Variable-interval Responding Research Articles

Traumatic brain injury substantially reduces the conditioned reinforcing effects of environmental cues in rats

Traumatic brain injury affects millions of people each year and is an established risk factor for addiction. Recent animal studies have causally demonstrated that injuries can increase drug self-administration across a variety of substances. One potential behavioral mediator for this finding is an increased responsivity to drug-associated cues. This endophenotype can be identified by profiling non-drug-related behaviors. The current study evaluated several paradigms (conditioned approach, conditioned reinforcement, extinction from variable interval responding, conditioned facilitation) to determine how rats with a frontal TBI differed in their response to Pavlovian conditioning in response to food-paired cues. Surprisingly, rats with a TBI demonstrated increased goal-tracking in a conditioned approach paradigm and exerted less effort for a conditioned reinforcer. Moreover, they had slightly facilitated extinction (as demonstrated by significantly larger interresponse times) in the face of reinforcer-associated cues. Despite these effects, TBI rats still demonstrated conditioned facilitation to an auditory stimulus. Together, these effects suggest a phenotype in the opposite direction of what might be anticipated. Cues still served a strong discriminative function and altered behavior; however, they did not function as strong conditioned reinforcers for TBI animals. One potential reason for this is that substantial changes to the dopamine system after TBI may reduce the conditioned reinforcing effects of cues, but sensitize the brain to potent drugs of abuse. More research will be needed to determine whether this is the case.

Repeated Operant Response Resurgence with a Peak-Interval Procedure

Resurgence is most often defined as the recurrence of previously reinforced behavior during extinction of another, more recently reinforced behavior. It provides a framework for deciphering the origin of operant responses, studying the effects of historical variables, and understanding socially relevant behaviors. The present manuscript proposes a novel procedure that incorporates the peak-interval procedure, typically used to examine temporal control, to examine resurgence repeatedly. Three experimentally naive Long Evans rats were trained using a concurrent variable-interval (VI) 30-s fixed-interval (FI) 30-s schedule, with each schedule programmed on a different lever. After stability was reached, responding under the VI 30-s schedule was extinguished for 15 sessions. A peak-interval procedure then was employed to examine resurgence of the previously VI responding. The peak-interval procedure consisted of FI 30-s trials intermixed with 300-s peak trials, each separated by 20-s blackouts. Resurgence of operant responding was obtained and examined within and across sessions.

AN ANALYTIC COMPARISON OF HERRNSTEIN'S EQUATIONS AND A MULTIVARIATE RATE EQUATION

Herrnstein's equations are approximations of the multivariate rate equation at ordinary rates of reinforcement and responding. The rate equation is the result of a linear system analysis of variable-interval performance. Rate equation matching is more comprehensive than ordinary matching because it predicts and specifies the nature of concurrent bias, and predicts a tendency toward undermatching, which is sometimes observed in concurrent situations. The rate equation contradicts one feature of Herrnstein's hyperbola, viz., the theoretically required constancy of k. According to the rate equation, Herrnstein's k should vary directly with parameters of reinforcement such as amount or immediacy. Because of this prediction, the rate equation asserts that the conceptual framework of matching does not apply to single alternative responding. The issue of the constancy of k provides empirical grounds for distinguishing between Herrnstein's account and a linear system analysis of single alternative variable-interval responding.

Response Rates on Variable-Interval Schedules: Sequential Interresponse Time Requirements and Reinforcement Rate

Pigeons’ response rates on three different sized variable-interval (VI) schedules were determined with three different interresponse time (IRT) contingencies. When the VI schedule arranged a reinforcement, either a single IRT between 1.0 and 1.5 sec. was required to produce the reinforcer or, in other experimental conditions, two or three such IRTs in succession were required to produce the reinforcer. Under these IRT contingencies, the observed response rates were not related to reinforcement rates (size of the VI) in any obvious orderly way. When only the final IRT concluded by the response immediately preceding each reinforcement was required to be between 1.0 and 1.5 sec. duration, the mean emitted IRT usually was not between 1.0 and 1.5 sec. When the next back IRT also was required to be between 1.0 and 1.5 sec., the mean emitted IRT was very near or between 1.0 and 1.5 sec. Results suggest that a comprehensive account of VI responding which emphasizes IRT reinforcement should consider not only the duration of the IRT immediately preceding each reinforcement but also the IRTs of preceding responses.

Variable interval responding maintained by intravenous codeine and ethanol injections in the rhesus monkey

Rhesus monkeys were trained to respond under a variable interval 2 min schedule for codeine or ethanol injections. Both codeine and ethanol were effective in the initiation of variable-interval responding; responding was maintained over a range of codeine (0.003–1.0 mg/kg/injection) and ethanol doses (32.0–560 mg/kg/injection). Maximum rates of responding were obtained at the 0.01 mg/kg/injection codeine dose (0.14 responses/sec) and at the 180 mg/kg/injection codeine dose (0.19 responses/sec). Rates of responsing were bitonic functions of the reinforcer dose for both codeine and ethanol; maximum rates were obtained at intermediate doses and lower rates occurred at the extremes of the dose range. Both codeine and ethanol showed within-session decreases in responding across the range of reinforcer doses. Codeine-reinforced responding declined in rate within the one-hour session without a similar change in the frequency of drug injection; in contrast, both ethanol-reinforced responding and the frequency of ethanol injections declined within each session across a range of doses. Increasing or decreasing the codeine dose half-way through the one-hour session resulted in increases or decreases in codeine responding compared to controls. These data indicate that the progressive decline in codeine-reinforced responding is not the result of a generalized disruption of responding.

Behavioral contrast in the pigeon depends upon the location of the stimulus

Three pigeons were exposed to a series of multiple schedules of reinforcement-both multiple variable interval extinction, and multiple variable interval variable interval. The stimuli which signaled the two components of the multiple schedule were sometimes located on the response key, and sometimes removed from the key. Positive behavioral contrast (increases in variable-interval responding which accompany decreases in extinction responding) was only observed when the signal stimuli were on the response key. These findings are consistent with the view that positive contrast results from the introduction of a differential stimulus-reinforcer dependency which directs behavior toward the stimulus, as in the phenomenon of autoshaping.

Long-term prereward suppression in monkeys unaccompanied by cardiovascular conditioning.

In Experiment I, a 3-min tone that preceded a free pellet of food suppressed variable-interval performances maintained by the same type of pellets, but failed to elicit conditioned changes in the heart rates and blood pressures of two rhesus monkeys. Initially severe, the prereward suppression became temporally discriminated to progressively later portions of the tone, and was maintained at an attenuated level for over four months. The suppression was apparently not caused by interfering autonomic respondents, nor was it superstitiously conditioned, since 21 of the initial 25 tone-food pairings took place outside of baseline sessions. In Experiment II, a 1-min light, paired with four free pellets of food, suppressed the variable-interval responding of a second pair of similarly trained monkeys. An interresponse-time analysis showed that in one subject, mild prereward suppression of responding developed through two stages. On early trials, response rate slowed by 10% throughout the prefood interval. On later trials, the animal suppressed by pausing for a like portion of the interval, most often near the end, but otherwise responded normally during the prefood signal.

Operant behavior and catecholamine-containing neurons: Prolonged increase in lever-pressing after 6-hydroxydopamine

Intraventricular administration of 6-hydroxydopamine (6-HDA) to rats produced a 4-fold increase in responding maintained by a variable interval (VI) schedule of reinforcement. This effect occurred whether 6-HDA was administered prior to training or after four weeks of training, and persisted for the duration of the experiment (over 5 months). The addition of a time-out or shock component to the VI schedule, procedures which decreased responding in the control groups, was effective in suppressing the high response output of the 6-HDA-treated rats. The increase in VI responding may be a consequence of the production by 6-HDA of a central supersensitivity to catecholamines.

Effects of deprivation and prefeeding on variable interval responding

Three pigeons with long operant conditioning histories were studied under a variable interval schedule as deprivation was varied. Periods of food deprivation up to 72 h and prefeeding up to 20 g of food had little effect upon responding. One pigeon did show a significant decrease in responding following a 10% increase in body weight. These findings indicate that variable interval responding in pigeons can become relatively insensitive to changes in deprivation.

Drug effects in squirrel monkeys trained on a multiple schedule with a punishment contingency.

The behavior of four monkeys trained on a multiple schedule was differentially sensitive to selected pharmacological agents. The three components of the multiple schedule were: (1) a variable-interval schedule in which responses were reinforced on the average of once per minute; (2) a concurrent schedule in which every tenth response was reinforced and every fifteenth response, on the average, was shocked; and, (3) a neutral stimulus in the presence of which responses were neither reinforced nor shocked. Pentobarbital, chlordiazepoxide, and meprobamate increased responding during each of the components. Scopolamine and d-amphetamine decreased variable-interval performance, had minimal effects on performance during the concurrent-schedule component, and increased responding in the presence of the neutral stimulus. Chlorpromazine decreased variable-interval responding and had slight effects on the responding during the other two components.

Conditioning history and maladaptive human operant behavior.

Two experiments are presented which examined the effects of different past experiences (histories of conditioning) upon maladaptive human behavior in an operant conditioning laboratory situation. In Exp. I, unnecessary punished responding under a fixed-interval reinforcement schedule was shown to result from a history of fixed-ratio conditioning. In Exp. II low rates of variable-interval responding which produced unnecessary reinforcement loss followed a history of conditioning under a differential reinforcement of low rate schedule.