Is The Corpus Callosum the Missing Link in Dyslexia?
---by Dorothy van den Honert

The terms 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:

• difficulty with sound-symbol matching     2,22,25,30,32
• disregard of punctuation
• omission or inappropriate insertion of functors
• omission of syllables in multisyllable words
• poor phonetic decoding skills
• poor comprehension in oral and silent reading
• poor letter-sequencing in spelling and reading
• omitting or miscalling syntactical endings such as -ed, or -s
• lack of phonemic awareness
• substituting semantic criterior for phonetic criterior: ripped for torn, large for big, etc.
• confusion of similar-looking abstract words, such as form and from

These 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.

In the 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:

• left-right confusion
• difficulty remembering ordered lists, such as the months of the year
• difficulty remembering a group of unrelated facts like the multiplication table
• deficit in tactile localization skills     14
• markedly slow interhemispheric transfer time     13,14,15
• abnormal distractibility, "twitchiness" and hyperactivity
• problems with fine motor control in handwriting
• visual 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.

Clearly 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

But how 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.

 One 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. 12

The corpus 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 things. 33   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.

Transfer of information on the locus of touch from the fingertips of one hand to the other without looking also requires use of the corpus collosum. The information on which finger was touched must cross this neural bridge to get to the opposite hand.

It seems 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.

Considering 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.

As for 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.

One of 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.

Another 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." 18  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 signal. 29

Unilateral delivery of input to each side is achieved in the auditory system if each signal originates in a single auditory field. 28  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.

Continuous unilateral delivery of input in the visual system can only be achieved with expensive, sophisticated equipment. The concept of isolating input to one side was developed for use in public schools, precluding the use of anything exotic. However, the problem caused by double, asynchronous input to the left hemisphere can still be minimized cheaply by cutting the inputs to the left language area down from four (two from each eye) to only two (from only one eye). This eliminates the asynchronous double signals. It can be accomplished by simply occluding one eye. The unobscured eye then sends half of what it sees to the right hemisphere, the other half to the left hemisphere. The right hemisphere then dutifully ships its half across the corpus callosum for assembly in the left language areas. 11  Glasses with the right lens popped out and the left lens darkened are the most comfortable way to achieve this. Of course the final processing in the left hemisphere will be slowed if the incoming contribution from across the way is chronically late, but the behavioral result is probably just that the dyslectic person's reading will always be somewhat slower and more labored that that of a standard reader. Training exercises done while wearing the glasses should again be phonics and sequencing exercises that the right hemisphere is incapable of doing, but the glasses can be worn for normal reading material as well.

Students 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.

It remains 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.

Best of 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 first place.