Overshadowing of explicitly unpaired conditioned inhibition is disrupted by preexposure to the overshadowed inhibitor

In two conditioned lick-suppression experiments with rats, the interaction of preexposure to a target stimulus and subsequent overshadowing of conditioned inhibition treatment was examined. Blaisdell, Bristol, Gunther, and Miller (1998) have demonstrated that stimulus preexposure and overshadowing counteract each other in their effects on Pavlovian excitation, producing strong excitatory responding to the target stimulus. In the present experiments, a conditioned inhibition analogue of this effect was examined. Overshadowing of conditioned inhibition (i.e., attenuation of inhibitory control of behavior by the target stimulus) was demonstrated when a more salient stimulus was compounded with the target inhibitor during conditioned inhibition training. However, the overshadowing deficit was attenuated by preexposing the target stimulus prior to overshadowing treatment. To account for this phenomenon, we contrast the comparator hypothesis (Miller & Matzel, 1988) with contemporary models that focus on associative acquisition.     

Savings in classical conditioning in the rabbit as a function of extended extinction

In the present experiments, savings phenomena following a limited amount of initial acquisition and extended extinction were examined. Experiments 1 and 2 compared rates of reacquisition following brief acquisition and various amounts of extinction in conditioning of the rabbit’s nictitating membrane and heart rate response, respectively. Experiment 3 compared rates of acquisition to a novel stimulus (e.g., light) following brief acquisition and various amounts of extinction to another stimulus (e.g., tone). In addition, in Experiment 3 recovery of responding to the extinguished stimulus during acquisition to the novel, cross-modal stimulus was examined. Experiments 1, 2, and 3 demonstrated that with a limited number of acquisition trials (1) there was a graded reduction in the rate of reacquisition as a function of the number of extinction trials in both conditioning preparations, (2) there was a graded reduction in the rate of cross-modal acquisition as a function of the number of extinction trials, but (3), in Experiment 3, recovery of responding to the extinguished stimulus during cross-modal training of the novel stimulus appeared uniformly robust even in the face of extended extinction.     

When more is less: Extending training of the blocking association following compound training attenuates the blocking effect

Three conditioned lick suppression experiments with rats were performed to assess the influence, following compound training of two stimuli (A and X) with the same outcome (AX-O trials), of extending training of the blocking association (i.e., A-O) on responding to the target stimulus (X) at test. In Experiment 1, backward blocking was attenuated when the blocking association was extensively trained. Experiment 2 showed that forward blocking was also attenuated by extensive further training of the blocking association following the AX-O trials. Experiment 3 contrasted candidate explanations of the results of Experiments 1 and 2 and demonstrated that these results are consistent with the framework of the extended comparator hypothesis (Denniston, Savastano, & Miller, 2001).     

Biological significance attenuates overshadowing, relative validity, and degraded contingency effects

Miller and Matute (1996) showed that blocking is attenuated when the blocked conditioned stimulus (CS) is “biologically significant” (i.e., when the CS has the potential to elicit vigorous responding of any kind). To the extent that blocking is representative of cue competition, this finding suggests that biological significance protects CSs against cue competition effects in general. In the present experiments, we tested this possibility by examining the influence of biological significance of CSs on other examples of cue competition, namely, overshadowing, the relative stimulus validity effect, and the degraded contingency effect in rats. In Experiment 1, we found that intense auditory stimuli induced transient unconditioned lick suppression, thereby indicating that intense sounds were of high inherent biological significance. In Experiment 2A, we found that cues with high inherent biological significance were protected from overshadowing. In Experiment 2B, this finding was extended to cues with high acquired biological significance, which was obtained through prior pairings with a reinforcer of the valence opposite to that used in the overshadowing treatment. In Experiments 3 and 4, we found that cues with high inherent biological significance attenuated the relative validity effect and the degraded contingency effect, respectively. These results lend support to the view that biological significance (inherent and acquired) protects stimuli from cue competition effects, a finding that is problematic for many contemporary theories of learning.     

The dual role of the context in postpeak performance decrements resulting from extended training

The context??s role in Pavlovian conditioning depends on the trial spacing during training, with massed trials revealing a function akin to that of discrete stimuli, and spaced trials revealing a modulatory function (Urcelay & Miller, Journal of Experimental Psychology. Animal Behavior Processes, 36, 268?C280, 2010). Here we examined the contextual determinants of a common but largely ignored effect: attenuated conditioned responding with extended reinforced training (i.e., a postpeak performance deficit [PPD]). Contextual sources of PPDs were investigated in four fear-conditioning experiments with rats. In Experiment 1, as the number of reinforced trials increased, conditioned responding decreased, even when testing occurred outside the training context. Experiment 2 revealed opposing influences of context on the PPD based on trial spacing, which interacted with whether testing occurred in the training context. This finding reconciles Experiment 1??s results with previous data (Bouton, Frohardt, Sunsay, Waddell, & Morris, Journal of Experimental Psychology. Animal Behavior Processes, 34, 223?C236, 2008). Experiment 3 suggested that extended training with these parameters did not lead to habituation to conditioned or unconditioned stimuli. In Experiment 4, few or many massed training trials were followed orthogonally by context extinction or no context extinction. After many pairings, context extinction reduced the PPD (i.e., enhanced responding), suggesting a competitive role of the context. These results, together with prior data suggesting that context modulates expressions of the PPD, are consistent with the view that contexts can play two distinctly different roles.     

Covariation in conditioned response strength between stimuli trained in compound

Conditioned lick suppression by water-deprived rats was used to elaborate on recent evidence that the attenuated conditioned response elicited by an overshadowed stimulus may be enhanced by extinction of the overshadowing stimulus with which it had been trained in simultaneous compound. Using a modified serial stimulus arrangement in which a light coexisted with the last half of a tone that terminated with footshock, it was found in Experiment 1 that the tone overshadowed the light. Extinction of the tone-shock association resulted in a virtually complete recovery of the response to the overshadowed light. Using this serial overshadowing procedure, the possibility that the strength of a conditioned response to an element trained in compound covaries as a function of the strength of the response to the other element was tested in Experiment 2. Following overshadowing training similar to that of Experiment 1, independent reinforcement of the overshadowed light, that is, associative inflation, was found to have no deleterious effect on the response to the overshadowing tone. This suggests that the effects of postconditioning extinction and inflation of one element do not have symmetrical effects upon responding to the other element. The results of Experiment 2 were replicated in Experiment 3 using a simultaneous compound stimulus as opposed to the serial compound of the previous studies. These results are discussed in terms of various associative and cognitive models of learning and performance.     

CS-duration and partial-reinforcement effects counteract overshadowing in select situations

Two experiments used rats in a conditioned lick suppression preparation to investigate how the conditioned stimulus (CS)-duration and partial-reinforcement effects (i.e., weakened responding due to conditioning with a CS of longer duration and presenting nonreinforced CSs intermingled with CS—unconditioned stimulus [US] pairings, respectively) interact with overshadowing. Experiment 1 found that when overshadowing treatment was combined with either extended CS duration or partial reinforcement, the response deficit was weaker than when either of these three treatments was administered alone. In Experiment 2, the generality of the findings in Experiment 1 was investigated by replicating it with various US—US intervals. This time counteraction was observed only when both the absolute duration of total CS exposure and the US—US interval were short. The results support neither the view that the ratio between the total CS exposure and total time in the context determines the CS-duration and the partial-reinforcement effects nor the view that these two effects arise from a loss of effectiveness of the excitatory CS—US association during CS-alone exposures in partial reinforcement or early periods of CS exposure with long CSs.     

Extinction of the overshadowing CS after overshadowing in conditioned taste aversion

Using a conditioned taste aversion preparation overshadowing of flavor-illness association was produced through the presentation of a second flavor during the interval between the first flavor and illness. The modulatory effects of extinguishing the association between the second (over-shadowing) flavor and illness on conditioned responding to the target flavor was investigated. In Experiment 1, we found that, following one-trial overshadowing, extinction of the overshadowing flavor had no effect on conditioned responding to the target flavor. In Experiment 2, we found a similar absence of an effect of extinction of the overshadowing stimulus in a multitrial over-shadowing paradigm. Experiment 3 confirmed the results of Experiments 1 and 2 using conditioning parameters that were designed to weaken the association between the overshadowed flavor and illness. In Experiments 4 and 5, we used simultaneous presentation of the flavors during conditioning and obtained a weakened aversion to the overshadowed flavor when the overshadowing CS was extinguished. These findings are inconsistent with previous observations in conditioned fear preparations that suggest that extinction of the association between the overshadowing stimulus and the unconditioned stimulus attenuates overshadowing. Possible reasons for the discrepant results are discussed.     

Overshadowing by food of stimulus control by a keylight in a two-element serial compound

Pigeons were given successive discrimination training in which pecking during a choice period when the key was white was either reinforced or not, depending upon the prior presence or absence of a discriminative stimulus, which was a two-element serial compound. The compound consisted of a keylight and food, with food presented second or first in a forward or backward pairing for different groups of pigeons. In Experiment 1, the sequence was an S+ indicating reinforced trials, while in Experiment 2, the sequence was an S? indicating nonreinforced trials. Following acquisition of discriminated operant behavior, a sequence generalization test was administered during which all possible orders of the two stimuli were presented on test trials prior to the onset of the choice period. The results showed that food overshadowed stimulus control by the color of the light on the key on the sequence-generalization test, independently of whether food was presented first or second during training and independently of whether food was associated with reinforcement or nonreinforcement. The similarity of results for the two experiments suggests that overshadowing occurs independently of whether the compound is a discriminative stimulus for reinforcement or nonreinforcement. Simultaneous presentation of elements of a compound stimulus is not necessary for overshadowing because the phenomenon was captured with sequentially presented stimuli.     

Recovery of the rabbit’s conditioned nictitating membrane response without direct reinforcement after extinction

Three experiments demonstrated that, following the extinction of an established conditioned stimulus (CSA—e.g., tone), the pairing of a novel, cross-modal stimulus (CSB—e.g., light) with the unconditioned stimulus (US) results in strong recovery of responding to the extinguished CSA. Experiment 1 demonstrated that the recovery of responding to CSA is not the result of US reinstatement but is attributable to pairings of CSB with the US. Experiment 2 demonstrated that the recovery of responding is specific to CSA and is not the result of cross-modal generalization. Experiment 3 revealed that a large number of CSB-US pairings in Stage 1 significantly reduced the amount of recovery to CSA during subsequent CSB-US trials. Experiment 3 also provided unexpected evidence of cross-modal secondary extinction. The extinction and subsequent recovery of responding seen in the present experiments is discussed with respect to possible contributions from contextual associations, CS processing, US processing, conditioned response expression, and layered excitatory associations.     

Stimulus competition in the absence of compound conditioning

Most associative theories have assumed that stimulus competition occurs only between conditioned stimuli (CSs) that are trained in compound. The present research investigated the possibility of competition between two CSs that were individually paired to the same unconditioned stimulus (US). We used human subjects in an anticipatory suppression analogue to Pavlovian conditioning. Experiment 1 showed that X+ training followed by A+ training resulted in impaired responding to X. This did not occur when A+ training preceded X+ training. Experiment 2 replicated the basic effect and showed that it did not occur when the Phase 2 training consisted of A? instead of A+ nor when the A+ pairings occurred in a second context. Experiment 3 showed that A+ pairings occurring in a second context could still produce the effect when X was tested in the context in which the A+ pairings had occurred, but not when X was tested in a context different from that used for A+ training. Collectively, these results suggest that individually trained CSs may compete with each other when one of those CSs is more strongly activated by the test context than the other one.     

Latent inhibition as a performance deficit resulting from CS—context associations

Treatments that attenuate latent inhibition (LI) were examined using conditioned suppression in rats. In Experiment 1, retarded conditioned responding was produced by nonreinforced exposure to the CS prior to the CS-US pairings used to assess retardation (i.e., conventional LI). In Experiment la, retarded conditioned responding was induced by preexposure to pairings of the CS and a weak US prior to retardation-test pairings of the CS with a strong US (i.e., Hall-Pearce [1979] LI). Both types of LI were attenuated by extensive exposure to the training context (i.e., context extinction) following the CS-US pairings of the retardation test. Experiment 2 examined the specificity of the attenuated LI effect observed in Experiment 1. After preexposure to two different CSs in two different contexts, each CS was paired with a US in its respective preexposure context. One of the two contexts was then extinguished. This attenuated LI to a greater degree for the CS that had been trained in the extinguished context. Experiment 3 differentiated the roles in LI of CS-context associations and context-US associations. Following preexposure to the CS in the training context, LI was reduced by further exposure to the CS outside the training context. This observation was interpreted as implicating the CS-context association as a factor in LI. Thus, the results of these experiments suggest that LI is a performance deficit mediated by unusually strong CS-context associations. Implications for Wagner’s (1981) SOP model and Miller and Matzel’s (1988) comparator hypothesis are discussed.     

Pavlovian conditioning in multiple contexts: Competition between contexts for comparator status

Water-deprived rats were used to investigate the effects of training a CS in more than one context on conditioned lick suppression. In each experiment, partial reinforcement of the CS was intermingled with unsignaled presentations of the US. In Experiment 1, subjects were either trained in one context alone, trained consecutively in two contexts (such that all training in one context occurred prior to any training in the second context), or trained alternately in two contexts. Following training, the first context, the second context, or neither context was extinguished. Testing of the CS occurred in a third (neutral) context. To the extent that either training context became established as a comparator stimulus for the CS, the comparator hypothesis (Miller & Matzel, 1988) predicts an increase in excitatory responding to the CS following extinction of that context. Subjects trained in a single context exhibited appreciable fear of the CS only when the CS’s training context had been extinguished. Additionally, subjects trained consecutively in the two contexts showed increased fear of the CS following extinction of the second, but not the first training context (i.e., a recency effect). Subjects trained alternately in the two contexts showed no increased fear of the CS as a result of either context alone being extinguished. In Experiment 2, subjects trained alternately in two contexts showed increased fear of the CS only when both training contexts were extinguished, suggesting that both training contexts had become comparator stimuli. These data indicate that multiple training contexts can either compete or act synergistic-ally in modulating responding to a Pavlovian trained CS as a function of the order of training in the different contexts.     

Overshadowing and CS duration: counteraction and a reexamination of the role of within-compound associations in cue competition

In the present experiments, we examined the role of within-compound associations in the interaction of the overshadowing procedure with conditioned stimulus (CS) duration, using a conditioned suppression procedure with rats. In Experiment 1, we found that, with elemental reinforced training, conditioned suppression to the target stimulus decreased as CS duration increased (i.e., the CS duration effect), whereas, with compound reinforced training (i.e., the overshadowing procedure), conditioned suppression to the target stimulus increased as CS duration increased. In subsequent experiments, we replicated these findings with sensory preconditioning and demonstrated that extinction of the overshadowing stimulus results in retrospective revaluation with short CSs and in mediated extinction with long CSs. These results highlight the role of the duration of the stimulus in behavioral control. Moreover, these results illuminate one cause (the CS duration) of whether retrospective revaluation or mediated extinction will be observed.     

The overshadowing of instrumental conditioning by a stimulus that predicts reinforcement better than the response

In an experiment designed to demonstrate the overshadowing of appetitive instrumental conditioning, three groups of rats were given 10 sessions of RI (random interval) training in which reinforcement was delivered 450 msec following the response. The correlated group experienced a stimulus during the response-reinforcer delay interval, while an uncorrelated control group experienced a similar brief stimulus occasionally following responses, but these were not responses that typically produced reinforcement. A no-stimulus control group did not experience the brief response-produced stimulus. The experiment was run in two replications. The first employed a light as the critical stimulus, the second a tone. Over the 10 RI sessions, subjects in the two control conditions increased their rate of responding significantly faster than subjects in the correlated condition in both replications. This finding was interpreted as an instance of the overshadowing of the acquisition of signal value by a response because of the presence of the stimulus, which, in the correlated condition, was a more reliable predictor of reinforcement than the response. A subsequent conditioned reinforcement test confirmed that, in the correlated condition, the stimulus had, indeed, become a signal for reinforcement as a function of RI training.     

Target-absent controls in blocking experiments with rats

In three between-groups blocking experiments with rats, two concurrent and one forward, several common control procedures were employed: Reinforced trials with the putative blocking stimulus were either omitted entirely (Kamin control), replaced by unsignaled reinforcements (Wagner control), or replaced by reinforced trials with a different stimulus (C1 control). In each experiment, parallel treatments with the target stimulus absent during training served to examine the possibility that differential responding in tests with the target stimulus might be traced solely to differential exposure to the nontarget stimuli. In Experiment 1, responding by a concurrent blocking group during the test was no different than responding by a Kamin control group, and responding by a Wagner control group was greater than that of either of the other groups—a pattern of results, mirrored in the performance of the target-absent groups, that could be attributed to the elevation of contextual excitation by unsignaled reinforcement. In Experiment 2, responding in the test by a concurrent blocking group was no different than that by a C1 control group. In Experiment 3, a finding of less responding by a forward blocking group than by a C1 control group when the target stimulus was present during training, but not when it was absent, provided plausible evidence of blocking.     

Summation: Assessment of a configural theory

In five experiments, rats were given Pavlovian pairings of auditory and visual stimuli with delivery of food pellets. Experiment 1 found greater responding to an AB compound after training with the individual A and B stimuli, compared with responding both to the A and B elements and to a separately trained CD compound. Experiment 2 found this enhanced responding to depend on the associative strengths of A and B. In Experiment 3, responding was greater to a CD compound than to the other compounds after an AB-, AD+, BC+ training procedure. In Experiment 4, responding to an AB compound was greater than that to the elements after A was reinforced on a 100% schedule and B on a 50% schedule. In Experiment 5, responding to an AC compound was greater than that to either A or C after an AB+, CD+, A-training procedure. A configural theory, such as that proposed by Pearce (1987), anticipates summation in none of these procedures, unless the conditioned context is assumed to have a salience greater than zero. In order to predict summation in Experiments 3, 4, and 5, a context salience greater than that of the elements must be assumed. However, such an assumption also anticipates that extinction of a 100% stimulus should eliminate responding to a 50% stimulus. The results of Experiment 3 contradicted that prediction. These results conform better to the expectations of elemental models of conditioning.     

Expression of a taste aversion conditioned with an odor-taste compound: Overshadowing is relatively weak in weanlings and decreases over a retention interval in adults

Adult rats were injected with lithium chloride (LiCl) after consumption of a novel flavor (chocolate milk) that either was or was not presented together with a novel ambient odor (banana) as a compound conditioned stimulus (CS). In Experiment 1, the adults’ consumption of the flavor 24 h after conditioning was compared with that of weanling rats given the same conditioning treatment on Postnatal Day 21. The results confirmed previous indications that the reduction in aversion observed for adults conditioned with the compound CS (overshadowing) was weak or nonexistent in weanlings. After a longer retention interval (21 days), there was no evidence of overshadowing in adults despite maintained retention of the basic conditioned aversion. In Experiment 2 this decrease in overshadowing after a long retention interval was replicated with adult animals and extended to a different method of testing. The form of the effect was the same as in Experiment 1: The decrease in overshadowing occurred over the retention interval without loss in retention of the basic taste aversion; the decrease in overshadowing was a consequence of anincrease in the flavor aversion displayed by animals conditioned with the compound CS. The impaired flavor aversion (i.e., the overshadowing) observed shortly after conditioning apparently was due to factors associated with memory retrieval, rather than to reduced attentional or associative strength.     

Recovery from blocking achieved by extinguishing the blocking CS

Extinction-induced attenuation of single-phase and two-phase blocking was examined with rats in a conditioned lick-suppression task. In Experiment 1, which compared the effectiveness of single- and two-phase blocking, it was found that single-phase blocking was facilitated by the initiation of training with an A-US trial rather than an AX-US trial. Single-phase (but not two-phase) blocking was attenuated as a result of 200 extinction trials with the blocking stimulus (Experiment 2). Experiment 3 revealed recovery from two-phase blocking after 800 extinction trials with the blocking stimulus. Recovery from both types of blocking was specific to the blocked CS trained in compound with the extinguished stimulus (Experiment 4). This is the first article to report that the blocking deficit can be reversed by extinguishing the blocking stimulus. These results are discussed in light of acquisition models (i.e., retrospective revaluation) and expression models (i.e., the comparator hypothesis).     

The effects of separate presentations of the US on conditioned suppression

Four experiments examined the effects of separate presentations of shock on conditioned suppression of instrumental responding evoked by a CS previously paired with shock. Experiment 1 showed that conditioned suppression of responding resulting from noise-shock pairings increased as a function of time after the initial noise-shock pairings. However, it also showed that this time-dependent increase in conditioned suppression of responding could be attenuated by presentations of light-shock pairings immediately prior to the test of the noise CS. Experiment 2 showed that this attenuation effect can be produced by presentations of either light-shock pairings or shock alone. Experiment 3 showed that the magnitude of this attenuation effect was directly related to the temporal proximity of the light-shock pairings to the test of the noise CS. Experiment 4 showed that the magnitude of this attenuation effect was inversely related to the intensity of separate shock presentations.