Cognition and control in schizophrenia: a computational model of dopamine and prefrontal function

Behavioral deficits suffered by patients with schizophrenia in a wide array of cognitive domains can be conceptualized as failures of cognitive control, due to an impaired ability to internally represent, maintain, and update context information. A theory is described that postulates a single neurobiological mechanism for these disturbances, involving dysfunctional interactions between the dopamine neurotransmitter system and the prefrontal cortex. Specifically, it is hypothesized that in schizophrenia, there is increased noise in the activity of the dopamine system, leading to abnormal "gating" of information into prefrontal cortex. The theory is implemented as an explicit connectionist computational model that incorporates the roles of both dopamine and prefrontal cortex in cognitive control. A simulation is presented of behavioral performance in a version of the Continuous Performance Test specifically adapted to measure critical aspects of cognitive control function. Schizophrenia patients exhibit clear behavioral deficits on this task that reflect impairments in both the maintenance and updating of context information. The simulation results suggest that the model can successfully account for these impairments in terms of abnormal dopamine activity. This theory provides a potential point of contact between research on the neurobiological and psychological aspects of schizophrenia, by illustrating how a particular physiological disturbance might lead to precise and quantifiable consequences for behavior.

Schizophrenic deficits in the processing of context. A test of a theoretical model

BACKGROUND

Schizophrenic patients show various deficits in cognitive functions that have been difficult to understand in terms of a common unifying hypothesis. Previously described neural network models of cognitive tasks suggest that several schizophrenic performance deficits may be related to a single function-an impairment in maintaining contextual information over time and in using that information to inhibit inappropriate responses.

METHODS

We tested first-episode schizophrenic patients and patients later in the course of their illness on a new variant of the Continuous Performance Test designed specifically to elicit deficits in the processing of contextual information.

RESULTS

Unmedicated schizophrenic patients showed a deterioration of their signal detection performance that followed the pattern predicted by the context hypothesis, ie, they responded inappropriately when correct responding required the maintenance of context information over time to inhibit the expression of a habitual response. This deficit correlated with positive symptoms. The results also suggested that the deficit may be worse in unmedicated patients who have had a longer course of illness. Medicated patients showed a more diffuse performance deficit.

CONCLUSIONS

These results support the view that a single deficit in the processing of context information may underlie various cognitive impairments observed in schizophrenia. They also suggest that such an impairment is associated with positive rather than negative symptoms, and that it may worsen with the course of the illness as in the kraepelinian view of schizophrenia.

Interference and facilitation effects during selective attention: an H215O PET study of Stroop task performance

To investigate the functional anatomy of interference and facilitation during selective attention, we studied 15 normal subjects using the H215O positron emission tomography technique and a computer presented single-trial Stroop task for cognitive activation. Increases in regional cerebral blood flow (rCBF) were observed in a network of structures that have been previously associated with selective attention, including the anterior cingulate gyrus, the frontal polar cortex, the inferior parietal lobule, and the thalamus, as well as the lingual gyrus. Furthermore rCBF decreases (compared to control states) were observed in lateral extra-striate cortex. rCBF changes in prefrontal and extra-striate regions varied with differences in the need to modulate the influence of word and color information while subjects responded to either incongruent or congruent Stroop stimuli. These results indicate the utility of Stroop procedures for investigating the functional anatomy of selective attention. Given recent interest regarding the role of the anterior cingulate gyrus in the pathophysiology of neuropsychiatric disorders, our results also suggest that the Stroop task can serve as a reliable neurobehavioral probe for this region. The significance of these results for understanding processing mechanisms underlying selective attention is discussed within the framework of a parallel distributed processing model of Stroop task performance.

Anterior cingulate gyrus dysfunction and selective attention deficits in schizophrenia: [15O]H2O PET study during single-trial Stroop task performance

OBJECTIVE

Attentional deficits are a prominent aspect of cognitive dysfunction in schizophrenia. The anterior cingulate gyrus is proposed to be an important component of frontal attentional control systems. Structural and functional abnormalities have been reported in this region in schizophrenia, but their relationship to attentional deficits is unknown. The authors investigated the function of the anterior cingulate gyrus and the related neural systems that are associated with selective attention in patients with schizophrenia.

METHOD

While subjects performed multiple blocks of a single-trial Stroop task, [15O]H2O positron emission tomography scans were obtained. Fourteen patients with schizophrenia were compared with 15 normal subjects matched for age, gender, and parental education.

RESULTS

The patients with schizophrenia responded at the same rate but made more errors in color naming during the color-incongruent condition. Consistent with the authors' hypothesis, patients with schizophrenia showed significantly less anterior cingulate gyrus activation while naming the color of color-incongruent stimuli.

CONCLUSIONS

Patients with schizophrenia fail to activate the anterior cingulate gyrus during selective attention performance. This finding adds to the understanding of the functional significance of the structural and metabolic abnormalities in schizophrenia that have been previously reported in this region of the brain.

Working memory for letters, shapes, and locations: fMRI evidence against stimulus-based regional organization in human prefrontal cortex

Investigations of working memory (WM) systems in the frontal cortex have revealed two stimulus dimensions along which frontal cortical representations may be functionally organized. One hypothesized dimension dissociates verbal from nonverbal WM processes, dividing left from right frontal regions. The second hypothesized dimension dissociates spatial from nonspatial WM, dividing dorsal from ventral frontal regions. Here we used functional magnetic resonance imaging to probe WM processes associated with three different types of stimuli: letters (verbal and nonspatial), abstract shapes (nonverbal and nonspatial), and locations (nonverbal and spatial). In a series of three experiments using the "n-back" WM paradigm, direct statistical comparisons were made between activation patterns in each pairwise combination of the three stimulus types. Across the experiments, no regions that demonstrated responses to WM manipulations were discovered to be unique to any of the three stimulus types. Therefore, no evidence was found to support either a left/right verbal/nonverbal dissociation or a dorsal/ventral spatial/nonspatial dissociation. While this could reflect a limitation of the present behavioral and imaging techniques, other factors that could account for the data are considered, including subjects' strategy selection, encoding of information into WM, and the nature of representational schemes in prefrontal cortex.

Functional hypofrontality and working memory dysfunction in schizophrenia

OBJECTIVE

Hypofrontality is a common but not invariable finding in schizophrenia. Inconsistencies in the literature may reflect, in part, the fact that abnormal physiological responses in the prefrontal cortex are best identified under conditions that place well-specified functional demands on this region.

METHOD

The authors studied eight patients with schizophrenia and eight matched comparison subjects using [(15)O]H2O positron emission tomography and the "N-back" task, which activates the prefrontal cortex as a function of working memory load in normal subjects.

RESULTS

Under low-working-memory-load conditions, the accuracy of both groups in the N-back task was equal, but when the memory load increased, the patients' performance deteriorated more than did that of the comparison subjects. The regional cerebral blood flow response to increased working memory load was significantly reduced in the patients' right dorsolateral prefrontal cortex.

CONCLUSIONS

These results confirm the importance of using tasks that tap specific cognitive functions, linked to specific neural systems, in studies of brain-behavior relationships in schizophrenia. Hypofrontality is reliably demonstrated in schizophrenia during tasks that engage working memory functions of the prefrontal cortex.

A computational approach to prefrontal cortex, cognitive control and schizophrenia: recent developments and current challenges

In this chapter we consider the mechanisms involved in cognitive control-from both a computational and a neurobiological perspective- and how these might be impaired in schizophrenia. By 'control', we mean the ability of the cognitive system to flexibly adapt its behaviour to the demands of particular tasks, favouring the processing of task-relevant information over other sources of competing information, and mediating task-relevant behaviour over habitual, or otherwise prepotent responses. There is a large body of evidence to suggest that the prefrontal cortex (PFC) plays a critical role in cognitive control. In previous work, we have used a computational framework to understand and develop explicit models of this function of PFC, and its impairment in schizophrenia. This work has lead to the hypothesis that PFC houses a mechanism for representing and maintaining context information. We have demonstrated that this mechanism can account for the behavioural inhibition and active memory functions commonly ascribed to PFC, and for human performance in simple attention, language and memory tasks that draw upon these functions for cognitive control. Furthermore, we have used our models to simulate detailed patterns of cognitive deficit observed in schizophrenia, an illness associated with marked disturbances in cognitive control, and well established deficits of PFC. Here, we review results of recent empirical studies that test predictions made by our models regarding schizophrenic performance in tasks designed specifically to probe the processing of context. These results showed selective schizophrenic deficits in tasks conditions that placed the greatest demands on memory and inhibition, both of which we have argued rely on the processing of context. Furthermore, we observed predicted patterns of deterioration in first episode vs multi-episode patients. We also discuss recent developments in our computational work, that have led to refinements of the models that allow us to simulate more detailed aspects of task performance, such as reaction time data and manipulations of task parameters such as interstimulus delay. These refined models make several provocative new predictions, including conditions in which schizophrenics and control subjects are expected to show similar reaction time performance, and we provide preliminary data in support of these predictions. These successes notwithstanding, our theory of PFC function and its impairment in schizophrenia is still in an early stage of development. We conclude by presenting some of the challenges to the theory in its current form, and new directions that we have begun to take to meet these challenges. In particular, we focus on refinements concerning the mechanisms underlying active maintenance of representations within PFC, and the characteristics of these representations that allow them to support the flexibility of cognitive control exhibited by normal human behaviour. Taken in toto, we believe that this work illustrates the value of a computational approach for understanding the mechanisms responsible for cognitive control, at both the neural and psychological levels, and the specific manner in which they break down in schizophrenia.

Brain regions responsive to novelty in the absence of awareness

Brain regions responsive to novelty, without awareness, were mapped in humans by positron emission tomography. Participants performed a simple reaction-time task in which all stimuli were equally likely but, unknown to them, followed a complex sequence. Measures of behavioral performance indicated that participants learned the sequences even though they were unaware of the existence of any order. Once the participants were trained, a subtle and unperceived change in the nature of the sequence resulted in increased blood flow in a network comprising the left premotor area, left anterior cingulate, and right ventral striatum. Blood flow decreases were observed in the right dorsolateral prefrontal and parietal areas. The time course of these changes suggests that the ventral striatum is responsive to novel information, and the right prefrontal area is associated with the maintenance of contextual information, and both processes can occur without awareness.

Activation of prefrontal cortex in children during a nonspatial working memory task with functional MRI

Functional magnetic resonance imaging (fMRI) was used to examine the pattern of activity of prefrontal cortex in prepubertal children during performance of a nonspatial working memory task. The children observed sequences of letters and responded whenever a letter repeated with exactly one nonidentical letter intervening. In a comparison task, subjects monitored similar sequences of letters for any occurrence of a single, prespecified target letter. Location of activation closely approximated that observed in a recent fMRI study with adults using exactly the same task. Activation of the inferior and middle frontal gyri was reliably observed within individual subjects during performance of the working memory task relative to the comparison task. Activation increased and decreased with a time course that was highly consistent with the task manipulations and correlated with behavioral performance. To our knowledge, this study is one of the first to demonstrate the applicability of fMRI to a normative developmental population. Issues of age dependence of the hemodynamic responses of fMRI are discussed.

Spiral K-space MR imaging of cortical activation

Brain function can be mapped with magnetic resonance (MR) imaging sensitized to regional changes in blood oxygenation due to cortical activation. Several MR imaging methods, including conventional imaging and echo-planar imaging, have been successfully used for this purpose. The authors investigated spiral k-space MR imaging, implemented with an unmodified 1.5-T clinical imager, for imaging of cortical activation. A gradient-echo, spiral k-space imaging method was used to measure activation in the primary visual cortex (number sequence task), primary motor cortex (fist-clenching task), and prefrontal cortex (verbal fluency task). Comparison of conventional and spiral k-space imaging in the visual and motor cortex, in which signal-to-noise ratio, voxel size, and imaging time were matched, showed that artifacts were reduced with the spiral k-space method, while the area and degree of activation were similar. The number of sections that could be imaged in a fixed time interval was increased by a factor of four with this implementation of spiral k-space imaging compared with conventional imaging.

Arabidopsis thaliana auxotrophs reveal a tryptophan-independent biosynthetic pathway for indole-3-acetic acid

We used tryptophan auxotrophs of the dicot Arabidopsis thaliana (wall cress) to determine whether tryptophan has the capacity to serve as a precursor to the auxin, indole-3-acetic acid (IAA). Quantitative gas chromatography-selected ion monitoring-mass spectrometry (GC-SIM-MS) revealed that the trp2-1 mutant, which is defective in the conversion of indole to tryptophan, accumulated amide- and ester-linked IAA at levels 38-fold and 19-fold, respectively, above those of the wild type. Tryptophan and free IAA were isolated from the trp2-1 mutant grown in the presence of [15N]anthranilate and [2H5]tryptophan, and the relative 15N and 2H5 enrichments of tryptophan and IAA were determined via GC-SIM-MS. The 15N enrichment of tryptophan, 13% +/- 4%, was less than the 15N enrichment of the IAA pool, 39% +/- 4%; therefore, IAA biosynthesis occurs via a tryptophan-independent pathway. The amount of 2H5 incorporated by the plant into IAA from tryptophan (9% +/- 4%) was low and only slightly above the level of spontaneous, nonenzymatic conversion of [2H5]tryptophan to [2H5]IAA. These results show that the dicot Arabidopsis is similar to the monocot Zea mays in that the major route of IAA biosynthesis does not occur through tryptophan.

Ischemic, hemodynamic, and neurohormonal responses to mental and exercise stress. Experience from the Psychophysiological Investigations of Myocardial Ischemia Study (PIMI)

BACKGROUND

The pathophysiology of mental stress-induced myocardial ischemia, which occurs at lower heart rates than during physical stress, is not well understood.

METHODS AND RESULTS

The Psychophysiological Investigations of Myocardial Ischemia Study (PIMI) evaluated the physiological and neuroendocrine functioning in unmedicated patients with stable coronary artery disease and exercise-induced ischemia. Hemodynamic and neurohormonal responses to bicycle exercise, public speaking, and the Stroop test were measured by radionuclide ventriculography, ECG, and blood pressure and catecholamine monitoring. With mental stress, there were increases in heart rate, systolic blood pressure, cardiac output, and systemic vascular resistance that were correlated with increases in plasma epinephrine. During exercise, systemic vascular resistance fell, and there was no relationship between the hemodynamic changes and epinephrine levels. The fall in ejection fraction was greater with mental stress than exercise. During mental stress, the changes in ejection fraction were inversely correlated with the changes in systemic vascular resistance. Evidence for myocardial ischemia was present in 92% of patients during bicycle exercise and in 58% of patients during mental stress. Greater increases in plasma epinephrine and norepinephrine occurred with ischemia during exercise, and greater increases in systemic vascular resistance occurred with ischemia during mental stress.

CONCLUSIONS

Mental stress-induced myocardial ischemia is associated with a significant increase in systemic vascular resistance and a relatively minor increase in heart rate and rate-pressure product compared with ischemia induced by exercise. These hemodynamic responses to mental stress can be mediated by the adrenal secretion of epinephrine. The pathophysiological mechanism involved are important in the understanding of the etiology of myocardial ischemia and perhaps in the selection of appropriate anti-ischemic therapy.

Indole-3-Acetic Acid Biosynthesis in the Mutant Maize orange pericarp, a Tryptophan Auxotroph

The maize mutant orange pericarp is a tryptophan auxotroph, which results from mutation of two unlinked loci of tryptophan synthase B. This mutant was used to test the hypothesis that tryptophan is the precursor to the plant hormone indole-3-acetic acid (IAA). Total IAA in aseptically grown mutant seedlings was 50 times greater than in normal seedlings. In mutant seedlings grown on media containing stable isotopelabeled precursors, IAA was more enriched than was tryptophan. No incorporation of label into IAA from tryptophan could be detected. These results establish that IAA can be produced de novo without tryptophan as an intermediate.

Functional topographic mapping of the cortical ribbon in human vision with conventional MRI scanners

The human brain has anatomically distinct areas in which processing is laid out in space at the millimetre level with substantial variation across individuals. Activity occurs along a cortical ribbon 1.5-3 mm thick in response to specific stimuli. Here we report the first use of cortical ribbon analysis on humans using non-invasive functional magnetic resonance imaging techniques performed with a conventional 1.5 T MRI scanner. Changes in activation were detected using T2*-weighted, gradient echo imaging sequences. Subjects observed partial field, flashing checkerboard patterns (left-right, top-bottom, half rings, and wedges). Stimuli produced magnetic resonance signal changes in the 1-8% range, varying at the millimetre scale, which showed contralateral vertically reflected patterns of activation in the visual cortex. To compare the spatial topographies across subjects, computer algorithms were used to control for the subject-unique folding of cortex, providing a flattened cortical ribbon identifying four topographically distinct areas.

Overt verbal responding during fMRI scanning: empirical investigations of problems and potential solutions

This paper presents a pair of studies designed to empirically explore the severity of potential artifacts associated with overt verbal responding during fMRI scanning and to examine several different solutions to these artifacts. In Study One, we compared susceptibility artifacts, signal-to-noise ratios, and activation patterns when overt versus covert verbal responses were elicited during fMRI scanning, using both individual and group analyses. The results indicated that different patterns of brain activation were elicited during covert as compared to overt verbal responses. This suggests that covert responses cannot be used as a simple substitute for overt verbal responses. Further, the results suggested that the use of overt verbal responses during fMRI scanning can produce interpretable results if: (1) the primary comparison is between two conditions that both use overt verbal responses, and (2) analyses are conducted on pooled group data rather than individual participant data. In Study Two, we evaluated the feasibility and validity of a method for acquiring participants' overt responses during fMRI scanning. The results indicated that our method was very accurate in acquiring the content of participant's responses. Further, inspection of the responses demonstrated that participants do not always comply with task instructions and highlighted the importance of obtaining behavioral performance measures during fMRI scanning.

Reproducibility of fMRI results across four institutions using a spatial working memory task

Four U.S. sites formed a consortium to conduct a multisite study of fMRI methods. The primary purpose of this consortium was to examine the reliability and reproducibility of fMRI results. FMRI data were collected on healthy adults during performance of a spatial working memory task at four different institutions. Two sets of data from each institution were made available. First, data from two subjects were made available from each site and were processed and analyzed as a pooled data set. Second, statistical maps from five to eight subjects per site were made available. These images were aligned in stereotactic space and common regions of activation were examined to address the reproducibility of fMRI results when both image acquisition and analysis vary as a function of site. Our grouped and individual data analyses showed reliable patterns of activation in dorsolateral prefrontal cortex and posterior parietal cortex during performance of the working memory task across all four sites. This multisite study, the first of its kind using fMRI data, demonstrates highly consistent findings across sites.

Stimulus generalization of fear responses: effects of auditory cortex lesions in a computational model and in rats

The conditioning of fear responses to a simple acoustic stimulus (pure tone) paired with footshock can be mediated by the transmission of auditory information to the lateral nucleus of the amygdala from either the auditory thalamus or the auditory cortex. We examined the processing capacity of the thalamo-amygdala pathway by making lesions of the auditory cortex and testing the extent to which conditioned fear responses generalized to tones other than the one paired with footshock. Two studies were performed, one in an anatomically constrained computational model of the fear conditioning network and the other in rats. Stimulus generalization was unaffected in both. These findings support the validity of the model as an approach to studying the neural basis of conditioned fear learning, and in addition suggest that the thalamo-amygdala pathway, possibly by the use of population coding, is capable of performing at least crude stimulus discriminations.

Semantic priming in schizophrenia: an examination of spreading activation using word pronunciation and multiple SOAs

Semantic priming in word pronunciation was examined at 5 stimulus onset asynchronies (SOAs) in 75 medicated and 25 unmedicated people with schizophrenia (SCZ) and in 10 depressed and 28 normal controls. At SOAs < 950 ms, SCZ displayed priming similar to that of normal and depressed controls. At the 950-ms SOA, SCZ displayed less priming than controls. Medication dosage, but not conceptual disorganization scores, was positively associated with priming at SOAs < 950 ms. These results suggest that prior reports of enhanced priming in schizophrenia may have been confounded by methodological problems and that automatic priming processes operate normally in SCZ. The failure of SCZ to display significant priming at the 950-ms SOA is consistent with a hypothesized disturbance in higher level processes.

Outcome of urgent percutaneous transluminal coronary angioplasty in acute myocardial infarction: comparison of single-vessel versus multivessel coronary artery disease

Despite recent clinical trials of percutaneous transluminal coronary angioplasty (PTCA) in acute myocardial infarction, specific groups of patients that may benefit from adjunctive or alternative therapy have yet to be adequately characterized. The in-hospital outcome of 151 consecutive patients treated for acute myocardial infarction with urgent PTCA of the infarct-related artery was studied to identify a subgroup of patients at high risk. Patients were divided into two groups based on the angiographic presence of either single-vessel (n = 86) or multivessel (n = 65) coronary artery disease. Despite PTCA of only the infarct-related artery and similar baseline clinical characteristics such as age, peak serum creatine kinase concentration, left ventricular ejection fraction, and time from the onset of chest pain to arrival at the hospital, the group with multivessel disease had a lower rate of successful angioplasty (75% vs 92%, p < 0.005), with higher incidences of persistent total occlusion of the infarct-related artery (14% vs 3%, p < 0.02) and procedural complications during PTCA (28% vs 13%, p < or = 0.02), and were more likely to have multiple complications (12% vs 1%, p < 0.004). In addition, the group with multivessel disease had a higher rate of urgent (< or = 24 hours) coronary artery bypass graft surgery (13% vs 2%, p < 0.05) and a trend toward a higher in-hospital mortality rate (6% vs 1%, p < or = 0.17). By stepwise logistic regression, only the presence of single-vessel versus multivessel disease was predictive of PTCA success (p < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

A theory of dopamine function and its role in cognitive deficits in schizophrenia

In spite of extensive research on the biological and behavioral correlates of dopamine (DA) function, little is known about the mechanisms by which DA may produce the cognitive deficits observed in schizophrenia. Neural network simulation models offer a framework for exploring how DA effects at the physiological level may influence behavior. We focus on findings suggesting that DA modulates neuronal activity by enhancing the ability of neurons to transmit signals and reduce distortion by noise. These phenomena can be captured in simulations by increasing the gain of individual units of a network. When gain is increased diffusely, improvement in signal detection performance of such a network parallels the improvement in performance of human subjects receiving DA agonists. Furthermore, decreasing gain in a network module supporting a memory function analogous to that of prefrontal cortex results in deterioration of performance equivalent to that of schizophrenic subjects. A test of predictions made by such networks about the performance of schizophrenic patients in a new variant of the Continuous Performance Test is discussed. Preliminary data are consistent with predictions and support the account of schizophrenic cognitive deficits in terms of the theory of DA function suggested by the models.

Left ventricular, peripheral vascular, and neurohumoral responses to mental stress in normal middle-aged men and women. Reference Group for the Psychophysiological Investigations of Myocardial Ischemia (PIMI) Study

BACKGROUND

The normal cardiovascular response to mental stress in middle-aged and older people has not been well characterized.

METHODS AND RESULTS

We studied 29 individuals 45 to 73 years old (15 women, 14 men) who had no coronary risk factors, no history of coronary artery disease, and a negative exercise test. Left ventricular (LV) volumes and global and regional function were assessed by radionuclide ventriculography at rest and during two 5-minute standardized mental stress tasks (simulated public speaking and the Stroop Color-Word Test), administered in random order. A substantial sympathetic response occurred with both mental stress tests, characterized by increases in blood pressure, heart rate, rate-pressure product, cardiac index, and stroke work index and rises in plasma levels of epinephrine and norepinephrine but not beta-endorphin or cortisol. Despite this sympathetic response, LV volume increased and ejection fraction (EF) decreased secondary to an increase in afterload. The change in EF during mental stress-varied among individuals but was associated positively with changes in LV contractility and negatively with baseline EF and changes in afterload. EF decreased > 5% during mental stress in 12 individuals and > 8% in 5; 3 developed regional wall motion abnormalities.

CONCLUSIONS

Mental stress in the laboratory results in a substantial sympathetic response in normal middle-aged and older men and women, but EF commonly falls because of a concomitant rise in afterload. These results provide essential age- and sex-matched reference data for studies of mental stress-induced ischemia in patients with coronary artery disease.

The "benefits" of distractibility: mechanisms underlying increased Stroop effects in schizophrenia

Recent studies of selective attention in schizophrenia patients suggest a particular pattern of single-trial Stroop performance: increased facilitation but not interference in reaction times (RTs), combined with increased error interference. Our Stroop task analysis suggests that this pattern can be explained by a selective attention deficit if one accounts for (1) performance in the congruent condition; (2) the nature of the neutral stimulus; (3) the relationship between accuracy and RT; and (4) response set effects. To test these hypotheses, we examined Stroop performance in 40 DSM-IV schizophrenia patients and 20 healthy control subjects, using a range of neutral stimuli (color patches, noncolor words, color words not in the response set). The findings confirmed several of our predictions and the results were consistent with the hypothesis that abnormal Stroop performance in schizophrenia reflects a failure to adequately attend to the task-appropriate stimulus dimension (color). This inattention affects both the congruent and incongruent conditions and multiple points in the information processing pathway.