High prevalence, high impact disorders like dyslexia are prone to sensational claims from quacks, scientists, journal editors and journalists. The latest is a sensational claim from an article in the high-profile journal Proceedings of the National Academy of Sciences (PNAS) that increasing letter spacing “ameliorated the reading performance of dyslexic children” (Zorzi et al., 2012).
The popular media has picked up these claims. For example the ABC quoted the lead author Marco Zorzi as saying: “Our findings offer a practical way to ameliorate dyslexics’ reading achievement without any training”. But are the claims fact or fiction?
The idea for the study seems to have been grounded in effects of crowding in dyslexia (see Martelli et al., 2009). Crowding occurs when stimuli from the periphery interfere with processing of stimuli in the focus of vision. I am not an expert in this aspect of the dyslexia literature and perhaps someone else may comment. However, my non-expert eye suggests two problems with this literature. First almost all studies (see here for an exception) have used word and/or letter stimuli which confounds reading ability and ‘crowding’ effects. Second, most studies have used age-matched controls rather than reading-age matched controls which leaves open the possibility that the effects on crowding tasks are the consequence of poor reading rather than the cause.
For the purposes of this post, let’s accept that crowding affects people with dyslexia more than good readers. Zorzi et al. (2012) predicted that widening the space between letters in words would decrease the effects of crowding and lead to better reading. They tested this idea in Italian and French people diagnosed with dyslexia (aged 8-14 years). The children had to read 24 short meaningful sentences taken from the Test for the Reception of Grammar. Print was Times New Roman 14-point. One reading condition had normal spacing between letters and the other had letter-spacing 2.5 points greater than normal (normal letter spacing is 2.7 pt in normal text; who knew?). Why they didn’t use single words and nonwords instead of the text-reading task is unclear given that dyslexia is widely acknowledged to be a deficit in single-word reading. People with dyslexia read better in context than they read words in lists (see here). Surely if crowding was the/a cause of dyslexia we would see it more in reading of word lists rather than stories and if increasing letter-spacing improved reading in dyslexia we would see larger effects in single word task?
Anyway….The results of one experiment showed that both the French and Italian groups with dyslexia made less reading errors in the condition in which letter-spacing was greater than normal. However, that on its own tells us nothing other than that doing something led to less errors. It doesn’t say that the specific manipulation (increased letter-spacing) was the key factor. It may be that chewing gum while reading does the same thing. Zorzi et al recognised this and suggested that if crowding really does affect reading and extra letter-spacing reduces crowding effects it is more important to show that people with dyslexia make less errors in the increased letter-spacing condition than reading-age matched controls. This they attempted to do in a second experiment.
The data from Experiment 2 (Zorzi et al) are shown in the figure below. Zorzi et al claimed that the increased letter-spacing condition improved the reading (i.e., they made fewer errors) of their French and Italian groups with dyslexia compared to the reading-age matched controls. These data are what the sensational claims reported in the media are based on. The problem is that their ‘control’ group were not of the same reading-age. Groups with the same reading ability should perform equally in the normal-spaced condition. The Figure below shows that this was not the case. The “reading-age matched controls” were significantly better readers in the first place.
What does this methodological flaw mean? Essentially it means that the claims Zorzi et al (or at least the media) made cannot be supported by the data. Using a group of younger children who were already better readers than the people with dyslexia is irrelevant to the research question. It leaves a data set that is subject to the same criticism as their first experiment. That is, it tells us nothing about the specific manipulation (increased letter-spacing) and it remains possible that any experimental manipulation, including the ridiculous like chewing gum, produces the same results.
Furthermore, it is possible, indeed likely in my view, that the reason the children in the “reading-age matched control” group did not improve as much in the increased-spacing condition is that they didn’t have much room to improve. They were already good readers and were at ceiling on the test. It is unlikely that any kind of experimental manipulation will make a good reader a better reader. Which leads me to my suggestion for replicating this study. Don’t replicate it!
I can’t see how using either age- or reading-age matched controls (i.e., good readers) will allow adequately testing of the hypothesis that increased letter-spacing results in improved reading ability in people with dyslexia because of the point I made above: it is unlikely that any kind of experimental manipulation will make a good reader a better reader. In my view, the next piece of research will need to use equivalent groups of people with dyslexia, one of which receives the extra-letetr spacing manipulation and who does not. It is also worth noting that recent research has shown that the effects of another visual manipulation (coloured overlays) on reading ability is not reliable on repeat testing (Henderson et al., 2012) so any future research should probably run the test multiple times for each condition. Finally, if the research is conducted in English, it would be interesting to see if increased letter-spacing changes error rates (for better or worse) for words that involve single grapheme-to-phoneme correspondence compared to those that have digraphs (e.g., chip and rain) or trigraphs (e.g., slight). It might also be interesting to see if increased letter-spacing reduces errors for words in which letter-positiuon errors can occur (e.g., trail-trial).
Until we see these data I’m keeping my ink dry.
The title of the original version of this post may have implied that vision therapy is an inappropriate treatment in general. Whereas, I intended only to refer to it as inappropriate for treating dyslexia/reading problems. There is evidence that vision therapy is an appropriate treatment for some vision problems such as convergence insufficiency. I regret that any adverse inference may have been drawn from the title.
Vision and reading
Many people have attributed reading problems to one or more subtle ocular or visual abnormalities, including Samuel Orton, who wrote about the difficulty he thought children with dyslexia had with reversible letters and words (eg. b/d, god, dog). However, scientific research has shown that Orton’s view and other views that reading problems are the result of issues with visual processing, visual perception or visual memory are almost certainly incorrect.
In the 1970′s Frank Vellutino and colleagues performed a series of studies in which they compared poor and good readers on a variety of visual processing tasks (e.g., visual discrimination, spatial orientation, visual memory, and visual learning). Most importantly, the tasks carefully controlled for verbal coding ability.
For example, Vellutino et al. found that memory for visually presented words and letters that were visually similar (e.g., b/d, was/saw) was the same in good and poor readers when a written rather than a verbal response was required. In other words, the kids with dyslexia see the same thing and can replicate the symbol but have more difficulty producing the letter or word name verbally. Another experiment showed that good and poor readers performed equally on visual recognition and recall of symbols from the Hebrew script with which both groups were equally unfamiliar. Finally, poor readers do make more ‘visual’ errors when reading compared to good readers of the same age. However, they DO NOT make more ‘visual’ errors than younger children of the same reading age. These data tell us that ‘visual’ errors are the result of poor reading, not the cause.
Even when individuals make errors that seem “visual”, such as migration errors within words (e.g., reading trail as trial) or between adjoining words (e.g., reading fig tree as fig free) what seems to be a visual or attention problem is actually a specific problem with the word-reading process. We know this because people who make these errors do not make the same errors for digit stimuli.
Eye movements and dyslexia
The eye movements of individuals who have dyslexia do differ from those of skilled readers (Rayner, 1998). While reading, the people with dyslexia exhibit longer duration of eye fixation, shorter saccades and a higher proportion of regressions (backward) saccades than controls (Huxler et al., 2006). However, research has demonstrated that abnormalities in eye movements occur specifically in reading tasks. When people with dyslexia and controls are compared on non-reading visual tasks that require similar perceptual and ocular motor demands to reading, there are no differences between eye movements of the groups. Hence the divergent eye movement patterns of people with dyslexia during reading reflect difficulties in the reading process rather than a primary impairment of ocular motor control (Huxler et al., 2006). This conclusion is supported by studies that have demonstrated that the eye movements of people with dyslexia do not differ from younger, reading age matched controls (Hyona & Olson, 1995) and that when people with dyslexia are given reading-level texts, their eye movements are comparable to controls (Olson et al., 1983).
Vision therapy involves eye exercises, eye-hand coordination tasks and other exercises designed to improve the individual’s motor memory activity. Although in widespread use, a number of reviews have concluded that vision therapy has limited evidence for efficacy (e.g., Barrett, 2009, Bishop, 1989, Wright, 2007). In response to concerns regarding the use of visual therapies, a number of influential bodies have conducted reviews and released policy statements for their members. For example, the joint statement of the Committee on Children With Disabilities, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus, and the American Academy of Ophthalmology states the following in regard to visual therapy:
“No scientific evidence supports claims that the academic abilities of children with learning disabilities can be improved with treatments that are based on 1) visual training, including muscle exercises, ocular pursuit, tracking exercises, or ‘training’ glasses; 2) neurological organisational training (laterality training, crawling, balance board, perceptual training).”
They go on to say that: “diagnostic and treatment approaches for dyslexia that lack scientific evidence of efficacy such as behavioral vision therapy and eye muscle exercises are not endorsed or recommended.”
Other recent reviews (e.g., The American Academy of Ophthalmology; Wright, 2007) have concluded that there is no scientific evidence that supports behavioural vision therapy or orthoptic vision therapy as effective treatments for reading difficulties. Claims of improvement after visual therapy have typically been based on poorly controlled studies and testimonials and reported benefits can often be explained by the traditional educational strategies with which vision therapies are usually combined or by placebo effects. Eye movements and visual perception are not critical factors in the reading impairment found in dyslexia and the majority of people with known ocular motility and eye movement defects read normally and even people with severely misaligned eyes can excel in reading and academics.
What is the evidence?
Evidence for any form of therapy can come from two sources: (a) theoretical evidence linking a treatment to a problem in a logical manner (e.g., excessive caloric intake results in weight gain so reducing caloric intake will probably result in weight loss) and (b) direct evidence for the efficacy of the treatment in reducing symptom severity (e.g., reducing caloric intake leads to greater weight loss in people with obesity compared to a placebo treatment). Of the two, the latter is a far stronger form of evidence. Both of these forms of evidence are missing for vision therapy as a treatment for dyslexia/reading problems.
Theoretical evidence linking vision and vision therapy to reading
It is likely true that people with dyslexia and other reading difficulties experience vision problems. However, it is also likely that there are good readers who experience vision problems. To prove causality between low scores on vision/visual processing tests one would have to demonstrate: (a) the vision/visual processing problems are specific to individuals who have reading problems, (b) that individuals who have vision/visual processing problems have different behavioural sequelae to individuals who do not have vision/visual processing problems, (c) that therapy that targets the putative visual problem leads to improvements in reading skills in the therapy group but not in an equivalent control group who received a placebo therapy and, (d) vision therapy produces greater gains in reading ability than reading intervention alone. There is currently no evidence showing a-d to be true. Therefore, one has to conclude that there is limited theoretical evidence for using vision therapy to treat reading problems.
Evidence for efficacy
The importance of demonstrating that vision therapy is an effective and therefore appropriate treatment for reading problems has been highlighted in a recent review (p. 5). It was noted that: “Demonstrating treatment efficacy is especially important here because these children and their parents represent a vulnerable group” and “the onus is clearly on treatment providers to produce the evidence in support of the treatment(s) that they are offering. Without such evidence, parents inevitably run the risk of wasting their time, effort and resources, and they and their children may become disillusioned if expectations are repeatedly raised and then dashed.”
There is currently no evidence that vision therapy improves reading ability directly or that greater reading growth occurs following vision therapy (i.e., that it enables better learning). The one study of sufficient scientific merit showed that vision therapy did not improve reading and spelling scores in poor readers compared to a control group of poor readers who received a placebo treatment.
Given the widespread use of vision therapy in a range of learning difficulties and its considerable cost, in terms of money and time, it is astounding that the clinicians and professional bodies that represent those clinicians have not invested in better research. The onus must surely be on clinicians and their professional bodies to prove scientifically that their treatments work. Until they manage that the conclusions of previous reviews that vision therapy is not recommended remain valid.