Birefringent properties of the human cornea in vivo : towards a new model of corneal structure

The fundamental corneal properties of mechanical rigidity, maintenance of curvature and optical transparency result from the specific organisation of collagen fibrils in the corneal stroma. The exact arrangement of stromal collagen is currently unknown but several structural models have been proposed. The purpose of the present study is to investigate inconsistencies between current x‐ray derived structural models of the cornea and optically derived birefringence data. Firstly, the thesis reviews the current understanding of corneal structure, particularly in relation to corneal birefringence. It also reviews and develops the different analytical approaches used to model optical biaxial behaviour, particularly as applied to predict corneal optical phase retardation. The second part develops a novel technique of elliptic polarization biomicroscopy (EPB), enabling study of corneal birefringence in vivo. Using EPB, the pattern of corneal retardation is recorded for a range of human subjects. This dataset is then used to investigate both central and peripheral corneal birefringence as well as the corneal microstructure. A key finding is that the central parts of the cornea exhibit a retardation pattern compatible with a negative biaxial crystal, whereas the peripheral corneal regions do not. Furthermore, within the central regions of the cornea, orthogonal confocal conic fibrillar structures are identified which resemble the analytically derived contours of equal refractive index of an ideal negative biaxial crystal. The third part of this work presents a synthesis of previous published experimental, anatomical and theoretical findings and the experimental results presented in this thesis. Based on these findings, a novel corneal structural model is proposed that comprises overlapping spherical elliptic structural units. Finally, ensuing biomechanical and clinical consequences of the spherical elliptic structural model and of the EPB technique are discussed including their potential diagnostic and surgical applications

Polarization perception in humans: on the origin of and relationship between Maxwell’s spot and Haidinger’s brushes

Under specific conditions of illumination and polarization, differential absorption of light by macular pigments is perceived as the entoptic phenomena of Maxwell’s spot (MS) or Haidinger’s brushes (HB). To simulate MS and HB, an existing computational model of polarization-dependent properties of the human macula was extended by incorporating neuronal adaptation to stabilized retinal images. The model predicted that polarized light modifies the appearance of MS leading to the perception of a novel phenomenon. The model also predicted a correlation between the observed diameters of MS and HB. Predictions were tested psychophysically in human observers, whose measured differences in the diameters of each entoptic phenomenon generated with depolarized and linearly polarized light were consistent with the model simulations. These findings support a common origin of each phenomenon, and are relevant to the clinical use of polarization stimuli in detecting and monitoring human eye disorders, including macular degeneration. We conclude: (i) MS and HB both result from differential light absorption through a radial diattenuator, compatible with the arrangement of macular pigments in Henle fibres; (ii) the morphology of MS is dependent on the degree of linear polarization; (iii) perceptual differences between MS and HB result from different states of neural adaptation

The Clinical Application of Polarization Pattern Perception

Purpose: Determine the repeatability of and optimum stimulus parameters for testing polarization pattern perception in a real-world clinical population, and assess the ability of polarization perception to distinguish normal from abnormal eyes. Methods: Polarization perception was evaluated in staff and patients attending ophthalmology clinics at Warwick Hospital, UK. A series of visual stimuli were presented in pseudorandom order using a liquid-crystal-display–based polarization pattern generator. Stimuli included geometric patterns, gratings, checkerboards, and optotypes. Participants had one or both eyes diagnosed as normal or abnormal following ophthalmic examination, optical coherence tomography, and measures of visual acuity. Measurement scores were assigned to the eye(s) of each participant depending on the total number of stimuli perceived or identified. Results: Stimuli covered the range of spatial scales resolvable within polarization perception by normal and abnormal eyes. Different stimuli had different saliencies. For each stimulus type, polarization perception in the abnormal group was significantly reduced compared with normal eyes (P < 0.001). Relative stimulus salience was broadly similar for normal-eye and abnormal-eye viewing groups, being greatest for radially symmetric patterns and least for optotypes. Checkerboard pattern salience had an inverse logarithmic relationship with check fundamental spatial frequency. A devised metric covering the dynamic range of polarization perception was repeatable, and the score derived from the metric was reduced in the abnormal group compared with the normal group (P < 0.001). Conclusions: Clinically useful metrics of polarization perception distinguish between normal and abnormal eyes. Translational Relevance: Perception of spatial patterns formed of non-uniform polarization fields has potential as a quantitative clinical diagnostic measuremen

The Differential Contribution of Macular Pigments and Foveal Anatomy to the Perception of Maxwell’s Spot and Haidinger’s Brushes

The relationship of macular pigments and foveal anatomy to the perception of Maxwell’s spot (MS) and Haidinger’s brushes (HB) entoptic phenomena were investigated. Dual-wavelength-autofluorescence and OCT were used to define macular pigment density and foveal anatomy in 52 eyes. MS was generated by alternating unpolarized red/blue and red/green uniform field illumination. HB was generated by alternating the linear polarization axis of a uniform blue field. In Experiment 1, horizontal widths of MS and HB were measured using a micrometer system and compared with macular pigment densities and OCT-defined morphometry. MS radius (mean 1.4°) was significantly less than HB radius (mean 1.6°), with the spatial extent of both phenomena falling between the boundaries of the foveola and foveal pit. Multiple regression showed MS and HB radii to be significantly associated with the macular pigment spatial profile radius. HB radius, but not MS radius, was also significantly associated with foveolar morphometry. Experiment 2 compared perceptual profiles of MS with macular pigment distribution patterns and demonstrated close agreement. The size and appearance of MS is a direct indicator of macular pigment density and distribution. Measures of HB radii are less specific, with dependence on both macular pigment density and foveal structure

The Effect of Age-Related Macular Degeneration on Polarization Pattern Perception

Purpose: The purpose of this study was to determine if a battery of polarization-modulated stimuli, quantified as a single metric, is effective in identifying macular disease in the presence/absence of cataract or pseudophakia. Methods: Using a modified liquid crystal display, polarization pattern perception (PPP) for a formulated battery of geometric and logMAR stimuli was evaluated in participants that had either no eye pathology (healthy participants) or were grouped according to the presence of cataract, pseudophakia, and/or age-related macular degeneration (AMD). PPP was quantified as response frequencies to individual stimuli, and as a novel monocular polarization sensitivity score (Ps) based on perception of the stimulus battery set. Results: Stimulus response frequencies were pattern-dependent and, compared with healthy participants, reduced for cataract and AMD groups but not for subjects with pseudophakia. Compared with healthy eyes (n = 47, median Ps = 17), Ps was significantly reduced by AMD (n = 59, median Ps = 1, P < 0.001) and, to a lesser extent, by cataracts (n = 80, median Ps = 6, P < 0.001). There was no significant difference between Ps for healthy and pseudophakic eyes (n = 47, median Ps = 13, P = 0.323). There was no significant correlation between Ps and logMAR visual acuity. Conclusions: In the absence of significant cataract, or in pseudophakia, a set of polarization-modulated visual stimuli, quantified as the Ps score, distinguishes AMD from healthy maculae. Translational Relevance: Perception of polarization-modulated stimuli, previously shown to be macula-dependent in a laboratory setting, is effective as a test of macular function in health and disease in a clinic setting

Computational simulation of Haidinger's brushes

Haidinger's brushes (HB) are entoptic phenomena resulting from differential absorption of linear polarized light by the human macula. Computational models have assisted in understanding the behavior of these subjective phenomena but have been limited in their application. This study presents a revised computational model that incorporates known determinants of the form and behavior of HB. The model generates both static and animated simulations of HB that can be quantified by their density, contrast, and radial/circumferential extent. Measured physiological parameters are used to demonstrate the dependency of HB on macular pigment (MP) density, MP distribution, and ocular retardation. Physiological variations in these parameters explain the reported variations in the perception of HB

Haidinger’s brushes elicited at varying degrees of polarization rapidly and easily assesses total macular pigmentation

Macular pigments (MPs), by absorbing potentially toxic short-wavelength (400–500 nm) visible light, provide protection against photo-chemical damage thought to be relevant in the pathogenesis of age-related macular degeneration (AMD). A method of screening for low levels of MPs could be part of a prevention strategy for helping people to delay the onset of AMD. We introduce a new method for assessing MP density that takes advantage of the polarization-dependent absorption of blue light by MPs, which results in the entoptic phenomenon called Haidinger’s brushes (HB). Subjects were asked to identify the direction of rotation of HB when presented with a circular stimulus illuminated with an even intensity of polarized white light in which the electric field vector was rotating either clockwise or anti-clockwise. By reducing the degree of polarization of the stimulus light, a threshold for perceiving HB (degree of polarization threshold) was determined and correlated (r2=0.66) to macular pigment optical density assessed using dual-wavelength fundus autofluoresence. The speed and ease of measurement of degree of polarization threshold makes it well suited for large-scale screening of macular pigmentation