Dyslexia

• Dyslexia


  Also called Developmental Language Disorder (DLD)
• Signs of DLD in the brain


  • Not specified

  
• It has been debated for decades what causes dyslexia
• Proposed causes of dyslexia


  • Genetic variation
  • Cognitive impairment
  • Working memory
• Blue cone-free area


  Dark region in the fovea, known as the Maxwell centroid
• Lack of eye dominance in dyslexics

  Maxwell centroid appears circular in both eyes

  
• Eye dominance in non-dyslexics

  Maxwell centroid appears circular in dominant eye, elliptical in weaker eye

  
• 27 out of 30 dyslexic observers were unable to detect any asymmetry in the Maxwell centroid

  
• Dyslexic observer no. 29 saw a superposition of the initial letter 'b' and its mirror image 'd' when using a continuous wave light

  
• Using a pulse-width modulation light, the extra mirror-image 'd' was suppressed and the reading skills were restored for dyslexic observer no. 29
• John Stein, emeritus professor of physiology at the University of Oxford: '"It brings back into focus the idea that vision has something to do with dyslexia, which tends to be ignored nowadays."'

  
• Dyslexic students' two eyes are equivalent and their brain has to successively rely on the two slightly different versions of a given visual scene, often inducing poor and unstable fixation

  
• The lack of asymmetry in dyslexics perturbs the complex connectivity and lateralization of the different modal and cross-modal regions of the brain involved in reading and other tasks
• The retinal connectivity, the organization and the detailed topography of the primary cortex, as well as numerous superior bundles such as the corpus callosum, the magnocellular pathway, and the left arcuate fasciculus can be affected in dyslexia

  
• Dyslexics show a decreased connectivity along the visual pathway as well as between visual and prefrontal regions

  
• Myelin


  Protects and insulates nerve fibers, aids in the quick and accurate transmission of electrical current carrying data from one nerve cell to the next

  
• Myelination


  1. Generating myelin

  2. Progresses rapidly during infancy

  3. Continues through adolescence and early adulthood

  4. Can be added in grey matter regions throughout life

  
• Purpose of myelin


  • Increases the speed at which electrical impulses propagate along the myelinated fibre
  • Permits larger body size by maintaining agile communication between distant body parts

  
• Hypermyelination of the left auditory cortex has been observed in developmental dyslexia
• Hypomyelination has been observed in children and adolescents with poor reading ability
• Hypomyelination has been observed in the corpus callosum, spinal thalamus, and medial geniculate body (MGB) in Developmental Language Disorder
• The cause(s) of Dyslexia are still unknown
• It is difficult to differentiate between cause and symptom in dyslexia
• Dyslexia may be related to the retina/fovea and symmetric processing of visual information

  
• Dyslexia may be related to hypermyelination and/or hypomyelination