Dyslexia is a specific learning difficulty (SpLD) affecting between 5-10% of people. The disorder is characterised by difficulties in phonological awareness (this refers to the ability to focus on and manipulate individual sounds in spoken words), verbal memory and verbal processing speed.
As well as difficulties with spelling and reading, there are a broad range of other symptoms – this can include concentration issues, trouble understanding certain jokes/expressions and difficulties with time management. Dyslexia is a broad spectrum, with some individuals experiencing some of the associated difficulties but not others, and with varying levels of severity. While some may have mild dyslexia, which can (personally, I was only diagnosed in my first year of university) be managed, others may always struggle significantly with reading and spelling. Intelligence is not affected.
For someone to be diagnosed with dyslexia, diagnostic tests are carried out, the content of which varies depending on the age of the individual. While these tests are very useful in giving information about an individual’s specific strengths and weaknesses, they can be very time-consuming.
Currently, there is no cure for dyslexia, but there are many strategies to help people, such as alternative exam arrangements and extra tutoring. With adequate support, many people with dyslexia go on to be very successful in life. The importance of early intervention is emphasised.
Dyslexia is widely believed to be a neurological problem, but a recent study suggests they may have found a possible cause of dyslexia – not in the brain, but the eyes!
Photoreceptors in the eye
In the eye, there are two types of photoreceptor (structures that respond to light) – the rod and cone cells. Rod cells, the more plentiful (around 120 million), respond to low levels of light but do not detect colour, which allows you to see in the dark. There is only one type of rod cell, and they are absent from the fovea (the region of the retina responsible for the highest visual acuity) but concentrated elsewhere. Cone cells (6-7 million) are only activated at higher concentrations of light, but detect colour. There are three types of cone cell – blue, red and green. There is a ‘blind spot’ in the fovea of about 0.1-0.15 millimetres, in which there are no blue cone cells.
Eye Dominance and Dyslexia
Similarly to the way in which most people have a dominant hand (apart from those who are ambidextrous), most people have a dominant eye. Both eyes record slightly different versions of the same image, so the brain decides which one is likely to be the most accurate. Signals from this dominant can override signals from the other. Lots more people are right-eyed than left.
This study investigated the presence or absence of eye dominance in 30 non-dyslexic students and 30 dyslexic students, using a method called the afterimage test. For the non-dyslexic participants, 19 were right-eye dominant and 11 were left-eye dominant – therefore, all had a dominant eye. On the other hand, 27/30 of the dyslexic participants had no dominant eye.
Furthermore, there were correlations between lack of eye dominant and apparent physical differences in eye. In the dominant eye, the shape of the blind spot is circular, while the shape in the non-dominant eye is elliptical. In the dyslexic participants with no eye dominance, however, the shape was circular in both eyes. For one of these participants, five family members who also had dyslexia were studied – there was no asymmetry in the arrangement of cone cells, as well as no eye dominance. This suggests a possible genetic cause of dyslexia, and could lead to new diagnostic strategies for dyslexia
Lack of asymmetry would mean the brain has to process two slightly different ‘mirror images’, which researchers believe would confuse the brain. Perhaps this could explain why dyslexic people commonly make ‘mirror image errors’ – for example, mistaking ‘b’ and ‘d’, or ‘3’ and ‘E’ – and often get confused between left and right.
So, what else does this study mean for dyslexic people? Firstly, lack of afterimage dominance could lead to a potential new, quicker way to diagnose the condition. In addition, researchers were able to use an LED lamp to “cancel” one of the images in the brains of the dyslexic participants, which reduced reading difficulty. Some participants referred to this as the “magic lamp”.
Considerations & Limitations
While this study seems very promising, it is important to remember that only 30 dyslexic participants were studied – this sample size is too small to draw any absolute conclusions. Also, all participants were students, so these would not have been representative of the whole dyslexic population.
A further problem is that the study cannot establish cause-effect relationships. It cannot be said whether the visual differences are the trigger of dyslexia, or simply a consequence.
As well as that, the findings from the study may explain some people’s dyslexia symptoms, but may not necessarily explain the symptoms of other people. As mentioned before, dyslexia has many symptoms and manifestations, which this study does not necessarily explain. For me, I don’t experience ‘mirror image’ distortions when reading, but the words sometimes start to go wobbly after I’ve been reading for a while. There are a range of other distortions experienced by other dyslexic people too, such size distortions of letters/words or gaps between words appearing narrower/wider.
In conclusion, while the study seems promising, significantly more work is needed before any proper conclusions about the cause of dyslexia can be drawn.