The effect of signaled reinforcement on rats' fixed-interval responding

Four experiments examined the effect on rats' response rate of presenting a brief (500 ms) stimulus simultaneously with the delivery of food on fixed-interval (FI) schedules. In Experiment 1, reinforcement signals that were spatially diffuse (both tones and lights) elevated rates of responding, but responding was attenuated by localized visual stimuli. The remaining experiments examined the signal-induced potentiation of responding. In Experiment 2, a tone reinforcement signal potentiated response rates on an FI schedule, but attenuated response rates on a variable-interval (VI) schedule. This difference was obtained even though the overall rate of responding was equated on the two schedules before the introduction of the signal. Signal-induced potentiation of responding occurred over a range of FI values employed in Experiment 3. In Experiment 4, presenting a reinforcement signal when high local rates of response had occurred immediately before reinforcement resulted in potentiated rates of responding on an FI schedule. The opposite effect on response rate occurred when the reinforcement signal followed only low local rates of response. These results indicate that a variety of factors influence the effects of a reinforcement signal. They imply, however, that the local rate of response at the time of reinforcement is a key factor in establishing the nature of the signaling effect.     

Stimulus control of responding during a fixed-interval reinforcement schedule

During training sessions, pecks by pigeons on a response key illuminated by a vertical line of white light resulted in reinforcement and an ensuing blackout according to a fixed-interval schedule. Training sessions were followed by dimensional stimulus control test sessions during which the orientation of the line present throughout the fixed interval was varied. Inverted U-shaped (excitatory) gradients of responding, with maximum responding occurring in the presence of the vertical line, were observed during the terminal part of the fixed interval. U-shaped (inhibitory) gradients of responding, with minimum responding occurring in the presence of the vertical line, were observed during the early part of the fixed interval when the preceding interval had terminated with reinforcement and blackout but not when the preceding interval had terminated with blackout only. These results suggest that the dimensional control by the stimulus present throughout the fixed interval is of a conditional variety. Whether the fixed-interval stimulus exerts inhibitory or excitatory dimensional control depends upon the presence and absence, respectively, of stimuli associated with reinforcement.     

Latency and frequency of responding under discrete-trial fixed-interval schedules of reinforcement

Pigeons' key pecking was studied under a number of discrete-trial fixed-interval schedules of food reinforcement. Discrete trials were presented by briefly illuminating the keylight repetitively throughout the interreinforcement interval. A response latency counterpart to the fixed-interval scallop was found, latency showing a gradual, negatively accelerated decrease across the interval. This latency pattern was largely invariant across changes in fixed-interval length, number of trials per interval, and maximum trial duration. Frequency of responding during early trials in the intervals varied, however, with different schedule parameters, being directly related to fixed-interval length, inversely related to number of trials, and complexly affected by conjoint variations of fixed-interval length and number of trials. Response latency thus was found to be simply related to elapsed time during the interval while response frequency was complexly determined by other factors as well.     

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.     

Studies on responding under fixed-interval schedules of reinforcement: the effects on the pattern of responding of changes in requirements at reinforcement

In pigeons responding under a 180-sec fixed-interval schedule of reinforcement, the frequency distribution of the duration of the final interresponse time before the reinforcer was compared with the distribution of the preceding two interresponse times. The results confirmed qualitatively and quantitatively the expected preferential reinforcement of longer interreinforcement times under fixed-interval reinforcement. Requirements at reinforcement were then changed to eliminate the preferential reinforcement of longer interresponse times. Local patterns and mean rate of responding could change, without the characteristic fixed-interval pattern of increasing responding through the interval (scalloping) being much affected. It is concluded that this characteristic pattern of fixed-interval responding does not depend crucially on effects of the reinforcer at the moment of reinforcement, but rather to effects extending over much longer periods of time than just the last interresponse time.     

Exteroceptive control of fixed-interval responding

Two pigeons were exposed to several fixed-interval schedules of food reinforcement. In some cases, exteroceptive stimuli associated with the passage of time were present. Such visual "clock" stimuli were found to gain almost complete control over the behavior, although at the longest fixed interval studied, the superposition of a new temporal discrimination upon the visual discrimination was observed. Where clock stimuli were made contingent upon the birds' behavior, a new form of responding was generated. This behavior was discussed in terms of positive and negative response-tendencies resulting from several stimulus factors: Some of these functioned as S(Delta)'s and secondary negative reinforcers; some functioned as S(D)'s and secondary positive reinforcers; and some were ambiguous with respect to reinforcement conditions. A "pure temporal" discrimination was superimposed upon these factors, but its exact nature was indeterminate from the present data.     

The microanalysis of fixed-interval responding

The fixed-interval schedule of reinforcement is one of the more widely studied schedules in the experimental analysis of behavior and is also a common baseline for behavior pharmacology. Despite many intensive studies, the controlling variables and the pattern of behavior engendered are not well understood. The present study examined the microstructure and superstructure of the behavior engendered by a fixed-interval 5- and a fixed-interval 15-minute schedule of food reinforcement in the pigeon. Analysis of performance typical of fixed-interval responding indicated that the scalloped pattern does not result from smooth acceleration in responding, but, rather, from renewed pausing early in the interval. Individual interresponse-time (IRT) analyses provided no evidence of acceleration. There was a strong indication of alternation in shorter-longer IRTs, but these shorter-longer IRTs did not occur at random, reflecting instead a sequential dependency in successive IRTs. Furthermore, early in the interval there was a high relative frequency of short IRTs. Such a pattern of early pauses and short IRTs does not suggest behavior typical of reinforced responding as exemplified by the pattern found near the end of the interval. Thus, behavior from clearly scalloped performance can be classified into three states: postreinforcement pause, interim behavior, and terminal behavior.     

Signal duration and suppression of operant responding by free reinforcement

Pigeons were trained on a VI (variable interval) schedule of food presentation with a superimposed schedule of response-independent food. Substantial suppression of the operant response rate occurred when the free food was presented without a signal. When the free food was preceded by a short (4 sec) signal, the degree of suppression was similar to that with unsignaled free food. But when the signal was lengthened to 12 sec, the degree of suppression was substantially reduced. Experiment 3 assessed the effect of signal duration using a baseline schedule of delayed reinforcement, in which contingent reinforcers were themselves preceded by a signal. The signal preceding the free reinforcers was then either the same as or different from this contingent signal. Signal duration effects occurred only when the two types of signals were different. These differences as a function of signal duration have implications for both “context-blocking” and “comparator” interpretations of the effects of noncontingent reinforcement in both Pavlovian and operant procedures.     

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.     

Interresponse time duration in fixed-interval schedules of reinforcement: control by ordinal position and time since reinforcement

The times between each of the first thirteen responses after reinforcement (the first twelve interresponse times) were determined for two pigeons whose pecking was reinforced on fixed-interval schedules of food reinforcement ranging from 0.5 min to 5 min. These interresponse times were classified with respect to their ordinal position in the sequence of responses and with respect to the time since the preceding reinforcement at which the initiating response occurred. The median interresponse time durations were essentially constant after the sixth response after reinforcement regardless of the time at which the interresponse time was initiated. The durations of the first few interresponse times after reinforcement decreased as the number of preceding responses increased and as the time since the preceding reinforcement increased.     

Schedule-induced escape from fixed-interval reinforcement

Pigeons trained to peck one of two keys for food were exposed to an ascending and descending series of fixed-interval values. A response on the second key produced an escape period consisting of a visual stimulus change. During escape periods, the fixed-interval timer continued to operate and even if it timed out, a response on the food key would not operate the feeder unless preceded by an escape-key response that terminated the escape condition. As the fixed-interval schedule was increased logarithmically through six values from 30 to 960 sec, the percentage of session time spent in escape as well as the frequency, duration, and rate of escape increased to a maximum and then decreased. One subject did not develop escape behavior to any significant degree. For all pigeons, escapes usually occurred after, rather than before, reinforcement.     

Studies on responding under fixed-interval schedules of reinforcement: II. The scalloped pattern of the cumulative record

Responding under fixed-interval schedules usually generates either scalloped or break-and-run cumulative records. Earlier, it was generally accepted that the characteristic pattern was the scallop, but in recent years there has been an increasing emphasis on the break-and-run pattern. The break-and-run pattern has been shown quantitatively to provide a good fit of certain fixed-interval patterns. In the present work, responding during fixed-interval 1000-second components of a multiple fixed-interval 1000-second fixed-ratio 50 responses schedule was examined in two rhesus monkeys. Even after responding had started in an interval, there was a high tendency for responding to accelerate over subsequent 100-second segments of the interval. In segments with responding, the rate increased from one segment to the next in 303 of 389 segments in one monkey and in 310 of 419 segments in the other. The size of the increase was substantial, the rate in the fifth segment after responding started being an average of 4.5 times higher than the rate in the first segment after responding started. Hence, the usual pattern of responding in individual intervals was of sustained and substantial acceleration, vindicating numerically the conclusion derived from inspection of the scalloped patterns of the cumulative records.     

Summation of responding maintained by fixed-interval schedules

A light and tone were separately correlated with responding maintained by fixed-interval schedules, in which the level of responding varied continuously throughout the duration of the interval. Responding during the presence of the single stimuli and their compound was compared during the successive segments of the interval. The following results were obtained: (1) more responses were emitted during compounding than were emitted during either stimulus alone in all segments of the interval; (2) increases in the number of responses across the interval during compounding paralleled increases during single-stimulus presentations. The sum of the responses emitted during the single stimuli was similar to the number of responses emitted during compounding, suggesting that the response tendencies correlated with the single stimuli combined in a summative or additive fashion.     

Preference for fixed-interval schedules of reinforcement

Pigeons were trained on a two-link concurrent chain schedule in which responding on either of two keys in the initial link occasionally produced a terminal link, signaled by a change in the color of that key and a darkening of the other. Further responding on the lighted key was reinforced with food according to a fixed-interval schedule. For one of the keys, this fixed interval was always 20 sec, while for the other it was held at values of 5, 14, 30, or 60 sec for several weeks. In the initial link, all pigeons responded relatively more often on the key with the shorter fixed interval than was predicted by the matching hypothesis. Responding in the initial link showed a large negative recency effect: pigeons responded less frequently on the key that provided their last reinforcement than predicted from the overall response rates.     

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.     

A response-initiated fixed-interval schedule of reinforcement

On a tandem fixed-ratio one fixed-interval schedule, the first response after reinforcement initiates a fixed interval of time and the first response after the interval has elapsed is reinforced. Pigeons trained with that schedule of food reinforcement paused after reinforcement for a period of time that approximated the fixed-interval duration for values of that duration ranging from 3.75 to 60 sec. Cumulative records revealed response patterns best described as break-and-run.     

Sequential effects of interval duration on fixed-interval performance

The bar pressing of rats was reinforced on a multiple fixed-interval schedule. The schedule intervals were 1 and 5 min long, and the sequence was such that intervals of either duration were equally likely to be followed by intervals of the same or of the other duration. Rates were higher during 1-min and after 5-min intervals. Best fit equations for cumulative responses during the 5-min intervals produced very similar exponents regardless of preceding duration. It was concluded that preceding duration may have affected the subjects' performances through direct effects on temporal discrimination.