Directional imaging of the retinal cone mosaic

We describe a near-IR scanning laser ophthalmoscope that allows the retinal cone mosaic to be imaged in the human eye in vivo without the use of wave-front correction techniques. The method takes advantage of the highly directional quality of cone photoreceptors that permits efficient coupling of light to individual cones and subsequent detection of most directional components of the backscattered light produced by the light-guiding effect of the cones. We discuss details of the system and describe cone-mosaic images obtained under different conditions.     

浅谈色觉的数学机制

正常人眼拥有色觉功能,其目的是为了辨识不同的光辐射能谱分布。色觉的生理基础是:在视网膜上具有三类视锥细胞,它们将光信号转换成为使大脑便于分析和处理的神经电信号。由于单个视锥细胞产生的神经电信号只反映光辐射的强度信息,所以光辐射的谱信息(波长分布)须由三类视锥细胞的响应比来反映。以光辐射中的单色辐射及线性连续辐射为辨识对象,构造了光辐射辨识的数学模型。基于该模型,对用三类视锥细胞的响应比来表征光辐射的谱信息这一命题的必要性与完备性进行了理论论证。     

Retinal cone mosaic imaged with transverse scanning optical coherence tomography

We improved our recently reported retinal OCT system based on transverse priority scanning to achieve high resolution in both the transverse and the axial directions. The implementation of an additional SLO channel enables precise on-line focusing. The system enables imaging of the human retinal cone mosaic off the foveal center without adaptive optics. We demonstrate, for what is believed to be the first time, cone mosaic imaging simultaneously in the scanning laser ophthalmoscope and optical coherence tomography (OCT) channels. OCT B-scan images demonstrate that the cone mosaic is observable in two adjacent layers. Furthermore, we present what are believed to be the first C-scan OCT images of the cone mosaic and show that the major part of light backscattered from below the photoreceptor layer is not guided back toward the pupil by the photoreceptors.     

Ultrastructure of the human retina in aging and various pathological states

Vision is hampered in aging and diseases, such as age-related macular degeneration, retinitis pigmentosa, diabetic retinopathy and glaucoma. This review collates the fine structural alterations of the human retina in aging and various pathological situations and their links to the disease pathogenesis. It transpires that most changes occur at the level of the retinal pigment epithelium -Bruch's membrane and the photoreceptor layer, causing visual problems to the sufferers. These changes include loss of normal, essential features of these cells and their gradual disappearance. It is important to understand in depth the selective vulnerability of this retinal region to alterations in aging and diseases. Evidence indicates that some of these changes may be mediated by the effects of oxidative stress, inflammation, and chronic light exposure. There are changes also in the inner retinal layers, wherein hypertension, auto-immunity, hypoxia and ischemia could play significant roles in disease pathogenesis. Results of extensive research utilizing animal models have broadened our idea about photoreceptor pathology. However, equivalent knowledge on various changes in aging human retina and in dystrophies that affect the macula is not complete. Since cone photoreceptor and ganglion cell death are a potential problem, it is imperative to know about the basic facts on how they are affected and the mechanisms involved in their death. Thus, prevention of cone and ganglion cell loss should be the target of choice. This review also highlights the significant role played by electron microscopy in understanding such ultrastructural changes and future strategies utilizing it and other techniques to fill some of the existing lacunae and advance our knowledge.     

Eyes of male and female Orgyia antiqua (Lepidoptera; Lymantriidae) react differently to an exposure with UV-A

The structural organization of the eyes belonging to 12 winged male and 12 wingless female Orgyia antiqua moths, exposed for 1 h to UV-radiation (lambda(max)=351 nm) of 1.4 kW/m2, was compared with that of 12 male and 12 female non-irradiated control specimens. Following the UV-exposure, the screening pigments were found in a position indicative of extreme light-adaptation. Extensive formations of vesicles along the perimeter of the cones as well as disintegrating ER in the cone cytoplasm were noticeable, especially in the eye of the female. On the retinal side of the clearzone, the microvilli of the rhabdoms had become affected by the UV in characteristic ways: in the male eye, retinal cell damage in the form of microvillar swellings and disintegrations were largely confined to just two cells per ommatidium, placed opposite to each other. The female eye, once again, exhibited greater vulnerability and more widespread microvillar disruptions that affected all of the ommatidial retinula cells. The greater resistance of the eye of the male to an exposure with UV makes sense, if we consider the consequences of the retinal damage, which would clearly be a more severe handicap for an actively flying individual than for an almost sedentary one like the wingless female.     

The photocurrent response of human cones is fast and monophasic

Background  

The precise form of the light response of human cone photoreceptors in vivo has not been established with certainty. To investigate the response shape we compare the predictions of a recent model of transduction in primate cone photoreceptors with measurements extracted from human cones using the paired-flash electroretinogram method. As a check, we also compare the predictions with previous single-cell measurements of ground squirrel cone responses.     

Morphology and spectral sensitivities of retinal and extraretinal photoreceptors in freshwater teleosts

Fish eyes possess a complicated morphological and neural organisation of retinal and extra-retinal receptors. Features such as photoreceptor mosaic array and photoreceptor grouping are unique among vertebrates. Spectral sensitivities of these photoreceptors range from UV to the red portion of the visible spectrum. Moreover, these sensitivities can change with the age of the animals. In this review we will examine thoroughly the morphology, and the spectral sensitivities of retinal and extra-retinal receptors and the influence upon them of factors such as hormones, ageing, season, habitat light conditions, and migration.     

Backscattering cross section of a rigid biconic reflector

Plane wave incidence on a rigid biconic target is considered. A biconic reflector consists of two cones truncated by planes perpendicular to their axes and joined at their smaller flat faces. The cone angles are allowed to be variable, provided their sum is equal to 90 degrees. The backscattering cross section is expressed in terms of a surface integral of the geometrical acoustics field, which results from incident singly and doubly reflected rays. A saddle-point calculation gives a first-order high-frequency approximation in which the backscattering cross section is proportional to the incident wave number and a function of the angle of incidence, cone angles, and inner and outer radii of the truncated cones. This expression is algebraically complex but easy to implement numerically. Results are presented that exercise the parameters of the problem. An interesting result of the solution is that for fixed outer radii there is a nonzero optimum inner radius for backscattering strength. For broadside incidence on 45 degrees cones with equal outer radii, this optimum value is approximately equal to 11% of the outer radius.     

Chloride equilibrium potential in salamander cones

Background  

GABAergic inhibition and effects of intracellular chloride ions on calcium channel activity have been proposed to regulate neurotransmission from photoreceptors. To assess the impact of these and other chloride-dependent mechanisms on release from cones, the chloride equilibrium potential (E_Cl) was determined in red-sensitive, large single cones from the tiger salamander retinal slice.     

Identification of a set of genes showing regionally enriched expression in the mouse brain

Background

We have recorded responses from single neurons in murine visual cortex to determine the effectiveness of the input from the two murine cone photoreceptor mechanisms and whether there is any unique selectivity for cone inputs at this higher region of the visual system that would support the possibility of colour vision in mice. Each eye was stimulated by diffuse light, either 370 (strong stimulus for the ultra-violet (UV) cone opsin) or 505 nm (exclusively stimulating the middle wavelength sensitive (M) cone opsin), obtained from light emitting diodes (LEDs) in the presence of a strong adapting light that suppressed the responses of rods.

Results

Single cells responded to these diffuse stimuli in all areas of striate cortex. Two types of responsive cells were encountered. One type (135/323 – 42%) had little to no spontaneous activity and responded at either the on and/or the off phase of the light stimulus with a few impulses often of relatively large amplitude. A second type (166/323 – 51%) had spontaneous activity and responded tonically to light stimuli with impulses often of small amplitude. Most of the cells responded similarly to both spectral stimuli. A few (18/323 – 6%) responded strongly or exclusively to one or the other spectral stimulus and rarely in a spectrally opponent manner.

Conclusion

Most cells in murine striate cortex receive excitatory inputs from both UV- and M-cones. A small fraction shows either strong selectivity for one or the other cone mechanism and occasionally cone opponent responses. Cells that could underlie chromatic contrast detection are present but extremely rare in murine striate cortex.     

Cones with a dual structure formed on Ar+-bombarded Si surfaces

Transmission electron microscopy disclosed that cones formed on crystalline Si surfaces bombarded with a few keV Ar⁺ ions were of a dual structure inexplicable by the existing models of cone formation. The outer region of the cones was composed of unoriented crystallites of Si, whereas the inner region was a single crystal oriented in the 〈111〉 direction. The polycrystalline region was too thick to explain its formation in terms of ion-induced disordering of the monocrystalline phase, suggesting that the redeposition of sputtered Si atoms was deeply concerned in evolving the cones. It is supposed that the present cone evolution involved particle supply processes underlying the growth of Si whiskers from the vapor phase.     

Retinal axial focusing and multi-layer imaging with a liquid crystal adaptive optics camera

With the help of adaptive optics(AO) technology, cellular level imaging of living human retina can be achieved.Aiming to reduce distressing feelings and to avoid potential drug induced diseases, we attempted to image retina with dilated pupil and froze accommodation without drugs. An optimized liquid crystal adaptive optics camera was adopted for retinal imaging. A novel eye stared system was used for stimulating accommodation and fixating imaging area. Illumination sources and imaging camera kept linkage for focusing and imaging different layers. Four subjects with diverse degree of myopia were imaged. Based on the optical properties of the human eye, the eye stared system reduced the defocus to less than the typical ocular depth of focus. In this way, the illumination light can be projected on certain retina layer precisely.Since that the defocus had been compensated by the eye stared system, the adopted 512 × 512 liquid crystal spatial light modulator(LC-SLM) corrector provided the crucial spatial fidelity to fully compensate high-order aberrations. The Strehl ratio of a subject with-8 diopter myopia was improved to 0.78, which was nearly close to diffraction-limited imaging. By finely adjusting the axial displacement of illumination sources and imaging camera, cone photoreceptors, blood vessels and nerve fiber layer were clearly imaged successfully.     

A structural study of the retinal photoreceptor, plexiform and ganglion cell layers following exposure to UV-B and UV-C radiation in the albino rat

Over the last two decades, ultraviolet radiation levels (UV), reaching the Earth's surface, have been increasing at a rate of 1.5% per each 1% loss of the ozone layer. Moreover, artificial UV-sources have also proliferated and contributed to the rising UV-stress that many organisms have to face. To assess how the vertebrate retina responds to an exposure of short wavelength UV, we focused our attention on the rat retina, observing photoreceptor (containing outer and inner segments of rods and cones), inner plexiform, and ganglion cell layers by light and transmission electron microscopy using conventional and cytochemical techniques. We analyzed how cells of the layers in question responded to a 30 min exposure to UV-C and UV-B radiation with doses of 7200 and 590 J/cm², respectively. The results show that there are significant changes in the nuclei and cytoplasmic organelles of the exposed retinae when compared with those of the unexposed controls. The changes include an increase in heterochromatin, distension of rough endoplasmic reticulum, mitochondrial disruptions, and increases in the number of myelin bodies. The recorded morphological changes, especially those of the ganglion cells, are suggestive of apoptotic processes and show that the exposure of vertebrate retina to wavelengths ranging from 254 to 312 nm can produce alterations that are likely to impact negatively on the retina's proper functioning.     

In vivo measurement of retinal physiology with high-speed ultrahigh-resolution optical coherence tomography

Noninvasive in vivo functional optical imaging of the intact retina is demonstrated by using high-speed, ultrahigh-resolution optical coherence tomography (OCT). Imaging was performed with 2.8 microm resolution at a rate of 24,000 axial scans per second. A white-light stimulus was applied to the dark-adapted rat retina, and the average reflectivities from different intraretinal layers were monitored as a function of time. A 10%-15% increase in the average amplitude reflectance of the photoreceptor outer segments was observed in response to the stimulus. The spatial distribution of the change in the OCT signal is consistent with an increase in backscatter from the photoreceptor outer segments. To our knowledge, this is the first in vivo demonstration of OCT functional imaging in the intact retina.     

Visualization of unsteady shock oscillations in the High-enthalpy flow field around double cones

The presence of an adverse pressure gradient, shock/shock interaction and shock wave/boundary layer interaction often induces flow separation around bodies. However, the effect of dissociated flow on separated flow characteristics, especially at hypersonic speeds, is still not clear, and considerable differences are observed between experiments and numerical simulations. In this investigation, the unsteady separated flow features around double cones are visualized in the Shock Wave Research Center (SWRC) free-piston driven shock tunnel at a nominal Mach Number of 6.99 using multiple optical techniques. The time resolved shock structure oscillations in the flow field around double cones (first cone, semi-apex angle = 25°; second cone, semi-apex angles=50°, 65°, 68° and 70°) have been visualized using a high-speed image converter camera (IMACON) at a nominal stagnation enthalpy of 4.8 MJ/kg. In addition, flow visualization studies around the double cone is also carried out using Schlieren and double exposure holographic interferometry in order to precisely locate the separation point and measure the separation length. The presence of a triple shock structure in front of the second cone and a non-linear unsteady shock structure oscillation in the flow field are the significant results from visualization studies on the 25° /65°, 25° /68° and 25°/70° double cones. On the other hand, the flow field around 25° /50° is relatively steady and Type V shock/shock interaction is observed. Illustrative numerical simulation studies are carried out by solving N-S equations to complement the experiments. The simulated flow features around a double cone agree well qualitatively with experiments.     

Optical Möbius strips in three-dimensional ellipse fields: I. Lines of circular polarization

The major and minor axes of the polarization ellipses that surround singular lines of circular polarization in three-dimensional optical ellipse fields are shown to be organized into Möbius strips (twisted ribbons). These strips can have either one or three half-twists, and can be either right- or left-handed. The normals to the surrounding ellipses generate cone-like structures. Two special projections, and eight new indices are developed to characterize the rather complex structures of the Möbius strips and cones. These eight indices, together with the two well-known indices used until now to characterize singular lines of circular polarization, could, if independent, generate 16,384 different index combinations. Geometric constraints and 15 selection rules are discussed that reduce the number of combinations to 1676. Of these 1150 have been observed in 10⁶ independent realizations of a simulated random ellipse field. Statistical probabilities are presented for the most important index combinations. It is argued that it is presently feasible to perform experimental measurements of the Möbius strips and cones described here theoretically.     

Therapeutic effect of the NMDA antagonist MK-801 on low-level laser induced retinal injury

The aim of this article was to explore the mechanism of injury in rat retina after constant low-level helium-neon (He-Ne) laser exposure and therapeutic effects of MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist, on laser-induced retinal injury. He-Ne laser lesions were created in the central retina of adult Wistar Kyoto rats and were followed immediately by intraperitoneal injection of MK-801 (2 mg/kg) or saline, macroscopical and microscopical lesion were observed by funduscope and light microscope. Ultrastructural changes of the degenerating cells were examined by electron microscopy. Photoreceptor apoptosis was evaluated by TdT-mediated dUTP nick end-labeling (TUNEL). mRNA levels were measured by in situ hybridization and NMDA receptor expression was determined by immunohistochemistry. Laser induced damage was histologically quantified by image-analysis morphometry. Electroretinograms (ERGs) were recorded at different time point after the cessation of exposure to constant irradiation. There was no visible bleeding, exudation or necrosis under funduscope. TUNEL and electron microscopy showed photoreceptor apoptosis after irradiation. MK-801-treated animals had significantly fewer TUNEL-positive cells in the photoreceptors than saline-treated animals after exposure to laser. In situ hybridization (ISH) showed that the NMDAR mRNA level of MK-801-treated rats decreased in the inner plexiform layer 6 h after the cessation of exposure to constant irradiation when compared with that of saline-treated rats. So did Immunohistochemistry (IHC). Electroretinogram showed that b-wave amplitudes of MK-801-treated group were higher than that of saline-treated group after laser exposure. These findings suggest that Low level laser may cause the retinal pathological changes under given conditions. High expression of NMDAR is one of the possible mechanisms causing experimental retinal laser injury of rats. MK-801 exhibits the therapeutic effect due to promote the recovery of structure and function of injured retina. Original Text ? Astro, Ltd., 2009.     

In vivo imaging of the human rod photoreceptor mosaic

Although single cone receptors have been imaged in vivo, to our knowledge there has been no observation of rods in the living normal eye. Using an adaptive optics ophthalmoscope and post processing, evidence of a rod mosaic was observed at 5° and 10° eccentricities in the horizontal temporal retina. For four normal human subjects, small structures were observed between the larger cones and were observed repeatedly at the same locations on different days, and with varying wavelengths. Image analysis gave spacings that agree well with rod measurements from histological data.     

Electric field manipulation of multiple nonequivalent Dirac cones in the electronic structures of hexagonal CrB_4 sheet

Two-dimensional materials with Dirac cones have significant applications in photoelectric technology. The origin and manipulation of multiple Dirac cones need to be better understood. By first-principle calculations, we study the influence of external fields on the electronic structure of the hexagonal CrB_4 sheet with double nonequivalent Dirac cones. Our results show that the two cones are not sensitive to tensile strain and out-of-plane electric field, but present obviously different behaviors under the in-plane external electric field(along the B-B direction), i.e., one cone holds while the other vanishes with a gap opening. More interestingly, a new nonequivalent cone emerges under a proper in-plane electric field. We also discuss the origin of the cones in CrB_4 sheet. Our study provides a new method on how to obtain Dirac cones by the external field manipulation, which may motivate potential applications in nanoelectronics.     

一种新的纳米结构--管状石墨锥

文章作者利用微波等离子体辅助化学气相沉积的方法在铁针尖上合成了一种新的纳米结构，并称之为管状石墨锥．管状石墨锥在外形上由多面锥体组成，其内部是同心的圆柱形石墨层，其空心的直径为几个纳米到几十个纳米．这些管状石墨层从内到外地逐渐变短，从而使得它们呈现出锥形外观．锥的顶角一般为6-7度左右，锥的尖端只有几个纳米大小，而锥的底部可达到微米量级．值得注意的是，组成管状锥体的石墨层具有惟一的手性，都表现为锯齿型。