[W6] - L6 Flashcards
Define Dyslexia
Dyslexia is a specific learning disability:
- Characterized by difficulties in single word decoding.
- Unexpected in relation to age or other cognitive abilities and the
provision of effective classroom instruction. - Typically resulting from a deficit in the phonological component of
language. - A language-based disorder of neurological origin.
[Composed of behavioural, cognitive, and neurobiological elements]
The Four Levels of Causation for Developmental Disorders
- Aetiology (genetic make-up/environment)
- Neurobiology.
- Cognition.
- Behavioral Symptoms.
[All levels interrelate downwards, with a link between behavior and the environment as well]
Early Single Cause Theories of Dyslexia
Dyslexia was conceptualized around the 1980s as having a single cause; research focused on finding a “dyslexia gene”.
Based on a view that the relationships between the levels were unidirectional.
Example: Phonological Deficit Hypothesis.
Exploring the Genetic Components of Dyslexia
Dyslexia has a significant heritable component; a large number of small genetic deficits play a role, with a wide range of severity and subtypes (and it has a high co-morbidity with ADHD)
Amongst monozygotic twins the concordance is 68%, and for fraternal twins 38%. If one parent has dyslexia the odds for the child having it also are increased by a factor of 7 to 12.
No single gene, but candidate genes have been identified in a molecular network involved in neuronal migration and neurite outgrowth (10 out of the 14 candidate genes identified are relevant in regulating neurite outgrowth - 45 out of 85 for ADHD)
Exploring the Neurobiological Components of Dyslexia
Neurons of relevance to dyslexia:
o Magnocels: Neurons with many connections, involved in fast information processing.
o Pyramidal cells: Typical to the motor system.
One of the causes of dyslexia is ectopia; certain cells migrate too far, such that they are no longer involved in their original network [note that neurons do typically migrate to outer layers during gestation (7 – 11 weeks) - and gestation can be influenced negatively by maternal alcohol use, medication, environmental toxins (lead) etc., - and stimulation at critical periods later in life is also important (attachment, social interaction, language/motor development etc.)]
In conclusion, dyslexia has been linked with early neurological maturation and subtle deficits in many areas (e.g., cerebellum)
Exploring the Cognitive Components of Dyslexia
One of the most reported cognitive factors of relevance is phonological insights. Phonological skills can be tested with the Foneem-Deletietest, or with auditory discrimination/categorization tests (POP vs. KOP – choose p or k).
Speech continuums are presented in which sounds gradually change – varying frequencies. At risk (for dyslexia) children and SLI-children show a less steep identification curve than control children do, indicating Poorer Phonemic Categorization.
Some children with dyslexia also struggle to discriminate between auditory patterns that are not based in speech. (tap tap tap versus tap… tap)
Rapid automatized naming (RAN) task: presented with sheet of colours and objects and the goal is to rapidly retrieve and name the presented stimuli. Some are more challenging (i.e., stroop variation - naming the colour of the ink, ignoring the written word)
Phonological knowledge/skill is a core ability. Reading is about the processing of language (i.e., its graphemes and phonemes)
Secondary language (writing) is built on primary language (speaking /listening) – but primary language is not affected by dyslexia.
Dyslexia features other subtle cognitive hindrances in rapid automatized naming; automatization, verbal memory, motor skills (cerebellum).
The severity of an individual’s cognitive deficits AND their external development determine their reading skill. There can be large variations in severity.
Exploring the Behavioral Components of Dyslexia
A diagnosis of dyslexia is purely behavioural based (even though there are causative factors at other levels) – it’s a persistent reading and spelling disorder with a basis in neurobiological elements that can not be explained by other factors [reading skill is measured via speed and accuracy]
Reading ability is a multifactorial concept – and therefore likely to be normally distributed. Some research suggests that dyslexia might be representative of the lower tail of this normal distribution.
Ellis and Young’s Dual Route Model
A model of processing that influences all language skills (listening/reading)
[Lexical Route] - There is a direct route (through the middle of the model) for the processing of words you are reading that you have seen repeatedly (that you’re familiar with) – the word is analyzed visually and the analysis proceeds to visual input lexicon.
[Phonological Route] - If the word is not familiar to you, then after visual analysis the grapheme-phoneme conversion route must be taken (i.e., early readers, or the reading of irregular/pseudo-words). This route is slower than the direct route.
In typically developing children, the reading of “irregular” words improves with age (words without a one-to-one correspondence with phonemes/graphemes). Children with dyslexia are poorer at irregular word reading.
Surface Dyslexic Pattern: Reading of irregular words is poorer than that of non-words.
Phonological Dyslexic Pattern (slightly more common): Non-word reading skills are inferior to irregular word reading skills.
Some individuals with dyslexia can compensate for their reading difficulties by reading more and therefore building up their visual input lexicon and their ability to utilize the direct route!
Orthography and Granularity
English has one of the most inconsistent orthographies (incongruence between graphemes and phonemes – think about how ord is pronounced in word vs. lord – you need more information to disambiguate the pronunciation of that phoneme!)
The more inconsistent a language’s orthography, the more difficult it is to acquire the spelling system of that language. A greater consistency between phonemes and graphemes makes it easier to acquire a language.
Granularity = How many graphemes do you need to know to know how to pronounce a given phoneme – in Finnish it would be 1: direct correlation.
Geschwind’s Area and Dyslexia
The Ventral neural pathway (Occipito-Temporal) for reading represents the Lexical/Direct Route.
The Dorsal neural pathway (Tempero-Parietal) for reading represents the Phonological/Indirect Route.
The Anterior Region and Dyslexia
Anterior Region: Relevant for fine grained articulatory recoding
(i.e. output phonology - speech)
Facts:
* More active for pseudoword reading > word reading
* Sensitive to pronunciation regularity
RD vs NI:
* RD > NI in word and pseudoword reading
The tempero-parietal (dorsal) region and dyslexia
Function: Rule based analysis function - integrating orthographic,
phonological, lexical-semantic dimensions [indirect route]
Facts:
* More active for pseudoword reading > word reading
* Increased activation during phonological analysis
* Relatively late response
* Decreased activity with faster presentation rates
RD vs NI:
* NI > RD activity in word and pseudoword reading
* RD: compensatory activity in right hemisphere.
The occipito-temporal (ventral) region and Dyslexia
Function: Linguistically structured word-form area [direct route]
Facts:
* More activation for word reading > pseudoword reading
* Consistent activation across tasks
* early response (150 – 180 ms)
* increased activity with faster presentation rates
RD vs NI:
* NI > RD in word and pseudoword reading
* RD: compensation in right hemisphere
Fawcett and Nicolson’s Ontogenetic Causal Chain for Dyslexia
Originated from the observation that individuals with dyslexia experience difficulty with automatizing skills (more disruptions in balance while balancing and counting backwards [i.e., engaging in a secondary task] than the control group).
They posited that in some children dyslexia is caused by a cerebellar impairment - with secondary effects on balance/motor skills.
OR they could have a cortico-cerebellar loop - with secondary effects on automatization difficulties (i.e., with word recognition)
Hence, the evidence would suggest that the cerebellum is relevant to the emergence of dyslexia. Note, however, that training of the cerebellum in one area does not result in improved performance in another (i.e., training motor to improve reading).
The Simple View of Reading and Dyslexia
The simple view of reading holds that skill in decoding and comprehension must be acquired for
success in reading.
Reading and listening are similar – decoding written versus spoken language.
Therefore, reading disabilities can result from an inability to decode (dyslexia), an inability to comprehend (hyperlexia), or both (common/garden variety reading disability).
If either decoding or comprehension ability are zero then the overall ability is zero (one cannot compensate for the other – both are needed).
