W5.2 (N) and 6.2 (N) : Acquired dyslexia syndromes and types of dyslexia Flashcards
(20 cards)
What symptoms did Dejerine’s patient ‘Mr C’ display and what do we now know this to be as a result of?
He had a stroke and woke up with the inability to read- oral language and spelling were intact, object, face and even some number recognition was largely preserved, tactile and letter/word recognition also remained intact
Post mortem revealed his stroke damaged the ventral occipito-temporal region, meaning that he was an alexic patient, with ‘pure alexia’ or ‘pure verbal blindness’
What are the two main classifications of acquired dyslexias?
Peripheral dyslexias- refers to any reading disorder in which seeing a word as a stable orthographic object fails (failure in an earlier stage, before recognising linguistic relevance of a stimulus)
Central dyslexias- refers to any reading disorder in which impairment occurs AFTER the stage of visual word form (psycho/neuro linguistic in nature)
Which parts of the system do peripheral and central dyslexias have an impairment in?
Peripheral- parallel letter perception (start)
Central- semantic, non semantic, grapho phonological (middle)
See figure 11!!
What are the three types of peripheral dyslexias?
- Pure alexia/letter by letter reading
- Attentional dyslexia
- Neglect dyslexia
What are the three types of central dyslexias?
- Phonological dyslexia
- Deep dyslexia
- Semantic dyslexia
- Surface dyslexia
Describe pure alexia aka alexia without agraphia
Mr C the first case but many have been described
Main symptom is: word identification impossible, except via explicit sequential identification of individual letters
Hence there is a substantial length effect and almost a linear relation between length in letters and reading time
Heavily affected by whether or not the font is ambiguous at the letter level- contextual information e.g. being able to fill the gap in a word, is not possible
What is the ‘Saffran effect’ in pure alexia?
Demonstrates that some parallel processing of the string is taking place, even though the patient is unable to read the word
Cerebral area involved: visual word form area is damaged or disconnected (see figures 12 and 13)
What is hemi-alexia?
Very rare
Patient A.C.
If the corpus collosum is damaged then the transfer of information between hemipsheres is often no longer possible, so the patient has difficulty reading words ONLY presented in the left VF- known as hemi alexia, where the visual form area is not activated by strings presented in the left visual field unlike controls (see figure 14)
This is not to be confused with hemi-neglect dyslexia
Describe attentional dyslexia
Very rare condition associated with a left parietal lesion
Involves the difficulty in identifying letters or words when flanked by other items of the same category, however, naming of a letter or a word in isolation is preserved e.g. the letter ‘o’ on its own is fine, but it becomes error prone in ‘word’, and ‘word on its own its fine but becomes error prime in ‘when the word is a sentence’ and so on
Essentially the patient is highly sensitive to crowding from same-level items e.g. words from words, letters from letters, numbers from numbers etc
Also involves the migration of letters to analogous position e.g. left hemipshere becomes heft hemipshere
Known as a disorder of the attentional filter, specific for reading
Describe neglect dyslexia
Main symptom is a failure to identify the initial or final letter(s) of a word or group, resulting in omissions, substitutions or additions
It is often but not always associated with spatial neglect
In the most common form, the initial portion of the word is problematic e.g. lend becomes end, wine becomes mine, oat becomes boat
Most often the amount of neglected letters to the left or right of the neglect point corresponds to the amount of neglected letters e.g. car -> bar but enigma -> stigma rather than sigma
Thus, some processing of the neglected letters occurs and suggests that although the position is preserved, identity is lost
Various levels possible e.g. different orientations: tied to retinal image location, independent of retinal image but orientation matters, independent of location and orientation
Describe phonological dyslexia
A form of central dyslexia that involves the impaired ability to read new or made up words (or non words) and to sound out individual graphemes
Lesion is often of the temporal lobe in the dominant hemipshere
Reading of real words is intact, visual processing of letter strings is intact and the immediate repetition of nonwords is perfect
Some patients can read concrete words better than abstract words and/or show difficulty with function (as opposed to content) words
Non words are read better if the items contain simple graphemes and are homophhoneous to real words e.g. BRANE, because whole word knowledge of an existing homophone can compensate for the failure to apply correspondence rules
Describe deep dyslexia
Associated with extensive damage to the dominant left hemisphere and consists of: impaired ability to read nonwords, semantically related errors e.g. ill>sick, visual errors, derivational errors e.g. card>’cards’, effect of syntactical class e.g. nouns>adjectives, effect of imageability e.g. concrete v abstract words
Which part of the reading route is disrupted in deep dyslexia?
Only the semantic lexical route is not impaired; all other routes inc nonsemantic lexical and graphophonological are impaired
implicates damage in both nonlexical route and direct non semantic lexical route
Why are semantic errors specifically common in deep dyslexia?
The semantic route is by default inaccurate when it comes to selecting words for speech production
Patient has no info about the target word other than its meaning which would explain the imageability effect and low performance on functors
Sign that this may be the only source of knowledge available to achieve pronunciation
Describe surface dyslexia
Main symptoms: regularisations (typical pronunciation chosen) so pint is rhymed with mint
Stress shift e,g, guiTAR becomes GUitar
Comprehension is based on pronunciation e.g. bear as ‘beer’
Failures to apply contextual rurles e.g. insect>insist
Incomplete decoding of diagram e.g. niece>nice
However, the reading of regular words and nonwords is spared
Traditional interpretation: inability to read via the lexical route, reading reflects the exclusive reliance on the nonlexical route and thus on GPCs
There is a strong sensitivity to regularity as ‘critical variable’
Surface dyslexia in itself is an argument for the existing of a nonlexical route
Better performance on frequent than rare irregular words interpreted in saying that representations of the latter are more likely to go
see figure 15
What does the extensive impairment of the DRC in surface dyslexia fail to explain?
Does not explain lexical errors that clearly are not just regularisations e.g. incense>increase, barge>bargain
Given the slowness of reading and occasional use of letter names, surface dyslexia possibly reflects a compensatory strategy
Damage to the lexical route does not explain why surface dyslexia tends to be associated with semantic dementia
Describe semantic dyslexia/reading without meaning
Disorder associated with neurological diseases such as Alzheimers or semantic dementia
Main symptom is the ability of patients to read fast and fluently (even irregular words in some patients) but the inability to comprehend what they are reading
Describe the case of WLP (Schwartz et al, 1980): a patient with semantic dementia
WLP can read pseudowords (made up words), match a spoke word ‘brain’ with a written pseudohomophone (brane) and can read aloud irregular words (pint) which by contrast she can not define or match with a picture
But she fails in an object naming task although can mimic the use of certain objects
Basically, has an operating DRC without a semantic system- her case led to a drastic revision of the dual route theory which needed to include a direct (nonsemantic) lexical route, to explain the performance on irregular words in the absence of semantic access
Before the discovery of semantic dementia, what was the reading route thought to only involve?
Thought to only involve a semantic route and grapho-phonological route; for this conception, reading irregular words without comprehension was impossible
So the route became a three route model
In DRC, what is the most straightforward explanation for semantic dyslexia?
Damage to the semantic system
