|Children acquire language by listening to those around them talking. In the first year of life they are building an ever-increasing store of speech sounds. This store is phonological memory — the units of sounds that make up words. If these sounds are stored in phonological memory in a faulty manner, the child’s perception of speech will be compromised, as will reading and spelling. Research by Paula Tallal shows that Dyslexic (and language impaired) children are unable to perceive fast sounds. These are the stop consonants that change to the vowel frequency before 40 milliseconds. Consonants such as b, t, k, d not perceived by the slow sensory processing system of the average Dyslexic and consequently auditory nerves are not stimulated into action in the same way. Many speech sound distinctions are lost.
Tallal, P. (1980) “Brain & Language” (9)
Tallal P & Piercy, M (1973) Nature, (241)
Nagarajan S et al “Cortical auditory processing in poor readers”
|This abnormal auditory processing is due to smaller neuronal fields in the left medial geniculate nuclei (MGN) according a post-mortem study of Dyslexics (Galaburda A et al, 1994, “Evidence for Aberrant Auditory Anatomy in Developmental Dyslexia” in Proceedings of the National Academy of Sciences, Vol. 91), and backed up by brain imaging studies showing the “knock-on” effect of auditory inefficiencies are weak phonological processing in Broca’s area (left frontal gyrus) which is often the target of stroke damage, which suggests this area is responsible for speech articulation. When speech is lost due to damage here it is called aphasia.
Other research suggests a timing circuit throughout the brain that simultaneously identifies letters (in the visual cortex) while the phonological analysis is progressing. After this meaning is mediated by the superior temporal gyrus and parts of the middle temporal and supramarginal gyri.
(Shaywitz S. “Dyslexia” Scientific American, Nov 1996).
Tallal P. The Science of Literacy Proceeding National Academy of Sciences 97: 2402
|Some Dyslexics show a pattern of under activation in the visual regions with a corresponding over activation in the phonological regions, when measured by Qeeg (quantitative eeg, a measure of brain activation). There seem therefore to be both sub-types and developmental stages of Dyslexia. The consistency of the brain based under and over activation in certain areas suggests a genetic causation and this is backed up by studies of families where Dyslexia affects generation after generation.
But early middle ear infections too, can cause a child to perceive speech sounds unevenly so that some are heard before others and the whole auditory system can be mistimed, and sounds misequenced. Thus there is a delay (in milliseconds) in the perception of speech sounds, which others notice as a time lag between their speaking and the child’s response. This affects the rate at which phonemes are matched to the syllable to recognize a word, the “inner voice” can’t keep up with the eyes and reading is inefficient.
Another environmental cause is premature birth This also is highly correlated with difficulties in the acquisition of literacy since the auditory nerve active in the last trimester, is not activated and primed as much as it would be if the baby was full term, especially if the baby is put into an incubator which cuts off sound. This has effects on the perception of speech sounds. Another environmental cause can be early exposure to the sounds of a second language before the child is secure about the sounds of the first language. While most children manage this quite successfully and go on to become bi-lingual, if there is any inefficiency in auditory perception it can inhibit the acquisition of literacy.