Reading and Dyslexia

• Neural basis of reading


  How the brain recognises letters, words, and sentences from print
• Reading and visual word recognition

  Reading involves visual processing of groups of letters across fixations, we can recognise written words rapidly and accurately in a series of fixations and saccades, with some words skipped altogether

  
• Spelling


  • Not as important for normal readers, only first and last letter need to be in right place to recognise word
• Word superiority effect

  Suggests there are units of representation corresponding to letter clusters (or known letter clusters comprising words themselves) that influence the visual recognition of letters and words
• Easier to detect the presence of a single letter if the letter is presented in the context of a word, rather than in isolation or within random string of letters
• Visual processing time NOT strongly affected by length of word, letters not analysed one by one
• Letters identification affected by whether it makes up word or not
• Implies top-down info from word knowledge influences letter identification (visual world recognition)

  
• Word blindness


  Incapability to recognise written words
• Visual Word Form Area (VWFA)

  Left ventral occipito-temporal cortex, likely to be area that processes word forms rather than a mere visual connection

  
• Only left vOTC seems to be specialised for word processing

  
• VWFA responds to range of non-visual-word stimuli e.g., pictures of objects, naming colours, repeat and think about meaning of heard words
• Visual word recognition, visual lexicon debatable; top-down influences (e.g., surrounding letters) unequivocal

  
• Neural basis for reading aloud

  No single reading centre, but network - different areas connecting sound - letters - words - meaning
• Types of acquired dyslexia

  • Peripheral (disruption of early visuo-attentional processing)
  • Central (disruption of phonological or semantic processing after visual word form processing)

  
• Pure alexia


  • Maps L-vOTC to Visual Word Form Processing, reading letter-by-letter, struggle with abstract letter identity

  
• Attentional dyslexia


  • Deficits in attentional filtering, difficulty separating constituent letters/words, letter migration errors

  
• Neglect dyslexia


  • Deficits in visuospatial attentional allocation, letter substitution errors on one side

  
• Surface dyslexia


  • Patients know the rules of regular pronunciation and follow them even with irregular words, knowledge of sound-letter mapping is damaged
• Phonological dyslexia


  • Patients fine with real words (regular + irregular), but tend to read non-word as real word, problems in phonological processing
• Deep dyslexia


  • Real word reading, but error-prone e.g., semantic errors, derivational errors, read concrete words better than abstract words, phonological retrieval affected by semantic impairment
• Dual-route model of reading aloud

  Lexico-Semantic route (word form - meaning - pronunciation), Phonological route (work out mapping between visual letters and sounds according to pronunciation route)

  
• Mapping brain damage with reading impairments reveal reading network
• Many cognitive functions are supported by complicated circuit of neural modules, damage to different part of circuit => different deficits
• No 'neural modules' unique, they're good at certain types of computation and can involve in multiple circuits for different functions