Effects of context novelty vs. familiarity on latent inhibition with a conditioned taste aversion procedure

The latent inhibition phenomenon is observed when a conditioned stimulus is preexposed without any consequence before conditioning. The result of this manipulation is a reduction in conditioned response intensity to such a stimulus. In this study, we analyse the role of context novelty/familiarity on LI modulation by changing the context using a three-stage conditioned taste aversion procedure. Experiment 1 revealed that, similar to other learning procedures, a context change between preexposure and conditioning/testing (but not between preexposure/conditioning and testing) resulted in LI attenuation when the experimental contexts were novel. Experiment 2, using animals’ home cages as one of the contexts, revealed a different pattern of results, with an unexpected increase in LI magnitude when the context change was introduced between conditioning and test stages. The Schmajuk et al. (1996) computational model explains these results in terms of the increased novelty of the conditioned stimulus during preexposure, conditioning, and testing.     

Salience modulation and associative inhibition interaction: Short but not long exposure to similar stimuli protects the salience of the unique elements

In three experiments, rats were given preexposure to two similar flavour compounds, AX and BX. Following preexposure, conditioning trials took place in which AX was paired with an illness-induced unconditioned stimulus. Animals that were given short alternated preexposure to AX and BX, showed higher generalization of conditioned aversion to AX to a new compound, AN, than animals that were given blocked preexposure (short and long) and long alternated preexposure (Experiments 1 and 2); and showed less preference for A when they were given a choice between A and X (Experiment 3). These results have been taken to indicate that the salience of the A element is well preserved after short alternated preexposure, but declines when preexposure goes on for some more trials. The results reported support the notion that perceptual learning is a multi-determined phenomenon that depends on salience modulation processes after relatively short preexposure, and on an associative inhibition mechanism after prolonged preexposure.     

Latent inhibition as a function of CS intensity in taste aversion learning

An experiment is reported in which the relationship between the intensity of a preexposed stimulus and latent inhibition was investigated, using the taste aversion learning paradigm in rats. Two concentrations of a saline solution (high, 1%; and low, 0.25%) were used during preexposure and conditioning phases in a factorial design. Two control conditions without preexposure were added, one for each stimulus concentration during conditioning. The known effect of conditioned stimulus (CS) intensity during conditioning was confirmed: the more concentrated the solution used in conditioning, the higher the acquisition rate. A direct relationship was observed between the CS intensity used during preexposure and the latent inhibition effect: the more concentrated the solution during preexposure, the lower the acquisition rate of conditioning. The implications of these results for latent inhibition theories are considered.     

Order effects after blocked preexposure to two compound flavors

In the first three experiments two groups of rats received prolonged blocked preexposure to AX–BX or to BX–AX. Experiment 1 showed that conditioning of AX after preexposure resulted in less generalization to BX in the AX–BX group than in the BX–AX group. Experiments 2 and 3 constituted, respectively, retardation and summation tests of the inhibitory properties acquired by B after preexposure and conditioning of A. Excitatory conditioning of B was retarded in the AX–BX group (Experiment 2) and only in this group B was able to alleviate the aversion conditioned to a new stimulus (Experiment 3). These results are explained as a consequence of the formation, in the AX–BX group, of inhibitory associations directed from B to A. A fourth experiment provided evidence that the A ↔ X association was preserved until the end of the preexposure phase in this group, which is a requisite for the formation of these inhibitory associations.     

Impairment of latent inhibition in OXYS rats with hereditary syndrome of premature aging

The effect of latent inhibition has been tested in OXYS (strain selected for accelerated aging) and Wistar (control) rats at the age of 4 months using a conditioned passive avoidance reaction. The OXYS rats displayed a higher level of anxiety in the elevated plus-maze as compared to Wistar strain. The experimental procedure consisted of preexposure stage (presentation of non-reinforced environmental stimulus), conditioning (presentation of a preexposed stimulus paired with shock), and testing the effect of latent inhibition. The effect of latent inhibition was revealed only in Wistar rats, in which the latency of transition to the dark compartment of the experimental chamber was reduced. Contrastingly, in OXYS rats the conditioned performance was not inhibited. It is suggested that aging-associated factors such as the low rate of habituation to experimental situation at the preexposure stage and the high general anxiety level of OXYS rats can interfere with inhibition of the attention to irrelevant information.     

Effect of the serotoninergic substrate of the nucleus accumbens on the latent inhibition

Latent inhibition (LI) is a reduction in the rate of acquisition of a conditioned response that results from repeated preexposure of an animal to a conditioned stimulus (CS). The present experiment was conducted to assess the effect of bilateral lesions of 5-OT terminals of the nucleus accumbens on LI in a conditioned passive avoidance response paradigm. The lesions were produced by administration of 5,7-DHT and resulted in disruption of LI. Sham-operated animals displayed the delay of conditioning (LI) in comparison with the non-preexposed controls. Disruption of the LI was prevented by systemic injection of haloperidol. Involvement of 5-HT substrate of the nucleus accumbens and its interaction with dopaminergic system in the process of the LI development is discussed.     

Appetitive latent inhibition in rats: preexposure performance does not predict conditioned performance

Nonreinforced preexposure to a conditioned stimulus impairs subsequent conditioning with that stimulus. The goal of these studies was to assess the extent to which acquisition performance could be predicted from preexposure performance using a correlational approach. For both preexposure and autoshaping, four measures of performance were computed, including overall average lever pressing, lever pressing in the initial session, percentage change in lever pressing, and slopes. These measures were correlated in a large sample of rats trained in an autoshaping situation. None of the three measures of autoshaping performance was consistently predicted by any of the three measures of preexposure performance. These results are consistent with the view that latent inhibition is not reducible to long-term habituation.     

Pavlovian backward conditioned inhibition in humans: summation and retardation tests

Two experiments using human participants investigated whether a Pavlovian backward inhibitory treatment (nonreinforced trials in phase 1 followed by reinforced trials in phase 2; i.e., AX- followed by A+) produces a stimulus which can pass summation and retardation tests for inhibition. The rationale for conducting these experiments was that previous demonstrations of Pavlovian backward inhibition informed participants about the nature of the outcome before starting the experiment. According to some theoretical views, this is a potential confound. In the present experiments we used a predictive task in which participants had no knowledge about the outcome until phase 2, when reinforcement occurred. The results of Experiment 1 (summation test) and Experiment 2 (retardation test) provide a clear demonstration of backward conditioned inhibition.     

Exposure to estradiol before but not during acquisition of LiCl-induced conditioned taste avoidance accelerates extinction

Estradiol accelerates extinction of LiCl-induced conditioned taste avoidance when it is present continuously before and during acquisition. We have suggested that the effect of estradiol on extinction is due to its illness-associated, rather than learning-associated, properties. If this were the case, then one would expect estradiol to act before but not during acquisition. This expectation is based on previous work showing attenuation of learned taste avoidance when rats are given distal preexposure (greater than 24 h before conditioning) or proximal preexposure (less than 24 h before conditioning) to the illness-inducing agent LiCl before acquisition of a LiCl-induced conditioned taste avoidance. In three separate experiments, estradiol was administered during three different time periods via subcutaneous implantation of a 10-mm estradiol-filled capsule. In each experiment, the extinction of estradiol-treated females was compared to that of females implanted with empty capsules. In the first experiment, female rats were given distal exposure to estradiol before acquisition. Capsules were implanted 11 days before acquisition and were removed 2 days before acquisition. In the second experiment, female rats were given proximal exposure to estradiol before acquisition. Capsules were implanted 2.5 h before LiCl was paired with a sucrose solution and were removed 16.5 h later. In the third experiment, female rats were given exposure to estradiol during acquisition. Capsules were implanted at the same time as LiCl administration and were removed 18 h later. The only estradiol-treated females to show accelerated extinction were those given distal preexposure to estradiol in Experiment 1. These data do not support a learning-associated hypothesis and only partially support an illness-associated hypothesis. The failure to find accelerated extinction following proximal preexposure may reflect an inappropriate choice of the parameters used in the experiment or a difference in the stimulus properties of LiCl and estradiol that allow each to serve as conditioning and preexposure agents in conditioned taste avoidance paradigms [corrected].     

Neuronal activity in the nucleus basalis magnocellularis during performance of an instrumental task

Spike activity of 95 neurons in the rabbit basal forebrain forebrain magnocellular nucleus was recorded during spontaneous behavior and instrumental conditioned performance. Almost half of the neurons (48.4%) displayed a significant (p > 0.05) negative correlation between their spontaneous discharge rate and the power of the delta rhythm in the EEG of the frontal cortex; most of these cells can be classified as projection cholinergic neurons. During instrumental conditioned performance, neurons of this subgroup responded with excitation to the conditioned stimulus, whereas presumably noncholinergic nonprojection neurons responded to the conditioned stimulus with inhibition. Excitatory response of cells in the basal forebrain magnocellular nucleus was significantly more intense as compared to trials without the conditioned reaction. On the whole, our data testify that the basal forebrain magnocellular nucleus maintains the level of arousal and attention required for the instrumental conditioned performance.     

Functional Difference between Sustained and Transient Modulations of Cognitive Control in the Simon Task: Evidence from False Alarm Responses on No-Go Trials

Cognitive control in response compatibility tasks is modulated by the task context. Two types of contextual modulations have been demonstrated; sustained (block-wise) and transient (trial-by-trial). Recent research suggests that these modulations have different underlying mechanisms. This study presents new evidence supporting this claim by comparing false alarm (FA) responses on no-go trials of the Simon task between the sustained and transient contexts. In Experiment 1, the sustained context was manipulated so that a block included a larger number of incongruent trials. Results showed that participants made more FA responses by the hand opposite to the stimulus location. This suggests a generation of response bias in which the task-irrelevant location information is utilized in a reversed manner (i.e., to respond with the right hand to a stimulus presented on the left side and vice versa). Next, Experiment 2 examined the effect of the transient context and found that overall FA rate was lower when a no-go trial was preceded by an incongruent trial than by a congruent trial, whereas such response bias as that shown in Experiment 1 was not demonstrated. This suggests that the transient conflict context enhances inhibition of the task-irrelevant process but does not make the task-irrelevant information actively usable. Based on these results, we propound two types of cognitive control modulations as adaptive behaviors: response biasing based on utilization of the task-irrelevant information under the sustained conflict context and transient enhancement of inhibition of the task-irrelevant process based on the online conflict monitoring.     

Lack of evidence for inhibitory processes in over-selectivity

Stimulus over-selectivity can be defined as control over behavior being exerted by one aspect of the environment at the expense of other equally salient aspects of the environment, and is a common problem for discrimination learning under conditions of cognitive strain, and in intellectual disorders. Non-clinical participants exposed to a concurrent task load were trained and tested on a two-component trial-and-error discrimination task to investigate whether inhibition plays a role in producing under-selectivity by using both summation and retardation tests. Experiment 1 found evidence for the over-selectivity effect, and replicated the finding that revaluation of a previously over-selected stimulus allows emergence of control by a previous under-selected stimulus, despite the latter stimulus receiving no direct conditioning. The under-selected cue was not found to gain any conditioned inhibitory status, as reflected by summation (Experiment 2), and retardation (Experiment 3), tests. The results extend the literature explaining over-selectivity as a post-acquisition failure.     

Repeated administration of LiCl produces an unconditioned stimulus preexposure effect in backward excitatory CTA but not habituation of the unconditioned increment in neophobia

In one experiment using conditioned taste aversion and the unconditioned stimulus (US) preexposure procedure, one group of rats was given LiCl exposure for 3 days, whereas two other groups received saline. Following this phase, all groups were given a novel flavour (saccharine) to drink following either LiCl (group preexposed and one of the control groups) or saline injections (the remaining control group) and the consumption of the flavour was assessed. After this neophobia test, the acquired saccharine aversion was evaluated. The results show that three LiCl injections are enough to produce a US preexposure effect on backward excitatory taste aversion conditioning, whereas this number of injections procedure does not produce habituation of the increment in neophobia, an unconditioned response to the LiCl. The results are discussed taking into account different mechanisms involved in US preexposure effect.     

Contrasting the overexpectation and extinction effects

After many target stimulus (X)-unconditioned stimulus (US) pairings, further conditioning of X in the presence of another well-established signal for the US (A) disrupts X's behavioral control. Some researchers have argued that the mechanism underlying this so-called overexpectation effect is similar to that underlying extinction (a reduction in X's behavioral control due to X-alone presentations). Three conditioned suppression experiments with rats as subjects compared overexpectation and extinction. Experiment 1 replicated the basic overexpectation effect by showing that A disrupts responding to X more than does a previously neutral stimulus. Experiment 2 found that posttraining context exposure disrupts extinction but not overexpectation. Experiment 3 suggested that overexpectation and extinction are differentially sensitive to the effects of overtraining (compound reinforced or nonreinforced, respectively), such that extinction is enhanced by increases in the amount of nonreinforced trials and overexpectation is unaffected. These results are inconsistent with the view that overexpectation and extinction are driven by a common mechanism.     

Effects of an extinguished CS on competition with another CS

Three experiments were conducted using a conditioned taste aversion procedure with rats to examine the effect of nonreinforced presentations of a conditioned stimulus (CS) on its ability to compete with a target stimulus for manifest conditioned responding. Two CSs (A and B) were presented in a serial compound and then paired with the unconditioned stimulus. CS A was first paired with the US and then presented without the US (i.e., extinction) prior to reinforced presentation of the AB compound. Experiment 1 showed that A was poor at competing with B for conditioned responding when given conditioning and extinction prior to reinforcement of AB relative to a group that received both A and B for the first time during compound conditioning. That is, an extinguished A stimulus allowed greater manifest acquisition to B. Experiment 2 found that extinction treatment produced a poor CR to the pretrained and extinguished CS itself following compound conditioning. Experiment 3 found that interposing a retention interval after extinction of A and prior to compound conditioning enhanced A's ability to compete with B. The results of these experiments are discussed with regard to different theories of extinction and associative competition.     

Perceptual learning in an appetitive Pavlovian procedure: Analysis of the effectiveness of the common element

Non-reinforced preexposure to two stimuli often enhances discrimination between them. Analyses of this perceptual learning phenomenon have mainly focused on the role played by the distinctive stimulus features; this study examined the contribution of the non-distinctive common elements. A standard appetitive Pavlovian procedure was used. Rats received two different schedules of exposure - alternated or blocked - to two compound auditory stimuli, AX and BX. In Experiment 1 a generalization test to BX that followed conditioning to AX showed that animals responded less, and hence discriminated better, following alternated exposure, thus extending the generality of this perceptual learning effect to standard appetitive Pavlovian procedures. The degree to which the common element X was mediating this effect was explored in the next three experiments. Experiment 2 assessed the effectiveness of X following conditioning to AX. Experiment 3 explored X's effectiveness throughout extensive conditioning to X. Experiment 4 tested the ability of X to overshadow a novel stimulus Y. The results were consistent with the suggestion that alternated preexposure can reduce the relative effectiveness of the common element.     

A comparison of the effects of discriminative and Pavlovian inhibitors and excitors on instrumental responding

In two experiments, the effects of Pavlovian or discriminative conditioned inhibitors on operant responding were investigated in rats. Experiment 1 found that a Pavlovian conditioned inhibitor for food suppressed food-reinforced lever pressing more than a non-differentially trained control stimulus did. Experiment 2 demonstrated that an operant discriminative inhibitor produced greater suppression of lever pressing than a Pavlovian conditioned inhibitor. Experiment 2 also found that compounding an operant discriminative stimulus (SD) for food-reinforced responding with another SD for food-reinforced responding resulted in more additive summation than when an SD was compounded with a Pavlovian conditioned excitor for food. The results of these experiments support two-factor theories that postulate that incentive and response discriminative processes summate algebraically when the processes are inhibitory or excitatory.     

When to Hold That Thought: An Experimental Study Showing Reduced Inhibition of Pre-trained Associations in Schizophrenia

Schizophrenia encompasses a wide variety of cognitive dysfunctions, a number of which can be understood as deficits of inhibition. To date, no research has examined ‘conditioned inhibition’ in schizophrenia - the ability of a stimulus that signals the absence of an expected outcome to counteract the conditioned response produced by a signal for that outcome (a conditioned excitor). A computer-based task was used to measure conditioned excitation and inhibition in the same discrimination procedure, in 25 patients with a confirmed diagnosis of schizophrenia and a community-based comparison sample. Conditioned inhibition was measured by a ratio score, which compared the degree to which the inhibitory stimulus and a neutral control stimulus reduced conditioned responding to the excitatory cue: the lower the ratio, the greater the inhibitory learning. At test the ratios were 0.45 and 0.39 for patient and control groups respectively, and the relevant interaction term of the ANOVA confirmed that the degree of inhibition was reduced in the patient group, with an effect size of r = 0.28.These results demonstrate for the first time that inhibitory learning is impaired in schizophrenia. Such an impairment provides an attractive framework for the interpretation of the positive symptoms of schizophrenia. However, we were unable to demonstrate any relationship between the level of conditioned inhibition and medication. Similarly, in the present study it must be emphasised that the available data did not demonstrate any relationship between individual variation in inhibitory learning and the level of positive symptoms as measured by the PANSS. In fact inhibitory learning impairment was relatively greater in participants with a predominantly negative symptom profile and their excitatory learning was also reduced. Accordingly the next step will be to investigate such relationships in a larger sample with a priori defined sub-groups displaying predominantly positive versus predominantly negative symptoms.     

Pavlovian conditioning of shock-induced suppression of lymphocyte reactivity: acquisition, extinction, and preexposure effects

Recent research has indicated that physical stressors, such as electric shock, can suppress immune function in rats. The present study investigated whether a nonaversive stimulus that had been associated with electric shock would also impair immune function. Presentation of that conditioned stimulus (CS) by itself produced a pronounced suppression of lymphocyte proliferation in response to the nonspecific mitogens, Concanavalin-A (ConA) and Phytohemagglutinin (PHA). In further evidence of a conditioning effect, the suppression was attenuated by extinction and preexposure manipulations that degraded the associative value of the CS. These results indicate that a psychological or learned stressor can suppress immune reactivity independently of the direct effect of physically aversive stimulation or of ancillary changes in dietary and health-related habits.     

Autoshaping in the rat: conditioned licking response to a stimulus that signals sucrose reinforcement

The present experiments were designed to determine if repeated presentations of an empty sipper tube (the conditioned stimulus or CS) with the response-independent delivery of a sucrose solution (the unconditioned stimulus or US) from a second spout results in the development of Pavlovian conditioned responding. In Experiment 1, rats in the experimental condition received paired CS-US presentations whereas subjects in the control condition were exposed to random presentations of CS and US. In Experiment 2, an additional control condition (CS alone) was included and, to encourage generalized responding between the US and CS, the CS tube was filled with water for all groups. The results of both experiments indicate that the CS-directed responding in the paired CS-US condition was Pavlovian in nature. Thus, the present procedure serves as an autoshaping task in which conditioned licking is generated.