Mechanisms underlying retarded emergence of conditioned responding following inhibitory training: Evidence for the comparator hypothesis.

The comparator hypothesis posits that conditioned responding is determined by a comparison at the time of testing between the associative strengths of the conditioned stimulus/stimuli (CS) and stimuli proximal to the CS at the time of conditioning. The hypothesis treats all associations as being excitatory and treats conditioned inhibition as the behavioral consequence of a CS that is less excitatory than its comparator stimuli. Conditioned lick suppression by rats was used to differentiate 4 possible sources of retarded responding to an inhibitory CS. These include habituation to the unconditioned stimulus/stimuli (UCS), latent inhibition to the CS, blocking of the CS-UCS association by the conditioning context, and enhanced excitatory associations to the comparator stimuli. Prior research has demonstrated the 1st 3 phenomena. Therefore, we employed parameters expected to highlight the 4th one—the comparator process. In Exp I, our negative contingency training produced a conditioned inhibitor that passed inhibitory summation and retardation tests. In Exp II we found transfer of retardation from an inhibitory CS to a novel stimulus when the location where retardation-test training occurred was excitatory. In Exp III, extinction of the conditioning context attenuated retardation regardless of whether extinction occurred before or after the CS-UCS pairings of the retardation test. Exp IV demonstrated that habituation to the UCS did not contribute to retardation in the present case. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Testing response generation rules.

Robbins (1988) reported data that he viewed as inconsistent with Miller and Schachtman's (1985a) comparator hypothesis of conditioned response generation. Here we explain why we do not find his experiments a compelling test of the comparator hypothesis. We also briefly review other studies that tested the same predictions of the comparator hypothesis that Robbins examined. We conclude that there is considerable evidence that following excitatory or inhibitory conditioning with a target conditioned stimulus (CS) and unconditioned stimulus (US), extinction of other cues that were present during CS training ordinarily increases excitatory responding and decreases inhibitory responding to the CS. However, consistent with Robbins's conclusion, there is scant evidence that after CS–US training, enhancing the associative value of other cues that were present during CS training influences excitatory or inhibitory responding to the CS. The implications of these conclusions for the comparator hypothesis as an explanation of differences in acquired behavior and as a heuristic tool are considered. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned inhibition produced by extinction-mediated recovery from the relative stimulus validity effect: A test of acquisition and performance models of empirical retrospective revaluation.

Empirical retrospective revaluation is a phenomenon of Pavlovian conditioning and human causal judgment in which posttraining changes in the conditioned response (Pavlovian task) or causal rating (causal judgment task) of a cue occurs in the absence of further training with that cue. Two experiments tested the contrasting predictions made by 2 families of models concerning retrospective revaluation effects. In a conditioned lick-suppression task, rats were given relative stimulus validity training, consisting of reinforcing a compound of conditioned stimuli (CSs) A and X and nonreinforcement of a compound of CSs B and X, which resulted in low conditioned responding to CS X. Massive posttraining extinction of CS A not only enhanced excitatory responding to CS X, but caused CS B to pass both summation (Experiment 1) and retardation (Experiment 2) tests for conditioned inhibition. The inhibitory status of CS B is predicted by the performance-focused extended comparator hypothesis (J. C. Denniston, H. I. Savastano, & R. R. Miller, 2001), but not by acquisition-focused models of empirical retrospective revaluation (e.g., A. Dickinson & J. Burke, 1996; L. J. Van Hamme & E. A. Wasserman, 1994). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Slow reacquisition following extinction: Context, encoding, and retrieval mechanisms.

Investigated the slow reacquisition (RAQ) of responding in rats that occurs when the conditioned stimulus/stimuli (CS) and unconditioned stimulus/stimuli (UCS) are paired again after prolonged extinction training. In Exp 1, an extinguished CS acquired less suppression than a novel CS during a final conditioning phase, but more suppression than CSs that had received comparable nonreinforcement without initial conditioning. In Exp 2, CS–UCS pairings resumed in the context of extinction caused the least RAQ of suppression: Pairings in a neutral context produced better RAQ, while return of the CS to the conditioning context caused an immediate renewal of responding to the CS. In Exp 3, a return of the CS to the extinction context after RAQ training caused renewed extinction performance and interfered with performance appropriate to RAQ. This effect was not due to demonstrable inhibitory conditioning of the extinction context. Results suggest that representations of conditioning and extinction (or CS–UCS and CS–no UCS relations) are both retained through extinction and that performance appropriate to either phase can be cued by the corresponding context. RAQ may thus be slow when the context retrieves an extinction memory. Similar mechanisms may also play a role in other Pavlovian interference paradigms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The comparator hypothesis of conditioned response generation: Manifest conditioned excitation and inhibition as a function of relative excitatory strengths of CS and conditioning context at the time of testing.

Water-deprived rats were used in a conditioned lick suppression paradigm to test and further develop Rescorla's (1968) contingency theory, which posits that excitatory associations are formed when a conditioned stimulus (CS) signals an increase in unconditioned stimulus (US) likelihood and that inhibitory associations develop when the CS signals a decrease in US likelihood. In Exp I we found that responding to a CS varied inversely with the associative status of the context in which the CS was trained and that this response was unaltered when testing occurred in distinctively dissimilar context with a different conditioning history, provided associative summation with the test context was minimized. Results suggest that manifest excitatory and inhibitory conditioned responding is modulated by the associative value of the training context rather than that of the test context. Exp II demonstrated that postconditioning decreases in the associative value of the CS training context reduced the effective inhibitory value of the CS even when testing occurred outside of the training context. This contextual deflation effect was specific to the CS training context. These studies support the comparator hypothesis, which states that conditioned responding is determined by a comparison of the associative strengths of the CS and its training context that occurs at the time of testing rather than at the time of conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of exposure to conditioned stimulus and control of its termination in the extinction of avoidance behavior.

Describes 2 experiments in which, following signaled shuttle box avoidance training, a total of 52 female Fischer344 rats were exposed to the conditioned stimulus (CS) during no-shock treatment trials and subsequently tested during extinction trials in which shock was also absent. In Exp I, Ss that could control the termination of the CS during treatment responded significantly more often during extinction than yoked partners that received the same pattern and duration of CS exposure but could not control its termination. Exp II revealed that the probability of responding during extinction was a decreasing function of the duration of CS exposure during treatment. Thus, in the absence of shock, both lack of control over CS termination and increasing CS exposure each independently facilitated the weakening of well-established avoidance responses. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of context in autoshaping.

Three experiments used an autoshaping procedure in 64 female White Carneaux pigeons to investigate the conditioning of the context and of a discrete CS with a food UCS. CS–UCS associations were measured by directed pecking at the key light CS; context–UCS associations were assessed by general activity in the context. Exp I investigated the influence of context–UCS associations on performance to a previously trained CS. The same CS produced greater keypecking in a context of higher associative strength. Exp II examined the influence of context–UCS associations on learning of CS–UCS associations. When tested in a context of fixed associative strength, a CS that had been trained in a context of high associative strength elicited less responding than one trained in a context of low associative strength. Exp III found that signaling a UCS by a discrete CS interfered with the formation of context–UCS associations, as measured both in terms of general activity and ability to promote responding to another CS. Results suggest that the context and the CS compete for association with the UCS. They also suggest that context–UCS associations facilitate the exhibition of CS–UCS associations. (17 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Degraded contingency revisited: Posttraining extinction of a cover stimulus attenuates a target cue's behavioral control.

In a Pavlovian conditioning situation, unsignaled outcome presentations interspersed among cue-outcome pairings attenuate conditioned responding to the cue (i.e., the degraded contingency effect). However, if a nontarget cue signals these added outcomes, responding to the target cue is partially restored (i.e., the cover stimulus effect). In 2 conditioned suppression experiments using rats, the effect of posttraining extinction of the cover stimulus was examined. Experiment 1 found that this treatment yielded reduced responding to the target cue. Experiment 2 replicated this finding, while demonstrating that this basic effect was not due to acquired equivalence between the target cue and the cover stimulus. These results are consistent with the extended comparator hypothesis interpretation of the degraded contingency and cover stimulus effects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

What starts an internal clock?

Five experiments with 52 male Charles River CD rats investigated under what conditions a stimulus is timed by the internal clock used in time-discrimination procedures. In Exps I–IV, Ss were trained to time one stimulus (e.g., light) and then tested as to whether they timed a stimulus from another modality (e.g., sound). The 2nd stimulus was treated in 3 ways: exposed (presented alone), paired with food, and extinguished. Exps I and II used the peak procedure, similar to a discrete-trial FI schedule, and paired the treated stimulus with food using instrumental training; Exps III and IV used a psychophysical choice procedure and paired the treated stimulus with food using classical conditioning. All 4 experiments found that there was cross-modal transfer of the time discrimination after pairing but not after exposure or extinction. This suggests that Ss' internal clock timed the treated stimulus after pairing but not after exposure or extinction. Exp V suggested that the decline of responding observed in extinction was not due to changes in timing. Thus, the internal clock apparently times stimuli with signal value (associative strength) and does not time stimuli without signal value. (46 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Facilitation of instrumental behavior by a Pavlovian appetitive conditioned stimulus.

In Exp I, 16 New Zealand white rabbits were trained to perform an instrumental head-raising response for sucrose reward. A jaw-movement CR was established to a 2-sec CS by pairing it with sucrose; a control stimulus was unpaired with sucrose. Instrumental responding maintained by a VI 40-sec schedule was enhanced during 10-sec presentations of the paired, but not the unpaired, CS. Responding on a VR 15 schedule was unaffected except on trials on which the pre-CS baseline response rate was low; in such cases the paired CS caused a long-lasting acceleration of responding. Noncontingent presentation of the sucrose reinforcer itself briefly suppressed responding but had no long-term effect. In Exp II (6 Ss), a CS that had been conditioned at a 10-sec duration produced the same pattern of effects as in Exp I, indicating that facilitation resulted from CS presentation rather than from the frustrative effects of nonreinforcement of the CS. In Exp III (16 Ss), an inhibitory CS blocked facilitation by the excitatory CS but did not itself affect instrumental responding. (53 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cerebellar stimulation as an unconditioned stimulus in classical conditioning.

Implanted rabbits with chronic stimulating electrodes in white matter underlying lobule HVI of the cerebellar cortex. Stimulation elicited movements of the face or neck and, when paired with a tone CS, produced learning comparable to that seen with peripheral unconditioned stimulus/stimuli (UCS). CS-alone trials produced extinction. Reinstatement of paired trials produced reacquisition with savings. Additional groups received either explicitly or randomly unpaired CS–UCS trials before paired conditioning. Low-frequency responding during these sessions indicated that the paired training results were associative and not due to pseudoconditioning or sensitization. Explicitly unpaired sessions retarded learning on subsequent paired trials compared with groups that received either randomly unpaired or no CS–UCS preexposure. These results are interpreted in terms of the role of the cerebellum and associated pathways in classical conditioning of motor responses. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pavlovian second-order conditioned analgesia.

Three experiments, with 118 Sprague-Dawley rats, assessed conditioned analgesia in a Pavlovian 2nd-order conditioning procedure by using inhibition of responding to thermal stimulation as an index of pain sensitivity. In Exp I, Ss receiving 2nd-order conditioning showed longer response latencies during a test of pain sensitivity in the presence of the 2nd-order conditioned stimulus (CS) than Ss receiving appropriate control procedures. Exp II found that extinction of the 1st-order CS had no effect on established 2nd-order conditioned analgesia. Exp III evaluated the effects of post 2nd-order conditioning pairings of subcutaneous morphine sulfate (10–20 mg/kg) and the shock unconditioned stimulus/stimuli (UCS). Ss receiving paired morphine–shock presentations showed significantly shorter response latencies during a hot-plate test of pain sensitivity in the presence of the 2nd-order CS than did Ss receiving various control procedures; 2nd-order analgesia was attenuated. Data extend the associative account of conditioned analgesia to 2nd-order conditioning situations and are discussed in terms of the mediation of both 1st- and 2nd-order analgesia by an association between the CS and a representation or expectancy of the UCS, which may directly activate endogenous pain inhibition systems. (52 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Counteraction between overshadowing and degraded contingency treatments: Support for the extended comparator hypothesis.

Four experiments using rats in a Pavlovian lick-suppression preparation investigated the effects of combining 2 treatments known for their response-decrementing effects, namely, overshadowing and degraded contingency. Contrary to most contemporary learning theories, the extended comparator hypothesis predicts that these 2 treatments will counteract each other, and therefore, less of a decrement in conditioned responding should be observed than with either treatment alone. Experiments 1 and 2 confirmed this prediction in first-order conditioning and sensory preconditioning preparations, respectively. Experiment 3 demonstrated that posttraining extinction of the training context resulted in a recovery from degraded contingency and reversed the counteractive effect on overshadowing. Finally, Experiment 4 demonstrated that posttraining extinction of the overshadowing stimulus resulted in recovery from simple overshadowing and also reversed the counteractive effect on degraded contingency. These results are consistent with the extended comparator hypothesis but not traditional or recent acquisition-focused models. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Overshadowing and latent inhibition counteract each other: support for the comparator hypothesis

In 4 conditioned lick suppression experiments with rats, the combined effects of latent inhibition treatment followed by overshadowing treatment were assessed as a test of the comparator hypothesis's (R.R. Miller & L.D. Matzel, 1988) explanations of overshadowing and latent inhibition. Experiments 1 and 2 confirmed the prediction of the comparator hypothesis that combined latent inhibition and overshadowing treatments attenuate the response deficit produced by either treatment alone. Furthermore, consistent with the comparator hypothesis, posttraining changes in the associative status of the putative comparator stimulus altered responding to the target conditioned stimulus (Experiment 3), and switching contexts between latent inhibition and overshadowing treatments (Experiment 4) eliminated the interaction between the latent inhibition and overshadowing treatments.     

Response strength as a function of stimulus novelty: Concept formation in the pigeon.

Cites a previous study which showed that a distinctive CS presented on the 1st trial of acquisition generated more responses after extinction than another CS presented regularly during acquisition. In the present study 140 pigeons were presented with a different distinctive CS on each 1st trial of 5 acquisition sessions. Ss were then broken down into 7 groups which were tested, following extinction, for residual response strength of (a) Day 1 novel CS, (b) Day 2 novel CS, (c) Day 3 novel CS, (d) Day 4 novel CS, (e) Day 5 novel CS, (f) the regular CS, and (g) a novel CS never presented before. Contrary to the previous study where a novel CS inhibited responding, Group 7 exhibited the most responding in this study. Results support a concept formation interpretation of the previous study and are difficult to explain from a "stimulus trace" or "identical elements" position. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of classical conditioning on an internal clock.

Six experiments with 53 male Charles River rats used a psychophysical choice procedure to study the internal clock used to discriminate duration and to investigate whether this clock is sensitive to the signal value (associative strength) of a stimulus. The experiments involved 2 types of trials: On choice trials, a stimulus lasted a short (e.g., 3-sec) or long (e.g., 12-sec) duration; Ss then chose between 2 levers. The rewarded choice depended on the duration of the stimulus. On conditioning trials, the stimulus used on choice trials was presented, but it ended without food (extinction trials) or with food (pairing trials) regardless of what the S did. The main measure of performance was short bias, defined as accuracy with the short stimulus without a corresponding accuracy with the long stimulus. Exp I showed that extinction trials increased short bias relative to training without conditioning trials or to training with pairing trials. Exps II–VI tested explanations of these results. The same results were found when extinction trials were the same duration as the short stimulus (Exp II), when extinction trials were a random duration (Exp V), and when the signal value of the CS was changed in another way (Exp VI). The effect of conditioning trials was modality specific (Exps III and IV). It is concluded that, of the explanations considered, the most valid is that changing the signal value of a stimulus changes how the clock times the stimulus. Reducing signal value reduces the measured duration. (54 ref) (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Trial spacing is a determinant of cue interaction.

Four conditioned lick suppression experiments with rats examined the effect of trial spacing on cue interaction. Experiments 1 and 2 found overshadowing to be eliminated with massed compound stimulus- outcome pairings and the usual trial spacing effect to be reversed with compound acquisition trials. Experiment 3 found that whether acquisition compound- outcome pairings were massed or spaced determined the effect of posttraining extinction treatment. Extinction of the overshadowing cue reduced responding following massed training and increased responding following spaced training. Extinction of the context decreased responding following massed training. Experiment 4 found the conditioning and devaluation results to be associative and stimulus specific. These results are in accord with the extended comparator hypothesis (J. C. Denniston, H. I. Savastano, & R. R. Miller, 2001). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of a stimulus intervening between CS and US in autoshaping.

Five experiments explored the effect of placing a stimulus between the termination of a key light (CS) and the onset of a food (UCS) with female Carneaux pigeons in an autoshaping preparation. Whether that stimulus was itself a key light evoking pecking or an auditory event failing to evoke pecking, it facilitated performance to the CS. The use of a within-Ss design made differential conditioning of background cues and stimulus generalization unlikely accounts. Exp III found that extinction of the intervening stimulus did not remove the facilitative effect. Exp IV found that pairing the CS with food and the intervening stimulus did not produce the effect unless both occurred on the same trial. Together, these experiments rule out an account in terms of 2nd-order conditioning of the CS by the intervening stimulus. Exp V used an intervening stimulus in a 2nd-order conditioning design to demonstrate that the stimulus acted to improve the association between the CS and the reinforcer. Results document a kind of catalytic effect of the intervening stimulus on the CS-reinforcer association. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Overshadowing effects in the stimulus control of morphine analgesic tolerance.

Attempted to replicate previous demonstrations of classical conditioning of morphine analgesic tolerance and to determine whether stimulus overshadowing effects might explain previous conflicting findings. In Exp I, 8 groups of male Sprague-Dawley rats received a series of 10 morphine (5 mg/kg) and/or saline injections, differing only with respect to the contingency between a compound visual-auditory CS and the substance injected. When tested for analgesic responding to morphine in the presence of the compound CS, only those groups for which the CS and morphine injections were paired during the acquisition sequence evidenced tolerance. In Exp II, tolerant Ss were tested in the presence of 1 component of the compound CS. When a loud tone (85 db) was used in the compound, less analgesic tolerance was elicited later by the weaker visual stimulus alone. This differential stimulus control of the analgesic response suggests that overshadowing may contribute to failures to replicate conditioned morphine tolerance. It is possible that internal morphine-produced stimuli may overshadow external cues. (18 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)