Behavioral contrast and the automaintained key peck

In two experiments behavioral contrast was demonstrated during discrimination training in a positive automaintenance procedure. During the baseline condition in each experiment, a key was transilluminated for eight seconds by one of two colors (CS) following a variable intertrial interval signaled by a dark key. Keylight transillumination terminated with a response-independent food presentation. In the first experiment, food was eliminated during one CS for up to fifty sessions. After reinstatement of food following each CS, the discrimination was reversed. Six of the eight subjects showed positive behavioral contrast, i.e., response rates increased during the CS associated with food as they decreased during the CS associated with no food. The effect was replicated in Experiment II, but it did not occur when both the food and its associated CS were eliminated. These results were comparable to those obtained with operant discrimination training procedures (behavioral contrast) and with Pavlovian discrimination training. The results suggest that additivity theories of behavioral contrast may be insufficient to account for these data.     

Behavioural contrast and autoshaping

This experiment was designed to investigate the importance of autoshaping to a signal for reinforcement in the production of behavioural contrast. Two groups of pigeons were given discrimination training on a mult VI-EXT schedule: the stimuli present in the two schedule components shared common attributes, but were distinguished by the presence or absence of a visual feature. For one group (the feature positive group) the feature signalled the availability of reinforcement. For the other group (the feature negative group) the feature signalled nonrein-forcement, and for this group there was no stimulus element which unambiguously signalled reinforcement. The feature positive group showed a higher response rate during the VI component of the mult VI-EXT schedule than the feature negative group. This finding was interpreted as support for the autoshaping explanation of behavioural contrast. The results differed from those of Jenkins and Sainsbury (1969, 1970) in that both the feature positive and the feature negative groups showed discrimination learning.     

Local contrast and Pavlovian induction

Two experiments examined the effects of number and similarity of stimuli on local contrast. In the first experiment, local contrast effects differed in magnitude as a function of the similarity among stimuli; greater positive local contrast appeared when stimuli were less similar, though this effect sometimes reversed for very dissimilar stimuli. In the second experiment, both positive and negative local contrast appeared transiently during the course of training a discrimination including two quite dissimilar stimuli. When two new stimuli were added, both effects reappeared in several cases. The effects remained when the discrimination was rendered more difficult by substituting a new stimulus very similar to one of the original pair. These and other data suggest that local contrast depends on the same factors that produce Pavlovian induction; in the absence of an alternative account, Pavlov's interpretation may be useful in suggesting further research that will help identify the mechanisms involved in both classical and operant discrimination learning.     

Ideological beliefs as determinants of discrimination in positive and negative outcome distributions

Social identity theory proposes that discrimination contributes favourably to group members' social identity. In minimal group paradigm (MGP) studies involving positive outcome distributions (e.g. money), discrimination is associated with a more positive social identity. But studies on the positive‐negative asymmetry effect show that categorization leads to less discrimination when negative (salary cuts) than when positive outcomes (salary increases) are distributed. Using structural equation modelling, this study (N = 279) tested whether discrimination involving negative outcome distributions could contribute as much to group members' positive social identity as discrimination on positive outcomes. The study also tested if ideological beliefs (i.e. social dominance orientation, authoritarianism), measured one month before the MGP experiment, could predict positive and negative outcome discrimination. While the fit of the hypothesized model was adequate, only social dominance orientation predicted both positive and negative outcome discrimination. Also, discrimination on positive outcomes but not on negative ones contributed to positive social identity. Copyright © 2004 John Wiley & Sons, Ltd.     

Procedural antecedents of behavioral contrast: a re-examination of errorless learning

Behavioral contrast reliably occurred in pigeons following errorless discrimination training, contrary to Terrace's (1963) observations. In the main experiment, a 60-sec green keylight, associated with a variable-interval 30-sec schedule of reinforcement alternated with a 60-sec period of extinction when the key was dark. Such aspects of the discrimination training procedure as: (1) the amount of prior nondifferential exposure to the positive stimulus before the discrimination was instituted, and (2) the rapidity with which the negative stimulus was introduced (whether progressively or abruptly) directly influenced the amount of behavioral contrast produced. This occurred independently of the number of errors made by a pigeon during acquisition of the discrimination. In a series of control experiments, substitution of a red keylight for the dark key during extinction resulted in greater behavioral contrast, while an increase to 3 min in the duration of the green keylight associated with reinforcement attenuated the behavioral contrast effect.     

Discrimination and the positive-negative asymmetry effect: ideological and normative processes

Research using the minimal group paradigm demonstrates that categorization and ingroup identification can foster intergroup discrimination. However, the positive-negative asymmetry effect shows that less discrimination occurs when negative rather than positive outcomes are distributed. The normative hypothesis explains this asymmetry by the stronger inappropriateness of discrimination in negative than in positive outcome distributions. Results obtained in this minimal group paradigm study (N = 257) did not replicate the asymmetry effect: discrimination occurred in both positive and negative outcome distributions, even if norms against discrimination were stronger in negative than in positive outcome distributions. The absence of the asymmetry effect is explained by the effect of the discrimination-justifying ideology.     

Orbeli’s experimental work on color discrimination in dogs

In the 1900’s, L. A. Orbeli, on I. P. Pavlov’s request, attempted to establish color discrimination in dogs. Previous experiments used the motor discrimination method and gave inconsistent results. In contrast, Orbeli used the salivary conditional reflex method, which he considered to be more precise than the method that relied on erratic movements of a dog. After experimentation that lasted about one and a half years, Orbeli failed to establish color discrimination. When subsequent experiments by Russians and Germans yielded positive results with the motor discrimination method, Orbeli switched to this method and also obtained positive results. These findings were confirmed by most subsequent experiments on color discrimination in dogs. The utility of Pavlovian conditioning in sensory experimentation was not universally justified and its importance was greater in the study of learning.     

Absolute and relative measures of dimensional contrast.

Two sets of experiments examined the discriminative performance of pigeons on a visual flicker-rate continuum using a maintained generalization procedure. In the first experiment, responses during the intermediate stimulus value were not reinforced, whereas responses during all other stimuli were reinforced periodically. In the second experiment, training was similar to the first, with the exception of one condition in which stimuli adjacent to the negative stimulus border were eliminated from the discrimination set. Results from both experiments show that positive dimensional contrast seems to represent a relative enhancement of discrimination gradient form, rather than an absolute increase in responding with respect to prior baseline. Further, the form and magnitude of positive dimensional contrast are not predictable from the form and magnitude of baseline response rates. Results from the second experiment indicate that eliminating border stimuli increased response-rate differences between positive and negative stimuli, but did not necessarily diminish the magnitude of positive dimensional contrast.     

A note on behavioural contrast and frustration

The suggestion has been made that positive contrast, the increase in response rate in one successively-presented stimulus following a change in conditions that decreases responding in the other, may not depend on differential stimulus control. Evidence is reviewed suggesting that contrast effects on most multiple interval schedules do depend upon discrimination, but that effects similar to contrast that result from the omission of reinforcement on fixed-interval schedules (the frustration/omission effect) reflect other factors. The link between contrast and the omission effect is the discrimination mechanism which allows animals to respond only at times or places correlated with food delivery. Contrast is a direct result of this mechanism; but the omission effect depends on the difference between the inhibitory discriminative after-effects acquired by food on fixed-interval schedules, and those acquired by a “neutral” stimulus presented in its place.     

Effect of various discriminative stimuli on choice behavior in male siamese fighting fish (Betta splendens)

The current experiment was an exploratory study empirically comparing three discrimination methodologies proposed for use in choice preparations with food rewards. Subjects were thirty-five, healthy, adult male Siamese fighting fish (Betta splendens). Subjects were randomly assigned to one of three discrimination groups: a Direction group (using left or right as discriminative stimuli), a Color group (using red or green as discriminative stimuli), and a Bubble group (using the presence or absence of air bubbles as discriminative stimuli). For all three discrimination groups, subjects chose between one or three food pellets in a submerged T-maze. The results from the experiment indicated a statistically significant preference for the three pellets of food over one pellet of food only for the Bubble group. Of particular note is the effect size and observed power obtained for the Bubble group, which was the only group which supported a reasonably powerful test for discrimination, given a sample size of 12 subjects and 0.94 effect size.     

Discrete and continuous measures of dimensional stimulus control

In two sets of experiments, we examined dimensional stimulus control of pigeons' responses to a visual flicker-rate continuum. In the first experiment, responses to a single key were reinforced periodically during stimuli from one half of the stimulus continuum, and responses during other stimuli were extinguished. In the second experiment, two response keys were simultaneously available, with reinforcement for each response alternative associated with different halves of the stimulus continuum. Conditions of the second experiment involved either free-operant or discrete-trial stimulus presentations. Results from these experiments show that positive dimensional contrast appeared in discrimination tasks with one or two response alternatives, but only with free-operant procedures. In addition, discrimination between stimulus classes established by differential reinforcement was assessed as accurately by continuous rate measures as by discrete response choice in the two-alternative situation. The general implication of these experiments is that response rate measures, when properly applied, may reveal sources of variation within stimulus classes, such as dimensional contrast, that are not evident with discrete measures.     

Categories and context in the perception of isolated steady-state vowels

The noncategorical perception of isolated vowels has been attributed to the availability of auditory memory in discrimination. In our first experiment, using vowels from an /i/-/I/epsilon) continuum in a same-different (AX) task and comparing the results with predictions derived from a separate identification test, we demonstrated that vowels are perceived more nearly categorically if auditory memory is degraded by extending the interstimulus interval and/or filling it with irrelevant vowel sounds. In a second experiment, we used a similar paradigm, but in addition to presenting a separate identification test, we elicited labeling responses to the AX pairs used in the discrimination task. We found that AX labeling responses predicted discrimination performance quite well, regardless of whether auditory memory was available, whereas the predictions from the separate identification test were more poorly matched by the obtained data. The AX labeling reponses showed large contrast effects (both proactive and retroactive) that were greatly reduced when auditory memory was interfered with. We conclude from the presence of these contrast effects that vowels are not perceived categorically (that is, absolutely). However, it seems that by taking the effects of context into account properly, discrimination performance can be quite accurately predicted from labeling data, suggesting that vowel discrimination, like consonant discrimination, may be mediated by phonetic labels.     

The minimal group paradigm: Categorization into two versus three groups

The minimal group paradigm (Tajfel, Billig, Bundy and Flament, 1971) has been influential in the study of intergroup relations. Thus far, most minimal group experiments have divided the subjects either into two groups, or have categorized them on two separate dichotomous dimensions in cross-categorization experiments. This study examines the minimal group paradigm using three distinct and independent groups. Comparison of the results with three minimal groups with those of a baseline two-group experiment shows that with a three-group structure there is no significant ingroup bias. It is suggested that the two-group minimal group experiment shows ingroup bias because subjects access a dichotomous categorization, and that this dichotomous categorization primes a competitive orientation. A two-group context may be particularly efective in evoking an ‘us versus them’ contrast. Self-categorization as a group member is more likely to occur in the presence of two groups whereas three minimal groups renders an ‘us–them’ contrastive orientation less salient. The absence of intergroup discrimination found in the present minimal group study may be limited to the behaviour of minimal or artificially created groups. In the real world of intergroup relations discrimination towards multiple outgroups is a well-known phenomenon. While this study should be regarded as only preliminary research, further elaboration and specification of the conditions under which multiple group contexts may hinder intergroup discrimination is required.     

Behavioral contrast without response-rate reduction

Behavioral contrast was obtained in two experiments, which both employed a standard free-operant successive discrimination (a multiple variable-interval extinction schedule), without the occurrence of reductions of response rate in the extinction component. In Experiment I, one group of four pigeons was trained on a multiple schedule in which one stimulus was associated with a variable-interval schedule and the second stimulus with response-independent reinforcement on a free variable-interval schedule. Though by the end of this training three pigeons were responding very little to the second stimulus, when this stimulus was associated with extinction all subjects showed a contrast effect. In Experiment II, eight pigeons were trained extensively to respond to a single stimulus on a variable-interval schedule, before a second stimulus associated with extinction was introduced. This second stimulus was dissimilar to the initial stimulus and five pigeons never responded in its presence. Nevertheless, all pigeons showed a contrast effect and there was no evidence that the effect was smaller in errorless subjects or smaller than in a subsequent discrimination where all subjects made many errors. Both experiments indicated that response reduction in one component of a multiple schedule is not a necessary condition for the occurrence of contrast.     

Finite mixture distribution models of simple discrimination learning

Through the application of finite mixture distribution models, we investigated the existence of distinct modes of behavior in learning a simple discrimination. The data were obtained in a repeated measures study in which subjects aged 6 to 10 years carried out a simple discrimination learning task. In contrast to distribution models of exclusively rational learners or exclusively incremental learners, a mixture distribution model of rational learners and slow learners was found to fit the data of all measurement occasions and all age groups. Hence, the finite mixture distribution analysis provides strong support for the existence of distinct modes of learning behavior. The results of a second experiment support this conclusion by crossvalidation of the models that fit the data of the first experiment. The effect of verbally labeling the values on the relevant stimulus dimension and the consistency of behavior over measurement occasions are related to the mixture model estimates.     

Stimulus contrast in tactile resolutlon

This experiment was designed to test the influence of contrast on tactile resolution by the use of different modes of distribution of pressure stimuli, with control of various factors such as intensity, area, rate, and time of application. The experimental task was two-point discrimination. The results showed that the use of annular stimuli result in a significantly higher number of resolutions than the use of circular stimuli, the intensity, total stimulating area, and pressure being controlled. The proposed interpretation of the results is that increase of stimulus contrast increases the resolving power of the somesthetic system.     

The peak shift in stimulus generalization: equivalent effects of errors and noncontingent shock

Terrace suggested that the peak shift in stimulus generalization occurs because the training stimulus not correlated with reinforcement has become aversive. This hypothesis is plausible in the light of instances where the peak shift is obtained compared with those where it fails to appear. The present experiment attempted to test implications of this hypothesis. Two groups of pigeons learned the same two-stimulus discrimination between colors by different training methods in a free-operant situation. When the discrimination was trained with many errors, a large peak shift was obtained in a subsequent generalization test of wavelength; after discrimination training with few errors, a negligible shift was observed. Half of each group then received noncontingent aversive shock during presentations of the stimulus not correlated with reinforcement in continued discrimination training. After this treatment, the errorless-shock subgroup showed a large peak shift and the error-shock subgroup tended to show a larger shift than before. Nonshocked control groups showed little change in the peak shift. It was concluded that pairing aversive shock with a stimulus not correlated with reinforcement is sufficient to produce or enhance a peak shift. In their effect on the peak shift, aversive shock and large amounts of nonreinforced responding appear to be equivalent.     

The role of age and prior beliefs in contingency judgment

This experiment investigated how prior beliefs affect young and older adults' ability to detect differences in objective contingency. Participants received new evidence that the objective contingency between two events was positive, negative, or zero when they believed that there was a positive or negative relationship between events, when they believed that the events were unrelated, and when they had no knowledge of the relationship between the events. They were then asked to estimate the objective contingency and recall the contingency evidence. Beliefs that events were or could be related improved young adults' contingency discrimination. Moreover, these beliefs did not produce biases in young adults' memory for the contingency evidence, but rather affected how they weighted this evidence at judgment. In contrast, these same beliefs did not improve older adults' contingency discrimination, but did produce biases in their memory for the evidence that were similar to those seen in their judgment. These findings are discussed in terms of age-related changes in working memory executive processes that impair older adults' ability to fully evaluate both belief-confirming and disconfirming contingency evidence and update their beliefs with this information.     

When does it hurt? Intergroup permeability moderates the link between discrimination and self‐esteem

Research shows that personal discrimination and group discrimination have distinct effects on personal self‐esteem. Specifically, whereas personal discrimination negatively impacts self‐esteem, group discrimination increases it. We suggest that this pattern is dependent on the socio‐structural context in which individuals experience discrimination. To test this hypothesis, we manipulate intergroup permeability and examine its impact on the link between personal/group discrimination and personal self‐esteem. Results show that a control condition replicates previous research, that is, a positive association between group discrimination and self‐esteem and a negative association for personal discrimination. The positive association of group discrimination disappeared in a permeable context and reversed when the context was presented as impermeable. Moreover, the deleterious effect of personal discrimination on self‐esteem vanished in impermeable contexts. Results are discussed in light of the literature on stigmatization. Copyright © 2015 John Wiley & Sons, Ltd.     

Behavioural contrast and preference in a double chained schedule

Pigeons peck faster during a signal for reward (S+) when that signal alternates with one for absence of reward (S-). This “contrast effect” has been shown to involve diminished preference for S + compared with a stimulus not involved in a discrimination. The present experiment demonstrates that the signal produced by pecks to S+ in a chained schedule is responded to in proportion to the contrast effect during S +. The result suggests that a prior interpretation of contrast, in terms of Amsel's frustration theory, is not the correct one.