Dyslexia

Question 1

What are the prominent theories of developmental dyslexia?

Answer 1

Phonological deficit hypothesis (Snowling, 1998)

Magnocellular hypothesis (Stein, 2001)

Cerebellar deficit hypothesis (Nicolson & Fawcett, 1990)

Question 2

What are the main assumptions of the Phonological Deficit Hypothesis?

Answer 2

• Suggests individuals have difficulties establishing, storing, and accessing adequate phonological representations
• Children with dyslexia perform poorly on tests that involve phonological awareness
• Rhyme (MacLean et al., 1987)
• Aliteration (Bryant et al., 1990)
• Repeating nonsense words (Snowling, 1981)
• Rapid picture naming (Denckla and Rudel, 1976)
• Word segmentation (Snowling et al., 1986)

Question 3

What have neuroimaging studies on the phonological deficit found?

Answer 3

• Differences in language areas of the brain

- Habib (2000) - reviewed 14 neuroimaging studies of dyslexia and concluded that key deficit was a phonological one

Question 4

What are the main assumptions of the Magnocellular Deficit Hypothesis?

Answer 4

• Explanation at the biological level suggesting that the magnocellular is impaired
• Magnocellular pathway carries information from large retinal ganglion cells through LGN to visual cortex
• This pathway helps control eye movements and is sensitive to motion - it is crucial in the direction of visual attention

Question 5

What have neuroimaging studies on the magnocellular deficit found?

Answer 5

• Neuroimaging has suggested magnocellular abnormalities in the LGN in dyslexic individuals
• Mechanism thought to depend on rapid dynamics of neuronal ion channels in turn dependent on normal processing of essential fatty acids

Question 6

What are the auditory assumptions of the magnocellular deficit?

Answer 6

• However, there are also magnocellular regions within the auditory pathway - dysfunction here would lead to an impaired ability to pick up changes in sound frequency and amplitude
• This may explain both the visual and auditory deficits in dyslexics
• In both cases the impairment is one of temporal resolution
• Wolff (1993) - children with dyslexia show deficits on timing tasks

Question 7

What are the main assumptions of the Cerebellar Deficit Hypothesis?

Answer 7

• This proposes that dyslexic individuals display a difficulty in automatising tasks
• Theory proposed due to the observation that the balance of children with dyslexia were poorer than that of controls when a 2nd task was introduced (dual-tasking) - It was concluded that dyslexic children had difficulties with the automatisation of the primary task, so the 2nd task ‘overloaded’ the processing capacity
• Nicolson and Fawcett (1990) argue that the difficulties with reading and spelling may be due to a general inability to automatise skills
• To account for this they propose an underlying low level impairment of the cerebellum - try to find level of explanation at both cognitive and biological levels
• Tested children with dyslexia on a range of motor tasks and found they were generally impaired on these

Question 8

What have neuroimaging studies on the cerebellar deficit found?

Answer 8

• Finch, Nicolson and Fawcett (2002) - found abnormalities in the cerebellar of dyslexic individuals
• Rae et al (1998) - metabolic abnormalities in this area

Question 9

What are the main points when evaluating the Phonological Deficit Hypothesis?

Answer 9

• Identifies underlying cognitive deficit as difficulties with phonological representations
• Some studies have found poor phonological awareness in all participants - appears to be a ‘core deficit’
• Makes no claims about the cause of the problem so lacks explanatory power at Morton/Friths biological level- leaves open the possibility that different biological deficits could underpin the cognitive one
• Intervention cannot target any underlying cause but can address the cognitive deficit
• Programme would be based on improving phonological representation
• This type of training is increasingly found in school intervention programmes

Question 10

What are the main points when evaluating the Magnocellular Deficit Hypothesis?

Answer 10

• Suggests a visuo-spatial processing deficit (and possibly auditory one) due to magnocellular impairment - explanatory power at both the cognitive and biological level
• May link to specific problems of visual resolution found in Irlen syndrome
• Leads to impairment in temporal processing
• NB - Magnocellular deficits seen in only a small subset of dyslexic individuals (Ramus et al., 2003)
• Conversely, Skoyles and Skutton (2004) note that there are probably more individuals without dyslexia who have a magnocellular deficit than individuals with dyslexia
• Throws doubt on the notion of a magnocellular deficit as an underlying cause of dyslexia

Question 11

What are some possible interventions for Magnocellular deficit dyslexia?

Answer 11

• Training in temporal processing
• Overy (2003) reports that the use of singing and rhythm games with children with dyslexia - Show improvements in phonological skills and spelling but not reading
• Essential Fatty Acids supplement
• Cyhlarova et al (2007) reading ability correlated with fatty acid metabolism in adults with and without dyslexia

Question 12

What are the main points when evaluating the Cerebellar Deficit Hypothesis?

Answer 12

• Involved in balance, sequence learning, timing, implicit learning, anticipation of error
• Overall, the cerebellum seems specialised for associative learning

Question 13

Is the cerebellum involved in speech?

Answer 13

Neuroimaging suggest that it is -

• FMRI studies show that different areas of the cerebellum are activated in different verbal tasks even though motor tasks are similar (Fulbright et al., 1999)
• Cerebellar damage affects phonemic but not semantic rule performance - tested by asking patients to generate as many words as possible beginning with a specific letter as opposed to as many as possible in specific categories (Leggio et al., 2000)
• So impairment of the cerebellum seems like a potential candidate for an underlying biological deficit in dyslexia

Question 14

What do Fawcett and Nicolson suggest?

Answer 14

Fawcett and Nicolson suggest that dyslexic children exhibit difficulties reminiscent of adults with cerebellar deficits’

• Difficulties with temporal estimation
• Balance
• Muscle tone
• Coordination

Question 15

Evaluation and Research

Answer 15

• A number of studies have failed to demonstrate much support for the cerebellar deficit hypothesis and many authors are dubious about the conclusions drawn by proponents of the theory
• Pope (2004) 53 dyslexic boys and control - and found any cerebellar impairment could equally be attributed to ADHD symptoms
• Wimmer et al (1999) - Balance impairments only found in children with comorbid ADHD
• Ramus et al (2003) investigated 3 theories with 16 dyslexics and found;
• 16 suffered from phonological deficit
• 5 ONLY from phonological deficit
• 10 from an auditory deficit
• 4 from a motor deficit
• 2 from visual magnocellular deficit
• The motor problems that were found did not look particularly ‘cerebellar’ in origin
• Bishop (2002) - suggest 2 other possibilities;
• Cerebellar deficit might be a non specific marker for neurodevelopment abnormality
• Cerebellar deficit may be a consequence, not a cause of dyslexia