Effects of stimulus spacing on steady state gradients of inhibitory stimulus control

When the response of pigeons is maintained to a number of stimulus wavelengths, but extinguished to one (S?), the birds peck more rapidly at stimuli near the S? than at more distant stimuli. The present study explores this “dimensional contrast” effect as a function of the number and spacing of test wavelengths. A fixed portion of the wavelength continuum was spanned by 5, 9, 13, or 49 stimuli, which appeared in random sequence behind a standard pecking key. At the end of each 20-sec trial, pecks to test stimuli produced a conditioned reinforcer (sometimes followed by food), while pecks to the S? stimulus produced only darkness. Dimensional contrast “shoulders” developed to test stimuli on either side of the S?; these shoulders were of approximately the same height and wavelength position for all but the 5-stimulus (widely spaced) condition, and were comparable to the original contrast results with 25 stimuli. The results strongly suggest that the extent and locus of contrast shoulders are largely independent of the number and spacing of test stimuli.     

Conditioning history and inhibitory instrumental stimulus control: Independent-groups and within-subjects measures

The effects of excitatory conditioning history on establishing inhibitory stimulus control have been investigated in classical conditioning, but not in the free-operant paradigm. The present experiments address this question within the context of discriminated free-operant avoidance in which rats’ barpressing postponed shock. When a stimulus with only a history of signaling safety was combined, on a summation test, with a stimulus that maintained avoidance, avoidance rate was reduced, on average, by 60%. In comparison, after a stimulus acquired an excitatory free-operant avoidance history, nonreinforcement alone was not adequate to make it a predictable and effective inhibitor of avoidance on a summation test. These results, consistent with the classical conditioning literature, were produced by both between-group (Experiment 1) and within-subject (Experiment 2) comparisons. These findings are discussed in terms of (1) Konorski’s distinction between “primary” and “secondary” inhibitory stimuli, (2) the Rescorla-Wagner model, (3) the potential contribution of the “reinstatement of fear” to the outcome of summation tests, and (4) their implications for assaying the effectiveness of behavior-modification treatments of phobias.     

Behavioral contrast redux

Behavioral contrast is defined as a change in response rate during a stimulus associated with a constant reinforcement schedule, in inverse relation to the rates of reinforcement in the surrounding stimulus conditions. Contrast has at least two functionally separable components: local contrast, which occurs after component transition, and molar contrast. Local contrast contributes to molar contrast under some conditions, but not generally. Molar contrast is due primarily to anticipatory contrast. However, anticipatory contrast with respect to response rate has been shown to be inversely related to stimulus preference, which challenges the widely held view that contrast effects reflect changes in stimulus value owing to the reinforcement context. More recent data demonstrate that the inverse relation between response rate and preference with respect to anticipatory contrast is due to Pavlovian contingencies embedded in anticipatory contrast procedures. When those contingencies are weakened, anticipatory contrast and stimulus preference are positively related, thus reaffirming the view that the reinforcing effectiveness of a constant schedule is inversely related to the value of the context of reinforcement in which it occurs. The underlying basis of how the context of reinforcement controls reinforcement value remains uncertain, although clear parallels exist between contrast and the effects of contingency in both Pavlovian and operant conditioning.     

Behavioral contrast and reinforcement value

Behavioral contrast was produced in two target components of a four-component multiple schedule by having two target stimuli followed either by a higher rate of reinforcement or by extinction. Response rate was higher in the target followed by extinction. Periodic probe trials were then presented in which the two target stimuli were presented together. Choice on these probe trials was in favor of the stimulus followed by the higher rate of reinforcement during regular training. Experiment 2 replicated this finding but with probe trials presented throughout training. Here, preference for the stimulus followed by the higher rate of reinforcement was evident early in training, substantially before the contrast effects developed. The results challenge interpretations of contrast based on the concept of relative value.     

Competing sources of stimulus value in anticipatory contrast

In previous studies of anticipatory contrast, identical target components (A and B) preceded either a lower (extinction) or a richer schedule. Higher response rates occurred during the target preceding the lower rate of reinforcement, whereas preference was in favor of the target preceding the richer schedule. In Experiment 1, the response and preference measures were positively related when additional stimuli, with no reinforcement of their own, preceded the target components. The effect of these additional stimuli was presumed to be due to their overshadowing of the Pavlovian association between the target components and their following schedules. Experiment 2 also demonstrated a consistent relation between response rate and preference in a conditioned reinforcement procedure. In the absence of a strong Pavlovian association, anticipatory contrast, like other forms of contrast in free-operant procedures, reflects an increase in the value of the target component with an unchanged reinforcement schedule.     

Behavioral contrast in pigeons and rats: A comparative analysis

The effects of reinforcement rate on behavioral contrast were examined in pigeons and rats. Each species was exposed to a series of 12 multiple variable-interval schedules, divided into four 3-schedule series. Each series consisted of a standard contrast manipulation, and baseline schedules provided a different rate of reinforcement in each of the series. The functions relating reinforcement rate to the magnitude of contrast were different across species. Rats showed a U-shaped function, with reliable contrast occurring only at high reinforcement rates. Pigeons showed an inverted U-shaped function, with contrast occurring on all schedules except the schedule providing the lowest rate of reinforcement. Pigeons discriminated between schedule components better than rats did, although differences in discrimination were probably not responsible for the differences in contrast. The results suggest that behavioral contrast in rats may be a different phenomenon from behavioral contrast in pigeons. The results cannot be explained by current theories, which view contrast as the product of a single general process.     

Mechanisms of inhibitory discriminative control

Rats were trained to discriminate between trials signaled by a tone, during which leverpressing was reinforced with food, and trials signaled by the tone in compound with a light stimulus, during which no reinforcers were delivered. A subsequent transfer test suggested that the light had acquired the ability to suppress operant responding; there was no evidence that this suppression could be attributed to Pavlovian inhibitory conditioning. It is argued that these data cannot be accommodated by current accounts of discriminative control.     

Overall and local contrast in multiple schedules: Effects of stimulus similarity and discrimination performance

An experiment with pigeons related overall and local behavioral contrast to similarity between stimuli signaling multiple-schedule components. Similarity was defined both physically and by discrimination performance. Initial and final baseline conditions used two equal random-interval schedules. During two intervening test periods, the schedule accompanying one component was changed to extinction. In the first test, components alternated strictly; in the second test, random component sequences were used. Signaling wavelength stimuli were separated by 1.5, 2, or 14 nm. Overall positive contrast occurred reliably, but its amount depended neither on wavelength difference nor on discrimination performance. Local positive contrast was less frequently observed when signaling stimuli were physically dissimilar; however, the effect was most closely related to actual discrimination performance. The relationship between discrimination and local contrast was nonmonotonic, indicating maximum local contrast at intermediate discriminations.     

On the acquisition and measurement of stimulus control in pigeons

Two experiments were performed to investigate the relationship between excitatory stimulus control (number of responses to a training stimulus) and dimensional stimulus control (generalization gradient slope). In experiment 1, after being trained to peck a green key, pigeons received either 20, 40, or 80 brief (.5, 2, 4, or 8 sec) presentations of a 45-deg line followed by reinforcement (12 groups) or 20, 40, or 80 reinforcements for pecking a continuously presented 45-deg line (3 groups). Number of reinforcements determined the slope (percent of total responses to 45 deg) of a subsequent line-angle generalization gradient, but number of responses to the 45-deg line in the test was controlled by total experience with 45 deg as measured by either total exposure time or total responses to 45 deg in training. In a second experiment, it was shown that increasing the number of days of pretraining to green decreased the slope of the gradient (in subjects given 2-sec presentations), but had no effect on number of responses to 45 deg in the test. Furthermore, continuous presentation yielded flatter gradients but more responding to the 45-deg line in the test than did 2-sec presentations. It was concluded that the measures of dimensional stimulus control and excitatory stimulus control reflect different processes because they vary differentially (sometimes in different directions) in response to the same independent variable manipulations.     

Sequential processes in the generalization and transfer of stimulus control

Prior research indicated that a training sequence consisting of a negative stimulus followed by a positive stimulus constitutes the minimal condition for the production of postdiscrimination phenomena typically observed after training with random sequences of the discriminanda. The present experiments, employing multiple schedules with pigeon subjects, confirmed the earlier findings but indicated that they are restricted to procedures in which the reinforcing stimulus may acquire a discriminative function that competes with the control exerted by the nominal discriminanda. The sequences in which the discriminanda were presented did not differentially affect subsequent measures of generalization and transfer if the discriminative function of reinforcement were degraded either by introducing some reinforcers during the negative stimulus (Experiment 1) or by omitting some reinforcers during the positive stimulus (Experiment 2). It was concluded that the sequence in which the discriminanda are presented during discrimination training does not contribute fundamentally to the processes responsible for discrimination formation with random training sequences.     

External stimulus control of target-directed attack and biting elicited by tailshock

Two experiments evaluating the effects of external stimuli on attack and biting in rats elicited by electric tailshock are reported. In the first experiment, stimuli presented to four groups of test subjects consisted of a stimulus animal, a stimulus animal plus taped vocalizations of a rat experiencing shock, an inanimate object, and an inanimate object plus taped vocalizations. Subjects in a fifth (control) group were tested in the absence of these stimuli. The presence of another animal in the test situation significantly increased the amount of target-directed responding. A decrease in responding, rather than increase, was shown by the subjects tested under the stimulus-object plus taped-vocalization conditions. Experiment 2 investigated the salient features of the stimulus animal and found a combination of both olfactory and visual cues to be most effective in eliciting target-directed responding. These studies indicate that the amount of target-directed attack and biting shown in this situation may be related causally to the type of sensory input received by the test animal.     

The acquisition of trace supraordinate stimulus control in pigeons

Match-to-sample and oddity-from-sample problems with four colors were acquired by two pigeons under the supraordinate control of a line tilt superimposed on samples, Since the supraordinate stimulus terminated before the comparison stimuli were presented, accurate matching and oddity performance indicated trace stimulus control as well, The temporal extent of trace control was assessed in one subject by presenting probes—trials without a line tilt on the sample—in which the basis of correct responding was the supraordinate stimulus presented on the previous trial, Trace supraordinate control did not extend between trials, Subsequently, the delay between the termination of the supraordinate stimulus and the presentation of the comparison stimuli was gradually increased within a trial, Both subjects were able to perform matching and oddity over longer delays, and eventually on probe trials, although accuracy decreased, Results were discussed in terms of instructional stimulus control and memory.     

Motivational cues as determiners of stimulus control in rats

Four rats were each trained to perform a light-intensity discrimination and a sound-intensity discrimination. For half of the subjects, light training sessions were preceded by food deprivation, and correct choices were reinforced with food. Sound training sessions, on the other hand, were preceded by water deprivation, and correct choices were reinforced with water. For the remaining subjects, light training sessions were associated with water deprivation, whereas sound sessions were associated with food deprivation. When the rats were tested in the presence of compounds of sound and light, choices tended to be controlled by light when testing was preceded by the deprivation condition associated with light discrimination task. Reliably fewer light-consistent choices were made under the other deprivation condition, though some preference for responding on the basis of light remained. Following extended training in the presence of all four combinations of light and sound stimuli, this preference was reduced somewhat. When additional testing sessions were preceded by combined food and water deprivation, the tendency to respond on the basis of either light or sound was shown to be related to both deprivation and reinforcement factors.     

Inhibitory control by one stimulus after an increase in the frequency of reinforcement associated with another stimulus

Pigeons were trained on a multiple variable-interval 5-min variable-interval 5-min schedule and then shifted to either a multiple variable-interval 1-min variable-interval 5-min or a multiple variable-interval 30-sec variable-interval 5-min schedule. A generalization test was subsequently administered along the dimension containing the stimulus associated with the variable-interval 5-min component. The generalization gradients for subjects that received multiple variable-interval 1-min variable-interval 5-min training were not consistent in shape. However, an incremental gradient was obtained from each subject that received multiple variable-interval 30-sec variable-interval 5-min training. Thus, a sufficiently large reduction in merely the relative frequency of reinforcement during a stimulus resulted in that stimulus’ acquiring inhibitory control over responding.     

Transfer of discriminative control during stimulus fading conducted without reinforcement

Using trial-and-error training, eight pigeons did not learn to discriminate between 45° and 135° lines, but did learn to discriminate between red and green colors. Control by line tilt was induced by stimulus fading that did not include reinforcement while fading out the colors. After establishing the red–green discrimination, low-intensity lines were superimposed on colors and were gradually faded in. All of this was done using reinforcement. At the end of the line fade-in, the lines had not acquired control of responding. Finally, color intensity was gradually faded out in the absence of reinforcement, and the lines acquired discriminative control by six of the eight pigeons. Thus, reinforcement during the color fade-out was not necessary for the acquisition of discriminative control by the lines during fading. Acquisition of control by lines was attributed to overshadowing, the reduction of stimulus blocking by generalization, and the evocation of correct responding by the colors while the participants were attending to the lines. This last process was also responsible for the induction of discriminative control during sensory preconditioning, higher order conditioning, and response transfer in equivalence classes. Errors, however, were not correlated with discrimination learning during stimulus fading. Finally, transfer of control occurred very quickly with or without errors.     

Individual differences in inhibitory control and children's theory of mind

This research examined the relation between individual differences in inhibitory control (IC; a central component of executive functioning) and theory-of-mind (ToM) performance in preschool-age children. Across two sessions, 3- and 4-year-old children (N = 107) were given multitask batteries measuring IC and ToM. Inhibitory control was strongly related to ToM, r = .66, p < .001. This relation remained significant controlling for age, gender, verbal ability, motor sequencing, family size, and performance on pretend-action and mental state control tasks. Inhibitory tasks requiring a novel response in the face of a conflicting prepotent response (Conflict scale) and those requiring the delay of a prepotent response (Delay scale) were significantly related to ToM. The Conflict scale, however, significantly predicted ToM performance over and above the Delay scale and control measures, whereas the Delay scale was not significant in a corresponding analysis. These findings suggest that IC may be a crucial enabling factor for ToM development, possibly affecting both the emergence and expression of mental state knowledge. The implications of the findings for a variety of executive accounts of ToM are discussed.     

Stimulus delay and the reduction of errors in the transfer of stimulus control

Pigeons initially trained on a simultaneous discrimination of line orientation (S1) were subsequently transferred to a wavelength discrimination (S2). Three transfer procedures were employed. The abrupt-transfer Ss were “abruptly” switched from S1 to the S2 dimension. The stimulus-compounding Ss were trained on a compound stimulus consisting of S1 and S2 displayed in superimposition prior to the presentation of S2 alone. The stimulus-delay Ss were trained on a compound stimulus in which the presentation of the S1 component was delayed for successively longer intervals as a result of a correct response to the preceding trial. Stimulus-delay Ss transferred by responding to S2 prior to the presentation of S1 and the resulting formation of the compound. Ss transferred by the stimulus-compounding and the abrupt-transfer procedures displayed 5 and 10 times as many errors to the S2 dimension, respectively, as Ss receiving the stimulus-delay procedure.

     

Effects of group rearing on the control exerted by an imprinting stimulus

Prior research suggests that group rearing may attenuate a young bird’s tendency to approach and follow an imprinting stimulus. The present work examined the effect of group rearing on a different measure of attachment, suppression by the imprinting stimulus of distress calling induced by abrupt reductions in group size. In Experiment 1, ducklings were reared in groups of 12 or 3 over Days 1–6 posthatch, and each group received a total of 3.5 h of exposure to an imprinting stimulus. Subsequent tests revealed that, when the groups of ducklings were separated into smaller subgroups: (1) the fewer the ducklings in a subgroup, the more distress calls emitted; (2) a given number of birds separated from a larger group emitted more distress calls than an equal number from a smaller group; and (3) regardless of the subgroup size, fewer distress calls occurred when the imprinting stimulus was present than when it was absent. A second experiment revealed that when a group of 12 birds was first confronted with an imprinting stimulus on Day 6 posthatch, they reacted with an increase in distress calling and corner huddling (an aversive reaction). After several hours of exposure to the stimulus, however, its presence exerted a powerful suppressive effect on distress calling. It is concluded that the social bonds between the members of a brood do not preclude the formation of a social attachment to an imprinting stimulus subsequently encountered.     

Additive summation by stimulus compounding irrespective of behavioral contrast during discrimination training: An investigation with positive reinforcement and avoidance schedules

The similarity in the discrimination training leading to behavioral contrast and that preceding tests producing response enhancement to combined discriminative stimuli suggested that the two phenomena might be related. This was investigated by determining if contrast indiscrimination training was necessary for this outcome of stimulus compounding. Responding to tone, light, and to the simultaneous absence of tone and light (T + L) was maintained during baseline training by food reinforcement in Experiment I and by shock avoidance in Experiment II. During subsequent discrimination training, responding was reduced in T + L by programming nonreinforcement in Experiment I and safety or response-punishment in Experiment II. In the first experiment, one rat exhibited positive behavioral contrast, i.e., tone and light rates increased while his T + L rate decreased. In Experiment II, rats punished in T + L showed contrast in tone and light, this being the first demonstration of punishment contrast on an avoidance baseline with rats. The discrimination acquisition data are discussed in the light of current explanations of contrast by Gamzu and Schwartz (1973) and Terrace (1972). During stimulus compounding tests, all subjects in both experiments emitted more responses to tone-plus-light than to tone or light (additive summation). An analysis of the terminal training baselines suggests that the factors producing these test results seem unrelated to whether or not contrast occurred during discrimination training. It was concluded that the stimulus compounding test reveals the operation of the terminal baseline response associations and reinforcement associations conditioned on these multicomponent free-operant schedules of reinforcement.