BEHAVIORAL BIOLOGY, 15, 65-72 (1975), Abstract No. 4289
The Effects of Crowding Under Different Rearing Conditions on Emotionality and Transfer of Perceptual Learning 1
L Y N N L E V I T T a n d T H O M A S L. B E N N E T T
Department of Psychology, Colorado State University, Fort Collins, Colorado
To examine the effects of population density on emotionality and transfer of perceptual learning, albino rats were either preexposed to circles and triangles in a visually sparse environment or received no early experience with the to-be-discriminated shapes. Within each early experience condition, the animals were initially placed in either a crowded or uncrowded environment. Approximately 1 mo later, half of the rats under each population condition were shifted to the opposite population density condition, while the other half remained under their original condition. Analysis of the data indicated that (1) the animals uncrowded for the entire experiment were less emotional than animals crowded for the same length of time; (2) there was no difference in emotionality produced by 1 mo versus 2 t o o of crowding; (3) animals uncrowded for 1 or more mo immediately after weaning were less emotional than animals crowded during this interval; (4) in general, animals preexposed to the to-be-discriminated forms learned the discrimination task in significantly fewer trials than did animals not receiving early stimulus preexposure; (5) animals that were crowded 1 mo immediately after weaning were poorer performers on the discrimination learning problem than were animals uncrowded during this same time period. Thus, the effects of population density on emotionality and transfer of perceptual learning were determined by population rearing conditions existing during the month immediately following weaning.
INTRODUCTION A n u m b e r o f i n v e s t i g a t o r s have e x a m i n e d t h e effects o f o v e r c r o w d i n g o n b e h a v i o r a n d a s s o c i a t e d p h y s i o l o g i c a l indices, b u t relatively few researchers have a t t e m p t e d t o e l u c i d a t e t h e effects o f o v e r c r o w d i n g o n l e a r n i n g ability, per se. I n general, t h o s e studies w h i c h have e x a m i n e d this q u e s t i o n have 1This article is based on a dissertation submitted by the ftrst author in partial fulfillment of the requirements for the Ph.D. degree at Colorado State University. Requests for reprints should be sent to: Lynn Levitt, Department of Psychology, William Paterson College of New Jersey, 300 Pompton Road, Wayne, New Jersey, 07470. 65 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
66
LEVITT AND BENNETT
shown that crowded animals exhibit decreased emotionality and in some instances, an increased acquisition rate CDolger, 1954; Taylor, 1969; Levitt and Bennett, 1972). Both Dolger and Taylor used a Hebb-Williams maze to assess learning ability while Levitt and Bennett used a transfer of perceptual learning paradigm. Leicht, King., and Harvey (1973), in contrast, found a decrease in T-maze learning and performance under high population density rearing conditions. Although previous researchers examining the effects of crowding on learning and emotionality have employed different methodologies, they have all tested for these effects immediately after the crowding period. No inquiry has assessed the effects on emotionality and learning of a period of delay between crowding and testing. The present research was addressed to this issue. Specifically, we attempted to test the effects of crowding on transfer of perceptual learning, emotionality, and associated physiological indices immediately after the crowding period and also after a period of delay between crowding and testing. It was hoped that this investigation would shed some light on both the permanency of the effects of crowding and the importance of the duration of the crowding period. Additionally, this study attempted to determine if there are sensitive periods during which crowding most significantly effects behavior. The experimental manipulations of population density during rearing were done within the framework of the transfer of perceptual learning paradigm of Gibson and Walk (1956). This early experience procedure involves rearing animals in a visually sparse environment, except that half of the subjects have cutout circles and triangles hanging in their cages from the time of weaning, 21 days of age, through the duration of the experiment. At 90 days of age, the early experience and nonexposed animals learn a circletriangle discrimination as a test of the effects of the preexposure conditions. It has generally been found that animals that receive early experience with the to-be-discriminated shapes reach criterion in significantly fewer trials than do nonexposed subjects. We have used this paradigm a number of times in recent years in attempts to elucidate this early experience effect (e.g., Bennett, Anton, and Levitt, 1971; Bennett and Ellis, 1968). This procedure allowed us to investigate the effects of variations in population density on the transfer, to a later discrimination learning problem, of information acquired in the early environment.
METHOD
Subjects. The subjects were 80 male Charles River albino rats randomly selected from 140 rats. The remaining 60 animals were used to provide a crowded environment.
OVERCROWDINGAND BEHAVIOR
67
Apparatus. At the age of 21 days, the animals were placed in rearing cages measuring 38.10 × 24.13 × 25.40 cm. Each cage was placed in a compartment painted flat white which measured 60.96 × 60.96 × 33.02 cm. A 25-W bulb located directly above the cage illuminated the compartment from 7:00 AM until 6:00 PM. The opening of each compartment was 60.96 cm from the white wall of the room in which the animals were housed, thus providing a visually sparse environment. A movable wire mesh screen measuring 24.13 × 25.40 cm served as the rear wall of the cage and was used to control the degree of crowding as the animals matured and increased in size. For those rats receiving early experience with shapes, the stimuli consisted of two circles, 7.62 cm in diameter, and two equilateral triangles, 8.89 cm per side. The forms were constructed from .32 cm thick black Plexiglas and were suspended from the cage ceiling by wires. A timidity apparatus (see Hunt and Otis, 1953) painted flat black was used to assess emotionality. The timidity apparatus consisted of a start box (12.70 X 30.48 × 25.40 cm) connected to a runway (12.70 X 76.20 cm) that was elevated 91.44 cm above the ground. For the discrimination learning problem, a modified Grice two-choice discrimination apparatus was employed (see Gibson and Walk, 1956). Rearing procedures. Beginning at 21 days of age, half of the subjects were preexposed to the to-be-discriminated stimuli, a circle and an equilateral triangle, and the other half received no early experience with these shapes. The population density rearing conditions varied within each of these early experience conditions as follows. Half of the animals within each early experience condition were initially placed in a crowded environment (10 rats per cage) and half in an uncrowded environment (2 rats per cage). At 55 days of age, half of the rats under each population density condition were shifted to the opposite condition. Hence, half of the uncrowded animals were shifted to a crowded condition, and half of the crowded subjects were shifted to an uncrowded condition. This was accomplished by taking five cages of two rats per cage and combining them into one cage of ten rats per cage, and taking one cage of ten rats and dividing it into five cages of two rats per cage. The other half of the rats remained under their initial population density rearing condition throughout the duration of the experiment. The two early experience conditions factorially combined with the four rearing conditions resulted in the following eight groups of animals with ten animals per group: 1. Gp E(U-U), early stimulus experience/uncrowded to uncrowded environment; 2. Gp E(C-C), early stimulus experience/crowded to crowded environment; 3. Gp E(U-C), early stimulus experience/uncrowded to crowded environment; 4. Gp E(C-U), early stimulus experience/crowded to uncrowded environment; 5. Gp NE(U-U), no early stimulus experience/ uncrowded to uncrowded environment; 6. Gp NE(C-C), no early experience/ crowded to crowded environment; 7. Gp NE(U-C), no early stimulus
68
LEVITT AND BENNETT
experience/uncrowded to crowded environment; and 8. Gp NE(C-U), no early stimulus experience/crowded to uncrowded environment. To maintain an approximatley equal degree of crowding, the sizes of the cages were varied according to the increasing size of the animals. At the onset of the experiment, a wire mesh screen was fastened 12.70 cm from the entrance of the cage, thereby decreasing the actual living space for all subjects to 12.70 × 24.13 × 25.40 cm. These dimensions were previously determined (Levitt and Bennett, 1972). When there was just enough floor space for all animals without any animal lying on top of another, the screen was fastened in place and all other cages were set at this distance. As the animals increased in size, the screens were moved farther back to allow more living space. At 55 days of age, the screens were moved from 12..70 cm to 20.32 cm from the entrance, and at 80 days, from 20.32 cm to 25.40 cm where they remained until the end of the experiment. Food and water were available ad lib. from 21 until 80 days of age (except for 1 day of food deprivation before administering the timidity test on day 54). A 23-hr food deprivation regimen was imposed at 80 days of age in preparation for the beginning of discrimination training. Assessment of emotionality. To assess the effects of various rearing conditions on emotionality, each rat was run in the timidity test on days 54, 81, 88, and 116 of age. All animals were placed on 23 hr of food deprivation 1 day prior to testing. During testing, each subject was initially placed in the start box for 5 min. Then the cage door was opened, and the rat was given 30 min to emerge and eat a pinch of wet mash placed along the elevated runway at a distance of 20.50 cm from the start box. Latency scores were obtained for the following three response measures: 1. time elapsing before a rat placed its forepaws on the runway; 2. time elapsing before a subject placed all four paws on the runway; 3. time elapsing before a rat emerged and also ate the pinch of wet mash. Low emotionality on the timidity test was operationally defined as a low latency score on each of the three measures. Measures of gross body weight were obtained on days 30, 55, 90, and 116. The animals were sacrificed 25 days after discrimination testing had begun, and body weight and adrenal weights were recorded. Discrimination training. At 80 days of age, the animals were placed on a 23-hr food deprivation schedule. At 90 days of age, the rats were given 3-5 days of discrimination pretraining. Following pretraining, the rats were administered a circle-triangle discrimination problem in a modified Grice discrimination apparatus (Gibson and Walk, 1956) as a test of the effects of the various rearing conditions. Details of the discrimination pretraining and training procedures are described by Bennett and Ellis (1968). The subjects were administered 12 trials per day until they reached a criterion of 18 out of 20 correct responses with the last 10 consecutive responses correct, or for a maximum of 300 trials.
OVERCROWDING AND BEHAVIOR
69
RESULTS Emotionality. The mean durations in minutes between the four population density rearing conditions for the three dependent measures are shown in Table 1. There were significant differences among the various population density rearing conditions on all three measures. These main effects and the significant sources of variance accounting for these observed differences may be summarized as follows: 1. Time elapsing before an animal placed its forepaws on the runway (F(3,72) = 3.42, P < .02). A Neuman-Keuls test indicated that Gp C-U took significantly more time to place its forepaws on the runway than either Gp U-C ( P < .05) or Gp U-U ( P < .05). 2. Time elapsing prior to an animal placing all four feet on the runway (F(3,72)= 3.32, P < .02). 3. The time elapsing before an animal ate one pinch o f wet mash (F(3,72)= 3.66, P < .02). The Neuman-Keuls test indicated that Gp C-U took significantly longer to eat one pinch of wet mash than did Gp U-U (P < .05). There were no significant differences between the four population density rearing conditions in absolute adrenal weight or relative adrenal weight (absolute adrenal weight/100 g body wt). As the animals matured, there was a significant increase in body weight ( F ( 3 , 2 1 6 ) = 2 6 4 . 1 1 , P < .005), but there were no significant differences in body weight of the animals between the four population density rearing conditions. Discrimination learning. The mean trials to criterion on the discrimination learning task for each group are shown in Table 2. A Kruskal-Wallis analysis of variance by ranks revealed a significant difference, in terms of trials to criterion on the discrimination problem, between the eight groups (H(7) = 14.27, P < .05). Mann-Whitney U tests (two-tailed) were applied to assess differences in trials to criterion between the groups. These analyses indicated that, regardless of population density in their early environments, animals preexposed to the to-be-discriminated forms learned the task in significantly fewer trials than did subjects receiving no early experience with the shapes (z(1) = 2.402, P < .02).
TABLE 1 Mean Time (in Minutes) Between the Four Rearing Conditions for the Three Dependent Measures on the Timidity Apparatus Group C-C Group C-U Group U-C Group U-U
P
Forepaws
1.54
2.19
0.50
0.47
The Effects of Crowding Under Different Rearing Conditions on Emotionality and Transfer of Perceptual Learning 1
L Y N N L E V I T T a n d T H O M A S L. B E N N E T T
Department of Psychology, Colorado State University, Fort Collins, Colorado
To examine the effects of population density on emotionality and transfer of perceptual learning, albino rats were either preexposed to circles and triangles in a visually sparse environment or received no early experience with the to-be-discriminated shapes. Within each early experience condition, the animals were initially placed in either a crowded or uncrowded environment. Approximately 1 mo later, half of the rats under each population condition were shifted to the opposite population density condition, while the other half remained under their original condition. Analysis of the data indicated that (1) the animals uncrowded for the entire experiment were less emotional than animals crowded for the same length of time; (2) there was no difference in emotionality produced by 1 mo versus 2 t o o of crowding; (3) animals uncrowded for 1 or more mo immediately after weaning were less emotional than animals crowded during this interval; (4) in general, animals preexposed to the to-be-discriminated forms learned the discrimination task in significantly fewer trials than did animals not receiving early stimulus preexposure; (5) animals that were crowded 1 mo immediately after weaning were poorer performers on the discrimination learning problem than were animals uncrowded during this same time period. Thus, the effects of population density on emotionality and transfer of perceptual learning were determined by population rearing conditions existing during the month immediately following weaning.
INTRODUCTION A n u m b e r o f i n v e s t i g a t o r s have e x a m i n e d t h e effects o f o v e r c r o w d i n g o n b e h a v i o r a n d a s s o c i a t e d p h y s i o l o g i c a l indices, b u t relatively few researchers have a t t e m p t e d t o e l u c i d a t e t h e effects o f o v e r c r o w d i n g o n l e a r n i n g ability, per se. I n general, t h o s e studies w h i c h have e x a m i n e d this q u e s t i o n have 1This article is based on a dissertation submitted by the ftrst author in partial fulfillment of the requirements for the Ph.D. degree at Colorado State University. Requests for reprints should be sent to: Lynn Levitt, Department of Psychology, William Paterson College of New Jersey, 300 Pompton Road, Wayne, New Jersey, 07470. 65 Copyright © 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
66
LEVITT AND BENNETT
shown that crowded animals exhibit decreased emotionality and in some instances, an increased acquisition rate CDolger, 1954; Taylor, 1969; Levitt and Bennett, 1972). Both Dolger and Taylor used a Hebb-Williams maze to assess learning ability while Levitt and Bennett used a transfer of perceptual learning paradigm. Leicht, King., and Harvey (1973), in contrast, found a decrease in T-maze learning and performance under high population density rearing conditions. Although previous researchers examining the effects of crowding on learning and emotionality have employed different methodologies, they have all tested for these effects immediately after the crowding period. No inquiry has assessed the effects on emotionality and learning of a period of delay between crowding and testing. The present research was addressed to this issue. Specifically, we attempted to test the effects of crowding on transfer of perceptual learning, emotionality, and associated physiological indices immediately after the crowding period and also after a period of delay between crowding and testing. It was hoped that this investigation would shed some light on both the permanency of the effects of crowding and the importance of the duration of the crowding period. Additionally, this study attempted to determine if there are sensitive periods during which crowding most significantly effects behavior. The experimental manipulations of population density during rearing were done within the framework of the transfer of perceptual learning paradigm of Gibson and Walk (1956). This early experience procedure involves rearing animals in a visually sparse environment, except that half of the subjects have cutout circles and triangles hanging in their cages from the time of weaning, 21 days of age, through the duration of the experiment. At 90 days of age, the early experience and nonexposed animals learn a circletriangle discrimination as a test of the effects of the preexposure conditions. It has generally been found that animals that receive early experience with the to-be-discriminated shapes reach criterion in significantly fewer trials than do nonexposed subjects. We have used this paradigm a number of times in recent years in attempts to elucidate this early experience effect (e.g., Bennett, Anton, and Levitt, 1971; Bennett and Ellis, 1968). This procedure allowed us to investigate the effects of variations in population density on the transfer, to a later discrimination learning problem, of information acquired in the early environment.
METHOD
Subjects. The subjects were 80 male Charles River albino rats randomly selected from 140 rats. The remaining 60 animals were used to provide a crowded environment.
OVERCROWDINGAND BEHAVIOR
67
Apparatus. At the age of 21 days, the animals were placed in rearing cages measuring 38.10 × 24.13 × 25.40 cm. Each cage was placed in a compartment painted flat white which measured 60.96 × 60.96 × 33.02 cm. A 25-W bulb located directly above the cage illuminated the compartment from 7:00 AM until 6:00 PM. The opening of each compartment was 60.96 cm from the white wall of the room in which the animals were housed, thus providing a visually sparse environment. A movable wire mesh screen measuring 24.13 × 25.40 cm served as the rear wall of the cage and was used to control the degree of crowding as the animals matured and increased in size. For those rats receiving early experience with shapes, the stimuli consisted of two circles, 7.62 cm in diameter, and two equilateral triangles, 8.89 cm per side. The forms were constructed from .32 cm thick black Plexiglas and were suspended from the cage ceiling by wires. A timidity apparatus (see Hunt and Otis, 1953) painted flat black was used to assess emotionality. The timidity apparatus consisted of a start box (12.70 X 30.48 × 25.40 cm) connected to a runway (12.70 X 76.20 cm) that was elevated 91.44 cm above the ground. For the discrimination learning problem, a modified Grice two-choice discrimination apparatus was employed (see Gibson and Walk, 1956). Rearing procedures. Beginning at 21 days of age, half of the subjects were preexposed to the to-be-discriminated stimuli, a circle and an equilateral triangle, and the other half received no early experience with these shapes. The population density rearing conditions varied within each of these early experience conditions as follows. Half of the animals within each early experience condition were initially placed in a crowded environment (10 rats per cage) and half in an uncrowded environment (2 rats per cage). At 55 days of age, half of the rats under each population density condition were shifted to the opposite condition. Hence, half of the uncrowded animals were shifted to a crowded condition, and half of the crowded subjects were shifted to an uncrowded condition. This was accomplished by taking five cages of two rats per cage and combining them into one cage of ten rats per cage, and taking one cage of ten rats and dividing it into five cages of two rats per cage. The other half of the rats remained under their initial population density rearing condition throughout the duration of the experiment. The two early experience conditions factorially combined with the four rearing conditions resulted in the following eight groups of animals with ten animals per group: 1. Gp E(U-U), early stimulus experience/uncrowded to uncrowded environment; 2. Gp E(C-C), early stimulus experience/crowded to crowded environment; 3. Gp E(U-C), early stimulus experience/uncrowded to crowded environment; 4. Gp E(C-U), early stimulus experience/crowded to uncrowded environment; 5. Gp NE(U-U), no early stimulus experience/ uncrowded to uncrowded environment; 6. Gp NE(C-C), no early experience/ crowded to crowded environment; 7. Gp NE(U-C), no early stimulus
68
LEVITT AND BENNETT
experience/uncrowded to crowded environment; and 8. Gp NE(C-U), no early stimulus experience/crowded to uncrowded environment. To maintain an approximatley equal degree of crowding, the sizes of the cages were varied according to the increasing size of the animals. At the onset of the experiment, a wire mesh screen was fastened 12.70 cm from the entrance of the cage, thereby decreasing the actual living space for all subjects to 12.70 × 24.13 × 25.40 cm. These dimensions were previously determined (Levitt and Bennett, 1972). When there was just enough floor space for all animals without any animal lying on top of another, the screen was fastened in place and all other cages were set at this distance. As the animals increased in size, the screens were moved farther back to allow more living space. At 55 days of age, the screens were moved from 12..70 cm to 20.32 cm from the entrance, and at 80 days, from 20.32 cm to 25.40 cm where they remained until the end of the experiment. Food and water were available ad lib. from 21 until 80 days of age (except for 1 day of food deprivation before administering the timidity test on day 54). A 23-hr food deprivation regimen was imposed at 80 days of age in preparation for the beginning of discrimination training. Assessment of emotionality. To assess the effects of various rearing conditions on emotionality, each rat was run in the timidity test on days 54, 81, 88, and 116 of age. All animals were placed on 23 hr of food deprivation 1 day prior to testing. During testing, each subject was initially placed in the start box for 5 min. Then the cage door was opened, and the rat was given 30 min to emerge and eat a pinch of wet mash placed along the elevated runway at a distance of 20.50 cm from the start box. Latency scores were obtained for the following three response measures: 1. time elapsing before a rat placed its forepaws on the runway; 2. time elapsing before a subject placed all four paws on the runway; 3. time elapsing before a rat emerged and also ate the pinch of wet mash. Low emotionality on the timidity test was operationally defined as a low latency score on each of the three measures. Measures of gross body weight were obtained on days 30, 55, 90, and 116. The animals were sacrificed 25 days after discrimination testing had begun, and body weight and adrenal weights were recorded. Discrimination training. At 80 days of age, the animals were placed on a 23-hr food deprivation schedule. At 90 days of age, the rats were given 3-5 days of discrimination pretraining. Following pretraining, the rats were administered a circle-triangle discrimination problem in a modified Grice discrimination apparatus (Gibson and Walk, 1956) as a test of the effects of the various rearing conditions. Details of the discrimination pretraining and training procedures are described by Bennett and Ellis (1968). The subjects were administered 12 trials per day until they reached a criterion of 18 out of 20 correct responses with the last 10 consecutive responses correct, or for a maximum of 300 trials.
OVERCROWDING AND BEHAVIOR
69
RESULTS Emotionality. The mean durations in minutes between the four population density rearing conditions for the three dependent measures are shown in Table 1. There were significant differences among the various population density rearing conditions on all three measures. These main effects and the significant sources of variance accounting for these observed differences may be summarized as follows: 1. Time elapsing before an animal placed its forepaws on the runway (F(3,72) = 3.42, P < .02). A Neuman-Keuls test indicated that Gp C-U took significantly more time to place its forepaws on the runway than either Gp U-C ( P < .05) or Gp U-U ( P < .05). 2. Time elapsing prior to an animal placing all four feet on the runway (F(3,72)= 3.32, P < .02). 3. The time elapsing before an animal ate one pinch o f wet mash (F(3,72)= 3.66, P < .02). The Neuman-Keuls test indicated that Gp C-U took significantly longer to eat one pinch of wet mash than did Gp U-U (P < .05). There were no significant differences between the four population density rearing conditions in absolute adrenal weight or relative adrenal weight (absolute adrenal weight/100 g body wt). As the animals matured, there was a significant increase in body weight ( F ( 3 , 2 1 6 ) = 2 6 4 . 1 1 , P < .005), but there were no significant differences in body weight of the animals between the four population density rearing conditions. Discrimination learning. The mean trials to criterion on the discrimination learning task for each group are shown in Table 2. A Kruskal-Wallis analysis of variance by ranks revealed a significant difference, in terms of trials to criterion on the discrimination problem, between the eight groups (H(7) = 14.27, P < .05). Mann-Whitney U tests (two-tailed) were applied to assess differences in trials to criterion between the groups. These analyses indicated that, regardless of population density in their early environments, animals preexposed to the to-be-discriminated forms learned the task in significantly fewer trials than did subjects receiving no early experience with the shapes (z(1) = 2.402, P < .02).
TABLE 1 Mean Time (in Minutes) Between the Four Rearing Conditions for the Three Dependent Measures on the Timidity Apparatus Group C-C Group C-U Group U-C Group U-U
P
Forepaws
1.54
2.19
0.50
0.47
