Determinants of value transfer and contrast in simultaneous discriminations by pigeons

In a simultaneous discrimination involving a positive (S+) and a negative (S₋) stimulus, positive value appears to transfer from the S+ to the S₋. However, negative value does not appear to transfer from the S₋ to the S+. Instead, when sufficient experience with the contingencies associated with responding to the S₋ is provided, it appears that the presence of the S₋ enhances the value of the S+ (i.e., a contrast effect is found). The purpose of the present experiments was to further examine the influence of the S+ on the S₋ in a simultaneous discrimination (between subjects in Experiment 1 and within subjects in Experiment 2). In both experiments, we found that under typical training conditions, given little direct experience with the value of the S₋, value transfers from the S+ to the S₋. If sufficient experience with the value of the S₋ is provided, however, contrast between the S+ and the S₋ can be demonstrated. Thus, in a simultaneous discrimination, value transfer from the S+ to the S₋ depends on the animal’s having responded relatively little to the S₋.     

Response modes in simultaneous and successive visual discriminations

Modes of responding in simultaneous and successive discrimination problems were investigated using a bidirectional transfer procedure. Rats receiving training on a simultaneous discrimination problem were transferred to successive discriminations. Rats trained on a successive problem were transferred to simultaneous problems. The transfer problems were either compatible or incompatible with the response mode present during training. The results suggest that a single response mode may underlie the solution of both simultaneous and successive discrimination problem.     

Within-event learning contributes to value transfer in simultaneous instrumental discriminations by pigeons

Following simultaneous discrimination training with pigeons, in which responding to the S−was reinforced on half of the trials and responding to the S− was never reinforced, we examined the effect on the S− of presenting the S− by itself and the effect on the S+ of presenting the S− by itself (relative to an S− or an S− for which there had been no single-stimulus presentations). For Group A−, responding to the S− presented by itself was always reinforced, whereas for Group A−, such responding was extinguished. For Group B−, responding to the S− presented by itself was always reinforced, whereas for Group B+, responding was extinguished. Although both Group A+ and Group A−tended to avoid their associated S− (relative to a control S−), Group A+ avoided its associated S− less than did Group A−. In contrast, although for Group B−, presentation of the S− alone increased the pigeons’ preference for its associated S−(relative to a control S+), for Group B−, presentation of the S−alone had little effect on its preference for its associated S+. These results suggest that presentation of one stimulus from a simultaneous discrimination has two independent and sometimes opposite effects on the other discriminative stimulus. First, it reduces the strength of within-event conditioning between the S+ and the S−, and second, if the value of the singly presented stimulus has increased, some of its newly acquired value will transfer retroactively to the stimulus with which it was originally paired.     

Interocular transfer of simultaneous but not successive discriminations in the pigeon

Pigeons were trained with one eye covered on each of two types of visual discriminations. They then were tested for interocular transfer with the previously covered eye. Transfer was shown by every pigeon trained on a simultaneous discrimination, while lack of transfer was shown by thesesame pigeons when trained on a spatial conditional (successive) discrimination. As opposed to the pigeon, animals with a larger proportion of ipsilateral (uncrossed) retinal fibers (e.g., cats) do show transfer of both discrimination problems. This difference in the decussation of the optic pathways may be a critical variable in interocular transfer in vertebrates. Furthermore, these studies demonstrate that interocular transfer in the pigeon depends upon the experimental paradigm.     

Apparent disinhibition of successive but not of simultaneous negative contrast

In a series of three experiments, rats shifted from a 32% to a 4% sucrose solution, after 10 days’ exposure to the 32% solution, exhibited a negative contrast effect in lick rate. In each experiment, shifted rats that received a novel stimulus (tone) during the postshift period exhibited a higher lick rate (smaller contrast effect) than shifted subjects not receiving the tone. This increase in lick rate resembles Pavlovian disinhibition and is interpreted as supporting an inhibitory view of successive negative contrast effects. Control conditions included in Experiments 2 and 3 favored the disinhibition interpretation of the effect of the tone, as opposed to a rate-dependency hypothesis or to the nonspecific energization of behavior. In Experiments 4–6, the tone was introduced coincident with the occurrence of a simultaneous negative contrast effect. Rather than disinhibition, a decrease in licking occurred. These results were discussed in terms of differences between successive and simultaneous contrast.     

Delayed discriminations in the pigeon: The role of within-trial location of conditional cues

Four pigeons were trained in a delayed conditional discrimination in which color and line cues jointly indicated trial outcome. These were either combined in advance of a retention interval (RI) or separately presented before and after the RI. The former procedure resulted in less forgetting over the RI, the difference increasing with longer RIs. In a second study, the line cue was presented redundantly before and after the RI, and then selectively omitted from either temporal location during probe tests. In general, the results indicated that the birds relied upon the line as a cue to a greater extent when it was compounded with the color in advance of the RI than when it was presented after the RI. The data support an interpretation based on anticipatory processing in working memory, which leads to better retention than retrospective remembering.     

Conditioning of simultaneous and serial feature-positive discriminations

Three experiments with rat subjects examined the development of simultaneous and serial feature-positive discriminations in appetitive conditioning. In Experiment 1, reinforced presentations of a simultaneous light-tone compound were intermixed with nonreinforced presentations of either the light or the tone. The compound stimulus acquired conditioned behaviors of a form characteristic of the predictive feature alone; the element common to reinforced and nonreinforced trials did not evoke conditioned behavior. In Experiment 2, reinforced presentations of a serial light-trace-tone compound were intermixed with nonreinforced tone-alone presentations. The light feature stimulus acquired conditioned behaviors characteristic of visual CSs. The tone stimulus, common to reinforced and nonreinforced trials, evoked conditioned behaviors characteristic of auditory CSs, but only when preceded by the light. In Experiment 3, variations in the interval between the light and tone on reinforced trials had little effect on responding to the light CS but substantially altered the pattern of responding to the tone CS. These results suggested that simultaneous and serial feature-positive discriminations may be solved differently. Performance in simultaneous feature-positive discriminations may be determined solely by associations between the feature stimulus and the reinforcer, but performance in serial discriminations may also involve the acquisition of a conditional cue function to the feature.     

Deprivation level and choice in pigeons: a test of within-trial contrast

Within-trial contrast has been proposed as a mechanism underlying preferences for stimuli that follow relatively more aversive events over stimuli that follow less aversive events. In this study, we manipulated deprivation level to test within-trial contrast predictions. In Experiment 1, pigeons encountered two discriminative stimuli, one presented when they were deprived and the other when they were prefed. When later given a choice between the two stimuli, pigeons strongly preferred the stimulus encountered when deprived, independently of their deprivation level at test. In Experiment 2, pigeons learned two simultaneous discriminations, one when deprived and the other when prefed. Here, subsequent tests between the two S⁺ or the two S⁻ stimuli revealed no consistent preferences. These contrasting findings suggest that differential aversiveness is necessary but not sufficient to induce preferences via within-trial contrast.     

Certainties and mysteries in the within-trial contrast literature: A reply to Zentall (2008)

Contrary to Zentall’s (2008) hypothesis that extensive overtraining is required in order to obtain the work ethic effect in pigeons, we find no evidence of this in two separate paradigms designed to assess preference for stimuli encountered after high effort. In light of these findings, along with our previously published results and those of Arantes and Grace (2008), we maintain that the work ethic effect is not a reliable phenomenon in pigeons. On the other hand, we are uncertain about the reason(s) underlying the different preference profiles for high- versus low-deprivation stimuli as a function of successive versus simultaneous discrimination training (Vasconcelos & Urcuioli, 2008). Nevertheless, we disagree with Zentall that additional analyses of the sort he suggests support the conclusion that both training regimens yield within-trial contrast effects.     

Simultaneous and successive contrast with delay of reward

In Experiment I, rats showed no evidence of a successive negative contrast effect (NCE) when shifted from immediate reward to a 15-sec delay of reward. Experiment II provided a direct comparison of NCE in both successive and simultaneous paradigms. As in Experiment I, there was no evidence of a successive NCE, but a reliable simultaneous NCE was observed. These results add support for the view that simultaneous and successive NCEs are not due to a single, common process.     

Changes in response to S+ and S− during acquisition and overtraining of simultaneous discriminations in rats

Two experiments with rats investigated learning about S+ and S? during various stages of discrimination training. Transfer tests, in which either S+ or S? was retained, or two new stimuli were presented, were used to assess learning early in acquisition, at criterion, or following overtraining. Both choice and latency measures were used. Results indicated that learning about S+ occurs early in acquisition when noncorrection training is given, and little improvement occurs after that time, at least up to 70 trials of overtraining under present conditions. Learning about S? begins somewhat later in acquisition and continues throughout overtraining. When correction training is used, learning about S? occurs earlier in acquisition, and further learning about both S+ and S? occurs during overtraining.     

Extinction and retraining of simultaneous and successive flavor conditioning

In three experiments, rats received pairings of flavor conditioned stimuli with polycose unconditioned stimuli, in either a simultaneous or a sequential relation. Both temporal relations produced excellent conditioned increases in consumption of the flavors. Separate presentation of the flavors resulted in extinction in both cases. However, restoring the pairing of the flavor with polycose resulted in reconditioning only with the sequential, not with the simultaneous, relation.     

Effects of effort and difficulty on human preference for a stimulus: Investigation of the within-trial contrast

The within-trial contrast hypothesis (WTC) provides a more parsimonious explanation for the phenomenon that humans and animals prefer outcomes that follow more effortful events to outcomes that follow less effortful events (Zentall, 2013). We conducted two WTC experiments with human adults. In Experiment 1, we manipulated the difficulty of a preceding event by varying the interresponse time and the limited-hold interval during differential reinforcement with a low response rate schedule, to examine the effect of effort on the preference for the subsequent stimuli. In Experiment 2, we attempted to identify the variables that had affected the results of Experiment 1, by manipulating time as the delay of reinforcement. The results showed preferences based on WTC only when participants made a high rate of incorrect responses in the preceding event, which was used as an index of the strength of individual effort. These results extend the findings of previous human WTC studies and suggest that the difficulty of a task could serve as an aversive event that affects the WTC effect. It is possible that an index based on performance in the preceding event would provide useful information for predicting the contrast effect.     

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.     

Repeated successive contrast in consummatory behavior with repeated shifts in sucrose concentration

Contrast in consummatory behavior was investigated following repeated shifts from 32% to 4% sucrose. In Experiment 1, contrast in licking and in open-field measures of activity occurred following the second and third downshifts. In Experiments 2a and 2b, equivalent contrast effects occurred following the first and second downshifts in sucrose. In Experiment 3, negative contrast remained unabated following nine downshifts in animals shifted between 32% and 4% sucrose on alternate days. Similar results were found for five downshifts in animals shifted every 2 days. In both of these latter conditions, positive contrast occurred over the first few shifts and was then lost as the 32% control group reached asymptote. These data show that repeated negative contrast effects in consummatory behavior are robust and enduring and occur under several different sets of experimental parameters. The results are discussed in terms of reinforcement level and emotional interpretations of contrast effects, and the possibility was suggested that the causal mechanism of contrast changes with repeated shifts.     

Value transfer in concurrent-schedule discriminations by pigeons

When pigeons are trained on a discrete-trial simultaneous discrimination, some of the value associated with the positive stimulus appears to transfer to the negative stimulus (Zentall & Sherburne, 1994). Pigeons preferred a negative stimulus that had been discriminated from an always-positive stimulus (S+) over a negative stimulus that had been discriminated from a sometimes-positive stimulus (S±). A very different finding (suggestive of transitivity of preference or contrast) was reported by Belke (1992). On concurrent probe tests of stimuli associated with equal variable interval (VI) schedules but originally trained in alternative concurrent pairs (one with a richer schedule, the other with a poorer schedule—VI 20 sec vs. VI 40 sec and VI 40 sec vs. VI 80 sec), the stimulus originally paired with the poorer schedule was preferred. But Belke’s results may have been obtained because the pigeons had been trained to peck the VI 40 sec paired with the poorer schedule and they had been trained not to peck the VI 40 sec paired with the richer schedule. In the present experiment, we avoided this bias by training pigeons on two concurrent schedules in which the tested stimuli both had been associated with the poorer schedule of the pair [A(VI 20 sec) vs. B(VI 80 sec) and C(VI 40 sec) vs. D(VI 80 sec)]. Evidence for value transfer was demonstrated when on probe trials pigeons preferred B over D.     

Effect of taste context and ambient context changes on successive negative contrast

Rats shifted from 32% sucrose to 4% sucrose lick less than rats that experience oniy the 4% solution. Previous experiments have found this negative contrast effect to be reduced (“disinhibited”) by the addition of a novel tone in the postshift period. In Experiment 1 of this paper, the negative contrast effect was enhanced when a novel flavor was added to the sucrose solution in the postshift period. In Experiments 2–4, changes in the ambient context, even changes sufficient to produce disruptions in licking, did not alter the degree of negative contrast. Tiese results suggest that (1) rats compare rewards across substantially different contexts, (2) contrast may serve to enhance taste neophobia, and (3) a disinhibitory effect may be confined to the presentation of punctate, nontaste, novel stimuli within a familiar context.     

Value transfer in a simultaneous discrimination by pigeons: The value of the S+ is not specific to the simultaneous discrimination context

During simultaneous discrimination training, there is evidence that some of the value of the S⁺ transfers to the S^?. When the value of the S+ is altered outside the context of the simultaneous discrimination, two very different predictions are made concerning its effect on its S^?, depending on whether one views the S⁺ as an occasion setter or as a stimulus capable of transferring value. In four experiments, pigeons were trained with two similar simultaneous discriminations, A⁺B^? and C⁺D^?, and two single-stimulus trial types, A and C, (in which A always had greater nominal value than C). According to value transfer theory, on test trials, B should always be preferred over D, because B and D should be affected by the net values of A and C, respectively. According to an occasion setting account, however, D should be preferred over B because the presence of D signals a higher probability of reinforcement for responding to C than when C is alone, and/or the presence of B signals a lower probability of reinforcement for responding to A than when A is alone. In all four experiments, the pigeons preferred B over D, a result consistent with value transfer theory. Thus, an S^? can acquire value from an S⁺ even when that value is conditioned in a “context” different from that of the simultaneous discrimination.