Investigations on the Reptilian Spectacle

The eyes of snakes and most geckos, as well as a number of other disparate squamate taxa, are shielded beneath a layer of transparent integument referred to as the “reptilian spectacle.” Derived from the embryonic fusion of palpebral tissues, the spectacle contains a number of specializations of the skin to benefit vision while still allowing it to function as the primary barrier to the environment. For example, in nearly all species that possess it, it is markedly thinned compared to the surrounding integument and its keratinized scale is optically transparent. While the spectacle may thus seem ideally adapted to vision in allowing the eyes to be always unoccluded, it does have a few drawbacks. One such drawback is its vascularity, the implications of which are still not fully understood, but are explored herein. As no recent synthesis exists of the body of knowledge on reptilian spectacles, the first chapter of this thesis consists of a review of spectacle anatomy, physiology, adaptive significance and evolution to help put into context the following chapters that present original research. The second chapter describes the dynamics of blood flow through the spectacle vasculature of colubrid snakes, demonstrating three main points: (1) that the spectacle vasculature exhibits cycles of regular dilation and constriction, (2) that the visual perception of a threat induces vasoconstriction of its vessels, and (3) that spectacle vessels remain dilated throughout the renewal phase. The implications of these points are discussed. The third chapter describes the spectral transmittance of the shed spectacle scale, the only keratinized structure in the animal kingdom to contribute to the dioptric apparatus of the eye, as well as its thickness. Spectacle scale transmittance and thickness was found to differ dramatically between snakes and geckos and found in snakes to vary between families. The adaptive significance of the observed variation is discussed. The fourth chapter describes biochemical analyses of the shed spectacle scales of snakes and geckos and compares their composition to other scales in the integument. Spectacle scales were found to differ significantly from other scales in their keratin composition, and gecko spectacle scales in particular were found to lack ß keratin, that hard corneous protein thought to be common to all reptile scales. The concluding chapter will discuss where this research has brought the state of our knowledge on the spectacle and offers thoughts on potentially useful avenues for further research

The ocular surface and blinking in children

Background: Ocular surface characteristics in children have not been as well investigated as in adults. Children’s digital device usage is rapidly increasing, and smartphones are the most commonly used device. The ocular surface impacts are unknown in children. In adults, use of digital devices induces ocular symptoms, adversely impacts blinking, and disrupts ocular surface homeostasis. Although blinking is integral to a healthy ocular surface, this is yet to be characterised in children, including the effects of digital devices. This thesis aims to characterise the ocular surface of children including blinking and to examine the impact of smartphone use on ocular surface homeostasis in children. Methods: The literature on ocular surface (symptoms, clinical indices, tear film function, blinking) of healthy children was reviewed and a meta-analysis of tear film stability and tear secretion was conducted. Cross-sectional studies of healthy school-aged children were conducted to examine the utility of commonly used adult-validated dry eye symptom questionnaires (SANDE, OSDI, NRS, OCI, DEQ-5, IOSS), and to characterise ocular surface clinical indices. This included blinking which was measured in situ using a novel eye tracking headset (Pupil Labs GmbH, Germany). The impact of one hour smartphone gaming on the ocular surface (including symptoms and blink parameters) of this paediatric population was examined with an intervention study. Blinking was also examined in situ under different conditions and tasks (reading from hard copy and on digital devices, conversation, walking) using the eye tracking headset in healthy adults. Repeatability of blink measurements (blink rate and interblink interval) in adults using the eye tracking headset was determined. Results: Ocular symptoms, tear film function and blinking were sparsely reported in children. The pooled mean by tear stability measurement methods in the meta-analysis were higher than previously reported in healthy adults while the pooled mean for tear secretion by methods were within the expected normal range for adults. Six existing dry eye questionnaires could be successfully used in paediatric eye care, and their repeatability was mostly comparable to that reported previously in adults. More time and assistance were at times required for younger children and specific terms such as ‘gritty’ and ‘foreign body sensation’ were not always well understood by younger children. The DEQ-5 and IOSS are recommended for use in younger age children. Blinking was associated with greater tear volume and worse meibomian gland expressibility but not digital device use, age, sex, or symptoms in children. One hour smartphone gaming led to increased symptoms of dryness, discomfort, and tiredness but did not impact tear film function. Blinking was rapidly reduced by a third within the first minute of gaming and this effect remained unchanged throughout one hour of gaming (p<0.001). Blink rate was consistently slower during all reading tasks compared to conversation (p≤0.002) and walking (p≤0.03), irrespective of task complexity, screen brightness, working distance or device used. Blinking could be reliably measured using a wearable eye tracking headset; the coefficient of repeatability for blink rate was ±12.4 blinks/min. Conclusions: This study established that existing dry eye questionnaires can be reliably used in children to examine the impact of challenges such as digital device use. An eye tracking headset reliably measured blink rate in situ in adults and detected differences in blinking during various real-life tasks. It was successfully used in children to measure blinking in situ showing an immediate and sustained slowing of blinking, evident after up to one hour of smartphone gaming. An hour of smartphone gaming worsened ocular comfort in children but did not appear to disturb the tear film. Given the ubiquitous use of smartphones by children, future work should examine whether effects reported herein persist or get worse over the longer term, potentially causing cumulative damage to the ocular surface. Blink amplitude and relationships with ocular surface clinical indices and digital device use may be explored using the methods established in this study

Astigmatism and Pseudoaccommodation in Pseudophakic Eyes

noAdvanced IOLs with circumferential zones of different power provide pseudoaccommodation. We investigated the potential for power variation with meridian, namely astigmatism, to provide pseudo-accommodation. With appropriate power and axis orientations, acceptable pseudo-accommodation can be achieved

What is a blink? Classifying and characterizing blinks in eye openness signals

Blinks, the closing and opening of the eyelids, are used in a wide array of fields where human function and behavior are studied. In data from video-based eye trackers, blink rate and duration are often estimated from the pupil-size signal. However, blinks and their parameters can be estimated only indirectly from this signal, since it does not explicitly contain information about the eyelid position. We ask whether blinks detected from an eye openness signal that estimates the distance between the eyelids (EO blinks) are comparable to blinks detected with a traditional algorithm using the pupil-size signal (PS blinks) and how robust blink detection is when data quality is low. In terms of rate, there was an almost-perfect overlap between EO and PS blink (F1 score: 0.98) when the head was in the center of the eye tracker’s tracking range where data quality was high and a high overlap (F1 score 0.94) when the head was at the edge of the tracking range where data quality was worse. When there was a difference in blink rate between EO and PS blinks, it was mainly due to data loss in the pupil-size signal. Blink durations were about 60 ms longer in EO blinks compared to PS blinks. Moreover, the dynamics of EO blinks was similar to results from previous literature. We conclude that the eye openness signal together with our proposed blink detection algorithm provides an advantageous method to detect and describe blinks in greater detail

Aerospace medicine and biology: A continuing bibliography with indexes, supplement 118

This special bibliography lists 338 reports, articles, and other documents introduced into the NASA scientific and technical information system in July 1973

In vivo Imaging of Light Induced Intrinsic Optical Signals in the Chicken Retina with a Combined Ultra-High Resolution Optical Coherence Tomography and Electroretinography System

The main objective of this thesis is to investigate the intrinsic optical signals (IOSs) with an ultra-high resolution optical coherence tomography system (UHROCT). In order to study the retinal IOSs evoked by visible light, an UHROCT and an Electroretinogram (ERG) system was combined. An animal model (chicken retina) based on its retinal avascularity and cone dominance, was selected. Imaging the chicken retina with OCT resulted in high contrast, high resolution (~3μm axial and ~5 μm lateral resolution) 2D and 3D volumetric tomograms, in which all retina layers were clearly distinguishable. Using the combined UHROCT and ERG system to image IOSs from the chicken retina exposed to visible light (7ms green flash) resulted in highly reproducible IOS recordings from all retinal layers for the first time. All inner retinal layers showed an initial increase and subsequently a decrease in the intensity of the backreflected imaging light within the first 100 ms after the onset of the stimulus. Outer segments of the photoreceptors also showed a decrease in the backreflected imaging light within 100 ms after the onset of the flash. All retinal layers showed a strong decrease in the backreflected light within 150 to 175 ms after the onset of the flash. Imaging the pupil dynamics of the chicken with a modified combined UHROCT and ERG system showed that part of the strong negative IOSs observed in all retinal layers resulted from the vignetting of the imaging beam due to the light induced pupil constriction. Thorough analysis of the pupil dynamics acquired with UHROCT showed a time dependent effect of the anesthesia agent on pupil constriction. Further experiments to investigate an anesthesia effects on retinal function showed significant changes in ERG components. Statistical analysis showed that Isoflurane anesthesia severely affects the inner retinal response. In conclusion, it was hypothesized that the fast IOSs within ~50-100 ms after the onset of the visual stimulus originated from the neuronal tissue in the retina and are related to tissue optical property changes as a result of the electrical signal propagation in the light activated retina. Longer term decreases in backreflected light are likely due to pupil changes

Optimizations and applications in head-mounted video-based eye tracking

Video-based eye tracking techniques have become increasingly attractive in many research fields, such as visual perception and human-computer interface design. The technique primarily relies on the positional difference between the center of the eye\u27s pupil and the first-surface reflection at the cornea, the corneal reflection (CR). This difference vector is mapped to determine an observer\u27s point of regard (POR). In current head-mounted video-based eye trackers, the systems are limited in several aspects, such as inadequate measurement range and misdetection of eye features (pupil and CR). This research first proposes a new `structured illumination\u27 configuration, using multiple IREDs to illuminate the eye, to ensure that eye positions can still be tracked even during extreme eye movements (up to ±45° horizontally and ±25° vertically). Then eye features are detected by a two-stage processing approach. First, potential CRs and the pupil are isolated based on statistical information in an eye image. Second, genuine CRs are distinguished by a novel CR location prediction technique based on the well-correlated relationship between the offset of the pupil and that of the CR. The optical relationship of the pupil and CR offsets derived in this thesis can be applied to two typical illumination configurations - collimated and near-source ones- in the video-based eye tracking system. The relationships from the optical derivation and that from an experimental measurement match well. Two application studies, smooth pursuit dynamics in controlled static (laboratory) and unconstrained vibrating (car) environments were conducted. In the first study, the extended stimuli (color photographs subtending 2° and 17°, respectively) were found to enhance smooth pursuit movements induced by realistic images, and the eye velocity for tracking a small dot (subtending \u3c0.1°) was saturated at about 64 deg/sec while the saturation velocity occurred at higher velocities for the extended images. The difference in gain due to target size was significant between dot and the two extended stimuli, while no statistical difference existed between the two extended stimuli. In the second study, twovisual stimuli same as in the first study were used. The visual performance was impaired dramatically due to the whole body motion in the car, even in the tracking of a slowly moving target (2 deg/sec); the eye was found not able to perform a pursuit task as smooth as in the static environment though the unconstrained head motion in the unstable condition was supposed to enhance the visual performance

Ocular and systemic vascular alterations in overweight and obese individuals undergoing weight-loss interventions

Obesity and its accompanying comorbidities play a crucial role in the pathogenesis of endothelial function which is a pre-cursor to atherosclerosis, subsequently leading to increases in cardiovascular risk. However, amelioration of these risk factors and improvements in endothelial function have never been fully explored in functional assessments of the retinal and peripheral microcirculations.The aim of this thesis was therefore to investigate the presence and impact of weight-loss interventions in overweight and obese individuals and also the relationships between functional measurements of different vascular beds. The principle findings of this work were:1. The relationship between retinal and peripheral vascular function in healthy individuals with low cardiovascular risk• Participants with higher peripheral vascular reactivity indices had a higher amplitude change from maximum to minimum and also showed enhanced reaction times to flicker provocation, which correlated to the degree of peripheral vascular function. 2. The effects of physical training on retinal and systemic microvascular function• Physical exercise positively influenced the retinal microcirculation through improvements in dilation and constriction reaction times to flicker provocation. 3. The long-lasting effects of fasting during the month of Ramadan on retinal and peripheral vascular function• Participants during fasting had a higher capacity to reach maximum dilation and also a greater percentage increase from baseline diameters. The retinal veins were also significantly less variable during baseline corrected measurements. 4. The effect of bariatric surgery on retinal vessels structure and systemic microvascular function• Increases were recorded for the diameter of retinal arteries but also for the veins. Peripheral vascular function was significantly improved and arterial stiffness was decreased. CVD risk was significantly decreased and also correlated with retinal vessel calibre measurements. 5. Is there an improvement in anterior ocular health after bariatric surgery?• Anterior surface health doesn’t necessarily cause ocular health problems in obese individuals nor can it be improved or ameliorated through bariatric surger

Novel photoreceptor cells, pupillometry and electrodiagnosis in orbital, vitreo-retinal and refractive disorders

Imperial Users onl