Blunted inhibition of return in schizophrenia-evidence from a longitudinal study

Inhibition of return (IOR) in patients with schizophrenia and cannabis use

Research concerning the spatial orientation in patients with schizophrenia has demonstrated a state independent deficit in inhibition of return (IOR), which has been discussed as a vulnerability marker for schizophrenia. Other recent investigations on brain structure and cognitive processing have revealed less deficits in schizophrenia patients with comorbid cannabis use (SCH + CUD) compared to abstinent schizophrenia patients (SCH). It was hypothesized that these results may reflect a premorbid lower vulnerability in at least a subgroup of comorbid patients. The aim of the present study is to extend previous work by investigating IOR functioning in patients with schizophrenia and cannabis use. This in turn should supplement the existing studies on the vulnerability of this patient group. Therefore, we compared IOR functioning in four groups: 62 patients with schizophrenia and 46 healthy controls, both with and without cannabis use. Participants underwent a covert orienting of attention task (COVAT) with peripheral cues and three stimulus onset asynchronies (SOAs: 200 ms, 400 ms and 800 ms). Both schizophrenia groups displayed delayed IOR with a more pronounced IOR effect in SCH + CUD compared to SCH. In healthy controls, IOR did not seem to be significantly affected by cannabis use. Significant IOR-differences between groups were only seen between SCH patients without cannabis use and both healthy groups at SOA 400 ms. Patterns of cannabis use as well as clinical parameters of psychoses did not affect IOR. Our results may support the hypothesis of IOR as a vulnerability marker for schizophrenia and of a lower biological vulnerability in at least a subgroup of SCH + CUD.

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.

Neural correlates of the preserved inhibition of return in schizophrenia

Inhibition of return (IOR) is an attentional mechanism that previously has been reported to be either intact or blunted in subjects with schizophrenia (SCZ). In the present study, we explored the neural mechanism of IOR in SCZ by comparing the target-locked N1 and P1 activity evoked by valid-cued trials with that evoked by invalid-cued trials. Twenty-seven schizophrenia patients and nineteen healthy controls participated in a task involving covert orienting of attention with two stimulus onset asynchronies (SOAs: 700 ms and 1200 ms) during which 64-channel EEG data were recorded. Behavioral reaction times (RTs) were longer in response to valid-cued trials than to invalid-cued ones, suggesting an intact IOR in SCZ. However, reduced N1 amplitude elicited by valid-cued trials suggested a stronger inhibition of attention from being oriented to a previously cued location, and therefore a relative inhibition of perceptual processing at that location in SCZ. These results indicate that altered N1 activity is associated with the preservation of IOR in SCZ and could be a sensitive marker to track the IOR effect.

Not all attention orienting is created equal: recognition memory is enhanced when attention orienting involves distractor suppression

Learning through visual exploration often requires orienting of attention to meaningful information in a cluttered world. Previous work has shown that attention modulates visual cortex activity, with enhanced activity for attended targets and suppressed activity for competing inputs, thus enhancing the visual experience. Here we examined the idea that learning may be engaged differentially with variations in attention orienting mechanisms that drive eye movements during visual search and exploration. We hypothesized that attention orienting mechanisms that engaged suppression of a previously attended location would boost memory encoding of the currently attended target objects to a greater extent than those that involve target enhancement alone. To test this hypothesis we capitalized on the classic spatial cueing task and the inhibition of return (IOR) mechanism (Posner, 1980; Posner, Rafal, & Choate, 1985) to demonstrate that object images encoded in the context of concurrent suppression at a previously attended location were encoded more effectively and remembered better than those encoded without concurrent suppression. Furthermore, fMRI analyses revealed that this memory benefit was driven by attention modulation of visual cortex activity, as increased suppression of the previously attended location in visual cortex during target object encoding predicted better subsequent recognition memory performance. These results suggest that not all attention orienting impacts learning and memory equally.

Inhibition of return: Twenty years after

When responding to a suddenly appearing stimulus, we are slower and/or less accurate when the stimulus occurs at the same location of a previous event than when it appears in a new location. This phenomenon, often referred to as inhibition of return (IOR), has fostered a huge amount of research in the last 20 years. In this selective review, which introduces a Special Issue of Cognitive Neuropsychology dedicated to IOR, we discuss some of the methods used for eliciting IOR and its boundary conditions. We also address its debated relationships with orienting of attention, succinctly review findings of altered IOR in normal elderly and neuropsychiatric patients, and present results concerning its possible neural bases. We conclude with an outline of the papers collected in this issue, which offer a more in-depth treatment of behavioural, neural, and theoretical issues related to IOR.

Abnormal inhibition of return: A review and new data on patients with parietal lobe damage

The study of the performance of patients with neurological disorders has been fruitful in revealing the nature and neural basis of inhibition of return (IOR). Thus, in recent years, studies have reported abnormal IOR in patients with Alzheimer's disease, patients diagnosed with schizophrenia, and brain-damaged patients. In the present study, we investigated the hypothesis that a spatial "disengagement deficit" (DD; Posner, Walker, Friedrich, & Rafal, 1984) contributed to the pattern of impaired IOR in the ipsilesional field of parietal patients, found in a previous work (Vivas, Humphreys, & Fuentes, 2003). In a first experiment, we replicated the attenuation of IOR for ipsilesional targets on those trials with a lateralized IOR procedure. With stimuli vertically aligned about fixation, we found intact IOR for both up and down targets. Most important, when we ameliorated the potential impact of a spatial DD by presenting both cues and target in the same hemifield, still we found impaired IOR in the ipsilesional field. We interpret these findings in terms of unilateral parietal damage leading to an imbalance of the relative salience of signals represented in a spatial map for directing attention.

Use of cannabis enhances attentional inhibition

OBJECTIVE

Orienting attention to an irrelevant location hampers the response to subsequent targets presented at that location in relation to novel, not previously attended, locations. This inhibitory effect has been named inhibition of return. We conducted an experiment to study the temporal course of inhibition of return in users of cannabis.

METHOD

Twenty-five cannabis users who self-reported a regular frequency of cannabis use in joints per month, and 26 drug-free controls participated in the study. We employed a typical inhibition of return task with a single cue and manipulated the time interval between the onset of the cue and the target (150, 350, 550, 1500, and 2550 ms). Participants were asked to detect the onset of the target regardless of its location.

RESULTS

The group of cannabis users showed a significantly greater overall inhibition relative to the group of nonusers. Furthermore, inhibition of return appeared earlier (at the 350 ms cue-target interval) in the user group.

CONCLUSIONS

This is the first study to show that attentional inhibition is enhanced in cannabis users. More research is needed to determine whether greater inhibition represents an advantage or disadvantage for visual search performance of cannabis users.

On the measurement of the effects of alcohol and illicit substances on inhibition of return

RATIONALE

Inhibition of return (IOR) refers to the delayed orienting of attention to previously inspected locations in favour of novel locations. Given its implications for visual attention and search, researchers have begun to investigate whether IOR may be impaired by the use of alcohol or illicit substances (e.g. d-amphetamine).

OBJECTIVES

The present paper reviews the existing literature exploring the impact of alcohol and other drugs on IOR through the use of the model spatial cueing task developed by Posner.

RESULTS

Studies were located that investigated IOR paradigm with respect to either (a) acute effects of alcohol or other psychoactive substances and (b) hallucinogenic drug states as models for psychosis. Findings suggest that alcohol, d-amphetamine and some hallucinogens may alter the timecourse of IOR. This review also yields a critical qualitative analysis of the methodology of studies in this field of research and the implications of particular methodological features for interpreting previous findings.

CONCLUSIONS

The importance of using multiple stimulus onset asynchronies, employing a cue-back to centre paradigm and distinguishing between acute and chronic substance use are emphasized. Furthermore, questions are raised as to whether findings suggest an impact of psychoactive substances on the subcortical mechanisms that play a critical role in the generation of IOR or are an indirect effect resulting from impairment of the cortical mechanisms responsible for voluntary disengagement of attention. Directions for future research and particular methodological approaches are highlighted.

Auditory orienting and inhibition of return in schizophrenia: a functional magnetic resonance imaging study

Patients with schizophrenia (SP) exhibit deficits in both attentional reorienting and inhibition of return (IOR) during visual tasks. However, it is currently unknown whether these deficits are supramodal in nature and how these deficits relate to other domains of cognitive dysfunction. In addition, the neuronal correlates of this pathological orienting response have not been investigated in either the visual or auditory modality. Therefore, 30 SP and 30 healthy controls (HC) were evaluated with an extensive clinical protocol and functional magnetic resonance imaging (fMRI) during an auditory cuing paradigm. SP exhibited both increased costs and delayed IOR during auditory orienting, suggesting a prolonged interval for attentional disengagement from cued locations. Moreover, a delay in the development of IOR was associated with cognitive deficits on formal neuropsychological testing in the domains of attention/inhibition and working memory. Event-related fMRI showed the characteristic activation of a frontoparietal network (invalid trials>valid trials), but there were no differences in functional activation between patients and HC during either attentional reorienting or IOR. Current results suggest that orienting deficits are supramodal in nature in SP, and are related to higher-order cognitive deficits that directly interfere with day-to-day functioning.

Inhibition of return and schizophrenia: a meta-analysis

Inhibition of return (IOR) is a phenomenon that involves inhibited or delayed orienting to previously cued locations in favor of attending to novel locations. To date, research on IOR in patients with schizophrenia has generated mixed, and seemingly conflicting, results. Some researchers report patients with schizophrenia exhibit blunted or delayed IOR, while other researchers report normal IOR, in terms of time course and magnitude. This meta-analysis summarizes the literature that has employed an IOR task in patients with schizophrenia and with controls while focusing upon a procedural feature, the use of a cue back to fixation, between the cue and target that is known to be important when executive control has been hampered in non-clinical populations. Fifteen experiments were located yielding a total sample of 362 patients with schizophrenia or schizoaffective disorder and 285 controls. Using a meta-analytic approach, results of the present analyses show that patients with schizophrenia demonstrate delayed IOR in the single cue procedure. In the cue back to fixation procedure, the time course of IOR among patients is more consistent with that of controls. Differences in measured IOR between patients with schizophrenia and controls are largely related to a deficit in endogenous disengagement of attention.

Psilocybin-induced deficits in automatic and controlled inhibition are attenuated by ketanserin in healthy human volunteers

The serotonin-2A receptor (5-HT(2A)R) has been implicated in the pathogenesis of schizophrenia and related inhibitory gating and behavioral inhibition deficits of schizophrenia patients. The hallucinogen psilocybin disrupts automatic forms of sensorimotor gating and response inhibition in humans, but it is unclear so far whether the 5-HT(2A)R or 5-HT(1A)R agonist properties of its bioactive metabolite psilocin account for these effects. Thus, we investigated whether psilocybin-induced deficits in automatic and controlled inhibition in healthy humans could be attenuated by the 5-HT(2A/2C)R antagonist ketanserin. A total of 16 healthy participants received placebo, ketanserin (40 mg p.o.), psilocybin (260 μg/kg p.o.), or psilocybin plus ketanserin in a double-blind, randomized, and counterbalanced order. Sensorimotor gating was measured by prepulse inhibition (PPI) of the acoustic startle response. The effects on psychopathological core dimensions and behavioral inhibition were assessed by the altered states of consciousness questionnaire (5D-ASC), and the Color-Word Stroop Test. Psilocybin decreased PPI at short lead intervals (30 ms), increased all 5D-ASC scores, and selectively increased errors in the interference condition of the Stroop Test. Stroop interference and Stroop effect of the response latencies were increased under psilocybin as well. Psilocybin-induced alterations were attenuated by ketanserin pretreatment, whereas ketanserin alone had no significant effects. These findings suggest that the disrupting effects of psilocybin on automatic and controlled inhibition processes are attributable to 5-HT(2A)R stimulation. Sensorimotor gating and attentional control deficits of schizophrenia patients might be due to changes within the 5-HT(2A)R system.

Deficient inhibition of return in chronic but not first-episode patients with schizophrenia

BACKGROUND

Inhibition of return (IOR) has been tested in patients with schizophrenia with contradictory results. Some studies indicated that patients with schizophrenia have normal levels of IOR; however, other studies reported delayed or blunted IOR. Inconsistency in findings might be due to differences across studies in relevant aspects associated with disease, such as heterogeneity of the disorder, different medications, onset and severity of the illness. The present study was to explore different patterns of IOR in antipsychotic medication free first-episode schizophrenia and chronic schizophrenia.

METHODS

Forty two patients with first-episode schizophrenia, 44 patients with chronic schizophrenia, and 38 healthy controls were included in the study. All subjects went through a covert orienting of attention task with seven stimulus onset asynchrony (SOA) intervals (400 ms, 500 ms, 600 ms, 700 ms, 800 ms, 1200 ms and 1500 ms).

RESULTS

Compared with healthy controls, the magnitude and onset of IOR in first-episode patients with schizophrenia were intact. However, in patients with chronic schizophrenia, there was an attenuated cuing effect especially at SOA 700 ms; in addition, there was a robust IOR until at SOAs 800 ms or above. Moreover, the illness duration and the number of psychotic episodes were significantly correlated with the validity effect at SOAs 400 ms and 600 ms.

CONCLUSION

Our study suggests that deficient IOR presents in chronic but not in first-episode patients with schizophrenia. IOR deficit in schizophrenia may begin during the course of illness and deteriorate over the course of illness. Our findings are consistent with the neurodegenerative model of schizophrenia.

Confirmation for a delayed inhibition of return by systematic sampling in schizophrenia

Inhibition of return (IOR) is a phenomenon thought to reflect a mechanism to protect the organism from redirecting attention to previously scanned insignificant locations. A number of studies reported altered IOR in schizophrenia patients with a reduction of its amplitude. However, incomplete sampling of stimulus onset asynchronies (SOAs) makes data on IOR time course incomplete. We examined 14 stabilized young patients with recent onset schizophrenia and 16 healthy controls matched for gender, age, and years of education. Schizophrenia patients (13 males, 1 female) had a mean age of 26.3+/-5.8 years and a mean number of years of study of 9.6+/-3.6. Their illness had a mean duration of 147 weeks. Patients displayed moderate overall slow reaction times (387 ms) in comparison with controls (322 ms). Onset of IOR was found to be delayed in schizophrenia patients appearing between 700 and 800 ms following the cue onset while it appeared at 300 ms in controls. In patients, IOR was constant up to 1100 ms; however, its amplitude was weak with an average of 6 ms. Validity effects (overall and at each SOA value) were uncorrelated to age, years of study, duration of illness, or total or subscale scores on the Positive and Negative Syndrome Scale.

Gaze cueing of attention in schizophrenia: individual differences in neuropsychological functioning and symptoms

Inhibition of return (IOR) represents a well-known mechanism of human perception that biases attentional orienting to novel locations in the environment. Behaviorally, IOR reflects slower reaction time (RT) to stimuli presented in previously cued locations. In this study, we examined within patients with schizophrenia this inhibitory aftereffect using two different cue types--eye gaze and standard peripheral cues. Results indicated that patients showed evidence of IOR, as reflected in a 3.2% slowing in RT to previously peripherally cued locations. However, for eye gaze, patients failed to show evidence of IOR and instead had 1.7% faster RT to targets presented following delay in locations that had been previously cued. This inhibitory failure correlated strongly with reduced neuropsychological performance and global symptoms ratings of attention and bizarre behavior. Reduced inhibitory aftereffect in RT for eye-gaze cues may reflect disease-related abnormalities in social attention.

The head-centered meridian effect: Auditory attention orienting in conditions of impaired visuo-spatial information

This paper examines the characteristics of spatial attention orienting in situations of visual impairment. Two groups of subjects, respectively schizophrenic and blind, with different degrees of visual spatial information impairment, were tested. In Experiment 1, the schizophrenic subjects were instructed to detect an auditory target, which was preceded by a visual cue. The cue could appear in the same location as the target, separated from it respectively by the vertical visual meridian (VM), the vertical head-centered meridian (HCM) or another meridian. Similarly to normal subjects tested with the same paradigm (Ferlazzo, Couyoumdjian, Padovani, and Olivetti Belardinelli, 2002), schizophrenic subjects showed slower reactions times (RTs) when cued, and when the target locations were on the opposite sides of the HCM. This HCM effect strengthens the assumption that different auditory and visual spatial maps underlie the representation of attention orienting mechanisms. In Experiment 2, blind subjects were asked to detect an auditory target, which had been preceded by an auditory cue, while staring at an imaginary point. The point was located either to the left or to the right, in order to control for ocular movements and maintain the dissociation between the HCM and the VM. Differences between crossing and no-crossing conditions of HCM were not found. Therefore it is possible to consider the HCM effect as a consequence of the interaction between visual and auditory modalities. Related theoretical issues are also discussed.

Response switching in schizophrenia patients and healthy subjects: effects of the inter-response interval

Schizophrenia patients show impaired saccadic response switching, pointing to action control deficits at the level of response selection. Previous studies on healthy subjects suggested that response switch effects might decrease if the prior response is longer ago, reflecting a slow dissipation of the response program persisting from the previous trial. The present study aimed at directly investigating whether response switch effects in schizophrenia patients and healthy subjects depend on the inter-response interval (IRI). Effects of response switching on pro- and antisaccade performance were analyzed in 19 schizophrenia patients and 19 healthy controls at 3 different IRIs (2,500, 3,000, 4,000 ms). Response switch effects of healthy subjects did not vary with the IRI, suggesting that the previous response program persists as long as no contrary response program is activated. In schizophrenia, response switch deficits were replicated at an IRI of 3,000 ms, whereas at IRIs of 2,500 and 4,000 ms, effects of response switching did not significantly differ from healthy subjects. This might suggest that there is a specific IRI range particularly sensitive to response switch deficits in schizophrenia. However, effects of response switching at different IRIs remain to be consolidated.

Turning it upside down: areas of preserved cognitive function in schizophrenia

Patients with schizophrenia demonstrate marked impairments on most clinical neuropsychological tests. These findings suggest that patients suffer from a generalized form of cognitive impairment, with little evidence of spared performance documented in several large meta-analytic reviews of the clinical literature. In contrast, we review evidence for relative sparing of aspects of attention, procedural memory, and emotional processing observed in studies that have employed experimental approaches adapted from the cognitive and affective neuroscience literature. These islands of preserved performance suggest that the cognitive deficits in schizophrenia are not as general as they appear to be when assayed with clinical neuropsychological methods. The apparent contradiction in findings across methods may offer important clues about the nature of cognitive impairment in schizophrenia. The documentation of preserved cognitive function in schizophrenia may serve to sharpen hypotheses about the biological mechanisms that are implicated in the illness.

Pharmacological modulation of the neural basis underlying inhibition of return (IOR) in the human 5-HT2A agonist and NMDA antagonist model of psychosis

RATIONALE

Attentional deficits are common symptoms in schizophrenia. Recent evidence suggests that schizophrenic patients show abnormalities in spatial orienting of attention, particularly a deficit of inhibition of return (IOR). IOR is mostly thought to reflect an automatic, inhibitory mechanism protecting the organism from redirecting attention to previously scanned, insignificant locations. Pharmacologic challenges with hallucinogens have been used as models for psychosis.

OBJECTIVES

The aim of this study was to investigate the neural correlates underlying orienting of attention in the human N-methyl-D-aspartic acid antagonist and 5-HT2A agonist models of psychosis.

MATERIALS AND METHODS

Fourteen healthy volunteers participated in a randomized, double-blind, cross-over event-related functional magnetic resonance imaging (fMRI) study with dimethyltryptamine (DMT) and S-ketamine. We administered a covert orienting of attention task with nonpredictive peripheral cues, and we scanned the subjects on two separate days at least 14 days apart with a placebo and a verum condition on each day.

RESULTS

DMT, but not S-ketamine, slowed down reaction times significantly. IOR was blunted after DMT, but not after S-ketamine. Relative to placebo, S-ketamine increased activation in the IOR condition in the right superior frontal gyrus, left superior temporal gyrus, and right midfrontal frontal gyrus.

CONCLUSIONS

The discrepancy between the behavioral and functional imaging outcome indicates that pharmacological fMRI might be a sensitive tool to detect drug-modulated blood oxygenation level-dependent signal changes in the absence of behavioral abnormalities. Our findings might help to further clarify the contradictory findings of IOR in schizophrenic patients and might, thus, shed more light on possible differential pathomechanisms of schizophrenic symptoms.

Orienting of attention in unmedicated patients with schizophrenia, prodromal subjects and healthy relatives

In typical orienting of attention tasks subjects have to respond as fast as possible to targets which appear in the periphery of the visual field and are preceded by spatial cues (e.g. brightening of a peripheral box where the target may subsequently appear). Reaction times (RT) are facilitated when cue and target appear at the same location (valid cueing) and the cue target interval is short (<250 ms). However, RTs slow down again when the target follows a valid cue after an interval of 250 ms and longer. This latter phenomenon is called Inhibition of Return (IOR) and is thought to reflect an automatic, inhibitory mechanism to protect the organism from redundant and distracting stimuli. Deficits of IOR were repeatedly reported in patients with schizophrenia. However, the role of medications and the nature of the deficit (trait or vulnerability indicator?) were unclear. In the present study we examined 15 unmedicated patients with schizophrenia (age: 31.2+/-11.1, m/f: 11/4, global scores SAPS: 48.33+/-33.09, SANS: 19.22+/-26.16), 29 subjects who were putatively in a prodromal state of psychosis, 30 first-degree relatives, another 8 first-degree relatives who had one child and at least one more relative with schizophrenia, and 50 healthy controls. We found an impairment of IOR only in the unmedicated patient group. In conclusion, blunted IOR in schizophrenia is not secondary to medications. According to this and previous studies blunted IOR may be most probably viewed as a trait cognitive feature of the schizophrenic disorder.

[Inhibition of return in schizophrenia: a review]

INTRODUCTION

Most visual environments contain more information than the human brain can process in real time. To overcome this limitation, the attention system acts as a filter by selectively orienting attention to specific regions of the visual field. This ability to orient attention can be reflected in covert shift processes of attention.

LITERATURE FINDINGS

In a typical covert orienting task, subjects have to maintain fixation on a central cross and respond as quickly as possible to a target, which appears in a peripheral box following a cue that summons attention to the direction where the target is going to appear (valid cueing) or to the contralateral direction (invalid cueing). When the cues are nonpredictive, the response characteristics critically depend on stimulus-onset asynchrony (SOA). With short SOAs (<300ms), valid cues result in a reaction time advantage over invalid trials, which is due to a reflexive shift of attention towards the source of stimulation. In contrast, with longer SOAs, valid cues result in longer reaction times to the subsequent target.

DISCUSSION

This phenomenon is known as the inhibition of return and is mostly thought to reflect an inhibitory mechanism protecting the organism from redirecting attention to previously scanned insignificant locations. Many studies have reported blunted or delayed inhibition of return in patients with schizophrenia. However, some authors reported normal amounts of inhibition of return. This can be partly explained by the use of manipulations of the covert orienting of the attention paradigm that is known to enhance the course of inhibition of return.

CONCLUSION

The deficit of inhibition of return seems to be time-stable and to be unrelated to psychopathology or length of illness. The contribution of neuroleptic medication to this deficit cannot be determined. Recent data suggest a deficit of inhibition of return in two human models of psychosis (dimethyltryptamine and ketamine). Further studies should clarify whether blunted inhibition of return might represent a trait marker of schizophrenia.

Schizophrenia patients show impaired response switching in saccade tasks

Action control deficits of schizophrenia patients result from frontostriatal brain abnormalities and presumably reflect an impairment of selective cognitive processes. This study aimed at dissociating two different levels of action control in saccades toward and away from visual stimuli (pro- and antisaccades). Results of previous studies suggested that task switch effects (between pro- and antisaccades) reflect the persistence of a task-specific production rule and refer to the level of task selection, whereas response switch effects (between leftward and rightward saccades) point to the persistence of a specific response program, referring to the level of response selection. In the present study, task switching and response switching were investigated in 20 schizophrenia patients and 20 control subjects. Groups did not differ concerning task switch effects. In contrast, response switching entailed a stronger enhancement of error rates in patients, suggesting a specific deficit on the level of response selection in schizophrenia. The deficit was associated with spatial working memory capacities, confirming and specifying existing hypotheses on a relationship between working memory and action control.

Neuroleptics reverse attentional effects in schizophrenia patients

Patients with schizophrenia show attention deficit characterized by a larger validity effect (fast responses to cued than uncued stimuli) in the right visual field than in the left visual field. In addition, schizophrenia patients do not show inhibition of return (IOR--a mechanism that enables efficient visual search), unless attention is summoned back to the center after the peripheral cue. The present study examined the short-term effect of neuroleptic medications on these two components of visual spatial attention in schizophrenia patients. In order to do this we tested schizophrenia patients that were treated with long-acting neuroleptic medication. These patients were treated once a month, which allowed us to test them with either low or high levels of medication. Here we show that neuroleptic medications reverse the attentional hemispheric asymmetry. In the group with a high level of medication fast RTs to cued trials were found in the right visual field, while in the group with a low level of medication the opposite pattern was found - fast RTs to cued trials were found in the left visual field. In addition, level of medication did not influence IOR - regardless of the level of medication, IOR was observed only when attention was summoned back to the center, unlike control group. These finding suggest an imbalance in dopaminergic activity, possibly in subcortical structures such as the basal ganglia. This study also shows a dissociation between the two components of visual orienting of attention and suggests that facilitation and inhibition are independent processes.

The inter-trial effects of stimulus and saccadic direction on prosaccades and antisaccades, in controls and schizophrenia patients

We investigated the influence of the direction of preceding saccadic trials on the latency of current prosaccades and antisaccades, in healthy subjects and patients with schizophrenia. When prosaccades and antisaccades were performed in separate, single-task blocks, we found that only prosaccades were delayed if the saccade in the prior trial was in the same direction, consistent with the expected directional effect from an 'inhibition of return'-like alternation advantage. However, both types of saccades were executed more quickly when the saccade in the penultimate trial was in the same direction, consistent with previous demonstrations of directional plasticity in monkeys. In blocks of randomly mixed prosaccades and antisaccades, the directional effects in healthy subjects were greatest when a prosaccade was preceded by an antisaccade, consistent with a summation of effects of alternation advantage (from the prior stimulus) and directional plasticity (from the prior saccade). Schizophrenic patients showed an additional phenomenon, a directionally specific inhibition of upcoming saccades by preceding antisaccades. These results suggest that saccades in humans are modulated by inter-trial effects attributable to both an 'inhibition of return'-like alternation advantage and directional plasticity.

Inhibition of return in the human 5HT2A agonist and NMDA antagonist model of psychosis

Patients with schizophrenia exhibit disturbances of orienting of attention. However, findings have been inconsistent. Pharmacologic challenges with hallucinogens have been used as models for psychosis. The NMDA antagonist state (PCP, ketamine) resembles undifferentiated psychoses with positive and negative symptoms, while the 5-HT(2A) agonist state (LSD, dimethyltryptamine (DMT)) is thought to be an appropriate model for psychoses with prominent positive symptoms. The aim of this study was to investigate orienting of attention in the human NMDA antagonist and 5-HT(2A) agonist models of psychosis. A total of 15 healthy volunteers participated in a randomized, double-blind, crossover study with a low and a high dose of DMT and S-ketamine, which elicited subtle 'prepsychotic' or full-blown psychotic symptoms (low and high dose, respectively). Nine subjects completed both experimental days with the two doses of both drugs. Overall, both hallucinogens slowed down reaction times dose dependently (DMT >S-ketamine) and DMT diminished the general response facilitating (alerting) effect of spatially neutral cues. Inhibition of Return (IOR), that is, the normal reaction time disadvantage for validly cued trials with exogenous cues and long cue target intervals, was blunted after both doses of DMT and the low dose of S-ketamine. IOR reflects an automatic, inhibitory mechanism of attention, which is thought to protect the organism from redundant, distracting sensory information. In conclusion, our data suggest a deficit of IOR in both hallucinogen models of psychosis, with the effect being clearer in the serotonin model. Blunted IOR may underlie or predispose to different psychotic manifestations, but particularly to those with prominent positive symptoms.

Deficient inhibition of return in schizophrenia-further evidence from an independent sample

Previous studies on spatial orienting of attention in schizophrenia demonstrated a deficit of Inhibition of Return (IOR). However, other studies reported a delay in the manifestation, but an overall normal amount of IOR in patients with schizophrenia. However, the latter studies used a cue-back manipulation which is known to reinstate or speed up IOR. Hence, it is not clear whether even very long cue target intervals would allow IOR to develop in patients with schizophrenia in the absence of a cue-back manipulation. The aim of the present study was to study IOR in patients with schizophrenia using a single cue paradigm and a very long cue target interval of >1 s in order to differentiate between blunted and delayed IOR. We examined 32 inpatients with schizophrenia and 16 healthy controls with a covert orienting of attention task (COVAT) with non-predictive peripheral cues and three stimulus onset asynchronies (SOA: 100 ms, 800 ms and 1050 ms). We found a lack of Inhibition of Return (IOR) in patients with schizophrenia with both long SOAs of 800 and 1050 ms. As in a previous study of our group, the IOR deficit was unrelated to psychopathology, length of illness, number of previous psychotic episodes and type of neuroleptic medication. In summary, our study confirms and extends previous reports of deficient IOR in patients with schizophrenia. IOR seems to be not just delayed, but rather profoundly disturbed in schizophrenia. Deficient IOR in patients with schizophrenia might be viewed as a trait or alternatively as a vulnerability marker of the disorder.