Disentangling Dyslexia - Phonological and Processing Impairment in Developmental Dyslexia

In the present dissertation I will review some of the most peculiar aspects concerning developmental dyslexia, focusing on its distribution and, especially, on its manifestations and possible causes. Observing that dyslexic individuals appear to manifest severe deficits in those cognitive tasks which require a fine phonological analysis and which are particularly demanding in terms of processing resources, I will propose an original hypothesis to account for the cognitive impairment underlying this disorder, the Phonological and Executive Working Memory Deficit Hypothesis. Working Memory is the brain system engaged in the temporary storage and manipulation of those information that are necessary for those cognitive tasks such as reasoning, learning, problem solving, language comprehension and comprehension and it plays a fundamental role in human cognition. Individual differences in cognitive tasks are determined by the general capacity of their Working Memory: people whose Working Memory is limited or less efficient are more likely to show lower speed and accuracy in the execution of those complex tasks which are demanding in terms of processing resources. In the Phonological and Executive Working Memory Deficit Hypothesis, I propose that developmental dyslexia is characterized by the presence of two main impairments affecting their Working Memory. On the one side, in fact, dyslexic individuals suffer from a phonological memory deficit, preventing them from correctly analyzing the internal structure of words and nonwords. On the other side, instead, they show an impairment affecting their executive functions and hampering their performance in complex and demanding tasks. The severity of these impairments determines the severity of the disorder itself. A clear consequence of this hypothesis is that dyslexic individuals are expected to exhibit difficulties whenever they are asked to perform complex operations or to execute more than one task simultaneously. Nevertheless, a compensation is allowed by the general plasticity of the system: an individual with an high IQ score, for instance, can learn to use alternative strategies to perform a task in order to circumvent her difficulties. Throughout this discussion, I will show that the Phonological and Executive Working Memory Deficit Hypothesis is able to account for all the principal manifestations of developmental dyslexia, explaining not only the well-known reading and spelling difficulties that characterize the disorder, but also the frequently reported phonological deficits, vocabulary and naming disorders, grammatical impairments and attention problems. The dissertation is organized as follows. In Chapter 1 I will present a detailed introduction to developmental dyslexia, discussing the manifestations of the disorder, and focusing on recent studies developed to identify the precursors of dyslexia. Moreover, I will briefly introduce the neurobiological aspects of the disorder. Chapter 2, instead, will be dedicated to the illustration of the main theories proposed to explain the causes of dyslexia, ranging from the Visual and Auditory Deficit Hypotheses and moving to the more recent approaches, such as the Magnocellular Deficit Hypothesis, the Phonological Deficit Hypothesis, the Double Deficit Hypothesis and the Phonological and Executive Working Memory Deficit Hypothesis. Discussing both strengths and weaknesses of each proposal, I will argue that none of them is able to capture all the difficulties associated with dyslexia, except for the Phonological and Executive Working Memory Deficit Hypothesis. However, I will suggest that this proposal should be reformulated more precisely and, first of all, strengthened by a further experimental protocol developed to test precisely dyslexic children’s and age-matched typically developing children’s Working Memory. The results of this experimental protocol will be presented in Chapter 3. As I will observe, findings provide uncontroversial evidence in favor of an impairment affecting dyslexics’ Phonological Loop and Central Executive, but leaving their Visuo-Spatial Sketchpad spared and normally functioning. Considering these results as a starting point, I will propose my hypothesis, the Phonological and Executive Working Memory Deficit Hypothesis, in Chapter 4. Specifically, I will argue that dyslexics’ poorly functioning phonological memory and executive functions hamper their performance in tasks requiring a good phonological competence and demanding a high amount of cognitive resources. I will note, therefore, that dyslexics’ deficits are more likely to arise in complex tasks. In order to further test the Phonological and Executive Working Memory Deficit Hypothesis I decided to assess dyslexic children’s performance in linguistically complex tasks, developing three experimental protocols whose results will be presented in the subsequent chapters. In Chapter 5 I will discuss the result of a first protocol testing dyslexic children’s ability to compute scalar implicatures, an operation remarkably expensive in terms of processing resources, comparing their performance to that shown by age-matched typically developing children, a group of younger children and a group composed by adults. In Chapter 6 I will present a second experiment testing the interpretation of negation in dyslexic children and age-matched typically developing children, considering their ability to comprehend negative sentences, negative quantifiers and negative concord. Finally, in Chapter 7 I will expose the results of a last protocol assessing dyslexic children’s competence in the interpretation of pronouns, comparing their performance to that shown by age-matched control children, control adults and two groups of younger children. As I will argue throughout the discussion, all three experiments provided results which are consistent with the Phonological and Executive Working Memory Deficit Hypothesis, demonstrating that dyslexics are indeed remarkably more impaired than their peers in the comprehension of complex sentences, and that their performance is similar to that shown by children who are 2 or 4 years younger than them. Finally, Chapter 8 will be dedicated to the concluding remarks: I will summarize the considerations put forward throughout the dissertation and I will propose a new definition of developmental dyslexia, which focuses on the phonological and executive Working Memory impairment exhibited by dyslexic individuals. I will also briefly introduce and discuss the Cerebellar Deficit Hypothesis developed by Nicolson and colleagues (1995, 2001, 2008) to explain dyslexia. I will argue that the Cerebellar Deficit Hypothesis and the Phonological and Executive Working Memory Deficit Hypothesis present both commonalities and differences and that further research is needed to analyze more thoroughly the distinct predictions made by the two proposals.