Multiple baseline investigation of stimulus functions in an FR chained schedule,

The performance of pigeons was studied on a multiple schedule in which a three-member FR chained schedule alternated with a three-member FR tandem schedule. The chain and tandem schedules contained identical response requirements. In the chained schedule, more pausing and lower response rates occurred in the first and second components than occurred in the tandem control, in which the same exteroceptive stimulus was associated with all components. Because the reinforcement and response contingencies were identical in the chain and tandem schedules, differences in performances can be attributed to stimulus control.     

Stimulus control of temporally spaced responding in second-order schedules

Second-order schedules consisting of sequences of component differential-reinforcement-of-low-rate schedules were used to investigate two different methods of introducing exteroceptive stimuli. When different exteroceptive stimuli were associated with each component (chained schedule), periods of pausing and lower response rates developed in the early components compared to the early components of the same sequence in the presence of a single exteroceptive stimulus (tandem schedule). When a brief stimulus change occurred at the completion of each component, response rates were higher in the early components compared to the tandem schedule. Changes in response rates in the early components were directly related to changes in reinforcement frequency in the terminal components produced by the two methods of presenting the exteroceptive stimuli.     

Conditioned suppression and variable ratio reinforcement

Conditioned suppression is a decrease in response rate during a relatively short duration stimulus that terminates independently of the animal's behavior and coincidentally with a brief unavoidable shock. The degree of conditioned suppression was measured for each of three birds on three variable ratio schedules; that is, the number of responses required for food reinforcement was varied around a mean of 50, 100, or 200. The results indicated a slight and possibly negligible decrease in the degree of suppression as the mean number of responses required on the schedule was increased from 50, to 100, and 200. In general, it was found that all of the variable ratio schedules tested were quite insensitive to the conditioned suppression procedure, although almost complete suppression was obtained on a few occasions. Since the reinforcement was contingent upon the emission of responses, the birds typically displayed a high rate of response during the pre-shock stimulus on all schedules. In addition, the rate during the pre-shock stimulus often changed abruptly independent of the presentation of a reinforcement. As a result of the high rate of response and the abrupt changes in rate, the degree of suppression from trial to trial was quite variable. A clear analysis of an experimental variable on this baseline is thus difficult.     

Dissociation of value and response strength

Four pigeons were exposed to multiple schedules and later to concurrent-chains schedules, with terminal links that had previously been multiple-schedule components. For 2 birds, the terminal-link schedules arranged an inverse relationship between response rate and reinforcement rate; for the other 2 birds a direct corresponding relationship was arranged. Those response rates were further modified by differentially reinforcing either longer or shorter interresponse times, relative to the current means. Although the birds' initial-link responses indicated preferences for terminal links with higher rates of reinforcement, in half the cases the birds responded during the terminal links in such a way as to produce lower rates of reinforcement, rates their initial-link behavior indicated they did not prefer. That outcome is inconsistent with maximization theory, but consistent with a strengthening analysis of behavior on single-key schedules.     

Selective punishment of fixed-ratio performance

Key-pecking of pigeons, maintained by an FR 50 grain reinforcement schedule, was punished by shocking the first, middle, or last response of the ratio. Under high shock levels, the three punishment conditions produced differential effects on the behavior. Punishment of the first response of the ratio resulted in consistent and extended post-reinforcement pausing and frequent extended breaks after the initial response(s) of the ratio. Punishment of the 25th response disrupted responding in the first half of the ratio with little effect on the last half of the ratio. Punishment of the final response resulted in breaks and local rate changes in various parts of the ratio. Durations of pauses after reinforcement were more variable when the 25th and 50th responses were punished relative to those when the first response was punished.     

Chained and tandem fixed-interval schedules of punishment

Pigeons' key pecks were both punished with electric shock on four-component chained and tandem fixed-interval schedules and reinforced on a variable-interval schedule of food presentation. Pecking was suppressed less in the early components of the chained schedule than in the early components of the tandem schedule. Related multiple and mixed schedules of punishment were also presented; these schedules were identical to the chained and tandem schedules, respectively, except that components changed independently of responding. Similar effects were obtained, in that responding was suppressed in all components of the mixed schedule and only in the fourth component of the multiple schedule of punishment. The performances maintained on the chained and tandem schedules of punishment were generally symmetrical to those found in analogous chained and tandem schedules of food reinforcement.     

Alternative reinforcement effects on fixed-interval performance

Pigeons' key pecks were reinforced with food on a fixed-interval schedule. Food also was available at variable time periods either independently of responding or for not key pecking (a differential-reinforcement-of-other-behavior schedule). The latter condition arranged reinforcement following the first pause of t seconds after it became available according to a variable-time schedule. This schedule allowed separation of the effects of pause requirements ≤ five-seconds and reinforcement frequency. The time spent pausing increased as the duration of the pause required for reinforcement increased from 0 to 30 seconds and as the frequency of reinforcement for pausing increased from 0 to 2 reinforcers per minute. Key pecking was more evenly distributed within each fixed interval with shorter required pauses and with more frequent reinforcement for pausing. The results complement those obtained with other concurrent schedules in which the same operant response was reinforced in both components.     

Drug effects on fixed-interval responding with pause requirements for food presentation.

In pigeons performing under a multiple schedule of food presentation, low key-pecking rates (0.18 to 0.29 responses per second) were maintained during 3-min fixed-interval components by requiring a 4-, 5-, or 6-sec pause preceding the food-delivery response (tandem DRL), while higher rates (0.70 to 1.37 responses per second) were maintained in alternative fixed-interval components by requiring a pause of no more than 40 msec preceding the food-delivery response (tandem DRH). Thus, reinforcement density was equal but overall response rates markedly different in the two schedule components. Pentobarbital (3, 10 mg/kg) had effects on overall rates of responding consistent with a rate-dependency interpretation (low rates were increased while higher rates were decreased), but d-amphetamine (0.03 to 3 mg/kg) either failed to increase low overall rates in the tandem DRL components or increased them only slightly. Effects of both drugs on local responding within the fixed-intervals were always related in an orderly way to control response rate, but the extent of rate increases produced by d-amphetamine was modifed in some birds by pause requirements such that the drug increased comparable rates less when these occurred in the tandem DRL component than when they occurred in the tandem DRH components. Control rate is an important determinant of drug effects, independent DRH components. Control rate is an important determinant of drug effects, independent of reinforcement density maintaining rates, and independent of environmental influences, such as response-spacing requirements for food presentation, that can modify the extent of some drug-produced rate changes.     

Response-dependent shock in second-order fixed-ratio schedules of food presentation

Three pigeons key pecked under second-order schedules in which the completion of two successive fixed-ratio 50 components constituted a reinforcement cycle. Tandem, chained, and brief-stimulus second-order schedules were studied when completion of the initial fixed-ratio 50 component delivered brief intense electric shock in every nth reinforcement cycle and n assumed values between one and nine. During sessions without shock, the brief-stimulus (unpaired with food) schedule generated higher rates of responding in the initial component than did the tandem schedule. Electric shock engendered increased time to the fifth response and a repeated pause-run pattern of not responding and responding, particuuarly in the initial component, even with shock scheduled in every ninth reinforcement cycle. The results were consistent with those reported for shock of a shorter duration scheduled in every reinforcement cycle. The overall rate of responding decreased as a function of increasing shock density and was lower in brief-stimulus than in tandem schedules.     

Extended pausing by humans on multiple fixed-ratio schedules with varied reinforcer magnitude and response requirements

We conducted three experiments to reproduce and extend Perone and Courtney's (1992) study of pausing at the beginning of fixed-ratio schedules. In a multiple schedule with unequal amounts of food across two components, they found that pigeons paused longest in the component associated with the smaller amount of food (the lean component), but only when it was preceded by the rich component. In our studies, adults with mild intellectual disabilities responded on a touch-sensitive computer monitor to produce money. In Experiment 1, the multiple-schedule components differed in both response requirement and reinforcer magnitude (i.e., the rich component required fewer responses and produced more money than the lean component). Effects shown with pigeons were reproduced in all 7 participants. In Experiment 2, we removed the stimuli that signaled the two schedule components, and participants' extended pausing was eliminated. In Experiment 3, to assess sensitivity to reinforcer magnitude versus fixed-ratio size, we presented conditions with equal ratio sizes but disparate magnitudes and conditions with equal magnitudes but disparate ratio sizes. Sensitivity to these manipulations was idiosyncratic. The present experiments obtained schedule control in verbally competent human participants and, despite procedural differences, we reproduced findings with animal participants. We showed that pausing is jointly determined by past conditions of reinforcement and stimuli correlated with upcoming conditions.     

Discrimination and differentiation of response number in stimulus directed pecking of pigeons.

In Experiment 1, autoshaping trials terminated with food only if pigeons emitted more than a target number of responses during a trial in one condition and fewer than a target number in another. The median number of responses per trial shifted in accordance wtih the requirements. The responding of yoked-control birds that received response-independent reinforcers did not vary with the response requirements. In Experiment 2, the number of responses in autoshaping trial became the discriminative stimulus for reinforcement in the second component of a chained schedule. In one condition, responding was reinforced only if the number of responses in the first component was above a target value; in the other condition, responding was reinforced only if the number was below the target value. The distribution of the first-component response numbers did not shift systematically between discrimination conditions, but response rates in the second component indicated that the number of responses in the autoshaping trial was a discriminable property behavior.     

Performances on ratio and interval schedules of reinforcement: Data and theory

Two differences between ratio and interval performance are well known: (a) Higher rates occur on ratio schedules, and (b) ratio schedules are unable to maintain responding at low rates of reinforcement (ratio “strain”). A third phenomenon, a downturn in response rate at the highest rates of reinforcement, is well documented for ratio schedules and is predicted for interval schedules. Pigeons were exposed to multiple variable-ratio variable-interval schedules in which the intervals generated in the variable-ratio component were programmed in the variable-interval component, thereby “yoking” or approximately matching reinforcement in the two components. The full range of ratio performances was studied, from strained to continuous reinforcement. In addition to the expected phenomena, a new phenomenon was observed: an upturn in variable-interval response rate in the midrange of rates of reinforcement that brought response rates on the two schedules to equality before the downturn at the highest rates of reinforcement. When the average response rate was corrected by eliminating pausing after reinforcement, the downturn in response rate vanished, leaving a strictly monotonic performance curve. This apparent functional independence of the postreinforcement pause and the qualitative shift in response implied by the upturn in variable-interval response rate suggest that theoretical accounts will require thinking of behavior as partitioned among at least three categories, and probably four: postreinforcement activity, other unprogrammed activity, ratio-typical operant behavior, and interval-typical operant behavior.     

Quantification of rats' behavior during reinforcement periods

What is treated as a single unit of reinforcement often involves what could be called a reinforcement period during which two or more acts of ingestion may occur, and each of these may have associated with it a series of responses, some reflexive, some learned, that lead up to ingestion. Food-tray presentation to a pigeon is an example of such a “reinforcement period.” In order to quantify this behavior, a continuous-reinforcement schedule was used as the reinforcement period and was chained to a fixed-ratio schedule. Both fixed-ratio size and reinforcement-period duration were manipulated. Rats were used as subjects, food as reinforcement, and a lever press as the operant. Major findings included (a) a rapid decline in response rates during the first 15 to 20 seconds of the reinforcement periods, and (b) a strong positive relationship between these response rates and the size of the fixed ratio. Also revealed was a short scallop not normally found in fixed-ratio response patterns, whose length was a function of fixed-ratio size and reinforcement-period duration. It is suggested that rapidly fluctuating excitatory processes can account for many of these findings and that such processes are functionally significant in terms of behavioral compensation.     

Suppression of behavior by timeout punishment when suppression results in loss of positive reinforcement

This investigation, using rats as subjects and punishment by timeout for responses maintained on a ratio schedule, sought to determine whether behavior would be suppressed by timeout punishment when such suppression also reduced reinforcement density or frequency. A series of experiments indicated that timeout punishment suppressed responding, with the degree of suppression increasing as a function of the duration of the timeout period. Suppressive effects were found to decrease as a function of increases in deprivation (body weight) and were eliminated when the punished response also was reinforced. It was concluded that timeout can produce aversive effects even when loss of reinforcement results. An alternative interpretation of the findings, based on the effects of extinction periods and delay of reinforcement on chained behavior, was discussed.     

Responding under chained and tandem fixed-ratio schedules

The role of stimuli in chained fixed-ratio schedules of reinforcement was examined. At various ratio values, responding on schedules consisting of three or five equal components, with a different colored light in each component (“block counter”) was compared with responding on tandem or simple fixed-ratio schedules having the same color present throughout the entire ratio. At all ratio values except the smallest, the chain stimuli resulted in longer pauses after reinforcement. The magnitude of this effect became greater as the size of the ratio was increased. Post-reinforcement pause durations were longer under five-component schedules than under three-component schedules. Running rates in the first component were lower on the chained schedules than on the tandem schedules; on both kinds of schedule, rates were lower in the first component than in the rest of the ratio. When the sequence of stimuli was reversed, the duration of the post-reinforcement pause dropped markedly and the running rate in the initial component increased, but these effects gradually disappeared after the first reversal session. When the final chain stimulus was substituted for the first component stimulus but continued to appear in the final chain component as well, the pause duration dropped and remained at this lower level during subsequent sessions.     

Reinforcement frequency and contingency as factors in fixed-ratio behavior

Two variables often confounded in fixed-ratio schedules are reinforcement frequency and response requirement. These variables were isolated by a technique that yoked the distributions of reinforcements in time for one group of pigeons to those of pigeons responding on various fixed-ratio schedules. The contingencies for the yoked birds were then manipulated by adding various tandem fixed-ratio requirements to their schedules. Post-reinforcement pause was approximately equal for the yoked and ratio pigeons, and was relatively insensitive to changes in the tandem requirement. Terminal response rate increased with increases in the tandem requirement, even though reinforcement rate was invariant. This increase was attributed to the progressive interference of the tandem requirement with the differential reinforcement of long interresponse times.     

A comparison of ratio and interval reinforcement schedules with comparable interreinforcement times

Pigeons were trained to peck keys on fixed-ratio and fixed-interval schedules of food reinforcement. Both schedules produced a pattern of behavior characterized as pause and run, but the relation of pausing to time between reinforcers differed for the two schedules even when mean time between reinforcers was the same. Pausing in the fixed ratio occupied less of the time between reinforcers for shorter interreinforcer times. For two of three birds, the relation was reversed at longer interreinforcer times. As an interreinforcer time elapsed, there was an increasing tendency to return to responding for the fixed interval, but a roughly constant tendency to return to responding for the fixed-ratio schedule. In Experiment 1 these observations were made for both single-reinforcement schedules and multiple schedules of fixed-ratio and fixed-interval reinforcement. In Experiment 2 the observations were extended to a comparison of fixed-ratio versus variable-interval reinforcement schedules, where the distribution of interreinforcement times in the variable interval approximated that for the fixed ratio.     

Pause relationships in multiple and chained fixed-ratio schedules

On a multiple fixed-ratio 10 fixed-ratio 100 schedule, pigeons pause for relatively long periods of time before the fixed-ratio 100 schedule. Only a short pause occurs before the fixed-ratio 10 schedule. A chain fixed-ratio 10 fixed-ratio 100 schedule produces the reverse pattern, i.e., a short pause before the fixed-ratio 100 schedule and a long pause before the fixed-ratio 10 schedule. Procedurally, the only difference between the two schedules is that the fixed-ratio 10 component is always terminated by some unconditioned reinforcer in the multiple schedule but never in the chained schedule. In the present experiment, the percentage of fixed-ratio 10 components which included reinforcement was gradually decreased for birds on the multiple schedule and gradually increased for birds on the chained schedule. It was found that percentage reinforcement within the fixed-ratio 10 component was inversely related to the duration of the pause before the fixed-ratio 10 component and directly related to the duration of the pause before the fixed-ratio 100 component. Thus, the relative rate of reinforcement paired with a particular stimulus was seen to be an important factor in determining response latency to that stimulus.     

Contingency and stimulus change in chained schedules of reinforcement

Higher rates of pecking were maintained by pigeons in the middle component of three-component chained fixed-interval schedules than in that component of corresponding multiple schedules (two extinction components followed by a fixed-interval component). This rate difference did not occur in equivalent tandem and mixed schedules, in which a single stimulus was correlated with the three components. The higher rates in components of chained schedules demonstrate a reinforcing effect of the stimulus correlated with the next component; the acquired functions of this stimulus make the vocabulary of conditioned reinforcement appropriate. Problems in defining conditioned reinforcement arise not from difficulties in demonstrating reinforcing effects but from disagreements about which experimental operations allow such reinforcing effects to be called conditioned.     

Tolerance to effects of cocaine on behavior under a response-initiated fixed-interval schedule

Tolerance to effects of cocaine can be modulated by schedules of reinforcement. With multiple ratio schedules, research has shown an inverse relationship between ratio requirement and amount of tolerance that resulted from daily administration of the drug. In contrast, tolerance to the effects of cocaine on behavior under multiple interval schedules generally has developed regardless of interval value. Under interval schedules reinforcement depends on the animal making one response following a time interval. Thus, as time to respond increases, the time to reinforcement decreases. On the other hand, fixed ratio schedules require a specified number of responses to be made prior to reinforcement. Therefore, delaying the initiation of responding does not coincide with a significant decrease in the time to reinforcement. In the current experiment, 6 pigeons were trained to respond under a three-component multiple schedule, with a different tandem fixed-ratio 1 fixed-interval schedule in each component. The multiple schedule required one response, which was followed by one of three fixed-interval values (5, 15, or 60 s). Thus, the multiple schedule was interval-like because after the fixed-ratio 1, only one more response was required for reinforcement, but it was also ratio-like because the length of the pause at the beginning of each interreinforcer interval affected the time until the next reinforcer. Acute administration of cocaine generally resulted in dose-dependent decreases in responding. Chronic (i.e., daily) administration of a rate-decreasing dose resulted in tolerance patterns similar to those usually obtained with multiple ratio schedules. That is, the magnitude of tolerance was related inversely to schedule size. These results suggest that delay to reinforcement from the initial response may play a role in the development of schedule-parameter-related tolerance.