Pattern Glare, Visual Stress and Dyslexia

The above three terms are often confused or misunderstood. The purpose of this article is to unravel them,  and show how they are (or are not) connected.

Pattern Glare

The following definition of Pattern Glare is from an optometrist’s website. The emphasis in bold is mine.:

“Pattern glare is a condition in which patients are abnormally sensitive to light, and to certain patterns. Printed reading can also often cause problems for people who are sensitive to light due to the spatial effect of words and rows.

Common issues relating to pattern glare include:

  • Significant eyestrain, headaches, words moving when reading, increased light sensitivity, flickering lights or reflections

Pattern glare can occur to anyone, however has been found to be more common in patients who have experienced:

  • Brain injury or stroke, concussion, photosensitive epilepsy, migraine, experience of fine flickering dots in both eyes, blepharospasm (involuntary tight closure of the eyelids), autism, reading problems and dyslexia”

The main point to note here is that the visual distortions with text that are often attributed to dyslexia are actually symptoms of pattern glare, not specifically of dyslexia. Pattern glare is the generic description of the symptoms of a condition variously known as scotopic sensitivity, Meares-Irlen syndrome, or visual stress. The term we use is visual stress

Visual Stress

Visual stress is the condition that causes pattern glare. It is caused by an over-reaction of certain neurons in the visual cortex to specific wavelengths of light, resulting in the visual disturbances and discomfort generically described above as pattern glare. Individuals are affected differently, and the pattern glare can be triggered by any wavelength in the spectrum, although some are more frequent than others. Stripy patterns at certain frequencies are most likely to trigger pattern glare when visual stress is present, and printed (or digital) text, especially in fonts with many vertical lines as opposed to more rounded characters, often falls within those frequencies. Visual Stress is a condition of the visual cortex, not of the retina, so the term scotopic sensitivity is actually a misnoma.


The definition of dyslexia adopted by the IDA (International Dyslexia Association) board of directors is as follows (My bold highlighting):

“Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.”

Dyslexia is essentially a phonological processing deficit, and not a visual-perceptual problem. Neuro-imaging has shown distinct differences between the “wiring” of dyslexic brains and neurotypical brains. Although visual stress is quite often co-morbid with dyslexia (figures vary, but the figure seems to be between 30-40%) it is a distinct and separate condition. To treat visual stress by removing or reducing pattern glare does not treat dyslexia: what it achieves is to make dyslexia more manageable by improving accessibility to the text that needs to be decoded. As an 11-year old non-reading boy said when his pattern glare was dealt with: “Is that what you mean by a word? Can I start learning to read now?”

How do you deal with pattern glare?

Since pattern glare is the result of specific wavelengths of light over-stimulating the visual cortex, the over-stimulation is reduced or removed by filtering out the wavelengths that cause the problem. This is done quite simply by using a transparent overlay or reading ruler of the right colour for the individual concerned, or having tinted lenses prescribed, either by an optometrist with Colorimitry equipment, or by an Irlen Center. We at Crossbow Education have been specialists in visual stress intervention for nearly twenty years, working closely with Prof Arnold Wilkins (cited below) in the development of our color range. Since we developed the  Eye Level Reading Ruler in 2004 hundreds of thousands of children around the world have benefited from our design.

The last words go to the scientists: (For the full article, see link below) 

”The theory of visual stress based on the above findings has been used to explain the benefits of coloured tints on reading. Text resembles a striped pattern, partly because of the horizontal rows of words and partly because of the periodic vertical strokes of letters in words such as mum. Both sources of stripes impair reading (Wilkins et al., 2007). Some individuals report visual perceptual distortions of text and these are similar to the illusions of colour, shape and motion reported in patterns of stripes (Wilkins and Nimmo-Smith, 19841987). People who experience visual stress often benefit from coloured overlays that are placed over passages of text (Jeanes et al., 1997Wilkins et al., 2001Hollis and Allen, 2006). The colour that provides maximum benefit is specific and individual (Wilkins, 1994). The explanation for this is based on the premise that a hyperexcitability of the visual cortex adversely affects visual processing, and coloured filters change the distribution of neural activity so as to reduce the activity in hyperexcitable regions. This hypothesis is difficult to test directly, but is consistent with the neuroimaging evidence that colored filters reduce cortical hyperexcitability (Huang et al., 2011).”

 (Extract from Pattern glare: the effects of contrast and color, Monger, Wilkins and Allen 2015)


Other articles that may be of interest:

A comparison of the effects of the colour and size of coloured overlays on young children’s reading
(Judith Veszeli, Alex J. Shepherd, 2019):

Colors, Colored Overlays and Reading Skills ( A critical review)
A systematic review of controlled trials on visual stress using Intuitive Overlays or the Intuitive Colorimeter (Bruce J.W. Evansa, and Peter M. Allenb)

Colorimetry and Pattern Glare (from an optometrist’s perspective)