Specific Learning Disability (SLD)
Learning disorders (LD) are the most frequently diagnosed disability in the US. Depending on reference: 4% of all school aged children. 1% of college freshman are self-identified as having an LD, or 7-15 percent of general school population. LD is a general term that refers to heterogeneous group of disorders manifested by significant difficulties with acquisition and use of listening, speaking, reading, writing, reasoning, or mathematical abilities.
Five factors are typically seen in the definition of the various LDs (including Reading, Mathematics, Disorder of Written Expression):
1) Failure to Achieve
2) Disorder of the psychological or cognitive processes – Attention, Learning, Concentration, Understanding and using oral/written language, conceptualization, information processing.
3) Exclusionary Criteria: visual/hearing/perceptual problems, mental retardation, emotional disturbance, environmental, cultural, or economic disadvantages
4) Etiology – Neurological, Brain injury, Delayed speech and language development, perinatal difficulties
5) Severe discrepancy between intellectual ability and achievement
LD are far more common in males, perhaps due to delayed myelination in left hemisphere. In girls, the left hemisphere typically myelinates faster, while boys it is right hemisphere.
Co-occuring disorders include Developmental Coordination Disorder (a motor skill disorder), and Communication Disorders (i.e., Expressive Language Disorder, Phonological Disorder (AKA Developmental Articulation Disorder), Stuttering, Attention Deficit Hyperactivity Disorder (ADHD), and Communication NOS.
Dyslexia (Reading Disorder).
The prevalence for all forms of dyslexia is high (20% of all US population or 3% to 6% of school-age children). Of those diagnosed, there are 50% more boys than girls but study showed that if research criteria were used, there were no gender differences. School referrals may be biased by confusion between achievement and behavioral problems. Types of Dyslexia are as follows:
- Developmental dyslexia refers to difficulties learning to read despite conventional instruction, adequate intelligence, and SES opportunity.
- Primary or specific dyslexia refers to reading disability of constitutional origin while acquired dyslexia refers to disorder resulting from cerebral damage e.g., perinatal, childhood brain damage. Deep dyslexia is adult form of dyslexia involving paralexia in which person makes semantic errors in single-word reading (e.g., inch as ruler). They have poor visual-semantic associations.
- Surface Dyslexics (may also be referred as “primary”): Over-rely on phonological rules, struggle with visual features of unfamiliar words, and exhibit impaired comprehension.
No one knows what causes dyslexia. Theories include postulating the cause for dyslexia include the possibility of incomplete or mixed cerebral dominance. That is, not from problems with visual images, but rather a developmental lag in left hemisphere development. Another theory states there is minimal brain damage in dyslexia because. Normal readers have asymmetric planum temporale with the left being longer and larger than the right. Dyslexics have symmetrical planum temporale. Moreover, minor cortical malformations, abnormal symmetry, and glial scarring in the perisylvian cortex are seen in dyslexics. Differences may also be shown in the anterior speech regions, auditory cortex, inferior parietal lobe, posterior thalamus. The Lateral Geniculate Bodies of the Thalamus were also shown to be less organized and smaller in dyslexics than normals. MRIs have indicated symmetry or reversed hemeispheric symmetry in dyslexics and smaller insular regions.
Genetics may also play a role, some forms of dyslexia may be autosomal dominant; there is some supportive evidence from twin studies and family history.
There is a link between dyslexia, left-handedness, and autoimmune and allergic disorders, which is correlated to prenatal testosterone exposure in utero.
Finally, perinatal stress such as premature birth, maternal smoking, birth weight, pregnancy and birth complications may also result in dyslexia. Hypoxic children in general show more general learning deficits.
The most reliable predictors of dyslexia is speech and language delays after the first year of life.
In general deficiencies in auditory and phonological processing that interferes with the recoding of written to spoken language has been described as the primary deficit underlying reading disability. However, dyslexia can be the result of left and right hemisphere deficits. These include:
- Left Hemisphere: auditory discrimination deficits, phonological coding problems, difficulties with morphonemic processing.
- Right Hemisphere: inadequate maturation of perceptual system, disturbance of visual perception, disturbance of eye movements.
Subtypes of Primary Dyslexia
- Dysphonetic reading disability – little understanding of letter-sound relationships
- Dyseidetic reading disability – inability to read words as a whole
- Dsyphonetic-dyseidedetic – problems with both
According to Bakker and Vinke’s “balance model,” in the early stages of learning to read a child uses a right hemisphere strategy that emphasis perceptual abilities but this shifts to a presumably faster more efficient linguistic strategy in normal reading development. Dyslexia can occur is over-reliance on either form develops.
- L-type dyslexia – too much on linguistic strategy. These individuals have speech mediated in left hemisphere but show weak right hemisphere specialization They read quickly but miss perceptual features and make substitution errors.
- P-type dyslexia – too much on perceptual strategy. These individuals show overdevelopment of right hemisphere function and depression of left-hemisphere mediated linguistic capabilities. They read slowly and are sensitive to perceptual features of script.
There are clear Neuropsychological aspects of dyslexia including subtle dysnomia (slow naming access speed individuals use on rapid naming tasks or to inadequate naming), a salient feature of children with dyslexia. These naming problems translated to other natural speech situations such as retelling stories. Other neuropsychological features of dyslexia include greater difficulty retrieving words, poorer verbal memory than visual memory, delayed processing speed, verbal dysfluency, poor verbal learning, and conceptual problem solving.
Dyscalculia or other Arithmetic Disorders
Epidemiology shows about 6% of school aged children with some form of dyscalculia. There is string evidence that dyscalculia is a brain disorder, likely tied to visual-perceptual and visuospatial deficits (right hemisphere) (e.g., reversal of numbers or signs) or language based with difficulties with comprehension of verbal problems (left hemisphere) (e.g., difficulty understanding word sequence). Indeed, 89% of pts with Right sided Temporal – Occipital lesions will have dyscalculia.
Neuropsychological Aspects of dyscalculia may include:
- Poor tactile perceptual tasks, bilaterally, delayed psychomotor abilities, and difficulties with sptial organizational and analysis skills.
- Spatial organization problems
- Inadequate attention to visual details
- Sequential difficulties in process
- Mental flexibility problems (set shifting)
- Poor graphomotor technique
- Compromised storage and retrieval of number facts
- Poor number logic
- Relationship to verbal attention and processing cannot be overlooked and probably play large roles in the presentation of dyscalculia.
Dysgraphia and other Writing Disorders
Disorders of written expression often co-occur with dyslexia and dysphonetic spelling disorders Unfortunately, there is great paucity of research, especially when compared to research conducted with dyslexia. Problems with Dysgraphia include:
- Graphomotor aspects (legibility, rate of production, spatial presentation, punctuation, capitalization, poor fine motor coordination)
- Phonological-linguistic features (grammar, sentence formulation, and spelling)
- Or both…
More complex abstract-imaginative writing (character and plot development, moral themes) may also be affected. This might not be evident right away since this is the highest stage of written language development, rarely sen in children. Semantic Dysgraphia may not be able to utilized metaphors and analogies, comprehension of words and events – may have compromised frontal lobes. These problems typically are not seen in the early school years and only manifest in later years, keeping them from developing to their full potential during later education.
Children may be incorrectly diagnosed as dyslexic but later make improvements in reading but continue to show dysgraphia. Dysgraphia brain pathology is similar to dyslexia.
Adult Learning Disabilities
Overall, learning disabilities seem to be persistent over the life span. Adults can show the same learning deficits seen in children. Subtypes of LDs that are used in children have been shown to be valid in adults. Adults with LD have high rates of depression, unemployment, and lower levels of educational and occupational attainment. Emotional problems are also common.