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Effect of shifts in body weight on rats' straight alley performance as a function of reward magnitude
LEARNING
AND
MOTIVATION
Effect
( 1973)
4, 229-2%
of Shifts
in Body
Alley
Performance of Reward ELIZABETH
Department
of Psychological Lafayette,
Weight
on Rats’
Straight
as a Function Magnitude1 D.
CAPALDI
Sciences, Purdue Indiana 47907
University,
The performance of rats trained in a straight alley for 55 trials under 75% body weight and then shifted to 90% body weight was compared to that of rats trained only under 90% body weight. When training was with a small reward the speed of the shifted group decreased to the level of the 90% small reward control groups, but when training was with a large reward the speed of the shifted group dropped below the level of the 90% large reward control group. Thus, the effects on performance of shifting deprivation level following extended training depend on the reward magnitude employed.
The question asked in the present investigation was: Does the effect on performance of changes in deprivation level depend on the reward magnitude employed? An examination of the literature suggests that the answer to this question may be yes. Mollenauer ( 1971) reported that when deprivation level was shifted late in training (after 75 trials) performance changed rapidly and there was some evidence of contrast effects. That is, rats shifted from high to low deprivation tended to run slower than rats trained only under low deprivation, and rats shifted from low to high deprivation tended to run faster than rats trained only under high deprivation. Zaretsky (XX%), on the other hand, found that when deprivation level was shifted, also late in training (after 69 trials), running speed adjusted gradually to the new deprivation conditions. It is possible that this difference in results is attributable to the different reward magnitudes employed. Zaretsky employed small reward magnitudes (either one 45mg pellet or one 260-mg pellet), whereas Mollenauer employed a large reward magnitude (1.5 g wet mash). However, as there were other differences in procedure between these two studies, it seemed desirable to directly determine the influence of reward ‘Requests for Sciences, Purdue Copyright All rights
reprints University,
should be Lafayette,
sent to Indiana
@ 1973 by Academic Press, of reproduction in any form
229 Inc. reserved.
author, 47907.
Department
of
Psychological
230
ELIZABETH
D.
CAPALDI
magnitude. Accordingly, in the present study rats were trained with either a small or large reward, and body weight was shifted following extended training. It was expected that for rats trained with a small reward the performance shift when body weight was changed would be gradual, as Zaretsky found, while for rats trained with a large reward the performance shift when body weight was changed would be rapid, as Mollenauer found, and that a contrast effect might occur for rats trained with a large reward. METHOD
Subjects The Ss were 58 naive male albino rats, about 90 days old on arrival from the Holtzman Co., Madison, Wisconsin. Apparatus The apparatus consisted of a runway painted a flat gray throughout, 144.8 cm long and 8.9 cm wide, with 10.2 cm high sides and covered with 1.3 cm hardware cloth. The start box was separated from the runway by a metal door, which the experimenter dropped by pushing a button. Dropping the door started the first of three O.Ol-set clocks. Clock 1 was stopped (start time) and CIock 2 started when the subject interrupted a photobeam 30.5 cm from the door. Clock 2 was stopped (run time) and Clock 3 started when the S broke the second photobeam located 43.2 cm from the first. Interruption of the third photobeam stopped Clock 3 (goal time). The third photobeam was 30.5 cm beyond the second photobeam and 7.6 cm in front of a gray block of wood with a 1.9 cm deep hole with a 3.17 cm diam used as a food cup. A manually lowered guillotine door, 34.3 cm from the rear of the goal compartment, served to confine the S to the goal area. The three times were summed to yield total time. Procedure The experiment was run by two experimenters, each experimenter running one-half the Ss in each experimental group, Ss were matched on weight and were assigned to 8 experimental groups of 6 Ss each (4 groups for each experimenter) and 1 control group of 10 Ss. The control Ss were fed ad lib. throughout the experiment and were used to estimate growth. During the course of the experiment the control Ss gained 144.6 g on the average. The design in Phase I was a 2 X 2 factorial combining two levels of reward magnitude (2 vs 22 45-mg pellets) with two levels of percentage body weight (75% vs 99%).
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
231
Groups are designated by the reward magnitude and percentage body weight experienced in Phase I. For their first 8 days in the laboratory the rats were fed ad lib. Each rat’s average weight on the last 3 of these days was taken at its original ad lib. weight. Beginning on the next day (day 4) an appropriate weight range was computed for each rat on each day. For animals in Groups 275% and 2275% this range was 7476% of each rat’s original ad lib. weight plus 75% of the mean weight gain of the control rats on each day. For animals in Groups 296% or 2296% this range was 89-91% of each rat’s original ad lib. weight plus 96% of the mean weight gain of the control rats on each day. All rats had reached their appropriate weight by day 19. On days 20, 21, and 22, each rat was handled individually for 90 sec. Fifteen min after handling, animals in Groups 275% and SW% were fed two 45mg Noyes pellets in their home cages, while animals in Groups 2275% and 22-90% were fed 22 pellets. On day 23, Phase I began. On days 1 and 2 of Phase I each animal received one trial, on day 3 of Phase I each rat received two trials, and on days 420 of Phase I each rat received three trials. One experimenter erroneously gave three additional trials on day 21. As this extra day did not affect performance in Phase II, these data are not presented or mentioned further. At the end of Phase I, animals in Groups 275% and 2275% were shifted to 96% body weight. To do this, 8991% of each rat’s original ad lib. weight was computed and 96% of what the control rats had gained since day 4 of pretraining was added to this range. On each subsequent day an acceptable weight range was computed for the previously 75% animals in the same manner as for Groups 296% and 2290%. After 17 days all animals were eating about the same amount of food each day and weights were in the appropriate weight range for all but three rats, two in Group 2275% and one in Group 275%. These rats were on the average 21 g below the appropriate weight, the largest discrepancy being 30 g. As these three rats had stopped gaining weight, Phase II was started the next day, 18 days after the end of Phase I. In Phase II each rat received three trials per day for 7 days. Animals in Groups 275% and 2-96% continued to receive 2 pellets on every trial, and animals in Groups 22-75X and 22-90X continued to receive 22 pellets on every trial. On every trial in Phases I and II the rat was removed from the goal box as soon as it had consumed all the pellets. Throughout the study, animals were run in squads of four composed of 1 S from each group, producing an intertrial interval of about 5-7 min. A given squad was fed its daily ration 15 min after completion of its trials. On each trial the rat was given 66 set in each alley section before it was gently guided to the next section, and 60 set was recorded for the untraversed section. During the course of the
232
ELIZABETH
D.
CAPALDI
experiment five rats became ill, two in Group 22-90%, two in Group 2275% and one in Group 2-90%, and their data were discarded. RESULTS
All times were converted to speeds (l/set). Speeds in Phases I and II were analyzed in a 2 X 2 X 2 between-within analysis of variance for unequal ns including magnitude of reward, Phase I body weight, and experimenter as factors. The difference due to experimenter occasionally reached significance in the run and goal sections but not in the start or total. However, no interaction involving experimenter was significant, thus this variable is not mentioned further. Results in all alley sections were substantially the same, although differences were smaller near the start; thus, only total speeds are presented here. Analyses of Phase II yielded the same results whether or not the three rats, which never reached the appropriate weight, were included; thus, results of analyses including these rats are presented. Mean total speeds’ for each group on the last day of Phase I and each day of Phase II are presented in Fig. 1. Throughout Phase I, 75% body weight animals ran faster than 99% body weight animals (overall Phase I, F(1,35) = 20.19, p < ,001; last day Phase I, F(1,35) = 9.13, p < .Ol). This difference decreased over days (F( 16,560) = 1.92, p < .02). Animals receiving 22 pellets ran faster than animals receiving 2 pellets early in the Phase I, however this difference decreased over days
\
_ 2-75~~ . 2-90x+ 22-75x 22-90x _ #,
\
/I PI
,
1 2
3
/ 4
I
I
5
1
6
7
Days
1. Mean of Phase II.
FIG.
day
’ Conversion
total
to ft/sec
speed
may
for
each
group
be accomplished
on the
last
day
by
multiplying
of Phase
total
I and
speed
by
each
3.42.
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
233
(F( 16,560) = 4.62, p < .OOl ). On the last day of Phase I there was no significant difference due to reward magnitude for 75% body weight animals, but for 90% body weight animals, 2 pellets produced faster running than 22 pellets (Newman-Me&, p < .05), The slow running of Group 2290% relative to Group 290% does not appear to be attributable to satiation within each day for Group 22-90%, as Group 290% ran significantly faster than Group 2290% on the first trial of the day (Newman-Keuls, p < .05) as well as on later trials. All analyses reported here were done on trial 1 of each day only as well as on all trials. The same relationships between groups and pattern of significant differences occurred when only trial 1 of each day was analyzed as occurred when all trials were analyzed, thus, only overall analyses are reported here. As may be seen in Fig. 1, when body weight was shifted from 75 to 90%, Group 2-75% ran slightly faster than Group 290% for the first four days of Phase II and then reached the level of Group 296%. However, Group 2.2-75% ran faster than Group 22-90% for only the first 2 days of Phase II and then dropped below the level of Group 22-90X These relationships are reflected in significant Phase I body weight level X Magnitude of reward X Trials interaction in Phase II (run-F( 20,700) = 1.76, p < .03: goal-F( 20,700) = 1.83, p < .02: total-F( 20,700) = 1.60, p < .05). The differential effects of shifting body weight as a function of reward magnitude were evaluated further by comparing the mean speed on the last 7 days of Phase I (speeds were stable on the last 7 days of Phase I) with the mean speed on the 7 days of Phase II; for Group 2-75% and Group 2275% these means were: Group 2-75%, Phase I 0.937, Phase II 0.759; Group 2275%, Phase I 0.863, Phase II 0.435. As may be seen, Group 2275% dropped in speed when its weight was shifted to a greater extent than did Group 2-75%. This interaction was significant (F( 1,18) = 13.08, p < .Ol). DISCUSSION
The effect of shifting body weight from 75 to 90%following extended training was shown in the present study to vary as a function of reward magnitude. Animals trained with a large reward decreased in speed rapidly to a level below that of the 90% large reward control group, whereas animals trained with a small reward decreased in speed to the level of the 90% small reward control group. Thus, the difference between the results of Mollenauer (1971) and those of Zaretsky (1966) appears to be attributable to the different reward magnitudes employed. It seems, then, that two conditions are necessary in order to obtain a contrast effect with a shift in deprivation level from high to low. Extended preshift training must be employed, as shown by Mollenauer
234
ELIZABETH
D.
CAPALDI
( 1971), and a large reward magnitude must be employed, as shown here. It is interesting to note that the present study repeated the results of both Mollenauer and Zaretsky despite the fact that here 18 days intervened between Phases I and II, this time being necessary to shift body weights, whereas in the studies of Mollenauer and Zaretsky, hours deprivation was used as a deprivation technique, and thus Phase II began the day after the end of Phase I. In the present study, when animals were maintained at 90% body weight animals receiving small reward ran faster than animals receiving large reward, and there was no significant difference due to reward magnitude at the end of Phase I for animals maintained at 75% body weight. A lack of significant differences due to reward magnitude following extended training under high drive has been obtained previously in many investigations (e.g., Gonzalez, Gleitman, & Bitterman, 1962; McCain, Dyleski, & McElvain, 1971). However, there appear to be no previous studies in which an adjusted percentage weight method was used and in which performance of animals run to large and small reward was compared under low drive. Thus, it is not known whether or not this result is a replicable one. However, as the other relationships between groups obtained here have been obtained previously, e.g., a contrast effect due to deprivation level shifts following extended training with a large reward ( Mollenauer, 1971), there seems no reason to question this particular result. That the influence of reward magnitude on the behavioral effects of deprivation level shifts may not be limited to the straight alley is indicated by the similarity of the present results to results obtained in the Skinner box by Reynolds, Marx, and Henderson (1952) and by Marx (1967). In these studies when training was with a small reward resistance to extinction under either low (Reynolds et al., 1952) or intermediate deprivation (Marx, 1967) was greater when training was under high deprivation than when it was under low (Reynolds et al., 1952) or these two groups were equal in resistance to extinction (Marx, 1967). On the other hand, if training was with a large reward, resistance to extinction under low deprivation was greater for animals trained under low deprivation (Marx, 1967; Reynolds et al., 1952). Many interpretations of the present results are possible. For example, it could be assumed that increasing reward magnitude accentuates the difference between stimuli due to different deprivation levels, and thus the decremental effects of shifting deprivation level downward are greater the larger the reward magnitude employed; or more generally, it could be suggested that increasing incentive by one sort of manipulation (e.g., reward magnitude) accentuates the difference between stimuli
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
235
provided by another source of incentive (e.g., deprivation level). Thus, it would also be expected that the decremental effects of shifting reward magnitude downward would be greater the higher the deprivation level employed, as has been reported by, for example, Ehrenfreund ( 1971). Ehrenfreund found that when the reward magnitude was shifted from large to small animals trained under high deprivation shifted rapidly to a level below that of the small reward control, whereas animals trained under low deprivation shifted to the level of the small reward control. Thus, it appears that responsivity to changes in reward is greater under high deprivation than under low deprivation, and responsivity to changes in deprivation level is greater when training is with a large reward than when it is with a small reward. At a general level, then, the effects of deprivation level on shifts in reward are similar to the effects of reward magnitude on shifts in deprivation level. However, there are many possible interpretations of Ehrenfreund’s results as well as of the present results. Thus, whether the similarity between the two is coincidental or reflects common underlying mechanisms remains a matter for future research. REFERENCES EHRENFREUND, D. Effect of drive on successive magnitude shift in rats. Journal of Comparative and Physiological Psychology, 1971, 76, 418423. GONZALEZ, FL C., GLEITMAN, H., & BITTERMAN, M. E. Some observations on the depression effect. Journal of Comparative and Physiological Psychology, 1962, 55, 578-581. MARX, M. H. Interaction of drive and reward as a determiner of resistance to extinction. Journal of Comparative and Physiological Psychology, 1967, 64, 488489. MCCAIN, G., DYLESKI, K., & MCELVAIN, G. Reward magnitude and instrumental responses: Consistent reward. Psychonomic Monograph Supplements, 1971, 3, No. 16 (Whole No. 48). MOLLENAUER, S. 0. Shifts in deprivation level: Different effects depending on amount of preshift training. Learning and Motivation, 1971, 2, 58-66. REYNOLDS, B., MARX, M. H., & HENDERSON, R. L. Resistance to extinction as a function of drive-reward interaction. Journal of Comparative and Physiological Psychology, 1952, 45, 3&42. ZARETSKY, H. H. Learning and performance in the runway as a function of the shift in drive and incentive. Jotmud of Comparative and Physiological Psychology, 1966, 62, 218-222. (Received
March
30,
1972)
AND
MOTIVATION
Effect
( 1973)
4, 229-2%
of Shifts
in Body
Alley
Performance of Reward ELIZABETH
Department
of Psychological Lafayette,
Weight
on Rats’
Straight
as a Function Magnitude1 D.
CAPALDI
Sciences, Purdue Indiana 47907
University,
The performance of rats trained in a straight alley for 55 trials under 75% body weight and then shifted to 90% body weight was compared to that of rats trained only under 90% body weight. When training was with a small reward the speed of the shifted group decreased to the level of the 90% small reward control groups, but when training was with a large reward the speed of the shifted group dropped below the level of the 90% large reward control group. Thus, the effects on performance of shifting deprivation level following extended training depend on the reward magnitude employed.
The question asked in the present investigation was: Does the effect on performance of changes in deprivation level depend on the reward magnitude employed? An examination of the literature suggests that the answer to this question may be yes. Mollenauer ( 1971) reported that when deprivation level was shifted late in training (after 75 trials) performance changed rapidly and there was some evidence of contrast effects. That is, rats shifted from high to low deprivation tended to run slower than rats trained only under low deprivation, and rats shifted from low to high deprivation tended to run faster than rats trained only under high deprivation. Zaretsky (XX%), on the other hand, found that when deprivation level was shifted, also late in training (after 69 trials), running speed adjusted gradually to the new deprivation conditions. It is possible that this difference in results is attributable to the different reward magnitudes employed. Zaretsky employed small reward magnitudes (either one 45mg pellet or one 260-mg pellet), whereas Mollenauer employed a large reward magnitude (1.5 g wet mash). However, as there were other differences in procedure between these two studies, it seemed desirable to directly determine the influence of reward ‘Requests for Sciences, Purdue Copyright All rights
reprints University,
should be Lafayette,
sent to Indiana
@ 1973 by Academic Press, of reproduction in any form
229 Inc. reserved.
author, 47907.
Department
of
Psychological
230
ELIZABETH
D.
CAPALDI
magnitude. Accordingly, in the present study rats were trained with either a small or large reward, and body weight was shifted following extended training. It was expected that for rats trained with a small reward the performance shift when body weight was changed would be gradual, as Zaretsky found, while for rats trained with a large reward the performance shift when body weight was changed would be rapid, as Mollenauer found, and that a contrast effect might occur for rats trained with a large reward. METHOD
Subjects The Ss were 58 naive male albino rats, about 90 days old on arrival from the Holtzman Co., Madison, Wisconsin. Apparatus The apparatus consisted of a runway painted a flat gray throughout, 144.8 cm long and 8.9 cm wide, with 10.2 cm high sides and covered with 1.3 cm hardware cloth. The start box was separated from the runway by a metal door, which the experimenter dropped by pushing a button. Dropping the door started the first of three O.Ol-set clocks. Clock 1 was stopped (start time) and CIock 2 started when the subject interrupted a photobeam 30.5 cm from the door. Clock 2 was stopped (run time) and Clock 3 started when the S broke the second photobeam located 43.2 cm from the first. Interruption of the third photobeam stopped Clock 3 (goal time). The third photobeam was 30.5 cm beyond the second photobeam and 7.6 cm in front of a gray block of wood with a 1.9 cm deep hole with a 3.17 cm diam used as a food cup. A manually lowered guillotine door, 34.3 cm from the rear of the goal compartment, served to confine the S to the goal area. The three times were summed to yield total time. Procedure The experiment was run by two experimenters, each experimenter running one-half the Ss in each experimental group, Ss were matched on weight and were assigned to 8 experimental groups of 6 Ss each (4 groups for each experimenter) and 1 control group of 10 Ss. The control Ss were fed ad lib. throughout the experiment and were used to estimate growth. During the course of the experiment the control Ss gained 144.6 g on the average. The design in Phase I was a 2 X 2 factorial combining two levels of reward magnitude (2 vs 22 45-mg pellets) with two levels of percentage body weight (75% vs 99%).
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
231
Groups are designated by the reward magnitude and percentage body weight experienced in Phase I. For their first 8 days in the laboratory the rats were fed ad lib. Each rat’s average weight on the last 3 of these days was taken at its original ad lib. weight. Beginning on the next day (day 4) an appropriate weight range was computed for each rat on each day. For animals in Groups 275% and 2275% this range was 7476% of each rat’s original ad lib. weight plus 75% of the mean weight gain of the control rats on each day. For animals in Groups 296% or 2296% this range was 89-91% of each rat’s original ad lib. weight plus 96% of the mean weight gain of the control rats on each day. All rats had reached their appropriate weight by day 19. On days 20, 21, and 22, each rat was handled individually for 90 sec. Fifteen min after handling, animals in Groups 275% and SW% were fed two 45mg Noyes pellets in their home cages, while animals in Groups 2275% and 22-90% were fed 22 pellets. On day 23, Phase I began. On days 1 and 2 of Phase I each animal received one trial, on day 3 of Phase I each rat received two trials, and on days 420 of Phase I each rat received three trials. One experimenter erroneously gave three additional trials on day 21. As this extra day did not affect performance in Phase II, these data are not presented or mentioned further. At the end of Phase I, animals in Groups 275% and 2275% were shifted to 96% body weight. To do this, 8991% of each rat’s original ad lib. weight was computed and 96% of what the control rats had gained since day 4 of pretraining was added to this range. On each subsequent day an acceptable weight range was computed for the previously 75% animals in the same manner as for Groups 296% and 2290%. After 17 days all animals were eating about the same amount of food each day and weights were in the appropriate weight range for all but three rats, two in Group 2275% and one in Group 275%. These rats were on the average 21 g below the appropriate weight, the largest discrepancy being 30 g. As these three rats had stopped gaining weight, Phase II was started the next day, 18 days after the end of Phase I. In Phase II each rat received three trials per day for 7 days. Animals in Groups 275% and 2-96% continued to receive 2 pellets on every trial, and animals in Groups 22-75X and 22-90X continued to receive 22 pellets on every trial. On every trial in Phases I and II the rat was removed from the goal box as soon as it had consumed all the pellets. Throughout the study, animals were run in squads of four composed of 1 S from each group, producing an intertrial interval of about 5-7 min. A given squad was fed its daily ration 15 min after completion of its trials. On each trial the rat was given 66 set in each alley section before it was gently guided to the next section, and 60 set was recorded for the untraversed section. During the course of the
232
ELIZABETH
D.
CAPALDI
experiment five rats became ill, two in Group 22-90%, two in Group 2275% and one in Group 2-90%, and their data were discarded. RESULTS
All times were converted to speeds (l/set). Speeds in Phases I and II were analyzed in a 2 X 2 X 2 between-within analysis of variance for unequal ns including magnitude of reward, Phase I body weight, and experimenter as factors. The difference due to experimenter occasionally reached significance in the run and goal sections but not in the start or total. However, no interaction involving experimenter was significant, thus this variable is not mentioned further. Results in all alley sections were substantially the same, although differences were smaller near the start; thus, only total speeds are presented here. Analyses of Phase II yielded the same results whether or not the three rats, which never reached the appropriate weight, were included; thus, results of analyses including these rats are presented. Mean total speeds’ for each group on the last day of Phase I and each day of Phase II are presented in Fig. 1. Throughout Phase I, 75% body weight animals ran faster than 99% body weight animals (overall Phase I, F(1,35) = 20.19, p < ,001; last day Phase I, F(1,35) = 9.13, p < .Ol). This difference decreased over days (F( 16,560) = 1.92, p < .02). Animals receiving 22 pellets ran faster than animals receiving 2 pellets early in the Phase I, however this difference decreased over days
\
_ 2-75~~ . 2-90x+ 22-75x 22-90x _ #,
\
/I PI
,
1 2
3
/ 4
I
I
5
1
6
7
Days
1. Mean of Phase II.
FIG.
day
’ Conversion
total
to ft/sec
speed
may
for
each
group
be accomplished
on the
last
day
by
multiplying
of Phase
total
I and
speed
by
each
3.42.
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
233
(F( 16,560) = 4.62, p < .OOl ). On the last day of Phase I there was no significant difference due to reward magnitude for 75% body weight animals, but for 90% body weight animals, 2 pellets produced faster running than 22 pellets (Newman-Me&, p < .05), The slow running of Group 2290% relative to Group 290% does not appear to be attributable to satiation within each day for Group 22-90%, as Group 290% ran significantly faster than Group 2290% on the first trial of the day (Newman-Keuls, p < .05) as well as on later trials. All analyses reported here were done on trial 1 of each day only as well as on all trials. The same relationships between groups and pattern of significant differences occurred when only trial 1 of each day was analyzed as occurred when all trials were analyzed, thus, only overall analyses are reported here. As may be seen in Fig. 1, when body weight was shifted from 75 to 90%, Group 2-75% ran slightly faster than Group 290% for the first four days of Phase II and then reached the level of Group 296%. However, Group 2.2-75% ran faster than Group 22-90% for only the first 2 days of Phase II and then dropped below the level of Group 22-90X These relationships are reflected in significant Phase I body weight level X Magnitude of reward X Trials interaction in Phase II (run-F( 20,700) = 1.76, p < .03: goal-F( 20,700) = 1.83, p < .02: total-F( 20,700) = 1.60, p < .05). The differential effects of shifting body weight as a function of reward magnitude were evaluated further by comparing the mean speed on the last 7 days of Phase I (speeds were stable on the last 7 days of Phase I) with the mean speed on the 7 days of Phase II; for Group 2-75% and Group 2275% these means were: Group 2-75%, Phase I 0.937, Phase II 0.759; Group 2275%, Phase I 0.863, Phase II 0.435. As may be seen, Group 2275% dropped in speed when its weight was shifted to a greater extent than did Group 2-75%. This interaction was significant (F( 1,18) = 13.08, p < .Ol). DISCUSSION
The effect of shifting body weight from 75 to 90%following extended training was shown in the present study to vary as a function of reward magnitude. Animals trained with a large reward decreased in speed rapidly to a level below that of the 90% large reward control group, whereas animals trained with a small reward decreased in speed to the level of the 90% small reward control group. Thus, the difference between the results of Mollenauer (1971) and those of Zaretsky (1966) appears to be attributable to the different reward magnitudes employed. It seems, then, that two conditions are necessary in order to obtain a contrast effect with a shift in deprivation level from high to low. Extended preshift training must be employed, as shown by Mollenauer
234
ELIZABETH
D.
CAPALDI
( 1971), and a large reward magnitude must be employed, as shown here. It is interesting to note that the present study repeated the results of both Mollenauer and Zaretsky despite the fact that here 18 days intervened between Phases I and II, this time being necessary to shift body weights, whereas in the studies of Mollenauer and Zaretsky, hours deprivation was used as a deprivation technique, and thus Phase II began the day after the end of Phase I. In the present study, when animals were maintained at 90% body weight animals receiving small reward ran faster than animals receiving large reward, and there was no significant difference due to reward magnitude at the end of Phase I for animals maintained at 75% body weight. A lack of significant differences due to reward magnitude following extended training under high drive has been obtained previously in many investigations (e.g., Gonzalez, Gleitman, & Bitterman, 1962; McCain, Dyleski, & McElvain, 1971). However, there appear to be no previous studies in which an adjusted percentage weight method was used and in which performance of animals run to large and small reward was compared under low drive. Thus, it is not known whether or not this result is a replicable one. However, as the other relationships between groups obtained here have been obtained previously, e.g., a contrast effect due to deprivation level shifts following extended training with a large reward ( Mollenauer, 1971), there seems no reason to question this particular result. That the influence of reward magnitude on the behavioral effects of deprivation level shifts may not be limited to the straight alley is indicated by the similarity of the present results to results obtained in the Skinner box by Reynolds, Marx, and Henderson (1952) and by Marx (1967). In these studies when training was with a small reward resistance to extinction under either low (Reynolds et al., 1952) or intermediate deprivation (Marx, 1967) was greater when training was under high deprivation than when it was under low (Reynolds et al., 1952) or these two groups were equal in resistance to extinction (Marx, 1967). On the other hand, if training was with a large reward, resistance to extinction under low deprivation was greater for animals trained under low deprivation (Marx, 1967; Reynolds et al., 1952). Many interpretations of the present results are possible. For example, it could be assumed that increasing reward magnitude accentuates the difference between stimuli due to different deprivation levels, and thus the decremental effects of shifting deprivation level downward are greater the larger the reward magnitude employed; or more generally, it could be suggested that increasing incentive by one sort of manipulation (e.g., reward magnitude) accentuates the difference between stimuli
BODY
WEIGHT
SHIFTS
AND
REWARD
SIZE
235
provided by another source of incentive (e.g., deprivation level). Thus, it would also be expected that the decremental effects of shifting reward magnitude downward would be greater the higher the deprivation level employed, as has been reported by, for example, Ehrenfreund ( 1971). Ehrenfreund found that when the reward magnitude was shifted from large to small animals trained under high deprivation shifted rapidly to a level below that of the small reward control, whereas animals trained under low deprivation shifted to the level of the small reward control. Thus, it appears that responsivity to changes in reward is greater under high deprivation than under low deprivation, and responsivity to changes in deprivation level is greater when training is with a large reward than when it is with a small reward. At a general level, then, the effects of deprivation level on shifts in reward are similar to the effects of reward magnitude on shifts in deprivation level. However, there are many possible interpretations of Ehrenfreund’s results as well as of the present results. Thus, whether the similarity between the two is coincidental or reflects common underlying mechanisms remains a matter for future research. REFERENCES EHRENFREUND, D. Effect of drive on successive magnitude shift in rats. Journal of Comparative and Physiological Psychology, 1971, 76, 418423. GONZALEZ, FL C., GLEITMAN, H., & BITTERMAN, M. E. Some observations on the depression effect. Journal of Comparative and Physiological Psychology, 1962, 55, 578-581. MARX, M. H. Interaction of drive and reward as a determiner of resistance to extinction. Journal of Comparative and Physiological Psychology, 1967, 64, 488489. MCCAIN, G., DYLESKI, K., & MCELVAIN, G. Reward magnitude and instrumental responses: Consistent reward. Psychonomic Monograph Supplements, 1971, 3, No. 16 (Whole No. 48). MOLLENAUER, S. 0. Shifts in deprivation level: Different effects depending on amount of preshift training. Learning and Motivation, 1971, 2, 58-66. REYNOLDS, B., MARX, M. H., & HENDERSON, R. L. Resistance to extinction as a function of drive-reward interaction. Journal of Comparative and Physiological Psychology, 1952, 45, 3&42. ZARETSKY, H. H. Learning and performance in the runway as a function of the shift in drive and incentive. Jotmud of Comparative and Physiological Psychology, 1966, 62, 218-222. (Received
March
30,
1972)