Filling-in of the foveal blue scotoma.

The blue-blindness (tritanopia) of the human foveola normally goes unnoticed but can be directly visualized by having observers view a flickering, monochromatic, short-wavelength field. The blue scotoma appears as a tiny dark spot in central vision, the visibility of which depends upon the wavelength of the field and the temporal frequency of modulation. Comparisons of fading times as a function of flicker frequency for the blue scotoma, foveal afterimages and optically stabilized images indicate a common time course, consistent with the hypothesis that perceptual filling-in of the foveal blue scotoma reflects the operation of neural processes similar to those involved in fading and regeneration of stabilized images.     

Role of the blue mechanism in wavelength discrimination

The role of blue cones as well as the pathways they supply (collectively called the "blue mechanism") is evaluated by comparing ordinary wavelength discrimination functions with those obtained using two methods designed to inhibit the blue mechanism selectively. These methods use a just-noticeable-border criterion (JNB), instead of the usual one of just-noticeable-difference, and a yellow preadapting field to induce transient tritanopia. Without transient tritanopia, the data obtained using the just-noticeable-border criterion reveal a small contribution of the blue mechanism to wavelength discrimination. Transient tritanopia, with JNB, produces an additional selective loss of wavelength discrimination in a spectral region flanking 460 nm, which yields a function resembling those for tritanopes previously examined.     

The relationship between the visual prognoses of branch retinal artery obstruction and foveal thickness on OCT

Purpose

To determine the correlation between the prognosis of branch retinal artery obstruction (BRAO) and the foveal thickness or outer nuclear layer (ONL) thickness on optical coherence tomography (OCT).

Methods

Twenty-one eyes (21 patients) in patients with resolved, non-complicated BRAO and a normal control of 10 eyes (10 volunteers) were used in this study. The average macular thickness, foveal thickness and ONL thickness at central fovea were measured in both the patients and the control group using spectral domain OCT. The thickness between the patient group and the control group were compared and correlation between the best corrected visual acuity (BCVA) and each thickness was determined.

Results

The average age of the patients was 52 ± 5.8 years. The average macular thickness, foveal thickness and ONL thickness at the central fovea of the patients were significantly (p < 0.001, p = 0.023, p = 0.021, respectively) thinner than that of the control group. Both the foveal thickness (r_s = 0.56, p = 0.008) and ONL thickness (r_s = 0.86, p < 0.001) were significantly correlated with BCVA. There was no significant correlation between the average macular thickness and BCVA.

Conclusions

The foveal thickness and ONL thickness at the fovea was positively correlated with the BCVA in patients with resolved BRAO.     

Multifocal electroretinogram (mfERG) in patients with diabetes mellitus and an enlarged foveal avascular zone (FAZ)

Purpose To assess the relationship between foveal microcirculation and central retinal function in diabetic patients having both an enlarged foveal avascular zone (FAZ) and a preserved visual acuity (0.6 or better). Methods Twenty-five patients with diabetes type 1 or 2 with an enlarged FAZ (largest diameter > 650 μm) measured in fluorescein angiograms were examined with multifocal ERG (mfERG). The largest FAZ diameter, the FAZ area as well as the adjacent perifoveal intercapillary area (PIA), was calculated from the fluorescein angiogram. The retinopathy level was mild to preproliferative. There was no macular edema and no eye had previously been treated with photocoagulation. Results The mean FAZ diameter was 0.92 ± 0.17 mm and the mean summed area (FAZ and PIA) was 0.74 ± 0.24 mm². There was a significant correlation between increasing FAZ diameter and increasing implicit time of the innermost concentric rings and of the third concentric ring in the first order kernel of the mfERG (P = 0.03 and P = 0.008, respectively). An increasing summed area (FAZ and PIA) was correlated to increasing implicit time in the same areas of the mfERG (P = 0.005 and P = 0.026, respectively). No correlation was seen between the ischemic areas and the mfERG amplitudes. Conclusion A correlation between the ischemic areas and prolonged implicit time in the mfERG indicates that alterations in neuronal macular function due to ischemia might precede the deterioration of visual acuity.     

The color-matching functions for lambda greater than or equal to 540 nm: incompatibility with the blue fundamental

The question of the blue fundamental contribution to foveal color matches for lambda greater than or equal to 540 nm is examined. It is shown that, with two almost universally accepted assumptions, such a blue fundamental contribution leads to a contradiction with the observed color-matching functions. It is concluded that the blue fundamental does not contribute to foveal color matches for lambda greater than or equal to 540 nm.     

Temporal modulation sensitivity of the blue mechanism: measurements made with extraretinal chromatic adaptation

Using the counterphase-modulated-tritan-metamers method of Wisowaty and Boynton [Vision Res. 20, 895-909 (1980)], we measured the modulation sensitivity function of the blue mechanism on dark backgrounds and on uniform yellow backgrounds that were produced either by illuminating the retina with yellow light or by filling-in of a stabilized retinal image. When measured on yellow backgrounds, temporal modulation sensitivity was markedly reduced whether the background was produced by retinal illumination or by filling-in. This sensitivity reduction has been attributed by other researchers to changes in the transmission characteristics of the blue/yellow pathway caused by yellow adaptation. Our results obtained with backgrounds that appeared yellow due to filling-in indicate that the site of adaptation must be extraretinal as there was no yellow light reaching the retina in the vicinity of the test probe.     

Butterflies and black lacy patterns: the prevalence and characteristics of Charles Bonnet hallucinations in an Australian population

Background: Charles Bonnet Syndrome (CBS) is characterized by vivid, elaborate and recurrent visual hallucinations in psychologically normal people. It most often occurs in older, visually impaired persons. The prevalence of the syndrome has been reported at 1–40% in Asia, Europe and North America. The aim of this study was to examine the prevalence and characteristics of CBS in the older aged, visually impaired population in Australia. Methods: Two hundred consecutive patients attending ophthalmology clinics aged more than 60 years with best‐corrected visual acuity of 6/12 or less were screened for CBS. Those who reported experiencing visual hallucinations were asked to participate in the project. They were then interviewed and asked demographic, general health and visual hallucination‐related questions. A group of 30 non‐hallucinating participants was chosen for comparison of demographic data. Results: The prevalence was found to be 17.5%. Participants experiencing hallucinations were predominantly female with a mean age of 77.7 years. Correlations were found between the living situation, level of education and characteristics of the hallucinations such as the duration, length of time the participant had been experiencing them and their frequency. Conclusion: The prevalence of CBS in the older‐aged, low‐vision population is 17.5%. Several demographic and syndrome‐specific characteristics were found to be consistent enabling a profile of a CBS sufferer in this group of participants to be compiled.     

Intrinsic tissue fluorescence in an organotypic perfusion culture of the porcine ocular fundus exposed to blue light and free radicals

BACKGROUND: A wide variety of pathological pathways may result in age-related macular degeneration. Because of its complexity, there is no comprehensive model of the disease yet. One key feature is the accumulation of the autofluorescent pigment lipofuscin in the retinal pigment epithelium (RPE). Thus, we developed an organotypic perfusion culture model of the porcine ocular fundus, generating lipofuscin under exposure to blue light and hydrogen peroxide. METHODS: Porcine fundi (choroid, Bruch's membrane, RPE, and retina) were explanted in toto, transferred into a perfusion culture chamber, perfused with cell culture medium and kept at 37 degrees C. Free radical stress was induced by supplementation of H(2)O(2), and/or the specimens were exposed to blue light, or kept untreated as controls. After a culture period of 7 days, the specimens were subject to microscopic inspection, histology, fluorescence microscopy, and measurement of fluorescence spectra as well as fluorescence decay times. RESULTS: Histology showed atrophic ganglion cells and rod outer segments. All other tissue structures were morphologically intact. Compared to the controls, RPE and retina exposed to light showed increased fluorescence, which was shifted towards shorter wavelengths. The fluorescence spectra and decays resembled that of lipofuscin granules isolated from human donor eyes. HPLC analysis revealed the abundance of the lipofuscin component N-retinylidene-N-retinylethanolamine (A2E), its precursor products, as well as two new, green-emitting fluorophores. CONCLUSIONS: Porcine ocular fundi were successfully preserved in an organotypic perfusion culture for 7 days, and exhibited remarkable autofluorescence after light and free radical exposure, making the model suitable for investigations of lipofuscinogenesis.     

Scene through the eyes of an apex predator: a comparative analysis of the shark visual system

The eyes of apex predators, such as the shark, have fascinated comparative visual neuroscientists for hundreds of years with respect to how they perceive the dark depths of their ocean realm or the visual scene in search of prey. As the earliest representatives of the first stage in the evolution of jawed vertebrates, sharks have an important role to play in our understanding of the evolution of the vertebrate eye, including that of humans. This comprehensive review covers the structure and function of all the major ocular components in sharks and how they are adapted to a range of underwater light environments. A comparative approach is used to identify: species‐specific diversity in the perception of clear optical images; photoreception for various visual behaviours; the trade‐off between image resolution and sensitivity; and visual processing under a range of levels of illumination. The application of this knowledge is also discussed with respect to the conservation of this important group of cartilaginous fishes.