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)     

Effects of hippocampal lesions in overshadowing and blocking procedures.

The effects of ibotenic acid lesions of the hippocampus on overshadowing and blocking were examined in a Pavlovian appetitive conditioning experiment with rats. In a standard test of performance to the overshadowed or blocked target stimulus, sham-lesioned rats displayed both of these stimulus-selection phenomena. Rats with hippocampal lesions showed normal blocking, but no overshadowing. Subsequent inhibitory learning about the target stimulus was slower after overshadowing or blocking procedures than after a control procedure in sham-lesioned rats, but not in lesioned rats. These results suggest that exposure to these procedures can induce hippocampally mediated losses in conditioned stimulus associability (learning rate parameter), even when those losses are not a major determinant of the stimulus-selection effects themselves. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Potentiation, overshadowing, and blocking of spatial learning based on the shape of the environment.

Rats were trained in Experiment 1 to find a submerged platform in 1 corner of either a rectangular or a kite-shaped pool. When the walls creating this corner were a different color than the opposite walls, then learning about the shape of the pool was potentiated in the kite but not in the rectangle. Experiments 2-4 revealed that learning about the rectangle can be overshadowed and blocked when information about the wall color indicates the location of the platform. The results mimic findings that have been obtained with Pavlovian conditioning, and they challenge the claim that learning about the shape of the environment takes places in a dedicated geometric module. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Blocking and overshadowing in human geometry learning.

In a two-dimensional computer-based search task, human participants were required to learn the location of a goal by using the geometric information available on the screen. When the goal location was defined by two shapes that differed in salience, the more salient shape overshadowed learning based on the less salient shape but not the other way round. Furthermore, when one shape was pretrained as a signal for the location of the goal, learning about the geometric cues of the other shape was blocked. These results suggest that spatial learning based on geometry is ruled by associative principles and support learning models that do not invoke a special status for geometric cues (e.g., Miller & Shettleworth, 2007). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An inhibitory within-compound association attenuates overshadowing.

According to the comparator hypothesis (Miller & Matzel, 1988), cue competition depends on the association between a target stimulus (X) and a competing cue (e.g., an overshadowing cue [A]). Thus, it was expected that overshadowing would be reduced by establishing an inhibitory-like relationship between X and A before compound conditioning. In three lever press suppression experiments with rats, this expectation was supported. Experiment 1 showed that establishing an inhibitory X-A relationship reduced overshadowing. In Experiment 2, degrading the inhibitory-like relationship before conditioning allowed reinforced AX compound trials to result in overshadowing. Experiment 3 replicated the results of Experiment 2 when the inhibitory relationship was degraded after compound conditioning. The results support the view that within-compound associations are necessary not only for retrospective revaluation, but also for conventional cue competition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Failure of retrospective revaluation to influence blocking.

In the blocking paradigm, subjects receive reinforced presentations of a compound, AX, after reinforced presentations of A alone. Following this training, responding to X is often diminished relative to a control group, which did not receive the prior training with A. Standard associative theories of learning such as the Rescorla–Wagner model (Rescorla & Wagner, 1972) explain this effect by assuming that A and X compete for control over behavior. In contrast, theories such as the comparator hypothesis assume that learning about X is unaffected by the properties of A, but it is the expression of this learning at test that is affected by the blocking manipulation. The aim of the 3 reported experiments was to distinguish between these 2 accounts. According to the comparator hypothesis, devaluing A following blocking should increase subsequent responding to X. In all 3 experiments the blocking effect was found to persist following devaluation of A, providing support for standard associative theories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A comparator view of Pavlovian and differential inhibition.

In 3 experiments using rats as subjects, the authors varied trial spacing to investigate the conditions under which Pavlovian and differential inhibition are observed. Experiment 1 compared Pavlovian and differential inhibition with spaced training trials. Spaced trials resulted in only the Pavlovian inhibitor passing both summation and retardation tests. Conversely, Experiment 2 compared these 2 types of inhibition with massed training trials. This training resulted in only the differential inhibitor passing both tests for conditioned inhibition. Finally, in Experiment 3 all subjects experienced Pavlovian inhibition training with massed trials. Although this training by itself did not result in behavior indicative of inhibition, subjects that also experienced posttraining extinction of the training context did pass both tests for inhibition. Overall, these results are anticipated by the extended comparator hypothesis (Denniston, Savastano, & Miller, 2001) but are problematic for most contemporary associative learning theories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The loss of latent inhibition across compound conditioning.

Six experiments used a magazine approach paradigm with rats to investigate latent inhibition (LI). Experiment 1 found that compound conditioning did not increase evidence for LI, in contrast to predictions from acquisition-deficit models that are based on a common error term (e.g., J. M. Pearce & G. Hall, 1980; A. R. Wagner, 1981). Instead, it appeared that preexposed and non-preexposed stimuli conditioned to the same asymptote following compound conditioning, as is the case when these stimuli are conditioned separately. This was confirmed in three further experiments that used probe trials to measure conditioning to each conditioned stimulus across the course of compound training. In these experiments, LI was observed during initial but not extended compound training, consistent with predictions derived from M. E. Bouton (1997) and the SLG model (N. A. Schmajuk, Y. Lam, & J. A. Gray, 1996). However, 2 further experiments did not support these models. Instead, these findings are most consistent with models that use separate error terms to compute the associative strength of conditioned stimuli conditioned in compound (S. E. Brandon, E. H. Vogel, & A. R. Wagner, 2003; M. E. Le Pelley, 2004; N. J. Mackintosh, 1975). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Medial dorsal thalamic lesions impair blocking and latent inhibition of the conditioned eyeblink response in rats.

The effects of lesions of the medial dorsal thalamic nucleus (MD) on blocking and latent inhibition (LI) of the rat eyeblink response were examined in the present study. Previous work has demonstrated that the cingulate cortex and related thalamic areas are involved in processing conditioning stimuli throughout training. The experiments in the present study tested the hypothesis that disruption of cingulothalamic stimulus processing produced by lesions of the MD would impair 2 types of associative learning that involve decremental changes in attention. In Experiment 1, MD lesions severely impaired blocking. In Experiment 2, NM lesions severely impaired LT. The results indicate that lesions of the NM impair incremental, decremental, or both types of changes in stimulus processing during learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Experimental challenges to theories of classical conditioning: Application of an attentional model of storage and retrieval.

Several studies have recently challenged the accuracy of traditional models of classical conditioning that account for some experimental data in terms of a storage deficit. Among other results, it has been reported that extinction of the blocking or overshadowing stimulus results in the recovery of the response to the blocked or overshadowed stimulus, backward blocking shows spontaneous recovery, extinction of the training context results in the recovery from latent inhibition, interposing a delay between conditioning and testing in latent inhibition increases latent inhibition, and latent inhibition antagonizes overshadowing. An existing neural network model of classical conditioning (N. A. Schmajuk, Y. Lam, & J. A. Gray, 1996), which includes an attentional mechanism controlling both storage and retrieval of associations, is able to quantitatively describe these results. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Potentiation and overshadowing in Pavlovian fear conditioning.

The present experiments addressed a fundamental discrepancy in the Pavlovian conditioning literature concerning responding to a target cue following compound reinforced training with another cue of higher salience. Experiment 1 identified one determinant of whether the target cue will be overshadowed or potentiated by the more salient cue, namely contiguity between compound CS termination and US presentation. Overshadowing and potentiation were observed with delay and trace procedures, respectively. Experiments 2 and 3 contrasted elemental and configural explanations of potentiation. Both experiments supported a configural account. Experiments 3 and 4, by manipulating prior learning experiences to bias subjects to encode the same compound elementally or configurally, demonstrated decreased potentiation and overshadowing, respectively. Overall, these experiments demonstrate potentiation with nontaste stimuli and identify one variable that determines whether overshadowing or potentiation will occur. Moreover, they show that prior experiences can determine how a compound is encoded and are compatible with the idea of flexible encoding as a principle of information processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Potentiation of latent inhibition.

Rats were given exposure either to an odor (almond) or a compound of odor plus taste (almond plus saline), prior to training in which the odor served as the conditioned stimulus. It was found, for both appetitive and aversive procedures, that conditioning was retarded by preexposure (a latent inhibition effect), and the extent of the retardation was greater in rats preexposed to the compound (i.e., latent inhibition to the odor was potentiated by the presence of the taste). In contrast, the presence of the taste during conditioning itself overshadowed learning about the odor. We argue that the presence of the salient taste in compound with the odor enhances the rate of associative learning, producing a rapid loss in the associability of the odor. This loss of associability will generate both overshadowing and the potentiation of latent inhibition that is observed after preexposure to the compound. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial learning based on boundaries in rats is hippocampus-dependent and prone to overshadowing.

In three experiments, rats were required to find a submerged platform by referring to the boundaries of a circular swimming pool. In the first experiment, rats with lesions of the hippocampus were impaired at finding the hidden platform, lending support for the proposal that learning to find a goal that is a certain direction and distance from a boundary is dependent upon the hippocampus. Experiments 2 and 3 offered preliminary tests to see if such boundary learning occurred incidentally, irrespective of the presence of a reliable landmark. In contrast to this proposal, a landmark hanging above the platform successfully restricted learning about the location of the platform with respect to the boundary of the arena. The discussion explores the capacity of the hippocampus to encode boundary information, as well as interprets the behavioral results on the basis of an associative learning framework. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contrasting predictions of extended comparator hypothesis and acquisition-focused models of learning concerning retrospective revaluation.

Three conditioned suppression experiments with rats investigated contrasting predictions made by the extended comparator hypothesis and acquisition-focused models of learning, specifically, modified SOP and the revised Rescorla–Wagner model, concerning retrospective revaluation. Two target cues (X and Y) were partially reinforced using a stimulus relative validity design (i.e., AX–Outcome; BX–No outcome; CY–Outcome; DY–No outcome), and subsequently one of the companion cues for each target was extinguished in compound (BC–No outcome). In Experiment 1, which used spaced trials for relative validity training, greater suppression was observed to target cue Y for which the excitatory companion cue had been extinguished in relation to target cue X for which the nonexcitatory companion cue had been extinguished. Experiment 2 replicated these results in a sensory preconditioning preparation. Experiment 3 massed the trials during relative validity training, and the opposite pattern of data was observed. The results are consistent with the predictions of the extended comparator hypothesis. Furthermore, this set of experiments is unique in being able to differentiate between these models without invoking higher-order comparator processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Latent inhibition and compound conditioning: A reply to Holmes and Harris (2009).

Schmajuk, Lam, and Gray (SLG, 1996) presented a neural network model of classical conditioning that addresses the multiple properties of latent inhibition (LI). According to the model, LI is the result of the decreased attention to the target stimulus during preexposure and testing. Recently, Holmes and Harris (2009) suggested that, although the model was able to describe their experimental results showing that LI to a preexposed stimulus disappears with extended compound conditioning, it could not describe the fact that LI is not affected by a delay following compound conditioning. However, computer simulations demonstrate that the SLG model describes and explains both results. Because the model also explains both the deleterious and the facilitating effects on LI of a delay following simple conditioning, the SLG model seems unique in explaining the complete range of reported effects of temporal delays on LI as well as most of the properties of LI. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Blocking of potentiation of latent inhibition.

We present a theory of latent inhibition based on the Pearce–Hall (Pearce & Hall, 1980) model for classical conditioning. Its central features are (1) that the associability of a stimulus declines as it comes to predict its consequences and (2) that nonreinforced exposure to a stimulus engages an associative learning process that makes the stimulus an accurate predictor of its consequences (in this case, the occurrence of no event). A formalization of this theory is shown to accommodate the finding that preexposure in compound with another cue can potentiate latent inhibition to the target cue. It further predicts that preexposure to the added cue will eliminate the potentiation effect. An experiment using rats and the flavor-aversion procedure confirmed this prediction. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Latent inhibition, but not prepulse inhibition, is reduced during withdrawal from an escalating dosage schedule of amphetamine.

The enhanced locomotor and stereotypic responses of the rat to repeated amphetamine (AMPH) administration are considered to be an animal model of positive schizophrenic symptoms. In contrast, behaviors observed during withdrawal from repeated AMPH are believed to model depression or anxiety. In the present study, the authors tested whether AMPH withdrawal might also elicit behaviors consistent with animal models of schizophrenia, specifically, disruptions in latent inhibition (LI) of 2-way active avoidance and prepulse inhibition (PPI) of startle. Rats treated with escalating doses of AMPH (6 days, 1–5 mg/kg ip) or saline were tested for LI and PPI during withdrawal. LI was eliminated by prior AMPH treatment in rats tested at 4, 13, and 28 days of withdrawal. In contrast, PPI did not differ between AMPH and control groups. These results support an interrelationship between repeated-AMPH and LI-disruption, but not PPI-disruption, models of schizophrenia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Breast-feeding is associated with reduced perceived stress and negative mood in mothers.

Two studies examined the effects of breast-feeding on maternal stress and mood. In Experiment 1, perceived stress in the past month was compared between 28 breast-feeding and 27 bottle-feeding mothers. Breast-feeding mothers reported less perceived stress, after controlling for demographic confounds. In Experiment 2, mood ratings were assessed in the same 24 mothers both before and then after 1 breast-feeding and 1 bottle-feeding session. Breast-feeding was associated with a decrease in negative mood, and bottle-feeding was associated with a decrease in positive mood from pre- to postfeeding. Results indicated that breast-feeding buffers negative mood. These effects appeared to be attributable to the effects of breast-feeding itself and not solely to individual-differences factors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Latent inhibition and contextual associations.

Attenuated responding to a CS after preexposure to that CS (latent inhibition) has traditionally been attributed to reductions in CS associability. Alternatively, CS-context associations formed during CS preexposure later interfere with the acquisition or expression of CS-outcome associations. Three lick suppression experiments with rats contrasted these accounts. Presumably, exposure to the context attenuates the CS-context association without altering CS associability. With a fixed amount of CS preexposure, latent inhibition decreased with increasing context exposure during or after (but not before) CS preexposure. When the ratio of context preexposure duration to CS preexposure was fixed, latent inhibition increased with CS preexposure. These results suggest that latent inhibition is a direct function of the strength of CS-context associations formed during preexposure. (PsycINFO Database Record (c) 2010 APA, all rights reserved)