Diseases and Disorders

Developmental Dyslexia

Priya Vijayakumar


Introduction

Learning disorders are more common than generally conceived: 15-20% of children and adults suffer from language-based learning disorders. Of those people, 5-10% have dyslexia [4][5]. Characterized by deficits in reading and interpreting words, dyslexia is a learning disability that affects 70-80% of all individuals with reading difficulties; the disorder is the result of phonological processing gone awry [3].

 

Neurological Underpinnings

    The past three decades have been significant in understanding the neurological mechanisms behind dyslexia. Imaging, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), taken while human subjects engaged in cognitive activity has led to the development of a few theories for the neurological underpinnings behind dyslexia [3].

    The phonological theory is the most popular explanation for dyslexia; this theory states that while humans are neurologically hardwired to recognize speech, reading itself is an acquired ability [3]. The process of reading requires the recognition of symbols, the order of which the symbols are in, and the pronunciation of phonemes. Phonemes are the fundamental auditory components of speech; in the English language, there are 44 phonemes. Dyslexics lack fluency in this phonological system and thus struggle to read or perform other language-based tasks such as math with accuracy and fluency [2].

    Brain-imaging studies have identified three regions in the left hemisphere of the brain that are implicated in dyslexia. Aligning with the notion that the left hemisphere is  dominant in processing language, dyslexics have decreased neural activity in the left parieto-temporal and occipito-temporal regions of their brains. The Broca’s area also exhibits decreased neural activity however, as dyslexia persists into adulthood,  activity may normalize due to maturation [3][4].

    A cell-level hypothesis for dyslexia asserts that the learning disability is due to a visuomotor coordination impairment. In other words, the eyes’ movement does not coordinate with the letters or words being read. As a result of this discoordination between eye movement and what is being read, dyslexics confuse the order of letters. Research evidences a deficit in the visual magnocellular system in dyslexics. The visual magnocellular system comprises of large neurons found in the retina that are responsible for coordinating movement with visual stimuli. For example, when reading, the eyes focus on the order of which the letters and words should be read in. If the eyes lose focus on their visual target, this information is fed to the magnocellular system which in turn, acts through the cerebellum and motor neurons in the eyes to return focus on reading. Dyslexics face impairment with this visuomotor system and thus, have less steady control of eye movement which results in the hallmark symptoms of dyslexia [2].

Courtesy of Gary Waters via Getty Images

 

 

Symptoms and Diagnosis

    Hallmark symptoms of dyslexia include morphological confusions between letters (such as confusing “b” and “d”), slow reading, mispronunciation of words, and even poor handwriting [2][7]. Poor reading skills can easily be confused with dyslexia so thorough screening for diagnosis is preferable.

    Dyslexia is easily diagnosable from reading tests. Because of this, dyslexia is often identified in young children if deficits in normal reading abilities are observed. However, in order to accurately differentiate dyslexia from other reading disabilities, professional screening is  typically conducted. Professional screening consists of a comprehensive evaluation of general intelligence, word recognition, word decoding, spelling, verbal fluency, reading comprehension, and vocabulary. Such screening for dyslexia is not always accurate; those with less-severe dyslexia often slip through without detection. Regardless, tests such as Predictive Assessment of Reading or Dynamic Indicators of Basic Early Literacy Skills serve as good starting points for diagnosing dyslexia in younger children [1][7]. Dyslexia is notorious for its inheritability thus, analyzing family histories of the disability can be helpful for confirming diagnoses [4].

 

Therapy

    Systematic and goal-oriented intervention serves as the best therapy for dyslexics. Consistent reading practice as well as understanding how phonemes correlate to their symbol counterparts are effective forms of intervention [4]. Thankfully, dyslexia is adequately compensated for when interventions take place during early childhood; 70% of at-risk dyslexics who received therapy prior to second grade are proficient readers [6].


References


  1. Testing and Evaluation. (n.d.). Retrieved August 23, 2016, from https://dyslexiaida.org/testing-and-evaluation/

  2. Tackling Dyslexia at an Early Age. (2014). On the Brain: The Harvard Mahoney Neuroscience Institute Letter, 20.

  3. Statistics on Dyslexia. (2010). Retrieved August 23, 2016, from http://www.dyslexiacenterofutah.org/dyslexia/statistics/

  4. Shaywitz, S. E., Mody, M., & Shaywitz, B. A. (2006). Neural Mechanisms in Dyslexia. Current Directions in Psychological Science, 15(6), 278-281. doi:10.1111/j.1467-8721.2006.00452.x

  5. Morris, R. G., Fillenz, M., & Dickenson, A. H. (1995). Neuroscience: Science of the Brain. Brain Research Association.

  6. Getting an Assessment for Dyslexia. (n.d.). Retrieved August 23, 2016, from http://www.bdadyslexia.org.uk/dyslexic/getting-an-assessment-for-dyslexia

Priya Vijayakumar

Priya Vijayakumar


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