Modelling ocular monochromatic aberrations using schematic eyes with homogeneous optical media

Previous research has indicated that schematic eyes incorporating aspheric surfaces but lacking gradient index are unable to model ocular spherical aberration and peripheral astigmatism simultaneously. This limits their use as wide-angle schematic eyes. This thesis challenges this assumption by investigating the flexibility of schematic eyes comprising aspheric optical surfaces and homogeneous optical media. The full variation of ocular component dimensions found in human eyes was established from the literature. Schematic eye parameter variants were limited to these dimensions. The levels of spherical aberration and peripheral astigmatism modelled by these schematic eyes were compared to the range of measured levels. These were also established from the literature. To simplify comparison of modelled and measured data, single value parameters were introduced; the spherical aberration function (SAF), and peripheral astigmatism function (PAF). Some ocular components variations produced a wide range of aberrations without exceeding the limits of human ocular components. The effect of ocular component variations on coma was also investigated, but no comparison could be made as no empirical data exists. It was demonstrated that by combined manipulation of a number of parameters in the schematic eyes it was possible to model all levels of ocular spherical aberration and peripheral astigmatism. However, the unique parameters of a human eye could not be obtained in this way, as a number of models could be used to produce the same spherical aberration and peripheral astigmatism, while giving very different coma levels. It was concluded that these schematic eyes are flexible enough to model the monochromatic aberrations tested, the absence of gradient index being compensated for by altering the asphericity of one or more surfaces.

Peripheral refraction validity of the Shin-Nippon SRW5000 autorefractor

PURPOSE: To investigate the operation of the Shin-Nippon/Grand Seiko autorefractor and whether higher-order aberrations affect its peripheral refraction measurements. METHODS: Information on instrument design, together with parameters and equations used to obtain refraction, was obtained from a patent. A model eye simulating the operating principles was tested with an optical design program. Effects of induced defocus and astigmatism on the retinal image were used to calibrate the model eye to match the patent equations. Coma and trefoil were added to assess their effects on the image. Peripheral refraction of a physical model eye was measured along four visual field meridians with the Shin-Nippon/Grand Seiko autorefractor SRW-5000 and a Hartmann-Shack aberrometer, and simulated autorefractor peripheral refraction was derived using the Zernike coefficients from the aberrometer. RESULTS: In simulation, the autorefractor's square image was changed in size by defocus, into rectangles or parallelograms by astigmatism, and into irregular shapes by coma and trefoil. In the presence of 1.0 D oblique astigmatism, errors in refraction were proportional to the higher-order aberrations, with up to 0.8 D sphere and 1.5 D cylinder for ±0.6 μm of coma or trefoil coefficients with a 5-mm-diameter pupil. For the physical model eye, refraction with the aberrometer was similar in all visual field meridians, but refraction with the autorefractor changed more quickly along one oblique meridian and less quickly along the other oblique meridian than along the horizontal and vertical meridians. Simulations predicted that higher-order aberrations would affect refraction in oblique meridians, and this was supported by the experimental measurements with the physical model eye. CONCLUSIONS: The autorefractor's peripheral refraction measurements are valid for horizontal and vertical field meridians, but not for oblique field meridians. Similar instruments must be validated before being adopted outside their design scope.

Accommodative response to electrical stimulation of the sclera of peripheral cornea in cats and porcines

Purpose: We have reported that the changes in the accommodative response to electrical stimulation of the branches of the ciliary nerves in cats. (Miyagawa et al, PLoS One, 2014). We have also reported that no robust accommodative responses to the electrical stimulations of the sclera of peripheral cornea (SSPC) were observed in enucleated porcine eyes (Mihashi et al, VPOptics, 2014). In this study, accommodative responses to SSPC stimulation in cats and porcines were investigated. Methods: Two eyes of two cats under anesthesia and after they were sacrificed were studied. Three enucleated porcine eyes obtained from a local slaughterhouse were also studied. Trains of biphasic pulses (current, 3 mA; duration, 2 ms/phase; frequency, 40 Hz) were applied using a tungsten electrode (0.3mm diameter) from several orientations. Wavefront sensing with a compact wavefront aberrometer (Uday et al J Cataract Refract Surg, 2013) were performed before and 4 s (cat) and 10 s (pig) after the stimulations and wavefront aberrations including spherical errors were analyzed over a 4-mm pupil area. Results: In the first cat under anesthesia, at three out of seven stimulus positions, 0.2 D hyperopic accommodative responses were observed and in two orientations, myopic responses were observed. For the other cat, weak accommodative responses including astigmatic changes were observed. In the sacrificed condition of the second cat, 0.1 D myopic response was observed for one stimulus orientation and the smaller responses were observed at six out of eight stimulus positions. No accommodative responses were elicited for the enucleated porcine eyes. Conclusions: In the anesthetized cats, electrical stimulation of the SSPC induced accommodative responses; the responses were unstable and weaker than the responses by the ciliary nerve stimulations we observed in our previous study. Small accommodative responses were observed after one of two cats had been sacrificed, but no accommodative responses were detected in the enucleated porcine eyes. Further studies are needed to confirm difference in the accommodation functions in the two species.

In vivo vitamin C supplementation increases phosphoinositol transfer protein expression in peripheral blood mononuclear cells from healthy individuals

Ascorbate can act as both a reducing and oxidising agent in vitro depending on its environment. It can modulate the intracellular redox environment of cells and therefore is predicted to modulate thiol-dependent cell signalling and gene expression pathways. Using proteomic analysis of vitamin C-treated T cells in vitro, we have previously reported changes in expression of five functional protein groups associated with signalling, carbohydrate metabolism, apoptosis, transcription and immune function. The increased expression of the signalling molecule phosphatidylinositol transfer protein (PITP) was also confirmed using Western blotting. Herein, we have compared protein changes elicited by ascorbate in vitro, with the effect of ascorbate on plasma potassium levels, on peripheral blood mononuclear cell (PBMC) apoptosis and PITP expression, in patients supplemented with vitamin C (0-2 g/d) for up to 10 weeks to investigate whether in vitro model systems are predictive of in vivo effects. PITP varied in expression widely between subjects at all time-points analysed but was increased by supplementation with 2 g ascorbate/d after 5 and 10 weeks. No effects on plasma potassium levels were observed in supplemented subjects despite a reduction of K+ channel proteins in ascorbate-treated T cells in vitro. Similarly, no effect of vitamin C supplementation on PBMC apoptosis was observed, whilst ascorbate decreased expression of caspase 3 recruitment domain protein in vitro. These data provide one of the first demonstrations that proteomics may be valuable in developing predictive markers of nutrient effects in vivo and may identify novel pathways for studying mechanisms of action in vivo.

Dietary supplementation with vitamin C but not vitamin E reduces constitutive expression of ICAM-1 in peripheral blood monocytes of normal subjects with low plasma vitamin C levels

Monocytes play a central role in inflammatory responses through systemic antigen presentation and cytokine secretion. Regulation of monocyte adhesion molecule and inflammatory gene expression is via redox sensitive transcription factors. Therefore we have investigated the hypothesis that dietary antioxidant supplementation with vitamins C (250mg/d) or E (400iU/d) for six weeks can modulate monocyte ICAM-1 expression in healthy male subjects with low plasma vitamin C at baseline. In a randomised, double-blind, crossover study, ICAM-1 mRNA and protein was analysed using quantitative RTPCR with ELISA measurement of PCR products and by flow cytometry and ELISA respectively. Monocyte numbers were unaltered by supplementation. Subjects with low plasma vitamin C (<50uM) prior to supplementation expressed higher levels of monocyte ICAM-1 mRNA, and showed a significant (50%) reduction in ICAM-1 mRNA expression after 6 weeks of 250mg/d vitamin C supplementation compared to subjects with normal plasma vitamin C. This was paralleled by a reduction in plasma sICAM-1. Vitamin E supplementation had no effect on ICAM-1 expression. For the first time, these results show that dietary vitamin C can modulate monocyte ICAM-1 gene expression in vivo, where regulation of gene expression represents a novel mechanism for benefit from dietary antioxidants.

The relative contributions of colour and luminance signals towards the visuomotor localisation of targets in human peripheral vision

We sought to determine the extent to which colour (and luminance) signals contribute towards the visuomotor localization of targets. To do so we exploited the movement-related illusory displacement a small stationary window undergoes when it has a continuously moving carrier grating behind it. We used drifting (1.0-4.2 Hz) red/green-modulated isoluminant gratings or yellow/black luminance-modulated gratings as carriers, each curtailed in space by a stationary, two-dimensional window. After each trial, the perceived location of the window was recorded with reference to an on-screen ruler (perceptual task) or the on-screen touch of a ballistic pointing movement made without visual feedback (visuomotor task). Our results showed that the perceptual displacement measures were similar for each stimulus type and weakly dependent on stimulus drift rate. However, while the visuomotor displacement measures were similar for each stimulus type at low drift rates (<4 Hz), they were significantly larger for luminance than colour stimuli at high drift rates (>4 Hz). We show that the latter cannot be attributed to differences in perceived speed between stimulus types. We assume, therefore, that our visuomotor localization judgements were more susceptible to the (carrier) motion of luminance patterns than colour patterns. We suggest that, far from being detrimental, this susceptibility may indicate the operation of mechanisms designed to counter the temporal asynchrony between perceptual experiences and the physical changes in the environment that give rise to them. We propose that perceptual localisation is equally supported by both colour and luminance signals but that visuomotor localisation is predominantly supported by luminance signals. We discuss the neural pathways that may be involved with visuomotor localization. © 2007 Springer-Verlag.

Languaging the worker:globalized governmentalities in/of language in peripheral spaces

In the introduction to the special issue “Languaging the worker: globalized governmentalities in/of language in peripheral spaces”, we take up the notion of governmentality as a means to interrogate the complex relationship between language, labor, power and subjectivity in peripheral multilingual spaces. Our aim here is to argue for the study of governmentality as a viable and growing approach in critical sociolinguistic research. As such, in this introduction, we first discuss key concepts germane to our interrogations, including the notions of governmentality, languaging, peripherality and language worker. We proceed to map out five ethnographically and discourse-analytically informed case studies. These examine diverse actors in different settings pertaining to the domain of work. Finally we chart how the case studies construe the issue of languaging the worker through a governmentality frame.

Influence of accommodation and refractive status on the peripheral refractive profile

AIM: The aim of the study was to determine, objectively and non-invasively, whether changes in accommodative demand modify differentially the peripheral refraction in emmetropic and myopic human eyes. METHODS: Forty subjects (19 male, 21 female) aged 20-30 years (mean 22.7 (SD 2.8) years), 21 emmetropes (mean spherical equivalent refractive error (MSE) -0.13 (SD 0.29) D) and 19 myopes (MSE -2.95 (SD 1.76) D) participated in the study. Ametropia was corrected with soft contact lenses (etafilcon A, 58% water content). Subjects viewed monocularly a stationary, high contrast (85%) Maltese cross at 0.0, 1.0, 2.0 and 3.0 D of accommodative demand and at 0, 10, 20 and 30 degrees field angle (nasal and temporal) through a +3.0 D Badal optical system. Static recordings of the accommodation response were obtained for each accommodative level, at each field angle, with an objective, open-view, infrared optometer. RESULTS: Peripheral mean spherical equivalent (M) data showed that the emmetropic cohort exhibited relative myopic shifts into the periphery, while the myopic group showed hypermetropic shifts. Increasing accommodative demand did not alter the peripheral refractive profile in either the temporal (p = 0.25) or nasal (p = 0.07) periphery with no differential accommodative effect between refractive groups in either the temporal (p = 0.77) or nasal (p = 0.73) field. Significant shifts in the J(0) astigmatic component were seen in the temporal (p<0.0005) and nasal (p<0.0005) fields with increasing eccentricity. Interaction effects between eccentricity and accommodative demand illustrated that increasing accommodative demand significantly altered the peripheral refractive profile in the temporal J(0) astigmatic component (p<0.0005). The nasal periphery, however, failed to show such an effect (p = 0.65). CONCLUSIONS: Alterations in peripheral refraction augmented by changes in ocular accommodation are relatively unaffected by refractive error for young, healthy human eyes.

Psychophysical investigations of human peripheral vision

This thesis investigates various aspects of peripheral vision, which is known not to be as acute as vision at the point of fixation. Differences between foveal and peripheral vision are generally thought to be of a quantitative rather than a qualitative nature. However, the rate of decline in sensitivity between foveal and peripheral vision is known to be task dependent and the mechanisms underlying the differences are not yet well understood. Several experiments described here have employed a psychophysical technique referred to as 'spatial scaling'. Thresholds are determined at several eccentricities for ranges of stimuli which are magnified versions of one another. Using this methodology a parameter called the E2 value is determined, which defines the eccentricity at which stimulus size must double in order to maintain performance equivalent to that at the fovea. Experiments of this type have evaluated the eccentricity dependencies of detection tasks (kinetic and static presentation of a differential light stimulus), resolution tasks (bar orientation discrimination in the presence of flanking stimuli, word recognition and reading performance), and relative localisation tasks (curvature detection and discrimination). Most tasks could be made equal across the visual field by appropriate magnification. E2 values are found to vary widely dependent on the task, and possible reasons for such variations are discussed. The dependence of positional acuity thresholds on stimulus eccentricity, separation and spatial scale parameters is also examined. The relevance of each factor in producing 'Weber's law' for position can be determined from the results.

Spatial scaling in human peripheral vision

The observation that performance in many visual tasks can be made independent of eccentricity by increasing the size of peripheral stimuli according to the cortical magnification factor has dominated studies of peripheral vision for many years. However, it has become evident that the cortical magnification factor cannot be successfully applied to all tasks. To find out why, several tasks were studied using spatial scaling, a method which requires no pre-determined scaling factors (such as those predicted from cortical magnification) to magnify the stimulus at any eccentricity. Instead, thresholds are measured at the fovea and in the periphery using a series of stimuli, all of which are simply magnified versions of one another. Analysis of the data obtained in this way reveals the value of the parameter E2, the eccentricity at which foveal stimulus size must double in order to maintain performance equivalent to that at the fovea. The tasks investigated include hyperacuities (vernier acuity, bisection acuity, spatial interval discrimination, referenced displacement detection, and orientation discrimination), unreferenced instantaneous and gradual movement, flicker sensitivity, and face discrimination. In all cases tasks obeyed the principle of spatial scaling since performance in the periphery could be equated to that at the fovea by appropriate magnification. However, E2 values found for different spatial tasks varied over a 200-fold range. In spatial tasks (e.g. bisection acuity and spatial interval discrimination) E2 values were low, reaching about 0.075 deg, whereas in movement tasks the values could be as high as 16 deg. Using a method of spatial scaling it has been possible to equate foveal and peripheral perfonnance in many diverse visual tasks. The rate at which peripheral stimulus size had to be increased as a function of eccentricity was dependent upon the stimulus conditions and the task itself. Possible reasons for these findings are discussed.

Peripheral vision and pattern recognition : a review

We summarize the various strands of research on peripheral vision and relate them to theories of form perception. After a historical overview, we describe quantifications of the cortical magnification hypothesis, including an extension of Schwartz's cortical mapping function. The merits of this concept are considered across a wide range of psychophysical tasks, followed by a discussion of its limitations and the need for non-spatial scaling. We also review the eccentricity dependence of other low-level functions including reaction time, temporal resolution, and spatial summation, as well as perimetric methods. A central topic is then the recognition of characters in peripheral vision, both at low and high levels of contrast, and the impact of surrounding contours known as crowding. We demonstrate how Bouma's law, specifying the critical distance for the onset of crowding, can be stated in terms of the retinocortical mapping. The recognition of more complex stimuli, like textures, faces, and scenes, reveals a substantial impact of mid-level vision and cognitive factors. We further consider eccentricity-dependent limitations of learning, both at the level of perceptual learning and pattern category learning. Generic limitations of extrafoveal vision are observed for the latter in categorization tasks involving multiple stimulus classes. Finally, models of peripheral form vision are discussed. We report that peripheral vision is limited with regard to pattern categorization by a distinctly lower representational complexity and processing speed. Taken together, the limitations of cognitive processing in peripheral vision appear to be as significant as those imposed on low-level functions and by way of crowding.

Efficacy of Adding 2% (w/v) Chlorhexidine Gluconate to 70% (v/v) Isopropyl Alcohol for Skin Disinfection Prior to Peripheral Venous Cannulation

We undertook a clinical trial to compare the efficacy of 2% (w/v) chlorhexidine gluconate in 70% (v/v) isopropyl alcohol with the efficacy of 70% (v/v) isopropyl alcohol alone for skin disinfection to prevent peripheral venous catheter colonization and contamination. We found that the addition of 2% chlorhexidine gluconate reduced the number of peripheral venous catheters that were colonized or contaminated. © 2008 by The Society for Healthcare Epidemiology of America. All rights reserved.

Development of areolae and growth of the peripheral prothallus in the crustose lichen Rhizocarpon geographicum:an image analysis study

Areolae of the crustose lichen Rhizocarpon geographicum (L.) DC., are present on the peripheral prothallus (marginal areolae) and also aggregate to form confluent masses in the centre of the thallus (central areolae). To determine the relationships between these areolae and whether growth of the peripheral prothallus is dependent on the marginal areolae, the density, morphology, and size frequency distributions of marginal areolae were measured in 23 thalli of R. geographicum in north Wales, UK using image analysis (Image J). Size and morphology of central areolae were also studied across the thallus. Marginal areolae were small, punctate, and occurred in clusters scattered over the peripheral prothallus while central areolae were larger and had a lobed structure. The size-class frequency distributions of the marginal and central areolae were fitted by power-law and log-normal models respectively. In 16 out of 23 thalli, central areolae close to the outer edge were larger and had a more complex lobed morphology than those towards the thallus centre. Neither mean width nor radial growth rate (RaGR) of the peripheral prothallus were correlated with density, diameter, or area fraction of marginal areolae. The data suggest central areolae may develop from marginal areolae as follows: (1) marginal areolae develop in clusters at the periphery and fuse to form central areolae, (2) central areolae grow exponentially, and (3) crowding of central areolae results in constriction and fragmentation. In addition, growth of the peripheral prothallus may be unrelated to the marginal areolae. © 2013 Springer Science+Business Media Dordrecht.

Impact of soft contact lens edge design and mid-peripheral lens shape on the epithelium and its indentation with lens mobility

Purpose. To evaluate the influence of soft contact lens midperipheral shape profile and edge design on the apparent epithelial thickness and indentation of the ocular surface with lens movement. Methods. Four soft contact lens designs comprising of two different plano midperipheral shape profiles and two edge designs (chiseled and knife edge) of silicone-hydrogel material were examined in 26 subjects aged 24.7 ± 4.6 years, each worn bilaterally in randomized order. Lens movement was imaged enface on insertion, at 2 and 4 hours with a high-speed, high-resolution camera simultaneous to the cross-section of the edge of the contact lens interaction with the ocular surface captured using optical coherence tomography (OCT) nasally, temporally, and inferiorly. Optical imaging distortions were individually corrected for by imaging the apparent distortion of a glass slide surface by the removed lens. Results. Apparent epithelial thickness varied with edge position (P < 0.001). When distortion was corrected for, epithelial indentation decreased with time after insertion (P = 0.010), changed after a blink (P < 0.001), and varied with position on the lens edge (P < 0.001), with the latter being affected by midperipheral lens shape profile and edge design. Horizontal and vertical lens movement did not change with time postinsertion. Vertical motion was affected by midperipheral lens shape profile (P < 0.001) and edge design (P < 0.001). Lens movement was associated with physiologic epithelium thickness for lens midperipheral shape profile and edge designs. Conclusions. Dynamic OCT coupled with high-resolution video demonstrated that soft contact lens movement and image-corrected ocular surface indentation were influenced by both lens edge design and midperipheral lens shape profiles. © 2013 The Association for Research in Vision and Ophthalmology, Inc.