Much interest has focused on reading difficulties in children with at least average intelligence and without "extraneous factors" accounting for their learning problems (sensory acuity deficits, socioeconomic disadvantage, severe emotional-behavioral disorders)--"exclusionary criteria"

They tend to have difficulties in learning basic reading subskills

word identification

phonological (letter-sound) decoding

Vellutino, Fletcher, Snowling, & Scanion (2004) estimate that 10% to 15% of school age children have such problems

word identification: learning to decode print

phonological awareness: recognition that spoken words are built of individual speech sounds (phonemes) and combinations of these sounds (syllables)

phonological coding: using speech codes to represent information in the form of words and parts of words

problem with phonological memory

phonological analysis and alphabetic mapping

orthographic awareness: how letters in written words are organized

poor readers are usually (not always) poor spellers

deficits in general learning abilities have not been supported in terms of explaining dyslexia

Intelligence and reading

deficits in visual perception and visual memory have not been supported in terms of explaining dyslexia

continuing issues and questions

These results primarily pertain to languages such as English

English is described as having an opaque (deep) orthography: many inconsistencies and exceptions in the relationships between letters and sounds

other languages (e.g., German or Italian) show transparent (shallow) orthographies: more consistent mapping between letters and sounds

rhyming skills predicts reading achievement in English but not German or Dutch (where rapid naming accounts for more variance in reading ability)

reading in non-alphabetic languages (Chinese, Japanese) may be associated with multiple deficits, rather than a core phonological deficit

Etiological variables

Neurobiological factors: possible constitutional factors intrinsic to the child

brain structure: left hemisphere

brain function: left hemisphere

genetics

parental history of reading difficulties increases risk of dyslexia approximately 8 times

various reports find 25%-60% of parents of children showing dyslexia having reading problems

concordance rates for identical twins typically greater than 80% for monozygotic twins and below 50% for dizygotic twins

approximately 50-60% of the variance in reading achievement can be accounted for by genetic factors

chromosome 6, 15, 1, 2

genes:

DCDC2 on chromosome 6 has been found to be highly expressed in brain regions used during times of reading, affects neuron migration during development in fetal rats, and about 17% of dyslexics are missing a short stretch of DNA within DCDC2--everyone with this deleltion had dyslexia (Science, 2005, v. 310, p. 759); but, Monaco & Williams failed to find association between DCDC2 and dyslexia in their British population samples.

ROBO1 on chromosome 3 has been linked to dyslexia: reduced ROBO1 activity in 21 dyslexic individuals from large Finnish family, fruit fly version helps shape neural connections between two hemispheres of brain (Science, 2005, v. 310, p. 759)

KIAA0319, a third candidate dyslexia gene, may play role in brain development (Science, 2005, v. 310, p. 759)

environmental interaction

risk for dyslexia appears to be best conceptualized as "continuous", rather an "all or none"

Environmental factors

pre-reading experience

inadequate instruction

Most consistently demonstrated remedial factor has been the amount of instruction in skill area

The method of identification of "dyslexic children", "poor readers", "children with specific reading disabilities" strongly influences the obtained sample and characteristics thereof

Skill discrepancy

Ability discrepancy

Statistical discrepancy

Response to experimental curriculum

Population

Prevalence: Reading problems are the most commonly identified learning disabilities in school age children

Sex Ration: More males than females are identified in most reports

Course: Varies most clearly with severity

milder cases may "disappear" for formal identification

severe cases appear to be more stable

Correlates:

Family histories of academic difficulties
Histories of early speech problems
Histories of early ear infections
Phonological processing difficulties

Comorbidity: often associated with other problems

ADD/ADHD

At least part of the comorbidity of Reading Disorder and ADHD appears related to pleiotropic effects of a quantitative trait locus (QTL) on chromosome 6p

pleiotropic: the same genes affect more than one phenotype

: producing more than one genetic effect; specifically : having multiple phenotypic expressions

behavior problems (ODD/Conduct Disorder)

substance use/abuse

Subtyping: highly controversal

Instruction