the Brain's Reading Network
from Science News, 3/7/98
study indicates that reading requires the angular gyrus, a
brain structure that maintains connections to areas involved
with speech comprehension and the integration of alphabet
letters with their corresponding sounds.
disorder known as dyslexia may often reflect an inability
of the angular gyrus to work in concert with these related
brain regions, hold neuroscientist Barry Horowitz of the National
Institute on Aging in Bethesda, MD., and his coworkers.
team administered positron emissions tomography, or PET scans
to 17 dyslexic men, all of whom had longstanding reading difficulties
despite having I.Q.'s in the normal range, and 14 men who
read well. Brain imaging was performed as participants read
difficult nonsense words (such as "phalbap" in which
the "ph" would be pronounced as "f") and
real words with unusual pronunciations (such as "choir").
readers, these tasks induced simultaneous blood flow surges
on the left side of the angular gyrus and in several brain
areas that integrate the visual and linguistic information
needed for effective reading, the scientists report in the
July 21 Proceedings of the National Academy of Sciences.
The men with dyslexia exhibited increased activity in all
of these areas except one - the left angular gyrus.
findings complement evidence that dyslexia may stem from disturbances
in various parts of a brain network that begin working together
during childhood as a result of continued exposure to written
language, according to the researchers.
3/7/98, p. 150).
Down the Roots of Dyslexia
1998, Newsweek Magazine
are reading these words easily, it's because your brain's
visual cortex is seeing the shapes of the letters accurately,
and the association cortex is recognizing these marks as letters.
A clump of nerve cells called the angular gyrus, toward the
back of your brain, is then linking letters with sounds, and
the striate cortex farther back is figuring out what the words
you are struggling to read, chances are regions in the back
of your brain - particularly areas that break down a word
into its sounds and put them back together again - are underactive.
Or so researchers led by Sally Shaywitz of Yale University
found when they compared functional magnetic resonance imaging
(fMRI) of the brains of dyslexics with those of nonimpaired
readers. Dyslexia is marked by "significant differences
in brain activation patterns," concluded the team, reporting
its discovery in the Proceeding of the National Academy of
in brain activity support the idea that dyslexia stems from
a problem translating letters into sounds. They also offer
the strongest evidence to date that chronic reading problems,
often called a hidden disability because they haven't shown
up on X-rays or other tests, arise from real neurological
abnormalities. One day fMRI might be used to diagnose
dyslexia early, as every parent of a child at risk hopes.
Even more tantalizing, the scans might be used to track what
kind of intervention might restore brain activity to normal
in children who are slow readers. "If we treat children
effectively," says Shaywitz, "it may change their
brain organization." Those studies are now underway.
New England Journal of Medicine - Letters
Dec. 31, 1987 - Dorothy van den
the Editor: Geiger and Lettvin found that dyslexic
subjects recognize letters better when they are presented
eccentrically as compared with foveally, and these workers
attribute this effect to "an interaction between foveal
and peripheral vision that degrades the ability to read in
the foveal field." A more parsimonious explanation is
available, which also explains various other oddities in dyslexia.
The real villain here may well be a defective corpus callosum.
Foveal vision involves interhemispheric transfer of information.
Peripheral vision goes to one hemisphere only. Perhaps the
left hemisphere can learn to deal with direct input from the
peripheral vision more easily than it can make sense of double,
non-matching input from both sides.
callosum is suspect in dyslexia in other ways. The only documented
visual problems in dyslexia appear in function that require
quick and efficient transfer of information across the commissures:
smooth convergence, stable ocular dominance, smooth tracking,
and matched focusing in the near vision. In addition, persons
with dyslexia are poor at tactile localization, which also
requires accurate transfer of information across the corpus
callosum. The corpus callosum helps control the arousal level
of the hemispheres, enabling one to choose which side to use
during cognitive tasks. People with dyslexia rely inappropriately
on right-brain processing in language handling, which suggests
that they have insufficient callosal management.
effective way to treat dyslexic persons is to send verbal
tasks to one hemisphere while distracting the other with qualitatively
different input. This supplies the missing focus and bypasses
the corpus callosum and its degraded secondary signals. All
this suggests that the time is ripe for a thorough study of
the role of the corpus callosum in dyslexia.