TB7 Lecture 4 BB; Central/Peripheral Dyslexias and Dysgraphias

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  • Created on: 21-12-15 15:56

Summary

  • Acquired Central Dyslexias
    • Surface dyslexia
    • Phonological dyslexia
    • Deep dyslexia
  • Central Dysgraphias
    • Deep dysgraphia
  • Peripheral ("Pure") Dysgraphias
    • Graphemic Buffer Disorder
    • Afferent dysgraphia
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Central Acquired Dyslexias

  • SURFACE dyslexia --> Problems with MEANING --> Damage to VENTRAL
  • PHONOLOGICAL dyslexia --> Problems with SOUND --> Damage to DORSAL
  • DEEP dyslexia --> Problems with BOTH meaning and sound --> Damage to BOTH routes

Perception of written words is intact. The issue lies in accessing semantics and/or phonology.

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Surface Dyslexia

  • Patients misread IRREGULAR words (spelling).
    • They are instead pronounced as if they are regular e.g leopard --> lee-o-pard.
    • Patients with surface dyslexia make regularisation errors.

Findings

  • Patients with surface dyslexia show a "regularity effect" in which words with regular spellings are read faster and more accurately than irregular spellings.
  • In particular, LOW frequency (less familiar) irregular words are read the slowest.
    • In this way, surface dyslexiacs show a frequency x regularity interaction.

This suggests an OVER-RELIANCE on grapheme-phoneme conversion rules i.e an over-reliance on the DORSAL phonological route following ventral (semantic) damage. Surface dyslexia is also comorbid with semantic dementia, L temporal pole damage --> loss of semantic representations.

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Phonological dyslexia

  • Patients read familiar words BETTER than invented nonwords.
    • Good comprehension of familiar words
  • Non-words are likely to be misread as SIMILAR-LOOKING real words.
    • e.g fint --> fine, poat --> boat
      • These are known as lexicalisation errors.
  • Phonological dyslexia is most disabling as a developmental disorder. This is because familar word knowledge in children, their 'sight vocabulary' is not as high as adults and they have difficulty with LETTER-SOUND CONVERSION.

Deficits in phonological dyslexia suggest a weak dorsal route with a relatively intact ventral route.

  • Phonological dyslexics also have verbal short term memory deficits e.g phono buffer and articulatory control and issues with manipulating phonemes in speech. The deficits may be to processes necessary for but not specific to reading
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Double disassociation between surface and phonolog

Non-words

Irregular words

Phonological Dyslexia

Impaired

Intact

Surface Dyslexia

Intact

Impaired

There is a DOUBLE DISASSOCIATION in the deficits found in phonological and surface dyslexia, phono have impaired non-word reading and intact irregular word reading, whilst surface have intact non-word reading and impaired irregular word reading.

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Deep Dyslexia

  • Patients completely unable to read non-words aloud.
    • Complete damage to the dorsal route.

Findings

  • Patients better at reading concrete words > abstract words.
  • Patients POOR at reading function words.
  • Patients also make characteristic errors in reading aloud;
    • Visual errors = signal --> single, bush --> brush
    • Semantic errors = arm --> finger, duel --> sword
    • Visual-then-semantic errors = favour --> taste, sympathy --> orchestra.

There are two main theories in explaining deep dyslexia...

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Theories to explain deep dyslexia

  • 1) Multiple impairments to the reading network
    • The complete inability to read nonwords aloud <-- the dorsal route is gone.
    • This means the patient is limited to reading via the ventral route alone BUT this is also impaired!
    • This causes;
      • Imageability errors, better at reading CONCRETE > ABSTRACT
      • Semantic errors
      • Visual errors?
  • 2) Right hemisphere hypothesis
    • Different versions, but all propose that due to extensive L hemispheric damage, processing may proceed as far as vwfa BUT access to meaning occurs in the right hemisphere.
      • Right hemisphere semantics are imprecise, leading to semantic errors and issues with abstract meaning.
    • The right hemisphere is unable to perform grapheme-phoneme conversion, there is no check on semantic errors and so patients cannot read unfamiliar words OR non-words.
    • Deep dyslexia is often COMORBID with deep dysgraphia, indicating shared processes
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The process of spelling and writing

  • Because we can spell in a variety of different ways, this suggests our word knowledge is ABSTRACT and can be outputted in a variety of formats.
  • Familiar words are processed in parallel when we read, number of letters does not affect reading speed.
  • In contrast, spelling is an inherently serial process, letters can only be produced one at a time. Therefore a greater emphasis is on STORAGE processes, selection and referencing.
    • Spelling places more of a demand on control processes than reading
  • Recent neuroimaging has shown that the vwfa (left occipitotemporal gyrus, fusiform) may be involved in spelling as well as reading, maybe it is the region from which abstract representations of the orthographic forms of familiar words are retrieved when we spell and write.
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The steps towards writing

  • These stages proceed from abstract graphemes, through letter forms (allographs) to movement commands, to graphomotor patterns.
  • 1) Select Grapheme; < F>
  • 2) Select Allograph; The letter form to be written (f)
  • 3) Select and execute Graphomotor pattern; Movement commands specifying the force, direction and duration of the movement.

Stage

Impairments in…

1)      Select grapheme

Graphemic Buffer disorder

2)      Select allograph

Alzheimers disease

3)      Select and execute graphomotor

Micrographia (Parkinsons), afferent dysgraphia

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Peripheral "Pure" Dysgraphias

  • Impairments in spelling and/or writing in patients with OTHERWISE INTACT spoken laguages and sometimes intact reading.

1) Graphemic Buffer disorder

  • Problems with retention and control of abstract letter representations leading to errors in all forms of output.
  • Patient BH, Sage and Ellis (2004).
    • Good speech comprehension and production
    • Well-formed letters
    • Reading was slow but intact, occaisional errors and poor non-word reading.
    • Spelling problems in spelling aloud and writing.
  • Spelling accuracy DECREASED as word length increased, with the same types of error in oral and written spelling. Patient BH and other have problems retaining and outputting letters from the graphemic buffer. Frequency and AoA DID HAVE EFFECTS <--- Top down influences?
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Afferent Dysgraphia

  • Problems in the use of feedback to control graphomotor patterns.
  • Patient VB (Ellis, Young & Flude, 1987).
    • Right hemisphere stroke causing visuo-spatial neglect, neglect dyslexia and hand-writing problems.
    • Spoken and oral language intact.

Patient VBs spelling errors often involved omission/repetition of strokes and letters. Indicates an impaired ability to utilise visual and motor feedback to control handwriting.

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