A search for contrast effects in discrete-trial discrimination learning by pigeons

Behavioral contrast has often been observed in free-operant experiments with pigeons, rarely in discrete-trial experiments with rats. Although Jenkins (1961) and Terrace (1963) have reported a discrete-trial contrast effect in pigeons, a series of experiments reported here found no evidence that latency of responding to S+ in a discrete-trial situation was reliably decreased by alternating reinforced trials to S+ with nonreinforced trials to S?. Latency of responding to S+ was affected neither by the length of the preceding intertrial interval (within the range of 10–60 sec), nor by whether the preceding trial had been to S+ or to S?. The results of two of these experiments suggested that the appearance of positive contrast in Jenkins's experiment was a consequence of differences in the variability of the intertrial interval experienced by control and discrimination groups. In two final experiments, employing standard free-operant procedures, contrast was observed as an increase in rate of responding to S+, but not as a decrease in latency of the first response on each S+ trial. The implication is that contrast effects are more readily observed with the rate measures of free-operant experiments than with the latency measures of discrete-trial experiments.     

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.     

Rapid acquisition of discrete-trial lever-press avoidance: effects of signal-shock interval

Acquisition of discrete-trial lever-press avoidance learning was studied in three experiments. Experiment I compared a new training procedure, which produces rates of lever-press avoidance learning comparable to those obtained in shuttle boxes, with a “conventional”, less efficient training procedure. A factorial design was used to compare continuous versus intermittent shock and a long-variable versus a short-fixed signal-shock interval. Learning was best in the groups trained with the long and variable interval and poorest in those trained with the short and fixed interval. Type of shock had no effect. Experiment II separated the effects of duration from those of variability of the signal-shock interval. Fixed and variable intervals of 10 and 60 sec were tested and duration was the only significant factor. Experiment III addressed the effect of the differential opportunity to avoid provided by long signal-shock intervals by varying this interval from 10 to 60 sec in 10-sec steps. Only the 10-sec group showed slow acquisition relative to the others. Analysis of avoidance response latencies showed that the distributions for all groups were positively skewed and that skewness increased with increasing duration of the signal-shock interval. At intervals longer than 20 sec, the animals made progressively less use of their increased opportunity to respond. The data do not support the opportunity-to-respond interpretation of the effects of duration of signal-shock interval and suggest that some type of inhibitory process may block lever-press avoidance learning at intervals as short as 10 sec. The significance of these findings for species-specific defense reaction and preparedness theories was emphasized.     

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.     

Auto-maintenance in the pigeon: sustained pecking despite contingent non-reinforcement

If a response key is regularly illuminated for several seconds before food is presented, pigeons will peck it after a moderate number of pairings; this “auto-shaping” procedure of Brown and Jenkins (1968) was explored further in the present series of four experiments. The first showed that pecking was maintained even when pecks turned off the key and prevented reinforcement (auto-maintenance); the second controlled for possible effects of generalization and stimulus change. Two other experiments explored procedures that manipulated the tendency to peck the negatively correlated key by introducing alternative response keys which had no scheduled consequences. The results indicate that pecking can be established and maintained by certain stimulus-reinforcer relationships, independent of explicit or adventitious contingencies between response and reinforcer.     

The effects of feature type in operant serial feature-positive discriminations

In two experiments, rats were trained on two operant serial feature positive discriminations in which one feature was a flavored solution and the second feature was a visual or auditory cue. As in a previous study ([Goddard and Holland, 1996]), transfer of a feature’s control to the target of the other discrimination was not observed when the flavor feature and the reinforcer were flavored sucrose solutions (Experiment 1). The performance of comparison groups showed that this lack of transfer was not due to confounded differences in the event contingencies resulting from having similar stimuli serve as feature and reinforcer. By contrast, in Experiment 2, transfer was observed between visual and flavor features when the flavor feature was unsweetened and the reinforcer was plain sucrose. These results suggest that the lack of transfer in Experiment 1 and in [Goddard and Holland, 1996] study were related to the biological significance or hedonic properties of the sucrose feature.     

Attentional effects on concurrent psychophysical discriminations: Investigations of a sample-size model

In two experiments, a concurrent discrimination paradigm was used to study the effects of visual attention on psychophysical judgments and the consistency of these effects with a sample-size model in which attention influences the variance of the internal representation used to make psychophysical judgments. Two pairs of lines were presented simultaneously—one on each side of fixation—and subjects had to indicate for each pair separately whether or not the lines had the same length. Attention was manipulated by instructing subjects to pay 100%, 75%, 50%, 25%, or 0% of their attention to the discrimination on one side, with the complementary amount of attention to the other side. In the first experiment, the relationship between attention and discrimination accuracy was consistent with the sample-size model both when attentional allocation varied from trial to trial and when it varied between blocks, and the relationship held over more widely varying attentional allocations than had previously been studied. In addition, discriminations were more accurate overall with varied than with blocked attentional allocation, suggesting that the two types of allocation do not merely differ in the degree to which attention is focused. The second experiment examined the effects of attentional allocation and stimulus variance, the latter being manipulated by randomly incrementing or decrementing line lengths. These manipulations had additive effects on total Thurstonian variance, and a version of the samplesize model gave an excellent quantitative fit to the obtained results. Besides supporting the samplesize model, the results of Experiment 2 suggest that criterion variance is at least as large as sensory variance and that criterion but not sensory variance increases with stimulus variance.     

Behavioral contrast: Pavlovian effects and anticipatory contrast.

Two sources of behavioral contrast have been identified previously: Pavlovian stimulus-reinforcer relations and component sequence effects (anticipatory contrast). This study sought to isolate these sources of control procedurally in a four-ply multiple schedule composed of two fixed two-component sequences. Different cues were associated with the first component of each sequence, and contrast effects were studied in these target components. In Experiment 1, differential cuing of Component 2 between sequences and availability of reinforcement during target components were varied across three groups of pigeons; the stimulus-reinforcer relation between target-component cues and schedule of reinforcement in Component 2 was varied within subjects. Control by the Pavlovian relation was demonstrated under all conditions, and anticipatory contrast was not observed. In Experiment 2, target-component duration was systematically varied in the three groups of Experiment 1. Control by the Pavlovian relation was reliably obtained only when target-component behavior was unreinforced, and diminished with increases in component duration. Anticipatory contrast emerged in the two groups for which target-component reinforcement was available. These and other data indicate that Pavlovian effects in multiple schedules may be obscured when the requisite conditions for anticipatory contrast are present.     

Specificity in contrast effects: Judgments of psychopathology

Three experiments were conducted to explore the specificity of the contrast effect in judgments of psychopathology. In the first two studies, respondents initially attempted to infer whether each item in a series of behavior samples (vocabulary definitions in one study, handwriting samples in the second) came from a schizophrenic or a nonschizophrenic patient. Some respondents were presented with highly pathological samples in this induction series, while others were presented with relatively nonpathological samples. These divergent experiences led to marked contrast effects in evaluating test stimuli from the same behavioral domain (e.g., additional vocabulary definitions), but had significantly less impact on the respondent's reactions to stimuli from a different domain (e.g., handwriting samples). A third experiment yielded similar results, using a paired-comparison methodology. In this study subjects first judged a series of high- vs low-pathology definitions. They were then presented with a series of matched stimulus pairs, each including one vocabulary definition and one hand-writing sample. Respondents were to indicate the member of each pair that seemed more indicative of schizophrenia. People assigned to the high-pathology induction group typically chose the handwriting samples as being more indicative of schizophrenia, compared with respondents who were assigned to the low-pathology conditions. These results were interpreted as supporting a representational (perceptual) theory of contrast.     

Contextual effects in vowel perception I: Anchor-induced contrast effects

Results from recent experiments using a selective adaption paradigm with vowels have been interpreted as the result of the fatigue of a set of feature detectors. These results could also be interpreted, however, as resulting from changes in auditory memory (auditory contrast) or changing response criteria (response bias). In the present studies, subjects listened to vowels under two conditions: an equiprobable control, with each of the stimuli occurring equally often, and an anchoring condition, with one vowel occurring more often than any of the others. Contrast effects were found in that vowel category boundaries tended to shift toward the category of the anchor, relative to the equiprobable control. Results from these experiments were highly similar to previous selective adaptation results and suggest that neither feature detector fatigue nor response criterion changes can adequately account for the adaptation/ anchoring results found with vowels.     

Comparing learned predictiveness effects within and across compound discriminations

In four human learning experiments, we examined the extent to which learned predictiveness depends upon direct comparison between relatively good and poor predictors. Participants initially solved (a) linear compound discriminations in which one or both of the stimuli in each compound were predictive of the correct outcome, (b) biconditional discriminations where only the configurations of the stimuli were predictive of the correct outcome, or (c) pseudodiscriminations in which no stimulus features were predictive. In each experiment, subsequent learning and test stages were used to assay changes in the associability of each stimulus brought about by its role in the initial discriminations. Although learned predictiveness effects were observed in all experiments (i.e., previously predictive cues were more readily associated with a new outcome than previously nonpredictive cues), the same changes in associability were observed regardless of whether the stimulus was initially learned about in the presence of an equally predictive, more predictive, or less predictive stimulus. The results suggest that learned associability is not controlled by competitive allocation of attention, but rather by the absolute predictiveness of each individual cue.     

Timing in free-operant and discrete-trial avoidance

A procedure (“discrete-trial” avoidance) was devised to differentiate between the two main theories of responding in Sidman's “free-operant” avoidance procedure. One theory, a version of two-factor theory, holds that responding is reinforced by the removal of a conditioned aversive stimulus. The conditioned aversive stimulus is held to be temporal, which accounts for the spaced responding, or timing, that Sidman's procedure produces. The other theory holds that the reinforcement for both responding and timing is shock-frequency reduction. The new procedure eliminated this reinforcement for timing, but retained the conditions for the formation of conditioned aversive temporal stimuli. According to one theory, the new procedure should have sustained timing as well as Sidman's, while according to the other, it should have sustained no timing. The results confirmed neither theory. Timing was found with both procedures, but unequally in degree and kind. Large variations in the precision of timing did not appear to be correlated with successful avoidance for either procedure.     

Behavioural contrast does not occur with continuous reinforcement in the positive component

An experiment reported by Jenkins (1961) obtained positive behavioural contrast in a discrimination with continuous reinforcement of the positive stimulus. A large body of research, however, has failed to obtain positive contrast with this type of schedule. Jenkins's experiment was repeated with an added control group matched to the discrimination group in number of nonreinforced components. No positive contrast was obtained, and it was concluded that Jenkins's result could not be interpreted as positive contrast. The implications of this result for some theoretical accounts of positive contrast were discussed.     

Responding under discrete-trial fixed-interval schedules of reinforcement

A fixed-interval schedule of reinforcement was modified by dividing each interval into 4-sec trial periods. No more than one response could occur during each trial because the operandum was inactivated for the remainder of any trial in which a response occurred. For example, under a 28-sec schedule, no more than seven responses could be emitted between reinforcements. Probabilities of responding by pigeons under six values of this discrete-trial fixed-interval schedule were best described by a two-state model: responding was either absent or infrequent immediately after reinforcement; then, at some variable time after reinforcement, there was an abrupt transition to a high and constant probability of responding on each trial. Performances under the discrete-trial procedure were less affected by uncontrolled sources of variance than performances under equivalent free-operant fixed-interval schedules.