an autodidact meets a dilettante…

the human brain- a very very rough guide

Jacinta: So we’re going to look at earlier ideas about dyslexia, before the recent revolution in neurology, if that’s not being too hyperbolic. These ideas tended to focus on known systems, before there were well-identified or detailed neural correlates. ‘Word-blindness’ was an early term for dyslexia, highlighting the visual system. This was partly based on the 19th century case of a French businessman and musician who, after a stroke, could no longer read words or musical notes or name colours. A second stroke worsened the situation considerably, eventually causing his death.

Canto: An autopsy revealed that the first stroke had damaged the left visual area and part of the corpus callosum, which connects the two hemispheres. It appears that what the man was seeing with his right hemisphere was not able to be ‘backed up’ by the left visual area, and/or connected to the left language area. The second stroke struck mainly the angular gyrus, a complex and vital integrating and processing region towards the back of the brain.

Jacinta: Yes, and before we go on, what we’re doing here is looking in more detail at the four potential sources of dyslexia set down at the end of the previous post. So in this post we’re focusing on 1. a developmental, possibly genetic, flaw in the structures underlying language or vision. 

Canto: Right, so there’ll be three more dyslexia posts after this. So this ‘Monsieur X’ case was one of ‘classic alexia’ or acquired dyslexia, and marked an important step forward in mapping regions in relation to the visual and processing aspects of language. Norman Geschwind described it as ‘disconnection syndrome’, when two brain regions essential to a function, in this case written language, are cut off from each other.

Jacinta: The auditory cortex became an important focus in the twentieth century, as researchers noted a problem with forming ‘auditory images’ – which sounds like a problem everyone would have! More specifically it means not being able to translate the images made by letters and phonemes into sounds.

Canto: Yes, so that a word like ‘come’ (which is actually quite complex – the hard ‘k’ followed by an ‘o’ which, orally, is neither the typically short nor long version, followed finally by the silent ‘e’ which has some quite strange effect on the previous vowel) would be quite a challenge. Perhaps the real surprise is that we have no trouble with it.

Jacinta: Yes, I prefer cum myself, but that’s a bit off-topic. Anyway, psycholinguistics, much derived from the work of Noam Chomsky, which came into prominence from the 1970s, tended to treat dyslexia more as specifically language-based rather than audio-visual. Taking this perspective, researchers found that ‘reading depended more on the linguistically demanding skills of phonological analysis and awareness than on sensory-based auditory perception of speech sounds’ (Wolf, p173). This was evidenced by the way impaired-reading children treated ‘visual reversal’ in letters (e.g p and q, b and d). They were able to draw the letters accurately, but had great trouble saying them (sounding them). This appears to be a spoken language problem, which carries over to writing.

Canto: Indeed, it highlighted a problem, which apparently had nothing to do with intelligence, or basic perception, but was more of a specific perception-within-language thing:

These children cannot readily delete a phoneme from the beginning or end of a word, much less from the middle, and then pronounce it; and their awareness of rhyme patterns (to decide whether two words like ‘fat’ and ’rat’ rhyme or not) develops much more slowly. More significantly, we now know that these children experience the most difficulties learning to read when they are expected to induce the rules of correspondence between letters and sounds on their own.

Phonological explanations of dyslexia have resulted in a lot of effective remedial work in recent decades, and a library of research in the field of reading deficits.

Jacinta: Yes, these are called structural hypotheses, noting deficits in awareness of phonemic structure, and phoneme-grapheme correspondences. And these deficits presumably have their home in specific neural regions and wiring. The executive processes of the frontal lobes may be at play, in terms of organised attention, the fixing of memory and the monitoring of comprehension, but also the more ‘basic’ processes of the cerebellum, involving timing and motor coordination. And co-ordination between these regions may also be an issue.

Canto: And, as Wolf points out, these structural hypotheses have sheeted home problems to so many brain regions – the frontal executive function region, the speech region close by, the central auditory region, the language and language/visual integration regions, the posterior visual cortex and the cerebellum – that it would be fair to say that ‘many of the collective hypothesised sources of dyslexia mirror the major component structures of the reading brain’ (Wolf, p176).

Jacinta: Which sounds pretty serious. Why is it happening? And why not for others…?

References

M Wolf, Proust and the squid: the story and science of the reading brain

https://www.kenhub.com/en/library/anatomy/angular-gyrus