Variable-interval 1-min Research Articles

AND and AND-OR drug mixture discriminations in rats: generalization to single drugs and drug mixtures.

Studies of the discriminative stimulus effects of drug mixtures provide an approach to polydrug abuse and studies on single drugs with multiple effects. This study was designed to investigate whether the use of the AND-OR procedure increases the specificity of drug mixture discriminations. Rats were trained to discriminate a mixture of amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) from saline (AND-discrimination, n = 8) or to discriminate the same mixture from its component drugs alone (AND-OR discrimination, n = 9). The studies used two-lever operant procedures with a tandem variable interval 1-min fixed ratio 10 schedule of food reinforcement. Under AND-discrimination conditions, there was partial generalization to nicotine and midazolam when each drug was administered singly, and there was no generalization to cocaine, caffeine or ethanol. With the AND-OR discrimination, there was no generalization to any of the preceding drugs administered singly. In "single substitution" tests, nicotine or midazolam was co-administered with the training doses of pentobarbitone and amphetamine, respectively; there was full generalization in the AND-discrimination and partial generalization under AND-OR conditions. Cocaine co-administered with pentobarbitone generalized fully under both procedures, but the dose of cocaine needed was much larger in the AND-OR than in the AND-discrimination. In "dual substitution" tests, mixtures of two novel substances were tested. Mixtures of either nicotine plus midazolam or caffeine plus ethanol produced very marked generalization under AND-discrimination conditions, but were without significant effect in the AND-OR procedure. Throughout the studies, in every instance where comparisons were made, generalization was greater or occurred at lower doses under AND- than under the AND-OR discrimination. The study yielded extensive evidence supporting the hypothesis that the AND-OR discrimination procedure increases the specificity of discriminations based on drug mixtures.

The effects of stopping and restarting a session on within-session patterns of responding

Rats pressed levers for sweetened condensed milk reinforcers delivered according to a multiple variable-interval 1-min, variable-interval 1-min schedule during 60-min baseline sessions. The obtained pattern of responding was an early-session increase in responding followed by a relatively constant rate of responding during the remainder of the session. In the experimental conditions of experiment 1, sessions were interrupted by a blackout or timeout lasting either 5, 10, or 30 min. Responding following 5- or 10-min interruptions resembled the constant rate of responding late in the baseline sessions. However, responding increased before 30-min interruptions and increased again after the interruption. This change in the pattern of responding was best described as a `restarting' of the within-session pattern of responding. In experiment 2, a 30-min blackout interrupted sessions. However, reinforcers were presented intermittently during the blackout. Responding did not restart following the 30-min blackout-with-reinforcers condition. These results suggest that the mere presentation of reinforcers is sufficient to maintain within-session changes in responding.

Within-session responding as a function of force required for lever press

Four male Sprague-Dawley rats pressed a lever for food reinforcers delivered by a multiple variable interval 1-min variable interval 1-min schedule. Components alternated every 5 min. The force required to move the lever was 0.25, 0.15, 1.10, 0.75, and 0.50 N in different conditions. Changes in the force requirement did not alter the within-session pattern of responding. These results are consistent with other experiments suggesting that muscle warm-up and fatigue are not sufficient explanations for the within-session pattern of responding.

Within-session patterns of responding when the operandum changes during the session

Eight pigeons and 10 rats responded on variable interval 1-min or variable interval 30-s schedules. During baseline, the same operandum produced reinforcers throughout the session. During the switch conditions, performing one response produced reinforcers early in the session and performing a different response produced reinforcers later. The switch between the two responses occurred 10, 20, 30, or 40 min into the session in different conditions. Within-session patterns of responding were not usually different when the same operandum produced reinforcers throughout the session and when the operandum changed from the right to the left key (pigeons) or lever (rats). Changes from a treadle to a key (pigeons) or from a lever to a key (rats) also failed to alter within-session patterns of responding. These results suggest that factors related to the form of the instrumental response (e.g., muscular warm-up, fatigue) contributed little to within-session changes in responding under the present circumstances.

Prospective factors contribute little to within-session changes in responding

Five rats pressed levers for food delivered by a multiple variable interval 1-min variable interval 1-min schedule. In theunpredictable conditions, sessions were 20, 40, 60, 80, or 100 min long, determined randomly at the beginning of each session. In thepredictable conditions, each of these session durations was presented for 15 consecutive sessions. Rate of responding changed systematically within the session even when the end of the session was unpredictable. This implies that prospective factors related to anticipation of the end of the session are not necessary for producing withinsession changes in responding. Within-session patterns of responding were also similar for the predictable and unpredictable conditions. This suggests that prospective factors contributed little to the form of the within-session patterns under the present conditions.

The effect of time between sessions on within-session patterns of responding

Four pigeons pecked keys for food reinforcers delivered by variable interval 1-min schedules during two successive 50-min sessions or one 100-min session. When 50-min sessions were conducted, they were separated by a 0-, 10- or 30-min delay, spent either inside or outside of the experimental enclosure. Responding usually increased to a peak and then decreased within sessions. This pattern was not altered by either the length of the delay between sessions or by the place where the subjects spent the delay. These results suggest that the beginning of the session or the passage of a short time between sessions restores the conditions necessary to produce within-session changes in responding. The results are incompatible with theories that explain these changes in terms of recovery from the handling routine, accumulation of arousal, priming, fatigue, or satiation. They can be reconciled with theories that attribute within-session changes in responding to reinstatement of memory or changes in attention only by arguing that memory and attentional adjustments are erased quickly between sessions.

Rate of reinforcement and session duration as determinants of within-session patterns of responding

Rats pressed levers for Noyes pellets or keys for sweetened condensed milk reinforcers delivered by multiple schedules. Session length and baseline rates of reinforcement were varied in two experiments. Rates of responding increased during the early part of the session and then decreased for both responses and reinforcers, as well as for all subjects and values of the independent variables. Changes in response rates across the session sometimes exceeded 500%. Respoiise rates peaked approximately 20 min after the beginning of the session, regardless of session duration, when subjects responded on a multiple variable interval 1-min variable interval 1-min schedule. The function was flatter for longer sessions than it was for shorter sessions. The function was flatter, more symmetrical, and peaked later for lower rates of reinforcement than for higher rates of reinforcement. The function appeared early in training, and further experience moved and reduced its peak. Variables related to reinforcement exerted more control over some aspects of this function than did variables related to responding. These within-session patterns of responding may have fundamental implications for experimental design and theorizing.

DSP4 alters the effect of d-amphetamine on variable-interval performance: analysis in terms of Herrnstein's equation.

The effect of d-amphetamine (0.1-3.2 mg/kg) on performance in variable-interval 1-min and variable-interval 12-min schedules of positive reinforcement was examined in ten rats treated with the selective noradrenaline neurotoxin DSP4 and 12 control rats. In the control group d-amphetamine had a dose-dependent suppressant effect on response rates maintained under variable-interval 1-min; under variable-interval 12-min, response rates were increased by low doses and suppressed by higher doses of the drug. In the case of both schedules, lower doses of d-amphetamine were more suppressant and higher doses less suppressant in the DSP4-treated group than in the control group. The levels of noradrenaline in the parietal cortex, hippocampus and cerebellum (determined by high-performance liquid chromatography) were reduced to approximately 15% of control levels in the DSP4-treated rats. The results indicate that treatment with DSP4 attenuated both the facilitatory and the suppressant effects of d-amphetamine on variable-interval performance. A formal model couched in terms of Herrnstein's (1970) equation is put forward to account for these results. It is suggested that the noradrenergic pathways emanating from the locus caeruleus are involved in both the facilitatory and suppressant effects of d-amphetamine on operant behaviour.

Rate of Change in Reinforcement Density as a Parameter of Behavioral Contrast

One component (variable component) of a three-ply multiple schedule was gradually changed over 55 sessions from variable-interval (VI) 1-min. to VI 5-min. by exposing pigeons to the following sequence of values: VI 1, VI 2, VI 3, VI 4, VI 5-min. Subsequently, the reinforcement density in a second component (following component) was abruptly changed from VI 1-min. to VI 5-min. The effects of both gradual and abrupt reductions in reinforcement density were examined in the third component (constant component) in which the schedule remained unchanged. For all subjects, the greatest increase over baseline values occurred during the abrupt reduction in reinforcement rate.

Inhibitory Stimulus Control Under Conditions of Signalled and Unsignalled Delay of Reinforcement

In Experiment 1, pigeons received delayed reinforcement during one component of a multiple variable-interval (VI) 1-min. variable-interval 1-min. schedule of food reinforcement. One unsignalled and four signalled delay schedules were examined, but in all cases a plain white key was associated with the immediate reinforcement schedule, and a black vertical line on a white background was associated with the delayed reinforcement schedule. The incremental generalization gradients that were obtained along the line-orientation dimension suggested that the stimulus associated with each delayed reinforcement schedule was exerting inhibitory control. The unsignalled delay schedule, however, generally maintained lower rates of responding than the signalled delay schedules. In addition, unsignalled delay did not produce behavioral contrast. In Experiment 2, pigeons were trained on a multiple schedule containing VI 1-min. schedules with signalled and unsignalled reinforcement delay. All subjects responded more slowly during the unsignalled than the signalled delay schedule, and the incremental generalization gradients that were obtained suggested that the stimulus associated with an unsignalled delayed reinforcement schedule was exerting inhibitory control.

Inhibitory control by one stimulus after an increase in the frequency of reinforcement associated with another stimulus

Pigeons were trained on a multiple variable-interval 5-min variable-interval 5-min schedule and then shifted to either a multiple variable-interval 1-min variable-interval 5-min or a multiple variable-interval 30-sec variable-interval 5-min schedule. A generalization test was subsequently administered along the dimension containing the stimulus associated with the variable-interval 5-min component. The generalization gradients for subjects that received multiple variable-interval 1-min variable-interval 5-min training were not consistent in shape. However, an incremental gradient was obtained from each subject that received multiple variable-interval 30-sec variable-interval 5-min training. Thus, a sufficiently large reduction in merely the relative frequency of reinforcement during a stimulus resulted in that stimulus’ acquiring inhibitory control over responding.

Rates of responding in the pigeon generated by simple and complex schedules which provide the same rates of reinforcement

Four pigeons pecked for food reinforcement on variable interval 1-min schedules and on the variable-interval 1-min components of multiple, concurrent, and pseudoconcurrent schedules. The pseudoconcurrent schedule provided only one schedule of reinforcement; but, any reinforcer could be collected by responding on either of two keys. The rate of responding generated by the variable interval schedule was not greater than the rates of responding generated by the components of the complex schedules. But, the rate of reinforcement obtained from the variable interval schedule was greater than the rates of reinforcement obtained from the components of the multiple schedule. These results may contradict the equation proposed by Herrnstein (1970). The equation predicts that the rate of responding generated by a schedule of reinforcement will be greater when the schedule appears alone, than when it appears as one component of a complex schedule.

Interactions Between Lever Pressing and Collateral Drinking During VI with Limited Hold

Lever pressing by rats was maintained by food pellets delivered according to a variable-interval 1-min. schedule. Water was concurrently available. The rate of lever pressing increased, the rate of water drinking decreased, and the rate of reinforcement for lever pressing decreased when a limited-hold contingency was added to the variable-interval schedule. The data suggest that interactions between collateral responses and reinforced responses have to be considered when either response is investigated.

Frequency of reinforcement as a determinant of extinction-induced aggression during errorless discrimination learning.

Seven pigeons were trained to discriminate without errors between a green keylight and a dark key. The key-pecking response was reinforced in the presence of green, and extinction was in effect in the presence of the dark key. The opportunity to attack a restrained target pigeon was present only during extinction. Both variable-interval 30-sec and fixed-ratio 1 schedules of reinforcement during the positive stimulus induced a higher rate of attack during extinction than a variable-interval 5-min schedule. The highest rate of attack during extinction occurred during the first 20 sec after the positive stimulus terminated. Hence, the withdrawal of the positive condition, rather than the consequences of the pecking response during extinction, appears to be one of the primary factors responsible for attack between pigeons during extinction. Behavioral contrast, defined as a decrease in the rate of responding when the positive stimulus was presented alone, was obtained from the four birds that displayed the lowest overall rates of attack while the three birds with the highest attack rates did not display behavioral contrast. For the birds without contrast, components of the attack response during the positive stimulus presumably competed with and reduced the rate of pecking the key, thereby recluding behavioral contrast.

PEAK SHIFT AS A FUNCTION OF MULTIPLE SCHEDULES OF REINFORCEMENT1

Pigeons were trained to respond to two stimuli on the wavelength continuum, 550 nm and 570 nm, each correlated with an independent schedule of reinforcement. The multiple schedule component in effect during 550 nm (S1) was always a variable-interval 1-min. During the 570-nm stimulus (S2) the second component of the schedule was either variable-interval 30-sec, 1-min, 2-min, 5-min, or extinction for different groups of birds. Generalization gradients were obtained after this training, with the following results: (1) response rate to S1 during training was related to the reinforcement frequency associated with S2; the distribution of responding during generalization testing was a function of the schedules of reinforcement used during training and the response rates they produced. Decreases in the relative frequency of reinforcement correlated with S2 resulted in increases in the distribution shift of responses away from S2 during generalization testing.

Contrast as a function of component duration

Behavioral contrast was studied as a function of the duration of two components of a multiple schedule: a constant component (a variable-interval 1-min schedule) alternated either every 5 sec or every 180 sec with a variable component (a variable-interval 1-min or an extinction schedule). The rate at which three pigeons pecked for food during the constant component revealed a strong reinforcement interaction, when component duration was 5 sec, but not when component duration was 180 sec. The results add to previous findings that component duration in concurrent and concurrent-like schedules of reinforcement affects both preference for a component and reinforcement interactions between components.

A LOCAL‐RATE‐OF‐RESPONSE AND INTERRESPONSE‐TIME ANALYSIS OF BEHAVIORAL CONTRAST1

Three pigeons were exposed to a two-component multiple schedule in which a variable-interval 3-min schedule was always in effect in one component. The schedule in the other component was either variable-interval 3-min or extinction in alternate blocks of sessions. When the schedule was changed from multiple variable-interval 3-min variable-interval 3-min to multiple variable-interval 3-min extinction in the second and fourth phases of the experiment, overall response rates in the unchanged variable-interval 3-min component increased in two pigeons. Response rate declined when the schedule was changed to multiple variable-interval 3-min variable-interval 3-min again. Correlated with increases in overall response rate in the unchanged component were increases in local response rates at the beginning of the unchanged component and immediately after food presentation. Local rates 40 sec after food presentation did not increase greatly in the presence of the multiple variable-interval 3-min extinction schedule. An interresponse time analysis of three local rate samples showed small increases in the relative frequency of short-duration interresponse times at the beginning of the unchanged component and immediately after food presentation. Neither the postreinforcement pause nor the latency to the first response in the unchanged component changed systematically.

CONTRAST EFFECTS IN MAINTAINED GENERALIZATION GRADIENTS1

In Experiment I, pigeons were given equal reinforcement (variable-interval 1-min) for responding during randomized presentations of eight line-orientation stimuli. Then, only responding in the vertical orientation was reinforced. Stable generalization gradients soon formed and persistent behavioral and local (transient) contrast effects appeared. Local contrast effects were not a function of relative reinforcement frequency or of any other variable known to produce contrast. Instead, they were related to average response rates associated with each stimulus. Experiment II showed that local contrast effects represent increases and decreases in response rates relative to baseline responding, and that these effects are relative; a given stimulus might enhance responding during a subsequent presentation of one stimulus, but depress responding when followed by another. These data indicate that discrimination learning is not adequately described as the acquisition of excitatory properties by some stimuli and inhibitory properties by others. A more adequate account implies that stimuli exert both excitatory and inhibitory effects related to their value.

SCHEDULE‐INDUCED LICKING DURING MULTIPLE SCHEDULES1

Schedule-induced polydipsia was studied in rats bar pressing under two-component multiple schedules of food reinforcement. The first component of the multiple schedule was a variable-interval 1-min schedule throughout the experiment. The schedule comprising the second component was varied over blocks of sessions in terms of rate and magnitude of reinforcement, and was either variable-interval 3-min (one pellet), variable-interval 3-min (three pellets), variable-interval 1-min (one pellet), or extinction. Water intake per session varied with the rate of reinforcement in the schedule comprising the second component and was highest when the schedule was variable-interval 1-min. Both bar-pressing behavior and licking behavior showed behavioral interactions between the two components of the multiple schedules. With magnitude of reinforcement held constant, a matching relationship was observed between lick rate and reinforcement rate; the relative frequency of licks in the constant component matched the relative frequency of reinforcement in that component. Bar pressing, however, showed only a moderate degree of relativity matching. During the schedule-induced licking, a burst of licking followed each delivery of a pellet (post-prandial drinking). The duration of these bursts of licking was observed to be a function of the inter-reinforcement interval.

TESTING FOR INHIBITORY STIMULUS CONTROL WITH S− SUPERIMPOSED ON S+1

Pigeons learned a successive discrimination between a positive stimulus (red) correlated with a variable-interval 1-min reinforcement schedule and a negative stimulus (vertical line) correlated with either a variable-interval 5-min schedule or extinction. Transfer tests measured the rate of responding to the positive stimulus alone, to various orientations of the negative stimulus, and to the same line orientations superimposed on the positive stimulus. Although there were no gradients with minima at the training value for the negative stimulus dimension, the addition of the negative stimulus dimension to the positive stimulus always resulted in a lower response rate than that for the positive stimulus alone. The results demonstrate that an operational definition of inhibitory stimulus control that requires increased responding to stimuli more distant from a negative stimulus (along some dimension) is not always consistent with a definition that requires the suppression of responding in the presence of one stimulus, the positive stimulus, by the simultaneous presentation of another, the negative stimulus.