Biology 202
2001 Third Web Report
On Serendip
 
 

The exact cause of dyslexia is unknown. However, researchers believe dyslexia is a reading disability with underlying genetic, developmental and neurological causes (8). People with dyslexia have trouble reading despite normal or high intelligence and exposure to sufficient language instruction. Specific reading problems apparent in dyslexia include reversal of words and letters, difficulty in pronouncing new words, difficulty in making a distinction between similarities and differences in words (on for no), and difficulty in discerning differences in letter sounds (ten, tin) (2).

In order to understand the specific reading problems associated with dyslexia, it is important to know how the brain conceptualizes language. The brain recognizes language in a hierarchical order. The upper levels of the hierarchy deal with semantics (the meaning of words), syntax (grammatical structure), and discourse (connected sentences). The lowest levels of the hierarchy deal with breaking words into separate small units of sound called phonemes. Thus, before words can be comprehended at higher levels in the hierarchy, they must be decoded at a phonological level (1)(9).

This phonological processing takes place automatically at a preconscious level in spoken language. A genetically determined phonological module automatically constructs words from phonemes for the speaker and deconstructs the words into phonemes for the listener. Speech is instinctive; it is the exemplary biological human trait. The alphabet, conversely, was created 5000 years ago to give speech concrete representation at the phonological level. Thus, reading is an invented artifact that must be learned on a conscious level. Reading is a difficult task because 'the reader must learn to listen with his eyes' (3). The reader must realize that the orthography, the sequence of letters on a page, represents the phonological structure of words (1) (4).

The phonological module of dyslexia argues that dyslexics have impaired reading ability because they have a deficit in phonological processing. According to this model, dyslexics have a difficult time with written language because they have an impaired ability to deconstruct written words into phonemes, thus preventing word identification. This low level phonological deficit prevents words from reaching high level linguistic processing, which would allow the reader to gain meaning from the text. Thus, dyslexics have intact memory and comprehension language processes that are not activated because they can only be activated after a word has been identified through phonological processing. The phonological model of dyslexia explains why dyslexics have difficulty with reading while remaining intellectually capable of processing very complex thoughts and ideas (1) (4).

The most compelling evidence for importance of phonological processing in reading ability are intervention studies indicating that phonological awareness training improves reading ability, while other language training programs do not (3) (1). A study conducted by Bradley and Bryant in 1980 included one group of children that worked on grouping words according to their sounds (phonological training), while another group worked on grouping words according to their meaning (holistic training). The reading ability of children in the phonological training group improved greatly, while the reading ability of children in the holistic training group only improved marginally. This study demonstrates that phonological training, above all other language training, improves reading skills (1).

The connection between phonological awareness and improved reading ability allowed researchers to investigate the possibility of phonological deficits in dyslexia. Several studies since the early 1990s have demonstrated that phonological deficits are the most significant and consistent marker of dyslexia in children. In these studies, dyslexic children are typically asked to divide specific words into phonemes and are then asked to delete a specific phoneme. For example, a child must say "rock" without the "r" sound. Dyslexic children, as compared to non-dyslexic children, have greater difficulty with this phoneme deletion task or are unable to do it. The results from these studies indicate that the deficiency in phonological awareness demonstrated by dyslexics is connected to their lower reading ability (1).

The research supporting the phonological model of dyslexia identifies a cognitive deficiency in phonological awareness and word identification that is carried out by a specific neural network. Until recently, however, researchers were unable to map cognitive functioning in the brain. The only avenue to understanding cognitive deficits in humans was through post-mortem examinations and studies of patients with brain injuries. Now, researchers are able to identify regions of the brain that are activated during cognitive processes through functional magnetic resonance imaging (fMRI). Functional magnetic resonance imaging is a noninvasive procedure that measures changes of metabolic activity in the brain while an individual engages in a task, such as reading (1) (10).

Through the use of fMRI, researchers at Yale University's Center for Learning and Attention have found that people with normal phonological awareness can rapidly process written words. When non-dyslexics are asked to imagine "block" without the "buh" sound, they quickly bring to mind "lock." The fMRI photographs show that phonological processing lights up the inferior frontal gyrus (Broca's area of the brain) in non-dyslexics. The brains of dyslexics, however, show less activation in the language centers of brain (Broca's area) during phonological awareness tasks. Other fMRI studies have indicated that letter identification lights up the extrastriate cortex in the occipital lobe and that the superior temporal gyrus and parts of the middle temporal and supramarginal gyri are activated during word meaning assessment (4).

Other research has shown that dyslexic children compensate for their deficiency in phonological processing in the Broca's area by using five times the brain area as non-dyslexics while engaging in simple oral language tasks. Using proton echo-planar spectroscopic imaging (PEPSI), a noninvasive technique that measure brain lactate (produced as a by-product of energy metabolism) activation, a group of researchers at the University of Washington looked at the metabolic brain activity of dyslexic and non-dyslexic boys during a simple oral rhyming tasks. A musical tone was also presented as a control measure. The PEPSI photographs illustrate that the dyslexic boys activated 4.6 times the brain area, specifically the left anterior or frontal lobe of the brain, as compared to non-dyslexic boys. The dyslexic and non-dyslexic boys did not differ in the brain areas activated by the musical tone. This indicates that brain activity in dyslexics is different from non-dyslexics specifically related to language processing, not nonlinguistic auditory processing (5).

The fMRI and PEPSI studies indicate that dyslexia is a neurodevelopmental disorder. These neurological findings support the phonological model of dyslexia by indicating that blocked phonological processing is a brain deficiency present in all dyslexics. Further neurological support for the phonological model has come from a recent study looking at dyslexia from a cross-cultural perspective. Paulesu et al. used positron emission tomography (PET) scans to monitor brain activity of English, French and Italian speaking subjects while they engaged in a reading task. The results indicate that, regardless of language, people with dyslexia or symptoms of dyslexia showed less neural activity in the language centers of the brain (6).

Despite the neurological similarity of dyslexic brain activity, the prevalence of dyslexia differs across culture. Researchers blame this discrepancy on the shallowness or complexity in orthography of different languages. English has a deep orthography, which consists of 44 phonemes that can be combined in over 1120 different ways. French has an equally deep orthography. Italian, on the other hand, has a shallow orthography, which consists of 25 phonemes that can be combined in just 33 ways. Thus, it is not surprising that dyslexia is more prevalent in English and French speaking countries in comparison to Italy. In fact, the researchers were not able to find any diagnosed Italian dyslexics are the university level. Instead, the researchers identified Italian "dyslexics" by selecting subjects with demonstrated verbal memory problems and slowed reading problems. Ken Spencer of the University of Hull in the UK contends that the shallowness of the Italian orthography allows the Italian dyslexics to cover up or compensate for their reading deficits, whereas the deep orthography of English and French "comes with a built in deficit" (7).

Paulesu et al.'s study provides support for the neurological component of the phonological model of dyslexia, but it does not offer a solution for dyslexics to overcome their reading disabilities. Dyslexic children in English and French speaking countries cannot up and move to Italy, where spelling is more basic and clear-cut. Instead, treatment for dyslexia should continue to focus on phonological language training. It is also important to factor the neurological component of dyslexia into better diagnostic processes and possible treatments. In addition to cognitive phonological language tasks, fMRI exams should be included diagnostic examination of dyslexia. This will allow researchers, psychologists, and physicians to more accurately separate dyslexia from other reading disabilities. It will also researchers to development better, more specific treatment, such as centrally acting drugs that increase the brain activity in the language centers of the brain.

WWW Sources

2) What is Dyslexia, by Roger P. Harrie and Carol Weller SITE, on the Kid Source web site

3) Advances in dyslexia research , on the Geocities web site

4)10 Years of Brain Imaging Research Shows The Brain Reads Sound by Sound , on the Healthy Place web site

5) Dyslexia and Brain Activity , on the Harvard web site

6) Dyslexia: Cultural Diversity and Biological Unity by Paulesu et al. , on the Science Magazine Online web site

7) Dyslexia: Same Brains, Different Languages by Laura Helmuth , on the Science Magazine Online web site

8) Fact Sheet: Dyslexia , on the Learning Disabilities Association web site

9) Beginning Reading And Phonological Awareness For Students With Learning Disabilities by Michael M. Behrmann , on the Kid Source web site

10) Brief Introduction to FMRI , on the FMRIB web site
 
 


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