Fixed-interval Schedule Of Food Reinforcement Research Articles

Behavioral effects of nicotine, amphetamine and cocaine under a fixed-interval schedule of food reinforcement in rats chronically exposed to caffeine.

Epidemiological surveys demonstrate that caffeine, the main psychoactive ingredient of coffee, is a positive correlate in drug abuse. To characterize the behavioral nature of caffeine interactions with other psychomotor stimulants, we examined the effects of chronic caffeine exposure on the behavioral responses to nicotine, amphetamine, cocaine, the selective D1 agonist SKF-82958 and the selective D2 receptor agonist NPA, in rats responding under a fixed interval (FI) schedule of food reinforcement. Following stabilization of rates and temporal patterns of responding (mathematically expressed as quarter-life values, QL), twenty-one Sprague-Dawley rats responding under a 5-min FI schedule of food reinforcement were divided into two groups; one (twelve rats) maintained on tap water (control) and the other (nine rats) on caffeine (3 mg/ml added to the drinking water). Following the substitution of caffeine solution for tap water, behavior was temporarily disrupted as evidenced by decreases in responding and QL values which reached a maximum after 72 h (rate 60% and QL 30% below baseline levels). Rats developed complete tolerance to these effects of caffeine over 5 days of caffeine exposure. After response rate and QL values stabilized, effects of drugs were evaluated. Nicotine (0.01-1.0 mg/kg; SC), amphetamine (0.1-5.6; IP), and cocaine (1.0-17; IP) each produced biphasic dose-dependent changes in response rate with maximum increases in response rate following intermediate doses and decreases in response rates following higher doses. The increase in rates of responding produced by amphetamine or cocaine (but not nicotine) were greater (P<0.05) in caffeine-drinking than in water-drinking rats. Both SKF-82958 (0.001-0.3 mg/kg; IP) and NPA (0.0001-0.1; IP) produced only dose-dependent decreases in rates of responding. Caffeine-drinking rats were less sensitive to the rate-depressant effects of SKF-82958 (P<0.05) than water-drinking rats. However, similar changes (P>0.05) were produced by NPA in both groups. Except for amphetamine, the remaining drugs produced similar (P>0.05) dose-dependent decreases in QL values in water- and caffeine-drinking rats. Amphetamine produced smaller decreases in QL values in caffeine-drinking rats than in water-drinking rats (P<0.05). Chronic exposure to caffeine produced complete insurmountable tolerance to the response-rate increasing (stimulant) effects of acute caffeine (3.0-17 mg/kg; IP) in caffeine-drinking rats. In conclusion, our study revealed that chronic caffeine exposure potentiates the behavioral response to amphetamine and cocaine but not to that of nicotine in rats responding under a FI schedule of food reinforcement. Thus, it is likely that these effects are mediated through different pharmacological mechanisms.

CHOICE BETWEEN FIXED‐INTERVAL SCHEDULES: GRADED VERSUS STEP‐LIKE CHOICE FUNCTIONS

Pigeons chose between two fixed-interval schedules of food reinforcement. A single peck on one of two lighted keys started the fixed-interval schedule correlated with that key. The schedule had to be completed before the next choice opportunity. The durations of the fixed intervals were varied over conditions from 15 s to 40 s. To maximize the rate of reinforcement, the pigeons had to choose exclusively the shorter of the two schedules. Nevertheless, choice was not all-or-none. Instead, relative choice, and the rates of producing the fixed intervals, varied in a graded fashion with the disparity between the two schedules. Choice ratios under this procedure (single response to choose) were highly sensitive to the ratios of the fixed-interval schedules.

Effects of diazepam on schedule-controlled and schedule-induced behavior under signaled and unsignaled shock.

Schedule-controlled lever pressing and schedule-induced licking were studied in rats under a multiple fixed-interval fixed-interval schedule of food reinforcement upon which was superimposed a multiple variable-time variable-time schedule of electric-shock delivery. Shocks were signaled in one component of the multiple schedule and unsignaled in the other. The effects of diazepam upon the suppression of behavior during the signal (conditioned suppression) and during signaled and unsignaled shock (differential suppression) were studied under several shock intensities (Experiment 1) and at increased body weight (Experiment 2). In each study, diazepam led to dose-dependent increases in the rate of pressing and licking during signaled and unsignaled shock, but had little effect on conditioned suppression. the rate-enhancing effects of diazepam depended upon the intensity of shock, nature of the response, and whether or not shocks were signaled. The data was discussed in terms of (1) implications for understanding the effects of signaled and unsignaled shock on behavior, (2) the effects of diazepam on behavior suppressed by response-independent shock, and (3) comparison between operant and schedule-induced behavior.

EFFECTS OF SIGNALED AND UNSIGNALED SHOCK ON SCHEDULE‐CONTROLLED LEVER PRESSING AND SCHEDULE‐INDUCED LICKING: SHOCK INTENSITY AND BODY WEIGHT

Schedule-controlled lever pressing and schedule-induced licking were studied in rats under a multiple fixed-interval fixed-interval schedule of food reinforcement. Following acquisition of stable rates of pressing and licking, a multiple variable-time variable-time schedule of electric-shock delivery was superimposed upon the baseline schedule. In only one component of the multiple schedule, a 5-sec stimulus preceded each shock (signaled shock). In the other component shock was unsignaled. Several shock intensities (Experiment 1) and body weights (Experiment 2) were studied. Lever pressing and licking were affected similarly by experimental manipulations, although with parametric differences. Depending upon shock intensity and body weight, rates of lever pressing and licking were hardly suppressed, suppressed primarily in the unsignaled shock component (differential suppression), or markedly suppressed in both components. Differential suppression during components with signaled and unsignaled shock and conditioned suppression of responding during the preshock stimulus appeared not to be functionally related. Differential suppression depended more on the discriminability of shock-free time, and on shock intensity, body weight, and the type of response than on the "preparatory" behavior preceding shock.

A COMPARISON OF RATIO AND INTERVAL REINFORCEMENT SCHEDULES WITH COMPARABLE INTERREINFORCEMENT TIMES

Pigeons were trained to peck keys on fixed-ratio and fixed-interval schedules of food reinforcement. Both schedules produced a pattern of behavior characterized as pause and run, but the relation of pausing to time between reinforcers differed for the two schedules even when mean time between reinforcers was the same. Pausing in the fixed ratio occupied less of the time between reinforcers for shorter interreinforcer times. For two of three birds, the relation was reversed at longer interreinforcer times. As an interreinforcer time elapsed, there was an increasing tendency to return to responding for the fixed interval, but a roughly constant tendency to return to responding for the fixed-ratio schedule. In Experiment 1 these observations were made for both single-reinforcement schedules and multiple schedules of fixed-ratio and fixed-interval reinforcement. In Experiment 2 the observations were extended to a comparison of fixed-ratio versus variable-interval reinforcement schedules, where the distribution of interreinforcement times in the variable interval approximated that for the fixed ratio.

Latencies on response-initiated fixed-interval schedules: Effects of signaling food availability

Three pigeons were trained on a response-initiated fixed-interval schedule of food reinforcement where the first keypeck in the presence of one key color changed the key color and started a fixed time interval, after which a keypeck produced food. The latency to begin the fixed-interval component was an increasing function of the fixed-interval duration, which was either 40 or 80 sec. At both interval durations, a condition was also studied where the key color changed again at the end of the fixed interval, signaling the availability of food reinforcement. Adding the signal reduced the rate of keypecking during the fixed-interval component but did not systematically affect the initial latency to start the fixed-interval component.

Levo-alpha-acetylmethadol and metabolites: Some effects on schedule-controlled behavior in the rat

The behavioral effects of acute IP administration of l-α-acetylmethadol, its metabolites, l-α-noracetylmethadol and l-α-dinoracetylmethadol, and morphine were studied in the rat using behavior controlled by a fixed-interval schedule of food reinforcement. Administration of all compounds produced a dose-related decrease in response rate. The metabolites were approximately three to four times the potency of the parent compound which was approximately five times the potency of morphine. Data obtained from cumulative response records suggested that the onset of effects for the metabolites was more rapid than for the parent compound.

Behavioral effects of chronic oral administration of levo-alpha-acetylmethadol in the rat

levo-alpha-Acetylmethadol was orally administered via a sucrose solution to rats in their drinking water for 24 days. A control group received only sucrose. Body weight and fluid consumption were monitored daily. The behavioral effects during chronic drug administration and during eight days of withdrawal were studied using behavior controlled by a fixed-interval schedule of food reinforcement. Body weights of treated animals remained stable during drug administration but decreased by approximately 25% during withdrawal. There were no significant differences in volume of fluid consumed by the two groups. Response rate and number of reinforcements were decreased during drug administration. During withdrawal, response rates were greater than pre-drug control rates.

Effects of salsolinol, a tetrahydroisoquinoline alkaloid, on multiple schedule performance in rats

Rats were trained to lever press under a multiple fixed-ratio fixed-interval schedule of food reinforcement. Intracerebroventricular injections of salsolinol (1-methyl-6,7-dihydroxytetrahydroisoquinoline), a condensation product of dopamine and acetaldehyde, caused a dose-related decrease in fixed-ratio responding over a dosage range of 15–120 μg. In contrast, fixed-interval responding was only reduced at the 120 μg dose and was increased in two of the animals after injection of 15–30 μg of salsolinol.

Response suppression on a mixed schedule of reinforcement during alcohol withdrawal

Two groups of rats were maintained on a mixed fixed-ratio, fixed-interval schedule of food reinforcement until performance was stable. The animals were intubated daily with ascending doses of alcohol or water 30 min before each session, starting with 4.0 g/kg and increasing to the final dose of 8.0 g/kg. The results show that withdrawal from alcohol suppresses responding in all components of a mixed schedule and this suppression lasts approximately 14 hours. These results support previous reports which suggest that the duration of the behavioral aberrations produced by alcohol withdrawal are most severe 6 to 14 hours post-withdrawal.

THE ROLE OF THE PECK‐FOOD CONTINGENCY ON FIXED‐INTERVAL SCHEDULES1

Pigeons were trained to peck on a fixed-interval schedule of food reinforcement and then exposed to three schedules in which there was either no, or an indirect, relation between pecking and food delivery: (a) a conjunctive schedule in which food was delivered at fixed intervals, providing at least one peck was emitted in the interval; (b) a recycling version of the conjunctive schedule that essentially eliminated occasional peck-food contiguities (recycling conjunctive); (c) delivery of food at fixed intervals independently of the birds' behavior (fixed time). The rates and patterns of pecking sustained by these procedures depended on interfood interval and relative proximity of pecks to food.

A detailed analysis of the effects of d-amphetamine on behavior under fixed-interval schedules.

Pigeons were exposed to fixed-interval schedules of food reinforcement with durations of 300 sec, 100 sec, or 40 sec. A range of doses of d-amphetamine was administered to each pigeon, and the resulting behavior was analyzed at several levels of detail. Average rates in different portions of the intervals predicted the magnitude of the drug's effect, but a finer analysis showed that average rates did not adequately characterize the behavior in some parts of the intervals. The probability of responding in different parts of an interval without drug was also a good predictor of the magnitude of the effect of d-amphetamine, and at the same time was more descriptive of the interval-to-interval performance. Analyses of the control performance indicated that responding in individual intervals could be described as consisting of two parts: a very low, or zero, rate at the beginning of the interval followed by an abrupt transition to a slightly, but reliably, positively accelerated rate maintained until reinforcement.

ON THE FORM OF THE RELATION BETWEEN RESPONSE RATES IN A MULTIPLE SCHEDULE1

Three pigeons received training on multiple variable-interval schedules with brief alternating components, concurrently with a fixed-interval schedule of food reinforcement on a second key. Fixed-interval performance exhibited typical increases in rate within the interval, and was independent of multiple-schedule responding. Responding on the multiple-schedule key decreased as a function of proximity to reinforcement on the fixed-interval key. The overall relative rate of responding in one component of the multiple schedule roughly matched the overall relative rate of reinforcement. Within the fixed interval, response rate during one multiple-schedule component was a monotonic, negatively accelerated function of response rate during the other component. To a first approximation, the data were described by a power function, where the exponent depended on the relative rate of reinforcement obtained in the two components. The relative rate of responding in one component of the multiple schedule increased as a function of proximity to fixed-interval reinforcement, and often exceeded the overall obtained relative rate of reinforcement. The form of the function relating response rates is discussed in relation to findings on rate-dependent effects of drugs, chaining, and the relation between response rate and reinforcement rate in single-schedule conditions.

LATENCY AND FREQUENCY OF RESPONDING UNDER DISCRETE‐TRIAL FIXED‐INTERVAL SCHEDULES OF REINFORCEMENT1

Pigeons' key pecking was studied under a number of discrete-trial fixed-interval schedules of food reinforcement. Discrete trials were presented by briefly illuminating the keylight repetitively throughout the interreinforcement interval. A response latency counterpart to the fixed-interval scallop was found, latency showing a gradual, negatively accelerated decrease across the interval. This latency pattern was largely invariant across changes in fixed-interval length, number of trials per interval, and maximum trial duration. Frequency of responding during early trials in the intervals varied, however, with different schedule parameters, being directly related to fixed-interval length, inversely related to number of trials, and complexly affected by conjoint variations of fixed-interval length and number of trials. Response latency thus was found to be simply related to elapsed time during the interval while response frequency was complexly determined by other factors as well.

Suppression of operant behavior and schedule-induced licking in rats.

The first experiment studied the effects of punishment on rats' lever pressing maintained by a fixed-interval schedule of food reinforcement and on the associated schedule-induced licking. When licking was followed by shock, licking was suppressed but lever pressing was largely unaffected. When lever pressing was followed by shock, lever pressing was suppressed but licking was unaffected. In both cases, the punished behavior recovered its previous unpunished level when the shocks were discontinued. In a second experiment, the rats' lever pressing was maintained by a variable-interval schedule of food reinforcement under which polydipsic licking also developed. Both lever pressing and licking were partially suppressed during a stimulus correlated with occasional unavoidable electric shocks. With a higher shock intensity, both behaviors were suppressed further. Both lever pressing and licking recovered their previous levels when shocks were discontinued. These results show that schedule-induced licking, which has been described as adjunctive behavior, can be suppressed by procedures that suppress reinforced lever pressing, an operant behavior.

Uncertainty Reduction and Conditioned Reinforcement

Pigeons were exposed to a single-key sequential choice procedure in which the choice phase was followed by either a multiple fixed-interval schedule of food reinforcement where different key-lights indicated which FI was in effect or by a mixed fixed-interval schedule of food reinforcement where the same key-light was present for either FI. In the choice phase a scrambled series of brief periods of blue and of green illumination of the key were presented. A single peck to blue produced the mult schedule; a single peck to green produced the mix schedule. The variable manipulated was the relative proportion of FI 10-sec. components in the mult and mix schedules. Pigeons pecked blue more frequently than green for proportions other than 0.00 and 1.00. Preference for blue was greater for the range of proportions from 0.03 to 0.50 than for the range from 0.50 to 0.97. Such asymmetry of preference is a problem for the information account of conditioned reinforcement.

INTERRESPONSE TIME DURATION IN FIXED‐INTERVAL SCHEDULES OF REINFORCEMENT: CONTROL BY ORDINAL POSITION AND TIME SINCE REINFORCEMENT1

The times between each of the first thirteen responses after reinforcement (the first twelve interresponse times) were determined for two pigeons whose pecking was reinforced on fixed-interval schedules of food reinforcement ranging from 0.5 min to 5 min. These interresponse times were classified with respect to their ordinal position in the sequence of responses and with respect to the time since the preceding reinforcement at which the initiating response occurred. The median interresponse time durations were essentially constant after the sixth response after reinforcement regardless of the time at which the interresponse time was initiated. The durations of the first few interresponse times after reinforcement decreased as the number of preceding responses increased and as the time since the preceding reinforcement increased.

Response force under fixed-interval reinforcement

The force of lever-pressing was measured under a 1-min. fixed-interval schedule of food reinforcement. The Ss, 6 rats, were tested under 4 conditions of response force requirement imposed in the following order: 22, 52, 22, and 7.4 gm. In one analysis, it was found that the average force of lever-pressing gradually increased in magnitude during the 1-min. intervals. Under the higher force requirements, it was found that criterion responses were less numerous and occurred closer to the end of the fixed-interval. Sub-criterion responses, however, remained numerous at higher force requirements. After the imposition of the highest (52 gm) requirement, it was found that a higher average force characterized the performance even after 5 weeks’ exposure to lower values.

EXTEROCEPTIVE CONTROL OF FIXED‐INTERVAL RESPONDING

Two pigeons were exposed to several fixed-interval schedules of food reinforcement. In some cases, exteroceptive stimuli associated with the passage of time were present. Such visual "clock" stimuli were found to gain almost complete control over the behavior, although at the longest fixed interval studied, the superposition of a new temporal discrimination upon the visual discrimination was observed. Where clock stimuli were made contingent upon the birds' behavior, a new form of responding was generated. This behavior was discussed in terms of positive and negative response-tendencies resulting from several stimulus factors: Some of these functioned as S(Delta)'s and secondary negative reinforcers; some functioned as S(D)'s and secondary positive reinforcers; and some were ambiguous with respect to reinforcement conditions. A "pure temporal" discrimination was superimposed upon these factors, but its exact nature was indeterminate from the present data.