Assessment of attack and drinking in White King pigeons on response-independent food schedules.

Four White King pigeons in Experiment I were exposed to a fixed-time 90-second food schedule with successive access to water and a conspecific target. Drinking per session was sporadic and minimal, while attack per session occurred during most interfood intervals for all animals. Analysis of the temporal distribution of attack showed that the typical postreinforcement pattern of attack developed over the course of the experiment. In Experiment II, the same animals were exposed to a series of fixed-time schedules ranging from 30 to 360 seconds with successive access to water and target. Time engaged in drinking and the number of interfood intervals with drinking were less than that of attack. Food and no-food baselines, which have been typically used to assess schedules-induced drinking and attack, respectively, were used to evaluate the effect of the schedule on attack and water ingestion. Relative to the no-food baseline, both attack and drinking were enhanced by the schedule in all birds. Relative to the food baseline, drinking was slightly suppressed in three birds and attack was enhanced in all. For all animals, the food baseline resulted in more attack and drinking than the no-food baseline.     

A temporal limit on the effect of future food on current performance in an analogue of foraging and welfare

Rats obtained access to food twice each 24-hour period. The first session was a work session in which food was available on a progressive-ratio schedule. During the second session, which occurred between 1 and 23 hours after the work session, food was freely available up to a fixed total intake each 24 hours. The situation resembled elements of several real world circumstances, including the choice between continuing to forage in a rapidly depleting patch and waiting for a better patch, and between working now and receiving a guaranteed income later. The purpose of the experiment was to explore the time period over which future access to reward could affect current responding. Contrary to what might be expected from recent theorizing, anticipation of future food delayed by an hour or more after the start of the work session had no effect on current performance. Food intake was high and constant during work sessions except for a prefeeding effect that occurred when the free session closely preceded the next day's work session. Also, an increase in the difficulty of the work schedule increased the amount of work and the maximum price paid for food as if the work session were the only time food was available. The results indicate the importance of considering temporal limits in theories that require animals to integrate input over time to determine the allocation of resources among alternatives.     

Effects of anorectic drugs on food intake under progressive-ratio and free-access conditions in rats

The effects of two anorectic drugs, dexfenfluramine and phentermine, on food intake under different food-access conditions were examined. Experiment 1 compared the effects of these drugs on food intake under a progressive-ratio (PR) schedule and free-access conditions. Dexfenfluramine decreased food intake under both conditions, but the doses required to decrease intake under free-access conditions were higher than those required to reduce intake under the PR condition. Intermediate doses of phentermine sometimes increased breaking points, and higher doses decreased them. Phentermine decreased food intake at the same doses under both access conditions. Thus the potency of dexfenfluramine, but not phentermine, to decrease food-maintained behavior depended upon the food-access condition. Experiment 2 used a novel mixed progressive-ratio schedule of food delivery to study the duration of drug effects. Sessions consisted of five components separated by 3-hr timeouts. The ratio requirement reset at the beginning of each component and a new breaking point was obtained. Both dexfenfluramine and phentermine dose-dependently decreased breaking points early in the session. In some rats, compensatory increases in breaking point were observed. That is, breaking points later in the session increased over control levels, resulting in no change in the total number of food pellets earned for the session compared to control. The present findings suggest that the effects of some anorectic drugs depend upon the access conditions for food; increasing the effort to obtain food may enhance their ability to decrease food-maintained behavior.     

Effects of reinforcer probability, delay, and response requirements on the choices of rats and pigeons: possible species differences

In Experiment 1 with rats, a left lever press led to a 5-s delay and then a possible reinforcer. A right lever press led to an adjusting delay and then a certain reinforcer. This delay was adjusted over trials to estimate an indifference point, or a delay at which the two alternatives were chosen about equally often. Indifference points increased as the probability of reinforcement for the left lever decreased. In some conditions with a 20% chance of food, a light above the left lever was lit during the 5-s delay on all trials, but in other conditions, the light was only lit on those trials that ended with food. Unlike previous results with pigeons, the presence or absence of the delay light on no-food trials had no effect on the rats' indifference points. In other conditions, the rats showed less preference for the 20% alternative when the time between trials was longer. In Experiment 2 with rats, fixed-interval schedules were used instead of simple delays, and the presence or absence of the fixed-interval requirement on no-food trials had no effect on the indifference points. In Experiment 3 with rats and Experiment 4 with pigeons, the animals chose between a fixed-ratio 8 schedule that led to food on 33% of the trials and an adjusting-ratio schedule with food on 100% of the trials. Surprisingly, the rats showed less preference for the 33% alternative in conditions in which the ratio requirement was omitted on no-food trials. For the pigeons, the presence or absence of the ratio requirement on no-food trials had little effect. The results suggest that there may be differences between rats and pigeons in how they respond in choice situations involving delayed and probabilistic reinforcers.     

Schedule-induced defecation: No-food and massed-food baselines

Defecation rate was monitored during daily 30-min periods as 16 rats were exposed to different sequences of the following three experimental conditions: (a) a fixed-time 60-s food delivery schedule, (b) a massed-food presentation baseline, and/or (c) a no-food baseline. All food delivery was response independent. Rate of defecation increased during fixed-time 60-s food delivery when compared to baseline rates of defecation established during no-food and massed-food baselines. This effect was present for 12 of 16 rats during four alternative sequences of experimental conditions. Within-subject reversals established reliability of this effect. Schedule induction of defecation is clearly demonstrated under these conditions.     

Time horizons in rats foraging for food in temporally separated patches

An important tenet of optimal foraging theory is that foragers compare prey densities in alternative patches to determine an optimal distribution of foraging behavior over time. A critical question is over what time period (time horizon) this integration of information and behavior occurs. Recent research has indicated that rats do not compare food density in a depleting patch with that in a rich patch delayed by an hour or more (Timberlake, 1984). In the present research we attempted to specify over what time period a future rich patch would affect current foraging. The effect of future food was measured by early entry into the rich patch (anticipation) and by a decrease in food obtained in the depleting patch (suppression). The rats showed anticipation of a rich patch up to an hour distant, but suppressed current feeding only if the rich patch was 16 min distant or less. The suppression effect appeared mediated by competition for expression between anticipatory entries into the rich patch and continued foraging in the depleting patch. These results suggest that optimal foraging is based on a variety of specific mechanisms rather than a general optimizing algorithm with a single time horizon.     

The effect of asymmetrical sample training on retention functions for hedonic samples in rats

Rats were trained in a symbolic delayed matching-to-sample task to discriminate sample stimuli that consisted of the presence of food or the absence of food. Asymmetrical sample training was provided in which one group was initially trained with only the food sample and the other group was initially trained with only the no-food sample. In addition, within each group half of the rats were trained with an illuminated intertrial interval (ITI) and the remaining rats with a dark ITI. While the retention functions did not differ as a function of which sample was trained first, they did differ as a function of the similarity in the illumination conditions during the ITI and the delay interval. Symmetrical retention functions were obtained when the lighting conditions were similar and slightly asymmetrical retention functions were obtained when the lighting conditions were dissimilar. Probe tests confirmed that features of the no-food sample were attended to and used to generate a memory representation for the no-food sample. The results are not consistent with the hypothesis that asymmetrical sample training encourages coding of the sample introduced initially and default responding to the subsequently introduced sample. Rats generate memory representations for both samples when asymmetrical sample training is given with hedonic samples.     

Drinking in a patchy environment: the effect of the price of water.

Rats in a laboratory foraging paradigm searched for sequential opportunities to drink in two water patches that differed in the bar-press price of each "sip" (20 licks) of water within a bout of drinking (Experiment 1) or the price and size (10, 20, or 40 licks) of each sip (Experiment 2). Total daily water intake was not affected by these variables. The rats responded faster at the patch where water was more costly. However, they accepted fewer opportunities to drink, and thus had fewer drinking bouts, and drinking bouts were smaller at the more costly patch than at the other patch. This resulted in the rats consuming a smaller proportion of their daily water from the more costly patch. The size of the differences in bout frequency and size between the patches appears to be based on the relative cost of water at the patches. The profitability of each patch was calculated in terms of the return (in milliliters) on either effort (bar presses) or time spent there. Although both measures were correlated with the relative total intake, bout size, and acceptance of opportunities at each patch, the time-based profitability was the better predictor of these intake measures. The rats did not minimize bar-press output; however, their choice between the patches and their bout sizes within patches varied in a way that reduced costs compared to what would have been expended drinking randomly. These data accord well with similar findings for choices among patches of food, suggesting that foraging for water and food occurs on the basis of comparable benefit-cost functions: In each case, the amount consumed is related to the time spent consuming.     

SCHEDULE-INDUCED POLYDIPSIA: ARE RESPONSE-DEPENDENT SCHEDULES A LIMITING CONDITION?1

The collateral water intake of albino rats was measured when the inter-pellet intervals in fixed-ratio and fixed-time schedules were equated. Fixed-ratio and fixed-time inter-pellet intervals were equated by dividing the average fixed-ratio session time of each subject by 150 (food pellets). The average inter-pellet interval obtained then defined the subsequent fixed-time schedule for each individual subject. Shifts to fixed-time schedules followed the completion of each fixed-ratio 20, 40, and 80 schedule. This procedure permitted an assessment of the extent to which excessive collateral drinking was associated with inter-pellet interval length or adventitious food reinforcement. For both the fixed-ratio and fixed-time schedules, drinking progressively increased as a function of increasing the duration of the inter-pellet interval and was a post-pellet event under the control of variables other than adventitious food reinforcement.     

Deprivation and satiation: The interrelations between food and wheel running

Two experiments were designed to assess whether depriving rats of food would increase the reinforcement effectiveness of wheel running (Experiment 1) and whether satiation for wheel running would decrease the reinforcement effectiveness of food (Experiment 2). In Experiment 1, a progressive-ratio schedule was used to measure the reinforcement effectiveness of wheel running when rats were deprived or not deprived of food. Completion of a fixed number of lever presses released a brake on a running wheel for 60 s, and the response requirement was systematically increased until the rat stopped pressing or until 8 hr had elapsed. The ratio value reached (and the total number of lever presses) was an inverted-U function of food deprivation (percentage body weight). In Experiment 2, when wheel running preceded test sessions, fewer food-reinforced lever presses were maintained by the progressive-ratio schedule, and responding occurred at a lower rate on a variable-interval schedule. An interpretation of these results is that deprivation or satiation with respect to one event (such as food) alters the reinforcement effectiveness of a different event (such as access to wheel running).     

The relationship between feeding rate and patch choice.

Rats in a laboratory foraging simulation searched for sequential opportunities to feed in two patches that differed in the rate at which food pellets were delivered (controlled by fixed-interval schedules) and in the size of the pellets. The profitability of feeding in each patch was calculated in terms of time (grams per minute) and in terms of effort (grams per bar press). These values were the result of the imposed fixed interval, the size of the pellets, and the rate at which the rats pressed the bar in each condition. The rats ate more food and larger meals, but not more frequent meals, at the patch offering the higher rate of food consumption, calculated as grams per minute. The relative intake at any patch was a function of the relative rate of intake during meals at that patch compared to the other patch. Rats respond to explicit manipulations of feeding time in the same manner as they respond to manipulations of feeding effort.     

Abdominal vagotomy disrupts food-related drinking in the rat

Rats with complete subdiaphragmatic bilateral transection of the abdominal vagus (Vgx-C) showed disordered food-related drinking when drinking water in temporal association with a meal of dry food after 5-hr food deprivation and when drinking water in association with a liquid meal after 24-hr food deprivation. The Vgx-C rats drank after significantly longer latencies and drank significantly less water in 1 hr than did sham-vagotomized (Sham) rats after eating the same size meal (solid or liquid) as Shams. Rats with incomplete vagal transection (Vgx-I) ate and drank like Shams. Water intake of Sham and Vgx-I rats correlated positively with the meal size of solid food, but the water intake of Vgx-C rats did not. The failure of Vgx-C rats to drink water normally when food was ingested was not due to failure of a food stimulus to reach the intestine, because Vgx-C and Sham rats emptied equivalent volumes of liquid food from the stomach into the intestine within 10 min of food entering the stomach. These results indicate that the abdominal vagus is an important neurological substrate for food-related drinking in the rat.     

Rapid acquisition of choice and timing in pigeons

Pigeons were trained on a concurrent-chains procedure in which the initial link associated with the shorter terminal-link delay to food changed unpredictably across sessions. In the minimal-variation condition, delays were always 10 s and 20 s, whereas in the maximal-variation condition delays were generated pseudorandomly for each session. On some terminal links, food was withheld to obtain measures of temporal control. Measures of choice (log initial-link response ratios) and timing (start and stop times on no-food trials) showed temporal control and stabilized within the 1st half of each session. In the maximal-variation condition, choice was a nonlinear function of the log delay ratio, consistent with a categorical discrimination but contrary to models based on the matching law. Residuals from separate regressions of log response and log start and stop time ratios on log delay ratios were positively correlated. Overall, results support cognitive models that assume that initial-link choice is based on an all-or-none decision process, and that choice and timing are mediated by a common representation of delay.     

Mediational use of internal representations of food and no-food events by pigeons

Two experiments were conducted with pigeons to determine whether internal representations of food and no-food events can act as mediators in the formation of new associations. Specifically we asked if the representation of an anticipated event can replace the event itself in an established conditional discrimination. In Phase 1 of both experiments pigeons were differentially autoshaped to peck hue stimuli, only one of which was followed by access to food. In Phase 2 they were trained on a symbolic 0-delay matching-to-sample (DMTS) task with food and no-food samples and line-orientation comparisons. In Phase 3 the birds were tested on a symbolic DMTS task in which the hue stimuli from Phase 1 were substituted for the Phase 2 food-event samples. For the Pos group, the hue followed by food in Phase 1 was substituted for the food sample and the hue associated with no-food was substituted for the no-food sample so that the resulting sample/comparison pairings were consistent with the food-event mediator. For the Neg group the pairings were reversed so that the sample/comparison pairings were inconsistent with the food-event mediators. In Experiment 1, when the no-food event was the absence of an event, acquisition of the transfer task was significantly more rapid in the Pos than in the Neg condition. In Experiment 2, when the no-food event was the presentation of an empty food hopper, the Pos group transferred at a significantly higher level than the Neg group. The two experiments provide evidence that in pigeons, representations involving event anticipations can be substituted for, and are thus similar to, the events themselves.     

Persistence of conditioned flavor preferences is not due to inadvertent food reinforcement

Almond preferences were produced by giving rats a mixture of almond and sucrose (Experiments 1-4) or saccharin (Experiment 4). A subsequent extinction procedure consisted of either repeated 2-bottle almond versus water tests (Experiment 1) or repeated exposure to almond alone (Experiments 2-4). The main independent variable was whether access to food following a session was given immediately, 30 min later, or 120 min later. No effect of extinction was found in any experiment. An important finding was that varying the delay until food access had no detectable effect. It was concluded that inadvertent flavor-food associations do not maintain preference for the flavor under extinction conditions.     

Polydipsia induced by intermittent delivery of salted liquid foods

Food-deprived rats given constant access to water were exposed to fixed-time presentations of soybean milk and diluted sweetened condensed cows' milk. In some conditions these liquid foods were adulterated with varying amounts of sodium chloride. Under a fixed-time 30-sec schedule of food delivery, little water was consumed when the food was soybean milk alone, or soybean milk with sodium chloride added in concentrations of .9, 1.8, or 3.6%. However, schedule-induced polydipsia appeared when soybean milk adulterated with 7.2 or 14.4% sodium chloride was delivered under this schedule. When soybean milk containing 7.2% sodium chloride was presented under fixed-time 15-, 30-, 60-, 120-, and 240-sec schedules, schedule-induced drinking increased with the fixed-time value from 15 to 120 seconds, and decreased at 240 seconds. Like soybean milk, diluted sweetened condensed milk delivered under fixed-time schedules of 30, 60, and 120 seconds failed to evoke schedule-induced polydipsia, but did so when adulterated with 7.2% sodium chloride. Drinking induced by salted liquid foods resembled the polydipsia engendered by spaced dry-food presentations in several ways, including temporal relation to food delivery, persistence within and across sections, sensitivity to interfood interval, and magnitude relative to intake evoked by bulk-food presentation.     

Foraging in a simulated natural environment: There's a rat loose in the lab

Rats were required to earn their food in a large room having nine boxes placed in it, each of which contained food buried in sand. In different phases of the experiment the amount of time allowed for foraging, the amount of food available in each food patch, and the location of the different available amounts were varied. The rats exhaustively sampled all patches each session but seemed to have fairly strong preferences for certain locations over others. If position preferences were for patches containing small amounts of food, the sensitivity to amount available was increased so that when location was compensated for, a pattern of optimal foraging was evident. The importance of environmental constraints in producing optimal behavior and the relation of the observed behavior to laboratory findings are discussed.     

Behavioral and pharmacological variables affecting risky choice in rats.

The effects of manipulations of response requirement, intertrial interval (ITI), and psychoactive drugs (ethanol, phencyclidine, and d-amphetamine) on lever choice under concurrent fixed-ratio schedules were investigated in rats. Responding on the "certain' lever produced three 45-mg pellets, whereas responding on the "risky" lever produced either 15 pellets (p = .33) or no pellets (p .67). Rats earned all food during the session, which ended after 12 forced trials and 93 choice trials or 90 min, whichever occurred first. When the response requirement was increased from 1 to 16 and the ITI was 20 s, percentage of risky choice was inversely related to fixed-ratio value. When only a single response was required but the ITI was manipulated between 20 and 120 s (with maximum session duration held constant), percentage of risky choice was directly related to length of the ITI. The effects of the drugs were investigated first at an ITI of 20 s, when risky choice was low for most rats, and then at an ITI of 80 s, when risky choice was higher for most rats. Ethanol usually decreased risky choice. Phencyclidine did not usually affect risky choice when the ITI was 20 s but decreased it in half the rats when the ITI was 80 s. For d-amphetamine, the effects appeared to he related to baseline probability of risky choice; that is, low probabilities were increased and high probabilities were decreased. Although increase in risky choice as a function of the ITI is at variance with previous ITI data, it is consistent with foraging data showing that risk aversion decreases as food availability decreases. The pharmacological manipulations showed that drug effects on risky choice may be influenced by the baseline probability of risky choice, just as drug effects can be a function of baseline response rate.     

Disrupted patterns of consummatory behavior in rats with fornix transections

The feeding and drinking behavior was examined in male rats with fornix transections and sham-operated control rats. Total food and water consumption was recorded but supplemented by a pattern analysis of feeding and drinking behavior. The behavior of the rats was continuously monitored during four hour morning and afternoon sessions under ad lib access and during a two hour session following adaptation to a restricted access feeding schedule. Rats with fornix transections were more active and exhibited increased frequencies of rearing, eating and drinking. The increased meal frequency in rats with fornix transections was accompanied by decreased meal durations and a reduction in the length of intermeal intervals. Total food and water consumption was unaffected by fornix transection as were the duration of sleep bouts and the frequencies of grooming, sleeping and carrying shavings. Fornix transections also reduced food carrying and food hoarding but only under conditions of restricted food access. The results suggest that fornix transection does not alter major homeostatic regulatory mechanisms nor does it alter the components of feeding and drinking behavior. Fornix transection alters, instead, the organization of microregulatory feeding and drinking patterns.     

A comparison of delays and ratio requirements in self-control choice.

In a discrete-trial procedure, pigeons could choose between 2-s and 6-s access to grain by making a single key peck. In Phase 1, the pigeons obtained both reinforcers by responding on fixed-ratio schedules. In Phase 2, they received both reinforcers after simple delays, arranged by fixed-time schedules, during which no responses were required. In Phase 3, the 2-s reinforcer was available through a fixed-time schedule and the 6-s reinforcer was available through a fixed-ratio schedule. In all conditions, the size of the delay or ratio leading to the 6-s reinforcer was systematically increased or decreased several times each session, permitting estimation of an "indifference point," the schedule size at which a subject chose each alternative equally often. By varying the size of the schedule for the 2-s reinforcer across conditions, several such indifference points were obtained from both fixed-time conditions and fixed-ratio conditions. The resulting "indifference curves" from fixed-time conditions and from fixed-ratio conditions were similar in shape, and they suggested that a hyperbolic equation describes the relation between ratio size and reinforcement value as well as the relation between reinforcer delay and its reinforcement value. The results from Phase 3 showed that subjects chose fixed-time schedules over fixed-ratio schedules that generated the same average times between a choice response and reinforcement.