Martha Escobar

Associative deficit accounts of disrupted latent inhibition and blocking in schizophrenia

Latent inhibition (attenuated responding to a signal due to signal-alone presentations preceding the signal-outcome pairings) and blocking (attenuated responding to Signal B due to Signal A being paired with the outcome prior to pairings of an AB compound with the outcome) are reportedly absent in acute schizophrenics. The common assumption that these phenomena reflect the normal functioning of attention and the observation that rats administered low doses of amphetamine show a similar disruption has resulted in the development of an animal model of attentional dysfunction in acute schizophrenia. Here, we selectively review the experimental and clinical literature concerning latent inhibition and blocking, their disruption in acute schizophrenia, and the current status of this model. We conclude that the construct validity of the model is compromised if latent inhibition and blocking are viewed in attentional terms because experimental data indicate both phenomena can be better understood in associative terms. We favor a framework in which disruption of latent inhibition and blocking in acute schizophrenics is viewed as an inability to compare and express stored representations (i.e. associative performance deficit). This change of perspective does not undermine the potential value of the model, but rather suggests that the nature of its validity should be reconsidered.

Cues trained apart compete for behavioral control in rats : Convergence with the associative interference literature

Contemporary theories of associative learning require cues be trained in compound for cue competition (interference) to occur. That is, Cues A and X should compete for behavioral control only if training consists of AX-outcome (O) trials and not if each cue is separately paired with O (i.e., X-O and A-O). Research with humans challenges this view by showing that A-O trials interpolated between training and testing of a X-O association impair responding to X (i.e., retroactive interference). In six conditioned suppression studies with rats, the authors demonstrate that two cues trained apart can each interfere with the potential of the other to predict the outcome. The authors conclude that this type of interference (a) reflects a failure to retrieve the target association due to priming at test of the interfering association and (b) is attenuated if the outcome is of high biological significance. These findings parallel previous reports in verbal learning research and suggest that a similar associative structure underlies some types of associations in nonverbal subjects.

A comparison between elemental and compound training of cues in retrospective revaluation

Associative learning theories assume that cue interaction and, specifically, retrospective revaluation occur only when the target cue is previously trained in compound with the to-be-revalued cue. However, there are recent demonstrations of retrospective revaluation in the absence of compound training (e.g., Matute & Pineño, 1998a, 1998b). Nevertheless, it seems reasonable to assume that cue interaction should be stronger when the cues are trained together than when they are trained apart. In two experiments with humans, we directly compared compound and elemental training of cues. The results showed that retrospective revaluation in the elemental condition can be as strong as and, sometimes, stronger than that in the compound condition. This suggests that within-compound associations are not necessary for retrospective revaluation to occur and that these effects can possibly be best understood in the framework of general interference theory.

Reversal from blocking in humans as a result of posttraining extinction of the blocking stimulus

In a blocking procedure, conditioned stimulus (CS) A is paired with the unconditioned stimulus (US) in Phase 1, and a compound of CSs A and X is then paired with the US in Phase 2. The usual result of such a treatment is that X elicits less conditioned responding than if the A-US pairings of Phase 1 had not occurred. Obtaining blocking with human participants has proven difficult, especially if a behavioral task is used or if the control group experiences reinforcement of a CS different from the blocking CS in Phase 1. In the present series, in which human participants and a behavioral measure of learning were used, we provide evidence of blocking, using the above described control condition. Most important, we demonstrate that extinction of the blocking CS (A) following blocking treatment reverses the blocking deficit (i.e., increases responding to X). These results are at odds with traditional associative theories of learning, but they support current associative theories that predict that posttraining manipulations of the competing stimulus can result in a reversal of stimulus competition phenomena.

Contrasting Acquisition-Focused and Performance-Focused Models of Acquired Behavior

What is encoded during a learning experience? In a learning situation, a training (acquisition) phase is often followed by a test phase. Acquisition-focused models (most associative models) emphasize information processing that occurs during training and assume that only summary statistics (associative values) are retained to influence behavior during testing. Performance-focused models (a k a “computational” models) emphasize information processing that occurs at test and often assume fairly comprehensive memory of prior experiences. In this article, we contrast these two families of models. We note that neither family can claim unique support from the various learning phenomena commonly cited as favoring one or the other position. Within each family, there are highly diverse models that preclude blanket generalizations. However, the acquisition-focused view that subjects retain only summary statistics suggests unrealistically impoverished memories relative to the fairly veridical memories that clearly are formed. But this oversimplifying assumption of acquisition-focused associative theories is exactly what has made them superior to performance-focused models to date in stimulating research.

Conditions favoring retroactive interference between antecedent events (cue competition) and between subsequent events (outcome competition)

Retrieval of a target association (A-B) is often impaired if training of a similar association is interpolated between target training and testing; this is known asretroactive interference. Two experiments, in which rats were used as subjects in a sensory preconditioning preparation, studied the associative nature of retroactive interference between antecedent events (i.e., A and C in the A-B, C-B paradigm) and between subsequent events (i.e., B and C in the A-B, A-C paradigm). With the present preparation, retroactive interference was equally strong between antecedent events and between subsequent events. Moreover, interference occurred only if (1) an association was trained in the interpolated phase and (2) the target and interpolated associations had a common element in a common temporal location.

Overshadowing as a function of trial number: dynamics of first- and second-order comparator effects.

In two conditioned lick suppression experiments with rats, we examined the permanence of the overshadowing effect as a function of the number of compound reinforced training trials. In Experiment 1, robust overshadowing was observed following 4 compound-US pairings but dissipated with 36 pairings. Overshadowing decreased because responding to the overshadowed stimulus increased, not because responding by the control group decreased. This dissipation was stimulus specific and not attributable to a response ceiling. Experiment 2 extended the generality of the effect to a sensory preconditioning design and further demonstrated that overshadowing lost through many compound-US pairings was restored by posttraining extinction of the training context. The results are explicable in terms of the extended comparator hypothesis (Denniston, Savastano, & Miller, 2001) under the assumption that the impacts of first- and second-order comparator processes grow differentially as a function of number of trials.

Comparator mechanisms and conditioned inhibition: Conditioned stimulus preexposure disrupts Pavlovian conditioned inhibition but not explicitly unpaired inhibition.

Three conditioned lick-suppression experiments with rats examined the effects of pretraining exposure to the conditioned stimulus (CS) on behavior indicative of conditioned inhibition. After CS-preexposure treatment, subjects received either Pavlovian conditioned inhibition training or explicitly unpaired inhibition training with the preexposed CS. The inhibitory status of the CS was then assessed with a retardation (Experiment 1) or a summation (Experiment 2) test. Experiment 3 controlled for the unconditioned stimulus-preexposure effect being a potential confound in Experiments 1 and 2. As predicted by the comparator hypothesis (R. R. Miller & L. D. Matzel, 1988), the CS-context association that developed during the CS-preexposure phase disrupted the expression of Pavlovian conditioned inhibition but not the expression of explicitly unpaired inhibition.