Learning disabilities, such as dyslexia, can be caused by premature birth or asphyxia at a younger age.
In other cases, the direct cause of a learning disability is not known. (Ellen B. Senisi/Photo
Researchers, Inc.)the prevalence of arithmetic LDs ranges from 1 per-
cent to 6 percent, it is not clear whether weak mathe-
matics performance is due to the quality of instruction
or an actual LD.
Nonverbal LDs are often overlooked, occur less
frequently than reading disorders, and are char-
acterized by problems in arithmetic computation,
graphomotor skills, reading comprehension, math
reasoning, science, complex concept formation, visu-
al memory, and social-behavioral skills; these are
often found in children with white-matter disorders,
and are assumed to be more right-hemisphere-based.
As of the late 1990s, a classification schema (based on
reading disability/dyslexia research) was applied to all
achievement domains included in federal and state
definitions of LD. Three major types of LDs were
identified: specific language impairment, specific
reading disability/dyslexia, and specific math disabili-
ty.
The area of greatest knowledge is reading disor-
ders. These fall into two main groupings: phonologi-
cal (dysphonetic) and orthographic (dyseidetic). The
former is more prevalent and is characterized by defi-
cits in decoding and word analysis, with guesses made
based on the initial letter of the word and misspell-
ings being phonetically inaccurate. Shaywitz wrote in
1998 that a deficit in basic phonemic awareness (in-
ability to segment phonemes [the smallest unit of
sound] into phonological units) is the underlying
cause in virtually all cases of dyslexia. The ortho-
graphic reading disability subtype involves an inabili-
ty to develop a memory for the whole word (gestalt),
with visuospatial reversals occurring (e.g., ‘‘was’’ read
as ‘‘saw’’) and misspellings being phonetically accu-
rate. There also is a mixed reading disorder, which
consists of characteristics found in both types of defi-
cit. The major new finding is that reading disabilities
are more strongly associated with auditory rather
than visual deficits.Causes and Diagnosis
With regard to causes, research extends to family,
genetic, and neuroanatomic bases, with most work
being done in language and reading disabilities.
There appears to be heritability in language and
reading LDs, with similar LDs being found in 35 per-
cent to 45 percent of first-degree relatives. Also, iden-
tical twins are more likely to have similar LDs than
fraternal twins. Chromosomes 6 and 15 have been im-
plicated frequently as possible genetic causes of LDs.
Neuroimaging techniques, such as functional mag-
netic resonance imaging, have documented differ-
ences among dyslexic and nonreading-disabled
individuals. Studies have found that in individuals
with dyslexia, certain areas of the brain are different
than in individuals without dyslexia. Nonetheless,
there are no neuroanatomic or neuroelectric diagnos-
tic tests that identify LDs in the brain. Although LDs
are found more frequently in children subject to brain
insult (such as premature birth and asphyxia), many
children display LDs without any identifiable cause.242 LEARNING DISABILITIES