Duration and rate of reinforcement as determinants of concurrent responding.

The duration and frequency of food presentation were varied in concurrent variable-interval variable-interval schedules of reinforcement. In the first experiment, in which pigeons were exposed to a succession of eight different schedules, neither relative duration nor relative frequency of reinforcement had as great an effect on response distribution as they have when they are manipulated separately. These results supported those previously reported by Todorov (1973) and Schneider (1973). In a second experiment, each of seven pigeons was exposed to only one concurrent schedule in which the frequency and/or duration of reinforcement differed on the two keys. Under these conditions, each pigeon's relative rate of response closely matched the relative total access to food that each schedule provided. This result suggests that previous failures to obtain matching may be due to factors such as an insufficient length of exposure to each schedule or to the pigeons' repeated exposure to different concurrent schedules.

An analysis of rats' drinking-tube contacts under tandem and fixed-interval schedules of food presentation.

Rats' lever presses and drinking-tube contacts were studied under fixed-interval schedules of food presentation and under a tandem schedule composed of three fixed intervals. One group of rats was exposed first to the tandem schedule, next to fixed-interval schedules of comparable interpellet intervals, and once again to the tandem schedule; a second group of rats was exposed first to a fixed-interval and then to the tandem schedule. Under the tandem schedule, lever presses occurred at a higher rate and were more uniformly distributed in time than under the fixed-interval schedule. Tube contacts emitted by rats exposed first to a fixed-interval schedule consisted mostly of tongue contacts, which occurred at a high rate shortly after food; tube contacts emitted by rats exposed first to the tandem schedule consisted mostly of paw contacts, which occurred at a lower rate at times other than shortly after food. Changing the schedule from fixed interval to tandem decreased the frequency of tongue contacts for all rats. Under schedules of food presentation with comparable interpellet intervals, the schedule of food presentation, rather than the rate of food delivery per se, determined the topography and temporal locus of drinking-tube contacts.

Short-component multiple schedules: effects of relative reinforcement duration.

Pigeons were exposed to multiple variable-interval 2-min variable-interval 2-min schedules of food presentation in which relative duration of food presentation was manipulated. When components alternated every 5 sec and were scheduled on separate response keys, relative response rates closely matched relative reinforcement duration in three of four pigeons. On the other hand, relative response rates were insensitive to relative reinforcement duration when components scheduled on a single response key alternated every 5 sec, and when components scheduled on separate response keys alternated every 2 min. Thus, both rapid alternation and spatial separation of components were necessary to produce approximate matching of relative responding to relative reinforcement duration. This finding contrasts with previous findings that only rapid component alternation is necessary for matching when relative rate of reinforcement is manipulated.

Separating the effects of response rate and reinforcement frequency in the rate-dependent effects of amphetamine and scopolamine on the schedule-controlled performance of rats and pigeons.

Rats and pigeons responded under three-component multiple schedules of food reinforcement. Each component was associated with a separate stimulus condition and a schedule that arranged food delivery intermittently for appropriately spaced responding. In the experiment with pigeons, three response rates were maintained with equal reinforcement frequency in the three components. With rats, the lowest response rate was associated with a higher reinforcement frequency. The effects of d-amphetamine in rats (0.05-0.6 mg) and pigeons (0.1-3.0 mg) were closely associated with the dose, and with the response rates that occurred under nondrug control conditions. Small doses of d-amphetamine increased low response rates proportionately more than moderate rates; moderate rates were increased proportionately more than were high rates. With larger doses, low rates were decreased proportionately less than were moderate rates, which in turn were decreased proportionately less than were high rates. Similar relations between drug effects and control rates were obtained in rats with scopolamine (0.0125-0.4 mg), with the exception that constant effects appeared at doses of 0.1 mg and greater. The present experiments, through manipulating response rate independently of reinforcement frequency, show that the rate-dependent effects of d-amphetamine and scopolamine are primarily response-rate-dependent drug effects.

Behavioral interactions in multiple variable-interval schedules.

In Experiment I, two groups of four pigeons each were exposed to multiple schedules in which one component was always a variable-interval schedule with a mean interreinforcement interval of 30 or 180 seconds. The other component was either an equal variable-interval schedule or extinction. Response rates in the unchanged component always increased when reinforcement was no longer scheduled in the changed component, and decreased in seven of eight cases when the variable-interval schedule was re-introduced. The per cent rate change in the unchanged component was inversely related to the frequency of reinforcement and to the ongoing response rate in the unchanged component. Rate changes in the unchanged component were not consistently correlated with changes in any single feature of the relative-frequency interresponse-time distributions. In Experiment II, the same pigeons were exposed to variable-interval schedules and multiple variable-interval variable-interval schedules with equal mean interreinforcement intervals. Response rates were similar under both conditions.

A detailed analysis of the effects of d-amphetamine on behavior under fixed-interval schedules.

Pigeons were exposed to fixed-interval schedules of food reinforcement with durations of 300 sec, 100 sec, or 40 sec. A range of doses of d-amphetamine was administered to each pigeon, and the resulting behavior was analyzed at several levels of detail. Average rates in different portions of the intervals predicted the magnitude of the drug's effect, but a finer analysis showed that average rates did not adequately characterize the behavior in some parts of the intervals. The probability of responding in different parts of an interval without drug was also a good predictor of the magnitude of the effect of d-amphetamine, and at the same time was more descriptive of the interval-to-interval performance. Analyses of the control performance indicated that responding in individual intervals could be described as consisting of two parts: a very low, or zero, rate at the beginning of the interval followed by an abrupt transition to a slightly, but reliably, positively accelerated rate maintained until reinforcement.

Accuracy of performance on a matching-to-sample procedure under interval schedules.

Correct matches on a matching-to-sample procedure were reinforced under fixed-interval, chained fixed-interval, and fixed-interval schedules with exteroceptive stimulus changes correlated with time since the last reinforcer (an added clock). For all four pigeons, accuracy changed within the fixed-interval and fixed-interval schedules with added clock, decreasing from the beginning of the interval to some point in the middle. The performance then became increasingly more accurate until the end of the interval. Under the chained schedules, accuracy also changed within the components. During the initial component, accuracy decreased from the beginning of the fixed interval to some point in the middle or at the end. During the middle component, the performance usually remained at an intermediate level of accuracy. During the terminal component, the initially inaccurate performance became increasingly more accurate throughout the interval. Systematic relationships between response rate and per cent error showed that all four pigeons performed most accurately at high rates. The accuracy of the performance at low rates was also quite high. These relationships held for all three types of schedules through an eight-fold variation in scheduled interreinforcement time.

Second-order schedules with fixed-ratio components: variation of component size.

Key pecking by pigeons was reinforced with food under second-order schedules with fixed-ratio units. A constant total number of key pecks was required for reinforcement under each condition, but the size and, inversely, number of fixed-ratio components were varied. The total response requirement of 256 pecks was divided into fixed-ratio units of 128, 64, 32, 8, and 2 responses. A brief stimulus, which always preceded food reinforcement, was presented upon completion of each fixed-ratio unit. Under most conditions, the pattern of within-unit responding was typical of that under simple fixed-ratio schedules. Overall response rate was an inverted U-shaped function of component size. That is, response rates were highest under moderate sized units (fixed ratio 128 and 64). This relationship is consistent with previous determinations of rate as a function of fixed-ratio value for simple fixed-ratio schedules.

Stimulus control and the response-reinforcement contingency.

Pigeons were trained under a schedule in which reinforcement was made available at varying periods of time after a prior reinforcement. The first key peck after a reinforcer was available began a timer and a second key peck, which exceeded a specified minimal time interval, produced the reinforcer. It was shown that a contingency which contains a minimal interresponse time does not necessarily weaken stimulus control by an exteroceptive stimulus.

The release of catecholamines by shocks and stimuli paired with shocks.

One monkey and five baboons were surgically prepared so that heart rate and blood pressure could be monitored continuously, and an extra-corporeal blood path was established to detect the secretion of epinephrine (adrenaline) and norepinephrine (noradrenaline). A respondent conditioning procedure was used in which a tone was paired with electric shocks. Epinephrine, but not norepinephrine was released by shocks, and a corresponding release was demonstrated by the tone alone. Heart rate and blood pressure changes were also elicited by shocks and by the tone.

Some effects of discriminative training with equated frequency of reinforcement.

Pigeons were exposed to a multiple schedule which provided equally frequent reinforcement in the presence of two stimuli but which produced markedly different rates of key-pecking. Generalization gradients were displaced away from the stimulus associated with the lower rate of key-pecking. Another group of pigeons had similar training, except that a low rate of key-pecking was established in a stimulus with a much higher frequency of food reinforcement. In this case, the generalization gradients were not affected by the training on the schedule producing a low response rate.

The relation between response rates and reinforcement rates in a multiple schedule.

In a multiple schedule, exteroceptive stimuli change when the reinforcement schedule is changed. Each performance in a multiple schedule may be considered concurrent with other behavior. Accordingly, two variable-interval schedules of reinforcement were arranged in a multiple schedule, and a third, common variable-interval schedule was programmed concurrently with each of the first two. A quantitative statement was derived that relates as a ratio the response rates for the first two (multiple) variable-interval schedules. The value of the ratio depends on the rates of reinforcement provided by those schedules and the reinforcement rate provided by the common variable-interval schedule. The following implications of the expression were evaluated in an experiment with pigeons: (a) if the reinforcement rates for the multiple variable-interval schedules are equal, then the ratio of response rates is unity at all reinforcement rates of the common schedule; (b) if the reinforcement rates for the multiple schedules are unequal, then the ratio of response rates increases as the reinforcement rate provided by the common schedule increases; (c) the limit of the ratio is equal to the ratio of the reinforcement rates. Satisfactory confirmation was obtained for the first two implications, but the third was left in doubt.

Some effects on generalization gradients of tandem schedules.

The relationship between training conditions and stimulus generalization gradients was examined using tandem schedules of reinforcement. Schedules were selected so that frequency of reinforcement and rate of responding were varied somewhat independently of each other. A peak-shift in the generalization gradient was obtained when extinction had been associated with one of the stimuli. No comparable peak shift was obtained when there were equal response rates in the training stimuli even with dissimilar frequencies of reinforcement. The data imply that response rates at the end of training, rather than reinforcement frequency per se, determine the characteristics of the generalization gradient.