Interaction of methadone, reinforcement history, and variable-interval performance.

In the present study, we examined how a reinforcement schedule history that generated high or low rates of responding influenced the effects of acute (Experiment 1) and chronic (Experiment 2) methadone administration. Initially, key-peck responses of pigeons were maintained under a variable-interval 90-s schedule of food presentation, and a methadone dose-response curve was determined with doses of 0.6, 1.2, and 2.4 mg/kg. The pigeons were then exposed, for at least 40 sessions, to either a fixed-ratio 50 schedule or a differential-reinforcement-of-low-rate 10-s schedule, or were given continued exposure to the variable-interval schedule. The methadone dose-response curve was redetermined after all pigeons again were responding under the variable-interval schedule. The effects of two different daily methadone doses (9.0 and 12.0 mg/kg/day) and withdrawal precipitated by naloxone also were assessed. Experience with a fixed-ratio or differential reinforcement of low rate schedule did not result in significantly different response rates under the variable-interval schedule and, in general, the acute effects of methadone did not have differential effects correlated with schedule history. However, for 2 of 4 subjects the rate-decreasing effects of methadone on rates of key pecking were greater following a history of low-rate responding, suggesting a possible interaction between schedule history and effects of methadone. Daily methadone administration under the variable-interval schedule revealed that pigeons with experience under the differential reinforcement of low rate schedule developed more rapid and complete tolerance to the rate-decreasing effects of methadone. Three of the 4 subjects in this group showed rate increases above drug-free baselines during chronic methadone dosing. Pigeons with a history of fixed-ratio responding also developed tolerance to the rate-decreasing effects of methadone but without the subsequent rate increases seen by subjects with low-rate histories. No subjects with variable-interval histories showed complete recovery of drug-free baselines, suggesting that interpolated training under other schedules may attenuate the rate-altering effects of chronically administered drugs. Naloxone (1.0 mg/kg), administered during the chronic methadone phase, resulted in greater disruption of responding by pigeons with a history of low-rate responding, as compared to subjects in the other two groups.(ABSTRACT TRUNCATED AT 400 WORDS)     

Consumption-leisure tradeoffs in pigeons: Effects of changing marginal wage rates by varying amount of reinforcement

Pigeons' rates of responding and food reinforcement under simple random-ratio schedules were compared with those obtained under comparable ratio schedules in which free food deliveries were added, but the duration of each food delivery was halved. These ratio-with-free-food schedules were constructed so that, were the pigeon to maintain the same rate of responding as it had under the simple ratio schedule, total food obtained (earned plus free) would remain unchanged. However, any reduction in responding would reduce total food consumption below that under the simple ratio schedule. These “compensated wage decreases” led to decreases in responding and decreases in food consumption, as predicted by an economic model of labor supply. Moreover, the reductions in responding increased as the ratio value increased (i.e., as wage rates decreased). Pigeons, therefore, substituted leisure for consumption. The relationship between these procedures and negative-income-tax programs is noted.     

Response-reinforcer relations and the maintenance of behavior

The effects on pigeons' key pecking of unsignaled delays of reinforcement and response-independent reinforcement were compared after either variable-interval or differential-reinforcement-of-low-rate baseline schedules. One 30-min session arranging delayed reinforcement and one 30-min session arranging response-independent reinforcement were conducted daily, 6 hr apart. A within-subject yoked-control procedure equated reinforcer frequency and distribution across the two sessions. Response rates usually were reduced more by response-independent than by delayed but response-contingent delivery of reinforcers. Under both schedules, response rates were lower when obtained delays were greater. These results bear upon methodological and conceptual issues regarding comparisons of contingencies that change the temporal response–reinforcer relations.     

A ROLE FOR NEGATIVE REINFORCEMENT OF RESPONSE OMISSION IN PUNISHMENT?

This experiment attempted to disentangle response-rate reductions controlled by the direct suppressive effects of a punisher from those due to negative reinforcement of response omission. Key-peck responding of pigeons was maintained by a conjoint variable-interval 3-min schedule of food presentation variable-interval 30-s schedule of response-dependent electric shock presentation. Omission of responses for 5, 10, or 30 s resulted in the possibility of canceling a scheduled shock. Response rates were a function of required pause duration, with lower rates occurring when longer periods of response omission were required for shock cancellation. These results show that, with several parameters of punishment held constant, response rates were controlled by the negative reinforcement contingency. Such a finding argues for renewed consideration of the role of negative reinforcement in punishment contingencies.     

A molecular analysis of choice on concurrent-chains schedules.

Six pigeons responded on concurrent-chains schedules with either independent or interdependent equal variable-interval schedules in the initial links and unequal variable-interval schedules, always in a 2:1 ratio, in the terminal links. Relative response rates in the initial links increased across conditions as initial-link duration was shortened and decreased across conditions as terminal-link duration was shortened, replicating previous findings. Responses in the initial links were recorded in 5-s bins, and local or molecular relative response rates were calculated in order to ascertain how relative response rate varied as a function of time since the onset of the initial links. Two distinct molecular patterns were found. With interdependent initial links, relative response rates for the preferred key were elevated for the first 10 or 20 s of the initial links and then declined to an asymptotic value. With independent initial links, a negative recency effect was found similar to that reported by Killeen (1970). These two molecular patterns were related to the different momentary reinforcement probabilities resulting from independent and interdependent scheduling.     

Effects on preference of reinforcement delay, number of reinforcers, and terminal-link duration

Three experiments used concurrent-chains procedures to examine the effects of reinforcement delay, number of reinforcers, and terminal-link duration on preference. In Condition 30 of Experiment 1, food was delivered after 30 seconds in each 150-second terminal link, with four additional food deliveries occurring at 30-second intervals in one of the links. In Condition 5, food was delivered after 5 seconds in each 25-second terminal link, and the four additional reinforcers were delivered at 5-second intervals. Preferences for the multiple-food chain were greater in Condition 30. In Experiment 2, the terminal link(s) providing only one reinforcer terminated immediately after delivery of the reinforcer. Preferences for the multiple-food chain were smaller than in Experiment 1. In Condition 5 of Experiment 3, food was delivered after 5, 75, 100, 125, and 150 seconds in one 150-second link and after 5 seconds in the other. Condition 50 differed only in that the first (or only) reinforcer in each link was delivered after 50 seconds instead of after 5 seconds. Preferences for the multiple-food chain were greater in Condition 50. Results of Experiments 1 and 2 do not correspond to results obtained by Moore (1979).     

Responding during reinforcement delay in a self-control paradigm.

Eight pigeons chose between a small, immediate reinforcer and a large, increasingly delayed reinforcer. Responding during the large-reinforcer delays was examined. During large-reinforcer delays, pecks on one key produced the small, immediate reinforcer; pecks on the other key had no effect. Thus, a pigeon could reverse its initial choice of the large, delayed reinforcer, or it could maintain its original choice. Pigeons that made a relatively high number of initial large-reinforcer choices tended to maintain these choices, and those pigeons that actually received a relatively high number of large reinforcers, tended to respond more frequently on the ineffective key during the delay periods. The findings suggest that some previous studies of self-control training in pigeons may have resulted in increased self-control partially due to a lack of opportunity for the pigeons to change their choices.     

Key pecking in pigeons produced by pairing keylight with inaccessible grain

In Experiment I, keylight was paired with inaccessible grain delivery (under two conditions of keylight intensity) to determine if autoshaping would occur in the absence of primary reinforcement. In Experiment II, the procedure was repeated with accessible grain, for comparison. In Experiment III, the procedures were repeated with explicitly unpaired presentations of keylight and either inaccessible or accessible grain. The results indicated that key pecking occurred as quickly in the presence of keylight pairings with inaccessible grain as with accessible grain, though (except for one bird) key pecking was not maintained with inaccessible grain. Furthermore, compared to the dim keylight, the bright keylight greatly suppressed key pecking when paired with inaccessible grain, and reduced the rate of key pecking when paired with accessible grain. Little key pecking occurred in groups exposed to explicitly unpaired presentations of keylight (whether bright or dim) and grain (whether accessible or inaccessible). When the birds in Experiment III were retested with explicitly paired presentations of keylight and grain, little key pecking was observed, suggesting suppressive effects of prior explicitly unpaired presentations. It is suggested that the effects of key-brightness manipulation were produced by the association of grain with cues other than the response key, or by distraction produced by partial illumination of the grain hopper.     

Behavioral contrast in rats with different reinforcers and different response topographies

Experiment I demonstrated positive behavioral contrast in rats when one of two qualitatively different reinforcers (milk and pellets) was removed from a component of a multiple schedule. The contrast effect was larger and more enduring when milk was removed. Experiment II showed that the rats spent more time on the side of a shuttle-box on which milk was freely available than on the side on which pellets were freely available. Experiment III, a partial replication of Experiment I, failed to demonstrate the contrast effect of Experiment I. Experiment IV demonstrated contrast when two topographically distinct responses, nose-key poking and lever pressing, were required in different components of a multiple schedule. These results extend the conditions that generate behavioral contrast in rats.     

The effects of varying the distribution of generalization stimuli within a constant range upon the bisection of a sound-intensity interval by rats

Two male, albino rats were trained on a two-valued, self-paced, discrete-trials auditory discrimination. In the presence of a high-intensity stimulus (90 decibels SPL, 4 kiloHertz), response A was reinforced; in the presence of a low-intensity stimulus (50 decibels SPL, 4 kiloHertz), response B was reinforced. When discrimination performance was asymptotic, stimuli intermediate in intensity were presented with the training stimuli in a maintained generalization paradigm. Generalization gradients were derived from the relative frequencies of response A in the presence of each stimulus. A relative frequency of 0.50 was then determined and used as the bisection point of the intensity interval defined by the 90- and 50-decibel stimuli. The bisection point varied with the distribution of the stimuli presented in generalization. This effect was similar to context effects seen in human psychophysics.