Conditioned reinforcement and discrimination in second-order schedules

Pigeons were exposed to multiple second-order schedules in which responding on the “main key” was reinforced according to either a variable-interval or fixed-interval schedule by production of a brief stimulus on the “brief-stimulus key”. A response was required to the brief stimulus during its fourth (final) presentation to produce food; responses to the earlier brief stimuli indicated the extent to which the final brief stimulus was discriminated from preceding ones. Main-key response rates were higher in early components of paired brief-stimulus schedules, in which each brief stimulus was the same as that paired with reinforcement, than in comparable unpaired brief-stimulus or tandem schedules. Poor discrimination occurred between paired brief stimuli (Experiment I). When chain stimuli on the main key induced a discrimination between the first two and second two brief stimuli, the response-rate enhancement in the paired brief-stimulus schedule persisted (Experiment II). Rate enhancement diminished when the initial link of the chain included the first three components (Experiment IV). Eliminating the contingency between responding and brief-stimulus production also diminished rate enhancement (Experiment III). The results show that the discriminative and conditioned reinforcing effects of food-paired brief stimuli may be selectively manipulated and suggest that the reinforcing effects are modulated by other reinforcers in the situation.     

Sign-tracking with an interfood clock

Food was presented to pigeons, irrespective of their behavior. The fixed 60-s interfood interval was segmented into ten 6-s periods, each signaled by a distinctive stimulus color, ordered by wavelength. This “interfood clock” reliably generated and maintained successively higher rates of key pecking at stimuli successively closer to food. Under extinction, key pecking ceased. When the standard stimulus sequence was changed to a different sequence for each bird, accelerated responding again emerged and was sustained under each of the new color sequences. However, responding was neither maintained nor acquired when each successive interfood interval provided a different random sequence of the ten stimuli. Thus, the interfood clock generated and maintained sign-tracking under stimulus control, and the resulting behavior was attributable neither to stimulus generalization nor to a simple temporal gradient.     

Some determiners of attention

Three experiments, using a total of 13 pigeons, examined the stimulus control acquired by the separate components of a compound visual stimulus transilluminating the pecking key. Experiment I measured the control acquired by components of compound discriminative stimuli used in discrimination training. Experiment II sought to demonstrate the effect of pretraining a single stimulus discrimination on control acquired by each component in a compound stimulus discrimination. It also investigated the effect of training the compound stimulus discrimination before the single stimulus discrimination. Experiment III sought a continuous stimulus control function when pretraining stimulus intensities were varied. The results suggest that the extent to which a bird "pays attention" to a stimulus, defined in terms of the degree of stimulus control acquired by that stimulus, is determined by how well it previously learned to discriminate that stimulus from other stimuli.     

Average uncertainty as a determinant of observing behavior

After discrimination training on a multiple variable-interval extinction schedule of food reinforcement, pigeons were placed on the uncued or mixed version of the same schedule and allowed to make an optional “observing response” that converted the uncued schedule to the corresponding cued schedule by providing a 20-sec exposure to the appropriate discriminative stimulus. The schedule consisted of one hundred 40-sec components, and the probability that any one of them would be a variable-interval component was systematically varied between 0.00 and 1.00. The results showed that the amount of observing behavior was an inverted “U” function of the probability of the variable-interval component. Few observing responses occurred at probabilities of 0.00 or 1.00, and maximum responding occurred at a value less than 0.50.     

Reinforcement by an imprinting stimulus versus water on simple schedules in ducklings

Ducklings (5 to 28 days old) were trained to peck a pole on fixed-ratio, fixed-interval, and multiple schedules using brief presentation of an imprinting stimulus as the response-contingent event. Other ducklings of the same age were trained similarly except that reinforcement consisted of access to water. With water reinforcement the typical fixed-ratio (“break-run”), fixed-interval (“scallop”), and multiple schedule response patterns were readily established and consistently maintained. With the imprinting stimulus these schedule effects were inconsistent in some subjects and virtually nonexistent in others, despite extended training. Schedule control with the imprinting stimulus was not improved by the use of a reinforcement signaling procedure which enhances responding reinforced by electrical brain stimulation on intermittent schedules. However, the overall rates of responding and the extinction functions generated after reinforcement with water versus the imprinting stimulus were comparable. These findings imply that control by temporal and discriminative stimuli may be relatively weak when a young organism's behavior is reinforced by presentation of an imprinting stimulus.     

The role of autoshaping in cooperative two-player games between starlings

We report a study of the behavior of starlings in laboratory situations inspired by the “prisoner's dilemma.” Our purpose is to investigate some possible mechanisms for the maintenance of cooperation by reciprocity and to investigate the process of autoshaping at a trial-by-trial level. In Experiment 1, pairs of starlings housed in adjacent cages played a discrete-trial “game” in which food could be obtained only by “cooperation.” In this game, pecking at a response key eliminated the opportunity to obtain food but produced food for the partner. If neither bird pecked, neither had the opportunity to obtain food in that trial. Some level of cooperation persisted for several sessions whether the birds had been pretrained for a high or low probability of pecking at the key. The probability of a cooperative response was higher after trials in which the partner responded (and a reward was obtained) than after trials in which neither bird responded (and no reward was obtained), but the probability of a response was even higher after trials in which the same bird had responded, even though no reward was obtained by the actor in these trials. This behavior did not require visual presence of another player, because similar results were obtained in Experiment 2 (a replicate of Experiment 1 in which the members of the pair could not see each other) and in Experiment 3, a game in which each starling played with a computer responding with “tit for tat.” Using an omission schedule, in which food was given in all trials in which the bird did not peck, Experiment 4 showed that pecking could be maintained by autoshaping. In this experiment, overall probability of pecking decreased with experience, due to a drop in the tendency to peck in consecutive trials. The probability of pecking in trials following a reinforced trial did not decrease with experience. An implementation of the Rescorla–Wagner model for this situation was capable of reproducing molar, but not molecular, aspects of our results. The results violate the predictions of several game-theoretical models for the evolution of cooperation, including tit for tat, generous tit for tat, and the superior win-stay-lose-shift.     

Schedule-induced mirror responding in the pigeon

Two pigeons that were previously exposed to a multiple schedule of reinforcement in the presence of a stuffed and a live pigeon, and two of three naive pigeons, responded on a mirror during exposure to multiple fixed-ratio, fixed-ratio schedules of reinforcement for key pecking. Both the topography and temporal pattern of mirror responding were comparable to schedule-induced “attack” on live and stuffed targets. Rate of target responding was reduced when either the mirror was covered with paper or when the multiple schedule was removed. A reversal in the relationship between reinforcement schedules and discriminative stimuli demonstrated that mirror responding was controlled by the stimulus correlated with the higher fixed-ratio schedule. With one component of the multiple schedule held constant at fixed ratio 25 and the ratio requirement of the other component varying from 25 to 150, there was an inverted U-shaped relationship between rate of mirror responding and fixed-ratio schedule in the varied component. As in Flory's study (1969b) there was an inverted U-shaped relationship between target responding and inter-food intervals. The combined results of these studies suggest that the relationship between rate of target responding and reinforcement schedules is controlled primarily by the inter-food intervals resulting from the schedules.     

The operant control of vocalization in the dog

Control over the vocal responses of three dogs was established using operant-conditioning procedures. Several points of interest were observed in the data. First, fixed-ratio schedules of reinforcement generated a vocal response topography which was similar in detail to that of a “motor” bar-nosing response. Second, vocal responding was brought under the control of external visual stimuli as a result of differential reinforcement. Third, good stimulus control was maintained on a multiple schedule containing a vocal-response component and a bar-response component. Fourth, the stimulus control on the multiple schedule transferred with minimal disruption to a chain schedule requiring a sequence of 10 bar responses followed by 10 vocal responses. Fifth, because vocal and bar responses are not mutually exclusive, concurrent responding tended to develop on the chain schedule.

These results were discussed with reference to the advisability of applying the terms operant and respondent to unconditioned behavior, and, particularly, to unconditioned verbal behavior.

     

Observational learning of two visual discriminations by pigeons: a within-subjects design.

Pigeon's observational learning of successive visual discrimination was studied using within-subject comparisons of data from three experimental conditions. Two pairs of discriminative stimuli were used; each bird was exposed to two of the three experimental conditions, with different pairs of stimuli used in a given bird's two conditions. In one condition, observers were exposed to visual discriminative stimuli only. In a second condition, subjects were exposed to a randomly alternating sequence of two stimuli where the one that would subsequently be used as S+ was paired with the operation of the grain magazine. In a third experimental condition, subjects were exposed to the performance of a conspecific in the operant discrimination procedure. After exposures to conspecific performances, there was facilitation of discriminative learning, relative to that which followed exposures to stimulus and reinforcement sequences or exposures to stimulus sequences alone. Exposure to stimulus and food-delivery sequences enhanced performance relative to exposure to stimulus sequences alone. The differential effects of these three types of exposure were not attributable to order effects or to task difficulty; rather, they clearly were due to the type of exposure.     

Effects of signaled and unsignaled shock on schedule-controlled lever pressing and schedule-induced licking: Shock intensity and body weight

Schedule-controlled lever pressing and schedule-induced licking were studied in rats under a multiple fixed-interval fixed-interval schedule of food reinforcement. Following acquisition of stable rates of pressing and licking, a multiple variable-time variable-time schedule of electric-shock delivery was superimposed upon the baseline schedule. In only one component of the multiple schedule, a 5-sec stimulus preceded each shock (signaled shock). In the other component shock was unsignaled. Several shock intensities (Experiment 1) and body weights (Experiment 2) were studied. Lever pressing and licking were affected similarly by experimental manipulations, although with parametric differences. Depending upon shock intensity and body weight, rates of lever pressing and licking were hardly suppressed, suppressed primarily in the unsignaled shock component (differential suppression), or markedly suppressed in both components. Differential suppression during components with signaled and unsignaled shock and conditioned suppression of responding during the preshock stimulus appeared not to be functionally related. Differential suppression depended more on the discriminability of shock-free time, and on shock intensity, body weight, and the type of response than on the “preparatory” behavior preceding shock.     

Conditioned suppression, punishment, and aversion

Three experiments were conducted to assess the aversive properties of a visual stimulus in the presence of which one group of birds received response-contingent shock (discriminated punishment) while a yoked group of birds received non-contingent shocks (conditioned suppression). In Experiment 1, presentation of the visual stimulus contingent on key pecking reduced the response rate (conditioned punishment effect) for birds under the conditioned suppression procedure but did not reduce the response rate of birds under the discriminative punishment procedure. Non-contingent shocks also produced greater suppression of responding maintained by positive reinforcement in the presence of a visual stimulus than did response-contingent shocks. In Experiment 2, a greater shock intensity (2 mA) was used. All the differences between the two groups found in Experiment 1 were also found in Experiment 2. Experiment 3 demonstrated that response-contingent shock did not result in a conditioned punishment effect even when positive reinforcers were unavailable during the discriminative punishment schedule. The exteroceptive stimulus that was paired with shock in the conditioned suppression procedure acquired the ability to punish behavior. The exteroceptive stimulus in the discriminative punishment schedule did not acquire this ability.     

Associative interaction: joint control of key pecking by stimulus-reinforcer and response-reinforcer relationships

The joint control of rate of key pecking in pigeons by stimulus-reinforcer and response-reinforcer relationships was studied in the context of a two-component multiple schedule of reinforcement. Food presentation was always associated with one component and extinction with the other. The stimulus-reinforcer relationship was manipulated by varying the relative durations of the two components. In the food-presentation component, a fixed rate of reinforcement, independent of rate of responding, was generated by a schedule referred to as “T*”. One aspect of the response-reinforcer relationship, contiguity, was manipulated by varying the percentage of delayed reinforcers. With the multiple T* extinction schedule, stimulus-reinforcer and response-reinforcer relationships could be varied independently of one another. Rate of key pecking was sensitive to manipulations of both relationships. However, significant differential effects due to either the stimulus-reinforcer or response-reinforcer relationship were obtained only when the other relationship was weak: stimulus-reinforcer and response-reinforcer relationships interacted in the joint control of responding.     

Food delivery as a conditional stimulus: Feature-learning and memory in pigeons

Three experiments investigated the learning and memory of discriminations based on presence versus absence of a pre-trial food delivery. In Experiment 1 half the illuminations of a response key were followed by food regardless of the subject's behavior. In one group an extra food delivery preceded only reinforced trials (feature-positive condition), whereas in a second group it preceded only nonreinforced trials (feature-negative condition). Key pecks and approaches revealed more rapid and superior discrimination learning in the first group. Experiment 2 replicated the results of Experiment 1 but yielded no evidence that greater “unexpectedness” of pretrial food conditions facilitates discriminative performance. In Experiment 3, individual pigeons trained on a conditional discrimination exhibited a within-subject feature-positive superiority. Delay between pretrial and trial stimuli interacted with feature-positive versus feature-negative training in both the between-group (Experiment 2) and within-subject (Experiment 3) procedures: performance was decremented at both short and long delays in the feature-positive condition but was decremented only at longer delays in the feature-negative condition. The feature-positive superiority obtained here is incompatible with explanations based on either the general concept of “perceptual organization” or on the conditional nature of feature-negative discriminations.     

A transfer of functions through derived arbitrary and nonarbitrary stimulus relations

During Experiments 1 and 2, subjects were trained in a series of related conditional discriminations in a matching-to-sample format (A1-B1, A1-C1 and A2-B2, A2-C2). A low-rate performance was then explicitly trained in the presence of B1, and a high-rate performance was explicitly trained in the presence of B2. The two types of schedule performance transferred to the C stimuli for all subjects in both experiments, in the absence of explicit reinforcement through equivalence (i.e., C1 = low rate and C2 = high rate). In Experiment 2, it was also shown that these discriminative functions transferred from the C1-C2 stimuli to two novel stimuli that were physically similar to the C stimuli (SC1 and SC2, respectively). For both these experiments, subjects demonstrated the predicted equivalence responding during matching-to-sample equivalence tests. In Experiments 3 and 4, the conditional discrimination training from the first two experiments was modified in that two further conditional discrimination tasks were trained (C1-D1 and C2-D2). However, for these tasks the D stimuli served only as positive comparisons, and ND1 and ND2 stimuli served as negative comparisons (i.e., C1 × ND1 and C2 × ND2). Subsequent to training, the negatively related stimuli (ND1 and ND2) did not become discriminative for the schedule performances explicitly trained in the presence of B1 and B2, respectively. Instead, the ND1 stimulus became discriminative for the schedule performance trained in the presence of B2, and ND2 became discriminative for the schedule performance trained in the presence of B1. All subjects from Experiment 4 showed that the novel stimulus SND1, which was physically similar to ND1, became discriminative for the same response pattern as that controlled by ND1. Similarly, SND2, which was physically similar to ND2, became discriminative for the same response pattern as that controlled by ND2. Subjects from both Experiments 3 and 4 also produced equivalence responding on matching-to-sample equivalence tests that corresponded perfectly to the derived performances shown on the transfer of discriminative control tests.     

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.     

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.     

Stimulus control of schedule-induced activity in pigeons during multiple schedules

Stimulus control of schedule-induced general activity was demonstrated with pigeons using multiple schedules of response-independent food delivery. In Experiment 1, the introduction of food during a multiple variable-time 30-second variable-time 30-second schedule produced a tenfold increase in activity above the no-food baseline. Each pigeon developed stable differential activity rates during the components (correlated with red and green lights) of a multiple variable-time 30-second extinction schedule. Lengthening the extinction component from 1 to 7 minutes increased the rate differences and produced a reliable pattern of responding during S− (the stimulus correlated with extinction): Activity rate was high immediately following the change from S+ (the stimulus correlated with variable-time 30-second) to S−, then decreased abruptly and remained low throughout the middle of the interval, and subsequently showed a positively accelerated increase until the stimulus changed to S+. In Experiment 2, three pigeons were exposed to a mixed variable-time extinction schedule prior to a multiple variable-time extinction schedule. Auditory rather than visual stimuli were used to determine the generality of Experiment 1 results. The multiple- versus mixed-schedule results indicated that stimulus control of activity occurred for two of the birds, but rate differences between S+ and S− were much less than those demonstrated with visual stimuli. A direct comparison of visual and auditory stimulus control in Experiment 3 supported this conclusion. These parallels between the stimulus control of reinforced responding and that of schedule-induced activity suggest that the stimulus control of induced activity may be a factor in operant stimulus control.     

Responding under sequence schedules of electric shock presentation

Lever pressing by squirrel monkeys was examined under second-order schedules of electric shock presentation in which different discriminative stimuli were associated with consecutive components (sequence schedules). Components were always two-minute fixed-interval schedules, and three different overall schedules were studied. Under an overall eight-minute fixed-interval schedule, the first component completion after at least eight minutes had elapsed produced electric shock. The number of components actually completed ranged from one to four; thus, different discriminative stimuli were occasionally associated with electric shock presentation. Under an overall “yoked” variable-ratio schedule, electric shock was presented after completion of a variable number of components; the required number and the distribution of components were matched to those obtained under the overall eight-minute fixed-interval schedule. Under an overall fixed-ratio schedule, electric shock was presented after completion of four components (chained schedule). Under all three sequence schedules, responding in early components was characterized by a pause followed by a single response after the end of the two-minute interval; responding in later components was characterized by a shorter pause followed by positively accelerated responding. Manipulation of overall schedules of shock presentation in these complex behavioral situations produced changes in responding comparable to those ordinarily obtained after similar manipulation of dependencies under both single and second-order schedules of food presentation. These experiments extend the range of conditions and levels of complexity under which responding can be maintained by presentation of electric shock.     

Discrimination learning as a function of stimulus location along an auditory intensity continuum

Eight groups of rats were trained on an auditory intensity discrimination in which the discriminative stimuli were separated by 10 decibels (db). Four pairs of stimuli were selected from different regions along a 60–100 db (SPL) intensity continuum. Counterpart groups were trained on each stimulus pair, with the relative intensity positions of the reinforced stimulus (S^D) and the non-reinforced stimulus (S^Δ) reversed for the two groups. Discrimination acquisition curves were compared to determine whether stimuli separated by equal logarithmic units were of comparable “difficulty”, and to determine the relative effectiveness of an S^D serving as the more versus less intense member of a stimulus pair. It was concluded that: (1) When S^D is the more intense, auditory intensities of constant logarithmic separation are graded in “difficulty” along the intensity continuum; high intensity discriminative stimuli are most readily discriminated. When S^Δ is the more intense, this graded effect is not evident. (2) For a given continuum location, discrimination is inferior when S^Δ is the more intense. This effect is most pronounced at the high intensity end of the continuum and is chiefly attributable to differences in the rate of S^Δ responding.     

Determinants of contrast in the signal-key procedure: Evidence against additivity theory

Two experiments are reported that challenge the interpretation of previous results with the signal-key procedure, in which the discriminative stimuli are located on a response key different from the key associated with the operant response requirement. Experiment 1 replicated the procedure of Keller (1974), and found that contrast effects on the operant key occurred reliably for only one of four subjects. High rates to the signal key initially occurred for only one subject, but modifications of the procedure produced substantial rates to the signal key for all subjects. In all cases, however, signal-key behavior was greatly reduced by the addition of a changeover delay which prevented reinforcement within 2 seconds of the last peck to the signal key, suggesting that signal-key pecking was maintained primarily by adventitious reinforcement. Experiment 2 modified the signal-key procedure by using three response keys, so that the discriminative stimuli on the signal key controlled different responses during all phases of training. With this modification, reliable contrast effects on the operant key occurred for all subjects, suggesting that the failure to find contrast in previous studies has been due to the confounding of changes in the discrimination requirements with changes in relative rate of reinforcement. The results challenge the additivity theory of contrast, and suggest that “elicited” behavior plays a minor role, if any, in the determination of contrast effects in multiple schedules.