Illusory conjunctions and perceptual grouping in a visual search task in schizophrenia

Impaired contingent attentional capture predicts reduced working memory capacity in schizophrenia

Although impairments in working memory (WM) are well documented in schizophrenia, the specific factors that cause these deficits are poorly understood. In this study, we hypothesized that a heightened susceptibility to attentional capture at an early stage of visual processing would result in working memory encoding problems. 30 patients with schizophrenia and 28 demographically matched healthy participants were presented with a search array and asked to report the orientation of the target stimulus. In some of the trials, a flanker stimulus preceded the search array that either matched the color of the target (relevant-flanker capture) or appeared in a different color (irrelevant-flanker capture). Working memory capacity was determined in each individual using the visual change detection paradigm. Patients needed considerably more time to find the target in the no-flanker condition. After adjusting the individual exposure time, both groups showed equivalent capture costs in the irrelevant-flanker condition. However, in the relevant-flanker condition, capture costs were increased in patients compared to controls when the stimulus onset asynchrony between the flanker and the search array was high. Moreover, the increase in relevant capture costs correlated negatively with working memory capacity. This study demonstrates preserved stimulus-driven attentional capture but impaired contingent attentional capture associated with low working memory capacity in schizophrenia. These findings suggest a selective impairment of top-down attentional control in schizophrenia, which may impair working memory encoding.

Event-related potential study of illusory contour perception in schizophrenia

Schizophrenic patients and healthy controls participated in event-related potential experiments, in which illusory contour (IC) and control objects [no contour (NC), real contour (RC)] were passively presented. As a result, P100 latency for IC in schizophrenic patients was significantly prolonged (+10.6 ms) compared to those for RC. The present findings indicate that an abnormality of IC processing, including 'bottom-up' as well as 'top-down' processing, may reflect basal pathogenesis of various clinical representations of schizophrenia. However, the P100 latency difference between IC and RC was very small in the patient group. Rather, 'cognitive' in the Positive and Negative Syndrome Scale (PANSS) model of Bell et al. significantly correlated with P100 latencies for NC. Such an association between PANSS and NC processing, where the shape must be inferred with increased attentional demands and 'top-down' processing, indicates that the abnormality of schizophrenic patients' preattentive process might be a problem of 'top-down' processing rather than 'bottom-up' processing.

Failure of schizophrenia patients to overcome salient distractors during working memory encoding

BACKGROUND

Prior demonstrations of impaired attentional control in schizophrenia focused on conditions in which top-down control is needed to overcome prepotent response tendencies. Attentional control over stimulus processing has received little investigation. Here, we test whether attentional control is impaired during working memory encoding when salient distractors compete with less salient task-relevant stimuli.

METHODS

Patients with schizophrenia (n = 28) and healthy control subjects (n = 25) performed a visuospatial working memory paradigm in which half of the to-be-encoded stimuli flickered to increase their salience. After a 2-second delay, stimuli reappeared and participants had to decide whether or not a probed item had shifted location.

RESULTS

In the unbiased condition where flickering and nonflickering stimuli were equally likely to be probed, both groups displayed a trend toward better memory for the flickering items. In the flicker-bias condition in which the flickering stimuli were likely to be probed, both groups displayed a robust selection advantage for the flickering items. However, in the nonflicker-bias condition in which the nonflickering stimuli were likely to be probed, only healthy control subjects showed selection of the nonflickering items. Patients displayed a trend toward preferential memory for the flickering items, as in the unbiased condition.

CONCLUSIONS

Both groups were able to select salient over nonsalient stimuli, but patients with schizophrenia were unable to select nonsalient over salient stimuli, consistent with impairment in the effortful control of attention. These findings demonstrate the generality of top-down control failure in schizophrenia in the face of bottom-up competition from salient stimuli as with prepotent response tendencies.

Examining the effects of two factors on working memory maintenance of bound information in schizophrenia

Integrating information in space and time is a central feature of episodic memory. Although disturbance of the binding processes in episodic memory is well established in patients with schizophrenia, data on working memory (WM) remain discrepant. In a change detection procedure, two target displays of pairs of letters located in cells of grid were successively presented. Participants attempted to detect changes in binding information (i.e., recombination of studied features) or feature information (i.e., a novel letter and/or a novel spatial location). Recombinations consisted of features belonging to the same display (intradisplay) or different displays (interdisplays). Results showed that patients demonstrated overall lower performance, with no specific deficit for recognizing bound information or feature information. In addition, patients did not demonstrate deficits for interdisplay recombinations or intradisplay recombinations. Patients' ability to remember temporal occurrence of stimuli was not affected. Together, these results suggest that in patients with schizophrenia, binding processes in WM are not specifically disturbed.

Contextual suppression and protection in schizophrenic patients

INTRODUCTION

Contextual processing is often strongly deteriorated in schizophrenic patients as found, for example, in higher cognitive as well as lower visual paradigms. In visual detection tasks, impoverished contextual facilitation was attributed to aberrant excitatory neural circuits. On the other hand, we found contextual suppression, possibly related to neural inhibition, to be fast and intact in a visual backward masking task. Here, we combine a suppressive with a "protective" paradigm to further our understanding of the contextual deficiencies of schizophrenic patients in visual information processing.

METHODS

Twenty three schizophrenic patients and 18 healthy controls were asked to discriminate the offset direction of a vernier target, which was followed by one of a variety of masks for several stimulus onset asynchronies (SOAs).

RESULTS

As in previous studies, patients needed clearly longer SOAs than controls. However, when longer SOAs were taken into account, increases as well as decreases in backward mask strength had comparable effects in patients and controls.

CONCLUSIONS

From these results, we suggest that complex spatial processing is fast and intact in schizophrenic patients.

Assessment of single and bound features in a working memory task in schizophrenia

If disturbance of binding in long term memory is well established in schizophrenia, data concerning working memory maintenance are less clear. Feature binding in working memory was investigated in 19 patients with schizophrenia and 19 healthy controls. Binding was assessed by comparing two conditions in which participants had to retain four letters and four spatial locations. These features were presented either bound or separate. Results showed that both groups had better performances for bound than separate features, despite the fact that patients performed significantly worse than controls. When maintenance for isolated features was assessed, patients were severely disturbed for spatial locations but not for letters. Such a result suggests that reduced working memory performance in patients with schizophrenia for bound features is probably a consequence of a spatial deficit rather than a specific deficit of the binding process. Thus, not all form of binding are disturbed in schizophrenia.

Visuo-spatial processing in a dynamic and a static working memory paradigm in schizophrenia

Recent findings suggest that the visuo-spatial sketchpad (VSSP) may be divided into two sub-components processing dynamic or static visual information. This model may be useful to elucidate the confusion of data concerning the functioning of the VSSP in schizophrenia. The present study examined patients with schizophrenia and matched controls in a new working memory paradigm involving dynamic (the Ball Flight Task - BFT) or static (the Static Pattern Task - SPT) visual stimuli. In the BFT, the responses of the patients were apparently based on the retention of the last set of segments of the perceived trajectory, whereas control subjects relied on a more global strategy. We assume that the patients' performances are the result of a reduced capacity in chunking visual information since they relied mainly on the retention of the last set of segments. This assumption is confirmed by the poor performance of the patients in the static task (SPT), which requires a combination of stimulus components into object representations. We assume that the static/dynamic distinction may help us to understand the VSSP deficits in schizophrenia. This distinction also raises questions about the hypothesis that visuo-spatial working memory can simply be dissociated into visual and spatial sub-components.

Is filtering difficulty the basis of attentional deficits in schizophrenia?

The distractibility that schizophrenia patients display may be the result of a deficiency in filtering out irrelevant information. The aim of the current study was to assess whether patients with schizophrenia exhibit greater difficulty when task-irrelevant features change compared to healthy participants. Thirteen medicated outpatients with a diagnosis of schizophrenia and thirteen age- and parental education-matched controls performed a target selection task in which the task-relevant letter or the task-irrelevant features of color, and/or location repeated or switched. Participants were required to respond by pressing the appropriate key associated with the target letter. These patients with schizophrenia were slower when the task-relevant target letter switched than when it repeated. In contrast, schizophrenia patients performed similarly to controls when task-irrelevant information changed. Thus, we found no evidence that patients with schizophrenia were impaired in inhibiting irrelevant perceptual features. In contrast, changes in task-relevant features were problematic for patients relative to control participants. These results suggest that medicated outpatients who are mild to moderately symptomatic do not exhibit global impairments of feature processing. Instead, impairments are restricted to situations when task-relevant features vary. The current findings also suggest that when a course of action is not implied by an irrelevant feature, outpatients' behavior is not modulated by extraneous visual information any more than in healthy controls.

Clear distinction between preattentive and attentive process in schizophrenia by visual search performance

Visual information-processing deficits were investigated in patients with schizophrenia using visual search tasks. Subjects comprised 20 patients with schizophrenia and 20 normal subjects. Visual search tasks were modified from those used previously to reveal more distinct differences between feature and conjunction search tasks. The presentation area of items in the present study was more than double the area used in our previous study [Mori, S., Tanaka, G., Ayaka, Y., Michitsuji, S., Niwa, H., Uemura, M., Ohta, Y., 1996. Preattentive and focal attentional processes in schizophrenia: a visual search study. Schizophrenia Research 22, 69-76], and items were distributed over the area randomly in each trial to produce a certain range of locational jitter for each item across trials that prevented a matrix-like presentation of items at fixed positions [Mori, S., Tanaka, G., Ayaka, Y., Michitsuji, S., Niwa, H., Uemura, M., Ohta, Y., 1996. Preattentive and focal attentional processes in schizophrenia: a visual search study. Schizophrenia Research 22, 69-76]. The target was a red square, and distractors were red circles in the feature search task and red circles and green squares in the conjunction search task. Slopes and intercepts of a linear function relating reaction times to set size were computed. In the feature search task, slopes for both groups were almost zero. In the conjunction search task, significant differences in slopes were seen between the two groups irrespective of target presence or absence. Moreover, the slopes were approximately twice as steep during target absence as during target presence. These results indicate more definitively than the results of our previous study [Mori, S., Tanaka, G., Ayaka, Y., Michitsuji, S., Niwa, H., Uemura, M., Ohta, Y., 1996. Preattentive and focal attentional processes in schizophrenia: a visual search study. Schizophrenia Research 22, 69-76] that patients with schizophrenia have deficits in focal attentional processing, although their preattentive processing functions at a normal level.

Visual feature conjunction in patients with schizophrenia: an event-related brain potential study

The neural mechanisms supporting performance during single feature and feature conjunction detection were investigated in patients with schizophrenia and age-matched controls using event-related brain potentials. In different blocks of trials, participants responded to visual targets defined by one of two colors, one of two orientations, or both color and orientation. All participants were faster and more accurate in detecting targets defined by a single feature than for targets defined by a conjunction of features. Relative to controls, patients made more errors and were slower in detecting targets defined by a combination of features. Patients also generated a smaller N2 wave to single and conjunctive targets, and showed greater P3b reduction over the right hemisphere for conjunctive targets than for targets defined by color only. In addition, target stimuli generated an increased negativity at the occipital sites that varied in scalp distribution with the attended features in controls but not in patients. Both behavioral and electrophysiological data provide converging evidence for deficits in integrating visual features in schizophrenia and suggest that processing features of visual objects in schizophrenia are partly supported by distinct functional networks.

Reduced or increased influence of non-pertinent information in patients with schizophrenia?

We tested the ability of 18 patients with schizophrenia and 18 matched controls to filter non-pertinent information in orientation matching tasks. The non-pertinent information consisted of pictures conveying semantic information. The pertinent information consisted of oriented line-segments drawn inside pictures or was represented by the overall orientation of the picture itself. The results showed that non-pertinent information interfered with performance of control subjects in all tasks. In contrast, non-pertinent information interfered with the performance of patients with schizophrenia to the same extent as that of matched controls only when non-pertinent and pertinent information was physically mingled, but not when they were separated. Yet, patients processed non-pertinent information in all experiments, as shown by reversed interference effects when non-pertinent and pertinent information was physically separated. These results suggest a deficit at linking physically distinct visual information in patients with schizophrenia. Additionally, performance was more impaired in patients with schizophrenia than in control subjects when the amount of displayed information increased. The results suggest the coexistence, and a possible relationship between attentional deficits and impairments at linking distinct visual information in patients with schizophrenia.