Effects of desalivation on schedule-induced polydipsia in the rat.

The development and magnitude of schedule-induced polydipisa (SIP) were examined in desalivate and control rats in two experiments. In Experiment 1 animals were placed on a noncontingent fixed time (FT) 60-sec schedule for 24 days. Results indicate speeded development but asymptotically lower levels of SIP in desalivate rats compared to controls. Home cage consumption, however, was elevated in desalivate rats. In Experiment 2 schedule length, pellet size, and the percentage of body weight were manipulated. Desalivate rats consumed less fluid during each schedule condition relative to controls, but the pattern of results across schedules was similar to controls. It was concluded that (a) desalivation attenuates SIP; (b) SIP is a learned phenomenon; (c) dry mouth theories of SIP appear inadequate; and (d) the developmental aspect of the SIP phenomenon deserves additional research attention.     

The effects of signalled and unsignalled lick-dependent delays on the development of schedule-induced drinking in rats

Food pellets were programmed to be delivered to rats every 60 sec (Fixed Time 60-sec schedule), and the development of schedule-induced drinking was measured in terms of the amount of water consumed and the number of licks per inter-pellet interval. For some rats (masters) 10-sec delays in food delivery were dependent on licks. Yoked-control rats received food at the same time as their masters and independently of their own behaviour. In Experiment 1, in which the delays were signalled by a blackout, the master rats began to drink, but this schedule-induced behaviour then decreased to levels lower than those shown by the yoked controls. When the signalled delays were discontinued, the drinking of the master rats recovered. In Experiment 2, in which the delays were not signalled, the master rats did not develop as much schedule-induced drinking as the yoked controls, and discontinuing the delays led to only small increases in drinking. These results support the view that schedule-induced drinking is subject to control by its consequences.     

Effects of response-dependent and independent electric shock on schedule-induced polydipsia

In Experiment I, lever pressing by rats was maintained by the delivery of food pellets under a 45-sec fixed-interval schedule. Fixed-time 180-sec and fixed-interval 180-sec schedules of shock delivery were systematically superimposed on the baseline food schedule to study effects on schedule-induced water intake. Response-dependent shock had little, if any, effect on water intake, whereas shocks independent of lever pressing attenuated fluid intake. In Experiment 2, rats received food pellets under a fixed-time 60-sec schedule. Electric shock delivered concurrently under a variable-time 180-sec schedule, but never while the animal was licking or within 5 sec after licking terminated, led to similar attenuation of water intake. These findings suggest that schedule-induced polydipsia is sensitive to differences in the functional properties of response-independent and dependent electric shock.     

Motivational properties of spaced food presentation in hungry rats: Long-lasting effects on drinking and eating

Hungry rats were given 60 food pellets either at spaced intervals or massed in one or two presentations, or no food, followed by a consumption test in the home cage. Drinking was higher after spaced food sessions than after massed food or no food sessions (Experiments 1, 2, 3, and 5) and higher after the presentation of standard food pellets than after quinine-flavored pellets (Experiment 2). An effect of spacing was found even with a delay of 1 hr between the end of the food session and the drinking test (Experiment 3). Spaced food presentation also produced an increase in the consumption of quinine-flavored food pellets after the end of the session (Experiment 4). These results suggest that the enhancement of behavior caused by intermittent reinforcement cannot be attributed entirely to the short-term eliciting effects of the reinforcer or to its thirst-inducing properties.     

SCHEDULE-INDUCED DRINKING: ELICITATION,ANTICIPATION, OR BEHAVIORAL INTERACTION?

We carried out five experiments with rats on fixed-time schedules in order to define the relation between drinking and individual food-pellet presentations. In Experiment 1, unsignaled extra food occurred at the end of occasional fixed intervals, and we compared subsequent drinking patterns with drinking before the extra food presentation. In Experiment 2 we presented signaled and unsignaled extra food and measured elicited and anticipatory drinking patterns. In Experiment 3, we observed the persistence of modified drinking patterns when several consecutive intervals ended with extra pellets. In Experiments 4 and 5, we varied the magnitude of food delivery across (rather than within) sessions to replicate published findings. Results show that schedule-induced drinking is neither elicited by food presentations nor induced by stimuli associated with a high food rate. All subjects seemed to follow a simple rule: during any stimulus signaling an increase in the local probability of food delivery within a session, engage in food-related behavior to the exclusion of drinking. Schedule-induced drinking appears to be the result of dynamic interactions among food-related behavior, drinking, and other motivated behavior, rather than a direct effect of the contingencies of food reinforcement.     

Schedule-induced kinesic and taxic behavioral stereotypy in the pigeon

Two experiments explored the functional character of 2 schedule-induced interim behaviors (pacing and retreat) and 1 terminal behavior (keypecking) that developed on fixed-time (FT) schedules of food delivery with a keylight that increased in brightness throughout the interstimulus interval. In Experiment 1, this ramp procedure was compared with a less discriminable ramp, and FT and Random Time (RT) procedures without a signal. Decreased discriminability expanded keypecking in the trial. The FT schedule eliminated only keypecking and the RT procedure eliminated keypecking and retreat while pacing remained. In Experiment 2, predictive and unpredictive ramps were added to the RT procedure. The data suggest that schedule-induced stereotypy can be divided into kinesic stereotypy (pacing), which arises from repeated reinforcer presentations, and taxic stereotypy, which is tied to an increase (keypecking) or decrease (retreat) in the momentary probability of reinforcement.     

Schedule-induced drinking as a function of interreinforcement interval in the rhesus monkey

Lever presses by two rhesus monkeys produced food pellets that were assigned by both an ascending and descending series of fixed-interval schedules whose values varied between 1 and 512 sec. The amount of schedule-induced drinking was bitonically related to interreinforcement interval, reaching a maximum at approximately 120 sec and declining at longer fixed intervals. The relation between water intake and interreinforcement interval was complexly related to two drinking measures: (1) the probability of drinking following a pellet and (2) the amount drunk per bout. Drinking rate was also bitonically related to interreinforcement interval.     

Control of schedule-induced polydipsi: type, size, and spacing of meals

Rats were given daily 1-min variable-interval sessions for several types of food delivered in various amounts per reinforcement and the concurrent, schedule-induced polydipsia was measured. Dry, solid food was neither a necessary nor sufficient condition for the development of polydipsia. Small portions of liquid Standard Monkey Diet produced polydipsia, but 45-mg dextrose or sucrose pellets did not. Within the range studied, smaller portions of both solid and liquid foods produced more drinking than larger portions per reinforcement. Two-min variable-interval sessions produced a greater polydipsic response than 1-min variable-interval, even though the number of 45-mg Noyes pellets allowed per session was held constant. Polydipsia was greatly attenuated on these schedules when the number of pellets remained constant, but were delivered two at a time. Within the ranges studied, the concurrent polydipsic response was increased by decreasing the rate of food acquisition, either by using smaller portions of food per reinforcement or by increasing the interreinforcement time.     

Operant hoarding: a new paradigm for the study of self-control.

In the first of four experiments, rats were exposed to a modified multiple continuous reinforcement-extinction schedule during 15-min daily sessions. In one condition (saves condition) with the cuelight on, a single lever press produced a food pellet, briefly extinguished the cuelight, and started a clock. Saves (additional lever presses with interresponse times less than 1 s) produced an additional food pellet, briefly extinguished the cuelight, and restarted the interresponse time clock. The cuelight was extinguished 1 s after the last lever press and remained off during a 10-s period of extinction, during which no food pellets were delivered. In the other condition (savings account condition), the contingencies were the same except that the cuelight was extinguished and was not reilluminated after the initial lever press, and the delivery of all food pellets in the reinforcement component was delayed until the onset of extinction. In both conditions, rats made saves, but mean saves (total saves divided by the number of reinforcement components) were slightly reduced in the savings account condition. In Experiment 2, using six equally spaced 15-min sessions per day on alternate days, saves were either followed immediately with food and brief cuelight offset (saves condition) or were not reinforced at all. Mean saves were much greater when saves were reinforced. In Experiment 3, during 5-min daily sessions, saves earned a single pellet (savings account condition) or a number of pellets equal to the ordinal number of the lever press (interest condition). Rats made fewer mean saves, with little change in the food rate, when saves earned interest. In Experiment 4, the rats earned all their food in the operant situation during 24 daily 5-min sessions, these separated by 55-min intersession intervals during which no food was available; otherwise, the conditions were the same as in Experiment 3. In Experiment 4, the shift to interest for saves led to an increase in mean daily mean saves (total daily mean saves divided by the number of daily sessions) as well as to an increase in the number of food pellets delivered in each session. The results are discussed in terms of self-control and behavioral economics.     

Immediate postsession feeding reduces operant responding in rats

Three experiments investigated the effects of immediate and delayed postsession feeding on progressive-ratio and variable-interval schedule performance in rats. During Experiments 1 and 2, immediate postsession feeding decreased the breakpoint, or largest completed ratio, under progressive-ratio schedules. Experiment 3 was conducted to extend the results of the first two experiments to responding maintained by variable-interval schedules with different session lengths (15 and 60 min). Response rates decreased in all 4 subjects when postsession feeding immediately followed a 15-min session and in 3 of 4 subjects when postsession feeding immediately followed a 60-min session. The implications of this research are twofold: (1) The functional context in which within-session reinforcers are embedded extends outside the experimental chamber, and (2) supplemental postsession feedings should be sufficiently delayed from the end of a session to avoid weakening operant behavior in the experimental sessions.     

Food and cocaine self-administration by baboons: effects of alternatives.

The effects of the availability of an alternative reinforcer on responding maintained by food pellets or drug solutions were examined in 8 adult male baboons (Papio hamadrayas anubis). During daily 23-hr experimental sessions, baboons had access to both food pellets and fluid under a two-choice procedure, in which the response requirement, under a fixed-ratio schedule, differed for the two commodities. There were no restrictions on access to water, which was continuously available from a spout at the rear of each cage. In Experiment 1, the fixed-ratio requirement, or cost, for fluid delivery remained constant while the fixed-ratio requirement for pellets was changed every 2 or 3 days when (a) no fluid, (b) a dilute dextrose vehicle, (c) 0.008 mg/kg per delivery cocaine, (d) 0.016 mg/kg per delivery cocaine, or (e) 0.032 mg/kg per delivery cocaine was available concurrently. In Experiment 1, progressively increasing the response requirement for pellets decreased pellet intake, but for 4 baboons pellet intake at maximum pellet cost was lower when cocaine, compared to the vehicle, was available. Increasing the response requirement for pellets had variable effects on vehicle intake. However, increasing the response requirement for pellets increased intake of at least one dose of cocaine to a greater extent than vehicle in all 8 baboons. Thus, cocaine could be considered a more effective economic substitute than vehicle for pellets. Experiment 2 systematically varied the order in which the response requirements for a pellet delivery were presented and added a control condition in which cocaine doses, yoked to the amount self-administered, were given three times during the session by the experimenter. Again, pellet intake at maximal pellet cost was lower when cocaine, compared to the vehicle, was available. In contrast, experimenter-given cocaine doses did not alter responding maintained by pellets. Thus, the effects of self-administered cocaine on responding maintained by food pellets differed from the effects of experimenter-given cocaine on responding maintained by food pellets.     

Punishment of Schedule-Induced Drinking by Lick-Dependent Delays in Food Presented at Different Frequencies

The Psychological Record - Three food-deprived rats (80% of their free-feeding weights) developed schedule-induced drinking after being exposed to a multiple fixed-time schedule (FT 60-s FT 18-s)...     

Suppression of schedule-induced drinking and food-reinforced bar pressing by tail-pinch is not reversed by naloxone

In Experiment 1 eleven food-deprived rats were tested in a schedule-induced drinking paradigm under both tail-pinch and non-tail-pinch conditions. Tail-pinch produced a strong suppression of schedule-induced drinking, licking, licks per milliliter, bar presses, and number of reinforcers received during 30-min test sessions. Experiment 2 showed that the narcotic antagonist naloxone (2 and 4 mg/kg) did not reverse the tail-pinch suppression of schedule-induced drinking and food-reinforced bar pressing. Experiment 3 demonstrated that the suppression of schedule-induced drinking by tail-pinch could not be attributed to a suppression of drinking behavior in general, as tail-pinch had no effect on deprivation-induced drinking. The failure of naloxone to reverse this blockade suggests that endogenous opiate systems do not play a very important role in the suppression of schedule-induced polydipsia by tail-pinch. It was suggested that the combination of both the schedule-induction paradigm and the tail-pinch procedure increased arousal levels to such a high level that all behaviors were suppressed.     

Risky choice as a function of amount and variance in food supply.

In Experiment 1, 4 rats earned their daily food ration by choosing on a trials basis between a "risky" and a "riskless" lever. The risky lever produced either 15 45-mg food pellets or no pellets, and on average provided five pellets per choice. The riskless lever always produced three pellets. Across conditions, the number of trials per session was varied. Body weight and choice of the risky lever decreased as the number of trials per session decreased, even though body weight could only be defended by increased choice of the risky lever. In Experiment 2, trials per session were fixed, but the number of pellets delivered by the risky and riskless levers was either at the same level as in Experiment 1 or tripled from those levels. Now choice of the risky lever was inversely related to the size of reinforcement and to body weight. The results of these experiments show that risk aversion covaries with the amount of food available in a session and the daily variance in the amount of food earned.     

Resistance to extinction following variable-interval reinforcement: reinforcer rate and amount

Rats obtained food-pellet reinforcers by nose poking a lighted key. Experiment 1 examined resistance to extinction following single-schedule training with different variable-interval schedules, ranging from a mean interval of 16 min to 0.25 min. That is, for each schedule, the rats received 20 consecutive daily baseline sessions and then a session of extinction (i.e., no reinforcers). Resistance to extinction (decline in response rate relative to baseline) was negatively related to the rate of reinforcers obtained during baseline, a relation analogous to the partial-reinforcement-extinction effect. A positive relation between these variables emerged, however, when the unit of extinction was taken as the mean interreinforcer interval that had been in effect during training (i.e., as an omitted reinforcer during extinction). In a second experiment, rats received blocks of training sessions, all with the same variable-interval schedule but with a reinforcer of four pellets for some blocks and one pellet for others. Resistance to extinction was greater following training with the larger (four pellets) than with the smaller (one pellet) reinforcer. Taken together, these results support the principle that greater reinforcement during training (e.g., higher rate or larger amount) engenders greater resistance to extinction even when the different conditions of reinforcement are varied between blocks of sessions.     

Choice between delayed reinforcers in an adjusting-delay schedule: The effects of absolute reinforcer size and deprivation level

Choice between two reinforcers differing in magnitude and delay was investigated in rats using an adjusting-delay discrete-trials schedule in which the two reinforcers were associated with two levers (A and B). The delay to Reinforcer A (the smaller reinforcer) was always 2 sec, whereas the delay to Reinforcer B was varied in accordance with the distribution of choices in successive blocks of trials. In Experiment 1, the mean delay to the large reinforcer during the last 5 of 60 training sessions was greater when the rats were maintained at 80% than when they were maintained at 90% of their free-feeding body weights. In Experiment 2, the delay to the larger reinforcer was greater when the two reinforcers consisted of one and two 45-mg food pellets than when they consisted of three and six pellets. The results are consistent with a model of “self-control” which posits hyperbolic relations between reinforcer value and reinforcer magnitude, and between reinforcer value and delay of reinforcement.     

Making the sour sweet? Upcoming food-pellet reinforcement produces positive induction when rats press a lever for unsweetened lemon juice

Research has suggested that rats increase their response rate for a low-valued reinforcer when a high-valued reinforcer will soon be available (i.e., positive induction) because the value of the low-valued substance has increased. The present study tested if such a procedure could be used to increase rats’ responding for a non-reinforcing food. Rats pressed a lever for unsweetened lemon juice in the first half of a 50-min session and, in treatment conditions, for food pellets in the second half. Experiment 1 demonstrated that rates of responding for the lemon juice generally varied directly with the upcoming rate of food-pellet reinforcement and that responding in lemon juice-only sessions did not differ significantly from that observed during extinction. Experiment 2 demonstrated that rats consumed more lemon juice following a condition in which they were displaying positive induction than following a condition in which they only responded for lemon juice. The present results are consistent with the increase in value account of positive induction. More importantly, they may indicate that certain environmental conditions can increase food-directed behavior for a non-reinforcing food, a finding which may have implications for our understanding of eating behavior and dysfunctions (e.g., overeating).     

Is the context specificity of latent inhibition a sufficient explanation of learned irrelevance?

In three experiments, rats were pre-exposed either to uncorrelated presentations of a light and sucrose pellets (group CS/US) or to equivalent presentations of the light and pellets in separate sessions (control). In Experiment 1, subsequent conditioning to the light proceeded more slowly in group CS/US than in the control group, whether this conditioning was excitatory, with the light signalling the delivery of pellets, or inhibitory, with the light signalling their absence. Bonardi and Hall (1996) have argued that this learned irrelevance effect may be reducible to latent inhibition, which would be stronger in group CS/US because they are both pre-exposed and conditioned to the CS in the presence of traces of previous USs occurring in the same session. This analysis implies that group CS/US should have conditioned more rapidly to the CS than controls on the first trial of each session in Experiment 1, but this did not happen. It also implies that the learned irrelevance effect should be reversed if conditioning trials are given at a rate of one per day . Experiments 2 and 3 found no support for this prediction. We conclude that learned irrelevance effects cannot always be reduced to latent inhibition.     

Investigating retrospective influences on induction in rats' responding for 1% sucrose when food-pellet reinforcement is upcoming

When rats lever press for 1% sucrose reinforcement in the first half of a 50-min session, response rates are higher when food-pellet reinforcement will be available in the second half than when 1% sucrose will be available. Results of past research have suggested that, under some conditions, this induction effect is prospective in nature (i.e., controlled by the conditions of reinforcement in the present session). However, that research did not rule out the possibility that, under other conditions, retrospective factors (i.e., the conditions of reinforcement in the previous session[s]) could contribute. In the present study, rats responded in two types of session, one in which 1% sucrose reinforcement was available in both halves of the session and one in which 1% sucrose and food-pellet reinforcement were available in the first and second halves, respectively. Which type of session was in effect unsignaled and session type alternated every session (Experiment 1), every second session (Experiment 2), or after at least 20 consecutive sessions of one type (Experiment 3). Across experiments, the results indicated that it takes several sessions of one type for observable retrospective effects to occur, but those effects are short lived. These results allow the authors to identify the mechanisms that must underlie induction. The authors also discuss induction as an animal model of anticipation.     

A developmental study of schedule-induced polydipsia in the rat

Rats 30, 60, 90, and 120 days of age were fed 105 45-mg food pellets on a noncontingent VT 70-sec schedule during 14 daily 2-hr sessions. Every animal became polydipsic on this schedule when compared with a schedule in which 105 pellets were presented at the beginning of the session. Mean polydipsic ratios (schedule intake/control intake) ranged from 2.9–5.4. Water intakes (milliliters per 100g body weight) did not differ during the last five sessions as a function of age. The results are discussed with respect to the generality of the class of adjunctive behaviors.