Successive discrimination training with equated reinforcement frequencies: failure to obtain behavioral contrast

In two experiments, pigeons were trained on two-component multiple schedules in which responding in one component (S₁) was always maintained by a variable-interval schedule. In Experiment I, low response rates were reinforced in the second (S₂) component for six master subjects. This schedule was adjusted to equate reinforcement frequencies in the two components. These subjects were compared to yoked partners, for which reinforcement in the S₂ component was made available on a variable-interval schedule whose value was determined by the master subjects. A similar procedure was used in Experiment II, where the S₂ schedule for master subjects made reinforcers contingent on the absence of responding. No evidence was found in either experiment for a behavioral contrast effect in the S₁ component attributable to response reduction in the S₂ component. A reliable contrast effect was obtained from a group of pigeons given extinction conditions in the S₂ component, which was compared to a group maintained throughout on a multiple variable-interval schedule. The results suggest that previous indications of behavioral contrast in similar situations were probably caused by uneven reinforcement distributions or reflect uncontrolled fluctuations in response rates.     

Behavior changes during repeated eight-day extinctions

Pigeons were given repeated two-day conditionings alternating with eight-day extinctions using a trial procedure. One group had different key colors during each of the first five conditioning-extinction pairs; another group had the same key color throughout. Total extinction responses of both groups were quite constant over successive extinctions. This finding differs from the rapid declines found in most previous studies with bar-press and key-peck responses. The difference probably was due to our longer extinctions, because responses early in each extinction did decrease. However, that decrease was neutralized by increases in responses late in each extinction. The two opposite changes indicate the influence of two different factors during repeated extinctions, with neither factor having much stimulus specificity. The reduction of early responses may result from feeding changes confounded with extinction. The increase in later extinction responses may result from a decrease in the effect of unreinforced responses after their repeated occurrence.     

Signalled reinforcement and multiple schedules

The responses of four pigeons were first reinforced in the presence of two different wave-lengths (green and red) on a two-ply multiple schedule with identical variable-interval 3-min schedules of reinforcement associated with each component. While the constant-component reinforcement schedule remained unchanged during the experiment, the schedule associated with the variable component was changed to (1) signalled variable time, (2) unsignalled variable time, or (3) signalled variable interval. The probability with which the availability of the reinforcer was signalled in the variable-interval schedules was either 0.5 or 1.0. Positive contrast occurred in both signalled variable-interval and variable-time schedules, but only when the availability of all the variable-component reinforcers was signalled. Signalling the availability of only 50% of the reinforcers in signalled variable-interval schedules resulted in negative induction. The present data suggest that positive behavioral contrast resulting from signalled reinforcer availability is due to the presence of an extinction-correlated stimulus.     

Factors influencing responding under multiple schedules of conditioned and unconditioned reinforcement

Two experiments examined pigeons' responses under multiple schedules of conditioned and unconditioned reinforcement. In one component, responses produced food according to a fixed-interval schedule; in a second component, responses produced brief stimuli according to a fixed-ratio schedule. When brief-stimulus presentations were paired with food in the first component, rates in the second component were usually higher than 10 responses per minute. When pairing in the first component was eliminated, responding continued to be maintained in the second component. Elimination of food presentation from the first component substantially decreased responding in the second component, even though the brief stimulus had not been paired with food. Experiment II demonstrated that response rate was affected by the duration of both the second component and the brief stimulus. The results suggest that three conditions are important in maintaining responding with brief-stimulus presentations: (1) pairing the brief stimulus, at least initially, with food, (2) maintaining unconditioned reinforcement in one component, and (3) employing optimal brief-stimulus and component durations.     

Fixed versus variable sequences of food and stimulus presentation in second-order schedules

Three pigeons were exposed to a second-order schedule in which the behavior specified by a fixed-interval component schedule was reinforced according to a ratio overall schedule. The completion of components not followed by food was signalled by a brief stimulus never paired with food. Food and the stimulus occurred in a random sequence or in fixed alternation, but the overall schedules (variable ratio 2 or fixed ratio 2) ensured that an equal number of food and brief-stimulus presentations occurred in each session. The control exerted by the food and by the brief stimulus was measured by overall response rates, mean pauses, frequency distributions of pauses, and response patterning across components. In general, the stimulus controlled patterns of behavior more similar to those controlled by food when food and the stimulus occurred in a random sequence than when they occurred in fixed alternation.     

Conjunctive schedules of response-dependent and response-independent reinforcement

Pigeons received food when they emitted the number of responses specified by a fixed-ratio schedule, and the time specified by a fixed-time schedule had elapsed. The order of meeting the response and time requirements was irrelevant. In different conditions, stimuli signalled completion of one, both, or neither requirement. Ratio size interacted with stimulus condition to determine performance. When a stimulus signalled the end of the fixed-time period, under all ratios the birds tended to respond after the stimulus appeared. When stimuli followed both components, small ratios produced responding during the fixed-time period, and other ratios resulted in responses after the time period had elapsed. With either no stimulus changes, or with a stimulus correlated with completion of the ratio alone, responding first increased and then decreased as the ratio increased. Low and high ratios produced stable response frequencies and patterns in successive intervals. Intermediate ratios resulted in two types of performance. Intervals with long initial pauses and few responses during the fixed-time period were followed by intervals with short pauses and numerous responses and vice versa. The source of these dynamic effects was hypothesized to be number of responses per reinforcer in one condition and response-reinforcer contiguity in the other.     

Acquisition of the autoshaped key peck as a function of amount of preliminary magazine training

Three experiments evaluated the effect of magazine training on acquisition of the pigeon's key peck during autoshaping. In Experiment I, pigeons were exposed to two days of extended magazine training, followed on the third day by keylight-only presentations. All pigeons pecked the keylight early in the keylight-only session. Experiment II examined the relationship between the number of magazine-training trials and trials to the first peck. Pigeons were given either 0, 3, 10, or 25 magazine-training trials followed by the standard autoshaping procedure. The number of trials to the first peck was related to the number of magazine-training trials. In Experiment III, pigeons were exposed to the standard autoshaping procedure without prior magazine training. The data from Experiment III suggested that key pecking will occur only after the response of eating from the lighted hopper has occurred. Taken together, these results suggest that initial magazine training is an important variable in autoshaping. Key pecking is discussed as a generalized consummatory response.     

The effects of brief-stimulus presentations in fixed-ratio second-order schedules

Pigeons' responses were reinforced according to a three-component multiple schedule. In Component 1, key pecks produced food according to a fixed-ratio second-order schedule with fixed-ratio units. Here, a fixed number of fixed-ratio units produced food, and the brief stimulus terminating each unit also accompanied food. Responses in Component 2 produced food on an identical schedule except that the brief stimulus was not paired with food. Component 3 contained a simple fixed-ratio schedule whose response requirement equaled that of Components 1 and 2. Across conditions the size of the fixed-ratio unit (five, ten, twenty, forty, and eighty responses) and the total number of responses per reinforcement were parametrically manipulated. The highest response rates and shortest preratio pauses were observed in Component 3 (no brief stimulus). The lowest rates and longest pauses were found in the component with paired brief-stimulus presentations, indicating that the food-paired brief stimulus suppressed responding. The suppressive effects were greatest when the fixed-ratio units were small (e.g., fixed-ratio 5) and the total fixed-ratio requirement was large (e.g., fixed-ratio 160). Under no conditions did the paired brief stimulus facilitate responding. The nonpaired brief stimulus also suppressed responding but to a lesser extent. The suppressive effects of nonpaired brief stimuli were greatest when the fixed-ratio units were small and the total response requirement was large. These data suggest that the suppressive effects of the brief stimuli may have masked the conditioned-reinforcing effects reported in other studies, and that conditions that maximize suppression in second-order schedules involve the use of fixed-ratio schedule units and the presentation of many brief stimuli per reinforcer.     

Paired baseline performance as a behavioral ideal

Several recent theories view performance under the constraints of a schedule as an attempt to approach the basepoint, the total amount of the instrumental response and the total amount of the contingent response seen in the absence of schedule constraint. Some new analyses of experiments on concurrent ratio schedules, and simple ratio schedules offering an optional magnitude of contingent reward, tested this view directly. In each of the five experiments examined the organism rejected the chance of a closer approach to the basepoint, and thereby failed in addition to maximize the rate of reinforcement.