 . Lester, D. (1967) . Sex differences in exploration of a familiar locale. Psycho!. Rec., 17, 63-65 . Luchins, A . S . & Forgus, R . H. (1955). The effect of differential post weaning environments on the rigidity of an animal's behaviour . J. genet . Psycho!., 86, 51-58 . McCall, R . B . & Lester, M . L . (1969). Differential enrichment potential of visual experience with angles versus curves . J. comp. physiol. Psycho!., 69, 644-648 . McReynolds, P . (1962) . Exploratory behaviour : A theoretical interpretation . Psycho!. Rep., 11, 311-318 . Montgomery, K. C. & Zimbardo, P . G. (1957) . Effects of sensory and behavioural deprivation upon exploratory behaviour in the rat . Percept. Mot . Skills, 7, 223-229 . Rosenzweig, M . R. (1966) . Environmental complexity, cerebral change, and behaviour. Am . Psycho!., 21, 321-332. Sackett, G . P. (1967). Response to stimulus novelty and complexity as a function of rats early experiences . J. comp. physiol. Psycho!., 63, 369-375. Sheldon, M. H . (1969) . The relationship between familiarity and two measures of the activity of rats in an elevated maze . Anim . Behav., 17,537-540. Tees, R. C. (1969) . Effect of early visual restriction on subsequent activity and emotionality in the rat . J. genet. Psycho!., 114, 193-202.

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ANIMAL BEHAVIOUR, 23, 4

Welker, W. I . (1957) . `Free' versus `Forced' exploration of a novel situation by rats . Psychol. Rep., 3,95-108 . Wells, P. A., Lowe, G ., Shelton, M . H. & Williams, D . I . (1969) . Effects of infantile stimulation and environmental familiarity on exploratory behaviour in the rat . Brit. J. Psychol., 60, 389-393 . Woods, P . J ., Ruckelhaus, S . I. & Bowling, D . M . (1960) . Some effects of `free' and `restricted' environmental

rearing conditions upon adult behaviour in the rat. Psychol. Rep., 6, 191-200 . Zimbardo, P. G . & Montgomery, K. C . (1957). Effects of `free-environment' rearing upon exploratory behaviour. Psychol . Rep ., 3, 589-594. (Received 26 August 1974 ; first revision 18 November 1974 ; second revision 3 December 1974 ; MS number : 1346)