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The effect of limited water availability on schedule-induced polydipsia
Physiology and Behavior Vol. 8, pp. 147-149. Brain Research Publications, Inc., 1972. Printed in U.S.A.
BRIEF COMMUNICATION The Effect of Limited Water Availability Schedule-Induced Polydipsia '
on
RANDALL K. FLORY AND MICHAEL K. O'BOYLE
Hollins College, Hollins College, Virginia and Florida State University Tallahassee, Florida, U.S.A.
(Received 9 August 1971) FLORY, R. K. AND M. K. O'BOYLE. The effect of limited water availability on schedule-induced polydipsia. PHYSIOL. BEHAV. 8 (1) 147-149, 1972.-Rats bar-pressing on a l-min fixed-interval schedule for 100, 45-rag food pellets per session became polydipsic when water was always concurrently available on a fixed-ratio 1 basis via a second response lever. When 15-see l~tiods of water unavailabifity were successively inserted in the first, second, third, and fourth quarters of the 1-min fixedqnterval food schedule, all rats remained polydipsic although water intakes were slightly lower than those in the control conditions of continuous water availability. Polydipsia
Tlmeout
Waterintake
Adjunctive behavior
Fixed interval
Rats
possible that post-reinforcement drinking is facilitated by the recent ingestion of food as well as by the relatively low probability of reinforcement at this time. If drinking occurs shortly after pellet delivery because probability of food reinforcement at this time is low, the postponement of water availability until later in the interreinforcement interval, when the probability of reinforcement is relatively higher, could be expected to decrease or eliminate polydipsia. Similarly, if drinking is facilitated most strongly immediately after reinforcement by stimulus factors associated with pellet ingestion, the postponement of water availability until later in the interreinforcement interval could be expected to attenuate or eliminate polydipsic drinking. To assess the effect on schedule-induced polydipsia of restricting water availability within portions of the interreinforaement interval, the present study successively inserted within the first, second, third, and fourth quarters of a 1-min fixed-interval food schedule 1 5-sec periods during which water was unavailable.
THE phenomenon of schedule-induced polydipsia was first reported by Falk [1]. He found that food-deprived rats, bar-pressing for 45-mg food pellets on a variable-interval 1-min schedule, consumed in a 3.2 hr session three to four times their normal, 24-hr water intakes. Schedule-induced polydipsia, which has been observed in the pigeon [9] and rhesus monkey [10] as well as in the rat, is not solely attributable to simple superstitious reinforcement [ 11 ] for it develops and persists even when substantial delays are imposed between drinking and pellet deliveries [2, 3, 5, 12]. Polydipsia is characterized by a period of drinking which occurs shortly after the ingestion of a food pellet [ t ] . This typical drinking pattern is observed with fixed or variable schedules whether or not pellet deliveries are contingent upon the emission of a specific operant response (see general reviews [5, 6] ). It is possible that polydipsic drinking, especially on fixed-interval food schedules, occurs shortly after food delivery because at this time the probability of reinforcement is relatively low [6 ]. This account is supported by the observation that post-reinforcement drinking did not occur following the completion of a 30-sec differential reinforcement of low rate schedule that alternated with a fixed-ratio 5 schedule but did occur following completion of the fixed-ratio component [5]. It is also possible that postreinforcement drinking is initiated or released in the manner of a fixed-action pattern [13] by the stimulus factors associated with food ingestion [6]. Finally, it is
METHOD
Animals Three, female, albino rats (Sprague-Dawley descendents) designated D1, D2, and D4 were used. Each experimentally-naive rat was approximately 120 days of age at the beginning of the study and weighed 256 g, 275 g, and
1These data are based on a portion of a thesis submitted by the second author to the Department of Psychology, Hollins College, in partial fulfillment of the requirements for the M.A. degree. Reprints may be obtained from Randall K. Flory, Department of Psychology, Hollins College, Hollins College, Virginia 24020. 147
148 245 g, respectively. They were housed individually in a temperature-controlled and constantly-illuminated room.
Apparatus The experimental space consisted of a Lehigh Valley Model 1417 small animal test cage equipped with two retractable levers, a 0.1-ml liquid dipper, a 45-mg pellet dispenser, and stimulus lights. Both levers were mounted on one wall 17.0 cm apart, thus preventing simultaneous responding on both. Each lever required a force in excess of 20 g for operation. The food-pellet chute was located to the right o f the left-hand lever (food), and the water dipper opening was located to the left of the second lever (water). Programming and recording were done by standard electromagnetic equipment. A Gerbrands cumulative recorder provided a moment-to-moment account of daily sessions.
Procedure Each animal was food-deprived and maintained at 80% of its free-feeding body weight during the entire experiment. With the water lever retracted, each rat was trained to bar-press for 45-rag Noyes food pellets on a fixed-ratio 1 schedule. During food-schedule training, water was freely available in the h o m e cages. Throughout the study, daily sessions consisted of the delivery of 100 45-rag Noyes f o o d pellets. When behavior on the fixed-ratio 1 food schedule was stable, the animals' home cage water tubes were removed, the food lever was retracted, and training on the water lever was initiated. Each bar press on the water lever operated a stimulus light above the bar and made available for 4 sec a 0.1-ml presentation of water. When behavior on the fixed-ratio 1 water schedule was stable, the home cage water tubes were replaced and were always accessible for the remainder of the experiment. With both response levers present, each rat was exposed to ascending fixed-interval food schedules until a 1-min schedule value was attained. In a fixed-interval schedule, the first lever press to be emitted after a specified period of time has elapsed is followed by the delivery of a reinforcer. Throughout the study, the water lever, when available, presented water on a fixed-ratio 1 schedule. The delivery of 100 food pellets on a l-rain fixed-interval basis made daily sessions approximately 100 rain in duration. Water intakes were obtained by weighing the water-dipper reservoir before and after each session. A blank session, during which the reservoir was filled and placed in the chamber for 2 hr, showed no measurable evaporation. When a 5-day 0eriod of stable session water intake was obtained, a period of water unavailability was inserted in the first quarter of the l-rain fixed-interval food schedule. Water intake was defined as stable when individual session intakes were within -+ 10% of the mean of those consecutive session intakes. During the first-quarter timeout period, the food schedule remained in effect and the water lever was retracted for a 15-sec period beginning with the delivery ot each food pellet. After a stable, 5-day, baseline water intake was obtained for the first-quarter timeout condition, the 15-sec period of water unavailability was removed and original conditions reinstated until session water intake stabilized over a period of at least three consecutive days. A 15-sec period of water unavailability was then inserted in the second quarter of the l-rain fixed-interval food schedule and maintained in effect until a stable, 5-day, water intake baseline was obtained. Original, no timeout condi-
FLORY AND O'BOYLt5 tions were then reinstated and a stable, 3-day, water intake baseline obtained. Similarly, 5-day session water intakes were obtained under conditions in which timeouts were successively inserted in the third and fourth quarters of the fixed-interval food schedule. Before changing the timeout placement, and after exposure to the fourth quarter timeout condition, 3-day intake baselines were obtained for each rat in the no-timeout condition. In order to determine the degree of overdrinking produced by the l-rain fixed-interval food schedule, a measure of drinking in a nonexperimental condition was obtained. Each rat was given 100, 45-rag food pellets in its home cage and water intakes measured 2 hr later. Following the determination of daily, 2-hr, home-cage intakes, each rat was weighed and fed an amount of Wayne Lab Blox sufficient to maintain its 80% free-feeding body weight. A 5-day, stable, home-cage control intake was obtained for each animal. 40. RAT DI
I
'0 30.
'°
o 40
RAT D2
0 Inl'iNll
20
I0.
50.
RAT D4
I
40 30 |
20 I
I !
o n o
g
I
/
'2 /,,.
FIG. 1. Mean water intakes (ml) and their standard errors for the 2-hr, home-cage control condition, for conditions in which 15-sec water timeout periods (TO) were inserted in each of the four quarters of a 1-rain fixed-interval food schedule, and for conditions in which water was continuously available during all four quarters of the food schedule. Each vertical bar represents the mean of five consecutive daily intakes.
POLYDIPSIA AND LIMITED WATER AVAILABILITY
149
RESULTS Figure 1 shows that mean water intakes in all session timeout and no timeout conditions for all rats were at least three times greater than mean intakes during the 2-hr, home-cage control condition. Polydipsia factors, obtained by dividing the mean session water intakes by the mean home-cage control intakes ranged from 3.2 - 5.3 for all rats. Although all three animals remained polydipsic throughout all phases of the experiment, drinking was somewhat attenuated during the water timeout conditions. A comparison of mean water intakes in all timeout conditions with those in all no timeout conditions revealed that only Rat D4 drank significantly more water during the no timeout conditions (t = 3.90, p < 0.05). For all three animals, water timeouts during the first quarter of the 1-min fixed-interval food schedule attenuated session water intakes more than did timeouts in any of the other three quarters of the food schedule. Observation of daily cumulative records revealed that in the no timeout condition, Rats D1 and D2 consistently drank after, rather than before, food reinforcements. Furthermore, both Rats D 1 and D2 showed scalloped food r e s p o n s e patterns consistent with a fixed-interval schedule [7]. Rat D4, however, responded on the water lever continuously during the no timeout sessions and emitted relatively few food responses. For all three animals, the introduction of the 1 5-sec periods of water unavailability generally resulted in the shifting of drinking to the time periods when the water lever was accessible. Whereas the insertion of water timeout periods resulted in relatively little disruption of food response patterns for Rats D 1 and D4, cumulative records of Rat D2 showed a more flattened pattern of food responding than during the conditions of continuous water availability. Throughout all experimental conditions, each rat obtained food pellets when or shortly after they were made available by the l-min fixed-interval schedule. Thus, the insertion of periods of water unavailability did not affect mean interreinforcement times in any systematic manner.
DISCUSSION The results support Falk's [41 finding that scheduleinduced polydipsia develops under an intermittent food
schedule when water is concurrently available in discrete portions under a fixed-ratio contingency. In the present study, all animals remained polydipsic when periods of water unavailability were inserted within various portions of a fixed-interval food schedule. That polydipsia was, for all three rats, attenuated but not eliminated when water was unavailable immediately following food consumption suggests that polydipsic drinking is partially influenced by ( a ) t h e relatively low probability of reinforcement at this time and/or (b) stimulus factors associated with recent pellet ingestion. This suggestion is also supported by the observation that periods of water unavailability later in the fixed-interval food schedule generally attenuated drinking less than those inserted relatively early in the food schedule. That the recency effect of pellet consumption is, however, not a necessary c o n d i t i o n for the occurrence of schedule-induced polydipsia is supported by the observation of drinking following non-food stimuli in a second-order schedule of food reinforcement [8]. If drinking had served to time food pellet availability, the interruption of drinking during latter portions of the food schedule could be expected to disrupt subjects' food-collecting performances. As indicated by the close correspondence of schedule value and interreinforcement times, such disruption was not observed for any of the rats in the present study. The results of the present study, while supporting Falk's [6] view that polydipsic drinking is not an unconditioned reflex elicited by schedule conditions, are perhaps best explained in motivational terms. If polydipsia is under the mutual control of (1) the thwarting properties of a schedule of food reinforcement, ( 2 ) t h e natural stimuli facilitating drinking after eating, and (3) the deprivation level of the animal [5], then the first-quarter period of water unavailability could be expected to curtail drinking more than any others since the natural facilitating stimuli for drinking would be stronger immediately after food ingestion than 15 sec later. Drinking, however, would not be severely attenuated or completely eliminated since the intermittent food schedule's "thwarting" properties and the animal's fooddeprivation level would still be operative in maintaining polydipsia. As such, the mutual controlling factors of schedule-induced polydipsia appear to serve as discriminative stimuli for an increase in behavior already present within the animal's repertoire.
REFERENCES
1. Falk, J.L. Production of polydipfia in normal rats by an intermittent food schedule. Science 133: 195-196, 1961. 2. Falk, J. L. The behavioral regulation of water-electrolyte. In: Nebraska Symposium on Motivation edited by M. R. Jones, Lincoln, Nebraska: University Press, 1961. 3. Falk, J. L. Studies on schedule-induced polydipsia. In: Thirst: first international symposium on thirst in the regulation o f body water, edited by M.J. Wayner. New York: Pergamon Press, 1964. 4. Falk, J.L. The motivational properties of schedule-induced polydipsia. J. exp. Analysis Behav. 9: 19-25, 1966. 5. Falk, J.L. Conditions producing psychogenic polydipsia in animals. Ann. N. Y. Acad. Sci., 157: 569-593, 1969. 6. Falk, J. L. A theoretical review: The nature and determinants of adjunctive behavior. Physiol. Behav. 6: 577-588, 1971.
7. Ferster, C. B. and Skinner, B. F. Schedules of Reinforcement. New York: Appleton-Century-Crofts, 1957. 8. Rosenblith, J.Z. Polydipsia induced in the rat by a secondorder schedule. J. exp. Analysis Behav., 14 139-144, 1970. 9. Shanab, M.E. and Peterson, J. L. Polydipsia in the pigeon. Psychonom. Sci. 15: 51-52, 1969. 10. Shuster, C. R. and Woods, J. H. Sd~lule-induced polydipsia in the rhesus monkey. Psychol. Rep. ,.~: 823-828, 1966. 11. Skinner, B. F. "Superstition" in the pigeon. J. exp. Psychol. 38: 168-172, 1948. 12. Stein, L. Excessive drinking in the rat: Superstition or thirst? J. comp. physiol. Psychol., 58: 237-242, 1964. 13. Tinbergen, N. The Study oflnstinct. Oxford: Oxford University Press, 1951.
BRIEF COMMUNICATION The Effect of Limited Water Availability Schedule-Induced Polydipsia '
on
RANDALL K. FLORY AND MICHAEL K. O'BOYLE
Hollins College, Hollins College, Virginia and Florida State University Tallahassee, Florida, U.S.A.
(Received 9 August 1971) FLORY, R. K. AND M. K. O'BOYLE. The effect of limited water availability on schedule-induced polydipsia. PHYSIOL. BEHAV. 8 (1) 147-149, 1972.-Rats bar-pressing on a l-min fixed-interval schedule for 100, 45-rag food pellets per session became polydipsic when water was always concurrently available on a fixed-ratio 1 basis via a second response lever. When 15-see l~tiods of water unavailabifity were successively inserted in the first, second, third, and fourth quarters of the 1-min fixedqnterval food schedule, all rats remained polydipsic although water intakes were slightly lower than those in the control conditions of continuous water availability. Polydipsia
Tlmeout
Waterintake
Adjunctive behavior
Fixed interval
Rats
possible that post-reinforcement drinking is facilitated by the recent ingestion of food as well as by the relatively low probability of reinforcement at this time. If drinking occurs shortly after pellet delivery because probability of food reinforcement at this time is low, the postponement of water availability until later in the interreinforcement interval, when the probability of reinforcement is relatively higher, could be expected to decrease or eliminate polydipsia. Similarly, if drinking is facilitated most strongly immediately after reinforcement by stimulus factors associated with pellet ingestion, the postponement of water availability until later in the interreinforcement interval could be expected to attenuate or eliminate polydipsic drinking. To assess the effect on schedule-induced polydipsia of restricting water availability within portions of the interreinforaement interval, the present study successively inserted within the first, second, third, and fourth quarters of a 1-min fixed-interval food schedule 1 5-sec periods during which water was unavailable.
THE phenomenon of schedule-induced polydipsia was first reported by Falk [1]. He found that food-deprived rats, bar-pressing for 45-mg food pellets on a variable-interval 1-min schedule, consumed in a 3.2 hr session three to four times their normal, 24-hr water intakes. Schedule-induced polydipsia, which has been observed in the pigeon [9] and rhesus monkey [10] as well as in the rat, is not solely attributable to simple superstitious reinforcement [ 11 ] for it develops and persists even when substantial delays are imposed between drinking and pellet deliveries [2, 3, 5, 12]. Polydipsia is characterized by a period of drinking which occurs shortly after the ingestion of a food pellet [ t ] . This typical drinking pattern is observed with fixed or variable schedules whether or not pellet deliveries are contingent upon the emission of a specific operant response (see general reviews [5, 6] ). It is possible that polydipsic drinking, especially on fixed-interval food schedules, occurs shortly after food delivery because at this time the probability of reinforcement is relatively low [6 ]. This account is supported by the observation that post-reinforcement drinking did not occur following the completion of a 30-sec differential reinforcement of low rate schedule that alternated with a fixed-ratio 5 schedule but did occur following completion of the fixed-ratio component [5]. It is also possible that postreinforcement drinking is initiated or released in the manner of a fixed-action pattern [13] by the stimulus factors associated with food ingestion [6]. Finally, it is
METHOD
Animals Three, female, albino rats (Sprague-Dawley descendents) designated D1, D2, and D4 were used. Each experimentally-naive rat was approximately 120 days of age at the beginning of the study and weighed 256 g, 275 g, and
1These data are based on a portion of a thesis submitted by the second author to the Department of Psychology, Hollins College, in partial fulfillment of the requirements for the M.A. degree. Reprints may be obtained from Randall K. Flory, Department of Psychology, Hollins College, Hollins College, Virginia 24020. 147
148 245 g, respectively. They were housed individually in a temperature-controlled and constantly-illuminated room.
Apparatus The experimental space consisted of a Lehigh Valley Model 1417 small animal test cage equipped with two retractable levers, a 0.1-ml liquid dipper, a 45-mg pellet dispenser, and stimulus lights. Both levers were mounted on one wall 17.0 cm apart, thus preventing simultaneous responding on both. Each lever required a force in excess of 20 g for operation. The food-pellet chute was located to the right o f the left-hand lever (food), and the water dipper opening was located to the left of the second lever (water). Programming and recording were done by standard electromagnetic equipment. A Gerbrands cumulative recorder provided a moment-to-moment account of daily sessions.
Procedure Each animal was food-deprived and maintained at 80% of its free-feeding body weight during the entire experiment. With the water lever retracted, each rat was trained to bar-press for 45-rag Noyes food pellets on a fixed-ratio 1 schedule. During food-schedule training, water was freely available in the h o m e cages. Throughout the study, daily sessions consisted of the delivery of 100 45-rag Noyes f o o d pellets. When behavior on the fixed-ratio 1 food schedule was stable, the animals' home cage water tubes were removed, the food lever was retracted, and training on the water lever was initiated. Each bar press on the water lever operated a stimulus light above the bar and made available for 4 sec a 0.1-ml presentation of water. When behavior on the fixed-ratio 1 water schedule was stable, the home cage water tubes were replaced and were always accessible for the remainder of the experiment. With both response levers present, each rat was exposed to ascending fixed-interval food schedules until a 1-min schedule value was attained. In a fixed-interval schedule, the first lever press to be emitted after a specified period of time has elapsed is followed by the delivery of a reinforcer. Throughout the study, the water lever, when available, presented water on a fixed-ratio 1 schedule. The delivery of 100 food pellets on a l-rain fixed-interval basis made daily sessions approximately 100 rain in duration. Water intakes were obtained by weighing the water-dipper reservoir before and after each session. A blank session, during which the reservoir was filled and placed in the chamber for 2 hr, showed no measurable evaporation. When a 5-day 0eriod of stable session water intake was obtained, a period of water unavailability was inserted in the first quarter of the l-rain fixed-interval food schedule. Water intake was defined as stable when individual session intakes were within -+ 10% of the mean of those consecutive session intakes. During the first-quarter timeout period, the food schedule remained in effect and the water lever was retracted for a 15-sec period beginning with the delivery ot each food pellet. After a stable, 5-day, baseline water intake was obtained for the first-quarter timeout condition, the 15-sec period of water unavailability was removed and original conditions reinstated until session water intake stabilized over a period of at least three consecutive days. A 15-sec period of water unavailability was then inserted in the second quarter of the l-rain fixed-interval food schedule and maintained in effect until a stable, 5-day, water intake baseline was obtained. Original, no timeout condi-
FLORY AND O'BOYLt5 tions were then reinstated and a stable, 3-day, water intake baseline obtained. Similarly, 5-day session water intakes were obtained under conditions in which timeouts were successively inserted in the third and fourth quarters of the fixed-interval food schedule. Before changing the timeout placement, and after exposure to the fourth quarter timeout condition, 3-day intake baselines were obtained for each rat in the no-timeout condition. In order to determine the degree of overdrinking produced by the l-rain fixed-interval food schedule, a measure of drinking in a nonexperimental condition was obtained. Each rat was given 100, 45-rag food pellets in its home cage and water intakes measured 2 hr later. Following the determination of daily, 2-hr, home-cage intakes, each rat was weighed and fed an amount of Wayne Lab Blox sufficient to maintain its 80% free-feeding body weight. A 5-day, stable, home-cage control intake was obtained for each animal. 40. RAT DI
I
'0 30.
'°
o 40
RAT D2
0 Inl'iNll
20
I0.
50.
RAT D4
I
40 30 |
20 I
I !
o n o
g
I
/
'2 /,,.
FIG. 1. Mean water intakes (ml) and their standard errors for the 2-hr, home-cage control condition, for conditions in which 15-sec water timeout periods (TO) were inserted in each of the four quarters of a 1-rain fixed-interval food schedule, and for conditions in which water was continuously available during all four quarters of the food schedule. Each vertical bar represents the mean of five consecutive daily intakes.
POLYDIPSIA AND LIMITED WATER AVAILABILITY
149
RESULTS Figure 1 shows that mean water intakes in all session timeout and no timeout conditions for all rats were at least three times greater than mean intakes during the 2-hr, home-cage control condition. Polydipsia factors, obtained by dividing the mean session water intakes by the mean home-cage control intakes ranged from 3.2 - 5.3 for all rats. Although all three animals remained polydipsic throughout all phases of the experiment, drinking was somewhat attenuated during the water timeout conditions. A comparison of mean water intakes in all timeout conditions with those in all no timeout conditions revealed that only Rat D4 drank significantly more water during the no timeout conditions (t = 3.90, p < 0.05). For all three animals, water timeouts during the first quarter of the 1-min fixed-interval food schedule attenuated session water intakes more than did timeouts in any of the other three quarters of the food schedule. Observation of daily cumulative records revealed that in the no timeout condition, Rats D1 and D2 consistently drank after, rather than before, food reinforcements. Furthermore, both Rats D 1 and D2 showed scalloped food r e s p o n s e patterns consistent with a fixed-interval schedule [7]. Rat D4, however, responded on the water lever continuously during the no timeout sessions and emitted relatively few food responses. For all three animals, the introduction of the 1 5-sec periods of water unavailability generally resulted in the shifting of drinking to the time periods when the water lever was accessible. Whereas the insertion of water timeout periods resulted in relatively little disruption of food response patterns for Rats D 1 and D4, cumulative records of Rat D2 showed a more flattened pattern of food responding than during the conditions of continuous water availability. Throughout all experimental conditions, each rat obtained food pellets when or shortly after they were made available by the l-min fixed-interval schedule. Thus, the insertion of periods of water unavailability did not affect mean interreinforcement times in any systematic manner.
DISCUSSION The results support Falk's [41 finding that scheduleinduced polydipsia develops under an intermittent food
schedule when water is concurrently available in discrete portions under a fixed-ratio contingency. In the present study, all animals remained polydipsic when periods of water unavailability were inserted within various portions of a fixed-interval food schedule. That polydipsia was, for all three rats, attenuated but not eliminated when water was unavailable immediately following food consumption suggests that polydipsic drinking is partially influenced by ( a ) t h e relatively low probability of reinforcement at this time and/or (b) stimulus factors associated with recent pellet ingestion. This suggestion is also supported by the observation that periods of water unavailability later in the fixed-interval food schedule generally attenuated drinking less than those inserted relatively early in the food schedule. That the recency effect of pellet consumption is, however, not a necessary c o n d i t i o n for the occurrence of schedule-induced polydipsia is supported by the observation of drinking following non-food stimuli in a second-order schedule of food reinforcement [8]. If drinking had served to time food pellet availability, the interruption of drinking during latter portions of the food schedule could be expected to disrupt subjects' food-collecting performances. As indicated by the close correspondence of schedule value and interreinforcement times, such disruption was not observed for any of the rats in the present study. The results of the present study, while supporting Falk's [6] view that polydipsic drinking is not an unconditioned reflex elicited by schedule conditions, are perhaps best explained in motivational terms. If polydipsia is under the mutual control of (1) the thwarting properties of a schedule of food reinforcement, ( 2 ) t h e natural stimuli facilitating drinking after eating, and (3) the deprivation level of the animal [5], then the first-quarter period of water unavailability could be expected to curtail drinking more than any others since the natural facilitating stimuli for drinking would be stronger immediately after food ingestion than 15 sec later. Drinking, however, would not be severely attenuated or completely eliminated since the intermittent food schedule's "thwarting" properties and the animal's fooddeprivation level would still be operative in maintaining polydipsia. As such, the mutual controlling factors of schedule-induced polydipsia appear to serve as discriminative stimuli for an increase in behavior already present within the animal's repertoire.
REFERENCES
1. Falk, J.L. Production of polydipfia in normal rats by an intermittent food schedule. Science 133: 195-196, 1961. 2. Falk, J. L. The behavioral regulation of water-electrolyte. In: Nebraska Symposium on Motivation edited by M. R. Jones, Lincoln, Nebraska: University Press, 1961. 3. Falk, J. L. Studies on schedule-induced polydipsia. In: Thirst: first international symposium on thirst in the regulation o f body water, edited by M.J. Wayner. New York: Pergamon Press, 1964. 4. Falk, J.L. The motivational properties of schedule-induced polydipsia. J. exp. Analysis Behav. 9: 19-25, 1966. 5. Falk, J.L. Conditions producing psychogenic polydipsia in animals. Ann. N. Y. Acad. Sci., 157: 569-593, 1969. 6. Falk, J. L. A theoretical review: The nature and determinants of adjunctive behavior. Physiol. Behav. 6: 577-588, 1971.
7. Ferster, C. B. and Skinner, B. F. Schedules of Reinforcement. New York: Appleton-Century-Crofts, 1957. 8. Rosenblith, J.Z. Polydipsia induced in the rat by a secondorder schedule. J. exp. Analysis Behav., 14 139-144, 1970. 9. Shanab, M.E. and Peterson, J. L. Polydipsia in the pigeon. Psychonom. Sci. 15: 51-52, 1969. 10. Shuster, C. R. and Woods, J. H. Sd~lule-induced polydipsia in the rhesus monkey. Psychol. Rep. ,.~: 823-828, 1966. 11. Skinner, B. F. "Superstition" in the pigeon. J. exp. Psychol. 38: 168-172, 1948. 12. Stein, L. Excessive drinking in the rat: Superstition or thirst? J. comp. physiol. Psychol., 58: 237-242, 1964. 13. Tinbergen, N. The Study oflnstinct. Oxford: Oxford University Press, 1951.