The Corpus Callosum the Missing Link in Dyslexia?
Dorothy van den Honert
dyslexia and learning -disability have been used interchangeably
and misused for mild retardation and ADD so often that it
behooves a writer on the subject to start by defining his
terms. Dyslexia, etymologically, merely means poor reading.
However, by now the term has come to mean a specific type
of poor reading which, in a shortened form of the official
definition, is present in a person of normal or better intelligence
who has no medical, psychological, or socio-economic condition
sufficiently acute to account for the deficit in reading.
The specific peculiarities that characterize dyslectic reading
are as follows:
with sound-symbol matching 2,22,25,30,32
or inappropriate insertion of functors
of syllables in multisyllable words
phonetic decoding skills
comprehension in oral and silent reading
letter-sequencing in spelling and reading
or miscalling syntactical endings such as -ed, or -s
of phonemic awareness
semantic criterior for phonetic criterior: ripped
for torn, large for big, etc.
of similar-looking abstract words, such as form and from
kinds of reading mistakes are peculiar to dyslexia. they are
not made routinely by children who are merely slow, disinterested
in reading, emotionally disturbed, or undereducated.
dyslectic person a variety of non-reading problems, co-occur
with poor reading. These are also problems for which no other
cause such as retardation, social or medical factors, is apparently
present. These include:
remembering ordered lists, such as the months of the year
remembering a group of unrelated facts like the multiplication
in tactile localization skills 14
slow interhemispheric transfer time
distractibility, "twitchiness" and hyperactivity
with fine motor control in handwriting
problems associated with motor control of the eyes: lack
of smooth tracking, lack of smooth convergence, unstable
ocular dominance, intermittent mismatched focusing and poor
stereopsis at the midline.
the findings from many years laboratory of work that the dyslectic
person does not properly utilize his left, language areas
of the brain during reading can account for his reading problems.
Dyslexia has a strong auditory component stemming from the
inability to match sounds with their corresponding letters
or separate a word into its auditory components, both specialties
of the angular gyrus in the left hemisphere. Indeed, some
of the most recent work shown that dyslectic individuals do
not use the angular gyrus when reading.
32,17 This fact helps explain why Enhanced Lateralization,
the technique by which the left language area is forced to
work by having the right distracted and out of the processing,
improves reading dramatically. 35,36,37
to account for the equally strong visual motor control, and
attentional problems that have nothing to do with sound-symbol
matching? Probably the most parsimonious explanation is that
the problem is one of poor cerebral organization.
section of the brain which is intimately involved in cerebral
organization, both during growth and all through adulthood,
is the corpus callosum. 23
This thick bridge of neural tissue in the middle of
the brain connects the two hemispheres, conveying information
from one side to the other. Far more than being a mere information
carrier, however, it seems to take an active role from infancy
in directing the development of the brain into the highly
lateralized organ it is. The function of the corpus callosum
during cognitive activity seems to be one of maintaining the
balance of arousal and attention between the two sides that
enables each side to contribute its part to achieve an integrated
whole. Thus it allocates each kind of processing to the area
of the brain which is programmed for the job, controls arousal
and the distribution of attention over the two hemispheres
and enables sustained attention during complex cognitive tasks.
callosum is also involved in the control of certain kinds
of eye movements. When the eyes move, as in making saccades,
or in convergence, information as to what to do comes to the
brain from two sources-- the eye muscles that tell where the
eyes are now, and the two retinas, which see slightly different
This input goes to both sides of the brain, and smooth
integration of this information requires fast and accurate
interhemispheric "chit-chat." Thus the corpus callosum
is intimately involved in smooth tracking, smooth convergence,
stable ocular dominance, and matched focusing.
of information on the locus of touch from the fingertips of
one hand to the other without looking also requires use of
the corpus callosum. The information on which finger was touched
must cross this neural bridge to get to the opposite hand.
reasonable to assume that without the fast, accurate guidance
of a central control mechanism, the brain might show the kinds
of symptoms which we see in dyslexia. For instance, poor allocation
of neural space and insufficient arousal of the left hemisphere
might encourage inappropriate reliance on right hemispheric
strategies during language and number processing. Without
the sustained attention and focus provided by a robust corpus
callosum, you would expect the kind of distractibility and
inattentiveness so often seen in dyslectic children.
the extremely fine and rapid interhemispheric transfer of
information necessary for smooth ocular functioning, it is
reasonable to assume that slow or degraded transfer of input
across the hemispheres might result in poor ocular motor skills.
tactile localization -- the transfer of the locus of touch,
unseen, from the fingertips of one hand to those of the other
-- it is poor in everybody below the age of about six, when
the corpus callosum is not yet fully myelinated. By the age
of ten, however, when myelinization is virtually complete,
normals have no trouble with this. Dyslectics score like everybody
else at age six, but they still score poorly in adulthood.
People without a corpus callosum cannot do it at all.
the most interesting pieces of evidence comes from some work
done with high speed photography which showed that dyslectics'
movements on one side of the body are out of synchrony with
movements on the other. 4
For instance, if a dyslectic child hears a click, his right
side turns toward the sound a fraction of a second before
his left side turns. When he blinks, the right eyelid starts
down before the left. When he smiles, the right side of his
mouth turns up before the left one. All this happens so fast
that it is unnoticeable to the naked eye, except for giving
the impression that the child is a bit "twitchy."
Apparently the child does not hear the sound twice,
but the secondary signal that comes across the corpus callosum
from the right hemisphere to the left is late getting there,
so in effect, the child is reacting twice to a single stimulus.
This effect could account for some of the hyperactivity and
distractibility so common in LD children and would certainly
make sustained attention tiring. It also could explain double
regressions in eye motions during linear scanning and other
defects in smooth ocular movements.
piece of evidence is perhaps the most telling. In 1995, Professor
George Hynd and his colleagues found that "subtle neurodevelopmental
variation in the morphology of the corpus callosum may be
associated with the difficulty that dyslectic children experience
in reading and on tasks involving interhemispheric transfer."
Now if you decide to teach someone to read while minimizing
the use of his corpus callosum during training, the results
are astounding; his learning speed increases by a factor of
somewhere between four and ten! 35
Part of the technique by-passes the corpus callosum
entirely by sending verbal exercises directly to the left
hemisphere only, while distracting the right side with qualitatively
different input. Thus the right hemisphere is kept out of
the verbal action by being occupied with suitable work of
its own so that the left is forced to process the language.
The technique mechanically supplies the allocation of space,
the arousal, focusing, and sustained attention that the corpus
callosum doesn't provide and cuts out that slow secondary
delivery of input to each side is achieved in the auditory
system if each signal originates in a single auditory field.
This can be done by putting the student into padded stereo
earphones, with the words going into the right ear and music
into the left for non-stop transfer to the opposite hemisphere.
The verbal input to the left hemisphere is a phonetic sound-symbol
matching task which the right hemisphere is incapable of doing.
unilateral delivery of input in the visual system can
be achieved with the use of the I-Card, as described in the "The Jigsaw Puzzle." It provides in visual system the single input that by-passes the corpus callosum.
taught under these conditions of "Enhanced Lateralization"
since 1972 have routinely achieved reading gains of two and
three years in one year of tutoring, but it is interesting
to note that there has been no obvious change in callosal
functioning. Their tactile localization remains poor and they
are still distractible. Without expensive equipment, unavailable
to a public school teacher, it is not possible to tell whether
eye motions have smoothed out. (However, you often get a startling,
serendipitous improvement in math skills.) It would seem that
there may be an improvement in the functioning of the left
hemisphere, but the corpus callosum probably remains unchanged.
Fortunately, the improvement in reading skill is retained
after training, even under normal free-field conditions. Spelling
improves, but not dramatically.
true that normal readers occasionally have some abnormal architectural
oddities in the language area. It is also true that people
without any corpus callosum, whether from birth or surgical
sectioning, can often read without showing the symptoms on
list #1, though, like dyslectics, they tire quickly.
Perhaps the most sensible conclusion is that a learning-disability
results only from a combination of the two problems: a defective
language area and a faulty corpus callosum. Certainly
a corpus callosum that does not effectively regulate arousal,
attention, and allocation of neural space and repeatedly sends
delayed or degraded signals to an already poorly developed
language area sounds like a recipe for verbal disaster. And
most of the sub-types, classifications, symptoms, deficits
and assorted peculiarities found in the learning-disabled
population can be accounted for by postulating varying degrees
of malfunction somewhere in those two areas.
all, the experts that hold out for visual problems in dyslexia
and those who opt for auditory ones are both right. The timing
problem in interhemispheric transfer of information can embrace
both views and suggests that future research might profitably
be concentrated on why the corpus callosum is so poky in the