Transfer of facilitation in the rat

In two experiments, rats solved two concurrent discrimination problems in which one stimulus (i.e., a facilitator) signaled the reinforcement of another stimulus (i.e., a target). Then a transfer test assessed the capacity of facilitators trained in one problem to promote responding to targets trained in the other. Experiment 1 found that a facilitator promoted as much responding to such a transfer target as to the target with which it was originally trained. Transfer was not obtained with a pseudofacilitator that was uninformative, in training, about the reinforcement of its target. Experiment 2 manipulated the stimulus modality of the targets and facilitators. Its results indicated that transfer performance was not due to generalization between training and transfer targets or facilitators. These results parallel those from comparable autoshaping paradigms with pigeons, and they agree with the view that facilitators promote responding by lowering the threshold for activation of the US representation.     

Effects of memory reactivation treatments on postdiscrimination generalization performance in pigeons

In Experiment 1, it was shown that generalization testing following successive discrimination training between two closely spaced wavelengths results in a sharp gradient with a peak of responding shifted from S+ so as to be further removed from S?. Testing after a 24-h delay resulted in a flatter gradient with greater peak (and area) shift. A 5-min pretest exposure to S+, reinforced or unreinforced, or to S? (unreinforced) reinstated immediate test performance; free reinforcement with no discriminative stimulus present had no such effect. Experiment 2 replicated the flattening of generalization gradients and enhanced peak shift in delayed testing. Free feeding in a pretest treatment with a distinctive food uniquely associated with the wavelength discrimination problem failed to reinstate immediate test performance. Experiment 3 tested the hypothesis that free feeding failed as a reactivation treatment because it did not engender keypecking. Subjects were trained to peck a vertical line stimulus before being given wavelength discrimination training. Again, the enhanced peak shift and greater flattening with delayed wavelength generalization testing was found. A pretest exposure to the vertical line stimulus elicited pecking but had no effect on subsequent wavelength generalization. Thus, only a reactivation treatment that included one of the discrimination training stimuli was effective in producing delayed test performance comparable to that obtained in an immediate test.     

Temporal generalization and peak shift in humans

Three experiments investigated temporal generalization in humans. In Experiment 1, a peak shift effect was produced when participants were given intradimensional discrimination training. In Experiment 2, after training with a standard S1 and generalization testing with an asymmetrical series of durations, generalization gradients moved toward the prevailing adaptation level. In Experiment 3, generalization gradients showed a central tendency shift and moved away from the S1 and toward the mean of the test series (of 11 stimulus durations) after participants received training with S1 and S2 durations that in one condition were the 2nd and 4th and in another condition were the 4th and 2nd stimuli in the series, respectively. The results were inconsistent with an absolute account of peak shift but were consistent with an adaptation level account of peak shift.     

Modulation of attention in discrimination learning: The roles of stimulus relevance and stimulus–outcome correlation

The advantage in learning after an intradimensional shift rather than an extradimensional shift has been widely used as a behavioural marker of attentional changes during discrimination learning in different fields of neuroscientific study. However, some of the factors assumed to guide these attentional changes have not been completely disentangled by previous research. In two predictive-learning experiments, we investigated the importance of stimulus relevance and of stimulus-outcome correlation for the modulation of attention. In each experiment, participants were trained on two discrimination problems given in successive order. Each problem required participants to differentiate stimuli varying on two dimensions. We found that acquisition of the second discrimination was influenced by whether its relevant dimension (Exp. 1) or its irrelevant dimension (Exp. 2) had previously been trained as relevant and uncorrelated or as irrelevant and uncorrelated. We also observed that acquisition of the second discrimination was independent of whether its relevant dimension (Exp. 1) or its irrelevant dimension (Exp. 2) had previously been trained as relevant and uncorrelated or as relevant and correlated. Our results indicated that the modulation of attention is guided by stimulus relevance and not by stimulus-outcome correlation.     

Preweanling and adult rats treat conditioned light-tone combinations differently

Ontogenetic differences in processing light-tone compounds were discovered in preweanling (17-day-old) and adult (60–80-day-old) rats. Suppression of general activity was used as an index of the magnitude of conditioned fear following a single training session in which a CS+ was paired with mild footshock. In Experiment 1, rats were trained on discriminations in which the CS? consisted of a light and the CS+ was either a tone alone (simple discrimination) or a light-tone compound (simultaneous feature-positive discrimination). Adults and preweanlings given each type of discrimination were then tested for fear of the CS? and a target stimulus (tone alone or light-tone compound). Adults in all groups displayed greater fear of the target than of the CS?. Preweanlings, however, discriminated the CS? from the target only when the target was the same as the original CS+. Experiment 2 revealed that age-related differences in conventional stimulus generalization is not a likely explanation for the pattern of results found in Experiment 1. Experiment 3 revealed age-related differences in expressed fear of a serial feature-positive discrimination; adults, but not preweanlings, showed greater fear of the compound than of the CS?. Alternative interpretations of the results from these experiments are discussed, and the general conclusion is that adults appear more inclined to process elements of a compound stimulus selectively, whereas preweanlings seem more likely to process the compound unselectively, with roughly equivalent processing of each element.     

Transfer of occasion setting: The role of generalization decrement

In Experiment 1, pigeons were trained on two feature-positive discriminations. A transfer test examined whether the feature from one discrimination enhanced responding to the target from the other. Transfer was obtained, but it was incomplete; the feature produced less responding to the transfer target than to its own. Experiments 2 and 3 examined whether this attenuation of responding was the product of generalization decrement induced by the novel combination of feature and target on transfer trials. Birds were trained on a pair of pseudo-occasion-setting discriminations in which each target was reinforced whether or not it was preceded by its feature. In a subsequent test, there was no loss of responding when novel combinations of features and targets were introduced; on the contrary, responding was, if anything, enhanced in this condition. This suggests that imperfect transfer is not due to generalization decrement but to the fact that an occasion setter is specific to its target stimulus.     

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.     

Contextual stimuli and memory retrieval in pigeons

Pigeons were given free-operant successive discrimination training in which, on alternate days, two different interdimensional problems were employed (color positive and line angle negative; a different color negative and a different line angle positive). Between days, these problems could be construed as intradimensional ones. For one group, training was conducted on each problem in the presence of the same ambient (contextual) stimuli, while for a second group each problem was trained in a different context. For a third group, these two contexts were randomly related to the problems. Postdiscrimination stimulus generalization gradients showed that peak shifts were obtained in both the same context and the random context groups, but no peak shifts were found for the group which learned each problem in a different context. Results were consistent with Spear’s (1973) treatment of animal memory which attributes retention test performance to the operation of a “context-elicited” retrieval process. An alternative uniprocess conditioning account of these and similar data was also discussed.     

Extradimensional transfer in the discriminative learning of goldfish

In Experiment I, one group of goldfish (TD) was trained to discriminate blue and green while a second group (PD) was exposed to the same colors in a “pseudodiscrimination,” after which both groups were reinforced for response to a tone. The TD group subsequently showed a sharper auditory discrimination gradient than the PD group and performed better in a differentially reinforced tone discrimination. The former PD animals then were given true discrimination training and the former TD animals pseudodiscrimination training with the colors, after which the first group showed better tone discrimination than the second. These results are analogous to those found in pigeons and rats. In Experiment II, goldfish which were trained in an easy color discrimination and shifted to a more difficult tone discrimination performed better than a control group trained from the outset with the tones. This result suggests that the dimensional specificity of the processes responsible for “transfer along a continuum” cannot safely be assumed in the absence of appropriate controls.     

The role of experimenter-odor cues in the performance of object-memory tasks by rats

Two experiments assessed whether odors left on stimulus objects by experimenters who handle them might confound the interpretation of ostensibly visually guided object-memory tasks for rats. In Experiment 1, rats were able to discriminate the relative recency with which an experimenter touched two otherwise identical objects (intertouch interval = 4 sec), presumably on the basis of an odorintensity discrimination. However, after the rats mastered the odor discrimination with no delay between when the second of the two stimulus objects was last touched by the experimenter and when the rats were permitted to attempt the discrimination, their performance dropped to chance levels when this delay was increased to 15 sec. In Experiment 2, rats were trained in two slightly different ways to perform a delayed-nonmatching-to-sample (DNMS) task, one that involved systematic differences in the temporal order in which the experimenter handled the sample and novel stimulus objects and one that did not. There were no significant differences in the rate at which rats mastered the DNMS task with these two procedures, and the performance of rats that were trained according to the former procedure was unaffected when they were switched to the latter procedure. Moreover, rats required considerably fewer trials to master the DNMS task than the rats in Experiment 1 required to master the odor discrimination. These findings demonstrate that, under certain circumstances, rats can discriminate the relative recency with which two objects are handled by an experimenter, but that this ability contributes little to their performance of conventional object-based DNMS tasks.     

Maintenance of responding when reinforcement becomes delayed

In four experiments with rats, we examined the persistence of behavior when reinforcement was switched from immediate to delayed. In Experiment 1, lever pressing elicited by instrumental training with immediate reinforcement continued when a 20-sec delay of reinforcement was introduced (easy-to-hard condition), whereas when the delay condition was introduced from the start (hard-to-hard condition), responding remained low throughout. A similar result was obtained in Experiment 2, in which lever pressing was elicited by a classical conditioning (autoshaping) procedure. In Experiment 3, rats initially trained with delayed reinforcement continued to respond at a low rate when switched to immediate reinforcement (hard-to-easy condition). By measuring magazine entry (goal tracking) as well as lever pressing (sign tracking) in Experiment 4, we confirmed that such transfer effects at least partly involve the persistence of whatever type of behavior was initially dominant.     

Intradimensional vs. extradimensional transfer in the discriminative learning of goldfish and pigeons

Evidence of better intradimensional than extradimensional transfer was sought in naive goldfish trained under free-operant single-stimulus conditions (Experiment I), sophisticated pigeons trained under free-operant single-stimulus conditions (Experiment II), sophisticated pigeons trained under discrete-trials choice conditions (Experiment III), and naive pigeons trained under discrete-trials choice conditions (Experiment IV). The results provide no support for attention theory.     

Transfer between Pavlovian facilitators and instrumental discriminative stimuli

Two experiments assessed the degree to which Pavlovian facilitators were interchangeable with instrumental discriminative stimuli (S^ds). In Experiment 1, rats were trained in a Pavlovian paradigm in which one stimulus (i.e., a facilitator) signaled the reinforcement of another stimulus (i.e., a target). Next, the rats were given instrumental discrimination training in which an S^d signaled the reinforcement of barpressing. A transfer test then assessed the capacity of the Pavlovian facilitator to promote barpressing. The results showed that the facilitator promoted significant barpressing, both when it was presented alone and when it was presented in compound with the S^d. Reliable transfer was not obtained with a “pseudofacilitator” control stimulus that, during training, was uninformative about the reinforcement of its target. Experiment 2 showed that a stimulus trained as an instrumental S^d reliably augmented responding to a stimulus previously trained as a target in a Pavlovian facilitation paradigm. A “pseudo-S^d” that, during training, was uninformative about the reinforcement of barpressing failed to promote such transfer. These results show that Pavlovian facilitators and instrumental S^ds are interchangeable to a significant degree, and suggest that facilitators and S^ds may act via similar mechanisms.     

Intrinsic reinforcing properties of putatively neutral stimuli in an instrumental two-lever discrimination task

Four experiments examined the influence of a stimulus presented after one response in a two-lever choice task. In Experiment 1, food-deprived rats trained on a concurrent variable-interval extinction schedule responded more often on the extinction lever when such responding periodically produced a visual stimulus than when it did not. In Experiments 2 and 3, a similar signal-induced enhancement effect was found even when food was delivered randomly with respect to responding on both levers or when no food was presented. In Experiment 4, a response-contingent visual stimulus elevated responding to the lever on which it was presented, but an auditory cue suppressed responding. These findings indicate that visual stimuli may possess intrinsically reinforcing properties for rats.     

The role of the mirror-image reversal effect in reducing the peak shift and area shift along the angularity dimension

Pigeons trained monocularly and tested with the trained eye along the angularity dimension were found to show enhanced area shifts when compared to pigeons trained and tested binocularly. Pigeons trained monocularly and tested with the untrained eye showed reversed peak shifts in the direction of S?. These findings are consistent with the hypothesis that the mirror image of S? in angularity gradients functions to reduce the likelihood of the peak shift and area shift along the angularity dimension in pigeons, and question the use of the angularity dimension for intradimensional discrimination training and generalization testing.     

Stimulus control of topographically tagged responding

Pigeons’ responses on an operant key were reinforced according to either multiple variable-interval variable-interval or multiple variable-interval extinction schedules. The multiple-schedule components were signaled by line-tilt stimuli on a second key (signal key). Signal-key responses never produced reinforcement, and operant-key responses were not reinforced if they followed within 1 sec of a signal-key response. Behavioral contrast was not observed on the operant key, although there was a small, but reliable, increase in signal-key responding in the variable-interval component of the multiple variable-interval extinction condition. Generalization tests were interspersed between sessions of multiple variable-interval extinction training. Generalization gradients along the line-tilt dimension exhibited peak shift for both operant-key and signal-key responding following intradimensional (line tilt) discrimination training. Line-tilt generalization gradients following interdimensional discrimination did not exhibit peak shift. Gradients following intradimensional discrimination were sharper than gradients following interdimensional discrimination for both operant-key and signal-key responding. It was concluded that dimensional stimulus control of topographically tagged responding maintained by the stimulusreinforcer relation parallels that maintained by the response-reinforcer relation.     

Flavor aversions and deprivation state

In the first four experiments, it was found that aversions to saccharin solution produced by contingent poisoning were similar regardless of whether the rats had been trained under the test deprivation or under a different deprivation; the two deprivation states used were thirst and satiety. In Experiment 5, rats were poisoned after drinking grape juice while hungry or poisoned after drinking milk while thirsty, but they were not poisoned after grape-thirst or milk-hunger combinations. In abstract terms, poisoning occurred after AX and BY stimulus combinations, but did not occur after AY and BX combinations. There was some learning under these discrimination conditions.     

The learned function of food-deprivation cues: A role for conditioned modulation

Rats trained in one context to use stimuli arising from food deprivation as discriminative signals for shock were tested in other contexts to assess the basis of conditioned responding (i.e., freezing or behavioral immobility). In Experiment 1, discriminative control by 24-h food-deprivation cues failed to promote transfer responding in a test context that had no association with shock. This indicated that food deprivation cues had little direct excitatory power. However, transfer of behavioral control by 24-h food-deprivation cues was obtained in a context paired with shock only when the rats were 19 h water deprived. This finding agrees with the idea that food-deprivation cues become conditioned modulators of the capacity of external stimuli to activate their association with an unconditioned stimulus. In Experiment 2, rats trained to use 24-h food-deprivation cues as signals for shock exhibited significantly greater transfer performance when the transfer context had undergone partial extinction relative to when the transfer context had undergone only simple excitatory training. This finding with deprivation cues and transfer contexts (1) paralleled earlier results obtained with discrete (auditory and visual) conditioned modulators and transfer targets, and (2) posed difficulties for associative summation and generalization interpretations of transfer performance.     

Contextual cues and the retrieval of competing memories of goal events

In Experiment 1, four groups of rats were initially trained on a discrimination which established a stimulus as a signal for reinforcement. That signal was then presented during subsequent partial reinforcement training in a way that could potentially interfere with retrieval of the memory of nonreinforcement (S^N) on the preceding trial either because (1) thestorage and retrieval contexts for S^N were different (retrieval failure hypothesis), or (2) the memory of reinforcement produced by the signal acted as a competing memory (competing memory hypothesis). Experiment 1 supported the competing memory hypothesis. In Experiment 2, we investigated the effect of stimulus change on the capacity of the context to retrieve a competing memory of a temporally remote reinforcement event with which the context was strongly associated. Retrieval of a competing memory was impaired by differences between the storage and retrieval contexts in a manner analogous to the effect of context on retrieval of a reinforcement event memory from an immediately preceding trial.     

The effect of the controllability of auditory discriminative stimuli in the performance of go/no-go discriminations by pigeons

Compared with their performance with localized (on-key) visual stimuli, pigeons are notoriously poor at performing go/no-go discriminations when keypecking for food in the presence of auditory discriminative stimuli. The difference might reflect the fact that an aversive visual onkey stimulus signaling nonreward can be escaped by looking away and not pecking, which contributes to the measure of good discriminative performance, while an auditory stimulus cannot be escaped. In Experiment 1, discriminative performance was significantly improved by providing pigeons with a response incompatible with keypecking by which they could escape a tone S+ and a tone S?. However, the pattern, frequency, and duration of escape responses were found to be insufficient to explain the improvement. In Experiment 2, it was found that the capacity to escape only S+ or only S? enhanced discriminative performance as much as the capacity to escape both. It is theorized that the Pavlovian relationship between the absence of the discriminative stimuli and the nonoccurrence of food might transfer to the instrumental relationships learned in a go/no-go discrimination. The possibility that intermittent stimuli command more attention than continuous stimuli is also considered.