Acquired dyslexia syndromes Flashcards
French neurologist Joseph-Jules Déjerine and Mr C. (1892)
Mr C. is a 68 year-old intelligent, cultured, wealthy retired Parisian textile merchant, whom following a stroke wakes up one day of 1887 with the inability to read. He is unable to recognize words or letters, however:
- First case of acquired dyslexia
Oral language AND spelling are intact.
Object, face, drawing and even number recognition largely preserved.
Surprisingly, tactile and gestural letter/word recognition remains intact.
Pure verbal blindness (alexia) – blind to orthographic strings
After postmortem, Déjerine concludes that Mr C.’s ‘pure verbal blindness’ results from a disconnection between primary visual areas and an other occipital area dealing specifically with letters and words.
- Bridge that links objects to linguistic system
We now know that this region is located in the ventral occipito-temporal region (Cohen et al., 2000)!
‘Pure verbal blindness’ or ‘pure alexia’ (i.e. without agraphia – without inability to write) typically results from this region being either damaged or disconnected and this is one type of acquired reading disorder.
- Damage in visual word form
- or disconnection between visual cortex and linguistic visual word form area on left side
fMRI data of patients w/ similar lesions
see slides
The convergence of the post-mortem and brain imaging data around the visual word-form area is just impressive.
The graph shows the linear relationship between word length (in number of letters) and reading time in ms.
Damage maps onto word form area
Linear r’ship to letters in word and RT to processing word for reading – decipher string one letter at a time
classification of acquired dyslexias
Two types of acquired reading disorder can be distinguished: the peripheral dyslexias and the central dyslexias.
For a word to be understood or pronounced, it must first be perceived as an orthographic entity, i.e., seen as a ‘legal’ visual word form.
Peripheral dyslexias refers to any reading disorder in which visual word form fails to be achieved.
- Something goes wrong with senses/attention
- Something breaks down/at level of gate to reading system – left evntral-occipital region – reading system not accessed
Central dyslexias refers to any reading disorder in which impairment occurs after the stage of visual word form.
In that sense, central dyslexias concern the reading system per se. They are therefore psycho/neurolinguistic in nature. In contrast, peripheral dyslexias are due to a failure at an earlier stage (i.e., before recognizing the linguistic relevance of the stimulus)
DRC
The visual word-form area is not to be confused with the input lexicon, which is right before the Y junction in the lexical route (in blue).
see notes
peripheral dyslexias
Pure alexia/alexia without agraphia/letter-by-letter reading (Déjerine, 1892)
Attentional dyslexia (Shallice & Warrington, 1977)
Neglect dyslexia (Ellis, Flude & Young, 1987)
central dyslexias
Phonological dyslexia (Beauvois & Derouesné, 1979)
Deep dyslexia (Marshall & Newcombe, 1973)
Semantic dyslexia (Schwartz, Marin & Saffran, 1980)
Surface dyslexia (Marshall & Newcombe, 1973)
pure Alexia (Déjerine, 1892)
Other names: ‘alexia without agraphia’ (i.e. no concurrent spelling impairment), ‘letter-by-letter reading’
Many cases described, and Mr C. was the first!
Main symptom: Word identification impossible, except via explicit sequential identification of individual letters (slow and painful).
Hence, substantial length effect and almost a linear relation between length in letters and reading time.
Word identification done in explicit manner
Letter-by-letter readers are heavily affected by whether or not the font is ambiguous at the letter level:
- e.g. m
- That means that in contrast to us, contextual information (e.g., s_ile) does not help them at all. This is another manifestation of the fact that they can process only one letter at a time.
Cerebral area involved: visual word form area (VWFA; ventral occipito-temporal region) damaged or disconnected.
the Saffran effect
Pure alexics are well above chance in categorizing the meaning of a word (is it an animal?) or lexical status of a letter string (is it a real word?), despite their inability to “consciously” identify the word.
- Make semantic decisions
demonstrates that some parallel processing of the string is taking place, even though the patient is unable to read the word.
damage of VWFA
see notes
Pure alexia is typically associated with either a damage directly to the visual word-form area (this slide), or a disconnection preventing information to reach that region (the next slide).
disconnection of VWFA
see notes
a special case: hemi-alexia (patient A.C.) - Molko et al. (2002)
see notes
As you know, information from the left visual hemi-field, initially processed in the right hemisphere, needs to cross to the left hemisphere to be combined with the right hemi-field information in the visual word-form area.
But if the corpus callosum, which allows communication between the hemispheres, is damaged, then this transfer may no longer be possible.
As a result, the patient has difficulty reading only words that are presented in the left visual field.
This condition has been termed ‘hemi-alexia’. It is NOT to be confounded with hemi-neglect dyslexia.
In patient AC the visual word-form area (i.e., left hemisphere) is NOT activated by strings presented in the left visual field, unlike controls.
By comparison, for strings presented in the right visual field, AC lights up his/her visual word form area normally.
attentional dyslexia (Shallice and Warrington, 1977)
Very rare!
Associated with left parietal lesion.
Difficulty in identifying letters or words when flanked by other items of the same category
- Attentional span is wider than it should be – suffer from info either side of the target
However, naming of a letter or a word in isolation preserved.
- ‘o’ on its own is fine, but errorprone in ‘word’;
- ‘word’ on its own is fine, but erroprone in ‘when the word is in a sentence’.
Basically, attentional dyslexics are highly sensitive to crowding from same-level items (e.g., words from words, letters from letters, numbers from numbers, etc.)
Also, migration of letters to analogous position in other words, i.e. left hemisphere > ‘heft hemisphere’.
- Not a problem with articulation
Disorder of the ‘attentional filter’: the window of attention is larger than the target part of the visual field, and this lets other information in.
It is specific to reading, as it does not hold for pictures (Warrington, 1993).
The nature of the flankers modulates the manifestation of the disorder: less interference with different-category than with same-category distractors!
Migration to an analogous position in another word indicates that knowledge of within-word letter position is preserved, despite the attentional deficit.
see notes for table
neglect dyslexia (Kinsbourne and Warrington, 1962)
Main symptom: failure to identify the initial or final letter(s) of a word or group of words, resulting in omissions, substitutions or additions.
Often but not necessarily associated with spatial neglect.
Typically, contralateral parietal lesion.
In the most common form, the initial portion of the word is problematic (i.e. right lesion).
- e.g. lend > ‘end’, wine > ‘mine’, oat > ‘boat’
Most often, there are as many erroneous letters to the left (or the right) of the neglect point as neglected letters in the target.
- car > ‘bar’, enigma > ‘stigma’
Thus, some processing of the neglected letters occurs: suggests position is preserved, but identity is lost!
see notes for disorder of orientation
- tied to retinal image location: syndrome connected to space
- Impairment tied to the left visual hemi-field. - independent of the retinal image, but orientation matters:
- Impairment tied to the left part of the stimulus, irrespective of its actual location in the visual field. - independent of location AND orientation:
- Impairment tied to the left part of the word, irrespective of its orientation. This is the most abstract form the impairment can take.
phonological dyslexia (Beauvois and Derouesné, 1979)
Main symptoms: Impaired ability to read new or made up word (or nonwords), and to sound out individual graphemes.
- Lose ability to generalise knowledge of English
The two symptoms point to a damage of print-to-sound correspondences (i.e., the nonlexical route in the DRC model): knowledge of the rules and application of the rules is faulty.
The fact that in non-imageable words may also be affected demonstrates that the meaning of words take an active part in supporting reading.
For those words that do not have a well defined meaning (e.g., abstract words, function words), an intact non-lexical route is crucial.
When the latter breaks down, these words are the first in line to suffer.
And of course the least frequent is such a word, the more like it will be misread, or not read at all.
Lesion of the temporal lobe of dominant hemisphere. (usually left)
But reading of words relatively intact, if not perfect (see below); visual processing of letter strings intact; and immediate repetition of nonwords perfect.
Some patients read concrete words better than abstract words, and/or show difficulty with function (as opposed to content) words.
Phonological alexics read nonwords better if the items (1) contain simple graphemes, and (2) are homophoneous to real words (i.e. BRANE).
- see notes for table
The complex correspondences between a group of letters and one individual sound (i.e., complex graphemes; e.g., IGH read ‘ai’) are acquired late in reading development.
Their less secured knowledge is therefore the first to go, compared to simple graphemes (e.g., P read ‘p’).
But sound similarity to existing words improves reading performance on nonwords (e.g., BRANE).
This is because whole-word knowledge of an existing homophone can compensate for the failure to apply correspondence rules.
DRC explains the poor performance on pseudowords by an impairment of the grapho-phonological conversion mechanism.
- see notes
- Can read anything acquired before the damage but can’t apply previous knowledge onto new strings
deep dyslexia (Marshall and Newcombe, 1973)
Widest impairment of reading
Associated with extensive damage to the dominant (L) hemisphere, and consists of:
Impaired ability to read nonwords.
Semantic errors: ill > ‘sick’, bush > ‘tree’, bad > ‘liar’
Visual errors: life >’wife’, sword >’words’ – perception not necessarily bad
Derivational errors: card > ‘cards’, fleeing > ‘flee’, entertain > ‘entertainment’, beg > ‘beggar’ – grammatical errors
Effect of syntactical class:
- nouns > adjectives > verbs > functors
Effect of imageability: concrete > abstract words
Read via semantic route
Deep dyslexia appears to be one step further compared to phonological dyslexia.
The impaired ability to read nonwords and sound out letters is present, but this is only one amongst a constellation of symptoms.
Semantic errors are perhaps the most intriguing errors, as the word that is pronounced bears no visual or sound resemblance to the target word.
Visual errors could be true perceptual errors, but they could also be the result of a compensation by lexical knowledge for a imprecise grapheme-phoneme conversion.
Effect of syntactical class and of imageability are likely to be one and the same thing, as verbs and functors are usually more abstract than adjectives and nouns.
These effects are also shared with phonological dyslexia (see earlier slides for explanations).
see notes
Most common explanation: reading only by the semantic lexical route, all other routes —nonsemantic lexical and graphophonological— being impaired.
- see notes
- The interpretation of this symptomatology in DRC would implicate a damage to both the nonlexical route and the direct (nonsemantic) lexical route: the only way to retrieve pronunciation is via the semantic system.
- Abstract words poorly read compared to concrete words
- Know category of words, but don’t know which one – e.g. furniture – chair/table
