EVOKED POTENTIALS AND DYSLEXIA

J. ERTL,PHd

E. DOUGLAS

Dyslexia, alternately labeled “primary reading disability, “word blindness”, or “reading retardation” is an idiopathic condition which may afflict as many as 20% of primary school pupils.   Newbrough and Kelly (1962) found 14% of a sample of 4000 primary school pupils with reading ability at least two grade level years below their actual grade placement. Incidence of dyslexia is known to be twice as great in males as in females.

While there is no universal agreement as to the nature, symptoms, etiology, or potential cure for dyslexia, the term is generally applied to children who are unable to learn to read with proper facility despite normal intelligence, no definite brain damage, intact senses, proper instruction and normal motivation.   Specific

symptoms and characteristics of the dyslexic child as summarized by Orton (1957) include;   marked confusion in remembering the orientation of letters (p-q, b-d, u-n) and the order of letters in words; confusion of words which have only a slightly different letter configuration;   poor oral reading and spelling;   irregular, confused penmanship;   generalized physical clumsiness;   no significant optical or ocular defects or obvious brain damage;   incomplete establishment of one-sided motor preferences (mixed dominance).   This latter symptom has been explored in a variety of studies (Zangwill, 1961In 1959;  Harris, 1957; Ettlinger and Jackson, 1955;   Cohn, 1961)   all of which found weak, mixed or inconsistent lateral preferences (cerebral dominance) in children with severe reading disability when compared with normal controls.

We have noted in our studies of hundreds of normal children that the photic evoked response from symmetrical placements is invariably in phase up to the first 250 msec, and the waveforms are almost identical. This observation is confirmed by Price, et al, 1966;   Werre and Smith, 1964;   and Eason et al,   1967.   It seemed reasonable to hypothesize that incomplete lateral specialization of cerebral functions may be linked with Dyslexia and that this should be reflected in differences of the visual evoked potential from the left and right hemispheres.

METHOD

Five dyslexic children age 9-12 were tested. All of the subjects had a complete neurological and psychological examination and they were of normal intelligence and exhibited the textbook symptoms of Dyslexia.   The neurological examination was negative. The control group consisted of several hundred subjects of approximately the same age who were studied for a different purpose previously Bi-polar electrodes were placed over the left and right parietal areas of all subjects.   200 photic stimuli and 200 auditory stimuli (clicks) were averaged by an Enhancetron   ND-801 computer. The EEG was filtered for all subjects in a bandwidth of 3 db down at 3 - 50 Hz with a 24 db per octave roll-off filter.   All data was initially recorded on a FM tape recorder and analysis performed later.   Results are displayed as X - Y plots showing the average responses to 200 photic and auditory stimuli from the left and right hemispheres.

RESULTS AND DISCUSSION

The average of several hundred records from normal children including computerized cross correlation analysis, indicates that in the case of photic stimulation with symmetrically placed electrodes, cross correlation coefficients of the order of .8 - .95 are obtained, in the first 250 msec.   For auditory stimulation, the cross correlations are lower, ranging from .6 - .87.   In the five dyslexic children studied, although no computerized cross correlation was performed, visual examination indicates that the evoked potentials, both auditory and visual from the two hemispheres, are badly out of phase in all cases.   With experience, it is possible to estimate cross correlation coefficients quite accurately. For the dyslexic subjects these coefficients are estimated at between .2-.5 at most.   In addition to phase differences, considerable amplitude differences were also noted.   In the normal control subjects both the phase and amplitude differences were quite small. Data from one subject to both photic and auditory stimulation are shown in Figures 1 and 2, and the results from a normal control subject to photic stimulation are shown in Figure 3. Numerous studies have indicated that the AEP may reflect neural correlates of information processing from the central nervous system.   Parameters of the AEP have been related to psychometric intelligence (Ertl, 1969 and Bragdon, 1964), decision making (Davis, 1964), Chapman and Bragdon, 1964), and  the perceptual content of presented stimuli (John, et al, 1967). Considering this evidence, it seems reasonable to expect AEP differences between normal and dyslexic subjects and our results indicate that this is the case. Speculation on the nature of these differences may be premature, but these findings may serve as a first step in the development of an objectively based diagnostic indicator of Dyslexia.   It is encouraging to note that the differences are so large, that even a visual examination is sufficient to be convincing.

SUMMARY

Evoked responses to light and sound were studied in five dyslexic subjects and compared with the evoked responses of a large group of control subjects.   Both the auditory and visual evoked responses of all the dyslexic subjects were greatly out of phase in the first 250 msec;   while the normal subjects exhibited a high degree of cross correlation “between AEP’s derived from right and left parietal areas.

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