Dyslexia: the Role of Vision and Visual Attention

This review comes from Professor John Stein who is Emeritus Professor of Physiology at Oxford. Of course, he is very much at the respectable end of the spectrum when it comes to 'visual stress' and the role of visual factors in reading difficulties. You can download the review here. Fortunately the article is open access so you can read it all if you wish.
I respectfully differ with some of the conclusions of this review.
In the section Visual Contribution to Reading Professor Stein states that 'about 5% of all children and 50% of dyslexic children complain of problems when they read; letters appear to blur, move around and go double, so that children can not see properly which often gives them eyestrain and headaches. Obviously such symptoms interfere with learning to read' In short 5% of normal readers and 50% of dyslexic children have 'visual stress' This is a very important claim and one of the key arguments of the review. If visual stress is a factor in reading difficulty we would expect it to be over-represented among reading impaired individuals. You might therefore expect some serious academic underpinning to support this argument. The only reference cited to support this claim is a book by Professor Wilkins. No primary sources of evidence are cited.
It is worth thinking what kind of evidence you might require to make this claim. Ideally a cross-sectional study of  a population containing normal readers and dyslexic readers comparing the prevalence of VS in the the two groups. Alternatively a case control study in which a group normal readers is compared with a selected group of reading impaired individuals. It is easy for bias to creep into the latter type of study. There are a small number of such studies none of which are referenced in this review. The study at the lowest risk of bias was published by Kruk et al in 2008. I have reviewed it in some detail in my blog post of May 2015. The study found no difference in the prevalence of visual stress in poor readers and good readers - approximately 50% in each group. Another case control study of sorts by Kriss and Evans in 2005 is reviewed in my blog post of May 25 2015. The study was at high risk of bias because the screeners were not blinded to the reading status of the subjects and because the subjects were not drawn from the same population. Normal readers were recruited from a school setting whereas the dyslexics were recruited from dyslexia clubs where some had previously been exposed to the idea of visual stress and the use of coloured overlays. Even though they found visual stress to be more prevalent in their dyslexia group the 95% confidence intervals were so wide that their data could also be consistent with VS being more prevalent among normal readers.

If have plotted the odds ratios for all the studies I could find. A value above one is consistent with VS being more common in dyslexics and below one more common in controls. In all cases the 95% confidence interval for the odds ratio crosses one meaning that the data is compatible with VS being over-represented in dyslexics or normal readers. It is striking that the study at the lowest risk of bias by Kruk had an odds ratio of less than one. Although it might be tempting to combine the data in a meta-analysis it is incorrect to combine studies at high risk of bias and only one study was of sufficient quality to justify inclusion - that by Kruk.
So, in conclusion, a key plank of Professor Stein's argument is not supported by evidence.

Yellow filters
In the section of the review on yellow filters its is stated that 'in a subsequent double blind, randomised, placebo controlled trial we showed that was indeed the case ........ and this improvement (in M cell function) was accompanied by improved single word reading'
The paper that is cited is
Ray NJ, Fowler S. Stein JF. Yellow filters can improve magnocellular function: motion sensitivity convergence accommodation and reading. Ann N Y Acad Sci. 2005;1039: 283-73

Unfortunately this is a conference proceeding rather than a true peer reviewed publication and the reporting of the study is very sketchy indeed. It certainly was not double blind. The authors compared a yellow overlay with a card with slit cut out to conceal all but one line. Participants would have been well aware of which intervention they were receiving. The nature of the sequence allocation and allocation concealment is not reported. Because only change scores are reported it is not clear whether the yellow filter group and the placebo group where comparable at the onset of the study.
The mean change of  standard score in the yellow filter group was only one which is tiny and amounts to about one more word read. However some children improved by 10 standard scores and one by 15 which would be a miraculous improvement! It seems more likely that placebo effects were playing a role given that this was not a double masked study.
Blue filters
In the next section Professor Stein reports that 'we were using blue filters as active controls for the effects of yellow filters. But, to our surprise we found that some children actually benefited more from using blue filters than the yellow filters. If he is still referring to the study above he has unreported data from the same study and it seems likely that the blue filter (placebo) group and the yellow filter group improved but there was no significant difference. The most plausible reason for both groups improving is not that both filters were effective, it is that placebo effect is greater for coloured filters than cardboard cutouts.
More seriously however, it appears that the reporting of the original study was incomplete and the original blue placebo control group was excluded.

Other colours
In this section Professor Stein refers to a randomised controlled trial published in 1994 by Arnold Wilkins and colleagues which I have reviewed in a previous blog it was a negative study which was

hampered by a high rate of attrition so no conclusions can be justified. Professor Stein goes on to state that ' interestingly, the chromaticities of the chosen filters mostly clustered around yellow or blue, and there was no evidence that simple yellow or blue would not have been equally or more efficacious'. First, since this was a negative study this is not saying much. Second, looking at the chromaticity diagram it doesn't look to me as though the 'effective chromaticities cluster around blue and yellow'. Two subjects chose blue and one yellow.  Indeed other studies seem to show a clustering around mint green for Intuitive Overlays and aqua for Irlen overlays. I will be returning to this issue in future post.

Conclusions
This was broad ranging review and I have only looked at one of the areas covered by Professor Stein. However, I do not think this review makes a convincing case for the use of colour to treat reading difficulties.