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Modulation of schedule-induced polydipsia by Pavlovian conditioned states
Physiology & Behavior,Vol. 24, pp. 411--414.Pergamon Press and Brain Research Publ., 1980. Printed in the U.S.A.
Modulation of Schedule-Induced Polydipsia by Pavlovian Conditioned States' ROBIN L. LASHLEY
AND ROBERT A. ROSELLINI
Department o f Psychology, State University of New York at Albany, Albany, N Y 12222 R e c e i v e d 10 S e p t e m b e r 1979 LASHLEY, R. L. AND R. A. ROSELLINI. Modulation tdschedule-inducedpolydipsia by Pavlovian conditioned states. PHYSIOL. BEHAV. 24(2) 411--414, 1980.--It has previously been porlSosed that Random-lnterval schedules fail to induce polydipsic drinking in rats because such schedules fail to provide gradients of reinforcement probability. In the present study, food-deprived rats received single pellet meals delivered on either a Fixed-Time or a Random-Time 120-sec schedule, with half of each group receiving signalled, and half unsignalled, pellet deliveries. Acquisition of polydipsia was retarded in animals exposed to the Random-Time Unsignalled condition. However, the imposition of a CS+ for pellet delivery on the identical Random-Time schedule resulted in levels of polydipsia comparable to those obtained in both Fixed-Time conditions. These results suggest that the establishment of a Pavlovian conditioned inhibitory signal, as well as local periods of low reinforcement probability, may be sufficient for the development of schedule-induced behaviors such as polydipsia. Schedule-induced polydipsia
Pavlovian conditioning
FOOD-deprived animals typically ingest excessive amounts of water when exposed to an intermittent schedule of food delivery [2]. This phenomenon, termed schedule-induced polydipsia, has been observed under a wide variety of deprivation conditions, magnitudes of reinforcement, and schedules of reinforcement [6]. However, it has recently been reported that a majority of rats fail to develop polydipsia when food pellets are delivered on a truly RandomInterval schedule [5]. The unique defining characteristic of such a Random-Interval schedule is a low, but constant, probability of reinforcement throughout the interpellet interval--that is, a response at any point in time is as likely to earn a pellet as is a response at any other point in time. In contrast, the more usual arithmetic Variable-Interval schedule includes, in practice, some minimum time period following each pellet delivery during which responses will never be reinforced. Thus, as time passes since the last reinforcement, the probability that the next response will also be reinforced is constantly increasing. Variable-Ratio, FixedRatio, Fixed-Interval, and Fixed-Time schedules likewise provide such gradients of reinforcement probability. Therefore, it appears that an important determinant of scheduleinduced polydipsia must be the establishment of the temporal period immediately following reinforcement as a C S - , indicating a locally low probability of reinforcement [5]. Similarly, it has been suggested [6] that such local periods of low reinforcement probability tend to induce a classically conditioned state by virtue of the fact that the immediate postpellet period ( C S - ) is negatively correlated with the next food delivery (US). The actual behavioral manifestations of this state are held to be a function o f the species, the reinforcer used, and the environmental stimuli which are pres-
Random-time schedules
Rats
ent. It has thus been proposed [5] that the provision of a water spout in close proximity to the food cup, combined with the rat's strong tendency to drink after eating, leads to the ingestion of water in the polydipsia situation. Such a Pavlovian conditioning view of schedule-induced behavior suggests that the inability of Random-Interval schedules to generate drinking should be overcome if pellet delivery were signalled. The imposition of such a C S + should establish the absence of the signal as a C S - , thereby providing the necessary conditions for the development of schedule-induced polydipsia. The primary purpose of the present investigation was to test the prediction that providing the rat with a C S + for pellet delivery and a C S - indicating pellet unavailability should allow Pavlovian conditioning to occur and induce polydipsic intakes even on random schedules. Acquisition of polydipsia was studied on a Random-Time (response independent) schedule, since the operant response would be expected to occur at a constant probability throughout the interpellet interval on Random-Interval schedules [5J--thus it is possible that the animal might not drink on such a schedule due to competition with the operant. An additional purpose of this study was to investigate the development of polydipsia on signalled versus unsignalled Fixed-Time schedules and compare this to development on Random-Time schedules. METHOD
Subjects Twenty-four male albino rats obtained from Holtzman Co. were used. They were approximately 120 days of age at
~This research was supported by UAC Award from the Research Foundation of the State University of New York to the second author.
C o p y r i g h t © 1980 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/80/020411-04502.00/1
412
I~ASHLEY AND R()St'II.t iT, f
the start of the experiment, and were maintained at 80c~ of their free-feeding body weight. Their deprivation weights ranged from 388 to 440 g. All animals were individually housed, had continuous access to water, and were run during the light phase of a 12 hr light/12 hr dark cycle.
ment. However, individual rats varied widcl~ bo~h L~ number of days to reach criterion and in final asymptotic intake levels attained, such that patterns of acquisition of polydipsia failed to differentiate among the experimental groups.
Apparattl~
Six operant chambers were used. Each chamber was 30.5 cm long, 27.9 cm high, and 25.5 cm wide. The two side walls were clear Plexiglas, and the front and back walls were aluminum. The floor consisted of stainless steel rods 0.32 cm in diameter, spaced 1.30 cm apart. A food cup 5.0×3.8×2.5 cm was centered on the front wall and 1.0 cm above the floor. A 2 cm diameter hole was located 5.5 cm to the right of the food cup. A 100 ml drinking tube with a 2.5 mm orifice was positioned 2 mm behind this aperture. Each chamber was housed in a sound and light attenuating container equipped with a house light, white masking noise, and a ventilation fan. Pro('edure
On Day 1, each animal was placed in an operant chamber and was allowed to consume 30 45 mg Noyes pellets presented en m a s s e . Water intake was measured during this session to serve as baseline intake level. On Days 2-29, each animal received one daily session which consisted of 30 pellet deliveries. The rats were divided into two groups, one of which received the pellets on a Fixed-Time 120-sec schedule, and the other on a Random-Time 120-sec schedule. Each of these groups was further subdivided with one half (n--6) of the animals receiving signalled pellet deliveries and the other half unsignalled deliveries. The signal (CS+) was a 5-sec absence of the house light and white noise, which terminated with a pellet delivery: consequently. house light illumination and white noise presence served as a compound C S - . The dependent measure was the total amount of water consumed during each session. RESULTS Figure 1 shows the amount of water consumed by each animal in the baseline and all experimental sessions. The columns represent the four experimental conditions, and the rows individual animals. It can be seen that initial baseline intakes were comparable across all four groups (mean=3.33, 3.17, 4.50, and 3.33 for groups Fixed-Time Signalled, Fixed-Time Unsignalled, Random-Time Signalled, and Random-Time Unsignalled, respectively). Significantly, the imposition of a CS for pellet delivery on a Random-Time 120-sec schedule (Column 3) resulted in polydipsic water intakes in five out of six rats. With the exception of R13, all animals reached and maintained intakes at least 200% of their initial baseline levels. In contrast, four of six animals (R19, R20, R21, and R23) which received unsignalled pellet deliveries on an identical Random-Time schedule (Column 4) failed to demonstrate polydipsia. F o r these rats, asymptotic intakes did not exceed intakes during the baseline session. Furthermore, the Fixed-Time Signalled (Column 1) and the Fixed-Time Unsignalled (Column 2) conditions were equally successful in generating polydipsic levels of water intake. Except for R2 in the former condition, and R7 in the latter, every animal reached and maintained the 200% criterion for polydipsia. All animals which became polydipsic showed gradually increasing intakes over the successive days of the experi-
DISCUSSION The results of the present study demonstrate that a truly Random-Time schedule fails to induce polydipsia in a majority of animals, consistent with previous findings [51. More significantly, it was shown that if the rat is provided with a CS+ for pellet delivery, drinking does develop and is maintained on such Random-Time schedules, suggesting the ira. portance of Pavlovian conditioned states to scheduleinduced behaviors such as polydipsia. It is unlikely that either dry-mouth [7] or post-prandial [3] factors played a major role in the control of drinking by these animals, since all four treatment groups received the same number of single-pellet " m e a l s " with an equal average duration between pellet deliveries. For the same reasons, our results can also not be explained by the consummatory rate hypothesis [1 ]. This hypothesis holds that water intake in the polydipsia situation is inversely related to rate of food consumption, for intervals up to 2 min in length. Development of polydipsia in the present study was attenuated in the Random-Time Unsignalled condition, even though all animals experienced the same rate of pellet delivery. Finally, any appeal to food motivation, as indexed either by degree of deprivation, or by quantity or quality of the pellet delivered [6], must likewise fail, since all animals in this study experienced these conditions equally. The literature contains a suggestion that scheduleinduced behavior occurs at maximum frequency at times when local probability of reinforcement is at a minimum [ 1]. Fixed-Time schedules are characterized by such periods occurring immediately following each pellet delivery, and drinking reaches a peak at such times, both because stimuli facilitating drinking are strong right "after eating, and because competition with the operant response (or food anticipation, in the case of Fixed-Time schedules) is low. This idea gains support from the fact that the imposition of a CS+ for pellet delivery on a truly Random-Time schedule produced polydipsic drinking in most animals. Presumably, the absence of this signal, in the period after pellet delivery, then served as a C S - indicating a momentary probability of reinforcement of zero. More recently, it has been proposed [6] that interim responses, such as drinking, tend to occur either: (1) when relative rate of reinforcement is low, but reinforcement is possible at any time (as would be the case on long Random-Time schedules), or (2) when there is a zero probability of reinforcement. The intervals employed in the present study were probably of insufficient length (and the constant probability of reinforcement therefore too high) to generate drinking in the Random-Time Unsignalted condition. But the addition of a signal to such a schedule, again, seems to have effectively created local periods of zero reinforcement probability ( C S - periods) immediately following each pellet delivery, This emphasizes the power such a signal gives the animal to predict when a reinforcement will or will not be forthcoming. Since, in this study, reinforcement was perfectly correlated with the occurrence of the signal, predictability of pellet deliveries was significantly greater for
SIP AND PAVLOVIAN
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SESSIONS FIG. I. MI water intake during the baseline and the 28 experimental sessions. Experimental conditions are represented by the four columns and subjects by rows.
414
i~ASHLEY AND ROS~LI t iN!
animals in the Random-Time Signalled as opposed to the Random-Time Unsignalled condition. The fact that little new information is provided an animal by the addition of a signal to a Fixed-Time schedule, which by virtue of its temporal regularity provides a sufficient opportunity for temporal conditioning to occur, may explain why the development of polydipsia was not facilitated in the Fixed-Time Signalled rats. It has further been suggested 161 that because at least post-prandial drinking is maintained even on Random-Time schedules, it is possible that animals on such schedules do not immediately eat any pellet which is delivered too closely upon the previous one. Thus, an animal may actually create a schedule which contains C S - periods, and this obtained schedule may then maintain the behavior which led to it. It is conceivable that such patterning of behavior may have produced the polydipsic drinking which was observed in two rats in the Random-Time Unsignalled condition: however, this possibility can not be evaluated, since latencies to eat each pellet were not recorded. Although in terms of total session intakes the RandomTime Signalled animals did not differ from those in either of the Fixed-Time groups, it is possible that the pattern of interpellet drinking might differentiate the groups. The imposition of a CS+ on a Random-Time schedule transforms it from one in which the momentary probability of reinforcement is constant, to one in which this probability is zero in the absence of the signal and changes abruptly to one at stimulus presentation. On a Fixed-Time schedule, however, the probability of reinforcement gradually increases from
zero to one over the course of each intervai. ~i ~Jct~ k,ca~ probabilities are perceived by the rat, its expect-tnc~ that ~t pellet will be forthcoming at any instant in time may be vet,, dil-Terent on Random- as opposed to Fixed-Time schedufl:~ Some support for this notion has been provided by a stud~ utilizing an aversive rather than an appetiti'~e context 14i. Rats were trained to barpress for food on a Random-lnler~ al 60-see schedule of reinforcement. Electric shocks were then superimposed at either fixed or random 60-see intervals, and response suppression was measured as an index of the ongoing level of fear. In the Fixed-Time condition, response rates systematically decreased as time for the next shock approached, whereas in the Random-Time condition, response rates remained at a fairly constant level. These results suggest that aversive events which are delivered randomly in time may indeed be phenomenologically random to the rat. Therefore, in the schedule-induced polydipsia situation, it might be hypothesized that for animals exposed to a Random-Time schedule with signalled pellet deliveries that drinking would occur at a constant rate in the absence of the signal, and be terminated at CS+ onset. In the case of a Fixed-Time schedule, with or without a signal, drinking should peak immediately following pellet consumption, and decline gradually as time for the next pellet approaches. Definitive conclusions concerning the rat's changing expectancy of reinforcement in such situations require further research which will continuously monitor the drinking of individual animals throughout the acquisition and maintenance of schedule-induced polydipsia.
REFERENCES
1. Falk, J. L. Conditions producing psychogenic polydipsia in animals. Ann. N, Y. Acad. Sci. 157: 569--593, 1969. 2. Falk, J. L. The nature and determinants of adjunctive behavior, Physiol. Behav. 6: 577-588, 1971. 3. Kissileff, H. R. Food associated drinking in the rat, J. ~'omp, physiol. Psychol. 67: 284--300, 1969. 4. LaBarbera, J. D. and R. M. Church. Magnitude of fear as a function of expected time to an aversive event. Anita. LearH. Behav. 2: 199-202, 1974.
5. Millenson, J. R., R. B. Allen and S. Pinker. Adjunctive drinking during variable and random-interval food reinforcement schedules. Anita. Learn. Behar. 5: 285-290, 1977. 6. Staddon, J. E. R. Schedule-induced behavior~ In: Handbook ~I Operant Behavior, edited by W. K. Honig and J. E. R. Staddon. New York: Prentice Hall, 1977. 7. Teitelbaum, P. The use of operant methods in the assessment and control of motivational states. In: Operant Behavior: Arc'as ~fResearch and Application. edited by W. K. Honig. New York: Meredith, 1966.
Modulation of Schedule-Induced Polydipsia by Pavlovian Conditioned States' ROBIN L. LASHLEY
AND ROBERT A. ROSELLINI
Department o f Psychology, State University of New York at Albany, Albany, N Y 12222 R e c e i v e d 10 S e p t e m b e r 1979 LASHLEY, R. L. AND R. A. ROSELLINI. Modulation tdschedule-inducedpolydipsia by Pavlovian conditioned states. PHYSIOL. BEHAV. 24(2) 411--414, 1980.--It has previously been porlSosed that Random-lnterval schedules fail to induce polydipsic drinking in rats because such schedules fail to provide gradients of reinforcement probability. In the present study, food-deprived rats received single pellet meals delivered on either a Fixed-Time or a Random-Time 120-sec schedule, with half of each group receiving signalled, and half unsignalled, pellet deliveries. Acquisition of polydipsia was retarded in animals exposed to the Random-Time Unsignalled condition. However, the imposition of a CS+ for pellet delivery on the identical Random-Time schedule resulted in levels of polydipsia comparable to those obtained in both Fixed-Time conditions. These results suggest that the establishment of a Pavlovian conditioned inhibitory signal, as well as local periods of low reinforcement probability, may be sufficient for the development of schedule-induced behaviors such as polydipsia. Schedule-induced polydipsia
Pavlovian conditioning
FOOD-deprived animals typically ingest excessive amounts of water when exposed to an intermittent schedule of food delivery [2]. This phenomenon, termed schedule-induced polydipsia, has been observed under a wide variety of deprivation conditions, magnitudes of reinforcement, and schedules of reinforcement [6]. However, it has recently been reported that a majority of rats fail to develop polydipsia when food pellets are delivered on a truly RandomInterval schedule [5]. The unique defining characteristic of such a Random-Interval schedule is a low, but constant, probability of reinforcement throughout the interpellet interval--that is, a response at any point in time is as likely to earn a pellet as is a response at any other point in time. In contrast, the more usual arithmetic Variable-Interval schedule includes, in practice, some minimum time period following each pellet delivery during which responses will never be reinforced. Thus, as time passes since the last reinforcement, the probability that the next response will also be reinforced is constantly increasing. Variable-Ratio, FixedRatio, Fixed-Interval, and Fixed-Time schedules likewise provide such gradients of reinforcement probability. Therefore, it appears that an important determinant of scheduleinduced polydipsia must be the establishment of the temporal period immediately following reinforcement as a C S - , indicating a locally low probability of reinforcement [5]. Similarly, it has been suggested [6] that such local periods of low reinforcement probability tend to induce a classically conditioned state by virtue of the fact that the immediate postpellet period ( C S - ) is negatively correlated with the next food delivery (US). The actual behavioral manifestations of this state are held to be a function o f the species, the reinforcer used, and the environmental stimuli which are pres-
Random-time schedules
Rats
ent. It has thus been proposed [5] that the provision of a water spout in close proximity to the food cup, combined with the rat's strong tendency to drink after eating, leads to the ingestion of water in the polydipsia situation. Such a Pavlovian conditioning view of schedule-induced behavior suggests that the inability of Random-Interval schedules to generate drinking should be overcome if pellet delivery were signalled. The imposition of such a C S + should establish the absence of the signal as a C S - , thereby providing the necessary conditions for the development of schedule-induced polydipsia. The primary purpose of the present investigation was to test the prediction that providing the rat with a C S + for pellet delivery and a C S - indicating pellet unavailability should allow Pavlovian conditioning to occur and induce polydipsic intakes even on random schedules. Acquisition of polydipsia was studied on a Random-Time (response independent) schedule, since the operant response would be expected to occur at a constant probability throughout the interpellet interval on Random-Interval schedules [5J--thus it is possible that the animal might not drink on such a schedule due to competition with the operant. An additional purpose of this study was to investigate the development of polydipsia on signalled versus unsignalled Fixed-Time schedules and compare this to development on Random-Time schedules. METHOD
Subjects Twenty-four male albino rats obtained from Holtzman Co. were used. They were approximately 120 days of age at
~This research was supported by UAC Award from the Research Foundation of the State University of New York to the second author.
C o p y r i g h t © 1980 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/80/020411-04502.00/1
412
I~ASHLEY AND R()St'II.t iT, f
the start of the experiment, and were maintained at 80c~ of their free-feeding body weight. Their deprivation weights ranged from 388 to 440 g. All animals were individually housed, had continuous access to water, and were run during the light phase of a 12 hr light/12 hr dark cycle.
ment. However, individual rats varied widcl~ bo~h L~ number of days to reach criterion and in final asymptotic intake levels attained, such that patterns of acquisition of polydipsia failed to differentiate among the experimental groups.
Apparattl~
Six operant chambers were used. Each chamber was 30.5 cm long, 27.9 cm high, and 25.5 cm wide. The two side walls were clear Plexiglas, and the front and back walls were aluminum. The floor consisted of stainless steel rods 0.32 cm in diameter, spaced 1.30 cm apart. A food cup 5.0×3.8×2.5 cm was centered on the front wall and 1.0 cm above the floor. A 2 cm diameter hole was located 5.5 cm to the right of the food cup. A 100 ml drinking tube with a 2.5 mm orifice was positioned 2 mm behind this aperture. Each chamber was housed in a sound and light attenuating container equipped with a house light, white masking noise, and a ventilation fan. Pro('edure
On Day 1, each animal was placed in an operant chamber and was allowed to consume 30 45 mg Noyes pellets presented en m a s s e . Water intake was measured during this session to serve as baseline intake level. On Days 2-29, each animal received one daily session which consisted of 30 pellet deliveries. The rats were divided into two groups, one of which received the pellets on a Fixed-Time 120-sec schedule, and the other on a Random-Time 120-sec schedule. Each of these groups was further subdivided with one half (n--6) of the animals receiving signalled pellet deliveries and the other half unsignalled deliveries. The signal (CS+) was a 5-sec absence of the house light and white noise, which terminated with a pellet delivery: consequently. house light illumination and white noise presence served as a compound C S - . The dependent measure was the total amount of water consumed during each session. RESULTS Figure 1 shows the amount of water consumed by each animal in the baseline and all experimental sessions. The columns represent the four experimental conditions, and the rows individual animals. It can be seen that initial baseline intakes were comparable across all four groups (mean=3.33, 3.17, 4.50, and 3.33 for groups Fixed-Time Signalled, Fixed-Time Unsignalled, Random-Time Signalled, and Random-Time Unsignalled, respectively). Significantly, the imposition of a CS for pellet delivery on a Random-Time 120-sec schedule (Column 3) resulted in polydipsic water intakes in five out of six rats. With the exception of R13, all animals reached and maintained intakes at least 200% of their initial baseline levels. In contrast, four of six animals (R19, R20, R21, and R23) which received unsignalled pellet deliveries on an identical Random-Time schedule (Column 4) failed to demonstrate polydipsia. F o r these rats, asymptotic intakes did not exceed intakes during the baseline session. Furthermore, the Fixed-Time Signalled (Column 1) and the Fixed-Time Unsignalled (Column 2) conditions were equally successful in generating polydipsic levels of water intake. Except for R2 in the former condition, and R7 in the latter, every animal reached and maintained the 200% criterion for polydipsia. All animals which became polydipsic showed gradually increasing intakes over the successive days of the experi-
DISCUSSION The results of the present study demonstrate that a truly Random-Time schedule fails to induce polydipsia in a majority of animals, consistent with previous findings [51. More significantly, it was shown that if the rat is provided with a CS+ for pellet delivery, drinking does develop and is maintained on such Random-Time schedules, suggesting the ira. portance of Pavlovian conditioned states to scheduleinduced behaviors such as polydipsia. It is unlikely that either dry-mouth [7] or post-prandial [3] factors played a major role in the control of drinking by these animals, since all four treatment groups received the same number of single-pellet " m e a l s " with an equal average duration between pellet deliveries. For the same reasons, our results can also not be explained by the consummatory rate hypothesis [1 ]. This hypothesis holds that water intake in the polydipsia situation is inversely related to rate of food consumption, for intervals up to 2 min in length. Development of polydipsia in the present study was attenuated in the Random-Time Unsignalled condition, even though all animals experienced the same rate of pellet delivery. Finally, any appeal to food motivation, as indexed either by degree of deprivation, or by quantity or quality of the pellet delivered [6], must likewise fail, since all animals in this study experienced these conditions equally. The literature contains a suggestion that scheduleinduced behavior occurs at maximum frequency at times when local probability of reinforcement is at a minimum [ 1]. Fixed-Time schedules are characterized by such periods occurring immediately following each pellet delivery, and drinking reaches a peak at such times, both because stimuli facilitating drinking are strong right "after eating, and because competition with the operant response (or food anticipation, in the case of Fixed-Time schedules) is low. This idea gains support from the fact that the imposition of a CS+ for pellet delivery on a truly Random-Time schedule produced polydipsic drinking in most animals. Presumably, the absence of this signal, in the period after pellet delivery, then served as a C S - indicating a momentary probability of reinforcement of zero. More recently, it has been proposed [6] that interim responses, such as drinking, tend to occur either: (1) when relative rate of reinforcement is low, but reinforcement is possible at any time (as would be the case on long Random-Time schedules), or (2) when there is a zero probability of reinforcement. The intervals employed in the present study were probably of insufficient length (and the constant probability of reinforcement therefore too high) to generate drinking in the Random-Time Unsignalted condition. But the addition of a signal to such a schedule, again, seems to have effectively created local periods of zero reinforcement probability ( C S - periods) immediately following each pellet delivery, This emphasizes the power such a signal gives the animal to predict when a reinforcement will or will not be forthcoming. Since, in this study, reinforcement was perfectly correlated with the occurrence of the signal, predictability of pellet deliveries was significantly greater for
SIP AND PAVLOVIAN
CONDITIONING
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;0
I
25
SESSIONS FIG. I. MI water intake during the baseline and the 28 experimental sessions. Experimental conditions are represented by the four columns and subjects by rows.
414
i~ASHLEY AND ROS~LI t iN!
animals in the Random-Time Signalled as opposed to the Random-Time Unsignalled condition. The fact that little new information is provided an animal by the addition of a signal to a Fixed-Time schedule, which by virtue of its temporal regularity provides a sufficient opportunity for temporal conditioning to occur, may explain why the development of polydipsia was not facilitated in the Fixed-Time Signalled rats. It has further been suggested 161 that because at least post-prandial drinking is maintained even on Random-Time schedules, it is possible that animals on such schedules do not immediately eat any pellet which is delivered too closely upon the previous one. Thus, an animal may actually create a schedule which contains C S - periods, and this obtained schedule may then maintain the behavior which led to it. It is conceivable that such patterning of behavior may have produced the polydipsic drinking which was observed in two rats in the Random-Time Unsignalled condition: however, this possibility can not be evaluated, since latencies to eat each pellet were not recorded. Although in terms of total session intakes the RandomTime Signalled animals did not differ from those in either of the Fixed-Time groups, it is possible that the pattern of interpellet drinking might differentiate the groups. The imposition of a CS+ on a Random-Time schedule transforms it from one in which the momentary probability of reinforcement is constant, to one in which this probability is zero in the absence of the signal and changes abruptly to one at stimulus presentation. On a Fixed-Time schedule, however, the probability of reinforcement gradually increases from
zero to one over the course of each intervai. ~i ~Jct~ k,ca~ probabilities are perceived by the rat, its expect-tnc~ that ~t pellet will be forthcoming at any instant in time may be vet,, dil-Terent on Random- as opposed to Fixed-Time schedufl:~ Some support for this notion has been provided by a stud~ utilizing an aversive rather than an appetiti'~e context 14i. Rats were trained to barpress for food on a Random-lnler~ al 60-see schedule of reinforcement. Electric shocks were then superimposed at either fixed or random 60-see intervals, and response suppression was measured as an index of the ongoing level of fear. In the Fixed-Time condition, response rates systematically decreased as time for the next shock approached, whereas in the Random-Time condition, response rates remained at a fairly constant level. These results suggest that aversive events which are delivered randomly in time may indeed be phenomenologically random to the rat. Therefore, in the schedule-induced polydipsia situation, it might be hypothesized that for animals exposed to a Random-Time schedule with signalled pellet deliveries that drinking would occur at a constant rate in the absence of the signal, and be terminated at CS+ onset. In the case of a Fixed-Time schedule, with or without a signal, drinking should peak immediately following pellet consumption, and decline gradually as time for the next pellet approaches. Definitive conclusions concerning the rat's changing expectancy of reinforcement in such situations require further research which will continuously monitor the drinking of individual animals throughout the acquisition and maintenance of schedule-induced polydipsia.
REFERENCES
1. Falk, J. L. Conditions producing psychogenic polydipsia in animals. Ann. N, Y. Acad. Sci. 157: 569--593, 1969. 2. Falk, J. L. The nature and determinants of adjunctive behavior, Physiol. Behav. 6: 577-588, 1971. 3. Kissileff, H. R. Food associated drinking in the rat, J. ~'omp, physiol. Psychol. 67: 284--300, 1969. 4. LaBarbera, J. D. and R. M. Church. Magnitude of fear as a function of expected time to an aversive event. Anita. LearH. Behav. 2: 199-202, 1974.
5. Millenson, J. R., R. B. Allen and S. Pinker. Adjunctive drinking during variable and random-interval food reinforcement schedules. Anita. Learn. Behar. 5: 285-290, 1977. 6. Staddon, J. E. R. Schedule-induced behavior~ In: Handbook ~I Operant Behavior, edited by W. K. Honig and J. E. R. Staddon. New York: Prentice Hall, 1977. 7. Teitelbaum, P. The use of operant methods in the assessment and control of motivational states. In: Operant Behavior: Arc'as ~fResearch and Application. edited by W. K. Honig. New York: Meredith, 1966.