A high quality, adequately powered study from a respected research group with no financial ties to visual stress treatment shows no link between the putative disorder visual stress and dyslexia
Visual stress is not the same thing as dyslexia – we all know that.
Dyslexia simply means an impaired ability to read and the prevalence depends on where you draw the line; the bottom 5% or 8% say. It is wrong to think in terms of ‘garden variety’ poor readers and poor readers with dyslexia.
The best evidence is that dyslexia is a language-based disorder that is based on the ability to link certain sequences of letters to sounds. Without this ability, it is more difficult to decode text and link it to the spoken word. However, that does not preclude the possibility that visual deficits can play some role in dyslexia. That said, the best evidence to date, from population-based studies, is that visual factors do not play a large role(1).
You might expect visual stress to be overrepresented in the population with dyslexia. Take the example of someone with borderline phonetic skills who also has visual stress. The additional handicap of visual problems while reading would be more like to tip them into the bottom 5% of readers. Furthermore, dyslexics with visual stress would be more resistant to existing teaching methods and would less likely to leave the category of impaired readers as a result of intense remedial tuition.
The claim that VS is over-represented among impaired readers has been made by VS enthusiasts such as Bruce Evans who has argued that VS is found 2-3 times more frequently in the population with dyslexia compared to normal readers(2). This seriously flawed piece of research is reviewed in my blog post of May 2015. Other research (admittedly underpowered) using Irlen methodology found no link between VS and reading ability(3). Also reviewed May 2015.
New Research
Recently an adequately powered study has been published in the journal Developmental Psychology(4). You can access the paper here.
The study comes from an internationally recognised research group based in France and Switzerland who have no financial ties to any visual stress diagnostic or treatment products.
The authors looked at 164 participants with dyslexia and 118 controls with normal reading. The authors did not rely on a self-reported diagnosis of dyslexia. Instead, all participants were tested with a bank of tests to diagnose dyslexia and also a bank of tests to exclude other co-morbidities including uncorrected sight problems, neurological disease and deafness.
Both groups were investigated for 3 possible causes of dyslexia- a phonological deficit, reduced visual attention span and visual stress.
The diagnostic criterion for visual stress was a response to gratings of a spatial frequency of 3 cycles per degree (CPD) and a control grating outside the range said to be aversive in VS (0.5 CPD) to check for response bias. The authors criticise tests based on increased reading fluency with overlays and argue that it does not prove that participants had visual stress or that their reading deficit was caused by visual stress, to begin with. Such tests also exclude subjects with visual stress who do not respond to overlays. The authors go on to make an analogy with attention deficit hyperactivity disorder (ADHD) arguing that response to methylphenidate would be an irrational diagnostic test for that disorder.
The results of this study came as no surprise to me. Most dyslexic children showed phonological deficits (92.5%) – in terms of response accuracy, speed or both. There was a small difference in visual attention span that affected 28.1% of dyslexic children (all of whom also had phonological deficits). There was no association between visual stress and dyslexia - 5.5% of dyslexics and 8.5% of controls had visual stress. The sample size of this study was big enough to look at the subgroup of patients in whom a phonological deficit could not be measured - the group in whom you might be most likely to detect a visual deficit - again no increased prevalence of VS could be found.
Conclusions.
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| An odds ratio above 1 would favour VS being more common among the population with dyslexia: below 1 in normal readers. Bars represent 95% confidence intervals. |
This study is a major problem for the visual stress/dyslexia hypothesis and needs to be put alongside previously published studies. Taken individually or together they do not make a convincing case for VS being more common in reading impaired individuals compared to normal readers. It is striking that the two studies at the lowest risk of bias, in the statistical sense of the word, offer the least support for the VS/dyslexia hypothesis.
How will enthusiasts and those with a financial interest in VS respond to these findings I wonder?
Probably by ignoring it or misrepresenting the findings altogether. That said there are signs that the claims being made are being toned down a bit. For example, by only suggesting that ‘visual-stress may co-occur with dyslexia’ a vacuous and meaningless statement if ever there was one.
1. Creavin AL, Lingam R, Steer C, Williams C. Ophthalmic Abnormalities and Reading Impairment. PEDIATRICS. 2015 Jun 1;135(6):1057–65.
2. Kriss I, Evans BJW. The relationship between dyslexia and Meares-Irlen Syndrome. J Res Read. 2005 Aug;28(3):350–64.
3. Kruk R, Sumbler K, Willows D. Visual processing characteristics of children with Meares-Irlen syndrome: Visual processing in Meares-Irlen syndrome. Ophthalmic Physiol Opt. 2008 Jan 14;28(1):35–46.
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