Etiology

There is strong empirical support for a genetic basis of Reading Disorder or
dyslexia from behavior genetic studies. John C. DeFries and his colleagues
indicate that heredity can account for as much as 60 percent of the variance
in Reading Disorders or dyslexia. As for the exact mode of genetic transmission,
Lon R. Cardon and his collaborators, in two behavior genetic studies,
identified chromosome 6 as a possible quantitative trait locus for a predisposition
to develop Reading Disorder. The possibility that transmission occurs
through a subtle brain dysfunction rather than autosomal dominance has
been explored by Bruce Pennington and others.

The neurophysiological basis of Reading Disorders has been explored in
studies of central nervous dysfunction or faulty development of cerebral
dominance. The hypothesized role of central nervous dysfunction has been
difficult to verify despite observations that many children with learning disorders
had a history of prenatal and perinatal complications, neurological
soft signs, and electroencephalograph abnormalities. In 1925, neurologist
Samuel T. Orton hypothesized that Reading Disorder or dyslexia results
from failure to establish hemispheric dominance between the two halves of
the brain. Research has yielded inconsistent support for Orton’s hypothesis
and its reformulation, the progressive lateralization hypothesis. However,
autopsy findings of cellular abnormalities in the left hemispheres of dyslexics
that were confirmed in brain imaging studies of live human subjects have
reinvigorated researchers. These new directions are pursued in studies using
sophisticated brain imaging technology.

Genetic and neurophysiological factors do not directly cause problems in
learning the academic skills. Rather, they affect development of neuropsychological,
information-processing, linguistic, or communication abilities,
producing difficulties or deficits that lead to learning problems. The most promising finding from research on process and ability deficits concerns
phonological processing—the ability to use phonological information (the
phonemes or speech sounds of one’s language)—in processing oral and
written language. Two types of phonological processing, phonological
awareness and phonological memory (encoding or retrieval), have been
studied extensively. Based on correlational and experimental data, there is
an emerging consensus that a deficit in phonological processing is the basis
of reading disorder in a majority of cases.