General research background
In the early 1980s, two professionals, Olive Meares in New Zealand and Helen Irlen in the USA independently observed that some people experience perceptual distortions when reading printed text. They also both discovered that this problem can generally be alleviated by using coloured overlays (sheets of transparent plastic that are placed upon the page). Later, Irlen summarised this work in her book Reading by the Colours (New York: Avery, 1991).
This condition is now usually called ‘visual stress’ but it has also been known by various other labels, including ‘Meares-Irlen syndrome’, ‘visual discomfort’ and ‘scotopic sensitivity syndrome’. The main symptoms include (i) sore, tired eyes; headaches; photophobia; (ii) visual perceptual distortions, including illusions of shape, motion and colour; and (iii) transient perceptual instability and double vision. As well as coloured overlays, coloured filters (i.e. spectacles with coloured lenses) are also now used as an effective treatment. A good general overview of the subject can be found in the following short review:
Singleton, C. & Henderson, L-M. Visual factors in reading. London Review of Education, 4(1), 2006, 89-98. This review puts visual stress in the context of various other visual factors that can influence reading. To download a copy of this review, click here for PDF
How and why does visual stress affect reading?
Childrens’ progress in reading depends not only on learning to decode text, but also to a considerable extent on practice at reading continuous text. In this way, the brain gradually learns to decode text with increasing fluency (i.e. speed plus accuracy) and to carry out the cognitive processes necessary for skilled reading largely automatically.
Because visual stress causes unpleasant visual symptoms, it disrupts the cognitive processes in reading, making it harder and harder for the person to read the text. As a consequence, people who suffer from visual stress often avoid reading as much as possible. When this happens during childhood the person typically fails to develop efficient reading skills and automaticity in reading, and is likely to remain a life-long poor reader.
This explains why visual stress is more likely to be seen in poor readers, both adult and child, than in good readers. However, visual stress is experienced by some adults who can read satisfactorily because they have been able to work out what strategies they need to minimise the unpleasant symptoms of the condition. These strategies include ensuring print size is large enough, the font clear and distinct, the lighting adequate and avoiding reading when they are tired. Many adults who suffer from this problem use coloured overlays or tinted glasses, and find that colour of text on the screen helps when working on a computer.
For further information see:
Singleton, C. Visual factors in reading. Educational & Child Psychology, 25(3), 2007, 8-20. To download a copy of this review, click here for PDF
How common is visual stress?
It is difficult to give precise figures of the incidence of visual stress because (a) it can be assessed in various ways, which can give slightly different results, and (b) there have not been any large scale epidemiological studies of this. A reasonable estimate is that between 10–20% of the population experience some degree of visual stress when reading. There is a very high incidence of visual stress in people who also suffer from migraine headaches, which can be triggered by visual factors. And there is also a fairly high incidence (30–50%) of visual stress in people who have dyslexia. For further information see:
Singleton, C. and Trotter, S. Visual stress in adults with and without dyslexia. Journal of Research in Reading, 28, 2005, 365-378. To download a copy of this paper, click here for PDF
Visual stress and dyslexia
A high proportion of dyslexics experience visual stress when reading. This is because the cognitive weaknesses in dyslexia (typically poor verbal memory and ability to process phonological information) means that dyslexics struggle to learn how to decode text and generally do not develop fluent or automatic word recognition. Their reading is stuck at the level of decoding individual words, which makes them more vulnerable to the visual characteristics of text (style and size of font, contrast, line spacing, etc.), which increases the likelihood of experiencing the unpleasant visual symptoms of visual stress. For an overview of this see:
Singleton, C. Visual stress and its relationship to dyslexia. In J. Stein and Z. Kapoula (Eds.) Visual Aspects of Dyslexia, Oxford: Oxford University Press, 2012, pp. 91-110. To download a copy of this chapter, click here for PDF
A useful book also covering this field up to 2001 is Dyslexia and Vision by Professor Bruce Evans (Whurr, 2001). It should be borne in mind that much has been discovered about visual stress since its publication.
Identifying visual stress
The traditional approach to identifying visual stress is either by using (a) a symptom questionnaire, or (b) a coloured overlay screening test. Symptom questionnaires asking questions such as ‘Does the print move around when you read?’ or ‘Do you eyes get tired when you read?’ may indicate susceptibility to visual stress in adults but they are entirely subjective and depend on the individual’s personal viewpoint. However, questioning children about suspected visual perceptual symptoms is recognised as potentially unreliable. Diagnosis by evaluation of the child’s responses to using an overlay was traditionally regarded as a safer option. If children express a preference for a coloured overlay when reading then this can be evaluated by measuring improvement in reading speed with the overlay and/or the voluntary sustained use of the overlay for at least half of a school term. Unfortunately, children may easily be influenced by other considerations, such as what colour they favour (or their best friend likes), which can give misleading results.
For further information see:
Wilkins, A. J., Lewis, E., Smith, F., Rowland, E. & Tweedie, W. (2001) Coloured overlays and their benefit for reading. Journal of Research in Reading, 24, 2001, 41–64. To download a copy of this paper, click here.
The only truly objective test for visual stress is Lucid’s product ViSS, a computer-based screening device that measures the effectiveness of a reading task when carried out under two different conditions: one condition being designed to create visual stress and the other not. Research studies have shown ViSS to be a highly efficient indicator of susceptibility to visual stress in children and adults. The efficacy of ViSS is supported by evidence published in several papers in international peer reviewed scientific journals. For research evidence on this see:
Singleton, C. & Henderson, L-M. Computerised screening for visual stress in reading. Journal of Research in Reading, 30, 2007, 316-331. To download a copy of this paper, click here for PDF
Singleton, C. & Henderson, L-M. Computerised screening for visual stress in children with dyslexia. Dyslexia: An International Journal of Research and Practice, 13, 2007, 130-151. To download a copy of this paper, click here for PDF