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Deficit of Visual Contour Integration in Dyslexia

From the Department of Visual Rehabilitation, Institute of Ophthalmology, University College London, United Kingdom.

PURPOSE. The visual processing of text occurs spontaneously in most readers. Dyslexic persons, however, often report both somatic symptoms and perceptual distortions when trying to read. It is possible that the perceptual distortions experienced by those with dyslexia reflect a disturbance in the basic mechanisms supporting perceptual organization at the early stages of visual processing. Integration of information over extended areas of visual space can be measured psychophysically in a task that requires the detection of a path defined by aligned, spatially narrow-band elements on a dense field of otherwise similar elements that are randomly oriented and positioned. In the present study a contour integration task was used to investigate such perceptual organization in dyslexia.

METHODS. The detection of contours or paths composed of Gabor micropatterns was performed within a field of randomly oriented distracter elements in a 2-alternate forced choice (AFC) task. The stimuli were manipulated by randomly varying both the density of the background noise elements and the number of elements that defined a path of constant length.

RESULTS. In all observers, sensitivity to the paths increased with the number of target elements comprising the path, and subjects in both groups exhibited similar trends in relative density of the stimuli. However, in all conditions, dyslexic observers were two to three times less sensitive to path stimuli than the control group.

CONCLUSIONS. In the present study the authors have described a visual deficit in a global integration task in dyslexia. The pattern of deficits reported suggest that abnormal cooperative associations may be present in dyslexia that are indicative of poor perceptual integration.

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