Impaired associative learning in chronic schizophrenics and their first-degree relatives: a study of latent inhibition and the Kamin blocking effect

Delusional Unreality and Predictive Processing

BACKGROUND

Phenomenological psychopathologists have recently highlighted how people with delusions experience multiple realities (delusional and nondelusional) and have suggested that this double bookkeeping cannot be explained via predictive processing. Here, we present data from Kamin blocking and extinction learning that show how predictive processing might, in principle, explain a pervasive sense of dual reality.

METHODS

This cross-sectional study involved 3 participant groups: patients with schizophrenia (SZ) (n = 42), healthy participants with elevated esoteric beliefs (EEBs) (clairaudient psychics) (n = 31), and healthy control participants (HCs) with neither illness nor significant delusional ideation (n = 62). We examined belief formation using a Kamin blocking causal learning task with extinction and delusions with the 40-item Peters Delusion Inventory, specifically the unreality item "Do things around you ever feel unreal, as though it was all part of an experiment?" as a proxy for unreality experiences and beliefs. A clinician also assessed symptoms with a structured clinical interview.

RESULTS

Some people with SZ did not report a sense of unreality, and some people with elevated esoteric beliefs (but no psychotic illness) reported unreality experiences. No HCs reported them (despite reporting other delusion-like beliefs). Unreality experiences in clinical delusions and nonclinical delusion-like beliefs were associated with different types of aberrant prediction error processing.

CONCLUSIONS

These data suggest how predictive processing may explain the sense of unreality. They indicate that different prediction error dysfunctions are associated with delusions with different contents. In this case, we have used predictive processing to address a salient issue raised by our phenomenological colleagues, namely the impact of psychosis on experiences of and beliefs about reality.

NrCAM-deficient mice exposed to chronic stress exhibit disrupted latent inhibition, a hallmark of schizophrenia

The neuronal cell adhesion molecule (NrCAM) is widely expressed and has important physiological functions in the nervous system across the lifespan, from axonal growth and guidance to spine and synaptic pruning, to organization of proteins at the nodes of Ranvier. NrCAM lies at the core of a functional protein network where multiple targets (including NrCAM itself) have been associated with schizophrenia. Here we investigated the effects of chronic unpredictable stress on latent inhibition, a measure of selective attention and learning which shows alterations in schizophrenia, in NrCAM knockout (KO) mice and their wild-type littermate controls (WT). Under baseline experimental conditions both NrCAM KO and WT mice expressed robust latent inhibition (p = 0.001). However, following chronic unpredictable stress, WT mice (p = 0.002), but not NrCAM KO mice (F < 1), expressed latent inhibition. Analyses of neuronal activation (c-Fos positive counts) in key brain regions relevant to latent inhibition indicated four types of effects: a single hit by genotype in IL cortex (p = 0.0001), a single hit by stress in Acb-shell (p = 0.031), a dual hit stress x genotype in mOFC (p = 0.008), vOFC (p = 0.020), and Acb-core (p = 0.032), and no effect in PrL cortex (p > 0.141). These results indicating a pattern of differential effects of genotype and stress support a complex stress × genotype interaction model and a role for NrCAM in stress-induced pathological behaviors relevant to schizophrenia and other psychiatric disorders.

Schizotypy dimensions do not predict overshadowing

When two cues are presented together and reliably predict an outcome (AB-O1) an "overshadowing" effect is typically observed. That is, the relationship between these cues and the outcome is learned about less well than a cue presented on its own with an outcome (e.g., C - O1). The current study sought to explore the relationship between overshadowing and the positive and negative dimensions of schizotypy. A total of 256 participants completed an overshadowing procedure embedded within a causal judgement task and the Short Oxford-Liverpool Inventory of Feelings and Experiences (O-LIFE) which measured the different dimensions of schizotypy. A unilateral overshadowing effect was observed, however, none of the dimensions of schizotypy predicted the magnitude of this effect. These results are the first to demonstrate this finding using an appropriately powered sample and reveal that a tendency to experience symptoms of schizophrenia does not impact upon the overshadowing effect.

Spurious Autobiographical Memory of Psychosis: A Mechanistic Hypothesis for the Resolution, Persistence, and Recurrence of Positive Symptoms in Psychotic Disorders

Psychotic disorders are complex disorders with multiple etiologies. While increased dopamine synthesis capacity has been proposed to underlie psychotic episodes, dopamine-independent processes are also involved (less responsive to dopamine receptor-blocking medications). The underlying mechanism(s) of the reduction in antipsychotic responsiveness over time, especially after repeated relapses, remain unclear. Despite the consistent evidence of dopamine overactivity and hippocampal volume loss in schizophrenia, few accounts have been provided based on the interactive effect of dopamine on hippocampal synapse plasticity mediating autobiographical memory processes. The present hypothesis builds upon previous works showing the potential effects of dopamine overactivity on hippocampal-mediated neuroplasticity underlying autobiographical memory, alongside known patterns of autobiographical memory dysfunction in psychosis. We propose that spurious autobiographical memory of psychosis (SAMP) produced during active psychosis may be a key mechanism mediating relapses and treatment non-responsiveness. In a hyperdopaminergic state, SAMP is expected to be generated at an increased rate during active psychosis. Similar to other memories, it will undergo assimilation, accommodation, and extinction processes. However, if SAMP fails to integrate with existing memory, a discontinuity in autobiographical memory may result. Inadequate exposure to normalizing experiences and hyposalience due to overmedication or negative symptoms may also impede the resolution of SAMP. Residual SAMP is hypothesized to increase the propensity for relapse and treatment non-responsiveness. Based on recent findings on the role of dopamine in facilitating hippocampal synapse plasticity and autobiographical memory formation, the SAMP hypothesis is consistent with clinical observations of DUP effects, including the repetition of contents in psychotic relapses as well as the emergence of treatment non-responsiveness after repeated relapses. Clinical implications of the hypothesis highlight the importance of minimizing active psychosis, integrating psychosis memory, avoiding over-medication, and fostering normalizing experiences.

Latent Inhibition in Schizophrenia and Schizotypy

Background

The Salience Hypothesis posits that aberrations in the assignment of salience culminate in hallucinations and unusual beliefs, the "positive symptoms" of schizophrenia. Evidence for this comes from studies on latent inhibition (LI), referring to the phenomenon that prior exposure to a stimulus impedes learning about the relationship between that stimulus and an outcome.

Design

This article reviewed all published studies examining the relationship between LI and both schizophrenia and schizotypy.

Results

Contemporary literature suggests that LI is attenuated in both people with schizophrenia and those loading highly on measures of schizotypy, the multidimensional derivative of schizophrenia. This suggests that these individuals assign greater salience to stimuli than healthy controls and people scoring low on measures of schizotypy, respectively. However, several confounds limit these conclusions. Studies on people with schizophrenia are limited by the confounding effects of psychotropic medications, idiosyncratic parsing of samples, variation in dependent variables, and lack of statistical power. Moreover, LI paradigms are limited by the confounding effects of learned irrelevance, conditioned inhibition, negative priming, and novel pop-out effects.

Conclusions

This review concludes with the recommendation that researchers develop novel paradigms that overcome these limitations to evaluate the predictions of the Salience Hypothesis.

The prediction-error hypothesis of schizophrenia: new data point to circuit-specific changes in dopamine activity

Schizophrenia is a severe psychiatric disorder affecting 21 million people worldwide. People with schizophrenia suffer from symptoms including psychosis and delusions, apathy, anhedonia, and cognitive deficits. Strikingly, schizophrenia is characterised by a learning paradox involving difficulties learning from rewarding events, whilst simultaneously 'overlearning' about irrelevant or neutral information. While dysfunction in dopaminergic signalling has long been linked to the pathophysiology of schizophrenia, a cohesive framework that accounts for this learning paradox remains elusive. Recently, there has been an explosion of new research investigating how dopamine contributes to reinforcement learning, which illustrates that midbrain dopamine contributes in complex ways to reinforcement learning, not previously envisioned. This new data brings new possibilities for how dopamine signalling contributes to the symptomatology of schizophrenia. Building on recent work, we present a new neural framework for how we might envision specific dopamine circuits contributing to this learning paradox in schizophrenia in the context of models of reinforcement learning. Further, we discuss avenues of preclinical research with the use of cutting-edge neuroscience techniques where aspects of this model may be tested. Ultimately, it is hoped that this review will spur to action more research utilising specific reinforcement learning paradigms in preclinical models of schizophrenia, to reconcile seemingly disparate symptomatology and develop more efficient therapeutics.

Neural Circuitry of Salience and Reward Processing in Psychosis

The processing of salient and rewarding stimuli is integral to engaging our attention, stimulating anticipation for future events, and driving goal-directed behaviors. Widespread impairments in these processes are observed in psychosis, which may be associated with worse functional outcomes or mechanistically linked to the development of symptoms. Here, we summarize the current knowledge of behavioral and functional neuroimaging in salience, prediction error, and reward. Although each is a specific process, they are situated in multiple feedback and feedforward systems integral to decision making and cognition more generally. We argue that the origin of salience and reward processing dysfunctions may be centered in the subcortex during the earliest stages of psychosis, with cortical abnormalities being initially more spared but becoming more prominent in established psychotic illness/schizophrenia. The neural circuits underpinning salience and reward processing may provide targets for delaying or preventing progressive behavioral and neurobiological decline.

BDNF haploinsufficiency induces behavioral endophenotypes of schizophrenia in male mice that are rescued by enriched environment

Brain-derived neurotrophic factor (BDNF) is implicated in a number of processes that are crucial for healthy functioning of the brain. Schizophrenia is associated with low BDNF levels in the brain and blood, however, not much is known about BDNF's role in the different symptoms of schizophrenia. Here, we used BDNF-haploinsufficient (BDNF^+/-) mice to investigate the role of BDNF in different mouse behavioral endophenotypes of schizophrenia. Furthermore, we assessed if an enriched environment can prevent the observed changes. In this study, male mature adult wild-type and BDNF^+/- mice were tested in mouse paradigms for cognitive flexibility (attentional set shifting), sensorimotor gating (prepulse inhibition), and associative emotional learning (safety and fear conditioning). Before these tests, half of the mice had a 2-month exposure to an enriched environment, including running wheels. After the tests, BDNF brain levels were quantified. BDNF^+/- mice had general deficits in the attentional set-shifting task, increased startle magnitudes, and prepulse inhibition deficits. Contextual fear learning was not affected but safety learning was absent. Enriched environment housing completely prevented the observed behavioral deficits in BDNF^+/- mice. Notably, the behavioral performance of the mice was negatively correlated with BDNF protein levels. These novel findings strongly suggest that decreased BDNF levels are associated with several behavioral endophenotypes of schizophrenia. Furthermore, an enriched environment increases BDNF protein to wild-type levels and is thereby able to rescue these behavioral endophenotypes.

22q11.2 deletion syndrome and schizophrenia

22q11.2 deletion is a common microdeletion that causes an array of developmental defects including 22q11.2 deletion syndrome (22q11DS) or DiGeorge syndrome and velocardiofacial syndrome. About 30% of patients with 22q11.2 deletion develop schizophrenia. Mice with deletion of the ortholog region in mouse chromosome 16qA13 exhibit schizophrenia-like abnormal behaviors. It is suggested that the genes deleted in 22q11DS are involved in the pathogenesis of schizophrenia. Among these genes, COMT, ZDHHC8, DGCR8, and PRODH have been identified as schizophrenia susceptibility genes. And DGCR2 is also found to be associated with schizophrenia. In this review, we focused on these five genes and reviewed their functions in the brain and the potential pathophysiological mechanisms in schizophrenia, which will give us a deeper understanding of the pathology of schizophrenia.

Personality, creativity and latent inhibition

The current study set out to investigate the relationship between creativity, multi‐dimensional schizotypy and personality more generally. This was achieved by analysing scores on a range of personality scales and measures of creativity, where it was found that the creativity measures were more closely related to asocial‐schizotypy than positive‐schizotypy. The study also sought to test Eysenck's prediction (1993, 1995) that, given the putative relationship between creativity and psychosis‐proneness, high psychosis‐prone scoring individuals and high creativity scoring individuals would demonstrate the same cognitive style of ‘overinclusiveness’ on latent inhibition. However, the results failed to demonstrate any evidence of a shared ‘widening of the associative horizon’ between high creativity and high psychosis‐prone scorers. The findings are discussed in relation to multi‐dimensional schizotypy. Copyright © 2006 John Wiley & Sons, Ltd.

Human latent inhibition: Problems with the stimulus exposure effect

Latent inhibition (LI) is a startlingly simple effect in which preexposure of a stimulus without consequence retards subsequent responding to a stimulus-consequence relation. The effect was first demonstrated with Pavlovian conditioning in animals and was later suggested to be a marker of human psychopathology such as schizophrenia. Individual differences in LI has supported the continued use of animal models to understand human mental health. In this review, we ask whether there is sufficient evidence to support the continued application of LI from animal models to human psychopathology because of the weak evidence for LI in humans. There is considerable variability in the methods used to assess LI, sustaining different theoretical accounts of the effects observed, which differ from the accepted accounts of LI as demonstrated in animals. The review shows that although there have been many experiments testing human LI, none provide the necessary experimental controls to support the conclusion that retarded responding is caused simply by preexposure to a stimulus, as has been demonstrated with animal models. Establishing this conflict, we set out a framework for future research.

Impaired contextual fear-conditioning in MAM rodent model of schizophrenia

The methylazoxymethanol acetate (MAM) rodent neurodevelopmental model of schizophrenia exhibits aberrant dopamine system activation attributed to hippocampal dysfunction. Context discrimination is a component of numerous behavioral and cognitive functions and relies on intact hippocampal processing. The present study explored context processing behaviors, along with dopamine system activation, during fear learning in the MAM model. Male offspring of dams treated with MAM (20mg/kg, i.p.) or saline on gestational day 17 were used for electrophysiological and behavioral experiments. Animals were tested on the immediate shock fear conditioning paradigm, with either different pre-conditioning contexts or varying amounts of context pre-exposure (0-10 sessions). Amphetamine-induced locomotor activity and dopamine neural activity was measured 1-week after fear conditioning. Saline, but not MAM animals, demonstrated enhanced fear responses following a single context pre-exposure in the conditioning context. One week following fear learning, saline rats with 2 or 7min of context pre-exposure prior to fear conditioning also demonstrated enhanced amphetamine-induced locomotor response relative to MAM animals. Dopamine neuron recordings showed fear learning-induced reductions in spontaneous dopamine neural activity in MAM rats that was further reduced by amphetamine. Apomorphine administration confirmed that reductions in dopamine neuron activity in MAM animals resulted from over excitation, or depolarization block. These data show a behavioral insensitivity to contextual stimuli in MAM rats that coincide with a less dynamic dopamine response after fear learning.

The effect of ketamine on the consolidation and extinction of contextual fear memory

Ketamine, principally an antagonist of N-methyl-ᴅ-aspartate receptors, induces schizophrenia-like symptoms in adult humans, warranting its use in the investigation of psychosis-related phenotypes in animal models. Genomic studies further implicate N-methyl-ᴅ-aspartate receptor-mediated processes in schizophrenia pathology, together with more broadly-defined synaptic plasticity and associative learning processes. Strong pathophysiological links have been demonstrated between fear learning and psychiatric disorders such as schizophrenia. To further investigate the impact of ketamine on associative fear learning, we studied the effects of pre- and post-training ketamine on the consolidation and extinction of contextual fear memory in rats. Administration of 25 mg/kg ketamine prior to fear conditioning did not affect consolidation when potentially confounding effects of state dependency were controlled for. Pre-training ketamine (25 mg/kg) impaired the extinction of the conditioned fear response, which was mirrored with the use of a lower dose (8 mg/kg). Post-training ketamine (25 mg/kg) had no effect on the consolidation or extinction of conditioned fear. These observations implicate processes relating to the extinction of contextual fear memory in the manifestation of ketamine-induced phenotypes, and are consistent with existing hypotheses surrounding abnormal associative learning in schizophrenia.

Cognitive function in schizophrenia: conflicting findings and future directions

INTRODUCTION

Schizophrenia is a severe mental disorder with multiple psychopathological domains being affected. Several lines of evidence indicate that cognitive impairment serves as the key component of schizophrenia psychopathology. Although there have been a multitude of cognitive studies in schizophrenia, there are many conflicting results. We reasoned that this could be due to individual differences among the patients (i.e. variation in the severity of positive vs. negative symptoms), different task designs, and/or the administration of different antipsychotics.

METHODS

We thus review existing data concentrating on these dimensions, specifically in relation to dopamine function. We focus on most commonly used cognitive domains: learning, working memory, and attention.

RESULTS

We found that the type of cognitive domain under investigation, medication state and type, and severity of positive and negative symptoms can explain the conflicting results in the literature.

CONCLUSIONS

This review points to future studies investigating individual differences among schizophrenia patients in order to reveal the exact relationship between cognitive function, clinical features, and antipsychotic treatment.

Chronic mild stress impairs latent inhibition and induces region-specific neural activation in CHL1-deficient mice, a mouse model of schizophrenia

Schizophrenia is a neurodevelopmental disorder characterized by abnormal processing of information and attentional deficits. Schizophrenia has a high genetic component but is precipitated by environmental factors, as proposed by the 'two-hit' theory of schizophrenia. Here we compared latent inhibition as a measure of learning and attention, in CHL1-deficient mice, an animal model of schizophrenia, and their wild-type littermates, under no-stress and chronic mild stress conditions. All unstressed mice as well as the stressed wild-type mice showed latent inhibition. In contrast, CHL1-deficient mice did not show latent inhibition after exposure to chronic stress. Differences in neuronal activation (c-Fos-positive cell counts) were noted in brain regions associated with latent inhibition: Neuronal activation in the prelimbic/infralimbic cortices and the nucleus accumbens shell was affected solely by stress. Neuronal activation in basolateral amygdala and ventral hippocampus was affected independently by stress and genotype. Most importantly, neural activation in nucleus accumbens core was affected by the interaction between stress and genotype. These results provide strong support for a 'two-hit' (genes x environment) effect on latent inhibition in CHL1-deficient mice, and identify CHL1-deficient mice as a model of schizophrenia-like learning and attention impairments.

Schizophrenia copy number variants and associative learning

Large-scale genomic studies have made major progress in identifying genetic risk variants for schizophrenia. A key finding from these studies is that there is an increased burden of genomic copy number variants (CNVs) in schizophrenia cases compared with controls. The mechanism through which these CNVs confer risk for the symptoms of schizophrenia, however, remains unclear. One possibility is that schizophrenia risk CNVs impact basic associative learning processes, abnormalities of which have long been associated with the disorder. To investigate whether genes in schizophrenia CNVs impact on specific phases of associative learning we combined human genetics with experimental gene expression studies in animals. In a sample of 11 917 schizophrenia cases and 16 416 controls, we investigated whether CNVs from patients with schizophrenia are enriched for genes expressed during the consolidation, retrieval or extinction of associative memories. We show that CNVs from cases are enriched for genes expressed during fear extinction in the hippocampus, but not genes expressed following consolidation or retrieval. These results suggest that CNVs act to impair inhibitory learning in schizophrenia, potentially contributing to the development of core symptoms of the disorder.

Recreational cocaine use is associated with attenuated latent inhibition

INTRODUCTION

Evidence has linked chronic cocaine use with various cognitive deficits; however few studies have investigated the effects of recreational (non-dependent) use. The present study aimed to assess whether recreational users show deficits in latent inhibition (LI: a measure of delayed learning of an association between 2 stimuli, one of which has been previously exposed (PE) without consequence and thus deemed irrelevant).

METHODS

Using a quasi-experimental between groups design, recreational cocaine users (n = 21), poly-drug users (n = 17) and drug-naive controls (n = 18) were compared on a LI task. Questionnaires assessing psychological health and drug use were also completed.

RESULTS

There was a statistically significant interaction between condition (PE vs non PE) and group (cocaine, polydrug and control); cocaine users scored lower in the PE condition compared to polydrug users and controls, indicating quicker learning.

CONCLUSIONS

Recreational cocaine users show attenuated LI reflecting reduced ability to filter out irrelevant stimuli enabling faster learning of a PE irrelevant and novel stimuli association. This does not appear to be a result of schizotypy and/or other drug use. Thus even at recreational levels, cocaine use may be sufficient to affect inhibitory attentional processes.

Delusions and prediction error: re-examining the behavioural evidence for disrupted error signalling in delusion formation

INTRODUCTION

There is now significant evidence that prediction error signalling is mediated by dopamine in the midbrain, and that dopamine dysfunction is implicated in people experiencing psychotic symptoms, including delusions. There has also been significant theorizing and experimentation concerning the remaining link in this triad, namely that deviant prediction error signalling produces or maintains psychotic symptoms.

METHODS

The research supporting the link between prediction error signalling and delusional symptoms was reviewed. Numerous studies indirectly support this link, but only one set of studies claim to directly test this hypothesis by combining three crucial elements: a patient sample, a manipulation of prediction error and neuroimaging. This particular set of studies were examined in detail.

RESULTS

Important methodological limitations in these studies were observed, and a reinterpretation of their data was offered.

CONCLUSIONS

Methodological inconsistencies significantly weaken the claims made by these studies, but their data are consistent with current theorizing and they are instructive for future lines of inquiry in this field.

Extreme Elemental Processing in a High Schizotypy Population: Relation to Cognitive Deficits

The cognitive deficits observed in schizophrenia have been characterized as a failure to utilize task-setting information to guide behaviour, especially in situations in which there is response conflict. Recently, we have provided support for this account; high schizotypy individuals demonstrated inferior biconditional discrimination performance compared to low scorers, but were not impaired on a simple discrimination that did not require the use of task-setting cues. These results may, however, also be explained by the way in which individuals with high schizotypy process stimulus compounds . Here, we examine the initial approaches to solving biconditional and control discrimination tasks of participants with high and low schizotypy scores. In particular, we focus on performance during the first block of training trials to capture processing style before the acquisition of the discrimination tasks. Participants scoring highly on the introvertive anhedonia subscale (which has been allied to the negative and cognitive symptoms of schizophrenia) demonstrated better biconditional performance during the first block of training trials than did low-schizotypy individuals, consistent with a highly elemental approach to stimulus processing. Subsequent recognition tests confirmed this analysis demonstrating that the pattern of performance observed in participants with high schizotypy was associated with a failure to discriminate conjunctions of items that had been seen before from those that had not. These results suggest that the negative/cognitive symptoms of schizophrenia may reflect an extreme bias towards elemental, as opposed to configural, processing of stimulus conjunctions.

Understanding the relationship between schizotypy and attention: dissociating stimulus- and dimension-specific processes

Two experiments investigated how the schizotypal characteristics of unusual experiences modulate changes in attention to alphanumeric stimuli. In the first stage of both experiments, participants were required to attend and respond to either Arabic numerals or Latin letters; with the four exemplars from each dimension being presented on a different number of occasions (0, 5, 10, 20). During the test in Experiment 1 (n=103), speeded alphanumeric decisions were more accurate for the novel than familiar exemplars, irrespective of whether they had been attended to or not. This influence of familiarity was not modulated by schizotypy. During the test in Experiment 2 (n=128), learning that the attended dimension predicted the presentation of the symbol X (or the absence of X) proceeded more rapidly than learning the corresponding predictions involving the unattended dimension. In the case of novel exemplars, but not familiar exemplars, this modulation of learning by attention was reduced as schizotypy scores increased. Taken together, these results show that schizotypal characteristics do not modulate the influence of familiarity on performance (Experiment 1), but do have an influence on attention, which is best characterised as one on tuning attention to stimulus dimensions rather than individual stimuli.

Attention to irrelevant cues is related to positive symptoms in schizophrenia

Many modern learning theories assume that the amount of attention to a cue depends on how well that cue predicted important events in the past. Schizophrenia is associated with deficits in attention and recent theories of psychosis have argued that positive symptoms such as delusions and hallucinations are related to a failure of selective attention. However, evidence demonstrating that attention to irrelevant cues is related to positive symptoms in schizophrenia is lacking. We used a novel method of measuring attention to nonpredictive (and thus irrelevant) cues in a causal learning test (Le Pelley ME, McLaren IP. Learned associability and associative change in human causal learning. Q J Exp Psychol B. 2003;56:68-79) to assess whether healthy adults and people with schizophrenia discriminate previously predictive and nonpredictive cues. In a series of experiments with independent samples, we demonstrated: (1) when people with schizophrenia who had severe positive symptoms successfully distinguished between predictive and nonpredictive cues during training, they failed to discriminate between predictive and nonpredictive cues relative to healthy adults during subsequent testing and (2) learning about nonpredictive cues was correlated with more severe positive symptoms scores in schizophrenia. These results suggest that positive symptoms of schizophrenia are related to increased attention to nonpredictive cues during causal learning. This deficit in selective attention results in learning irrelevant causal associations and may be the basis of positive symptoms in schizophrenia.

The positive symptoms of acute schizophrenia and latent inhibition in humans and animals: underpinned by the same process(es)?

INTRODUCTION

It has been suggested that the positive symptoms of acute schizophrenia are a consequence of a disruption of the process that produces latent inhibition (slower acquisition of conditioned responding after preexposure to the conditioned stimulus) and that this effect can be modelled by pro- and antipsychotic compounds in healthy participants and in nonhuman animals. This idea assumes that latent inhibition in humans and animals is underpinned by the same process(es).

METHOD

First, we question the equivalence of human and animal latent inhibition. Second, we review the studies that have examined latent inhibition in populations with schizophrenia and in healthy populations after administration of amphetamine or haloperidol.

RESULTS

Theoretical analysis of the similarities and differences in latent inhibition effects, and the procedures used to generate them, in humans and animals renders the suggested equivalence between them unconvincing. The studies examining latent inhibition in populations with schizophrenia and in healthy populations after administration of amphetamine or haloperidol are marked by a number of methodological shortcomings and reveal discrepant results.

CONCLUSIONS

The theoretical and empirical analyses provide little support for a common process underlying deficits of latent inhibition in patients exhibiting positive symptoms of acute schizophrenia, and such deficits in experimental models in healthy humans and infrahumans.

Kamin blocking is associated with reduced medial-frontal gyrus activation: implications for prediction error abnormality in schizophrenia

The following study used 3-T functional magnetic resonance imaging (fMRI) to investigate the neural signature of Kamin blocking. Kamin blocking is an associative learning phenomenon seen where prior association of a stimulus (A) with an outcome blocks subsequent learning to an added stimulus (B) when both stimuli are later presented together (AB) with the same outcome. While there are a number of theoretical explanations of Kamin blocking, it is widely considered to exemplify the use of prediction error in learning, where learning occurs in proportion to the difference between expectation and outcome. In Kamin blocking as stimulus A fully predicts the outcome no prediction error is generated by the addition of stimulus B to form the compound stimulus AB, hence learning about it is "blocked". Kamin blocking is disrupted in people with schizophrenia, their relatives and healthy individuals with high psychometrically-defined schizotypy. This disruption supports suggestions that abnormal prediction error is a core deficit that can help to explain the symptoms of schizophrenia. The present study tested 9 healthy volunteers on an f-MRI adaptation of Oades' "mouse in the house task", the only task measuring Kamin blocking that shows disruption in schizophrenia patients that has been independently replicated. Participant's Kamin blocking scores were found to inversely correlate with Kamin-blocking-related activation within the prefrontal cortex, specifically the medial frontal gyrus. The medial frontal gyrus has been associated with the psychological construct of uncertainty, which we suggest is consistent with disrupted Kamin blocking and demonstrated in people with schizophrenia. These data suggest that the medial frontal gyrus merits further investigation as a potential locus of reduced Kamin blocking and abnormal prediction error in schizophrenia.

Disambiguating ventral striatum fMRI-related BOLD signal during reward prediction in schizophrenia

Reward detection, surprise detection and prediction-error signaling have all been proposed as roles for the ventral striatum (vStr). Previous neuroimaging studies of striatal function in schizophrenia have found attenuated neural responses to reward-related prediction errors; however, as prediction errors represent a discrepancy in mesolimbic neural activity between expected and actual events, it is critical to examine responses to both expected and unexpected rewards (URs) in conjunction with expected and UR omissions in order to clarify the nature of ventral striatal dysfunction in schizophrenia. In the present study, healthy adults and people with schizophrenia were tested with a reward-related prediction-error task during functional magnetic resonance imaging to determine whether schizophrenia is associated with altered neural responses in the vStr to rewards, surprise prediction errors or all three factors. In healthy adults, we found neural responses in the vStr were correlated more specifically with prediction errors than to surprising events or reward stimuli alone. People with schizophrenia did not display the normal differential activation between expected and URs, which was partially due to exaggerated ventral striatal responses to expected rewards (right vStr) but also included blunted responses to unexpected outcomes (left vStr). This finding shows that neural responses, which typically are elicited by surprise, can also occur to well-predicted events in schizophrenia and identifies aberrant activity in the vStr as a key node of dysfunction in the neural circuitry used to differentiate expected and unexpected feedback in schizophrenia.

Dopamine in nucleus accumbens: salience modulation in latent inhibition and overshadowing

Latent inhibition (LI) is demonstrated when non-reinforced pre-exposure to a to-be-conditioned stimulus retards later learning. Learning is similarly retarded in overshadowing, in this case using the relative intensity of competing cues to manipulate associability. Electrolytic/excitotoxic lesions to shell accumbens (NAc) and systemic amphetamine both reliably abolish LI. Here a conditioned emotional response procedure was used to demonstrate LI and overshadowing and to examine the role of dopamine (DA) within NAc. Experiment 1 showed that LI but not overshadowing was abolished by systemic amphetamine (1.0 mg/kg i.p.). In Experiment 2, 6-hydroxydopamine (6-OHDA) was used to lesion DA terminals within NAc: both shell- and core- (plus shell-)lesioned rats showed normal LI and overshadowing. Experiment 3 compared the effects of amphetamine microinjected at shell and core coordinates prior to conditioning: LI, but not overshadowing, was abolished by 10.0 but not 5.0 µg/side amphetamine injected in core but not shell NAc. These results suggest that the abolition of LI produced by NAc shell lesions is not readily reproduced by regionally restricted DA depletion within NAc; core rather than shell NAc mediates amphetamine-induced abolition of LI; overshadowing is modulated by different neural substrates.

Learned irrelevance and associative learning is attenuated in individuals at risk for psychosis but not in asymptomatic first-degree relatives of schizophrenia patients: translational state markers of psychosis?

Learned irrelevance (LIrr) refers to a form of selective learning that develops as a result of prior noncorrelated exposures of the predicted and predictor stimuli. In learning situations that depend on the associative link between the predicted and predictor stimuli, LIrr is expressed as a retardation of learning. It represents a form of modulation of learning by selective attention. Given the relevance of selective attention impairment to both positive and cognitive schizophrenia symptoms, the question remains whether LIrr impairment represents a state (relating to symptom manifestation) or trait (relating to schizophrenia endophenotypes) marker of human psychosis. We examined this by evaluating the expression of LIrr in an associative learning paradigm in (1) asymptomatic first-degree relatives of schizophrenia patients (SZ-relatives) and in (2) individuals exhibiting prodromal signs of psychosis ("ultrahigh risk" [UHR] patients) in each case relative to demographically matched healthy control subjects. There was no evidence for aberrant LIrr in SZ-relatives, but LIrr as well as associative learning were attenuated in UHR patients. It is concluded that LIrr deficiency in conjunction with a learning impairment might be a useful state marker predictive of psychotic state but a relatively weak link to a potential schizophrenia endophenotype.

Contribution of nonprimate animal models in understanding the etiology of schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by positive and negative symptoms and cognitive impairments. The etiology of the disorder is complex, and it is thought to follow a multifactorial threshold model of inheritance with genetic and neurodevelop mental contributions to risk. Human studies are particularly useful in capturing the richness of the phenotype, but they are often limited to the use of correlational approaches. By assessing behavioural abnormalities in both humans and rodents, nonprimate animal models of schizophrenia provide unique insight into the etiology and mechanisms of the disorder. This review discusses the phenomenology and etiology of schizophrenia and the contribution of current nonprimate animal models with an emphasis on how research with models of neuro transmitter dysregulation, environmental risk factors, neurodevelopmental disruption and genetic risk factors can complement the literature on schizophrenia in humans.

Ignoring irrelevant stimuli in latent inhibition and Stroop paradigms: the effects of schizotypy and gender

Latent inhibition (LI), poor evidence of learning following preexposure to a task-irrelevant stimulus, reflects the ability to ignore inconsequential events. Stroop interference represents a failure to inhibit processing of a task-irrelevant word when it is incongruent with the required naming of the word's print color. The apparent commonality between the two effects is in contradiction to the literature, which indicates that LI is affected by schizotypy and schizophrenia, and perhaps gender, while Stroop interference generated by the trial-to-trial procedure is unaltered by those variables. In the present experiment, low schizotypal healthy males, but not females, exhibited LI. The same groups did not differ on Stroop interference. The results are discussed in terms of different processing requirements for task-irrelevant stimuli that are an integral part of the task-relevant target stimulus (as in Stroop) or separated from it in space (as in LI).

Intact associative learning in patients with schizophrenia: evidence from a Go/NoGo paradigm

OBJECTIVE

Schizophrenia is associated with deficits in executive control and associative learning. In the present study, we investigated the effect of associative learning during a Go/NoGo task in healthy controls subjects and patients with schizophrenia.

METHODS

Thirty patients with schizophrenia and 30 age-and-gender matched healthy control subjects performed 15 blocks of training and 3 blocks of test trials. The trials consisted of responding to words denoting either living or non-living objects. In the training condition, subjects were instructed to respond by pressing the space bar (Go-task) to one of the word types (living or non-living objects), but not the other. In the test phase, the Go/NoGo mapping was reversed. Subjects were instructed to respond as quickly and as accurately as possible. Reaction times (RT) and accuracy were recorded for each trial and all subjects were debriefed upon completion of the test trials.

RESULTS

Patients with schizophrenia had significantly longer Go RTs when compared to the control group, during both training and test trials. However, the two groups did not differ on any measure of associative learning.

CONCLUSIONS

Our findings suggest that associative learning is intact in schizophrenia patients during the performance of a relational Go/NoGo paradigm.

The neural underpinnings of associative learning in health and psychosis: how can performance be preserved when brain responses are abnormal?

Associative learning experiments in schizophrenia and other psychoses reveal subtle abnormalities in patients' brain responses. These are sometimes accompanied by intact task performance. An important question arises: How can learning occur if the brain system is not functioning normally? Here, we examine a series of possible explanations for this apparent discrepancy: (1) standard brain activation patterns may be present in psychosis but partially obscured by greater noise, (2) brain signals may be more sensitive to real group differences than behavioral measures, and (3) patients may achieve comparable levels of performance to control subjects by employing alternative or compensatory neural strategies. We consider these explanations in relation to data from causal- and reward-learning imaging experiments in first-episode psychosis patients. The findings suggest that a combination of these factors may resolve the question of why performance is sometimes preserved when brain patterns are disrupted.

Abnormal prediction error is associated with negative and depressive symptoms in schizophrenia

Prediction error in learning is where learning occurs to the degree to which an outcome consequent to a stimulus is surprising. It has been suggested that abnormal use of prediction error in schizophrenia may underlie the formation of inappropriate associations giving rise to psychotic symptoms. Kamin blocking is a phenomenon that demonstrates prediction error. Kamin blocking is shown where prior learning about a stimulus A paired with an outcome retards learning about a stimulus B when presented subsequently as part of a stimulus compound AB paired with the same outcome. Prior studies have indicated reduced Kamin blocking in schizophrenia specifically in non-paranoid patients. It is however unclear how reduced Kamin blocking is associated with specific symptoms in schizophrenia. The present study examined Kamin blocking performance in a high functioning community-based sample of 34 people with schizophrenia and 48 controls closely matched for pre-morbid IQ. In these patients we measured Kamin blocking and symptoms using positive and negative symptom scales (PANSS) and Scale for the Assessment of Negative Symptoms (SANS) and Scale for the Assessment of Positive Symptoms (SAPS). Results confirmed that people with schizophrenia had significantly reduced Kamin blocking. Kamin blocking performance was associated with negative and depressive symptoms. These associations with symptoms were crucially not found with baseline associative learning or unblocking measures, confirming specificity to the Kamin blocking effect. These data demonstrate first that abnormal prediction error as assessed in the Kamin blocking task is associated with negative and depressive symptoms rather than positive symptoms in high functioning schizophrenia patients. Second this strongly suggests that reduced Kamin blocking may be useful as an animal model of specific relevance to negative and depressive symptoms in schizophrenia.

From prediction error to psychosis: ketamine as a pharmacological model of delusions

Recent cognitive neuropsychiatric models of psychosis emphasize the role of attentional disturbances and inappropriate incentive learning in the development of delusions. These models highlight a pre-psychotic period in which the patient experiences perceptual and attentional disruptions. Irrelevant details and numerous associations between stimuli, thoughts and percepts are imbued with inappropriate significance and the attempt to rationalize and account for these bizarre experiences results in the formation of delusions. The present paper discusses delusion formation in terms of basic associative learning processes. Such processes are driven by prediction error signals. Prediction error refers to mismatches between an organism's expectation in a given environment and what actually happens and it is signalled by both dopaminergic and glutamatergic mechanisms. Disruption of these neurobiological systems may underlie delusion formation. We review similarities between acute psychosis and the psychotic state induced by the NMDA receptor antagonist drug ketamine, which impacts upon both dopaminergic and glutamatergic function. We conclude by suggesting that ketamine may provide an appropriate model to investigate the formative stages of symptom evolution in schizophrenia, and thereby provide a window into the earliest and otherwise inaccessible aspects of the disease process.

Increased concentration of cerebral kynurenic acid alters stimulus processing and conditioned responding

Kynurenic acid (KYNA) is a tryptophan metabolite synthesized and released by glia and recently shown to be a non-competitive antagonist of alpha7 nicotinic acetylcholine receptors at physiologically relevant concentrations, and NMDA receptors at higher concentrations. KYNA concentration is elevated in individuals with schizophrenia and those with Alzheimer's disease, two populations exhibiting cholinergic-related cognitive impairments. The present study investigated the effects of elevated KYNA concentration on conditioned stimulus processing in rats. For the first 2 days of the experiment, a subset of rats received intracerebroventricular infusions of either KYNA (0.1 microM) or vehicle and were either returned to the home cage or received non-reinforced presentations of a visual stimulus. All rats subsequently received presentations of the same visual stimulus followed by food reward during a 6-day training phase. In vehicle-treated rats, pre-exposure to the visual stimulus reduced orienting behaviour to the light (standing on the hind legs and orienting towards the visual stimulus) when it was later reinforced (i.e., conditioned orienting). In contrast, pre-exposure to the visual cue or 2 days of KYNA pretreatment reduced conditioned orienting behaviour. Finally, the reduction of orienting in KYNA-treated rats following pre-exposure was not as robust as in vehicle-treated rats. These results suggest that elevated KYNA levels can alter specific aspects of attentional processing of environmental stimuli and are discussed in terms of the potential contribution of KYNA to cognitive function and dysfunction.

The role of CA3 hippocampal NMDA receptors in paired associate learning

The hippocampus is necessary for declarative memory in humans and episodic memory in rodents. Considerable current research is focused on the role of plasticity within specific subfields of the hippocampus. Here, using a viral vector to temporally control a focal deletion of the NR1 gene, we show that learning novel paired associations between specific cues and their context is dependent on CA3 NMDA receptors. Deletion of CA3 NR1 genes in <30% of the dorsal hippocampus was sufficient to disrupt new learning, whereas the same treatment does not prevent expression of previously acquired paired associates and does not affect the ability to discriminate contexts or paired associate learning when either the cues or the context is familiar. The findings suggest that CA3 NMDA receptors specifically support the encoding of new experiences to involve incidental and contingent associations.

The visual search analogue of latent inhibition: implications for theories of irrelevant stimulus processing in normal and schizophrenic groups

Latent inhibition (LI) is a robust phenomenon that is demonstrated when a previously inconsequential stimulus is less effective in a new learning situation than a novel stimulus. Despite LI's simplicity, there is considerable disagreement as to its theoretical basis. Attentional theories claim that unattended stimulus preexposures reduce stimulus associability. Alternatively, it has been asserted that associability is unaffected and that LI is a result of competition/retrieval processes. The present article reviews a series of visual search studies, some with normal subjects, both undifferentiated and divided into low and high schizotypals, and others with pathologies that entail dysfunctional attention, such as schizophrenia, Parkinson's disease, and anxiety. The visual search conditions were designed to model those of traditional LI experiments, while tapping attentional processes independently of the learning scores that index LI. A variety of evidence from these and other studies is used to support the involvement of attentional and retrieval processes in LI. A model of the mechanism of action of these processes in LI is presented, together with its application to schizophrenia.

Stimulus processing and associative learning in Wistar and WKHA rats

This study assessed basic learning and attention abilities in Wistar-Kyoto hyperactive (WKHA) rats using appetitive conditioning preparations. Two measures of conditioned responding to a visual stimulus, orienting behavior (rearing on the hind legs), and food cup behavior (placing the head inside the recessed food cup) were measured. In Experiment 1, simple conditioning, but not extinction, was impaired in WKHA rats compared with Wistar rats. In Experiment 2, nonreinforced presentations of the visual cue preceded the conditioning sessions. WKHA rats displayed less orienting behavior than Wistar rats but comparable levels of food cup behavior. These data suggest that WKHA rats exhibit specific abnormalities in attentional processing as well as in learning stimulus-reward relationships.

Disruption of learned irrelevance in acute schizophrenia in a novel continuous within-subject paradigm suitable for fMRI

Learned irrelevance (LIrr) is closely related to latent inhibition (LI). In LI a to-be-conditioned stimulus (CS) is prexposed alone prior to the opportunity to learn an association between the CS and an unconditioned stimulus (UCS). In LIrr preexposure consists of intermixed presentations of both CS and UCS in a random relationship to each other. In both paradigms preexposure leads in normal subjects to reduced or retarded learning of the CS-UCS association. Acute schizophrenics fail to show LI. LI is usually demonstrated as a one-off, between-groups difference in trials to learning, so posing problems for neuroimaging. We have developed a novel, continuous, within-subject paradigm in which normal subjects show robust and repeated LIrr. We show that this paradigm is suitable for functional magnetic resonance imaging (fMRI) and gives rise, in normal subjects, to activation in the hippocampal formation, consistent with data from animal experiments on LI. We also report, consistent with previous studies of LI, loss (indeed, significant reversal) of LIrr in acute (first 2 weeks of current psychotic episode) schizophrenics. Chronic schizophrenics failed to demonstrate learning, precluding measurement in this group of LIrr. These findings establish the likely value of the new paradigm for neuroimaging studies of attentional dysfunction in acute schizophrenia.

The molecular genetics of the 22q11-associated schizophrenia

Schizophrenia has a strong genetic component but the mode of inheritance of the disease is complex and in all likelihood involves interaction among multiple genes and also possibly environmental or stochastic factors. A number of studies have shown that the 22q11 deletion syndrome (22q11DS) is a true genetic subtype of schizophrenia and as such may play an extremely important role in deciphering the genetic basis of schizophrenia. Microdeletions of the 22q11 locus are associated with a staggering increased risk to develop schizophrenia. The same locus has also been implicated by some linkage studies. Systematic examination of individual genes from the 1.5 Mb critical region has identified so far the PRODH and ZDHHC8 as strong candidate schizophrenia susceptibility genes from this locus. Discovery of these genes implicates neuromodulatory aminoacids and protein palmitoylation as important for disease development. Other genes, including the gene encoding for COMT, have been implicated by candidate gene approaches. It therefore appears that the 22q11-associated schizophrenia may have the characteristics of a contiguous gene syndrome, where deficiency in more than one gene contributes to the strikingly increased disease risk. Mouse models for individual candidate genes will provide the investigators with the opportunity to start understanding the function of these genes and how they may impact on schizophrenia. Mouse models that carry long-range deletions will likely capture the interactions among the culprit genes and help explain the genetic contribution of this locus to the high risk for schizophrenia. In-depth human and animal model studies of 22q11DS promise to answer critical questions relating to the devastating illness of schizophrenia, whose causes remain largely unknown.

Learned irrelevance is disrupted in first-episode but not chronic schizophrenia patients

Learned irrelevance (LIrr) is a pre-exposure effect in which uncorrelated presentations of a conditioned stimulus (CS) and an unconditioned stimulus (US) retard subsequent CS-US association. LIrr is closely related to the phenomenon of latent inhibition (LI). LI refers to the retarding effects of inconsequential stimulus pre-exposure on subsequent conditioning to that stimulus, and is considered to reflect the organism's capacity to ignore irrelevant stimuli. LI is disrupted in schizophrenia patients, due to faster learning of the association between the preexposed CS and the US. A new within-subject target-recognition LIrr procedure was applied. The target was either cued by a priming signal or appeared at random, and priming signals were novel or preexposed cues. Schizophrenia patients were compared to age- and sex-matched control subjects. Normal subjects (n = 24) have shown robust LIrr, namely, faster cue-target associations of novel compared to preexposed cues. Schizophrenia patients at the early stages of their first episode (n = 7) showed LIrr disruption, namely, cue-target associations to preexposed cues were as fast as for novel cues. Chronic patients during an acute phase (n = 18) did not show LIrr as they failed to learn the cue-target association. In addition to the LIrr paradigm the same subjects were tested in a covert-orientation task. No differences were observed between the groups on this task. The possible advantages of the new LIrr paradigm are discussed.

The relevance of irrelevance to schizophrenia

Jeffrey Gray's neuropsychological theory of the positive symptoms of schizophrenia has been highly influential by enabling a strong link between animal and human research. Central to the development and testing of this theory has been the phenomenon and paradigm of latent inhibition (LI-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of the first stimulus). We review findings relating to its alteration in patients with schizophrenia (acute and chronic), people high on dimensions of schizotypy and the effects of amphetamine and anti-psychotic medication in humans. We suggest that many human-LI paradigms still suffer from theoretical and practical limitations, but that recent developments are beginning to address these. Finally we explore the idea that the paradigm of Learned Irrelevance (LIRR-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of both stimuli but in an unrelated manner) might be used to complement studies on LI in exploring the cognitive distortions suffered by patients with schizophrenia.

Intact visual latent inhibition in schizophrenia patients in a within-subject paradigm

People are normally slower to learn a CS-UCS association if they first experience the CS without the UCS. This normal slowing, termed "latent inhibition" (LI), is reported by some to be absent in schizophrenia patients. Our previous studies detected generalized learning deficits but not LI deficits in schizophrenia patients, using between-subject auditory and visual LI paradigms. To understand our divergent results, we developed a within-subject visual LI paradigm that detects LI in normal male subjects that we previously reported to be disrupted by acute treatment with dopamine agonists. In the present study, we verified the ability of this dopamine-sensitive within-subject LI paradigm to detect LI among both male and female normal control subjects, and then used this paradigm to assess LI in schizophrenia patients. Among normals, LI exhibited no sex differences or menstrual cyclicity. Compared to normals, schizophrenia patients exhibited learning deficits with both preexposed (PE) and non-preexposed (NPE) stimuli. Despite these generalized deficits, both acutely hospitalized patients and stable outpatients with schizophrenia exhibited robust LI, as evidenced by significantly faster learning with NPE than PE stimuli. LI deficits in schizophrenia may be paradigm-specific and are not detected by a paradigm that we previously reported to be sensitive to disruption by dopamine agonists.

Transient disruption of attentional performance following escalating amphetamine administration in rats

RATIONALE

Attentional deficits are thought to be critically involved in the development of positive symptoms in schizophrenia. The present experiment tests the general hypothesis that sensitization of the mesolimbic dopaminergic system contributes to the attentional deficits in schizophrenia.

OBJECTIVES

The present study assessed attentional performance following administration of an escalating amphetamine regimen and subsequent "challenge" amphetamine administration in rats.

METHODS

Rats were trained to perform a two-lever sustained attention task that involved discrimination of visual signals and no signal presentation. After reaching criterion, subjects were assigned to receive escalating amphetamine or saline. Attentional performance was assessed immediately following escalating amphetamine, following "challenge" amphetamine administration (1.0 mg/kg) to amphetamine-pretreated rats, and for 3 days after the challenge session. At the end of this experiment, a dose-response study was conducted with saline-pretreated rats to confirm the appropriateness of the challenge dose.

RESULTS

Amphetamine-pretreated animals demonstrated a transient increase in errors on nonsignal trials following escalating amphetamine administration. The latency to press a lever was decreased during and after challenge amphetamine administration. Administration of 1.0 mg/kg amphetamine did not alter accuracy of amphetamine-pretreated animals or of saline-pretreated animals in the dose-response experiment.

CONCLUSIONS

Prior escalating amphetamine administration transiently disrupted attention, increasing incorrect "claims" for a signal on trials when no signal was presented. The present data support the existing literature that escalating amphetamine regimens may be useful to model the attentional deficits that contribute to the psychotic symptoms in schizophrenia.

Effects of amisulpride, risperidone and chlorpromazine on auditory and visual latent inhibition, prepulse inhibition, executive function and eye movements in healthy volunteers

In view of the evidence that cognitive deficits in schizophrenia are critically important for long-term outcome, it is essential to establish the effects that the various antipsychotic compounds have on cognition, particularly second-generation drugs. This parallel group, placebo-controlled study aimed to compare the effects in healthy volunteers (n = 128) of acute doses of the atypical antipsychotics amisulpride (300 mg) and risperidone (3 mg) to those of chlorpromazine (100 mg) on tests thought relevant to the schizophrenic process: auditory and visual latent inhibition, prepulse inhibition of the acoustic startle response, executive function and eye movements. The drugs tested were not found to affect auditory latent inhibition, prepulse inhibition or executive functioning as measured by the Cambridge Neuropsychological Test Battery and the FAS test of verbal fluency. However, risperidone disrupted and amisulpride showed a trend to disrupt visual latent inhibition. Although amisulpride did not affect eye movements, both risperidone and chlorpromazine decreased peak saccadic velocity and increased antisaccade error rates, which, in the risperidone group, correlated with drug-induced akathisia. It was concluded that single doses of these drugs appear to have little effect on cognition, but may affect eye movement parameters in accordance with the amount of sedation and akathisia they produce. The effect risperidone had on latent inhibition is likely to relate to its serotonergic properties. Furthermore, as the trend for disrupted visual latent inhibition following amisulpride was similar in nature to that which would be expected with amphetamine, it was concluded that its behaviour in this model is consistent with its preferential presynaptic dopamine antagonistic activity in low dose and its efficacy in the negative symptoms of schizophrenia.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

An investigation into age and gender differences in human Kamin blocking, using a computerized task

Kamin blocking (KB) is a function within selective attention found to be deficient in schizophrenia. Disparate results in the KB literature, specifically regarding age and gender effects, suggest that we do not yet have a comprehensive understanding of KB in normal human development. The aim of this study is to provide a thorough investigation into the development and occurrence of KB in a normal population. The design replicated and extended a study by Oades, Roepcke, and Schepker (1996). KB is measured using a computer game called the "mouse in the house." Participants must use a joystick to move an icon around a set floor plan, to find a hidden location. These locations are cued by sets of colors, which denote the KB paradigm. Data was collected on 222 participants across 5 age groups (6-8 years; 9-12 years; 13-17 years; 18-21 years; 22+ years). Comparisons were carried out for age and gender effects. KB was observed in all age groups, but there was no significant effect of age on mean KB score. A measure of frequency of participants who showed KB did show a significant increase with age. A significant difference was found between males and females, with females having higher KB score than males. The gender difference was present from the earliest age tested. Our findings suggest significant age and gender differences in the manifestation of selective attention and information processing abilities. This has implications for understanding the development of attention and the understanding of the age and gender dependence of the development of schizophrenia.

Reduced Kamin blocking in non paranoid schizophrenia: associations with schizotypy

Kamin blocking (KB) is an attentional phenomenon whereby prior learning about a stimulus (A) retards learning about a new stimulus (B) when later presented in compound (AB) with the original stimulus A. KB has been shown to be reduced in patients with schizophrenia. Using Oades' KB paradigm it has been suggested that drug treatment may influence the expression of KB abnormalities in patients. It is therefore unclear whether Reduced KB are due to drug treatment or to the illness itself. One experimental approach that circumvents drug treatment confounds is to study schizotypal traits in healthy volunteers. In the present study we investigated KB using the Oades paradigm in 27 healthy volunteers and 21 schizophrenic patients. We additionally investigated the relationship between KB performance and measures of schizotypal traits and a number of factors relevant to the experience of schizophrenia using the O-LIFE questionnaire. Our results indicate first a clear negative relationship between general schizotypy and more specifically, Unusual experiences (UNEX) and cognitive disorganisation (COGDIS) and KB performance. This relationship was qualitatively and quantitatively similar in both healthy volunteers and schizophrenic patients. Second we have independently replicated reduced KB in non-paranoid patients and no change in KB in paranoid patients using the Oades KB task. This study also confirms that reduced KB in non-paranoid patients is confined to early test trials (3-4) while the negative relationships with schizotypy scales UNEX and COGDIS that we have found are also confined to these early test trials confirming the psychological relevance of this specificity.