Visual attention and perception models for assessing quality in 2D and 3D stereoscopic video

La medida de calidad de vídeo sigue siendo necesaria para definir los criterios que caracterizan una señal que cumpla los requisitos de visionado impuestos por el usuario. Las nuevas tecnologías, como el vídeo 3D estereoscópico o formatos más allá de la alta definición, imponen nuevos criterios que deben ser analizadas para obtener la mayor satisfacción posible del usuario. Entre los problemas detectados durante el desarrollo de esta tesis doctoral se han determinado fenómenos que afectan a distintas fases de la cadena de producción audiovisual y tipo de contenido variado. En primer lugar, el proceso de generación de contenidos debe encontrarse controlado mediante parámetros que eviten que se produzca el disconfort visual y, consecuentemente, fatiga visual, especialmente en lo relativo a contenidos de 3D estereoscópico, tanto de animación como de acción real. Por otro lado, la medida de calidad relativa a la fase de compresión de vídeo emplea métricas que en ocasiones no se encuentran adaptadas a la percepción del usuario. El empleo de modelos psicovisuales y diagramas de atención visual permitirían ponderar las áreas de la imagen de manera que se preste mayor importancia a los píxeles que el usuario enfocará con mayor probabilidad. Estos dos bloques se relacionan a través de la definición del término saliencia. Saliencia es la capacidad del sistema visual para caracterizar una imagen visualizada ponderando las áreas que más atractivas resultan al ojo humano. La saliencia en generación de contenidos estereoscópicos se refiere principalmente a la profundidad simulada mediante la ilusión óptica, medida en términos de distancia del objeto virtual al ojo humano. Sin embargo, en vídeo bidimensional, la saliencia no se basa en la profundidad, sino en otros elementos adicionales, como el movimiento, el nivel de detalle, la posición de los píxeles o la aparición de caras, que serán los factores básicos que compondrán el modelo de atención visual desarrollado. Con el objetivo de detectar las características de una secuencia de vídeo estereoscópico que, con mayor probabilidad, pueden generar disconfort visual, se consultó la extensa literatura relativa a este tema y se realizaron unas pruebas subjetivas preliminares con usuarios. De esta forma, se llegó a la conclusión de que se producía disconfort en los casos en que se producía un cambio abrupto en la distribución de profundidades simuladas de la imagen, aparte de otras degradaciones como la denominada “violación de ventana”. A través de nuevas pruebas subjetivas centradas en analizar estos efectos con diferentes distribuciones de profundidades, se trataron de concretar los parámetros que definían esta imagen. Los resultados de las pruebas demuestran que los cambios abruptos en imágenes se producen en entornos con movimientos y disparidades negativas elevadas que producen interferencias en los procesos de acomodación y vergencia del ojo humano, así como una necesidad en el aumento de los tiempos de enfoque del cristalino. En la mejora de las métricas de calidad a través de modelos que se adaptan al sistema visual humano, se realizaron también pruebas subjetivas que ayudaron a determinar la importancia de cada uno de los factores a la hora de enmascarar una determinada degradación. Los resultados demuestran una ligera mejora en los resultados obtenidos al aplicar máscaras de ponderación y atención visual, los cuales aproximan los parámetros de calidad objetiva a la respuesta del ojo humano. ABSTRACT Video quality assessment is still a necessary tool for defining the criteria to characterize a signal with the viewing requirements imposed by the final user. New technologies, such as 3D stereoscopic video and formats of HD and beyond HD oblige to develop new analysis of video features for obtaining the highest user’s satisfaction. Among the problems detected during the process of this doctoral thesis, it has been determined that some phenomena affect to different phases in the audiovisual production chain, apart from the type of content. On first instance, the generation of contents process should be enough controlled through parameters that avoid the occurrence of visual discomfort in observer’s eye, and consequently, visual fatigue. It is especially necessary controlling sequences of stereoscopic 3D, with both animation and live-action contents. On the other hand, video quality assessment, related to compression processes, should be improved because some objective metrics are adapted to user’s perception. The use of psychovisual models and visual attention diagrams allow the weighting of image regions of interest, giving more importance to the areas which the user will focus most probably. These two work fields are related together through the definition of the term saliency. Saliency is the capacity of human visual system for characterizing an image, highlighting the areas which result more attractive to the human eye. Saliency in generation of 3DTV contents refers mainly to the simulated depth of the optic illusion, i.e. the distance from the virtual object to the human eye. On the other hand, saliency is not based on virtual depth, but on other features, such as motion, level of detail, position of pixels in the frame or face detection, which are the basic features that are part of the developed visual attention model, as demonstrated with tests. Extensive literature involving visual comfort assessment was looked up, and the development of new preliminary subjective assessment with users was performed, in order to detect the features that increase the probability of discomfort to occur. With this methodology, the conclusions drawn confirmed that one common source of visual discomfort was when an abrupt change of disparity happened in video transitions, apart from other degradations, such as window violation. New quality assessment was performed to quantify the distribution of disparities over different sequences. The results confirmed that abrupt changes in negative parallax environment produce accommodation-vergence mismatches derived from the increasing time for human crystalline to focus the virtual objects. On the other side, for developing metrics that adapt to human visual system, additional subjective tests were developed to determine the importance of each factor, which masks a concrete distortion. Results demonstrated slight improvement after applying visual attention to objective metrics. This process of weighing pixels approximates the quality results to human eye’s response.

Diagnostic validity of clinical signs associated with a large exophoria at near

Purpose. To analyze the diagnostic validity of accommodative and binocular tests in a sample of patients with a large near exophoria with moderate to severe symptoms. Methods. Two groups of patients between 19 and 35 years were recruited from a university clinic: 33 subjects with large exophoria at near vision and moderate or high visual discomfort and 33 patients with normal heterophoria and low visual discomfort. Visual discomfort was defined using the Conlon survey. A refractive exam and an exhaustive evaluation of accommodation and vergence were assessed. Diagnostic validity by means of receiver operator characteristic (ROC) curves, sensitivity (S), specificity (Sp), and positive and negative likelihood ratios (LR+, LR−) were assessed. This analysis was also carried out considering multiple tests as serial testing strategy. Results. ROC analysis showed the best diagnostic accuracy for receded near point of convergence (NPC) recovery (area = 0.929) and binocular accommodative facility (BAF) (area = 0.886). Using the cut-offs obtained with ROC analysis, the best diagnostic validity was obtained for the combination of NPC recovery and BAF (S  =  0.77, Sp = 1, LR+ = value tending to infinity, LR− = 0.23) and the combination of NPC break and recovery with BAF (S  =  0.73, Sp = 1, LR+ = tending to infinity, LR− = 0.27). Conclusions. NPC and BAF tests were the tests with the best diagnostic accuracy for subjects with large near exophoria and moderate to severe symptoms.

Evaluation of Binocular Vision Therapy Efficacy by 3D Video-Oculography Measurement of Binocular Alignment and Motility

Objective: To evaluate two cases of intermittent exotropia (IX(T)) treated by vision therapy the efficacy of the treatment by complementing the clinical examination with a 3-D video-oculography to register and to evidence the potential applicability of this technology for such purpose. Methods: We report the binocular alignment changes occurring after vision therapy in a woman of 36 years with an IX(T) of 25 prism diopters (Δ) at far and 18 Δ at near and a child of 10 years with 8 Δ of IX(T) in primary position associated to 6 Δ of left eye hypotropia. Both patients presented good visual acuity with correction in both eyes. Instability of ocular deviation was evident by VOG analysis, revealing also the presence of vertical and torsional components. Binocular vision therapy was prescribed and performed including different types of vergence, accommodation, and consciousness of diplopia training. Results: After therapy, excellent ranges of fusional vergence and a “to-the-nose” near point of convergence were obtained. The 3-D VOG examination (Sensoro Motoric Instruments, Teltow, Germany) confirmed the compensation of the deviation with a high level of stability of binocular alignment. Significant improvement could be observed after therapy in the vertical and torsional components that were found to become more stable. Patients were very satisfied with the outcome obtained by vision therapy. Conclusion: 3D-VOG is a useful technique for providing an objective register of the compensation of the ocular deviation and the stability of the binocular alignment achieved after vision therapy in cases of IX(T), providing a detailed analysis of vertical and torsional improvements.

Análisis de la eficacia de la terapia visual en el tratamiento de la exotropía intermitente mediante videooculografía 3D

Objetivo: Evaluar la eficacia del tratamiento en 3 casos de exotropia intermitente (XT(i)) mediante ejercicios de terapia visual, completando la exploración clínica con Videooculografia-30 y evidenciar la potencial aplicabilidad de esta tecnología para dicho propósito. Métodos: Exponemos los cambios ocurridos tras ejercicios de terapia visual en una mujer de 36 años con XT(i) de -25 dioptrías prismáticas (dp) de lejos y 18 dp de cerca; Un niño de 10 años de edad con 8 dp de XT(i) en posición primaria, asociados a +6 dp de hipotropia izquierda; y un hombre de 63 años con XT(i) de 6 dp en posición primaria asociada a +7 dp de hipertropia derecha. Todos los pacientes presentaron buena agudeza visual corregida en ambos ojos. La inestabilidad de la desviación ocular se evidenció mediante análisis de VOG-30, revelando la presencia de components verticales y torsionales. Se realizaron ejercicios de terapia visual, incluyendo diferentes tipos de ejercicios de vergencias, acomodación y percepción de la diplopía. Resultados: Tras la terapia visual se obtuvieron excelentes rangos de vergencias fusionales y de punto próximo de convergencia («hasta la nariz»). El examen mediante VOG-3D (Sensoro Motoric lnstruments, Teltow, Germany) confirmó la compensación de la desviación con estabilidad del alineamiento ocular. Se observó una significativa mejora después de la terapia en los components verticals y torsionales, lo cuales se hicieron más estables. Los pacientes se mostraron muy satisfechos de los resultados obtenidos. Conclusión: La VOG-3D es una técnica útil para dotamos de un método objetivo de registro de la compensación y estabilidad de la desviación ocular después de realizar ejercicios de terapia visual en casos de XT(i), ofreciéndonos un detallado análisis de la mejoría de los components verticales y torsionales.

Comparisons between analogue and numerical models of thrust wedge development

Analogue and finite element numerical models with frictional and viscous properties are used to model thrust wedge development. Comparison between model types yields valuable information about analogue model evolution, scaling laws and the relative strengths and limitations of the techniques. Both model types show a marked contrast in structural style between ‘frictional-viscous domains’ underlain by a thin viscous layer and purely ‘frictional domains’. Closely spaced thrusts form a narrow and highly asymmetric fold-and-thrust belt in the frictional domain, characterized by in-sequence propagation of forward thrusts. In contrast, the frictional-viscous domain shows a wide and low taper wedge and a thrust belt with a more symmetrical vergence, with both forward and back thrusts. The frictional-viscous domain numerical models show that the viscous layer initially simple shears as deformation propagates along it, while localized deformation resulting in the formation of a pop-up structure occurs in the overlying frictional layers. In both domains, thrust shear zones in the numerical model are generally steeper than the equivalent faults in the analogue model, because the finite element code uses a non-associated plasticity flow law. Nevertheless, the qualitative agreement between analogue and numerical models is encouraging. It shows that the continuum approximation used in numerical models can be used to model frictional materials, such as sand, provided caution is taken to properly scale the experiments, and some of the limitations are taken into account.

The relationship between CA/C ratio and individual differences in dynamic accommodative responses while viewing stereoscopic images

The oculomotor synergy as expressed by the CA/C and AC/A ratios was investigated to examine its influence on our previous observation that whereas convergence responses to stereoscopic images are generally stable, some individuals exhibit significant accommodative overshoot. Using a modified video refraction unit while viewing a stereoscopic LCD, accommodative and convergence responses to balanced and unbalanced vergence and focal stimuli (BVFS and UBVFS) were measured. Accommodative overshoot of at least 0.3 D was found in 3 out of 8 subjects for UBVFS. The accommodative response differential (RD) was taken to be the difference between the initial response and the subsequent mean static steady-state response. Without overshoot, RD was quantified by finding the initial response component. A mean RD of 0.11 +/- 0.27 D was found for the 1.0 D step UBVFS condition. The mean RD for the BVFS was 0.00 +/- 0.17 D. There was a significant positive correlation between CA/C ratio and RD (r = +0.75, n = 8, p <0.05) for only UBVFS. We propose that inter-subject variation in RD is influenced by the CA/C ratio as follows: an initial convergence response, induced by disparity of the image, generates convergence-driven accommodation commensurate with the CA/C ratio; the associated transient defocus subsequently decays to a balanced position between defocus-induced and convergence-induced accommodations.

The effect of ageing on in vivo human ciliary muscle morphology and contractility

Purpose. To assess the effect of ageing on in vivo human ciliary muscle morphology and contractility during accommodation. Methods. Seventy-nine subjects, aged 19–70 years were recruited. High-resolution images were acquired of nasal and temporal ciliary muscle in the relaxed state, and at stimulus vergence levels of -4 and -8 D, using anterior segment optical coherence tomography (AS-OCT). Objective refractions and axial lengths were also recorded. Linear regression analysis was performed to determine the effect of age on nasal and temporal ciliary muscle morphologic characteristics. Results. Ciliary muscle anterior length decreased significantly with age both nasally (R = 0.461, P = 0.001) and temporally (R = 0.619, P < 0.001) in emmetropic eyes. In a subset of 37 participants, ciliary muscle maximum width increased significantly with age, by 2.8 µm/year nasally (R = 0.54, P < 0.001) and 3.0 µm/year temporally (R = 0.44, P = 0.007), while the distance from the inner apex of the ciliary muscle to the scleral spur decreased significantly with age on both the nasal and temporal aspects (R = 0.47; P = 0.004 and R = 0.43; P = 0.009, respectively). During accommodation, changes to ciliary muscle thickness and length remained constant throughout life. Conclusions. The human ciliary muscle undergoes age-dependent changes in morphology that suggest an antero-inwards displacement of muscle mass, particularly in emmetropic eyes. However, the morphologic changes observed appear not to affect the ability of the muscle to contract during accommodation, even in established presbyopes, thus supporting a lenticular model of presbyopia development.

In vivo analysis of ciliary muscle morphologic changes with accommodation and axial ametropia

Purpose. To use anterior segment optical coherence tomography (AS-OCT) to analyze ciliary muscle morphology and changes with accommodation and axial ametropia. Methods. Fifty prepresbyopic volunteers, aged 19 to 34 years were recruited. High-resolution images were acquired of nasal and temporal ciliary muscles in the relaxed state and at stimulus vergence levels of -4 and -8 D. Objective accommodative responses and axial lengths were also recorded. Two-way, mixed-factor analyses of variance (ANOVAs) were used to assess the changes in ciliary muscle parameters with accommodation and determine whether these changes are dependent on the nasal–temporal aspect or axial length, whereas linear regression analysis was used to analyze the relationship between axial length and ciliary muscle length. Results. The ciliary muscle was longer (r = 0.34, P = 0.02), but not significantly thicker (F = 2.84, P = 0.06), in eyes with greater axial length. With accommodation, the ciliary muscle showed a contractile shortening (F = 42.9. P < 0.001), particularly anteriorly (F = 177.2, P < 0.001), and a thickening of the anterior portion (F= 46.2, P < 0.001). The ciliary muscle was thicker (F = 17.8, P < 0.001) and showed a greater contractile response on the temporal side. Conclusions. The accommodative changes observed support an anterior, as well as centripetal, contractile shift of ciliary muscle mass.

Phakometric measurement of ocular surface radii of curvature, axial separations and alignment in relaxed and accommodated human eyes

Measurements (autokeratometry, A-scan ultrasonography and video ophthalmophakometry) of ocular surface radii, axial separations and alignment were made in the horizontal meridian of nine emmetropes (aged 20-38 years) with relaxed (cycloplegia) and active accommodation (mean ± 95% confidence interval: 3.7 ± 1.1 D). The anterior chamber depth (-1.5 ± 0.3 D) and both crystalline lens surfaces (front 3.1 ± 0.8 D; rear 2.1 ± 0.6 D) contributed to dioptric vergence changes that accompany accommodation. Accommodation did not alter ocular surface alignment. Ocular misalignment in relaxed eyes is mainly because of eye rotation (5.7 ± 1.6° temporally) with small amounts of lens tilt (0.2 ± 0.8° temporally) and decentration (0.1 ± 0.1 mm nasally) but these results must be viewed with caution as we did not account for corneal asymmetry. Comparison of calculated and empirically derived coefficients (upon which ocular surface alignment calculations depend) revealed that negligible inherent errors arose from neglect of ocular surface asphericity, lens gradient refractive index properties, surface astigmatism, effects of pupil size and centration, assumed eye rotation axis position and use of linear equations for analysing Purkinje image shifts. © 2004 The College of Optometrists.

Effect of β-adrenoceptor antagonists on autonomic control of ciliary smooth muscle

Purpose: Pharmacological intervention with peripheral sympathetic transmission at ciliary smooth muscle neuro-receptor junctions has been used against a background of controlled parasympathetic activity to investigate the characteristics of autonomic control of ocular accommodation. Methods: A continuously recording infrared optometer was used to measure accommodation on a group of five visually normal emmetropic subjects under open- and closed-loop conditions. A double-blind protocol between saline, timolol and betaxolol was used to differentiate between the localised action on ciliary smooth muscle and effects induced by changes in stimulus conditions. Data were collected before and 45 min following the instillation of saline, timolol or betaxolol. Open-loop post-task decay was investigated following 3 min sustained near fixation of a stimulus placed 3 D above the subject's pre-task tonic accommodation level. Closed-loop dynamic responses were recorded for each treatment condition while subjects viewed sinusoidally (0.05-0.6 Hz) or stepwise vergence-modulated targets over a 2 D range (2-4 D). Results: Open-loop data demonstrate a rapid post-task regression to pre-task tonic accommodation levels for saline and betaxolol control conditions. A slow positive post-task shift was induced by timolol indicating that sympathetic inhibition contributes to accommodative adaptation during sustained near vision. Closed-loop accommodation responses to temporally modulated sinusoidal stimuli showed characteristic features for both saline and betaxolol control conditions. Timolol induced a reduced gain for low- and mid-temporal frequencies (< 0.3 Hz) but did not affect the response at higher temporal frequencies. Response times to stepwise stimuli increased following the instillation of timolol for the near-to-far fixation condition compared with the controls and was related to the period of sustained prior fixation. Conclusions: Modulation of accommodation under open- and closed-loop conditions by a non-selective β-blocker is consistent with the temporal and inhibitory features of sympathetic innervation to ciliary smooth muscle. Although parasympathetic innervation predominates there is evidence to support a role for sympathetic innervation in the control of ocular accommodation. © 2002 The College of Optometrists.

Assessment of the oculomotor response in human factor environments

The need to measure the response of the oculomotor system, such as ocular accommodation, accurately and in real-world environments is essential. New instruments have been developed over the past 50 years to measure eye focus including the extensively utilised and well validated Canon R-1, but in general these have had limitations such as a closed field-of-view, a poor temporal resolution and the need for extensive instrumentation bulk preventing naturalistic performance of environmental tasks. The use of photoretinoscopy and more specifically the PowerRefractor was examined in this regard due to its remote nature, binocular measurement of accommodation, eye movement and pupil size and its open field-of-view. The accuracy of the PowerRefractor to measure refractive error was on averaging similar, but more variable than subjective refraction and previously validated instrumentation. The PowerRefractor was found to be tolerant to eye movements away from the visual axis, but could not function with small pupil sizes in brighter illumination. The PowerRefractor underestimated the lead of accommodation and overestimated the slope of the accommodation stimulus response curve. The PowerRefractor and the SRW-5000 were used to measure the oculomotor responses in a variety of real-world environment: spectacles compared to single vision contract lenses; the use of multifocal contact lenses by pre-presbyopes (relevant to studies on myopia retardation); and ‘accommodating’ intraocular lenses. Due to the accuracy concerns with the PowerRefractor, a purpose-built photoretinoscope was designed to measure the oculomotor response to a monocular head-mounted display. In conclusion, this thesis has shown the ability of photoretinoscopy to quantify changes in the oculomotor system. However there are some major limitations to the PowerRefractor, such as the need for individual calibration for accurate measures of accommodation and vergence, and the relatively large pupil size necessary for measurement.

Oculomotor function in incipient presbyopia

The incipient phase of presbyopia represents a loss in accommodative amplitude of approximately 3 dioptres between the ages of 35 and 45 and is the prelude to the need for a reading addition. The need to maintain single binocular vision during this period requires re-calibration of the correspondence between accommodation and vergence response. No previous study has specifically attempted to correlate change in accommodative status with the profile of oculomotor responses occurring within the incipient phase of presbyopia. Measurements were made of the amplitude of accommodation, stimulus and response AC/A ratios, CA/C ratio, tonic accommodation, tonic vergence, proximal vergence, vergence adaptation and accommodative adaptation of 38 subjects. Twenty subjects were aged 35 to 45 years of age and 10 subjects were aged 20 to 30 years of age at the commencement of the study. The measurements were repeated at four-monthly intervals for a total of two years. The results of this study fail to support the Hess-Gullstrand theory of presbyopia with evidence that the effort to produce a unit change in accommodation increases with age. The data obtained has enabled the analysis of how each individual oculomotor function varies with the decline in amplitude of accommodation. MATLAB/SIMULINK software has been used to assist in the analysis and to allow the amendment of existing models to represent accurately the ageing oculomotor system. This study has proposed that with the decline in the amplitude of accommodation there is an increase in the accommodative convergence response per unit of accommodative response. To compensate for this increase, evidence has been found of a decrease in tonic vergence with age. If this decline in tonic vergence is not sufficient to counteract the increase in accommodative convergence, it is proposed that the near vision response is limited to the maximum vergence response that can be tolerated, with the resulting lower accommodative response being compensated for by an increase in the subjective depth-of-focus. When the blur due to the decrease in accommodative response can no longer be tolerated, the first reading addition will be required.

Measurement of ocular component contributions to residual astigmatism in adult human eyes

The aim of this study was to determine whether an ophthalmophakometric technique could offer a feasible means of investigating ocular component contributions to residual astigmatism in human eyes. Current opinion was gathered on the prevalence, magnitude and source of residual astigmatism. It emerged that a comprehensive evaluation of the astigmatic contributions of the eye's internal ocular surfaces and their respective axial separations (effectivity) had not been carried out to date. An ophthalmophakometric technique was developed to measure astigmatism arising from the internal ocular components. Procedures included the measurement of refractive error (infra-red autorefractometry), anterior corneal surface power (computerised video keratography), axial distances (A-scan ultrasonography) and the powers of the posterior corneal surface in addition to both surfaces of the crystalline lens (multi-meridional still flash ophthalmophakometry). Computing schemes were developed to yield the required biometric data. These included (1) calculation of crystalline lens surface powers in the absence of Purkinje images arising from its anterior surface, (2) application of meridional analysis to derive spherocylindrical surface powers from notional powers calculated along four pre-selected meridians, (3) application of astigmatic decomposition and vergence analysis to calculate contributions to residual astigmatism of ocular components with obliquely related cylinder axes, (4) calculation of the effect of random experimental errors on the calculated ocular component data. A complete set of biometric measurements were taken from both eyes of 66 undergraduate students. Effectivity due to corneal thickness made the smallest cylinder power contribution (up to 0.25DC) to residual astigmatism followed by contributions of the anterior chamber depth (up to 0.50DC) and crystalline lens thickness (up to 1.00DC). In each case astigmatic contributions were predominantly direct. More astigmatism arose from the posterior corneal surface (up to 1.00DC) and both crystalline lens surfaces (up to 2.50DC). The astigmatic contributions of the posterior corneal and lens surfaces were found to be predominantly inverse whilst direct astigmatism arose from the anterior lens surface. Very similar results were found for right versus left eyes and males versus females. Repeatability was assessed on 20 individuals. The ophthalmophakometric method was found to be prone to considerable accumulated experimental errors. However, these errors are random in nature so that group averaged data were found to be reasonably repeatable. A further confirmatory study was carried out on 10 individuals which demonstrated that biometric measurements made with and without cycloplegia did not differ significantly.

Oculomotor responses in emmetropia and myopia

It has been proposed that early-onset myopia (EOM) i.e. myopia onset before the age of 15 is primarily inherited whereas late-onset myopia (LOM) i.e. myopia onset after the age of 16 is induced by environmental factors, principally sustained near vision. No consensus exists as to which aspect of the near vision response; accommodation, vergence or their synergistic cross links promotes LOM development. Furthermore, the mechanism by which near vision could induce elongation of posterior chamber is obscure although there is evidence to show that ciliary muscle tone plays an important role. By comparing accommodation and vergence responses of emmetropes (EMMs), EOMs and LOMs under both open- and closed-loop conditions, this thesis aims to define further the oculomotor correlates of myopic development. A Canon Autoref R-1 optometer was used to measure accommodation responses while an Apple IIe controlled the flashed Maddox Rod sequence used when measuring vergence. Both techniques permitted open- and closed-loop measures to be obtained. The results presented demonstrate that it is unlikely that those individuals susceptible to LOM can be distinguished with regard to oculomotor responses or innervational characteristics of the ciliary muscle. The aetiology of LOM may be associated with ciliary muscle function but account needs to be taken of interactions between the ciliary muscle, choriod, sclera and introcular pressure and further research is necessary before those EMMs susceptible to LOM can be identified.

Ophthalmic factors in dyslexia

Although the role of ophthalmic factors in dyslexia remains the subject of controversy, recent research has indicated that the correlates of dyslexia may include binocular dysfunction, unstable motor ocular dominance, a deficit of the transient visual subsystem, and an anomaly that can be treated with tinted lenses. These features, typically, have been studied in isolation and their inter-relationship has received little attention. The aim of the present research was to investigate ophthalmic factors in dyslexia, with a particular emphasis on the interaction between optometric variables. Further aims were to establish the most appropriate investigative techniques for optometric practice and to explore the relationship between optometric and psychometric variables. A pilot study was used to refine the experimental design for a subsequent detailed study of 39 children with a specific reading disability and 43 good readers, who were selected from 240 children. The groups were matched for age, sex, and performance IQ. The following factors emerged as correlates of dyslexia: slight impaired visual acuity; reduced vergence amplitudes; increased vergence instability; decreased accommodative amplitude; poor peformance at tests that were designed to assess the function of the transient visual system; and slightly slower performance at a non-verbal simulated reading visual search task. The `transient system deficit', as measured by reduced flicker sensitivity, was significantly associated with decreased accommodative and vergence amplitudes. This links the motor and sensory visual correlates of dyslexia. Although the binocular dysfunction was correlated with increased symptoms, the difference in the groups' simulated reading visual search task performance was largely attributable to psychometric variables. The results suggest tht optometric problems may be a contributory factor in dyslexia, but are unlikely to play a key causative role. Several optometric variables were confounded by psychometric parameters, and this interaction should be a priority for future investigation.