Visual scan paths are abnormal in deluded schizophrenics
Introduction
Delusions are a core feature of psychosis and are particularly evident in paranoid psychoses such as schizophrenia. There have been several cognitive models proposed to explain delusion formation. One popular explanation is that they result from the rational interpretations of abnormal experiences [16]. Other studies have emphasized abnormalities in attention or information processing in subjects with persecutory delusions, who selectively attend to threatening events 14, 25, or have highlighted the role of abnormal reasoning 11, 13, with deluded subjects requiring less information before reaching a judgement compared with non-deluded subjects.
Monitoring directed attention in deluded patients in real time would help to identify processing strategies employed by such subjects when comprehending stimuli. Although this would not distinguish between different cognitive theories of delusion formation, or between cause and effect with respect to delusions and abnormal information processing pathways, such an `on-line' measurement would be able to detect `abnormal' strategies, i.e. strategies differing significantly from those employed by normal subjects. The measurement of visual scan paths is one method that has potential as a monitor of real-time visual information processing.
The visual scan path is essentially a map that traces the direction and extent of gaze when an individual comprehends a complex scene [20]. The meaurements include fixations, defined as consecutive gaze positions within 1° of visual field for a duration of 200 msec or more 12, 22, and voluntary saccades, i.e. voluntary eye movements in between fixations. Fixations as defined above represent `points of attention' by a subject on viewing the stimulus. The importance of both the external features of the visual stimulus and the presence of internal schemata as influences on the visual scan path pattern have been noted [23].
Section snippets
Clinical studies
A small number of studies have investigated visual scan paths in schizophrenic subjects [21]. Gaebel et al. [10]demonstrated a relationship between symptomatology and viewing pattern in schizophrenia, with the presence of positive symptoms associated with increased scanning (i.e. reduced fixation), and negative symptoms with increased staring (i.e. increased fixation duration). Another study [12]demonstrated that schizophrenics, on viewing a facial stimulus, attended less initially to facial
The cognitive neuropsychiatric approach
The approach taken in the current study [7]was to monitor visual attention in deluded patients rather than schizophrenic patients as a group. Delusions often emerge as misinterpretations of social interactions and events, frequently revolving around a person's relationship to others and their role in society rather than neutral or impersonal themes 4, 5. It is, therefore, important to utilize socially-relevant stimuli when investigating delusion formation. Human faces are ideal stimuli in that
Subjects
Patients with a diagnosis of schizophrenia (DSM-III-R criteria) [1]were recruited from the inpatient and outpatient populations of the Maudsley and Bethlem Royal Hospitals. Seven deluded schizophrenics [scoring 3 or more on the delusion section of the Scale for the Assessment of Positive Symptoms (SAPS)] [3]and seven non-deluded schizophrenics (scoring less than 3 on SAPS) were tested. The latter acted as a patient control group. In addition, 10 age-matched normal controls were recruited from
Results
There was no significant difference among the three subject groups on score for either space or object perception (VOSP). There was a trend for normal controls to have a higher mean IQ compared with the two patient groups [F(2,21)=3.08; P=0.07], but no significant difference in IQ between the two patient groups. There were significant differences between the two patient groups in SAPS delusion score (SAPSdel) (P<0.01) and total SAPS score minus the delusion score (SAPSoth) (P<0.05). There was,
Temporal analysis of scan paths
There was a trend for a greater mean duration of fixation for viewing single faces, and a significantly greater mean duration of fixation on viewing face pairs [F(2,21)=3.53; P=0.05] in deluded subjects compared with non-deluded and normal controls. There was also a non-significant trend for a greater mean number of fixations in deluded subjects compared with the other two groups on viewing both single faces and face pairs. There was no significant difference in total fixation duration among
Single faces
MANOVA revealed a highly-significant difference in the time spent fixating the three different face areas (feature, non-feature and outside), with all subjects spending more time viewing outside areas compared with the other two areas [F(2,21)=14.69; P<0.001]. There was a near-significant interaction between this and diagnosis. Post-hoc two-tailed t-tests demonstrated a significantly greater percentage fixation time for viewing facial features in normal controls compared with deluded subjects (P
Discussion
The aim of the current study was to investigate information processing in deluded schizophrenic patients. Visual information processing was monitored using the visual scan path, a physiological marker of visual attentive processes. The results can be summarized as follows:
- 1.
Deluded schizophrenic patients had longer and fewer fixations compared with non-deluded subjects and normal controls when viewing single faces and also when viewing face pairs in the face recognition test, i.e. deluded
Acknowledgements
MLP is supported by an MRC (U.K.) clinical training fellowship. We would like to thank Dr A. Reveley for her support of the study and Prof. E. Warrington for permission to employ stimuli from the Recognition Memory Test. We would also like to thank T. Ribchester, T. Hewitt and G. Costello for technical advice, and the Mason Medical Research Foundation for additional financial support.
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