Evidence for frequent divergence impairment in French dyslexic children: deficit of convergence relaxation or of divergence per se?

Age-Matched Comparative Analysis of Binocular Vision Anomalies among Children with Dyslexia in Northern Nigeria

Background: Dyslexia, a neurodevelopmental disorder affecting reading skills, poses significant challenges to children's academic performance and quality of life. Despite its rising prevalence and adverse effects, understanding of its relationship with vision anomalies remains limited, particularly in low-resource settings like Nigeria. This study aims to assess the prevalence of binocular vision anomalies (BVAs) among children with and without dyslexia in Kano, Nigeria. Methods: This is a hospital-based, cross-sectional, matched-paired, controlled study conducted at the Aminu Kano Teaching Hospital (AKTH) Eye Clinic in Northern Nigeria. The study included school children who visited the AKTH Eye Clinic from January 2018 to December 2022. Visual acuity tests, external eye examinations and accommodative, binocular vision and oculomotor skills tests were conducted. Descriptive statistics, independent t-tests, Mann-Whitney U tests and Fisher's exact tests were conducted, with a significance level set at p < 0.05. Results: Forty-four children aged 12 ± 2 years participated. Children with dyslexia reported higher rates of visual symptoms than those without dyslexia, Blurring vision, visual distortion and eye strain were the most prevalent (p < 0.05) BV symptoms. Accommodative insufficiency), was the most common visual abnormality, and was significantly higher in children with dyslexia than those without dyslexia (45.5% vs. 18.2%). However, other visual anomalies showed no significant difference between groups. There was a high prevalence of binocular vision anomalies in both groups. Binocular test findings showed dyslexic children had significantly lower distance positive fusional vergence recovery values (p = 0.005). All cases of convergence insufficiency alone were found in the non-dyslexic group. Conclusions: The study found that children with dyslexia residing in Northern Nigeria demonstrated higher rates of visual symptoms, more accommodative insufficiency and lower distance positive fusional vergence recovery values compared to their non-dyslexic counterparts.

Identifying dyslexia in school pupils from eye movement and demographic data using artificial intelligence

This paper represents our research results in the pursuit of the following objectives: (i) to introduce a novel multi-sources data set to tackle the shortcomings of the previous data sets, (ii) to propose a robust artificial intelligence-based solution to identify dyslexia in primary school pupils, (iii) to investigate our psycholinguistic knowledge by studying the importance of the features in identifying dyslexia by our best AI model. In order to achieve the first objective, we collected and annotated a new set of eye-movement-during-reading data. Furthermore, we collected demographic data, including the measure of non-verbal intelligence, to form our three data sources. Our data set is the largest eye-movement data set globally. Unlike the previously introduced binary-class data sets, it contains (A) three class labels and (B) reading speed. Concerning the second objective, we formulated the task of dyslexia prediction as regression and classification problems and scrutinized the performance of 12 classifications and eight regressions approaches. We exploited the Bayesian optimization method to fine-tune the hyperparameters of the models: and reported the average and the standard deviation of our evaluation metrics in a stratified ten-fold cross-validation. Our studies showed that multi-layer perceptron, random forest, gradient boosting, and k-nearest neighbor form the group having the most acceptable results. Moreover, we showed that although separately using each data source did not lead to accurate results, their combination led to a reliable solution. We also determined the importance of the features of our best classifier: our findings showed that the IQ, gender, and age are the top three important features; we also showed that fixation along the y-axis is more important than other fixation data. Dyslexia detection, eye fixation, eye movement, demographic, classification, regression, artificial intelligence.

Routine binocular examination of young Taiwanese adults as a predictor of visual behavior performance

BACKGROUND

Morgan and Scheiman's Optometric Extension Program (OEP) expected binocular vision findings have longstanding use in optometry. With technological advances, the demands and standards of binocular function have changed. This study aimed to investigate which binocular visual functions can effectively predict visual behavior performance.

METHODS

Participants aged 15-24 years were recruited from two colleges and two universities. After completing the CSMU-Visual Behavioral Performance questionnaire (CSMU-VBP, with four components: near work, visual perception, visual comfort, and whole-body balance), participants were divided into symptomatic and asymptomatic groups based on questionnaire findings (cutoff: < 12 vs. ≥ 12 symptoms). Then a 24-step binocular visual examination was undertaken. Data were analyzed with one-sample, Student's, and paired t-tests. Additionally, receiver operating characteristic analysis was used to determine the predictors of binocular visual function required for near work, visual perception, visual comfort, and body balance dimensions.

RESULTS

Among 308 participants, 43 (14%) and 265 (86%) were symptomatic and asymptomatic, respectively. Among the 46 participants with abnormal binocular vision, 36 (78%) reported that they had no obvious symptoms. The commonest dysfunctions were accommodative excess and convergence excess. Most of the binocular visual findings significantly diverged from traditional normal values: amplitude of accommodation, as well as base-in prism to break and recovery points at distance were higher than traditional normal values, whereas others were lower than traditional normal values. Total CSMU-VBP scores indicated that the asymptomatic and symptomatic groups had significant differences in DBO recovery (t = 2.334, p = 0.020) and BAF (t = 1.984, p = 0.048). Receiver operating characteristic curve analysis yielded the following binocular visual functional cutoff points: near work (DBO blur < 7, DBO recovery < 5.5), visual perception (MAF < 10.5, BAF < 10.25), visual comfort (DLP <  - 2.25, DBI break > 11.5, NBI blur > 15, NBI break > 17.5, NBI recovery > 13, NPC < 5.75), and body balance (NFD_H >  - 0.5, gradient AC/A [minus] > 2.25, NPC < 4.75).

CONCLUSIONS

The mean values of binocular visual function among young Taiwanese adults were statistically different from traditional normative values. Further research is required to confirm whether these findings reflect impaired binocular vision or stringent criteria. Assessments of binocular visual function, especially binocular accommodation sensitivity, are crucial in routine optometric examination.

Predicting Dyslexia and Reading Speed in Adolescents from Eye Movements in Reading and Non-Reading Tasks: A Machine Learning Approach

There is evidence that abnormalities in eye movements exist during reading in dyslexic individuals. A few recent studies applied Machine Learning (ML) classifiers to such eye movement data to predict dyslexia. A general problem with these studies is that eye movement data sets are limited to reading saccades and fixations that are confounded by reading difficulty, e.g., it is unclear whether abnormalities are the consequence or the cause of reading difficulty. Recently, Ward and Kapoula used LED targets (with the REMOBI & AIDEAL method) to demonstrate abnormalities of large saccades and vergence eye movements in depth demonstrating intrinsic eye movement problems independent from reading in dyslexia. In another study, binocular eye movements were studied while reading two texts: one using the "Alouette" text, which has no meaning and requires word decoding, the other using a meaningful text. It was found the Alouette text exacerbates eye movement abnormalities in dyslexics. In this paper, we more precisely quantify the quality of such eye movement descriptors for dyslexia detection. We use the descriptors produced in the four different setups as input to multiple classifiers and compare their generalization performances. Our results demonstrate that eye movement data from the Alouette test predicts dyslexia with an accuracy of 81.25%; similarly, we were able to predict dyslexia with an accuracy of 81.25% when using data from saccades to LED targets on the Remobi device and 77.3% when using vergence movements to LED targets. Noticeably, eye movement data from the meaningful text produced the lowest accuracy (70.2%). In a subsequent analysis, ML algorithms were applied to predict reading speed based on eye movement descriptors extracted from the meaningful reading, then from Remobi saccade and vergence tests. Remobi vergence eye movement descriptors can predict reading speed even better than eye movement descriptors from the meaningful reading test.

Dyslexics' Fragile Oculomotor Control Is Further Destabilized by Increased Text Difficulty

Dyslexic adolescents demonstrate deficits in word decoding, recognition, and oculomotor coordination as compared to healthy controls. Our lab recently showed intrinsic deficits in large saccades and vergence movements with a Remobi device independent from reading. This shed new light on the field of dyslexia, as it has been debated in the literature whether the deficits in eye movements are a cause or consequence of reading difficulty. The present study investigates how these oculomotor problems are compensated for or aggravated by text difficulty. A total of 46 dyslexic and 41 non-dyslexic adolescents' eye movements were analyzed while reading L'Alouette, a dyslexia screening test, and 35 Kilos D'Espoir, a children's book with a reading age of 10 years. While reading the more difficult text, dyslexics made more mistakes, read slower, and made more regressive saccades; moreover, they made smaller amplitude saccades with abnormal velocity profiles (e.g., higher peak velocity but lower average velocity) and significantly higher saccade disconjugacy. While reading the simpler text, these differences persisted; however, the difference in saccade disconjugacy, although present, was no longer significant, nor was there a significant difference in the percentage of regressive saccades. We propose that intrinsic eye movement abnormalities in dyslexics such as saccade disconjugacy, abnormal velocity profiles, and cognitively associated regressive saccades can be particularly exacerbated if the reading text relies heavily on word decoding to extract meaning; increased number of regressive saccades are a manifestation of reading difficulty and not a problem of eye movement per se. These interpretations are in line with the motor theory of visual attention and our previous research describing the relationship between binocular motor control, attention, and cognition that exists outside of the field of dyslexia.

Distribution of Visual and Oculomotor Alterations in a Clinical Population of Children with and without Neurodevelopmental Disorders

A prospective, non-randomized comparative study was conducted to compare the distribution of oculomotor and visual alterations in children with neurodevelopmental disorders and healthy children without such disorders. Sixty-nine children (aged 6-13 years) were enrolled and divided into three groups: a control group (CG) of 23 healthy children; a group of 18 healthy children with oculomotor abnormalities (OAG); and a group of 28 children with a neurodevelopmental disorder (NDDG), with 15 cases of dyslexia, 7 cases of developmental coordination disorder (DCD) and 6 cases of attention deficit/hyperactivity disorder (ADHD). Significantly worse near stereopsis was found in NDDG compared with CG (p < 0.001) and OAG (p = 0.001). Likewise, a significantly lower amplitude of accommodation was found in NDDG compared with CG in both the right (p = 0.001) and left eyes (p < 0.001). No statistically significant differences between groups were found in the measurement of near and distance phoria (p ≥ 0.557), near point of convergence (p = 0.700) and fusional vergences (p ≥ 0.059). Significantly impaired oculomotor test scores were found in NDDG compared with CG (p < 0.001), with no significant differences between OAG and NDDG (p ≥ 0.063). The comparison between the three types of neurodevelopmental disorders included revealed the presence of a significantly lower amplitude of accommodation in children with DCD compared with dyslexics. Furthermore, less exophoria at near was present in children with dyslexia compared with children with ADHD (p = 0.018) and DCD (p = 0.054). In conclusion, children with dyslexia, ADHD and DCD show an altered oculomotor pattern and a more reduced amplitude of accommodation, not always compatible with the diagnostic criteria of an accommodative insufficiency. Accommodative and binocular vision problems are not always present in these children and cannot be considered an etiologic factor.

Differential diagnosis of vergence and saccade disorders in dyslexia

Previous studies suggest vergence and saccade abnormalities in dyslexic adolescents. However, these studies are mainly clinically based and do not provide objective measurements of eye movements, but rather subjectively evaluate vergence using haplosopic conditions in which the two eyes are dissociated (via polarizers, prisms, or intermittent spectacles). Other studies have identified deficits with binocular coordination during reading in dyslexics. Yet, there are few studies that provide objective measurements of eye movements in the dyslexic population to help provide more information regarding if these deficits could be due to an intrinsic motor problem or if they are the consequence of poor reading. 47 dyslexic adolescents (18 female, 29 male; mean age 15.5) and 44 non-dyslexic adolescents (22 female, 22 male; mean age 14.8) wore a head-based eye tracker (PupilCore, Pupil Labs, Berlin) which recorded wide angle saccade and vergence eye movements at 200 Hz. Tests were run using the REMOBI device, which produced a saccade or vergence audiovisual target. Analysis of eye movements was performed with lab-developed software, AIDEAL. The results showed statistically significant abnormalities in vergence and saccades. In vergence, dyslexics displayed a reduced amplitude of the visually driven portion of convergence and a longer duration in the initial phase of divergence. In saccades, dyslexic adolescents demonstrated slower saccades in both directions. They also had an increased disconjugate drift in the first 80 or 160 ms following saccades to the right, suggesting poor binocular coordination. For both vergence and saccades, the peak velocity and time to peak velocity was higher and earlier, respectively, in non-dyslexics compared to dyslexics; yet the average velocity of both movements was lower in dyslexics. Thus, these results indicate peculiar velocity profiles in dyslexics, particularly a slow deceleration phase in both vergence and saccades. The study provides an objective method to diagnose vergence and saccade abnormalities while viewing targets in the real three-dimensional space in a dyslexic population. Vergence abnormalities are demonstrated to be a problem in dyslexics, occurring independently from reading. We hypothesize these disconjugate drifts following saccades are the result of slow vergence capacity. Rehabilitation programs, such as those using REMOBI, should aim to target these deficits in vergence velocity, as this has been shown to improve binocular control.

Clinical Characterization of Oculomotricity in Children with and without Specific Learning Disorders

Children with specific learning disorders have been associated with oculomotor problems, with their analysis even suggested to be a potential diagnostic tool. A prospective non-randomized comparative study evaluating 59 children (6–13 years old) divided into three groups was conducted: a control group (CG) including 15 healthy emmetropic children; a group of 18 healthy children with oculomotor abnormalities (OAG); and a group of 26 children diagnosed with specific learning disorders (LDG). In all groups, besides a complete eye exam, oculomotricity was characterized with two clinical tests: Northeastern State University College of Optometry’s Oculomotor (NSUCO) and Developmental Eye Movement (DEM) tests. Concerning the NSUCO test, lower ability, precision, and head/body movement associated scorings were obtained for both smooth pursuits and saccades in OAG and LDG when compared to the CG (p < 0.001). Likewise, significantly longer time needed to read the horizontal sheet of the DEM test and a higher DEM ratio were found in OAG and LDG compared to CG (p ≤ 0.003). No differences between LDG and OAG were found in the performance with the two oculomotor tests (p ≥ 0.141). Oculomotor anomalies can be present in children with and without specific learning disorders, and therefore cannot be used as diagnostic criteria of these type of disorders.

Children with Dyslexia Have Altered Cross-Modal Processing Linked to Binocular Fusion. A Pilot Study

Introduction

The cause of dyslexia, a reading disability characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities, is unknown. A considerable body of evidence shows that dyslexics have phonological disorders. Other studies support a theory of altered cross-modal processing with the existence of a pan-sensory temporal processing deficit associated with dyslexia. Learning to read ultimately relies on the formation of automatic multisensory representations of sounds and their written representation while eyes fix a word or move along a text. We therefore studied the effect of brief sounds on vision with a modification of binocular fusion at the same time (using the Maddox Rod test).

Methods

To check if the effect of sound on vision is specific, we first tested with sounds and then replaced them with proprioceptive stimulation on 8 muscular sites. We tested two groups of children composed respectively of 14 dyslexic children and 10 controls.

Results

The results show transient visual scotoma (VS) produced by sensory stimulations associated with the manipulation of oculomotor balance, the effect being drastically higher in the dyslexic group. The spatial distribution of the VS is stochastic. The effect is not specific for sounds but exists also with proprioceptive stimulations.

Discussion

Although there was a very significant difference between the two groups, we were not able to correlate the (VS) occurrence with the dyslexic’s reading performance. One possibility to confirm the link between VS and reading impairment would be to find a specific treatment reducing the occurrence of the VS and to check its effect on dyslexia.

Diagnosis of oculomotor anomalies in children with learning disorders

This systematic review presents the existing scientific evidence for oculomotor anomalies in children with three different types of learning disorders - namely, dyslexia, dyspraxia and attention deficit-hyperactivity disorder (ADHD). This review was registered in the PROSPERO database (registration number: 139317). The QUADAS-2 tool was used to systematically evaluate the quality of the diagnostic tests used in the evaluated studies and to confirm whether the oculomotor alterations observed in the different groups of children with various learning disorders had a consistent diagnostic basis. Using this tool, the design of the articles was well elaborated, although concerns exist regarding the selection of patients and the diagnostic criteria for the binocular conditions. All the studies reviewed conclude that a pattern of oculomotor anomalies exist in the groups of children with these three types of learning disorders compared to healthy children. However, there is a concern regarding the diagnostic methodology, as no clear range of normality for the parameters used to characterise ocular motility was identified and no gold standard or reference test has been defined. In future studies, this range of normality must be developed for different oculomotor skills, and a reference test (possibly video-oculography) for the measurement of these skills must be established.

Accommodation, Convergence, and Stereopsis in Dyslexic Schoolchildren

BACKGROUND

Previous studies have shown a correlation between reading problems and binocular function, but few studies have assessed visual skills in dyslexic students, particularly in Iranian Farsi-language students. This study is aimed to determine some of the binocular functions of dyslexic children and compare it with a group of age, sex, and social class-matched control children.

MATERIALS AND METHODS

We conducted a case-control study on 27 dyslexic children and 40 age, sex, and social class-matched control children (all between 7 and 13 years old) in grades one to six. This study was performed at three elementary schools in Tehran, Iran. Monocular and binocular near point of accommodation (NPA) were measured using the subjective push-up method, near point of convergence (NPC) was determined using the standard push-up technique, and stereoacuity was tested with the Titmus stereotypes in all of the children.

RESULTS

Mean NPA of the right eye was 6.90 cm in dyslexic group and 5.98 cm in the control group (P = 0. 003). Mean NPA of the left eye in dyslexic children was 7.32 cm and in the control group was 6.23 cm (P = 0. 003). Mean binocular NPA was 6.66 cm in dyslexic and 6 cm in the control group (P = 0. 049). However, mean NPC (P = 0.33) and mean stereoacuity (P = 0.785) did not differ significantly between the dyslexic and control groups.

CONCLUSION

Our findings showed a reduced monocular and binocular NPA in dyslexic children so that this function should be assessed by an optometric clinician in children with dyslexia.

Diagnosing the Neural Circuitry of Reading

We summarize the current state of knowledge of the brain's reading circuits, and then we describe opportunities to use quantitative and reproducible methods for diagnosing these circuits. Neural circuit diagnostics-by which we mean identifying the locations and responses in an individual that differ significantly from measurements in good readers-can help parents and educators select the best remediation strategy. A sustained effort to develop and share diagnostic methods can support the societal goal of improving literacy.

Looking for ideas: Eye behavior during goal-directed internally focused cognition

Humans have a highly developed visual system, yet we spend a high proportion of our time awake ignoring the visual world and attending to our own thoughts. The present study examined eye movement characteristics of goal-directed internally focused cognition. Deliberate internally focused cognition was induced by an idea generation task. A letter-by-letter reading task served as external task. Idea generation (vs. reading) was associated with more and longer blinks and fewer microsaccades indicating an attenuation of visual input. Idea generation was further associated with more and shorter fixations, more saccades and saccades with higher amplitudes as well as heightened stimulus-independent variation of eye vergence. The latter results suggest a coupling of eye behavior to internally generated information and associated cognitive processes, i.e. searching for ideas. Our results support eye behavior patterns as indicators of goal-directed internally focused cognition through mechanisms of attenuation of visual input and coupling of eye behavior to internally generated information.

A systematic review of controlled trials on visual stress using Intuitive Overlays or the Intuitive Colorimeter

Claims that coloured filters aid reading date back 200 years and remain controversial. Some claims, for example, that more than 10% of the general population and 50% of people with dyslexia would benefit from coloured filters lack sound evidence and face validity. Publications with such claims typically cite research using methods that have not been described in the scientific literature and lack a sound aetiological framework. Notwithstanding these criticisms, some researchers have used more rigorous selection criteria and methods of prescribing coloured filters that were developed at a UK Medical Research Council unit and which have been fully described in the scientific literature. We review this research and disconfirm many of the more extreme claims surrounding this topic. This literature indicates that a minority subset of dyslexics (circa 20%) may have a condition described as visual stress which most likely results from a hyperexcitability of the visual cortex. Visual stress is characterised by symptoms of visual perceptual distortions, headaches, and eyestrain when viewing repetitive patterns, including lines of text. This review indicates that visual stress is distinct from, although sometimes co-occurs with, dyslexia. Individually prescribed coloured filters have been shown to improve reading performance in people with visual stress, but are unlikely to influence the phonological and memory deficits associated with dyslexia and therefore are not a treatment for dyslexia. This review concludes that larger and rigorous randomised controlled trials of interventions for visual stress are required. Improvements in the diagnosis of the condition are also a priority.

Can vergence training improve reading in dyslexics?

BACKGROUND

Dyslexia affects 5%-8% of the population of the Western world. While reading, different eye movements are required. Compared to other persons, dyslexics have more and longer fixations, shorter saccade amplitude, a higher percentage of regression, and more fixation disparity when reading. In non-reading situations, dyslexics do not have more binocular problems than others. The aim of the present study was to investigate whether computerized orthoptic vergence training could improve reading ability for dyslexic children.

METHODS

The study was conducted at Ängkärrskolan, Solna, an elementary school exclusively for dyslexic children. Twelve subjects, aged 13-14 years, were trained with RetCorr, a computerized vergence training program. Reading speed was assessed before and after treatment. The results were compared with an age-matched control group.

RESULTS

The dyslexic subjects conducted on average 11.75 sessions (±2.53 SD) of orthoptic training over a 5-week period. On average, the number of words read per minute before training were 87.83 (±16.80 SD) and after training 95.58 words (±18.08 SD). The difference was statistically significant (p=0.0066). In the control group, the change was from 85.00 (±19.68 SD) words to 89.37 words (±19.71 SD) over the same time period. This difference was not significant (p=0.1235).

DISCUSSION

Most scientists agree that dyslexia is mainly a phonological impairment. Nevertheless, the results show that vergence treatment might help dyslexics. Larger studies are required to provide guidance in this area.

Evaluation of aspects of binocular vision in children with dyslexia

BACKGROUND

Prevalence of dyslexia is 5%-10% of the population. Opinions differ on how binocular function affects dyslexia. The aim of the present study was to evaluate the binocular function in dyslexic children and compare it with a group of age-matched control children.

METHODS

The study was performed at Ängkärrskolan and at Kungsholmen elementary schools in Stockholm, Sweden. Sixty-three children with dyslexia and 60 control children between fourth and ninth grade participated. Monocular and binocular visual acuity, refractive error, best corrected visual acuity at distance and near, near point of convergence, amplitude of accommodation, stereopsis, phorias, and fusional reserves were evaluated in all of the children.

RESULTS

The results show that there was no difference in the visual functions tested except for the amplitude of accommodation, which was found to be reduced both monocularly and binocularly in the dyslexic children.

CONCLUSION

This study showed that only the amplitude of accommodation seems to differ in children with dyslexia as compared with the control children; however, the ability to accommodate was still good and is unlikely to hamper reading and learning ability. The results therefore support that the recent findings of binocular deficits in dyslexic children are a result of the phonological deficit of dyslexia and not an underlying cause of dyslexia.

Developmental dyslexia and vision

Developmental dyslexia affects almost 10% of school-aged children and represents a significant public health problem. Its etiology is unknown. The consistent presence of phonological difficulties combined with an inability to manipulate language sounds and the grapheme-phoneme conversion is widely acknowledged. Numerous scientific studies have also documented the presence of eye movement anomalies and deficits of perception of low contrast, low spatial frequency, and high frequency temporal visual information in dyslexics. Anomalies of visual attention with short visual attention spans have also been demonstrated in a large number of cases. Spatial orientation is also affected in dyslexics who manifest a preference for spatial attention to the right. This asymmetry may be so pronounced that it leads to a veritable neglect of space on the left side. The evaluation of treatments proposed to dyslexics whether speech or oriented towards the visual anomalies remains fragmentary. The advent of new explanatory theories, notably cerebellar, magnocellular, or proprioceptive, is an incentive for ophthalmologists to enter the world of multimodal cognition given the importance of the eye's visual input.

Dyslexic children are confronted with unstable binocular fixation while reading

Reading requires three-dimensional motor control: saccades bring the eyes from left to right, fixating word after word; and oblique saccades bring the eyes to the next line of the text. The angle of vergence of the two optic axes should be adjusted to the depth of the book or screen and - most importantly - should be maintained in a sustained manner during saccades and fixations. Maintenance of vergence is important as it is a prerequisite for a single clear image of each word to be projected onto the fovea of the eyes. Deficits in the binocular control of saccades and of vergence in dyslexics have been reported previously but only for tasks using single targets. This study examines saccades and vergence control during real text reading. Thirteen dyslexic and seven non-dyslexic children read the French text “L'Allouette” in two viewing distances (40 cm vs. 100 cm), while binocular eye movements were measured with the Chronos Eye-tracking system. We found that the binocular yoking of reading saccades was poor in dyslexic children (relative to non-dyslexics) resulting in vergence errors; their disconjugate drift during fixations was not correlated with the disconjugacy during their saccades, causing considerable variability of vergence angle from fixation to fixation. Due to such poor oculomotor adjustments during reading, the overall fixation disparity was larger for dyslexic children, putting larger demand on their sensory fusion processes. Moreover, for dyslexics the standard deviation of fixation disparity was larger particularly when reading at near distance. We conclude that besides documented phoneme processing disorders, visual/ocular motor imperfections may exist in dyslexics that lead to fixation instability and thus, to instability of the letters or words during reading; such instability may perturb fusional processes and might – in part - complicate letter/word identification.

[Ocular movements and reading: a review]

A literature review of the important findings discovered over the past three decades on ocular movements during reading is presented herein. This fairly recent function in human evolution is a sophisticated sensorimotor and cognitive activity that brings very complex neurological and motor mechanisms into play. However, knowledge in this field is limited, even though reading problems are very common in children. We collected all the references in PubMed dating from 1969 to 2009 using the following "binocular coordination", "eye movements", "reading", and "dyslexia". When reading, the visual axes move in a very particular way, notably with regard to the parallelism of the ocular axes when saccades are triggered to reach the words to be read and during fixations, which enable decoding. In fact, when reading, the visual axes are often disassociated, even going as far as to intersect in a considerable number of cases. There are relatively few studies that have examined binocular coordination during reading. We are beginning to understand how the ocular axes move during horizontal saccades. Three-dimensional studies could be the next step to providing more precise data.

Normal speed and accuracy of saccade and vergence eye movements in dyslexic reader children

Objective. Latency of eye movements depends on cortical structures while speed of execution and accuracy depends mostly on subcortical brainstem structures. Prior studies reported in dyslexic reader children abnormalities of latencies of saccades (isolated and combined with vergence); such abnormalities were attributed to deficits of fixation control and of visual attention. In this study we examine speed and accuracy characteristics of horizontal eye movements in natural space (saccades, vergence and combined movements) in dyslexic reader children. Methods. Two paradigms are tested: gap paradigm (fixation offset 200 ms prior to target onset), producing shorter latencies, in both non-dyslexic reader and dyslexic reader children and simultaneous paradigm. Seventeen dyslexic reader children (mean age: 12 ± 0.08 years) and thirteen non-dyslexic reader children (mean age: 12 ± 1 years) were tested. Horizontal eye movements from both eyes were recorded simultaneously by a photoelectric device (Oculometer, Dr. Bouis). Results. For all movements tested (saccades, vergence, isolated or combined) and for both paradigms, the mean velocity and accuracy were similar in dyslexic readers and non-dyslexic readers; no significant difference was found. Conclusion. This negative but important result, suggests no dysfunction of brainstem ocular motor circuits in dyslexic readers. It contrasts results on latencies related to visual attention dysfunction at cortical level.

Binocular function in school children with reading difficulties

INTRODUCTION

Prior findings suggest that poor readers tend to have poor binocular vision skills, but data on the binocular abilities of children with poor reading skills are lacking. Our aim was to characterize distance and near horizontal heterophoria, distance and near horizontal fusional vergence ranges, accommodative convergence/accommodation (AC/A) ratio, near point of convergence, and stereopsis in poor-reading school-age children without dyslexia selected from a non-clinical population.

METHODS

We conducted a cross-sectional study on 87 poor readers and 32 control children (all 8-13 years of age) in grades three to six recruited from eleven elementary schools in Madrid, Spain. With best spectacle correction in each subject, distance and near horizontal heterophoria measurements were obtained using the von Graefe technique, distance and near horizontal fusional vergence ranges were obtained using Risley rotary prisms, the AC/A ratio was measured using the gradient method, near point of convergence (NPC) was evaluated by the standard push-up technique using a transilluminator, and stereoacuity was tested with the Randot stereotest.

RESULTS

Mean distance base-in break and base-in recovery values were nearly 2 Delta lower (p < 0.01) in the poor readers than those recorded in the control group. However, mean distance base-out vergences (blur, break and recovery), mean distance and near horizontal heterophoria, mean near horizontal fusional vergence ranges, mean AC/A ratio, mean near point of convergence (NPC), and mean stereoacuity did not differ significantly between the poor readers and controls.

CONCLUSIONS

This study provides information on the binocular ability of children with poor reading skills but without dyslexia. Our findings suggest reduced distance base-in break and base-in recovery, such that distance fusional vergence ranges should always be assessed in children who complain of reading difficulties.

Free exploration of painting uncovers particularly loose yoking of saccades in dyslexics

Binocular yoking of saccades is essential for single vision of words during reading. This study examines the quality of binocular coordination in individuals with dyslexia, independent of the process of reading. Fifteen dyslexia children (11.2+/-1.4 years) and 15 non-dyslexia individuals (8 children, aged 11.1+/-1.3 years, and 7 adults, 24+/-3 years) were studied. Eye movements were recorded in two conditions. In the control condition, participants made saccades to a single target where the saccade direction and magnitude were controlled. In the experimental condition saccades were allowed to move freely while viewing paintings. The results indicated that, compared with the non-dyslexia group, the dyslexia group showed a larger saccade amplitude difference between the two eyes, as well as a larger conjugate post-saccadic drift, during painting exploration than that for saccades to a single target. While both groups showed a larger disconjugate post-saccadic drift during painting exploration relative to the control condition, this showed a negative correlation with saccade disconjugacy (i.e. disconjugate drift reduced the disparity) only for the non-dyslexia group. These results indicate that individuals with dyslexia have problems of binocular coordination, both during the saccade and fixations, which are independent of the reading process. It is suggested that this reflects an immaturity of the normal oculomotor learning mechanisms.

Postural control in dyslexic and non-dyslexic children

Postural control relies to visual-motion processing (afferent or efferent) and this is thought to be deficient in dyslexics. There is a controversy between clinic and fundamental studies as to the presence of posture abnormalities in dyslexics. To explore further this issue, this study examines posture stability in quite stance in 13 dyslexics (mean age: 13.5 years) and in 13 non-dyslexics (mean age: 13 years). Experiment 1 shows that, similarly to adults and elderly, all children (dyslexics and non-dyslexics), present better stability at near distance (i.e. smaller surface area of the COP, smaller lateral and antero-posterior oscillations). This could be due to reduced angular size of retinal motion signals at far, but also to convergence relaxation. Importantly, the surface area of the COP, lateral and antero-posterior oscillations are significantly higher in dyslexics. Experiment 2 examines posture stability while subjects make active vergence movements between a far and a near target. For many dyslexics, moving the eyes back and forth in depth rather improved postural stability. The only significant difference was that the lateral oscillations were still higher in dyslexics. Experiment 3 uses eye movement recordings (video-oculography) and demonstrates that dyslexics have problems with maintaining stable the angle of vergence for a prolonged period. We conclude that mild postural instability may exist in dyslexics but it could be improved by oculomotor and attention processes.