Coloured filters and reading: reasons for an open mind

Dear Editor-in-Chief, Griffiths et al. are well known for their sceptical views on the existence and treatment of visual stress (VS). It is good to see such a comprehensive review of this fascinating and controversial subject, but it is unfortunate and disappointing that their review seeks to obscure positive findings in the literature. The reviewers apply a high level of critical appraisal for the many studies with positive results and take a ‘light touch’ when dealing with the few with negative findings. I have concentrated my comments on the section of the review relating to Intuitive Overlays and Tints as this is the research with which I am most familiar. The review authors report improvements in reading continuous or discontinuous text with coloured filters in many studies but criticise effect sizes as being small. Since reading disability is primarily attributable to language skills and not visual problems, the effect of visual interventions on reading will on average be modest. This is why I have always argued that filters are not a treatment for dyslexia, but rather can alleviate co-occurring symptoms of VS in a proportion of people with dyslexia. The Griffiths et al. review conceals from readers the key summary statistic of results of the studies it reviews. This is the case in both the main table 1 reproduced in the manuscript and in the supplementary table 2. Just considering the 22 studies reported relating to Intuitive Overlays and Lenses in Griffiths et al.’s table 2, 18 (82%) found statistically significant improvements in reading continuous or discontinuous text with coloured filters. Of the four studies that did not find such an improvement, two found a significant reduction in symptoms and a third found a significant improvement with one reading test (WRRT) but not another (Neale). Whilst I accept that an over-reliance on p-values is inappropriate, to completely ignore the key findings obtained by the original researchers is to conceal an important feature of the literature. The review authors confuse steps in the diagnostic process of VS with diagnostic criteria for VS. For example, they list ‘subjective reports of perceptual distortions while reading’ and ‘immediate subjective benefit from overlays’ as diagnostic criteria. These are not recommended in isolation as diagnostic criteria because they will over-diagnose VS. The paper that the authors’ cite in support of the latter diagnostic criterion clearly makes this point, criticising the ‘immediate benefit’ criterion because it is likely to lead to a high proportion of ‘just placebo’ cases. The reviewers argue that studies comparing chosen overlay/lens with a placebo colour show a difference between optimal colour and no colour but no difference between optimal colour and control colour. The reviewers cite two papers when making this point. These two studies both used a reading test with large font and widely spaced lines that is now known to be insensitive for demonstrating the benefit from filters in VS. All practitioners who manage VS will be familiar with sufferers typically reporting problems with crowded text and finding widely-spaced text much easier to read. This has been demonstrated in a small research study which the reviewers cite and has been stated in other publications. Both studies that the reviewers cite are now quite dated and use selection criteria that are less rigorous than those in use nowadays. This means that both studies are likely to have over-diagnosed VS which will weaken the statistical power of the studies. This is especially true of the more recent study which used an Irlen diagnostic method that over-diagnoses VS and which was acknowledged by the study authors as being prone to placebo effects. These authors highlighted that ‘individual participants did show dramatic improvements’ which may indicate that ‘there are some people who will benefit more from these lenses, so research is needed to identify such candidates’. The review authors ignore this point. Similarly, in the study by Wilkins et al. (1994) there were seven participants who individually had significantly fewer symptoms with an optimal coloured pair of glasses than with a control pair of similar colour. There were none who had significantly fewer symptoms with the control colour. Griffiths et al. accept that in this study ‘effective masking was maintained’, but conceal this important finding whilst instead misrepresenting a successful check that the placebocontrol was maintained as a negative finding. In the review discussion the authors highlight that some diagnostic procedures would appear to produce a large number of false positives. Yet, the reviewers do not consider that over-diagnosis of VS will increase the risk of negative results in studies with small sample sizes. I agree that improvements in the diagnosis of VS are required and a paper on this is currently in press. Yet the literature already contains guidelines on diagnosis which Griffiths et al. ignore. By including in a systematic review studies that clearly did not select people with the target condition the reviewers will bias their review towards a negative conclusion. Griffiths et al.’s claim of detecting a trend in the research whereby trials with better masking are more likely to reach negative conclusions is also questionable. For the four trials that they cite when making this statement, it could also be said that all used selection criteria that are now known to over-diagnose VS and used reading tests that are now