I make a point of not commenting on research into medical and psychological conditions. However, I am dyslexic and feel able to comment on research into that specific condition. I recognize that there are probably many types, levels and causes of dyslexia and so my reaction might not be the same as others. But I still automatically judge the research by ‘does it feel like it is right in my case?’
Several theories have fit with my experience of dyslexia. The idea that there is a problem with the corpus callosum, the nerves that connect the two hemispheres, in the region where the sound processing is done so that the left and right hemispheres do not properly cooperate for auditory information. This fits with my brother’s cleft pallate and more severe dyslexia and with my high pallate. It might explain the lack of consciousness of what I am going to say that often happens to me. (It has only been on rare occasions that I have disagreed with that I have said.) I am left-handed and perhaps am not conscious of what the other hemisphere is preparing to say due to a lack of communication at some area along the corpus callosum.
Another theory points to a fault in the dorsal/ventral streams. This idea is that sensory information leaves the primary sensory areas via two paths called the dorsal and ventral streams, also called the ‘where/how’ and the ‘what’ streams. The dorsal (where) path leads to motor speech areas, is very fast, and not very conscious. The ventral (what) path leads to more cognitive areas where auditory information is converted into semantic information, is slower, and more conscious. These streams interact in some ways – they both map phonemes but in two different maps and those maps need to be consistent with one another. We need to recognize a phoneme and we need to speak a phoneme. Dyslexics appear to have great difficulty consciously recognizing individual phonemes. They also appear to have difficulty with very short phonemes in particular. This appears to have something to do with a lack of communication between the streams.
Reasonable oral skill (as opposed to written) is possible without phonological awareness by dealing with syllables as entities that are not divided into individual phonemes. The vowel in the syllable is modified by the consonants that proceed or follow it. So the a in bat is different than the a in cap. It is not necessary to recognize the individual b, t, c or p in order to recognize the two words and produce them in speech because the short consonants modify the vowel. This also rings true to me – it is like it feels. The inability to consciously recognize things as separate if they are close together and very poor reflex times also indicate this time problem with short consonants. It is odd, but I find it hard to explain to people how it is to hear a syllable clearly but not hear its components. I seems such a simple obvious perception to me, a single indivisible sound.
Neither of these theories explain the symptoms of mixing up left and right, clockwise and counterclockwise, confusing something with its mirror image and the ‘was’ and ‘saw’ problem. Nor do they explain the slight lag between knowing something was said and hearing what it was.
Theories that have to do with vision or with short-term memory do not seem to apply to me. Although I have to admit that I am not sure what a bad short-term memory would feel like. I certainly have an excellent long-term memory.
Recently there has appear a paper with a new theory. (Perrachione, Del Tufo, Winter, Murtagh, Cyr, Chang, Halverson, Ghosh, Christodoulou, Gabrieli; Dysfunction of Rapid Neural Adaptation in Dyslexia ; Neuron 92, 1383–1397, December 2016) They looked at perceptual adaption in dyslexics and non-dyslexics. Perceptual adaption is the attenuation in perceptual processing of repetitive stimuli. So for example if the same voice says a list of words, there is less activity in parts of the brain than if a different voice delivers each word. The brain has adapted to the voice and that makes processing easier. They measured the adaptation using fMRI and used procedures featuring spoken words, written words, objects and faces with adult subjects and children just starting to read. Always the adaption was weaker for dyslexics then for controls. Also the differences were in the areas involved in processing the particular type of stimulus (such as in visual areas for visual stimuli). The amount of adaptation in these areas correlated with the level of reading skill of the dyslexic. The research supports the idea that dysfunction in neural adaptation may be and important aspect of dyslexia.
Here is part of their conclusion:
“Dyslexia is a specific impairment in developing typical reading abilities. Correspondingly, structural and functional disruptions to the network of brain areas known to support reading are consistently observed in dyslexia. However, these observations confound cause and consequence, especially since reading is a cultural invention that must make use of existing circuitry evolved for other purposes. In this way, differences between brains that exert more subtle influences on non-reading behaviors are likely to be the culprit in a cascade of perceptual and mnemonic challenges that interfere with the development of typical reading abilities. Recent research has begun to elucidate a cluster of behaviorally distinct, but potentially physiologically related, impairments that are evinced by individuals with reading difficulties and observable in their brains. Through this collection of neural signatures—including unstable neural representations, diminished top-down control, susceptibility to noise, and inability to construct robust short-term perceptual representations—we are beginning to see that reading impairments can arise from general dysfunction in the processes supported by rapid neural adaptation.”
Does the theory ring true? It certainly fits with the feeling that the problem is wider than just language. I have to say that I have always found it difficult to mimic other people’s speech and that would fit with a weak adaptation. The theory does not seem impossible to me but it also does not seem to fit closely to how I feel about being dyslexic. I feel a kind of wall between what I hear and written language; I have never felt that I have overcome the wall; but I have felt that I worked around it.
I have to give the paper respect for the convincing data even if it does not seem to be the whole story. The picture may be about some aspect of the dyslexic developmental fault but not actually have much to do with the main symptom, difficulty with phoneme awareness.