Studies on responding under fixed-interval schedules of reinforcement: II. The scalloped pattern of the cumulative record

Responding under fixed-interval schedules usually generates either scalloped or break-and-run cumulative records. Earlier, it was generally accepted that the characteristic pattern was the scallop, but in recent years there has been an increasing emphasis on the break-and-run pattern. The break-and-run pattern has been shown quantitatively to provide a good fit of certain fixed-interval patterns. In the present work, responding during fixed-interval 1000-second components of a multiple fixed-interval 1000-second fixed-ratio 50 responses schedule was examined in two rhesus monkeys. Even after responding had started in an interval, there was a high tendency for responding to accelerate over subsequent 100-second segments of the interval. In segments with responding, the rate increased from one segment to the next in 303 of 389 segments in one monkey and in 310 of 419 segments in the other. The size of the increase was substantial, the rate in the fifth segment after responding started being an average of 4.5 times higher than the rate in the first segment after responding started. Hence, the usual pattern of responding in individual intervals was of sustained and substantial acceleration, vindicating numerically the conclusion derived from inspection of the scalloped patterns of the cumulative records.     

Acquisition of cocaine self-administration with unsignaled delayed reinforcement in rhesus monkeys

Six experimentally naive rhesus monkeys produced 0.01 mg/kg/infusion cocaine by lever pressing under a tandem fixed-ratio 1 differential-reinforcement-of-other-behavior schedule. One lever press initiated an unsignaled 15- or 30-s delay culminating in cocaine delivery. Each press made during the delay reset the delay interval. With two exceptions, responding was acquired and maintained at higher rates than responding on a second (inoperative) lever. For the exceptions, a cancellation contingency was arranged in which each formerly inoperative-lever response reset the tandem schedule. This manipulation reduced presses on the inoperative lever. Subsequently, the consequences of responding on the two levers were reversed, and the monkeys again responded at higher rates on the operative lever. As a comparison, 3 additional experimentally naive monkeys received response-independent cocaine deliveries. Although lever pressing was observed, it extinguished and was subsequently reestablished under the tandem schedule. The results suggest that although response-reinforcer contiguity is not required for cocaine to acquire reinforcing functions, a response-reinforcer relation appears necessary.     

Oral self-administration of pentobarbital by rhesus monkeys: maintenance of behavior by different concurrently available volumes of drug solution.

For 4 rhesus monkeys, mouth-contact responses with either of two brass spouts were reinforced according to fixed-ratio schedules by 0.65-mL liquid deliveries during daily 3-hr sessions. Three experiments were conducted. In each experiment, independent fixed-ratio schedules were concurrently in effect at the two spouts. Following completion of each fixed ratio on a spout, a specified number of liquid deliveries were available from that spout under a continuous-reinforcement schedule. The number of such deliveries available at each spout was manipulated independently. In Experiment 1, a 1-mg/mL pentobarbital solution was simultaneously available with water (the drug vehicle) under concurrent fixed-ratio schedules of 32 responses for 3 subjects and 64 responses for the remaining subject. The number (N) of liquid deliveries that were available after completion of each fixed ratio was varied in the following order: 8, 4, 2, 1, and 8 (retest). For each subject at each condition, drug maintained more responding than water. The number of drug deliveries obtained per session was directly related to the amount of drug available per fixed ratio (i.e., to N), whereas the number of fixed ratios completed per session generally was inversely related to the value of N. In Experiment 2, fixed-ratio size was the same for each subject as in Experiment 1, but deliveries of a 1-mg/mL pentobarbital solution were available at both spouts. The number of drug deliveries available under one fixed-ratio schedule (Ns, the "standard" reinforcer amount) was held at eight, and the number of drug deliveries available under the second schedule (Nc, the "comparison" reinforcer amount) was changed across blocks of six sessions of stable responding in the following order: 1, 2, 4, 8, 4, 2, and 1. The identical series of comparison reinforcer amounts (Nc) was then tested twice more, but with the standard reinforcer (Ns) held first at four and then at two deliveries. Across the three choice series, reinforcing effects were directly related to reinforcer magnitude. In Experiment 3, deliveries of a 1-mg/mL pentobarbital solution again were available at both spouts. However, the two reinforcer amounts were held constant at N = 8 deliveries under one schedule and N = 4 deliveries under the second schedule, and fixed-ratio size was systematically varied. Across the range of fixed-ratio sizes from low to high, the degree to which behavior was better maintained by the larger of the two drug quantities was an inverted U-shaped function of fixed-ratio size.(ABSTRACT TRUNCATED AT 400 WORDS)     

Concurrent phencyclidine and saccharin access: presentation of an alternative reinforcer reduces drug intake.

Six monkeys self-administered orally delivered phencyclidine ("angel dust") and saccharin under concurrent fixed-ratio 16 schedules during daily three-hour sessions. Liquid deliveries were contingent upon lip-contact responses on solenoid-operated drinking spouts. Three saccharin concentrations (0.003%, 0.03% and 0.3%, wt/vol) were tested in a nonsystematic order. For each saccharin concentration, the following series of phencyclidine concentrations (mg/ml) was presented: 0.25, 0.5, 1, 0.25 (retest), 0.125, 0.0625, 0.0312, 0.25 (retest) and 0 (water with stimuli signaling phencyclidine). As the saccharin concentration increased, the number of drug deliveries decreased, and the peaks of the concentration-response functions were shifted to the right. The lowest saccharin concentration (0.003%, wt/vol) maintained responding in excess of phencyclidine levels in only one monkey. The two higher saccharin concentrations maintained behavior far in excess of phencyclidine, but saccharin deliveries decreased in some monkeys as phencyclidine concentration and intake (mg/kg) increased. The time course and patterns of phencyclidine-reinforced responding were also altered when saccharin was concurrently available. The results are discussed in terms of strategies to reduce drug-reinforced behavior, preference between different reinforcers, and measures of reinforcing efficacy.     

Continuity and Change in Emotional Reactivity in Rhesus Monkeys Throughout the Prepubertal Period

We examined continuity and change in emotional reactiviy as assessed by hypothalamic–pituitary–adrenal (HPA) activity in rhesus monkeys. Plasma samples were obtained from mother-peer–reared and surrogate-peer–reared monkeys and assayed for cortisol concentrations at 5 time points in infancy. Both plasma and saliva samples were collected for cortisol assay from the same monkeys as 1-, 2-, and 3-year-old juveniles, respectively. Plasma cortisol concentrations increased during the first 5 months of life and decreased from 1 to 3 years of age. Females consistently had higher plasma cortisol concentrations than males. Plasma cortisol concentrations were lower in surrogate-peer–reared than mother-peer–reared monkeys during the first month of life. Juvenile plasma cortisol concentrations were significantly correlated with infant cortisol levels at all but the earliest time point. Analysis of salivary cortisol from a subset of animals showed a similar age-related decline and the presence of a significant rearing effect, with lower concentrations in surrogate-peer–reared juveniles, but no sex difference. Salivary and plasma cortisol concentrations in these animals were significantly correlated.     

Effects of daily morphine administration and deprivation on choice and demand for remifentanil and cocaine in rhesus monkeys

Choice procedures have indicated that the relative reinforcing effectiveness of opioid drugs increases during opioid withdrawal. The demand curve, an absolute measure of reinforcer value, has not been applied to this question. The present study assessed whether mild morphine withdrawal would increase demand for or choice of remifentanil or cocaine. Four rhesus monkeys chose between remifentanil and cocaine during daily sessions. Demand curves for both drugs were subsequently obtained. The effects of daily injections of 3.2 mg/kg morphine on both choice and demand for these drugs was assayed 3 and 20.5 hr after each morphine injection, and then during a postmorphine period. Three hours following morphine injections, choice of remifentanil over cocaine decreased and demand for remifentanil--but not cocaine--became more elastic. During morphine withdrawal (20.5 hr postinjection), choice of remifentanil increased and remifentanil demand became more inelastic in 3 of 4 monkeys. Cocaine demand also became more inelastic during this period. Four to five weeks following the morphine regimen, demand for both drugs was more inelastic relative to the initial determination. The results suggest that both the relative and absolute reinforcing effectiveness of remifentanil decreased following morphine administration and increased during morphine withdrawal. The absolute reinforcing effectiveness of cocaine also increased during morphine withdrawal. In addition, extended exposure to drug self-administration and/or exposure to the morphine regimen produced long-term increases in demand for both drugs.     

Similar stimulus features control visual classification in orangutans and rhesus monkeys

Many species classify images according to visual attributes. In pigeons, local features may disproportionately control classification, whereas in primates global features may exert greater control. In the absence of explicitly comparative studies, in which different species are tested with the same stimuli under similar conditions, it is not possible to determine how much of the variation in the control of classification is due to species differences and how much is due to differences in the stimuli, training, or testing conditions. We tested rhesus monkeys (Macaca mulatta) and orangutans (Pongo pygmaeus and Pongo abelii) in identical tests in which images were modified to determine which stimulus features controlled classification. Monkeys and orangutans were trained to classify full color images of birds, fish, flowers, and people; they were later given generalization tests in which images were novel, black and white, black and white line drawings, or scrambled. Classification in these primate species was controlled by multiple stimulus attributes, both global and local, and the species behaved similarly.     

Concurrent nonindependent fixed‐ratio schedules of alcohol self‐administration: Effects of schedule size on choice

Choice behavior was studied under concurrent nonindependent fixed‐ratio fixed‐ratio (nFR) schedules of reinforcement, as these schedules result in frequent changeover responses. With these schedules, responses on either operandum count toward the completion of the ratio requirements of both schedules. Five monkeys were subjects, and two pairs of liquid reinforcers were concurrently available: 16% (w/v) and 0% ethanol or 16% and 8% ethanol. For each pair of reinforcers, the nFR sizes were systematically altered across sessions while keeping the schedule size equal for both liquids. Responding varied as a function of reinforcer pair and nFR size. With the 16% and 0% pair, higher response rates were maintained by 16% and were an inverted U‐shape function of nFR size. With 16% and 8%, a greater number of responses initially occurred on the schedule that delivered 8% ethanol. However, as nFR size increased, preference reversed such that responses that delivered 16% ethanol were greater. When the nFR size was subsequently decreased, preference reverted back to 8%. Number of responses emitted per delivery was a dependent variable and, in behavioral economic terms, was the price paid for each liquid delivery. With 16% and 0%, changeover responses initially increased and then decreased as schedule size became larger. In contrast, with the 16% and 8% pair, changeover responses increased directly with schedule size. Responding under nFR schedules is sensitive to differences in reinforcer magnitude and demonstrates that relative reinforcing effects can change as a function of schedule size.     

Sex and aggression during rhesus monkey group formation

Observational data from 16 newly formed social groups of rhesus monkeys were analyzed to identify patterns of behavior that correlated with the frequency and intensity of aggression directed toward females. The results revealed significant correlations between heterosexual behavior and female-female noncontact aggression and between female homosexual behavior and female-female contact aggression. The role of female homosexual behavior and its relationship to aggression in the formation of new groups is discussed.     

Oral self-administration of pentobarbital by rhesus monkeys: relative reinforcing effects under concurrent fixed-ratio schedules.

During daily 3-hr sessions, orally delivered pentobarbital solutions and water, or two separate pentobarbital solutions, were concurrently available to rhesus monkeys according to fixed-ratio schedules of mouth contacts with a spout. First water, and then each of four "comparison-concentration" pentobarbital solutions (0.0625, 0.25, 1, and 4 mg/mL), was successively available from one spout for a block of sessions under a fixed-ratio-64 (three monkeys) or fixed-ratio-16 (one monkey) schedule. Under an identically sized fixed-ratio schedule, deliveries of a "standard-concentration" pentobarbital solution were concurrently available from a second spout. The concentration of the standard solution remained unchanged throughout testing of the series of comparison solutions. Each of three pentobarbital concentrations (4, 1, and 0.25 mg/mL) in turn served as the standard concentration. Within each pair of concurrently available solutions, the higher drug concentration maintained more behavior than the lower concentration. Thus when monkeys were provided with concurrent access to different pentobarbital concentrations, relative reinforcing effects were directly related to drug concentration. Further, the amount of behavior maintained by a particular drug concentration was dependent on the concentration of the concurrently available drug solution. Thus, the relative effectiveness of a reinforcer in maintaining behavior is a function of both the reinforcer's magnitude and the availability of alternative reinforcers in the environment.