DYSLEXIA AND THE PHONOLOGICAL MODE LOver one hundred years ago, in November 1896, a doctor in Sussex, England, published the first description of the learning disorder that would come to be known as developmental dyslexia. 'Percy F. , ... aged 14, ... has always been a bright and intelligent boy,' wrote W.
Pringle Morgan in the 'British Medical Journal,' 'quick at games, and in no way inferior to others of his age. His great difficulty has been -- and is now -- his inability to learn to read." (Sec 3) In that brief introduction, Morgan captured the illness that has intrigued and frustrated scientists for a century. In 2000 as in 1896, reading ability is taken as a substitute for intelligence; most people assume that if someone is smart, motivated and schooled, he or she will learn to read. But the experience of millions of dyslexics, like Percy F. , has shown that assumption to be false. In dyslexia, the relation between intelligence and reading ability breaks down.
Early explanations of dyslexia in the 1920 s, held that defects in the visual system were to blame for the reversals of letters and words thought to typify dyslexic reading. Eye training was often prescribed to overcome these alleged visual defects. Later research has shown, however, that children with dyslexia are not unusually prone to reversing letters or words and that the deficit responsible for the disorder is related to the language system. In particular, dyslexia reflects a deficiency in the processing of the distinctive linguistic units, called phonemes that make up all spoken and written words.
Current linguistic models of reading and dyslexia now provide an explanation of why some very intelligent people have trouble learning to read and performing other language-related tasks. Over the past twenty years, a consistent model of dyslexia has emerged that is based on phonological processing. The phonological model is consistent both with the clinical symptoms of dyslexia and with what neuroscientist's know about brain organization and function. To understand how the phonological model works, one first has to consider the way in which language is processed in the brain. Researchers theorize the language system as a hierarchical series of modules or components, each devoted to a particular aspect of language.
At the upper levels of the hierarchy are components involved with semantics (vocabulary or word meaning), syntax (grammatical structure) and discourse (connected sentences). At the lowest level of the hierarchy is the phonological module, which is dedicated to processing the distinctive sound elements that constitute language. The phoneme, defined as the smallest meaningful segment of language, is the fundamental element of the linguistic system. Different combinations of just 44 phonemes produce every word in the English language. The word 'cat,' for example, consists of three phonemes: 'k uh,' 'aah,' and 't uh.' (Linguists indicate these sounds as |k|, |ae| and |t|. ) Before words can be identified, understood, stored in memory or retrieved from it, they must first be broken down, or parsed, into their phonetic units by the phonological module of the brain.
In spoken language, this process occurs automatically, at a preconscious level. As Steven Pinker of the Massachusetts Institute of Technology has argued, "language is instinctive -- all that is necessary is for humans to be exposed to it" (Sec 6). A genetically determined phonological module automatically assembles the phonemes into words for the speaker and translates the spoken word back into its underlying phonological components for the listener. In producing a word, the human speech mechanism -- the larynx, palate, tongue and lips -- automatically compresses and merges the phonemes. As a result, information from several phonemes is combined into a single unit of sound. Because there is no obvious clue to the underlying nature of speech, spoken language appears to be seamless.
Therefore, an oscilloscope would register the word 'cat' as a single burst of sound; only the human language system is capable of distinguishing the three phonemes embedded in the word. "Reading reflects spoken language", as Alvin M. Liber man of Haskins Laboratories in New Haven, Conn. , points out, "but it is a much harder skill to master" (Sec 3).
Although both speaking and reading rely on phonological processing, there is a significant difference: speaking is natural, and reading is not. Reading is an invention and must be learned at a conscious level. The task of the reader is to transform the visual percepts of alphabetic script into linguistic ones, that is, to recode graphemes (letters) into their corresponding phonemes. To accomplish this, the beginning reader must first come to a conscious awareness of the internal phonological structure of spoken words. Then he or she must realize that the orthography -- the sequence of letters on the page -- represents this phonology.
That is precisely what happens when a child learns to read. In contrast, when a child is dyslexic, a deficit within the language system at the level of the phonological module impairs his or her ability to segment the written word into its underlying phonological components. This explanation of dyslexia is referred to as the phonological model, or sometimes as the phonological deficit hypothesis. According to this theory, a circumscribed deficit in phonological processing impairs decoding, preventing word identification.
This basic deficit in what is essentially a lower- order linguistic function blocks access to higher-order linguistic processes and to gaining meaning from text. Therefore, although the language processes involved in comprehension and meaning are intact, they cannot be called into play, because they can be accessed only after a word has been identified. The impact of the phonological deficit is most obvious in reading, but it can also affect speech in predictable ways. If dyslexia is the result of an under developed phonological specialization, other consequences of impaired phonological functioning should also be apparent -- and they are.
In 1986, the scientist Robert B. Katz documented the problems poor readers have in naming objects shown in pictures. Katz showed that when dyslexics misname objects, the incorrect responses tend to share phonological characteristics with the correct response. Furthermore, the misnaming is not the result of a lack of knowledge. For example, a girl shown a picture of a volcano calls it a tornado.
When given the opportunity to elaborate, she demonstrates that she knows what the pictured object is -- she can describe the attributes and activities of a volcano in great detail and point to other pictures related to volcanoes. She simply cannot summon the word 'volcano.' Of course, many dyslexics do learn to read and even to excel in academics despite their disability. These so-called "compensated dyslexics" perform as well as non dyslexics on tests of word accuracy -- they have learned how to decode or identify words, thereby achieving higher levels of the language system. Timed tests reveal that decoding remains very difficult for compensated dyslexics; they are not automatic in their ability to identify words. Many dyslexics have told how tiring reading is for them, reflecting the amount of energy they must expend on the task. In fact, extreme slowness in making phonologically based decisions is typical of compensated dyslexics.
In the late 1980 s came the invention of functional magnetic resonance imaging (FMRI). Using the same scanning machine that has changed clinical imaging, FMRI can measure changes in the metabolic activity of the brain while an individual performs a cognitive task. Therefore, it is ideally suited to mapping the brain's response to activity such as reading. As a result of this program, we can now suggest a tentative neural architecture for reading a printed word. In particular, the identification of letters activates sites in the extra striate cortex within the occipital lobe; phonological processing takes place within the inferior frontal gyrus; and access to meaning calls on areas within the middle and superior temporal gyro of the brain. Studies with FMRI have discovered a difference between men and women in phonological representation.
It turns out that in men phonological processing engages the left inferior frontal gyrus, whereas in women it activates not only the left but the right inferior frontal gyrus as well. These differences in lateralization provide the first concrete proof of gender differences in brain organization. The fact that women's brains tend to have bilateral representation for phonological processing answers several former questions: why, for example, after a stroke involving the left side of the brain, women are less likely than men to have significant decrements in their language skills, and why women tend more often than men to compensate for dyslexia. The phonological model sums up exactly what we mean by dyslexia: a brain relay deficiency often surrounded by strengths in problem solving, concept formation, critical thinking and vocabulary. It's true that compensated dyslexics may use the 'big picture' of theories, models and ideas to help them remember specific details.
Even if compensated dyslexics succeed in memorizing lists, they will still have trouble producing the words on demand. The phonological model predicts, and experimentation has shown, that routine memorization and rapid word retrieval are particularly difficult for dyslexics. Even when the individual knows the information, needing to retrieve it rapidly and present it orally often results in calling up a related phoneme or incorrectly ordering the retrieved phonemes. On the other hand, when not pressured to provide instant responses, the dyslexic can deliver an excellent oral presentation. Similarly, in reading, whereas non impaired readers can decode words automatically, individuals frequently need to resort to the use of context to help them identify specific words. This method slows them further and is another reason that the provision of extra time is necessary if dyslexics are to show what they actually know.
However, Research at Oxford University suggests that many compensated dyslexics have a distinct advantage over non dyslexics in their ability to reason and conceptualize and that the phonological deficit masks what are often excellent comprehension skills. Many schools and universities now understand the nature of dyslexia and offer to evaluate the achievement of their dyslexic students with essays and prepared oral presentations rather than tests of routine memorization or multiple choices. Just as researchers have begun to understand the neural substrate of dyslexia, educators are beginning to recognize the alternative teaching methods of the disorder. A century after W. Pringle Morgan first described dyslexia in Percy F.
, society may at last understand the complexity of the disorder.