Dyslexia is a learning disability and derives from the Greek word dys meaning poor or inadequate and the word lexis meaning words. Dyslexia affects reading, spelling, writing, speaking, listening or even memory. Some people call dyslexia a specific learning difficulty'. (Medical dictionary)
According to Carlson there are two major distinctions of people. Those, who have developmental specific learning difficulties and those who have acquired specific learning difficulties. Developmental specific learning difficulties are caused by biological anomalies, usually genetic, in the brain at various levels before the birth and through childhood development. Acquired specific learning difficult is caused by brain trauma that may occur before birth or later in life. There are four subgroups of dyslexia. Surface dyslexia refers to reading words phonetically correct, but people experience problems with whole word recognition. Phonological dyslexia refers to the ability of a person to read familiar words by using whole word method but facing difficulties with "sounding out" words that are new or letter-to-sound decoding problems. Another subgroup of dyslexia is spelling dyslexia and its specific characteristic is the ability of a person to read individual letters that lead to reading words if given enough time but has problems recognizing the word as a whole and phonetically. The last subgroup is direct dyslexia, which is when a person can read aloud without comprehension.
Deep dyslexia is an acquired reading disorder in which readers can not read aloud function words such us the words the, while, when and facing a difficulty in reading abstract words such us truth, skill. Semantic errors, visual errors and visual-semantic errors are potential errors that can occur to deep dyslexics. Deep dyslexia is thought to occur after a brain damage. (Dictionary of biological psychology)
There are two different theories supporting deep dyslexia. The first one was proposed by Price et al (1998) and states that dyslexia results from a residual left-hemisphere reading system that has lost the ability to pronounce a printed word without reference to meaning. The second was proposed by Coltheart (2000), who states that deep dyslexia reflects right-hemisphere word processing. For both of the two theories there is scientific evidence.
Price et al (1998) contacted an experiment with two deep dyslexics. The first one was a 66 year old man, known as JG, with a right hemiparesis and aphasia, caused from a left hemisphere CVA 6 year ago. The second one was CJ, a 29 year old man who became aphasic after a head injury in a RTA 6 years ago. JG's speech production was influenced and agrammatic and he could be described as a Broca's aphasic. Broca's aphasic's lack language comprehension, they can not speak fluently and skip grammatic words. CJ's speech production was fluent and grammatical but marked by word-finding difficulties. For the study was used a PET ( positron emission tomography) to explore the brain areas activated when words were read aloud by the two deep dyslexic patients CJ and JG. The pet was restricted to two conditions each with six observations. One resting with eyes closed and the other one reading aloud words with concrete referents. The words were matched according the number of letters, phonemes, syllables, frequency, familiarity, and imageability, in each condition. When participants could not read the word showed, were instructed to say the word
The results showed that both participants had extensive left temporal lobe damage. Patients show normal left posterior basal temporal lobe activation a region that is associated with phonological retrieval and regions associated with semantic processing. It appears that even in deep dyslexics patients, activation of the inferior temporal cortices in the left hemisphere is accomplished by visual word recognition, semantic processing and phonological retrieval. In conclusion the above results show a poorly right-hemisphere processing.
There are also the studies, where the findings support Price's state. Such are the experiments of Laine et al (2000), and Roeltgen (1987).
Roeltgen (1987) supported that dyslexia is right hemisphere until he observed a patient who was deep dyslexic after a stroke at the left hemisphere and following a new stroke at the left hemisphere he lost his remaining reading ability. The reading of this patient was depended on the left hemispheric structures.
Furthermore Laine et al (2000), observed a 46-year old man who, suffered extensive left-hemisphere damage. Two reading tasks were used, a silent reading of sentences and oral reading of single words. While the deep dyslexic patient was reading sentences and single words, an MEG, (magnetoencephalographic) was performed in order to combine accurate temporal and spatial resolution in charting the patients reading related cortical activation patterns. The oral reading task included monomorphemic and case-inflected nouns. The results showed that a slow-activity wave was generated in the left-hemisphere, which was the lesion. Also lexical-semantic processing was related to the left superior-temporal cortex activation as measured by sentence-final semantic-in congruency detection.
In contrast with the above approach the right hemisphere theory suggests that in deep dyslexia access related stages of reading, orthographic and semantic processing are obliged by the right hemisphere.
According to Coltheart (1987) deep dyslexics make noumerous errors and omissions when attempting to read abstract words aloud. This claims that deep dyslexics use their right hemisphere to read.
Weeks et al (1997) investigated a deep dyslexic, a surface dyslexic and two normal controls. For his experiment while participants were performing different types of readings, the regional cerebral blood flow was measured using a Xenon-133. It was found that the regional cerebral blood flow related to orthographic processing of was weaker at the left cortical areas rather than in the right cortical areas.
Moreover at Michel's et al (1996) study a 23 year old college student suffering from a lesion to the posterior part of the corpus callossum was tested. In order to test his reading words and non words were presented to him to the left or to the right visual hemi-field. The results showed that there were no abnormalities in his reading with the right hemisphere and that left hemi-field reading was similar to the readings of CJ and JG.
Also the split-field studies of Saffran et al (1980) in a patient with deep dyslexia support the right hemisphere theory. The first study was a pilot study. The two hemifields of the patient V.S was tested tachistoscopically in letter recognition and counting dots tasks. The association of the two fields for the ability to process graphemic input similar in complexity to the stimuli to be used in the experimental task was the goal of the study. Later on the patient was also given lexical decision tasks, which were consisted of 40 four letter concrete nouns and 40 pronounceable pseudo words. Each word was presented once at the left visual field and once at the right visual field. The patient was administered to say yes every time that the stimulus was a word and no when the stimulus was a pseudo word. The results showed that when the stimulus was presented to the left visual field the lexical decision performance was very good. However when the word was presented to the right visual field participants were tended to say No, whether the stimulus was word or pseudo word.
In contrast to all the above studies which support either the one or the other theory this study of split-field lexical decision can not either support or not the right hemisphere theory. The study was contacted in three deep dyslexics including the V.S patient. The other two patients B.L and H.T were agrammatic as well as dyslexics like V.S and they were tested in the lexical decision task with four-letter concrete words after some practise on the task. Like in the pilot study 40 four letter concrete nouns and 40 pronounceable pseudo words were presented to the patients and each word was presented twice, once to each visual field. The results were presented separately for each patient. Patients V.S and B.L showed a slight right visual-field superiority and patient H.T performed poorly in both hemi-fields, with a small favour of the right visual-field. The three dyslexics combined results fail to show significant left visual-field advantage. The surprising thing in this study was the finding that V.S patient showed only small left visual-field advantage after showing large effects in the pilot study.
From the above information it can be concluded that there is scientific evidence for both of the two theories. As in any complex scientific field, different approaches are always welcomed and useful, as the competition between them leads to further and more accurate investigation and knowledge of the subject.
- Carlson, NR. Physiology of Behavior, Sixth ed. Boston: Allyn and Bacon, 1998.
- Coltheart, M., Patterson, K., & Marshall, J, C. (1987). Deep dyslexia (2nd ed.). International library of Psychology. London: Routledge & Kegan Paul.
- Coltheart, M. (2000). Deep dyslexia is right hemisphere reading. Brain and Language, 71, 299-309.
- Laine, M., Salmelin, R., Helenius, P., & Marttila, R. (2000). Brain activation during reading in deep dyslexia: An MEG study. Journal of Cognitive Neuroscience, 12, 622-634.
- Michel, F., Henaff, M. A., & Intrilligator, J. (1996). Two different readers in the same brain after a posterior callosal lesion. NeuroReport, 7, 786-788.
- Price, C. J., Howard, D., Patterson, K., Warburton, E. A., Friston, K.J., & Frackowiak, R. S. J. (1998). A functional neuroimaging description of two deep dyslexic patients. Journal of Cognitive Neuroscience, 10 (3), 303-315.
- Roeltgen, D.P. (1987). Loss of deep dyslexic reading ability from a second left-hemispheric lesion. Archives of Neurology, 44, 346-348.
- Saffran, E.M., Bogyo,L.C., Schwartz, M. F., & Marin, O. S. M. (1980). Does deep dyslexia reflect right-hemisphere reading? In M. Coltheart, K. Patterson, & J.C. Marshall (Eds.), Deep dyslexia (pp. 381-406). London Routledge & Kegan Paul.
- Schwartz, M. F., Saffran, E.M., & Marin, O. S. M. (1977). An analysis of agrammatic reading in aphasia. Paper presented to the International Neuropsychology Society, Santa Fe, New Mexico, February, 1977.
- Vandenberghe, R., Price, C. J., Wise, R., Josephs, O., & Frackowiak, R. S. J,. (1996) semantic systems for words for pictures: Functional anatomy. Nature, 383, 254-256
- Weeks, B., Coltheart, M., & Gordon, E. (1997). Deep dyslexia and the right hemisphere reading. A regional cerebral blood flow study. Aphasiology, 11, 1139-1158.