Theories Explaining The Cause Of Dyslexia Psychology Essay

Theories Explaining The Cause Of Dyslexia Psychology EssayAccording to this shot dyslexia is caused due to the impairment of the phonologic module which disrupts the ability to sequence the written expression into its phonetic constituents D. Shankweiler et al,1995. Reading is a complex mental process that involves two steps decoding and comprehensionP.B. Gough,1986. The process of comprehension requires higher order cognitive abilities like intelligence, vocabularyD.L. Shargon,1995 which atomic number 18 left unaffected in dyslexiaShaywitz SE,2001. Decoding involves the conversion of graphemes to phonemes. As a result the written word is non decoded properly which makes the individual difficult to comprehend it. Support for this hypothesis comes from an fmri study conducted by GeorgiewaP. Georgiewa,2002 in which both the dyslexic and control subjects were asked to read silently the given linguistic stimuli. There was a significant activating of wanting(p) frontal gyrus(IFG) in the control subjectsP. Georgiewa,2002. In the dyslexic subjects there was activation in 3 areas in addition to significant activation in the Brocas area. The hyper activation of Brocas area in dyslexics when compared to the controls could possibly be due to increased effort in phonological decoding. Recent anatomical point shows that there is a significant underactivation in Wernickes area, angular gyrus and striate cortex and overactivation in the Inferior frontal Gyrus in dyslexics when performing a phonological undertakingShaywitz SE,1998. Critics of this hypothesis argue that it doesnt account for symbols not related to phonological decoding. An some other criticism of this theory is that these phonological acquirements can be recovered on extensive training.This hypothesis states that the symptoms in dyslexia are causes due to the impairment of the magnocellular pathway of the Lateral Geniculate nucleus(LGN) Livingstone M,1991. In their post mortem study of 5 dyslexic br ains, Galaburda and their colleagues found that the M cells of the LGN were 20% smaller than that in ordinary people. The LGN is a six layered structure with the layers 1 and 2 know as magnocellular layer and the layers 3,4,5,6 known as the paarvocellular layerCarlson,2007. The Magnocellular layer is known as the immediate processing pathway as they respond faster and responsible for contrast sensitivity at low spatial frequencies and low luminosity levelsXuX, IchidaJM 2001, WIESELT,. N. D. H. HUBEL1. 1966.As this functional sequestration of the LGN is maintained upto the cortical association areas,(posterior parietal cortex)this M-layer defect could cause impairments in readingM. Kinsbournen 1962, Morris, R.K 1991, Olson 1991. The theory is supported by the fact that the flicker fusion rate in dyslexics is low at conditions operated by the magnocellular layerW.J. Lovegrove,1980.Since the major orient of M cells is the Cerebellum there are possibilities where this defect in sp ecific line of M-layer could account for the motor symptom defects in dyslexicsJohn Stein and Vincent Walsh,1997.The Magnocellular hypothesis has some criticisms. There is exhibit against the theory that dyslexics perform worse not barely in low spatial frequencies and luminance but also on high frequencies and luminance levelsSkottun, B. C, 2000.THE DOUBLE DEFICIT HYPOTHESIS(DDH)According to this hypothesis the reading impairment caused in dyslexia is due to two dearths which are phonological and rapid naming and both are differentWolf, M,1999. Rapid naming deficit is a deficit in which the subjects have problem in naming different classes of stimuli when presented visually. A typical test which assesses rapid naming is Rapid Automatized naming(RAN) Denckla MB,1974. According to WolfWolf, M,1999, Dyslexics perform poorly on RAN. Support for this theory also comes from other researchers like Tallal, TempleTemple 2000 who argue that dyslexic children process sounds very slowly th an normal people. But Critics say that the task of rapid naming is also phonological since naming is done by spelling and producing sounds and so it involves the whizs phonological system.THE AUTOMATISATION DEFICIT HYPOTHESIS(ADH)Automatization is a process by which the process of acquisition of skill becomes finer and refined so that the skill can be performed easily. This hypothesis states that the dyslexics perform badly in any task that requires skill automatizationNicolson,1990. For example dyslexic children perform poorly in balancing task and the operation is poor only when they are not allowed to compensate for it consciouslyNicolson R.I and Angela J. Fawcett(1995). A normal child would automatize the process of hearing phonemes in a word after some period, which is difficult for dyslexic children.THE CEREBELLAR DEFICIT HYPOTHESIS(CDH)The poor performance of dyslexics on tasks that require motor skill automatization(balance task) raises a possibility that Cerebellum might the involved in Dyslexia since cerebellum is principally involved in motor learningJ.F. Stein and M. Glickstein, 1992 and delusion based learning. This theory states that Dyslexia is characterised by cerebellar impairmentNicolson, 2001. about 80% of the cases of Dyslexia is characterised by cerebellar impairment. If the cerebellum is impaired in Dyslexia then the dyslexic patients should show classic cerebellar syndromes like dystonia and ataxiaR.S. Dow and G. Moruzzi, 1958. It was found that the performance of the dyslexics were worse than the control subjects on all the cerebellar testsA.J. Fawcettet al., 1996. Direct evidence for the involvement of cerebellum in dyslexia is based on the experimental paradigm of a PET study conducted by JenkinsI.H. Jenkinset al, 1994. In their study the subjects were made to learn a sequence of key presses by trial and error and using auditory feedback. It was found that the Cerebellum was active when the subjects learned new sequence and when the subjects were performing a prelearned sequence. The dyslexics showed slight cerebellar activation (ipsilateral) both during learning of new sequence and performance of a prelearned sequenceNicolson, 2001.One important query to answer is how cerebellar impairment could cause specific cognitive deficits in Dyslexia. The answer would be Cerebellar impairment causes problem in the childs articulatory speed which leads to reduced working memory which in minute causes language acquisition problemsS.A. Gathercoleet al, 1992. A recent study on the morphology of Cerebellum correlates cerebellar symmetry with the decimal point of phonological deficit in dyslexicsRae C,2002 indicating that there is a connection between cerebellum and phonological deficit. A propagate of neuroimaging studies say that the Cerebellum is involved in language processing tasksJ.E. Desmond and J.A. Fiez 1998, S.G. Kim, K. Ugurbil and P.L. Strick 1994.Also abnormalities in the fronto cerebellar network is rel ated to picture deficit in dyslexicsMark A. Eckert and Christiana M. Leonard 2003.Given all these evidences it becomes clear that cerebellum is involved in Dyslexia. But the Cerebellar deficit hypothesis also has some unanswered questions. The Cerebellum is a large structure receiving inputs from a lot of regions of the brain and so the primary impairment might be located somewhere else in the brain causing a disruption of Cerebellar processingT. Zeffiro and G. Eden,2001.Recent research also suggests that the actual impairment might be located in the perisylvian neocortical regionsEden, G.F. and Zeffiro, T.A. 1998, Klingberg, T. et al. 2000, which sends and receives projections from the Cerebellum.CONCLUSIONDyslexia is characterised by a wide range of symptoms and so it highly possible that the underlying defects are in multiple brain systems. Given the role of Cerebellum in Language and motor tasks, it is kinda clear that cerebellum is involved in dyslexia. That said one cannot deny the PDH and MDH. Many problems in dyslexia are phonological which is in accord with the PDH. The MDH accounts for rapid processing deficits. Also Anatomical evidences described above shows that there is comfortable underactivation in the angular gyrus, Wernickes area and the magnocellular layer and the striate and extra striate cortices in dyslexics which account for the phonological and rapid processing deficits. One way of unifying these three theories is by using the fact that the Cerebellum is machine-accessible with many parts of the brain J.D. Schmahmann and D.N. Pandya 1997, H.C. Leiner, A.L. Leiner and R. Dow 1993. Thus I conclude by stating that Dyslexia is an impairment of multiple Brain systems.