Some determinants of inhibitory stimulus control

Interspersed reinforcement and extinction during discrimination learning generate a U-shaped gradient of inhibition about the stimulus correlated with extinction. The present work showed that extinction is not a necessary determinant of inhibitory stimulus control. In Exp. I, a reduction in the rate of reinforcement, through a shift from a multiple variable-interval 1-min variable-interval 1-min schedule to a multiple variable-interval 1-min variable-interval 5-min schedule, resulted in a post-discrimination line orientation gradient of inhibition about the stimulus correlated with the variable-interval 5-min schedule. In Exp. II, the rates of reinforcement, correlated with a pair of stimuli, were held constant during a shift from a multiple variable-interval 1-min variable-interval 1-min schedule to a multiple variable-interval 1-min differential-reinforcement-of-low-rate schedule. Inhibitory stimulus control about the stimulus correlated with the differential reinforcement of low rate was obtained. In both experiments, a reduction in the rate of responding during one stimulus and behavioral contrast during the other stimulus preceded the observation of inhibitory stimulus control.     

Discrimination of a response-independent component in a multiple schedule

Pigeons were trained to respond in non-differential reinforcement pre-discrimination training, with a multiple variable-interval 1-min variable-interval 1-min schedule. Each bird then received discrimination training with a multiple variable-interval 1-min variable-time 1-min schedule. Thus, discrimination training was between response-dependent (variable-interval) and response-independent (variable-time) schedules with the rate of reinforcement equated. In Experiment I, only three sessions of non-differential reinforcement preceded discrimination training and for half the birds, a 0° line was correlated with the response-dependent schedule; for the remaining birds the 0° line was correlated with the response-independent schedule. Post-discrimination gradients of excitatory stimulus control were obtained from the former group, while the latter group showed little evidence of post-discrimination stimulus control by the 0° line. Differential responding to the variable-time schedule was not accompanied by behavioral contrast to the variable-interval schedule. In Experiment II, 20 sessions of non-differential reinforcement preceded discrimination training and the 0° line was correlated with variable-time reinforcement for each bird. Differential responding to the 0° line was accompanied by negative induction to the variable-interval schedule and by inhibitory stimulus control about the 0° line during a post-discrimination generalization test.     

Aversive properties of the negative stimulus in a successive discrimination

Experiment I sought to determine if the stimulus correlated with extinction in a successive discrimination was an aversive stimulus. An escape response provided an index of aversive control. Two groups of pigeons were exposed to a multiple variable-interval 30-sec extinction schedule. For the experimental group, a single peck on a second key produced a timeout during which all lights in the chamber were dark. For the control group, pecks on the second key had no contingency. The rate of responding on the timeout key during extinction for the experimental group was higher than that of the control group during all sessions of discrimination training except the first. In Exp. II, green was correlated with variable interval 30-sec and red was correlated with variable-interval 5-min. Timeouts were obtained from variable-interval 5-min. There were more timeouts from extinction in Exp. I than from variable-interval 5-min in Exp. II. Experiment III showed that not presenting the positive stimulus reduced the number of timeouts from the negative stimulus for the two birds from Exp. I that had the highest rate of timeouts from extinction, but had little effect on the two birds that had the lowest rate of timeouts. These results suggest that in a multiple schedule, the stimulus correlated with extinction, or the lower response rate, functions as a conditioned aversive stimulus. Explanations of the timeout response in terms of extinction produced variability, displaced aggression, and stimulus change, were considered but found inadequate.     

Peak shift as a function of multiple schedules of reinforcement

Pigeons were trained to respond to two stimuli on the wavelength continuum, 550 nm and 570 nm, each correlated with an independent schedule of reinforcement. The multiple schedule component in effect during 550 nm (S1) was always a variable-interval 1-min. During the 570-nm stimulus (S2) the second component of the schedule was either variable-interval 30-sec, 1-min, 2-min, 5-min, or extinction for different groups of birds. Generalization gradients were obtained after this training, with the following results: (1) response rate to S1 during training was related to the reinforcement frequency associated with S2; the distribution of responding during generalization testing was a function of the schedules of reinforcement used during training and the response rates they produced. Decreases in the relative frequency of reinforcement correlated with S2 resulted in increases in the distribution shift of responses away from S2 during generalization testing.     

Behavioral contrast in one component of a multiple schedule as a function of the reinforcement conditions operating in the following component

The key pecks of four pigeons were reinforced on a variable-interval 5-min schedule which operated in each of the four components of a multiple schedule, indicated by red, green, yellow, and blue stimuli and presented in such an order that the red stimulus always preceded the yellow and the green stimulus always preceded the blue. After establishing baseline rates, the reinforcement schedule associated with the blue and yellow components was altered so that one was now an extinction schedule and the other was a variable-interval 1-min schedule. In a second experimental stage, the blue stimulus was interchanged with the yellow so that the red stimulus preceded the blue and the green stimulus preceded the yellow. In both experimental stages the response rate in the variable-interval 5-min component that preceded the extinction component was higher than the response rate in the variable-interval 5-min component that preceded the variable-interval 1-min component. The results were discussed in relation to the importance of stimulus ordering in experiments concerned with investigating behavioral contrast.     

Variable-interval schedules of timeout from avoidance

Rats were trained on concurrent schedules in which pressing one lever postponed shock and pressing the other occasionally produced a 2-min timeout during which the shock-postponement schedule was suspended and its correlated stimuli were removed. Throughout, the shock-postponement schedule maintained proficient levels of avoidance. Nevertheless, in Experiment 1 responding on the timeout lever was established rapidly, was maintained at stable levels on variable-interval schedules, was extinguished by withholding timeout, was reestablished when timeout was reintroduced, and was brought under discriminative control with a multiple variable-interval extinction schedule of timeout. These results are in contrast with Verhave's (1962) conclusion that timeout is an ineffective reinforcer when presented to rats on intermittent schedules. In Experiment 2 the consequence of responding on the timeout lever was altered so that the shock-postponement schedule remained in effect even though the stimulus conditions associated with timeout were produced for 2 min. Responding extinguished, indicating that suspension of the shock-postponement schedule, not stimulus change, was the source of reinforcement. By establishing the reinforcing efficacy of timeout with standard variable-interval schedules, these experiments illustrate a procedure for studying negative reinforcement in the same way as positive reinforcement.     

Effects of conditioned reinforcement frequency in an intermittent free-feeding situation,

Key-pecking intermittently produced a set of brief exteroceptive stimulus changes under two-component multiple schedules of conditioned reinforcement. Throughout the study, free access to grain was concurrently provided on an intermittent basis via a variable-interval tape. Free food presentations scheduled by the tape were delivered if no peck had been emitted for 6 sec, and the brief stimulus changes produced by responding under the multiple schedules were those which accompanied food presentation. The second component of each multiple schedule was always associated with a 1-min, variable-interval schedule of conditioned reinforcement. The schedule associated with the first component was systematically varied and conditioned reinforcement was either absent (extinction) or programmed on a 1-, 3-, 6-, or 12-min variable-interval schedule. Under these conditions, rate of responding in the manipulated component decreased monotonically with a decrease in the frequency of conditioned reinforcement. In addition, contrast effects were often obtained in the constant, second component. These results are similar to those obtained with similar multiple schedules of primary reinforcement.     

Peak shift in concurrent schedules

Pigeons were exposed to two keys, a main key and a changeover key. Initially non-differential training was given in which pecking the main key was reinforced on a variable-interval 2-min schedule when the key displayed the first stimulus, a black line on a blue background, and was reinforced on an identical but independent variable-interval 2-min schedule when the key displayed a plain blue stimulus. Later, differential training was given in which pecking the main key was reinforced on a variable-interval 2-min schedule when the first stimulus was displayed; and was reinforced on a variable-interval 10-min schedule when a second stimulus, a black line of another orientation on a blue background, was displayed. During non-differential and differential training, each peck on the changeover key changed the stimulus on the main key. Generalization tests were given before and after the differential training. These consisted of presentations on the main key of seven orientations of the black line on the blue background, including the first and second stimuli, with no reinforcements being given. Changeover-key pecks changed the stimuli on the main key. Generalization gradients were obtained using three measures: time spent, responses, and response rate in the presence of each test stimulus. Typically, maximum values on these measures occurred to stimuli away from the first in a direction opposite the second stimulus, and minimum values occurred to stimuli away from the second in a direction opposite the first.     

Factors influencing inhibitory stimulus control: differential reinforcement of other behavior during discrimination training

Pigeons were trained to respond on a multiple variable-interval 1-min variable-interval 1-min schedule, then switched to a multiple variable-interval 1-min differential-reinforcement-of-other-behavior discrimination training. The rate of reinforcement was held constant during the shift from non-differential reinforcement to discrimination training. Behavioral contrast and post-discrimination inhibitory stimulus control followed the observation of a reduction in the rate of responding to the stimulus correlated with reinforcement for the non-occurrence of pecking.     

Stimulus generalization and delay of reinforcement during one component of a multiple schedule

The key pecking of six pigeons was reinforced according to a variable-interval 1-min schedule during each of two successively presented stimuli. When the key was illuminated by a black line on a white background, reinforcement was delayed for 10 sec. When the key was illuminated by a plain white light, reinforcement was not delayed. All subjects responded at a lower rate during the presentation of the black line. A subsequent generalization test along the line-orientation dimension produced a U-shaped gradient, with the nadir located at or near the training stimulus, for each subject. These gradients suggested that the lower rate of response during the stimulus associated with delayed reinforcement may have been due to an inhibition of responding.     

Facilitation of food-reinforced responding by a signal for response-independent food

Five pigeons whose key pecking was maintained by 4-sec access to grain on a variable-interval 2-min schedule received Pavlovian differential conditioning trials superimposed upon the instrumental baseline. The conditioned stimuli were changes in the stimulus on the key from white to red, or to a white horizontal line against a dark background. The positive conditioned stimulus was 20 sec long, and was followed immediately by 8-sec access to grain. The negative conditioned stimulus, also 20 sec long, was never paired with response-independent food. All pigeons responded more rapidly in the presence of the positive conditioned stimulus than in the presence of the negative one. The positive conditioned stimulus produced an increase in response rate over the pre-conditioned stimulus period. The negative conditioned stimulus had no marked effect upon response rate. When the roles of the positive and negative stimuli were reversed, and the duration of the response-independent reinforcement was reduced to 4 sec, the new positive conditioned stimulus came to facilitate responding, and the new negative conditioned stimulus no longer produced facilitation. A second discrimination reversal produced similar outcomes. When a third reversal was initiated, and the duration of response-independent reinforcement was reduced to 2 sec, the difference between the effects of the positive and negative stimuli diminished.     

Concurrent performances: stimulus-control gradients during schedules of signalled and unsignalled concurrent reinforcement

On one key, pigeons' pecks were reinforced according to a variable-interval schedule in the presence of vertical lines, and were not reinforced in the presence of oblique lines. On a second key, pecks were reinforced according to a variable-interval schedule in the presence of blue, according to a signalled variable-interval schedule in the presence of red, and were not reinforced in the presence of white. Subsequently, during extinction, stimulus-control gradients were obtained by presenting eight different line orientations on the first key concurrent with each of the three colors on the second key. On the first key, line-orientation gradients tended to be lower, narrower, and less shifted in peak or area when the second-key stimulus was blue or red, the stimuli respectively correlated with unsignalled and signalled reinforcement, than when it was white, the stimulus correlated with extinction. Thus, the effect on first-key line-orientation gradients depended on second-key stimuli correlated with concurrent reinforcement, whether or not these stimuli were also correlated with concurrent responding. As a function of first-key line orientation, an inverted gradient was obtained on the second key during blue; during both red and white, rates of pecking on the second key were near zero.     

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.     

Discrimination learning, the peak shift, and behavioral contrast

A discrimination between two successively alternating stimuli was trained under conditions that maintained equal frequencies of reinforcement in the presence of each of the discriminative stimuli (S1 and S2) but that also reduced the rate of responding to S2. These conditions included a multiple variable-interval differential-reinforcement-of-low-rate schedule and a multiple variable-interval variable-interval schedule in which responses to S2 were punished. Whenever the rate of responding to S2 was reduced, rate of responding to S1 (behavioral contrast) increased, and the peak of a subsequently obtained generalization gradient did not occur at the expected location (between S1 and S2) but was displaced away from S2, below S1. Discrimination training in which the frequencies of reinforcement earned in S1 and S2 were not equal (variable-interval 1-min variable-interval 5-min training) produced contrast and the peak shift only if the rate of responding to S2 had been reduced, as after non-differential reinforcement in which variable-interval 1-min schedules were correlated with S1 and with S2. It was concluded that a sufficient condition for the occurrence of behavioral contrast and the peak shift was reduction of the rate of responding to one of two alternating discriminative stimuli and that a peak shift will occur only if contrast had occurred during discrimination training.     

Frequency of reinforcement as a determinant of extinction-induced aggression during errorless discrimination learning.

Seven pigeons were trained to discriminate without errors between a green keylight and a dark key. The key-pecking response was reinforced in the presence of green, and extinction was in effect in the presence of the dark key. The opportunity to attack a restrained target pigeon was present only during extinction. Both variable-interval 30-sec and fixed-ratio 1 schedules of reinforcement during the positive stimulus induced a higher rate of attack during extinction than a variable-interval 5-min schedule. The highest rate of attack during extinction occurred during the first 20 sec after the positive stimulus terminated. Hence, the withdrawal of the positive condition, rather than the consequences of the pecking response during extinction, appears to be one of the primary factors responsible for attack between pigeons during extinction. Behavioral contrast, defined as a decrease in the rate of responding when the positive stimulus was presented alone, was obtained from the four birds that displayed the lowest overall rates of attack while the three birds with the highest attack rates did not display behavioral contrast. For the birds without contrast, components of the attack response during the positive stimulus presumably competed with and reduced the rate of pecking the key, thereby recluding behavioral contrast.     

Fading and errorless transfer in successive discriminations

A successive discrimination between red positive and green negative stimuli was established with pigeon subjects. Then, lines with different angular orientations were superimposed on one of the colors to form compound stimuli. Finally, either the colored element of the positive compound, the colored element of the negative compound, or both colored elements together, were gradually attenuated. Before each attenuation, the line elements were presented alone against dark backgrounds as probes to assess the degree to which they had acquired control of responding. When the positive color was attenuated alone or in conjunction with the negative color, angular orientation acquired control of responding in an errorless fashion. Lines, however, did not acquire control when only the negative component was attenuated. These results were interpreted in terms of changes in the predictability of reinforcement by color and line elements during stimulus attenuation. Finally, attenuation of the negative stimulus influenced the number of “dimensions” of the new line stimuli that acquired control of responding. When the positive stimulus was attenuated with the negative, only one dimension of the lines acquired control. When the positive stimulus was attenuated without the negative, however, more than one dimension of the lines acquired control of responding. These results were interpreted in terms of how errorless performance can be maintained while an organism attends to different dimensions of the new stimuli.     

Concurrent schedules of primary and conditioned reinforcement in rats

Rats responded on a fixed-interval schedule during which a 3-sec stimulus preceded each water reinforcement. The stimulus was then scheduled concurrently for responses on the same lever according to either a variable ratio. Although water reinforcement continued on a fixed-interval schedule, the pattern of responding became typical of a variable-interval or variable-ratio schedule. When the 3-sec stimulus was presented on a variable-interval or variable-ratio schedule, but was omitted on the fixed-interval schedule, the response rate decreased. When the stimulus occurred after the same time periods as those of the variable-interval schedule, but at least 7-sec after the last response, the rate decreased. The rate became higher when the fixed-interval schedule was discontinued and each presentation of the 3-sec stimulus was followed by water on a variable-interval schedule. When both water and the 3-sec stimulus were discontinued for a period of time, resulting in extinction of the lever response, and the 3-sec stimulus alone then presented on a variable-interval or variable-ratio schedule after lever responses, rate increased and then gradually decreased.     

Maintenance and suppression of responding under schedules of electric shock presentation

In squirrel monkeys previously trained under a continuous avoidance schedule, characteristic patterns of responding were maintained under a 3-min variable-interval schedule of shock presentation (response-produced shock). Responding in the presence of a periodically presented stimulus, the termination of which coincided with the delivery of a response-independent electric shock (Estes-Skinner procedure), was not reliably affected. When shocks followed every response during certain signalled portions of the session, and were presented under the variable-interval schedule during the rest of the session (multiple 1-response fixed-ratio, 3-min variable-interval schedule of shock presentation), responding was suppressed during the fixed-ratio component and maintained during the variable-interval component. Environmental consequences do not have immutable properties, and may either support or suppress behavior, depending on the schedule of presentation.     

Response rate, reinforcement frequency, and behavioral contrast

Pigeons responded for food on a multiple schedule in which periods of green-key illumination alternated with periods of red-key illumination. When behavior had stabilized with a variable-interval 2-min schedule of reinforcement operating during both stimuli, low rates of responding (interresponse times greater than 2 sec) were differentially reinforced during the green component. Conditions during the red stimulus were unchanged. Response rates during the green component fell without changing the frequency of reinforcement but there were no unequivocal contrast effects during the red stimulus. The frequency of reinforcement during the green component was then reduced by changing to a variable-interval 8-min schedule without reducing the response rates in that component, which were held at a low level by the spacing requirement. Again, the conditions during the red stimulus were unchanged but response rates during that stimulus increased. These results show that reductions in reinforcement frequency, independently of response rate, can produce interactions in multiple schedules.     

A facilitative effect of punishment on unpunished behavior

The key pecking of two pigeons was reinforced on a variable-interval schedule of reinforcement during the presentation of each of two stimuli. In various phases of the experiment, punishment followed every response emitted in the presence of one of the stimuli. In general, when the rate of punished responding changed during the presentation of one stimulus, the rate of unpunished responding during the other stimulus changed in the opposite direction. This sort of change in rate is an example of behavioral contrast. When punishment was introduced, the rate of punished responding decreased and the rate of unpunished responding increased as functions of shock intensity. When the rate of previously punished responding increased after the termination of the shock, the rate of the always unpunished responding decreased. When the procedure correlated with a red key was changed from variable-interval reinforcement and punishment for each response to extinction and no punishment, the rate of reinforced responding during presentations of a green key decreased and then increased while the rate of the previously punished responding during red first increased and then decreased during extinction.