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Visual Aspects of Dyslexia$
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John Stein and Zoï Kapoula

Print publication date: 2012

Print ISBN-13: 9780199589814

Published to Oxford Scholarship Online: September 2012

DOI: 10.1093/acprof:oso/9780199589814.001.0001

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Origins of Visual Stress

Origins of Visual Stress

(p.63) Chapter 4 Origins of Visual Stress
Visual Aspects of Dyslexia

Arnold Wilkins

Oxford University Press

The chapter begins with a description of the characteristics that make visual stimuli uncomfortable to look at. It is shown that certain spatial and temporal periodicity, and strong colour contrast are responsible for discomfort. These particular characteristics are rare in the natural images to which the human visual system has adapted and, perhaps for this reason, natural images are generally more comfortable than those that are unnatural. Text has the characteristics of an uncomfortable image, and there are simple steps that can be taken to reduce the discomfort. Uncomfortable (and unnatural) visual stimulation gives rise to strong physiological excitation in the visual cortex of the brain and some people are far more susceptible to such strong excitation than others. Individuals with migraine are among those who are most susceptible. The excitation induced by strong visual stimulation can be reduced by coloured filters individually chosen to reduce discomfort, and other coloured filters are without effect. The benefits of coloured filters on reading speed, on accommodation and on other higher perceptual function can be understood as due to a reduction of over-excitation, with implications for treatment in a variety of medical conditions in which the cortex is hyperexcitable, and with implications also for the idiosyncratic manner in which colour is coded within the higher levels of the visual system. There are large individual differences in the colours optimal for therapy.

Keywords:   stimuli, spatial, discomfort, migraine, colour

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