Visual dysfunction in schizophrenia assessed with a novel continuous performance test
Legatt, Michael E.
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In patients with schizophrenia, well-documented deficits are found in some visual pathways as demonstrated with visual evoked potentials and psychophysical measures (i.e., threshold responses in a backward masking paradigm and reaction times on continuous performance tests (CPTs) of attentional mechanisms). Using the variable-contrast continuous performance test (VC-CPT), a novel task designed to present stimuli at varying contrast levels and shapes (squares as targets, circles as noise) in order to target particular visual pathways, this study sought to determine the role that low-order visual pathway function has on CPT response measures. It was determined that, due to several characteristics of the stimuli, the test largely targets the magnocellular pathway, and both the dorsal and ventral visual streams. Both ON and OFF divisions of the magnocellular pathway were examined. Fifteen inpatients with schizophrenia and seventeen controls of similar age received this seven-minute test.;Generally, patients with schizophrenia had higher variability of performance than did the controls. Controls' sensitivities to the stimuli, as measured by the nonparametric measure A', were highest at an ISI of 2 seconds, as compared with 1 and 4 s, and for high contrast stimuli. Controls' reaction times to targets were also significantly faster for high-contrast stimuli than for low-contrast stimuli, while patients performed about equally for the two levels. Using measures of sensitivity (A') and mean reaction time to targets in a logistic regression, individuals were classified with an accuracy (estimated area under the ROC curve) of 93.7%, for the difference between bright checks between a 4 s ISI and 1 s ISI.;The results of this behavioral task were interpreted to indicate that patients with schizophrenia have a deficit in the magnocellular pathway and/or reduced contrast gain, attributed to reduced NMDA-receptor activation in the cortex and lateral geniculate nucleus. The visual pathway deficits identified with the VC-CPT may involve both the dorsal and ventral visual streams. Thus, CPT tasks on individuals with schizophrenia may detect deficits in the early visual pathways or in neural circuitry in the primary visual cortex, rather than in higher-order pathways that mediate executive functions such as attention.
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