IN DYSLEXIC PEOPLE'S BRAINS
Dpt. of EEG and CNF, Neurological Institute, Montevideo - Uruguay
Conference presented in ISBET/96,October 1996, Rio de Janeiro
In fact, that is what reviews about applications of quantitative electroencephalography on neurologic pathologies display.
In a paper published on Brain Topography ,1996, M.Newer outlines this situation, by expressing: "Quantified EEG techniques are unable to diagnose dyslexia."
We would like to raise our point of view about the apparent failure of the qEEG in the study of dyslexia and introduce you to our research into this field.
The first level of complexity in this pathology starts with the definition and classification of dyslexia. In fact, its definition, classification and aetiopathogeny is still being studied.
Differences of dyslexia in each language must be added to this variety of definitions and classifications. Actually, English and Spanish differ in the phonetic and visual aspects of their language structures.
For that reason, the proportion of each subtype of dyslexia is different in anglo-saxon countries than in hispanic nations. While in Spanish language there is a great proportion of disorders on visual and spatial decodification, English has a wider proportion of phonetic and secuential trouble. That's because Spanish is a more phonological language than English, and its teaching at school applies an analytic- synthetic methodology.
During the last decade, important advances on dyslexic brain's anatomical characteristics have been achieved by using Nuclear Magnetic Resonance (NMR); also, studies of altered regional metabolism have progressed using the Positron Emission Tomography (PET), (Rumsey JM, 1992-4).
Several facts cause disparity among discoveries in clinical neurophysiology. Many of the presented cognitive tests showed some methodological problems, and the studied populations were too heterogenous.
Another level of complexity lies on the fact that dynamic neurophysiology involves new technical and methodological problems, not only because face or eyes's movement may contaminate the records, but also the selection of references may cause trouble. Moreover, there may be some difficulties in sampling the cognitive activity, mostly because it's related whether with variations on attention, efforts or frustration towards the task.
On the other hand, classic EEG bands - created several decades ago-, have not been adjusted to the kind of variables qEEG and cognitive phenomenons study.
Finally, two kids who carry the same subtype of dyslexia but have diverse educational levels would answer differently to the same test. The child who has the lowest educational level would add other variations to the activity caused by the reading. Those variations would arise as a result of making harder efforts -by paying more attention or feeling more anxious and frustrated than normally.
This work will outline some of our results as well as other author's who seem to be in agreement with some aspects of our concept of dyslexic people's qEEG.
There is a first level of consent to the analysis of the EEG activity at rest in pure dyslexias; according to Yingling and cols and Ortiz and cols, they show no differences with the reference groups.
From 1991 to 1996, we have studied two groups of patients in co-operation with the Neurophysiology, Neuropediatrics and Neuropsychology Departments of the Montevideo's Neurology Institute.
1) A group of 15 "pure" dyslexic children (from 8 to 13 years old) were classified according to E.Boder's test (which was adapted to Spanish by Rebollo and cols) and the behaviour of their alpha rhythm was studied in relation to reading, writing and drawing tasks.
2) Another group of 20 "pure" dyslexic children and teenagers (from 8 to 20 years old) was diagnosed and classified according to E.Boder's and neuropsycho logycal tests.
Both studies had control groups of normal children and teenagers who matched in number and age.
With regard to the selection of cognitive tasks, three continous and verifiable tests were selected. In fact, such tests might be as continous and verifiable as the reality of reading could be.
These three tests lasted approximately 1 minute and were:
* reading in soft voice
* taking dictation
* drawing a clock
A pedagogue adjusted each task to every child's educational level, with the purpose of avoiding whether any psychologycal blockage during the reading task or high levels of reading difficulties.
Only the alpha rhythm variations have been studied in this research.
In fact, Pfurtscheller and cols' works prove -by using ERD- such variations are the best indicators of the subjacent cerebral cortex's physiology.
We calculated the percentages of the alpha power's variations for the cognitive tasks of the control situation. The "control" situation is defined as an "attention without operation" situation, and is obtained by recording each child's EEG activity 1 minute before and 1 minute after they make the reading, writing and drawing tasks. They are asked to focus their sight on a dot on a plain piece of paper.
So, we studied the variations in the "control" situation , as well as the reading, writing and drawing tests -which are considered "attention with operation" kind of tasks.
The first two images shows average desynchronization in the alpha rhythm of normal teenagers, who were doing the reading and drawing tasks.
You can see that both hemispheres take part in this activity . (yellow-white colors represents alpha desynchronization between 0 to 30% from the control situation)
The two following figures presents particular findings in pure dyslexias. There are areas without alpha desynchronization, or little synchronization with the cognitive tasks.
The next two images displays the average maps of the dyslexic population's alpha variations during the reading and drawing tasks. The maps belong to a population of 14 disgestaltic dyslexias (according to E.Boder's test).
On the right hemisphere you can see an area of lack of desynchronization.
In the "dysphonetic" subtype there is some lack of desynchronization at the left temporal area.
The alpha variation maps of the mixed type of dyslexia shows an abnormal functionning in the alpha rhythm of both hemispheres, which consists in an increase of the alpha power during the task's performance.
Twenty years ago, P.Fuller had already described the abnormal functionning of alpha rhythm that happens during the performance of cognitive tasks by children with learning disorders.
In Duffy and cols' original works on brain mapping of dyslexia (1980) , two basic truths arose. Firstly, there is evidence of physiological participa tion from areas of both hemispheres.
On the other hand, an abnormal functionning of the alpha rhythm is manifested either by some kind of "excess" of the alpha fre quency, or a low variation in the alpha activity.
Pfurtscheller's works have shown a sequential desynchronization of the alpha activity from the occipital area to the sensory-motor cortex, during a visuo-verbal task.
The process occurs during the first second after the stimulus is presented. Our one minute reading tasks implies the repeated activation of this circuit. In both hemispheres, our activated areas coincide with those Pfurscheller determined.
Ortiz and cols. selected a population of dyslexic people with phonetic discrimina tion disorders and, in most cases, no desynchronization in the left temporo-parietal cortex areas was found. Our findings about dysphonetic dyslexias -based upon the E.Boder's test- agree with these results.
Seri and cols' work (Brain Top.1993) coincides with our results as well. It actually shows a lack of alpha desynchronization in the right hemisphere of dyseidetic dyslexic patiens. Their population was classified also with the E.Boder's test as our.
Findings on mixed dyslexias are the most difficult to interpret. The increase of alpha activity has already been described by Galin and Duffy. Nowadays, there is no clear explanations of this phenomenon.
Several qEEG studies coincide in showing correlations among highly selected dyslexic populations and the alterations in the alpha band.
The anormalities found are SECTORIAL ALTERATIONS OF ALPHA RHYTHM . (areas of underreactivity of alpha rhythm).
Such focal alterations occur by virtue of a lack of normal alpha rhythm desynchronization during the performance of cognitive tasks.
There is a good correlation between each subtype of dyslexia and the cortical topography where the altered alpha rhythm is located. For instance, visuo-spatial dyslexias present "patches of arreactive alpha rhythm" in right hemisphere, whereas analytic, secuential and phonological disorders present this alteration in the left hemisphere.
These results indicate the altered physiology of some areas in the cerebral cortex coincide with some alterations shown in the PET. Actually, Rumsey JM (1992-4) shows a focal diminution of flux in the left angular gyrus and a lack of activation of the left temporo-parietal cortex, during the syntactic process.
Another finding consists in the increase of alpha activity during the performance of cognitive tasks by people who are the most serious subtype of dyslexia (Boder's mixed type). Such special functionning of the alpha rhythm deserves future researches.
Finally, we think the disparity among the results is caused by several unavoida ble theoretic and methodological problems in the neurological, neuropsychological and neurophysiological fields, and explains much of the scepticism towards the qEEG.
If we take into account the passing of time since Fuller's works, (20 years), we can conclude there is little neurophysiological literature on dyslexia today.
We believe the mentioned literature itself contains corresponding discoveries about the brain's cognitive processes of dyslexic people.
Neurophysiological focal anormalities correlate with clinical and other paraclinical findings , and also enable the sketch of this pathology's profile.
INSTITUTE OF NEUROLOGY, SCHOOL OF MEDICINE
MONTEVIDEO - URUGUAY
FAX : + 598 2 474786
email: dcibils@chasque.apc.org