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We measure the effect of defocus blur on contrast sensitivity with Sloan letters in the 0.75–2.00 arc min range of letter gaps. We compare our results with the prediction of the Dalimier and Dainty model [J. Opt. Soc. Am. A 25, 2078 (2008)JOAOD60740-323210.1364/JOSAA.25.002078] and propose a new metric of retinal image quality that we define as the model limit for very small letters. The contrast sensitivity is measured for computationally blurred Sloan letters (0, 0.25, and 0.50 diopters for a 3 mm pupil) of different sizes (20/40 to 20/15 visual acuity), and subjects look through a small (2 mm) diaphragm to limit the impact of their own aberration on measurements. Measurements and model predictions, which are normalized by the blur-free condition, weakly depend on letter size and are in good agreement with our metric of retinal image quality. Our metric relates two approaches of modeling visual performance: complete modeling of the optotype classification task and calculation of retinal image quality with a descriptive metric.

The focusing distance of the eye fluctuates during accommodation. However, the visual role of these accommodation fluctuations is not yet fully understood. The fluctuation complexity is one of the obstacles to this long standing challenge in visual science. In this work we seek to develop a statistical approach that i) accurately describes experimental measurements and ii) directly generates randomized and realistic simulations of accommodation fluctuations for use in future experiments. To do so we use the random walk approach, which is usually appropriate to describe the dynamics of systems that combine both randomness and memory.

This article introduces a copy of The Woorkes of Geffrey Chaucer (1561) formerly belonging to the writer, cleric, limner, and book-collector Stephan Batman (c. 1542–1584). The volume is currently held at the Guildhall Library (SR 2.3.3), and contains Batman’s annotations and manicules throughout the text. It also features a 28-line poem in Batman’s hand, a short booklist of medieval chronicles, and five line drawings. The book thus offers a fresh insight into the reading practices of one of the most industrious English antiquaries of the sixteenth century, and sheds new light on Chaucer’s sixteenth-century reception.

Stephen Hawes’s fusion of chivalric romance and personification allegory has long been recognized as his chief claim to originality in English literary history. The generic fusions of his work, however, have often led him to be viewed merely as a distant precur-sor to Edmund Spenser’s Faerie Queene rather than as an innovative poet in his own right. This article resituates Hawes’s poetic originality in its early Tudor contexts and brings his poetry back into conversation with the metrical romances, devotional trea-tises, and dreams of spiritual warfare that shaped his creative aspirations.

W. H. Auden’s debts to Old English and Old Norse literature have long been celebrated, but his lifelong interest in Middle English has received far less critical attention. This article examines how Auden’s formative encounters with Middle English verse and prose influenced his poetry between 1922 and 1930. It surveys the different anthologies, editions, and critical perspectives that shaped Auden’s early medievalism, and through an analysis of the 1927 poem ‘Out of sight assuredly, not out of mind’, it explores many of the difficulties that Auden had in turning England’s medieval past into a modern poetic resource. Finally, using the evidence of Auden’s surviving lecture notes, this article uncovers an important new source for the seminal poem ‘Doom is dark and deeper than any sea-dingle’: two lines from Laamon’s Brut: ‘vppen Þan Þe hit falleð | he scal uaren of londe’ [upon whomever it fall, he must leave his land].

To evaluate macular shape in normal eyes using swept-source optical coherence tomography (SS-OCT). Retrospective cross-sectional study. We retrospectively evaluated 77 normal eyes of 48 subjects. Curvature of retinal pitment epithelium (RPE) and choroid/scleral interface (CSI) was measured in vertical and horizontal SS-OCT 16-mm scanned images. After correcting the optical distortion of OCT images, curvatures of superior, central, and inferior sectors in the vertical scan, and temporal, central, and nasal sectors in the horizontal scan (each 4-mm length) were compared. Factors associated with overall RPE and CSI curvatures were investigated. RPE and CSI curvatures of superior, central, and inferior sectors in the vertical scan were 16.6±3.1, 13.8±2.1, 17.7±3.2 and 17.8±3.0, 13.8±3.3, 18.4±3.3 (×10−5 μm−1), respectively. Central curvature was significantly flatter than superior and inferior curvatures in both RPE and CSI (all P

Despite the prevalence of optical imaging techniques to measure hemodynamics in large retinal vessels, quantitative measurements of retinal capillary and choroidal hemodynamics have traditionally been challenging. Here, a new imaging technique called dynamic contrast optical coherence tomography (DyC-OCT) is applied in the rat eye to study microvascular blood flow in individual retinal and choroidal layers in vivo. DyC-OCT is based on imaging the transit of an intravascular tracer dynamically as it passes through the field-of-view. Hemodynamic parameters can be determined through quantitative analysis of tracer kinetics. In addition to enabling depth-resolved transit time, volume, and flow measurements, the injected tracer also enhances OCT angiograms and enables clear visualization of the choriocapillaris, particularly when combined with a post-processing method for vessel enhancement. DyC-OCT complements conventional OCT angiography through quantification of tracer dynamics, similar to fluorescence angiography, but with the important added benefit of laminar resolution.

Although microvascular dysfunction accompanies cognitive decline in aging, vascular dementia, and Alzheimer's disease, tools to study microvasculature longitudinally in vivo are lacking. Here, we use Doppler optical coherence tomography (OCT) and angiography for noninvasive, longitudinal imaging of mice with chronic cerebral hypoperfusion for up to 1 month. In particular, we optimized the OCT angiography method to selectively image red blood cell (RBC)-perfused capillaries, leading to a novel way of assessing capillary supply heterogeneity in vivo. After bilateral common carotid artery stenosis (BCAS), cortical blood flow measured by Doppler OCT dropped to half of baseline throughout the imaged tissue acutely. Microscopic imaging of the capillary bed with OCT angiography further revealed local heterogeneities in cortical flow supply during hypoperfusion. The number of RBC-perfused capillaries decreased, leading to increased oxygen diffusion distances in the days immediately after BCAS. Linear regression showed that RBC-perfused capillary density declined by 0.3% for a drop in flow of 1 mL/100 g per minute, and decreases in RBC-perfused capillary density as high as 25% were observed. Taken together, these results demonstrate the existence of local supply heterogeneity at the capillary level even at nonischemic global flow levels, and demonstrate a novel imaging method to assess this heterogeneity.

A method of measuring cortical oxygen metabolism in the mouse brain that uses independent quantitative measurements of three key parameters: cerebral blood flow (CBF), arteriovenous oxygen extraction (OE), and hemoglobin concentration ([HbT]) is presented. Measurements were performed using a single visible light spectral/Fourier domain OCT microscope, with Doppler and spectroscopic capabilities, through a thinned-skull cranial window in the mouse brain. Baseline metabolic measurements in mice are shown to be consistent with literature values. Oxygen consumption, as measured by this method, did not change substantially during minor changes either in the fraction of inspired oxygen (FiO2) or in the fraction of inspired carbon dioxide (FiCO2), in spite of larger variations in oxygen saturations. This set of experiments supports, but does not prove, the validity of the proposed method of measuring brain oxygen metabolism.

Here, we present high-speed spectral domain OCT in the visible spectral range using a supercontinuum source that performs angiography, oximetry, and speed assessment of red blood cells in individual vessels of the mouse pinna in vivo.

Trait divergence between co-occurring individuals could decrease the strength of competition between these individuals, thus promoting their coexistence. To test this hypothesis, we manipulated establishment timing for four congeneric pairs of perennial plants and assessed trait plasticity. Because soil conditions can affect trait expression and competition, we grew the plants in field-collected soil from each congener. Competition was generally weak across species, but the order of establishment affected divergence in biomass between potmates for three congeneric pairs. The type of plastic response differed among genera, with trait means of early-establishing individuals of Rumex and Solanum spp. differing from late-establishing individuals, and trait divergence between potmates of Plantago and Trifolium spp. depending on which species established first. Consistent with adaptive trait plasticity, higher specific leaf area (SLA) and root–shoot ratio in Rumex spp. established later suggest that these individuals were maximizing their ability to capture light and soil resources. Greater divergence in SLA correlated with increased summed biomass of competitors, which is consistent with trait divergence moderating the strength of competition for some species. Species did not consistently perform better in conspecific or congener soil, but soil type influenced the effect of establishment order. For example, biomass divergence between Rumex potmates was greater in R. obtusifolius soil regardless of which species established first. These results suggest that plant responses to establishment timing act in a species-specific fashion, potentially enhancing coexistence in plant communities.

We present volumetric imaging and computational techniques to quantify neuronal and myelin architecture with intrinsic scattering contrast. Using spectral / Fourier domain Optical Coherence Microscopy (OCM) and software focus-tracking we validate imaging of neuronal cytoarchitecture and demonstrate quantification in the rodent cortex in vivo. Additionally, by ex vivo imaging in conjunction with optical clearing techniques, we demonstrate that intrinsic scattering contrast is preserved in the brain, even after sacrifice and fixation. We volumetrically image cytoarchitecture and myeloarchitecture ex vivo across the entire depth of the rodent cortex. Cellular-level imaging up to the working distance of our objective (~3 mm) is demonstrated ex vivo. Architectonic features show the expected laminar characteristics; moreover, changes in contrast after the application of acetic acid suggest that entire neuronal cell bodies are responsible for the “negative contrast” present in the images. Clearing and imaging techniques that preserve tissue architectural integrity have the potential to enable non-invasive studies of the brain during development, disease, and remodeling, even in samples where exogenous labeling is impractical.

The time-evolution of ocular aberrations has been the subject of many studies, but so far there has been little discussion involving the modelling of the underlying temporal statistics. This paper presents a non-stationary modelling approach based on a coloured-noise generator, which can be applied to ocular aberration dynamics. The model parameters are computed from measured ocular aberration data. A custom-built aberrometer based on a Shack-Hartmann sensor was used for measurement. We present simulations based on our modelling approach, and validate them through comparison to real data. This work could be useful in areas such as the testing of ophthalmic devices and the development of improved algorithms for laser refractive surgery.

Recently, optical coherence tomography (OCT) angiography has enabled label-free imaging of vasculature based on dynamic scattering in vessels. However, quantitative volumetric analysis of the vascular networks depicted in OCT angiography data has remained challenging. Multiple-scattering tails (artifacts specific to the imaging geometry) make automated assessment of vascular morphology problematic. We demonstrate that dynamically focused optical coherence microscopy (OCM) angiography with a high numerical aperture, chosen so the scattering length greatly exceeds the depth-of-field, significantly reduces the deleterious effect of multiple-scattering tails in synthesized angiograms. Capitalizing on the improved vascular image quality, we devised and tailored a self-correcting automated graphing approach that achieves a reconstruction of cortical microvasculature from OCM angiography data sets with accuracy approaching that attained by trained operators. The automated techniques described here will facilitate more widespread study of vascular network topology in health and disease.

Inner retinal metabolism requires a continuous supply of oxygen and other nutrients, as well as waste removal, by blood flow through a network of vessels.1,2 Vascular architecture constrains how blood flow can be controlled to meet metabolic needs in the inner retina.3 Moreover, vascular structure in the retina may aid the diagnosis and management of diseases that involve vascular dysfunction, such as diabetic retinopathy,4 glaucoma,5,6 and cardiovascular disease.7,8 Thus, comprehensive architectural analysis of the inner retinal vasculature will impact both basic research and clinical practice. Optical imaging through the transparent ocular media offers the opportunity to quantitatively assess vasculature noninvasively in vivo. The blood vessels of the inner retina exhibit a characteristic three-dimensional (3D) layered structure, with superficial, intermediate, and deep vessel layers, corresponding to the ganglion cell layer/nerve fiber layer, the inner plexiform layer, and the outer plexiform layer, respectively.9 A fundamental limitation of 2D photography-based techniques, such as fluorescein angiography,10 is the lack of depth discrimination,11 due to overlapping signals from the individual retinal layers and the choroid. Optical coherence tomography (OCT)12 enables 3D optical sectioning of the retina with micron-scale resolution,13 employing only intrinsic contrast without the requirement for additional contrast agents. The superior axial resolution of OCT distinguishes the layered architecture of the retina, including the capillary plexuses located in the plexiform layers.14 Optical coherence tomography angiography techniques,15–18 based on enhancing contrast arising from motion and scattering, were recently developed for selective imaging of only the “functional” vasculature perfused with moving blood cells. Doppler OCT19,20 has also been shown to perform quantitative assessment of blood flow.21 Optical coherence tomography–based studies involving 2D segmentation have proven effective for visualization22 and feature detection,23 however, 3D analysis is required to assess the true vascular topology, more directly related to function. In this work, we derive a 3D representation, or “graph,” of the vasculature in the rat inner retina based on OCT angiography data. Unlike previous segmentation efforts in the retina, the interconnected nature of the multilayered vascular structure is implicitly incorporated into the derived representations and analysis. Using the graph, in conjunction with experimentally-quantified blood flow data (computed using Doppler OCT methods24) and a simple theoretical network model of blood flow, the capability of blood vessel dilation to produce localized and lamina-specific changes in blood flow is examined.

Quantification of chromophore concentrations in reflectance mode remains a major challenge for biomedical optics. Spectroscopic Optical Coherence Tomography (SOCT) provides depth-resolved spectroscopic information necessary for quantitative analysis of chromophores, like hemoglobin, but conventional SOCT analysis methods are applicable only to well-defined specular reflections, which may be absent in highly scattering biological tissue. Here, by fitting of the dynamic scattering signal spectrum in the OCT angiogram using a forward model of light propagation, we quantitatively determine hemoglobin concentrations directly. Importantly, this methodology enables mapping of both oxygen saturation and total hemoglobin concentration, or alternatively, oxyhemoglobin and deoxyhemoglobin concentration, simultaneously. Quantification was verified by ex vivo blood measurements at various pO2 and hematocrit levels. Imaging results from the rodent brain and retina are presented. Confounds including noise and scattering, as well as potential clinical applications, are discussed.

This paper presents an optical system called MacPI, which implements a two-color reflectance technique in combination with various hardware and software tools to assess objectively the macular pigment (MP) optical density in vivo. The system consists of a bespoke optical design, a control architecture, driver electronics, a collection of image-processing techniques, and a graphical user interface. The deficiencies of the technique employed and the solutions implemented in the MacPI system to confront those inherent frailties are presented. An overview of the effective interpretation of the acquired data and the techniques employed by MacPI in the acquisition of that data is discussed. The result of a comparison trial with an alternative device is also presented. We suggest that appropriate design of the hardware and an efficient interpretation of the acquired data should produce a system capable of consistent, accurate, and rapid measurements, while retaining the distinction of ease of use, portability, comfort for the subject, and a design that is economic to produce. Its versatility should allow both for a clinical screening application and for further investigation and establishment of the physiological role of the MP in a laboratory-based environment.

Retinal images are frequently corrupted by unwanted variations in intensity that occur due to general imperfections in the image acquisition process. This inhomogeneous illumination across the retina can limit the useful information accessible within the acquired image. Specifically, this can lead to serious difficulties when performing image processing tasks requiring quantitative analysis of features present on the retina. Given that the spatial frequency content of the shading profile often overlaps with that of retinal features, retrospectively correcting for inhomogeneous illumination while maintaining the radiometric fidelity of the real data can be challenging. This paper describes a simple method for obtaining an estimate of the illumination profile in retinal images, with the particular goal of minimizing its influence upon features of interest. This is achieved by making use of Laplace interpolation and a multiplicative image formation model.

Microfluctuations of accommodation have been the subject of many studies. New technological developments now permit us to study the dynamics of the microfluctuations with unprecedented resolution and accuracy. We aim to characterise their temporal statistics for different levels of accommodative effort, using a custom-built aberrometer. We conducted 46 s long measurements on the dominant eye of 9 young, healthy subjects. The ocular wavefront was sampled every 250 microm across the 3.9 mm measured pupil, at a frame rate of 173 Hz. This enabled us to obtain high resolution estimates of the Power Spectral Density (PSD). Results show that the shape of the estimated PSD for a 4 D effort is distinct from the shape for the two extrema of the accommodation range. The autocorrelation function of the increments of the accommodation signal is also affected by the level of effort, regardless of the refractive error of the subject.

Pre-clinical quantitative imaging endpoints have been challenging in mouse models of cerebrovascular disease. Here we present optical coherence imaging platforms that can quantify blood flow, capillary perfusion, cellular status, and oxygen extraction based on intrinsic scattering signatures.