Physiology& Behavior.Vol. 52, pp. 1029-1035, 1992

0031-9384/92 $5.00 + .00 Copyright © 1992PergamonPress Ltd.

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BRIEF COMMUNICATION

Cognitive and Noncognitive Processes Involved in Selective Object Exploration: Comparison Between Young Adult and Old Rats MONIQUE

SOFFII~, *~ M A R I E C H R I S T I N E

BUHOTt

AND BRUNO

POUCETI"

*Psychobiology Unit, University o f Louvain-la-Neuve, Belgium and ?C.N.R.S., Marseille, France R e c e i v e d 25 M a r c h 1992 SOFFII~, M., M. C. BUHOT AND B. POUCET. Cognitiveand noncognitiveprocesses involvedin selectiveobject exploration: Comparison between young adult and old rats. PHYSIOL BEHAV 52(5) 1029-1035, 1992.--The age effects on locomotor activity, object-oriented exploration, habituation, and response to a spatial change were studied in young adult and old rats using an object exploration test. In this test the spatial response was evaluated by the renewal of exploration of a familiar object after its repositioning. The specificity of the spatial response was determined by comparison with control animals not submitted to a spatial change. Male Wistar rats 6 and 24 months old were used. Results showed a significant decrement in locomotor activity, object exploration, and spatial reactivity in old rats. The habituation curve and the reactivity to a new object were preserved. Detail analyses suggest that the spatial deficit of old rats is due to an incapacity to detect the spatial change and not to their poor locomotor or exploratory activity. These results corroborate those obtained in spatial orientation tasks and support the idea that the lack of spatial response observed in old animals is more related to cognitive impairments than to other factors such as sensory, motor, or motivational differences. Spatial response

Exploration

Habituation

Locomotor activity

FOR many years, spatial tasks have been used to analyse cognitive processes in rodents. Spatial orientation tasks (e.g., the radial maze, the circular platform, or the water maze) have shown that aged rats are particularly impaired when they have to use a spatial response to succeed [reviews in (1-4,10-13,22)]. However, several experiments have reported that aged rats acquire the spatial response with extended training (13,22). Therefore, the age-related deficit appears sometimes as a slowness in learning probably due to a greater difficulty to master the task, rather than as a permanent or complete inability to learn. Even though some studies (10,1 l) have clearly shown that the spatial deficit is not correlated with the sensorimotor differences during aging, one cannot totally rule out the possibility that the cognitive impairment observed in aged animals is partly due to a difference in motivational level or to a decrease in visual acuity (2,25). For example, fear or hunger may greatly interfere with the animal's capacity to lend attention to the multiplicity of environmental stimuli and, consequently, can prevent the registering of spatial stimuli. Similarly, diminished visual acuity can interfere with the optimal use of distal cues necessary for adequate spatial performance in mazes.

Aging

Rats

Accordingly, the present study analysed the effects of aging in a spatial task that does not consist in going to a goal, as the orientation tasks, but that implies to identify stimuli according to their position in space. Moreover, the test minimizes both the manipulation of animal motivation and the relevance of distal cues to accurate spatial performance. As a matter of fact, the experiment presented here used the object exploration test previously described by Poucet et al. (21 ). This test was proposed as a relevant and powerful paradigm for the analysis of spatial knowledge acquired by animals (24). During exploration in an open field, the animal has the opportunity to encode some characteristics of the environment. These characteristics include, in particular, the nature of the objects and their position in space. That animals encode object positions is indicated by the observation that object repositioning usually results in a renewal of exploration. This reaction suggests that the animal has compared the current spatial arrangement with an internal representation of the past arrangement and, accordingly, detected a discordance between both. On the contrary, when there is no discordance be-

This research was partially presented at the third IBRO World Congress of Neuroscience, Montreal, August 1991. Request for reprint should be addressed to M. Sotfi6, Psychobiology unit, Place Croix du Sud l, B-1348 Louvain-la-Neuve, Belgium.

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SOFFII~, BUHOT AND POUCET

tween the previous and the present arrangements (both are similar), the animal usually responds by a decrease in exploration (habituation) rather than by an increase. These different reactions suggest that the animal is able to build up a mental representation, i.e., a cognitive map, of the environment. Finally, this kind of task has been reported/to be sensitive to both hippoeampal damage (23,27) and cholinergic blockade (8), a brain area and a neuromediator specifically affected during aging (5,17,28).

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EXPERIMENT I METHOD

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Naive male Wistar-derived rats bred in the laboratory colony were reared as groups of four per cage (35 x 20 X 15 cm), in the animal room (temperature 21-23°C) and maintained on a 12/12 normal light/dark schedule. In the case of mortality in old rats, a minimum of two rats per cage was maintained. Adults 5-6 months old (young n = 18) and 24-27-month-old rats (old n = 19) were used. Their mean weights in g (+SEM) were 333 (+ 11) and 427 (+__11), respectively.

s5-s6

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Apparatus The apparatus was an open field (100 X 100 cm with 45 cm high walls) made of wood. The open field was painted grey and was surrounded by a brown curtain so that the environment was visually homogeneous except for a striped pattern (25 X 25 cm) placed in the middle of one of the walls, as shown in Fig. 1. The objects placed in the open field were two identical wooden cylinders painted clark grey (10 cm in diameter, 10 cm high). The objects occupied positions A and B, and the distance between the objects was always 20 cm throughout the experiment. The apparatus was illuminated by a light which lit the floor homogeneously (10 lux). The floor was divided into squares (10 X l0 cm). A camera was placed 2 m above the open field, and a recorder placed outside the curtain registered the events.

Procedure Preexperimental familiarization. Preliminary observations showed that rats confronted to a novel object often exhibit avoidance reactions rather than curiosity (approach). This had been reported previously (9). In order to suppress such avoidance reactions, the animals were familiarized with different novel objects for 3 weeks before the experiment. The familiarization consisted in transferring rats, similar in age, from their rearing home cage to a larger home cage (50 X 30 X 20 cm) where they were maintained until the experiment, in groups of six. During this period, an object was placed in the cage. The object was changed every day. Five different objects were used (boxes in wood or metal of different sizes but approximately similar in volume). The objects employed during the familiarization period were different from those employed during the experiment. The day before testing, the two experimental objects were impregnated with familiar odor. It consisted in placing the two experimental objects in sawdust coming from cages where the experimental animals were kept. This odor impregnation was carried out because it has been reported that odor impregnation increases exploratory behavior (l 8,19). Finally, again the day before testing, two rats coming from the same home cage were placed together in the open field, without any object, for two 15-min sessions with a 3-min intersession interval. This constituted the apparatus familiarization. The ex-

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FIG. 1. Schematic representation of the objects configuration during successive sessions (SI to $6) for experimental and control groups. The left side of the figure shows the objects positioned accordingto sequence 1, and the right side according to sequence 2. perimental sessions with the two objects were conducted between 0900 and 1600 h on the following day. Experiment. The experiment consisted in placing each animal individually in the open field for six successive 9-min sessions separated by 3-min intervals. During sessions 1-4, the two objects remained in the same position, but before session 5 object B was repositioned. Half of the experimental animals had sequence l, half had sequence 2. Control animals were submitted to the six sessions without any object repositioning (detailed illustration is given in Fig. 1). During the first two sessions of the experiment, half of the animals were introduced into the apparatus first near object A (session l) and then near object B (session 2), the other half was submitted to the inverse sequence. During the four subsequent sessions (sessions 3-6), all animals were introduced at another position, i.e., on the opposite side of the open field facing the visual striped pattern. This procedure was adopted in order to ascertain that each animal had noticed both objects (sessions 1-2) and to avoid possible object preference induced by an experimental bias (sessions 3 to 6). During the intervals between the sessions, the animal was placed into an individual waiting cage outside the curtain of the apparatus in the experimental room. The apparatus was cleaned between subjects but not between sessions for a given animal.

OBJECT EXPLORATION AGING

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DATA COLLECTIONAND STATISTICALANALYSIS Both locomotoractivity,(i.e., the numberof squarescrossed), and exploratory activity directed towards the objects (i.e., the mean time spent exploring objects A and B), were registered. The experimental animals were classified as good or bad learners. The good learners were the animals which exhibited a significant renewal of exploration after the repositioning of object B. On the contrary, the bad learners were the animals which did not display an increase of exploration after the spatial change. The comparison of the number of good and bad learners according to their age was analysed by the ×2 test. The habituation process and the response to spatial change were analysed by pooling data of two successive sessions, i.e., S 1 + $2, $3 + $4, and $5 + $6. The habituation was evaluated by the decrease of exploration or locomotor activity between S 1 + $2 and $3 + $4. The response to spatial change was measured by the time spent exploring the objects during $5 + $6 compared to $3 + $4. ANOVAs for repeated measures (age X sessions) were computed for the total of young (n = 12) and the total of old (n = 12) experimental animals as for the total of young (n = 6) and the total of old (n = 7) control rats.

RESULTS

Locomotor Activity As shown in Fig. 2, locomotion was greater in young than in old rats during the first two sessions and, consequently, the decrease (habituation) was greater in young than in old rats [ANOVA: S1 + $2 and $3 + $4, age effect: F(1, 35) = 26.6, p < 0.0001, session effect: F(1, 35) = 120.6, p < 0.0001, interaction (age X session): F = 34.4, p < 0.001]. In no group was locomotor activity affected by the spatial change [ANOVA: $3 + $4 and $5 + $6, experimental animals with spatial change, age effect: F(1, 22) = 7.93, p < 0.001, session effect: NS, interaction: NS; control animals without spatial

change, age effect: E(1, 11) = 5.8, p < 0.05, session effect: NS, interaction: NS].

Object Exploration In Fig. 3, separate curves were drawn according to the spatial reaction of the animals (good and bad learners). The number of young rats reacting to the repositioning was significantlyhigher than the number of old rats, X2 (1) = 4.29, p < 0.05. Habituation (SI + $2 vs. $3 + $4). Both young and old animals of the experimental and control groups exhibited a decrease in exploration of the two objects (A + B) during sessions 1 to 4, during which there was no spatial change [ANOVA, session effect: F(1, 35) = 40.05, p < 0.0001 ]. As for locomotion, the drop of exploration between sessions was greater for young than for old rats, probably because young animals displayed more investigation of the objects during the first two sessions than old ones [age effect: F(I, 35) = 8.36, p < 0.01, interaction (age X session): F = 6.81, p < 0.05]. Response to spatial change ($5 + $6 vs. $3 + $4). The response to the spatial change was assessed by the presence of a renewal of exploration of at least one object after the repositioning of object B. Of course, control animals (i.e., with no change) were expected to stabilize or decrease their exploration levels. In order to estimate whether the reactivity was restricted to object B (repositioned) or extended also to object A (never moved), the times spent exploring objects A and B were analysed separately. These results are illustrated in Fig. 4. The spatial reaction was significantly stronger in young than in old rats and was restricted to object B (i.e., the repositioned object). The control animals did not show any renewal of exploration during $5 + $6 [ANOVA, spatial change group, age effect object B: F( 1, 22) = 5.36, p < 0.05; other comparisons are not significant]. EXPERIMENT II In order to determine whether the poor reactivity of old rats to the spatial change could be due to fatigue or to a failure to

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maintain attention, the following control experiment was carried out: it consisted of replacing object B by a new object C (a yellow cylinder made in porcelain, dia. 5 cm, height 5 cm) without any spatial change, after the four habituation sessions. Eight naive old rats similar in age and weight to those of Experiment I and familiarized in the same way, were used. RESULTS

Locomotor activity, object exploration, habituation, and response to object change were registered and analysed by Student's t-tests. The results are illustrated in Fig. 5.

and was not affected by object change [$3 + $4 4:$5 + $6, t(7) = 0.17,

NS].

Object Exploration Habituation curves were observed for both objects A and B [S1 + $2 vs. $3 + $4, t(7) = -3.59, p < 0.01 for object A; and t(7) = -3.83, p < 0.01 for object B]. The increase of exploration after the object change was restricted to the changed object C [$3 + $4 4= $5 + $6, t(7) = 1.12 NS for object A; and t(7) = 3.98, p = 0.005 for the changed object C]. G E N E R A L DISCUSSION

Locomotor Activity As in Experiment I, locomotor activity decreased during the first four sessions [S1 + $2 :/: $3 + $4, t(7) = 7.05, p < 0.001],

The present experiment shows that old rats move less and investigate the objects less than young adult rats. However, old rats display habituation curves for both behavioral aspects. The

OBJECT EXPLORATION A G I N G

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weaker level of initial investigation of the novel objects accords with other studies (6,7,14,15,26), but the decrease in exploration, i.e., the habituation, contrasts with the results of the studies of Brennan et al. (7) who observed an increase (rather than a decrease) in exploration over sessions in old mice. In agreement with Brennan et al. (6), our experiment supports the idea that the increase in exploration observed in old mice is a reflection of the suppression of exploratory behavior induced by stress rather than an impairment of habituation. They reduced the differences in habituation caused by age with preexposure to the test environment. In our experiment, it seems plausible that the preexperimental familiarization, as described in the procedure, has attenuated the stress induced by the novel environment in the aged rats, and has consequently enhanced exploration during the first sessions and restored the habituation processes. This interpretation is supported by preliminary experiments in which nonfamiliarized old rats explored less, presented a random increase in exploration over the sessions, and consequently failed to habituate. In both young adult and old rat the spatial change did not result in any increase in locomotor activity. This result strongly supports the notion that exploration is distinct from locomotion and that exploratory behavior plays a predominant part in the acquisition of spatial knowledge (8,16,21). Most of the old rats did not react to the spatial change, while most of the young animals detected the new spatial arrangement, as shown by a renewal of exploration after the displacement of one object. The renewal of exploration was significantly oriented towards the displaced object. This selectivity suggests that young rats, in our experimental conditions, had accurately stored the two object positions. All animals, young and old, that reacted to the spatial change explored the repositioned object as much as they had at the beginning of the experiment when the object was new. This intense reaction suggests that the position in space, when detected,

is as important as other attributes (form, material, and all other stimuli). One possible explanation of the failure of old rats to react to the spatial change could lie in their low level of initial exploration. In this hypothesis, old rats would not have the opportunity to encode the relevant information required to build an internal representation of the object positions in space. However, detailed examination of the results does not support this interpretation. As shown in Fig. 3, the four old rats which detected the spatial change (good learners) did not explore more than the old ones, which did not react to the repositioning (bad learners). Consequently, our results suggest that locomotor activity is not related to spatial reactivity and that, even though the acquisition of spatial knowledge is mediated by the quality of exploration, the quantity, or the level of exploration, seems not to be the critical factor to explain the age-related deficit. These results are in accordance with those obtained by Gage et al. (l 1), having shown that, in old rats, both locomotor activity and exploration levels are not correlated to spatial memory performances. As far as young bad learners are concerned, they did not present a habituation curve similar to the other subgroups for the investigation of object B. Therefore, it cannot be ruled out that these two animals had not reached a normal habituation level before the spatial change. The fact that old animals reacted to the novel object C (Experiment 2) weakens the hypothesis that the lack of spatial reaction could be due to increased fatigue or to an attention reduction over the sessions. All these elements taken together, in addition with the low aversive properties and the poor relevance of distal external cues in the test, support the idea that the spatial deficit in old rats is more related to a reduced ability to detect the relations between the objects and the environment, i.e., to acquire spatial knowledge, than to other factors such as sensory motor and motivational differences.

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In conclusion, the results o b t a i n e d in the present e x p e r i m e n t c o r r o b o r a t e those reported in spatial orientation tasks and show that the age-related cognitive spatial deficit extend to other procedures in which the spatial response consists in reacting to a spatial change in the environment,

ACKNOWLEDGEMENTS This research was supported by UCB-Rrgion Wallonne grants and carried out within the framework of the Centre Interdisciplinaire sur le Vieillissement (CRIV). The authors wish to thank Vrronique Sober for active collaboration in the preliminary experimentation and Marie Bronchart for the computation of the statistical analyses.

REFERENCES 1. Barnes, C. A.; Memory deficits associated with senescence: A behavioural and neurophysiological study in rats. J. Comp. Physiol. Psychol. 93:74-104; 1979. 2. Barnes, C. A.; Nadel, L.; Honig, W. K. Spatial memory deficit in senescent rats. Can. J. Psychol. 34(1):2-39; 1980. 3. Barnes, C. A.; Green, E. J.; Baldwin, J.; Johnson, W. E. Behavioural and neurophysiological examples of functional sparing in senescent rat. Can. J. Psychol. 41(2):131-140; 1987. 4. Barnes, C. A. Aging and the physiology of spatial memory. Neurobiol. Aging 9:563-568; 1988. 5. Bartus, R. T.; Crook, T. H.; Dean, R. L. Current progress in treating age-related memory problems: A perspective from animal preclinical and human clinical research. In: Gibson Wood, W.; Strong, R., eds. Geriatric clinical pharmacology. New York: Raven Press; 1987:71-94. 6. Brennan, M. J.; Blizard, D. A.; Quartermain, D. Amelioration of an age-related deficit in exploratory behavior by preexposure in the test environment. Behav. Neural Biol. 34:55-62; 1982. 7. Brennan, M. J.; Allen, D.; Aleman, D.; Azmitia, E. C.; Quartermain, D. Age differences in within-session habituation of exploratory behavior: Effects of stimulus complexity. Behav. Neural Biol. 42:6172; 1984.

8. Buhot, M. C.; Soflir, M.; Poucet, B. Scopolamine affects the cognitive processes involved in selective object exploration more than locomotor activity. Psychobiology 17:409-417; 1989. 9. Cowan, P. E. Neophobia and neophilia: New-object and new-place reactions of three Rattus species. J. Comp. Physiol. Psychol. 91(1): 63-74; 1977. 10. Gage, F. H.; Dunnett, S. B.; Bjrrklund, A. Spatial learning and motor deficits in aged rats. Neurobiol. Aging. 5:43-48; 1984. 11. Gage, F. H.; Dunnett, S. B.; Bjrrklund, A. Age-related impairments in spatial memory are independent of those in sensorimotor skills. Neurobiol. Aging. 10:347-352; 1989. 12. Gallagher, M.; Pelleymounter, A. Spatial learning deficits in old rats: A model for memory decline in the aged (review). Neurobiol. Aging. 9:549-556; 1988. 13. Gallagher, M.; Burwell, R. D. Relationship of age-related decline across several behavioral domains. Neurobiol. Aging 10:691-708; 1989. 14. Goodrick, C, L. Free exploration and adaptation within an open field as a function of trials and between-trial-interval for matureyoung, mature-old, and senescent Wistar rats. J. Gerontol. 26(1): 58-62; 1971.

OBJECT E X P L O R A T I O N A G I N G 15. Goodrick, C. L. Variables affecting free exploration responses of male and female Wistar rats as a function of age. Dev. Psychol. 4(3): 440-446; 1971. 16. Leyland, M.; Robbins, T.; Iversen, S. D. Locomotor activity and exploration: The use of traditional manipulations to dissociate these two behaviors in the rat. Anim. Learn. Behav. 4(3):261265; 1976. 17. Meencke, H. J.; Ferszt, R.; Gertz, H. J.; Cervos-Navarro, J. Hippocampal pathology in normal aging and dementia. In: CervosNavarro, J.; Sarkander, H. I., eds. Brain aging: Neuropathology and neuropharmacology. Vol. 21. New York: Raven Press; 1983: 13-26. 18. Misslin, R.; Ropartz, Ph. Responses in mice to a novel object. Behaviour 78:169-177; 1981. 19. Misslin, R.; Ropartz, P. Olfactory regulation of responsiveness to novelty in mice. Behav. Neural Biol. 33:230-236; 1981. 20. PeUeymounter, M. A.; Beatty, G.; Gallagher, M. Hippocampal SHCPP binding and spatial learning deficits in aged rats. Psychobiology 18(3):298-304; 1990. 21. Poucet, B.; Chapuis, N.; Durup, M.; Thinus-Blanc, C. A study of exploratory behavior as an index of spatial knowledge in hamsters. Anim. Learn. Behav. 14(1):93-100; 1986.

1035 22. Rapp, P. R.; Rosenberg, R. A.; Gallagher, M. An evaluation of spatial information processing in aged rats. Behav. Neurosci. 101(1):3-12; 1987. 23. Save, E.; Poucet, B.; Foreman, N.; Buhot, M. C. Object exploration and reactions to spatial and non spatial changes in hooded rats following damage to parietal cortex or hippocampal formation. Behav. Neurosci. 106:447-456; 1992. 24. Thinus-Blanc, C. Animal spatial cognition. In: Weiskrantz, L., ed. Thought without language. Oxford: Clarendon Press; 1988:372-395. 25. Wallace, J. E.; Krauter, E. E.; Campbell, B. A. Animals models of declining memory in the aged: Short-term and spatial memory in the aged rat. J. Gerontol. 15(3):355-363; 1980. 26. Willig, F.; Palacios, A.; Monmaur, P.; M'Harzi, M.; Laurent, J.; Delacour, J. Short-term memory, exploration and locomotor activity in aged rats. Neurobiol. Aging. 8:393-402; 1987. 27. Xavier, G. F.; Stein, C.; Bueno, O. F. A. Rats with dorsal hippocampal lesions do react to new stimuli but not to spatial changes of known stimuli. Behav. Neural Biol. 54:172-183; 1990. 28. Zornetzer, S. F.; Rogers, J. Animal models for assessment of geriatric mnemonic and motor deficits. In: Crook, T.; Ferris, S.; Bartus, R., eds. Assessment in geriatric psychopharmacology. New Canaan: Mark Powley Assoc., 1983:301-322.

Cognitive and noncognitive processes involved in selective object exploration: comparison between young adult and old rats.

The age effects on locomotor activity, object-oriented exploration, habituation, and response to a spatial change were studied in young adult and old ...
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