The dyslexia research world has many divisions. One of those divides is between those who think that subtypes of dyslexia exist and those that consider ‘subtypes’ to be an individual variation arising from a core deficit in phonological processing (see Stanovich et al., 1997; Thomson, 1999).
Pure subtypes exist in acquired dyslexia which occurs following an acquired brain injury (ABI). ABIs can produce clear dissociations in reading skill in previously good readers producing numerous subtypes. These include but are not limited to:
While these pure subtypes certainly exist in acquired dyslexia, as I mentioned, there is considerable controversy about whether they exist in developmental cases. For mind, they can be found but they are unlikely to be as pure as in acquired dyslexia. For example, I recently saw an 11 year old who read irregular- and regular-words well but had great difficulty reading non-words. This pattern indicates that the lexical pathway works quite nicely but that he has deficits in the non-lexical pathway. However, he could, of course, read some non-words which is not what one usually sees in a pure case of acquired phonological dyslexia.
One really interesting subtype of developmental dyslexia that has been identified in Hebrew and Arabic, but never before in English has been reported by Kohnen et al (2012). Letter position dyslexia is a peripheral dyslexia in which the major reading errors are characterised by migrations of middle letters within words (e.g., ‘smile’ is read as slime). They make no more letter identity errors (e.g., reading ‘form’ as farm) or between word migrations (e.g., ‘dark part’ read as park dart) than expected for age. Using various tasks, Kohnen et al showed that the problem is specific to the letter position encoding function in the visual analyser part of the word-reading system.
I suspect that children who have weaknesses in the peripheral parts of the reading system, as in letter position dyslexia, and in the non-lexical pathway as in the case of the 11 year old I mentioned above, are quite common in the population. We just don’t see them much because they probably still read ‘okay’ and aren’t referred to specialist clinics as a result. It is probably only in the odd case where the child has severe spelling problems or similar that they present to a clinician skilled enough to identify the problem.
Regarding letter position dyslexia, we currently don’t have normed tests that allow us to accurately measure migration errors. I wait on the wonderful Saskia Kohnen to produce such a test; a feat she will inevitably accomplish sometime soon.
I probably have a thing variously called Irlen-Meares syndrome, scotopic sensitivity syndrome, visual stress or visual discomfort. I use the term visual discomfort in deference to my old PhD supervisor Liz Conlon (my PhD is old not Liz) who is a leader in the field although she hardly ever gets cited. Here are some of her papers you bums! (Conlon et al, 1999; Conlon, Sanders, & Wright, 2009; Conlon & Humphreys, 2001; Conlon et al., 1998; Conlon & Sanders, 2011).
What is visual discomfort?
No one really knows what visual discomfort is. My own view, somewhat consistent with the literature, is that it is an abnormal sensory sensitivity to stimuli of high contrast, and/or low spatial and/or high temporal frequencies. Black text on a white page is an example of a high contrast stimulus. Single spaced text with small font size is an example of a stimulus of low spatial frequency. One of the reasons universities require assignments to be typed in 12-point font, double spaced is that double spaced text is more comfortable to read than single spaced. High temporal frequency stimuli are characterised by rapid flashing. Think of the rapid flicker emitted by strobe lights or a fluorescent bulb. These stimuli lead to excessive neuronal firing that can lead to perceptual distortions or, in my experience, simply make the stimuli uncomfortable to be around.
My own visual discomfort manifests as light sensitivity, too much time in harsh sunlight sans sunglasses results in eye strain and a headache (although this may be psychosomatic as I spent many years as a kid surfing and playing cricket sans sunglasses with no ill effects). I dislike fluorescent lights, which unfortunately light our clinic offices. One office has a bulb that runs directly along my left eye line as I sit in the therapy chair. After a heavy day of consulting I can actually feel a “buzzing” and some days a bad headache in the part of my head that seems to match where the light runs. Again, this may be neurotic but needless to say I attempt to avoid this room. I had trouble with the old CRT computer screens at university; LCD screens were still rare back then. Flashing lights drive me nuts. Laser shows, strobe lights at concerts, and my 3-year old son’s flashing Batman toothbrush (a light on the brush flashes, not Batman) all make me a little more grumpy than usual.
Visual discomfort, reading and dyslexia
There is a theory that visual discomfort causes print to become distorted, which affects word reading and comprehension in turn. Visual discomfort is also claimed to affect reading efficiency such that sufferers can only read for short periods and are prone to reading related headaches.
Visual discomfort has been reported to be more prevalent in dyslexic populations. However, the relationship between dyslexia and visual discomfort remains controversial. Visual complaints are made by many healthy people and visual discomfort also exists in skilled readers (I’m an example). That visual discomfort exists in skilled readers makes a nonsense of claims that it is a form of “visual dyslexia”. Dyslexia and visual discomfort are separate conditions.
Visual discomfort, dyslexia and coloured overlays/lenses
Coloured overlays or lenses are a common treatment for visual discomfort (Allen, Gilchrist and Hollis, 2008; Wilkins, 1995; 2003; Wilkins, Huang and Cao, 2004). Coloured overlays are thin transparent coloured films that are placed over a page of text. They are designed to colour the page without affecting clarity of the text.
The evidence for whether coloured overlays improve reading is mixed. A lot of the existing data published in “scientific” journals are plagued by methodological concerns, including no controls on other therapies/intervention or poorly matched intervention groups. Of the better studies Singleton and Trotter (2005) used undergraduate students with (n = 10) and without (n = 10) dyslexia. Each group had 5 students with high visual discomfort (HVD) scores and 5 with low visual discomfort (LVD) scores. All participants read faster using their chosen overlay. The dyslexics with HVD scores made significant gains in reading speed with an overlay while the other groups made non-significant change (gains of 3-4%). Singleton and Henderson (2007) showed children (6-14 yoa) made greater improvement in reading rate with coloured overlays relative to reading-age matched controls. In contrast, Ritchie, Della Sala and McIntosh (2011) reported on 61 children (7-12 yoa) with reading difficulties (77% were diagnosed by an Irlen diagnostician as having the visual discomfort). There was limited evidence that individually prescribed Irlen coloured overlays had any immediate benefit for reading rate.
A recent study from the lab of respected reading scientist, Maggie Snowling, investigated the effect of coloured overlays in a well-designed experiment. They took 26 controls and 16 people with dyslexia, all undergraduate students, matched for IQ. Both were tested on two reading tests. The Wilkins Rate of Reading Test (WRRT) measures the impact of overlays on reading. The WRRT requires speeded oral reading of a passage of text comprising 15 high-frequency words (familiar to children from 7 years) that are repeated in random order, ensuring that no word can be guessed from the context. The test was administered with and without the chosen overlay placed over the text to test for an immediate benefit in reading rate. Reading rate was calculated as the number of words read correctly per minute (wpm) not including errors, omitted words and omitted lines. They also used two passages adapted from passages in the secondary school edition of the York Assessment of Reading for Comprehension (YARC). Passage 1 consisted of 311 words, and Passage 2 consisted of 302 words. Five comprehension questions followed each passage.
Both groups read more words per minute in the Overlay versus No Overlay condition. The group with dyslexia showed marginally greater gains relative to controls. However, these data need to interpreted with a healthy dose of salt because the dyslexics were slower readers to begin with and therefore had more room to improve.
When reading real text (YARC), there was an effect for Group on passage reading time. Unsurprisingly, the dyslexic group was slower than controls in both Overlay and No Overlay conditions. But there was no effect for Overlay (the overlay made no difference to reading rate for either group) or a Group by Overlay interaction (there was no relative advantage for the dyslexic group in the Overlay condition v No Overlay). Reading comprehension scores did not change in either group as a result of using an overlay. These data are consistent with those reported by Ritchie, Della Sala and McIntosh (2011) in children. They suggest that coloured overlays are not as effective as claimed for improving reading accuracy or fluency.