A twin study involving 457 pairs has found that ADHD on its own was associated with a reduced ability to inhibit responses to stimuli, while reading disabilities were associated independently with weaknesses on measures of phoneme awareness, verbal reasoning, and working memory. Both disorders were associated with a slow processing speed, and there was a significant genetic correlation between RD and ADHD.
However, just to remind us that genetics are rarely solely the answer, another twin study, involving 271 pairs of 10-year-old identical and fraternal twins, has found evidence that the associations between ADHD symptoms, reading outcomes and math outcomes are a product of both genetic and common environmental influences. The researchers speculate that such environmental influences may include aspects of the classroom and homework environment.
While brain laterality exists widely among animal species, the strong dominance of right-handedness in humans is something of an anomaly. As this implies a left-hemisphere dominance for motor function, it’s been suggested that the evolution of language (also mainly a function of the left hemisphere) may be behind the right-handed bias, leading to a search for a connection between hand preference and language disorders. To date, no convincing evidence has been found.
However, a genetic study of 192 dyslexic children has now revealed a strong link between a variant of a gene called PCSK6 and relative hand skill in these children. Specifically, those who carried the variant in PCSK6 were, on average, more skilled with their right hand compared to the left than those not carrying the variant. However, among the general population, this gene variant is associated with less right-hand skill.
The findings provide evidence for a link between brain lateralization and dyslexia. The gene’s protein is known to interact with another protein (NODAL) that plays a key role in establishing left-right asymmetry early in embryonic development, suggesting that the gene may affect the initial left-right patterning of the embryo, with consequences for cerebral lateralization.
The ongoing 12-year Connecticut Longitudinal Study, involving a representative sample of 445 schoolchildren, has found that in typical readers, IQ and reading not only track together, but also influence each other over time. But in children with dyslexia, IQ and reading are not linked over time and do not influence one another. Although this difference has been assumed, this is the first direct evidence for it. It should also be noted that the language problem is not confined to reading: those with dyslexia take a long time to retrieve words, so they might not speak or read as fluidly as others.
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Unraveling the roots of dyslexia
There is some evidence that dyslexia is distinguished by a basic deficit in phonological processing, characterized by difficulties in segmenting spoken words into their minimally discernable speech segments (speech sounds, or phonemes). A new study investigating brain activity of dyslexics and normal adult readers when presented with letters, speech sounds, or a matching or non-matching combination of the two, has revealed that dyslexic adults showed lower activation of the superior temporal cortex when needing to integrate letter and speech sounds. The findings point to reading failure being caused by a neural deficit in integrating letters with their speech sounds.
Blau, V. et al. 2009. Reduced Neural Integration of Letters and Speech Sounds Links Phonological and Reading Deficits in Adult Dyslexia. Current Biology, 19 (6), 503-508.
Chinese and English dyslexias stem from different brain abnormalities.
Dyslexia involves impairment in connecting the sight and sound of a word. In English, this is commonly seen in transpositions of letters, while in Chinese, the problem can affect how a person converts a symbol into both sound and meaning. Following an earlier study in which the brain areas involved in dyslexia were found to be different for English and Chinese readers, a new technique has confirmed and clarified the results. Chinese children with dyslexia had a significantly smaller left middle frontal gyrus than did Chinese children without the disorder, even though both groups had the same overall volume of gray matter. Intriguingly, this area is not associated with symbol recognition, but with working memory. Earlier research has found English-speaking dyslexics have less gray matter in the left parietal region. The findings also suggest that dyslexics in one language will probably not be dyslexic in the other.
New evidence for the cause of dyslexia
A new study casts new light on the cause of dyslexia. Recent research has tended to focus on the magnocellular (M) pathway, one of two visual pathways in the brain that processes motion and brightness. The other visual channel, the parvocellular (P) pathway, processes detail and color. Although some studies have implicated an impaired M channel, showing that dyslexic children have trouble seeing rapidly changing or moving stimuli, results have been inconsistent. A new study suggests that the problem is rather a more general problem in basic sensory perception — an inability to shut out “noise”, that is, the sounds and patterns surrounding the target information.
Dyslexia doesn't have a universal biological cause
While most of the latest research focuses on the biological causes of dyslexia, a new study reveals that the disorder affects the brains of Chinese and English speakers differently, suggesting that the neural basis of reading differs depending on the nature of the writing system. The findings have enormous implications for helping impaired readers in China, where 2% to 7% of children are dyslexic. The study also highlights the importance of paying attention to differences in languages, even languages as similar as English and Italian. It has been shown that the degree of impairment when reading can differ depending on the language.
Dyslexics have less gray matter in the brain's language centers
A new imaging study involving people with a family history of dyslexia confirms earlier research suggesting dyslexics have a significant reduction of gray matter in centers associated with language processing. The study lends credence to earlier studies that suggested intensive reading therapy activates parts of the brain needed for decoding words.
Brain development and puberty may be key factors in learning disorders
New research suggests that the brains of children with learning problems not only appear to develop more slowly than those of their unaffected counterparts but also actually may stop developing around the time of puberty's onset. In the study, children with impairments started out about three years behind, but their rate of improvement was very similar to that of the children without impairments — until around 10 years, when further development in the children with learning problems stopped. The researchers suggest that delayed brain development and its interaction with puberty may be key factors contributing to language-based learning disabilities such as dyslexia. This hypothesis suggests a completely new approach to the study of learning problems. It also points to the importance of early intervention.
Sensory processing different in people with dyslexia
An imaging study of dyslexics has found that dyslexic readers appear to process auditory and visual sensory cues differently than do normal readers. In the study 30 dyslexic readers and 30 normal readers performed three matching tasks — an auditory task, a visual task and a multisensory task — involving consonant-vowel pairs. During the auditory matching task the dyslexic readers showed increased activity in the visual pathway of the brain, while that same region deactivated in the normal readers. The dyslexic readers' increased activation patterns in the visual pathway corresponded to poorer performance on the matching task.
Burdette, J.H., Laurienti, P.J., Flowers, L., Kraft, R., Maldjian, J. & Wood, F.B. 2003. Report presented at the 89th Scientific Assembly and Annual Meeting of the Radiological Society of North America (RSNA).
Imaging study confirms theory of dyslexia
Functional magnetic resonance imaging (fMRI) has confirmed part of an eighty-year-old theory on the neurobiological basis of dyslexia. Dr Orton theorized that normally developing readers learn to suppress the visual images reported by the right hemisphere of the brain because these images potentially interfere with input from the left. The imaging study found that children do in fact "turn off" the right side of the visual parts of the brain as they become accomplished readers, and also demonstrated that different phonological skills relate to activity in different parts of the brain when children read. This observation lends support to the theory that there may be several neurobiological profiles that correspond to different subtypes of dyslexia, each associated with varying deficits in one or more of these different phonological skills.