Some effects of fixed-interval duration on response rate in a two-component chain schedule

In Exp. I three pigeons were trained on a two-component chain schedule. Responding on a 1-min variable-interval schedule in the initial component led to a sequence of two fixed-interval schedules in the terminal component. The rate of reinforcement in the terminal component was kept constant while the values of the two fixed intervals were varied. Three combinations of fixed-interval schedules were studied, FI 0.25, FI 1.75 (minutes) or FI 1.00, FI 1.00, or FI 1.75, FI 0.25. The rate for each subject declined in the initial component as the value of the first fixed interval was increased. Experiment II was conducted to assess the role of the second fixed-interval schedule in the terminal component in determining the rate of responding in the initial component. For each chain schedule the rate of responding in the initial component was determined both with and without the second of the sequence of fixed intervals. In all three cases the rate of responding in the initial component decreased when the second fixed interval was removed. Increasing the first fixed interval in Exp. I had a greater effect on variable-interval performance than did the removal of the second fixed interval in Exp. II.     

Intractable properties of responding under a fixed-interval schedule

The behavior engendered by the fixed-interval schedule is characterized by its variability within and across intervals. The present experiment was designed to assess further the magnitude of interval-to-interval dynamics and to explore conditions which might enhance control by response number for subsequent output. Pigeons were exposed to three experimental manipulations after responding had stabilized under a fixed-interval five-minute schedule. First, a discrete five-stimulus counter was added so that the key color changed after a fixed number of responses. Then additional grain presentations were made at the end of the interval so that high response output was differentially reinforced in the presence of the counter stimuli. Finally, the counter stimuli were presented as an irregular clock (i.e., independently of responding), but the durations were yoked to performance under the counter condition. The data show that response number can exert influence from one interval to the next, but this source of control is weak and not influenced by the experimental manipulations. Results from the clock arrangement indicate that behavior is controlled largely by the stimulus conditions prevailing at the time of interval onset.     

Schedules using noxious stimuli. I. Multiple fixed-ratio and fixed-interval termination of schedule complexes

The termination of a schedule complex, comprising a stimulus in the presence of which brief presentations of electric shocks are scheduled, is a reinforcer. Conditions were studied under which schedule-controlled patterns of responding characteristic of fixed-interval, fixed-ratio, and multiple fixed-interval fixed-ratio schedules can be maintained in the squirrel monkey by terminating a schedule complex. The schedule of shock presentation was a critical determinant of the patterns of responding, especially under fixed-interval schedules of termination. The rates and patterns of responding under various schedules of termination of schedule complexes were generally akin to those maintained under comparable schedules of food presentation. The findings suggest a general similarity in the dynamic aspects of performances under schedules of schedule-complex termination and comparable schedules of food presentation. The schedule of reinforcement is more important than the nature of the reinforcer in the control of behavior.     

Aversive aspects of a fixed-interval schedule of food reinforcement

The key pecking of pigeons was reinforced according to a fixed-interval schedule of reinforcement. The pigeons were also given the opportunity to attack a restrained target pigeon. The attack rates during the sessions of fixed-interval reinforcement were higher than during the operant level sessions in four of the five pigeons. Most attack occurred during the post-reinforcement pause in key pecking. It was suggested that a fixed-interval schedule of positive reinforcement possesses aversive properties, the most aversive of which are located during the post-reinforcement pause.     

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.     

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.     

The effects of schedule history and the opportunity for adjunctive responding on behavior during a fixed-interval schedule of reinforcement.

The effects of schedule history and the availability of an adjunctive response (polydipsia) on fixed-interval schedule performance were investigated. Two rats first pressed levers under a schedule of food reinforcement with an interresponse time greater than 11 s, and 2 others responded under a fixed-ratio 40 schedule. All 4 were then exposed to a fixed-interval 15-s schedule. Water was continuously available under these conditions, but after responding became stable on the fixed-interval schedule, access was experimentally manipulated. With water freely available, subjects did not display characteristic fixed-interval response rates and patterns (i.e., scalloping or break-and-run). Instead, they exhibited predictable, stable patterns of behavior as a function of their schedule histories: Subjects with the interresponse-time history exhibited low response rates, and those with the fixed-ratio history exhibited high rates. Manipulating the amount of water available resulted in marked changes in response rates for rats with the interresponse-time history but not for those with the fixed-ratio history. The results illustrate the multiple causation of behavior by its previous and current schedules of reinforcement and other concurrent factors.     

Effects of a DRL contingency added to a fixed-interval reinforcement schedule

Following 30 days of reinforcement for the bar press response of two white rats on 30-sec fixed-interval (FI), a DRL component was added so that a minimal interresponse time (IRT) for the reinforced response, in addition to the FI variable, was necessary for reinforcement. Marked control over response rate by the superimposed DRL requirement was demonstrated by an inverse hyperbolic function as the DRL component was increased from 1 to 24 sec within the constant 30-sec FI interval. Interresponse time and post-reinforcement (post-S^R) “break” distributions taken at one experimental point (DRL = 24 sec) suggested that a more precise temporal discrimination was initiated by an S^R than by a response, since the relative frequency of a sequence of two reinforced responses appeared greater than that of a sequence of a non-reinforced response followed by a reinforced one. This latter finding was confirmed with new animals in a follow-up experiment employing a conventional 24-sec DRL schedule.     

Varying temporal placement of an added stimulus in a fixed-interval schedule

One paradigm for exploring stimulus effects on behavior is defined for steady state experiments. The paradigm is illustrated by a 60-sec fixed-interval reinforcement schedule wherein a 6-sec light is introduced into each interval. The temporal relation of this stimulus to the reinforcer is the independent variable that is systematically explored. Two experiments studied this temporal relation under two parametric conditions: (a) when the 6-sec light occurs once in each 60-sec interval, (b) when the 6-sec light occurs twice in each interval, the second time always during the 6 sec immediately preceding the reinforcer. Functions are presented showing the effect of the 6-sec light on responding at all points in the fixed-interval.     

Preference for fixed-interval terminal links in a three-key concurrent chain schedule

Pigeons were trained on three-key concurrent chain schedules in which the initial links were variable-interval schedules and the terminal links were fixed-interval schedules. In the first experiment, the initial links were all equal and the terminal-link schedule on one key only was varied. In the second part of the experiment, the terminal-link schedules were all fixed, but different, and the initial-link schedule on one key was varied. Relative response rates in the initial links did not match either the relative arranged, nor the relative obtained, terminal-link reinforcement rates. The relations between independent and dependent variables in three-key concurrent chains were similar to, but not identical with, those found in two-key chains comprising the same schedule types.     

Varying the temporal placement of a drinking opportunity in a fixed-interval schedule.

Three rats, lever pressing for food delivered on a fixed-interval 128-s schedule, were presented with a 16-s opportunity to drink from a retractable water source. The temporal placement of the water probe within the reinforcement cycle was varied sequentially, in steps of 16 s. Although the lever-pressing pattern was modulated by the intercalated water probe, water consumption during the probe itself was a decreasing function of time from the following reinforcer. These results were interpreted as evidence against the notion that schedule-induced drinking is a "ubiquitous" phenomenon and are congruent with results from other "intruded stimulus" experiments.     

A functional analysis of chained fixed-interval schedule performance

Three pigeons were trained on two-link chained fixed-interval fixed-interval schedules. Numbers of responses, time spent responding, and the total time spent in each component were measured. The data were analyzed according to the matching law for multiple and concurrent schedules. In most conditions, the ratio of response rates in the two links was a constant proportion of the ratio that would be predicted in a multiple schedule with the same components. Data on pauses during the interval schedules showed that, in most conditions, the pause duration was a linear function of the interval length, and greater in the initial link than in the terminal link. The experiment thus demonstrated a quantitative functional analysis of performance on a chained schedule.     

Molecular analyses of the effects of d-amphetamine on fixed-interval schedule performances of rats.

A series of doses (0.5 to 2.0 mg/kg) of d-amphetamine was administered to rats whose lever pressing was maintained by fixed-interval 30-s, 60-s, or 120-s schedules of reinforcement by sucrose delivery. Under both saline and d-amphetamine conditions, molecular features of responding were reliably described in terms of the distribution of postreinforcement pauses and local response rate following the onset of responding. Postreinforcement pause always varied from interval to interval but, on average, shortened under the drug. Local response rate (response rate exclusive of pause time) tended to decrease under the drug, and where acceleration occurred within runs of responses, it was reduced by the drug. All of these effects were dose-related. These findings suggest that fixed-interval behavior can be analyzed effectively at a molecular level, and that the effects of d-amphetamine are best described as disruption of temporal discrimination.     

Effects of reinforcement amount on attack induced under a fixed-interval schedule in pigeons

Key pecking by pigeons was maintained on a chained fixed-interval 4-min (12-min for 1 subject) fixed-ratio 1 schedule of food presentation. Attacks toward a restrained and protected conspecific were recorded. In the first experiment, the amount of food presented per interval was manipulated across phases by varying the number of fixed ratios required in the terminal link of the chain. Measures of attack for all pigeons during the fixed-interval component increased monotonically as a function of food amount. In the second experiment, two different food amounts alternated within each experimental session under a multiple schedule. For both pigeons in this experiment, measures of attack were higher during the component that delivered the larger food amount per interval. The differences in levels of attack induced by the two food amounts in Experiment 2, however, were not as great as in Experiment 1; apparently this was because attack during the first interval of each component was controlled in part (P-5626) or entirely (P-7848) by the reinforcement amount delivered at the end of the previous component. Attack was also a function of the location of the interfood interval within the session. For both pigeons, attack tended to decrease throughout the session. The results of both experiments suggest that attack is an increasing function of reinforcement amount under fixed-interval schedules, but that this function may be influenced by the manner in which reinforcement amount is manipulated, by the duration of the interfood interval, and by the location of the interfood interval within the experimental session. In general, these results are compatible with theories of induced attack and other schedule-induced behavior that emphasize aversive after-effects of reinforcement presentation.     

Effects of concurrent response-independent reinforcement on fixed-interval schedule performance

In three experiments, behavior maintained by fixed-interval schedules changed when response-independent reinforcement was delivered concurrently according to fixed- or variable-time schedules. In Experiment I, a pattern of positively accelerated responding during fixed interval was changed to a linear pattern when response-independent reinforcement occurred under a variable-time schedule. Overall response rates (total responses/total time) decreased as the frequency of response-independent reinforcement increased. Experiment II showed that the response-rate changes in the first experiment were controlled by the response-reinforcer relation, but the changes in patterns of responding were similar whether concurrently available reinforcement at varying times was response-dependent or response-independent. In the final experiment, the addition of response-independent reinforcement at fixed times to a fixed-interval schedule resulted in changes in both local and overall response rates and in the occurrence of positively accelerated responding between reinforcements. These results suggest that the temporal distribution of reinforcers determines response patterns and that both the response-reinforcement dependency and the schedule of reinforcement determine overall response rates during concurrently scheduled response-dependent and response-independent reinforcement.     

Changes in rate of schedule-induced behaviour in rats as a function of fixed-interval schedule

Two experiments were conducted to determine the way in which rate of adjunctive drinking and food-tray responding vary as a function of interreinforcement interval duration. In Experiment I rats were tested under fixed-interval schedules ranging from 1-60 s in duration, and in Experiment II under fixed-interval schedules ranging from 1-180 s in duration, with food as the reinforcer. The rate of drinking increased and then declined as interreinforcement interval increased, reaching a maximum under intervals of about 45 s. The rate of food-tray responding declined over the whole range of schedules. It is concluded that drinking can meaningfully be described as “schedule-induced”, in the sense of being directly facilitated by intermittent schedules of reinforcement; but this is less certain in the case of food-tray responding.     

The effects of brief stimuli presented under a multiple schedule of second-order schedules

The effects of briefly presented stimuli paired or not paired with food reinforcement were investigated in the pigeon on a multiple schedule containing second-order schedules. A stimulus paired with food reinforcement was presented on a variable-interval schedule in one unit of the multiple schedule and either a stimulus not paired with food reinforcement or no stimuli were scheduled in the other unit. Response rates were highest when behavior was followed by the food-paired stimulus. Presentation of the food-paired stimulus at completion of each 1-min variable-interval component maintained a steady rate of responding between consecutive food presentations. Pausing following food reinforcement was greatest in the second-order schedule not containing the paired stimulus. Reversing the stimulus pairings led to a reversal of the relative response rates and patterns of responding for each stimulus.     

Conditioning of the aggressive behavior of pigeons by a fixed-interval schedule of reinforcement

Operant reinforcement of aggression was studied in food-deprived pigeons by delivering food for attacks against a target pigeon. The food was delivered according to a fixed-interval schedule and attack behavior was recorded automatically. Attack could be conditioned and extinguished, and the proportion of time spent in attack was a direct function of the frequency of reinforcement of the attack. The fixed-interval schedule produced an increasing rate of attack during the interval between food reinforcements. This positive curvature was an inverse function of the duration of the interval. The findings revealed that the duration and temporal patterning of the complex social behavior of attack can be influenced in a substantial and predictable manner by the schedule and frequency of operant reinforcement.