Effects of lick-contingent timeout on schedule-induced polydipsia

Rats bar pressing on a 1-min fixed-interval schedule for 45-mg food pellets became polydipsic when water was concurrently available. They were then exposed to conditions in which each lick on the drinking tube produced a timeout period during which the food-schedule lever was retracted and the fixed-interval timer either did or did not continue to operate. Licks occurring within a timeout period extended its duration. As the duration of the lick-initiated timeout period was increased logarithmically through four values from 10 sec to 80 sec, lick rates as well as water intake rates generally decreased for all three subjects. As timeout duration was progressively increased, the rate of licks occurring in the absense of, but producing, timeouts decreased for all three rats, whereas the rate of licks occurring in the presence of timeout periods remained essentially constant. Water-intake rates and, with one exception, lick rates were suppressed more by timeout periods during which the fixed-interval timer did not continue to operate. These results indicate that lick-contingent timeout from positive reinforcement reduces schedule-induced drinking, and this suppressive effect is greater when the timeout period necessarily increases the interreinforcement interval beyond its minimum duration than when it does not.     

Conjunctive schedules of reinforcement: I. Rate-dependent effects of pentobarbital and d-amphetamine

Key pecking of pigeons was maintained under conjunctive schedules of food presentation in which both a fixed-interval and a fixed-ratio schedule had to be completed before a peck produced food. For two pigeons, pecks on a single key completed both schedule requirements (fixed-interval 3-min, fixed-ratio 50 for one bird, fixed-interval 5-min, fixed-ratio 50 for the second). For two other pigeons, each requirement was scheduled on a separate key. On the two-key schedule, a peck after 5 min on the key scheduling the fixed-interval requirement produced food if at least 10 pecks had occurred on the ratio key (conjunctive fixed-interval 5-min, fixed-ratio 10). When each requirement was scheduled on a separate key, response rates on the fixed-ratio key were generally higher in the early portion of the interval and declined as the interval progressed; responding on the fixed-interval key, once initiated, typically remained at a constant rate throughout the interval. Responding under the single-key schedule was characterized by a high rate early in the interval; this then changed to a lower rate that continued until a peck produced food. For all pigeons, increases in response rates with pentobarbital and d-amphetamine were inversely related to the control rate of responding. When equivalent rates on each key of the two-key schedule were compared, both drugs increased rates on the fixed-ratio key less. Although the effects of both drugs were rate dependent, each drug differentially modified the pattern of responding under the single-key schedule.     

Motivational aspects of escape from punishment

Punishment and escape were studied simultaneously by allowing a subject to escape from a stimulus situation in which responses were punished, into a stimulus situation in which responses were not punished. The frequency of the punished responses was found to be an inverse function of the intensity of punishment, whereas the frequency of the escape response was a direct function of the intensity of punishment. Both of these functions were obtained under three different schedules of food reinforcement. The strength of the escape behavior was evidenced by (1) the emergence of the escape response even when the frequency of food reinforcement decreased as a consequence of the escape response, (2) the maintenance of the escape response by fixed-interval and fixed-ratio schedules of escape reinforcement, and (3) the occurrence of escape responses at intensities of punishment that otherwise produced only mild suppression of the punished response when no escape was possible. This last finding indicates that a subject may be driven out of a situation involving punishment even though the punishment is relatively ineffective in suppressing the punished responses when no escape is possible.     

Rates and patterns of responding with concurrent fixed-interval and variable-interval reinforcement

Pigeons were exposed to concurrent fixed-interval and variable-interval schedules of food reinforcement on two keys. The times between reinforcement were varied systematically on both keys. The overall relative frequency of responding on the fixed-interval key depended on the relative frequency of reinforcement, but did not match it. Instead, the ratio of responses on the fixed-interval key to responses on the variable-interval key was a power function of the ratio of reinforcements, with an exponent of 0.5. Patterns of responding between reinforcements on the fixed-interval key depended on both relative and absolute values of the reinforcement schedules. Similar overall relative responding was obtained at different absolute schedule values with equal relative reinforcement, despite some differences in patterns of responding.     

Procrastination by pigeons with fixed-interval response requirements.

Two experiments studied the phenomenon of procrastination, in which pigeons chose a larger, more delayed response requirement over a smaller, more immediate response requirement. The response requirements were fixed-interval schedules that did not lead to an immediate food reinforcer, but that interrupted a 55-s period in which food was delivered at random times. The experiments used an adjusting-delay procedure in which the delay to the start of one fixed-interval requirement was varied over trials to estimate an indifference point--a delay at which the two alternatives were chosen about equally often. Experiment 1 found that as the delay to a shorter fixed-interval requirement was increased, the adjusting delay to a longer fixed-interval requirement also increased, and the rate of increase depended on the duration of the longer fixed-interval requirement. Experiment 2 found a strong preference for a fixed delay of 10 s to the start of a fixed-interval requirement compared to a mixed delay of either 0 or 20 s. The results help to distinguish among different equations that might describe the decreasing effectiveness of a response requirement with increasing delay, and they suggest that delayed reinforcers and delayed response requirements have symmetrical but opposite effects on choice.     

Fixed-interval behavior: effects of percentage reinforcement

The percentage of fixed intervals terminating with food presentation was varied parametrically. Intervals that did not end with food were terminated by a stimulus uncorrelated with food presentation (a timeout stimulus). In Experiment I, the pigeons' response rates were an inverted U-shaped function of the percentage of food presentations: decreasing the percentage from 100% to 90%, 70%, or 50% produced an increase in response rates; lower percentages decreased the rates. The patterns of responding in the 100% condition differed from those of the other conditions. In Experiment II, the chamber was darkened after food presentations and timeouts. Response rate was directly related to the percentage of food presentations: decreasing the percentage decreased the response rate. Characteristic fixed-interval patterns of responding were maintained as long as there were occasional food presentations; pausing followed by positively-accelerated responding occurred in percentage conditions ranging from 7% to 100%. The ability to maintain fixed-interval performance with percentage reinforcement suggested that the behavioral sequences occurring in each interval may operate as unitary responses.     

Food deliveries during the pause on fixed-interval schedules

Pigeons were trained on fixed-interval schedules of food delivery. In Experiments I and II, the fixed interval was initiated by the previous fixed-interval reinforcer; in Experiment III, the fixed interval was initiated by the first key peck following the preceding fixed-interval reinforcer (a chain fixed-ratio one, fixed-interval schedule). During the postreinforcement pause, variable-time schedules delivered food independent of any specific response. Rate of food delivery during the pause had only small effects on pause duration in Experiments I and II. In Experiment III, however, pause duration increased systematically with the rate of food delivery during the pause. These data suggest that the momentary proximity to reinforcement delivered via the fixed-interval schedule exerts potent control over pause termination. Additional analysis revealed that pause termination was unaffected by the intermittent delivery of food during the pause. Such data suggest that the temporal control by fixed-interval schedules is highly resistant to interference.     

Second-order schedules: brief shock at the completion of each component

Pigeons worked on second-order schedules in which completion of fixed-interval component schedules was reinforced with food according to a variable-interval schedule of reinforcement. The completion of each fixed-interval component resulted in the presentation of a brief electric shock. In one condition (shock-paired), the completion of every fixed-interval component, including those that ended in food, resulted in the shock. In another condition (shock-nonpaired), completion resulted in shock except for those components that ended in food. Shock presentations resulted in a positively accelerated rate within fixed-interval components. This patterning within components was similar whether the shock was intermittently paired with food or not. Response rates tended to decrease as shock intensity increased. The characteristic fixed-interval response pattern within components did not occur when shock presentations were omitted at the end of each component (tandem schedule). When shocks were scheduled but food was no longer presented (extinction) response rates declined to a near-zero level. The performance under shock conditions is similar to that in other studies in which visual and auditory stimuli are presented at the completion of component schedules.     

Changing the response unit from a single peck to a fixed number of pecks in fixed-interval schedules

Each of three pigeons was studied first under a standard fixed-interval schedule. With the fixed interval held constant, the schedule was changed to a second-order schedule in which the response unit was the behavior on a small fixed-ratio schedule (first a fixed-ratio 10 and then a fixed-ratio 20 schedule). That is, every completion of the fixed-ratio schedule produced a 0.7-sec darkening of the key and reset the response count to zero for the next ratio. The first fixed-ratio completed after the fixed-interval schedule elapsed produced the 0.7-sec blackout followed immediately by food. These manipulations were carried out under two different fixed-interval durations for each bird ranging from 3 min to 12 min. The standard fixed-interval schedules produced the typical pause after reinforcement followed by responding at a moderate rate until the next reinforcement. The second-order schedules also engendered a pause after reinforcement, but responding occurred in bursts separated by brief pauses after each blackout. For a particular fixed-interval duration, post-reinforcement pauses increased slightly as the number of pecks in the response unit increased despite large differences in the rate and pattern of key pecking. Post-reinforcement pause increased with the fixed-interval duration under all response units. These data confirm that the allocation of time between pausing and responding is relatively independent of the rate and topography of responding after the pause.     

Preference for mixed-interval versus fixed-interval schedules

Pigeons were trained on a two-link concurrent chain schedule in which responses on two keys were reinforced according to independent variable-interval schedules by the production of a change in key color. Further responses on the key on which the stimulus change had been produced gave a single food reinforcement and a return to concurrent variable-interval conditions. On one key the terminal link was a two-valued mixed-interval schedule, while on the other, the terminal link was a fixed-interval schedule. When the mixed-interval values were kept constant and the fixed-interval values varied, relative response rates in the initial concurrent links matched relative reinforcement rates in the terminal links when these were computed from cubic transformations of the reciprocals of the intervals comprising the terminal link schedules.     

Second-order schedules: discrimination of components

Pigeons were exposed to a series of second-order schedules in which the completion of a fixed number of fixed-interval components produced food. In Experiment 1, brief (2 sec) stimulus presentations occurred as each fixed-interval component was completed. During the brief-stimulus presentation terminating the last fixed-interval component, a response was required on a second key, the brief-stimulus key, to produce food. Responses on the brief-stimulus key before the last brief-stimulus presentation had no scheduled consequences, but served as a measure of the extent to which the final component was discriminated from preceding components. Whether there were one, two, four, or eight fixed-interval components, responses on the brief-stimulus key occurred during virtually every brief-stimulus presentation. In Experiment 2, an attempt was made to punish unnecessary responses on the brief-stimulus key, i.e., responses on the brief-stimulus key that occurred before the last component. None of the pigeons learned to withhold these responses, even though they produced a 15-sec timeout and loss of primary reinforcement. In Experiment 3, different key colors were associated with each component of a second-order schedule (a chain schedule). In contrast to Experiment 1, brief-stimulus key responses were confined to the last component. It was concluded that pigeons do not discriminate well between components of second-order schedules unless a unique exteroceptive cue is provided for each component. The relative discriminability of the components may account for the observed differences in initial-component response rates between comparable brief-stimulus, tandem, and chain schedules.     

Concurrent fixed-interval variable-ratio schedules and the matching relation

Five rats responded under concurrent fixed-interval variable-ratio schedules of food reinforcement. Fixed-interval values ranged from 50-seconds to 300-seconds and variable-ratio values ranged from 30 to 360; a five-second changeover delay was in effect throughout the experiment. The relations between reinforcement ratios obtained from the two schedules and the ratios of responses and time spent on the schedules were described by Baum's (1974) generalized matching equation. All subjects undermatched both response and time ratios to reinforcement ratios, and all subjects displayed systematic bias in favor of the variable-ratio schedules. Response ratios undermatched reinforcement ratios less than did time ratios, but response ratios produced greater bias than did time ratios for every subject and for the group as a whole. Local rates of responding were generally higher on the variable-ratio than on the fixed-interval schedules. When responding was maintained by both schedules, a period of no responding on either schedule immediately after fixed-interval reinforcement typically was followed by high-rate responding on the variable-ratio schedule. At short fixed-interval values, when a changeover to the fixed-interval schedule was made, responding usually continued until fixed-interval reinforcement was obtained; at longer values, a changeover back to the variable-ratio schedule usually occurred when fixed-interval reinforcement was not forthcoming within a few seconds, and responding then alternated between the two schedules every few seconds until fixed-interval reinforcement finally was obtained.     

Schedule-induced biting under fixed-interval schedules of food or electric-shock presentation

Squirrel monkeys pressed a lever under fixed-interval schedules of food or of electric-shock presentation. Both schedules induced repeated biting on a latex hose. Whether lever pressing was controlled by food or by electric shock, a pattern of decreasing hose biting and increasing lever pressing occurred within fixed-interval cycles. As the fixed-interval duration was increased from 6 to 600 sec, average rates of lever pressing decreased under both schedules. Average rates of hose biting first increased with increasing parameter value, reaching a maximum at values that varied from 60 to 337 sec in different monkeys, and then declined at higher values. d-Amphetamine at appropriate doses increased overall rates of lever pressing maintained by food or by shock, but either did not affect or decreased overall rates of hose biting. When no timeout period occurred between fixed-interval cycles, the monkeys bit most frequently immediately after food or electric shock was presented. When there was a timeout period, hose biting began shortly after the start of the fixed-interval cycles, with little or no hose biting immediately after food or electric shock was presented. Most hose biting appeared to be schedule-induced rather than food- or shock-induced.     

On the form of the relation between response rates in a multiple schedule

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.     

Alternative fixed-ratio fixed-interval schedules of reinforcement

Five rats were trained under alternative fixed-ratio fixed-interval schedules, in which food reinforcement was provided for the completion of either a fixed-ratio or a fixed-interval requirement, whichever was met first. Overall response rate and running rate (the rate of responding after the postreinforcement pause) decreased for all subjects as the fixed-ratio value increased. As the proportion of reinforcements obtained from the fixed-ratio component increased and the alternative schedule approached a simple fixed ratio, overall response rate and running rate both increased; conversely, as the proportion of reinforcements obtained from the fixed-interval component increased and the alternative schedule approached a simple fixed interval, response rates decreased. Postreinforcement pause length increased linearly as the average time between reinforcements increased, regardless of the schedule parameters. A break-run pattern of responding was predominant at low- and medium-valued fixed ratios. All subjects displayed at least occasional positively accelerated responding within interreinforcement intervals at higher fixed-ratio values.     

Magnitude and duration of the effects of cocaine on conditioned and adjunctive behaviors in the chimpanzee.

Characteristic patterns of conditioned key-pressing were maintained in the chimpanzee under a multiple 30-response fixed-ratio, 10-minute fixed-interval schedule of food presentation. Adjunctive drinking occurred with regularity during the fixed-interval schedule and, with less frequency, during 1-minute timeout periods that followed each food presentation; drinking seldom occurred during the fixed-ratio schedule. Cocaine increased key pressing under the fixed-interval schedule at doses between .1 and 3.0 mg/kg, but adjunctive drinking and key pressing under the fixed-ratio schedule did not increase at any dose. Conditioned and adjunctive behaviors were disrupted and suppressed for different durations at 10,0 mg/kg, a dose which induced convulsive seizures within 10 minutes after intramuscular injection. A time-course analysis showed the magnitude and duration of the effects of cocaine on key pressing under the fixed-interval schedule and on adjunctive drinking to be dose-related. Moreover, a given dose of cocaine had diverse effects, depending on the behavior and the time since drug administration.     

Conjunctive schedules of reinforcement: III. A fixed-interval adjusting fixed-ratio schedule

Key pecking of three pigeons was studied under a conjunctive schedule that specified both a fixed-interval and an adjusting fixed-ratio requirement. The fixed-interval schedule was 6 min for one pigeon and 3 min for the other two. The size of the ratio requirement was determined within each cycle of the fixed interval by the duration of the pause before responding began. The fixed-ratio value was at maximum at the start of each fixed interval and decreased linearly until the first response occurred (adjusting fixed-ratio schedule). A peck produced food when the number of responses remaining on the fixed-ratio schedule was completed and when the fixed interval had elapsed. If no response occurred during the interval, the fixed-ratio requirement decreased to one and a single response after the interval elapsed produced food. The initial value of the adjusting fixed-ratio schedule was studied over a range of 0 to 900. Increases in the adjusting fixed-ratio schedule to about 300 responses increased both pause duration and running response rate and also modified the pattern of responding from that obtained under the fixed-interval schedule. Higher values of the adjusting fixed ratio generally decreased pause duration and running response rate and also disrupted responding. Interreinforcement time under the conjunctive schedule was increased substantially when the adjusting fixed-ratio size exceeded 300 responses.     

The effects of morphine on fixed-interval patterning and temporal discrimination

Changes produced by drugs in response patterns under fixed-interval schedules of reinforcement have been interpreted to result from changes in temporal discrimination. To examine this possibility, this experiment determined the effects of morphine on the response patterning of 4 pigeons during a fixed-interval 1-min schedule of food delivery with interpolated temporal discrimination trials. Twenty of the 50 total intervals were interrupted by choice trials. Pecks to one key color produced food if the interval was interrupted after a short time (after 2 or 4.64 s). Pecks to another key color produced food if the interval was interrupted after a long time (after 24.99 or 58 s). Morphine (1.0 to 10.0 mg/kg) decreased the index of curvature (a measure of response patterning) during fixed intervals and accuracy during temporal discrimination trials. Accuracy was equally disrupted following short and long sample durations. Although morphine disrupted temporal discrimination in the context of a fixed-interval schedule, these effects are inconsistent with interpretations of the disruption of response patterning as a selective overestimation of elapsed time. The effects of morphine may be related to the effects of more conventional external stimuli on response patterning.     

Notes on fixed-ratio and fixed-interval escape responding in the pigeon

After learning to peck a key when each peck removed a slowly increasing series of electric shocks, pigeons were placed on fixed-ratio and fixed-interval escape schedules. The resulting behavior was comparable to that of other species on ratio and interval escape schedules. Thus, while the pigeon apparently requires special techniques for the initial shaping of a key-peck response with negative reinforcement, this response, once obtained, can be subjected to intermittent schedules of negative reinforcement with no great difficulty.     

Negatively reinforced key pecking

A reinforcement-switching procedure was used to produce negatively reinforced key pecking in pigeons. First, key pecking on a chain schedule (fixed-interval 10-sec variable-interval 60-sec) was conditioned using grain reinforcement. Second, intermittent shock in the initial link was introduced at a low intensity and gradually increased. Third, food reinforcement in the terminal link was eliminated. With shock at 90 V occurring on the average every 3 sec, initial-link pecking was maintained with no terminal-link food. Three of four pigeons responded consistently at shock intensities of 90, 70, and 50 V but not at 30 V. A fourth pigeon responded at but not below 90 V. Rate of response was directly related to shock frequency. Eliminating food deprivation did not affect the negatively reinforced performance.