Extra large letter spacing improves reading in dyslexia. Or does it?

June 12th, 2012

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.