312
DYSLEXIA AND EYE MOVEMENTS*
D. R. DOSSETOR and
J. PAPAIOANNOU† University ofCambridge
The mean saccadic reaction time (SRT) of a group of dyslexic children was compared the SRT of a group of normal children and to another group of normal adults. The mean SRT of the dyslexic group was significantly longer than that of the other two groups. The saccades of the dyslexic group had a shorter SRT when they were directed towards the right than when they were directed towards the left. For the other two groups the converse was true. There were no qualitative or quantitative differences between the optokinetically-evoked nystagmus of dyslexic children and to
that of normal adults.
Dyslexia might be defined as an impairment in the ability to read which is not attributable to any obvious sensory, emotional or intellectual deficits. This disability varies in severity and manifests itself in letter or word reversals, lack of word recognition and reading backwards. Further difficulties often experienced by dyslexics include retarded speech acquisition, stuttering, poor spelling and reduced visuo-spatial perception. Although dyslexia was first recognized at the turn of the century (Hinshelwood, 1896), no pathological or physiological correlates of the condition have been described with one notable exception. More specifically, Sklar, Hanley and Simmons (1972) described some interesting differences between the electroencephalogram (EEG) of dyslexic and normal children. EEG recordings were taken from several skull positions. under conditions of rest as well as during reading tests. Spectral analysis of the records revealed higher theta-band activity and greater coherence between regions in the same hemispheres among dyslexic children. The purpose of the present study was to examine another physiological parameter associated with reading, namely eye movements, and to determine possible differences between dyslexic and normal children. Since reading basically involves laterally asymmetrical eye movements we felt that such a comparison might yield information useful for the early diagnosis of dyslexia.
We are grateful to Professor O. L. Zangwill and I. Clifton Everest for helpful advice and to the Cambridge Dyslexics Association and St. John’s College School for providing the subjects. &dag er; We regret to have to record the death of Dr. Papaioannou on 28th September, 1973 (Ed.). *
313
SUBJECTS,
MATERIALS AND METHODS
The dyslexic group comprised 10 subjects ranging in age from 6 to 15 years. The control group consisted of 10 normal subjects which were matched for age and sex to the dyslexic group. In order to assess possible developmental changes m eye movements a third group of 10 normal adults, ranging in age from 20-50 years, was examined. Electro-oculograms (E.O.G.) were obtained from the subjects via suction-cup electrodes. The electrodes were placed lateral to the two lateral canthi, while a third reference electrode was placed on the left mastoid process. The E.O.G. signals were passed through a pre-amplifier, amplifier and a 50 c.p.s. filter in order to minimize mains interference. The amplified signal was then fed into an ultra-violet (U.V.) recorder. The U.V. recorder was used because of the small inertia of its galvanometer which produces minimal distortion during the recording of fast eye movements.
The gain of the system was set so that a 40° saccade produced a 4 cm. deflection of the trace (minor adjustments of the sensitivity were necessary to accommodate individual differences). Electro-oculograms were recorded from the subjects during saccadic eye movements and during optokinetic nystagmus. For the measurement of saccadic reaction times, each subject was seated at a table in a darkened room, facing a big matt-black screen and was asked to fixate To prevent contamination on a luminous spot directly in front of him on the screen. the head was immobilized by the use of a bite bar. of the data by head movements, On either side of the fixation spot and at an angular distance from it of 40° was The subject was a small amber xenon light, which was invisible when not lit. instructed to fixate the central spot until one of the two xenon lights lit up, at which time he was to fixate the xenon light as quickly as possible. When the xenon light went off the subject was to return his fixation to the central luminous spot. During a given session each xenon light came on 10 times, for 0.7 sec. each time, the right-left sequence of presentation being random to eliminate guessing. In this way 20 saccades of the same magnitude were produced, 10 towards the right and 10 towards the left. A white-noise generator was switched on continuously during this part of the experimental session in order to mask various equipment-
produced auditory
cues.
During the second part of each session optokinetic nystagmus was elicited by the use of a 9 foot long revolving band of vertically-striped paper, in a well-lit The subject was instructed to fixate as many black stripes as possible. room. Nystagmus was recorded for 20 seconds in each direction at two rates of paper Head movements were again eliminated by the movement (30 and 50 cm/sec.). use of a bite bar. Each subject had two experimental sessions on different days. Since there was little variation within a given subject from day to day, mean scores are presented and used for subsequent analysis.
314 RESULTS
Saccadic reaction time It can be seen from Table 1 that the reaction times of most dyslexic children shorter for saccades towards the right than for saccades towards the left. The These differences between saccadic converse is true in the case of normal children. reaction times to the right and to the left are significantly different for the two groups (Wilcoxon matched pairs p < 0.01, 2-tailed). When the saccadic reaction times are averaged for both directions, dyslexic children are again significantly different from normal children, the former having longer reaction times (Wilcoxon matched pairs, p < 0.01, 2-tailed). The differences between the saccades of dyslexic children and normal adults are qualitatively similar to those found between dyslexic and normal children, but they are more pronounced in magnitude. Thus, dyslexic children have shorter saccadic reaction times towards the right whereas normal adults have shorter reaction times towards the left, the difference being significant (t-test, p < 0.001, 2-tailed). Averaged over both directions, dyslexic children have much longer saccadic reaction times than normal adults (t-test, p < 0.001, 2-tailed). There are no significant differences between normal children and normal adults either in the lateral asymmetry or mean reaction time of saccades.
are
Optokinetic nystagmus The records of optokinetic nystagmus in either direction were subdivided into 5 second sections. The number of ocular flashbacks occurring in each section was used as an index of the subject’s tracking facility. As there was considerable variation in the amplitude of the ocular flashbacks, an arbitrary criterion was established. Thus only eye movements at least one-fifth of the amplitude of the average deflection were counted as representing ocular flashbacks. This procedure was adopted in order to prevent contamination of the data by the inclusion of irrevelant ocular movements such as tremors. The number of flashbacks elicited by stripedpaper movement to the right was compared to the number of flashbacks elicited by paper movement in the opposite direction, with the non-parametric MannWhitney U-test. Three out of the 10 dyslexic children examined were significantly (p < 0.5) better at tracking the stripes when the stripes were moving towards the right than when they were moving towards the left. A similar lateral asymmetry was found in 4 of the 10 normal adults. Thus, no difference exists between the two groups in this respect. When the number of flashbacks for both directions was averaged, there were again no significant differences between dyslexic children and normal adults. In view of these negative findings, it was deemed unnecessary to perform a similar comparison between the data of dyslexic and normal children.
315 TABLE 1
Saccadic reaction times
*
based
on
(S.R.T.)
40 saccades
316 DISCUSSION There are several lines of evidence suggesting that dyslexia is due to the operation of constitutional factors, although environmental factors are not excluded. Hallgren’s (1950) monograph based on the family histories of 276 dyslexics presents strong evidence in favour of a genetic contribution to the aetiology of dyslexia. More specifically, in 88% of the cases investigated there was a family history of reading disability. If this is so, how is the genetic contribution effected? The most obvious suggestions is to postulate a deficit in visual perception. This possibility has been considered by Fildes (1921). Following tests of visual discrimination and retention on dyslexic and normal children, she concluded that the reading difficulties of dyslexic children were not due to a general perceptual deficit. Instead, she suggested that dyslexic children suffer from a specific deficit in the comprehension and retention of symbols. It is difficult to reconcile the above suggestion with the experimental findings of Bachmann (1927) who presented words tachistoscopically to dyslexic and normal children. He found that the limit of the readability of words was reached with the same number of letters in the case of both dyslexic and normal children. Since in this experiment ocular movements were precluded by the tachistoscopic presentation, one wonders whether perhaps one of the causes of dyslexia might not be abnormal eye movements. Such a possibility is supported by the fact that eye movement records of dyslexics during the act of attempting to read are markedly different from those of normal subjects: there is constant oscillation and ocular movements are generally small in amplitude (for example, see Hallpike’s records in Critchley, 1970). Abnormal eye movements could be due to at least two factors. The first could be a motor deficit or malfunction of the &dquo; final common pathway &dquo;; the second could be malfunction of the cortical &dquo; centres &dquo; initiating voluntary eye movements. The results of the present study are consistent with the second alternative, since dyslexic and normal subjects exhibit no differences in optokinetic nystagmusbasically a subcortical phenomenon-but do differ in their saccadic reaction time. The cause of the postulated malfunction of cortical centres initiating voluntary It is possible that this malfunction is a by-product of movements is not clear. imperfect cerebral dominance. It is relevant in this context that often, though not always, imperfect cerebral lateralization is associated with reading problems (Roudinesco, 1950; Zangwill, 1960; Zurif and Carson, 1970). Early diagnosis of dyslexia, when coupled with subsequent special attention to the child’s problem, can be a great help to the child and its family. The present study could be ultimately helpful in this direction, if followed up by more comprehensive, developmental studies.
317 REFERENCES F. (1927). Über kongenitale Wortblindheit. Abhandl. Psych. Psychol. Grenzgeb. 40. 1. CRITCHLEY, M. (1970). The Dyslexic Child (London). FILDES, L. G. (1921). A psychological inquiry into the nature of the condition known as congenital word-blindness. Brain, 44, 286. HALLGREN, B. (1950). Specific dyslexia. Acta. psych. neurol., Suppl., 65, 1. HINSHELWOOD, (1896). A case of dyslexia: a peculiar form of word-blindness. Lancet, 2, 1451 ROUDINESCO, J. (1950). Note sur la dyslexie. Bull. Soc. med, Hôp. Paris, 66, 1451. SKLAR, B., HANLEY, J. and SIMMONS, W. W. (1972). An EEG experiment aimed toward identifying dyslexic children. Nature, 240, 414. ZANGWILL, O. L. (1960). Cerebral Dominance and its Relation to Psychological Function
BACHMANN,
(Edinburgh). ZARIF, E. B. and CARSON, G. (1970). Dyslexia analysis. Neuropsych., 8, 351.
in relation
to
cerebral dominance and
temporal
DYSLEXIA AND EYE MOVEMENTS*
D. R. DOSSETOR and
J. PAPAIOANNOU† University ofCambridge
The mean saccadic reaction time (SRT) of a group of dyslexic children was compared the SRT of a group of normal children and to another group of normal adults. The mean SRT of the dyslexic group was significantly longer than that of the other two groups. The saccades of the dyslexic group had a shorter SRT when they were directed towards the right than when they were directed towards the left. For the other two groups the converse was true. There were no qualitative or quantitative differences between the optokinetically-evoked nystagmus of dyslexic children and to
that of normal adults.
Dyslexia might be defined as an impairment in the ability to read which is not attributable to any obvious sensory, emotional or intellectual deficits. This disability varies in severity and manifests itself in letter or word reversals, lack of word recognition and reading backwards. Further difficulties often experienced by dyslexics include retarded speech acquisition, stuttering, poor spelling and reduced visuo-spatial perception. Although dyslexia was first recognized at the turn of the century (Hinshelwood, 1896), no pathological or physiological correlates of the condition have been described with one notable exception. More specifically, Sklar, Hanley and Simmons (1972) described some interesting differences between the electroencephalogram (EEG) of dyslexic and normal children. EEG recordings were taken from several skull positions. under conditions of rest as well as during reading tests. Spectral analysis of the records revealed higher theta-band activity and greater coherence between regions in the same hemispheres among dyslexic children. The purpose of the present study was to examine another physiological parameter associated with reading, namely eye movements, and to determine possible differences between dyslexic and normal children. Since reading basically involves laterally asymmetrical eye movements we felt that such a comparison might yield information useful for the early diagnosis of dyslexia.
We are grateful to Professor O. L. Zangwill and I. Clifton Everest for helpful advice and to the Cambridge Dyslexics Association and St. John’s College School for providing the subjects. &dag er; We regret to have to record the death of Dr. Papaioannou on 28th September, 1973 (Ed.). *
313
SUBJECTS,
MATERIALS AND METHODS
The dyslexic group comprised 10 subjects ranging in age from 6 to 15 years. The control group consisted of 10 normal subjects which were matched for age and sex to the dyslexic group. In order to assess possible developmental changes m eye movements a third group of 10 normal adults, ranging in age from 20-50 years, was examined. Electro-oculograms (E.O.G.) were obtained from the subjects via suction-cup electrodes. The electrodes were placed lateral to the two lateral canthi, while a third reference electrode was placed on the left mastoid process. The E.O.G. signals were passed through a pre-amplifier, amplifier and a 50 c.p.s. filter in order to minimize mains interference. The amplified signal was then fed into an ultra-violet (U.V.) recorder. The U.V. recorder was used because of the small inertia of its galvanometer which produces minimal distortion during the recording of fast eye movements.
The gain of the system was set so that a 40° saccade produced a 4 cm. deflection of the trace (minor adjustments of the sensitivity were necessary to accommodate individual differences). Electro-oculograms were recorded from the subjects during saccadic eye movements and during optokinetic nystagmus. For the measurement of saccadic reaction times, each subject was seated at a table in a darkened room, facing a big matt-black screen and was asked to fixate To prevent contamination on a luminous spot directly in front of him on the screen. the head was immobilized by the use of a bite bar. of the data by head movements, On either side of the fixation spot and at an angular distance from it of 40° was The subject was a small amber xenon light, which was invisible when not lit. instructed to fixate the central spot until one of the two xenon lights lit up, at which time he was to fixate the xenon light as quickly as possible. When the xenon light went off the subject was to return his fixation to the central luminous spot. During a given session each xenon light came on 10 times, for 0.7 sec. each time, the right-left sequence of presentation being random to eliminate guessing. In this way 20 saccades of the same magnitude were produced, 10 towards the right and 10 towards the left. A white-noise generator was switched on continuously during this part of the experimental session in order to mask various equipment-
produced auditory
cues.
During the second part of each session optokinetic nystagmus was elicited by the use of a 9 foot long revolving band of vertically-striped paper, in a well-lit The subject was instructed to fixate as many black stripes as possible. room. Nystagmus was recorded for 20 seconds in each direction at two rates of paper Head movements were again eliminated by the movement (30 and 50 cm/sec.). use of a bite bar. Each subject had two experimental sessions on different days. Since there was little variation within a given subject from day to day, mean scores are presented and used for subsequent analysis.
314 RESULTS
Saccadic reaction time It can be seen from Table 1 that the reaction times of most dyslexic children shorter for saccades towards the right than for saccades towards the left. The These differences between saccadic converse is true in the case of normal children. reaction times to the right and to the left are significantly different for the two groups (Wilcoxon matched pairs p < 0.01, 2-tailed). When the saccadic reaction times are averaged for both directions, dyslexic children are again significantly different from normal children, the former having longer reaction times (Wilcoxon matched pairs, p < 0.01, 2-tailed). The differences between the saccades of dyslexic children and normal adults are qualitatively similar to those found between dyslexic and normal children, but they are more pronounced in magnitude. Thus, dyslexic children have shorter saccadic reaction times towards the right whereas normal adults have shorter reaction times towards the left, the difference being significant (t-test, p < 0.001, 2-tailed). Averaged over both directions, dyslexic children have much longer saccadic reaction times than normal adults (t-test, p < 0.001, 2-tailed). There are no significant differences between normal children and normal adults either in the lateral asymmetry or mean reaction time of saccades.
are
Optokinetic nystagmus The records of optokinetic nystagmus in either direction were subdivided into 5 second sections. The number of ocular flashbacks occurring in each section was used as an index of the subject’s tracking facility. As there was considerable variation in the amplitude of the ocular flashbacks, an arbitrary criterion was established. Thus only eye movements at least one-fifth of the amplitude of the average deflection were counted as representing ocular flashbacks. This procedure was adopted in order to prevent contamination of the data by the inclusion of irrevelant ocular movements such as tremors. The number of flashbacks elicited by stripedpaper movement to the right was compared to the number of flashbacks elicited by paper movement in the opposite direction, with the non-parametric MannWhitney U-test. Three out of the 10 dyslexic children examined were significantly (p < 0.5) better at tracking the stripes when the stripes were moving towards the right than when they were moving towards the left. A similar lateral asymmetry was found in 4 of the 10 normal adults. Thus, no difference exists between the two groups in this respect. When the number of flashbacks for both directions was averaged, there were again no significant differences between dyslexic children and normal adults. In view of these negative findings, it was deemed unnecessary to perform a similar comparison between the data of dyslexic and normal children.
315 TABLE 1
Saccadic reaction times
*
based
on
(S.R.T.)
40 saccades
316 DISCUSSION There are several lines of evidence suggesting that dyslexia is due to the operation of constitutional factors, although environmental factors are not excluded. Hallgren’s (1950) monograph based on the family histories of 276 dyslexics presents strong evidence in favour of a genetic contribution to the aetiology of dyslexia. More specifically, in 88% of the cases investigated there was a family history of reading disability. If this is so, how is the genetic contribution effected? The most obvious suggestions is to postulate a deficit in visual perception. This possibility has been considered by Fildes (1921). Following tests of visual discrimination and retention on dyslexic and normal children, she concluded that the reading difficulties of dyslexic children were not due to a general perceptual deficit. Instead, she suggested that dyslexic children suffer from a specific deficit in the comprehension and retention of symbols. It is difficult to reconcile the above suggestion with the experimental findings of Bachmann (1927) who presented words tachistoscopically to dyslexic and normal children. He found that the limit of the readability of words was reached with the same number of letters in the case of both dyslexic and normal children. Since in this experiment ocular movements were precluded by the tachistoscopic presentation, one wonders whether perhaps one of the causes of dyslexia might not be abnormal eye movements. Such a possibility is supported by the fact that eye movement records of dyslexics during the act of attempting to read are markedly different from those of normal subjects: there is constant oscillation and ocular movements are generally small in amplitude (for example, see Hallpike’s records in Critchley, 1970). Abnormal eye movements could be due to at least two factors. The first could be a motor deficit or malfunction of the &dquo; final common pathway &dquo;; the second could be malfunction of the cortical &dquo; centres &dquo; initiating voluntary eye movements. The results of the present study are consistent with the second alternative, since dyslexic and normal subjects exhibit no differences in optokinetic nystagmusbasically a subcortical phenomenon-but do differ in their saccadic reaction time. The cause of the postulated malfunction of cortical centres initiating voluntary It is possible that this malfunction is a by-product of movements is not clear. imperfect cerebral dominance. It is relevant in this context that often, though not always, imperfect cerebral lateralization is associated with reading problems (Roudinesco, 1950; Zangwill, 1960; Zurif and Carson, 1970). Early diagnosis of dyslexia, when coupled with subsequent special attention to the child’s problem, can be a great help to the child and its family. The present study could be ultimately helpful in this direction, if followed up by more comprehensive, developmental studies.
317 REFERENCES F. (1927). Über kongenitale Wortblindheit. Abhandl. Psych. Psychol. Grenzgeb. 40. 1. CRITCHLEY, M. (1970). The Dyslexic Child (London). FILDES, L. G. (1921). A psychological inquiry into the nature of the condition known as congenital word-blindness. Brain, 44, 286. HALLGREN, B. (1950). Specific dyslexia. Acta. psych. neurol., Suppl., 65, 1. HINSHELWOOD, (1896). A case of dyslexia: a peculiar form of word-blindness. Lancet, 2, 1451 ROUDINESCO, J. (1950). Note sur la dyslexie. Bull. Soc. med, Hôp. Paris, 66, 1451. SKLAR, B., HANLEY, J. and SIMMONS, W. W. (1972). An EEG experiment aimed toward identifying dyslexic children. Nature, 240, 414. ZANGWILL, O. L. (1960). Cerebral Dominance and its Relation to Psychological Function
BACHMANN,
(Edinburgh). ZARIF, E. B. and CARSON, G. (1970). Dyslexia analysis. Neuropsych., 8, 351.
in relation
to
cerebral dominance and
temporal
