When smile becomes slime: Subtypes of dyslexia

September 22nd, 2012

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:

  1. Surface dyslexia – the non-lexical reading route (the pathway we use for ‘sounding out’) is spared and the patient can therefore read nonwords and regular words. However, access to the orthographic lexicon (our ‘word dictionary’) is impaired resulting in problems with reading irregular words.
  2. Phonological dyslexia – Lexical access is spared so the patient can read words they had in their ‘word dictionary’ before the ABI. In contrast, they can’t read new words or nonwords because the non-lexical (sounding out) pathway is impaired.
  3. Mixed dyslexia – a combination of surface and phonological dyslexia.
  4. Deep dyslexia – difficulty reading nonwords and the presence of semantic reading errors. For example, they might read ‘orchestra’ as symphony or ‘river’ as ocean (see Plaut & Shallice, 1993)
  5. Attentional dyslexia – a deficit in which letters migrate between neighbouring words but are correctly identified and keep their relative position within the word. For example, ‘fig tree’ can be read as fig free or even tie free.
  6. Neglect dyslexia – results in omissions or substitutions of letters at the beginning or end of words depending on whether the patient has left or right neglect.
  7. Letter position dyslexia – the patient predominately makes errors of letter migration within words, such as reading ‘board’ as broad or ‘pirates’ as parties. The migrations occur for medial letters within the word.

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.



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