Color and motion: which is the tortoise and which is the hare?

Recent psychophysical studies have been interpreted to indicate that the perception of motion temporally either lags or is synchronous with the perception of color. These results appear to be at odds with neurophysiological data, which show that the average response-onset latency is shorter in the cortical areas responsible for motion (e.g., MT and MST) than for color processing (e.g., V4). The purpose of this study was to compare the perceptual asynchrony between motion and color on two psychophysical tasks. In the color correspondence task, observers indicated the predominant color of an 18 degrees x 18 degrees field of colored dots when they moved in a specific direction. On each trial, the dots periodically changed color from red to green and moved cyclically at 15, 30 or 60 deg/s in two directions separated by 180 degrees, 135 degrees, 90 degrees or 45 degrees. In the temporal order judgment task, observers indicated whether a change in color occurred before or after a change in motion, within a single cycle of the moving-dot stimulus. In the color correspondence task, we found that the perceptual asynchrony between color and motion depends on the difference in directions within the motion cycle, but does not depend on the dot velocity. In the temporal order judgment task, the perceptual asynchrony is substantially shorter than for the color correspondence task, and does not depend on the change in motion direction or the dot velocity. These findings suggest that it is inappropriate to interpret previous psychophysical results as evidence that motion perception generally lags color perception. We discuss our data in the context of a "two-stage sustained-transient" functional model for the processing of various perceptual attributes.     

Epithelial plugs in radial keratotomy: the origin of incisional keratitis?

PURPOSE: Incisions in radial keratotomy (RK) patients were examined histopathologically to explore the cause of delayed infections occurring following this procedure. METHODS: Four corneas from two successful RK patients were obtained postmortem and 1 cornea from an unsuccessful RK was collected following lamellar keratoplasty. The tissue was prepared for light and transmission electron microscopy. RESULTS : All incisions examined contained an epithelial plug. The average depth of the plugs for left and right corneas in the 2 successful cases were 142.58 microm (range: 36.46-183.04 microm) and 58.44 microm (range: 29.27-115.66 microm), whereas the unsuccessful case these measurements were 121.06 microm (range: 70.03-172.01 microm). In the successful cases, the plugs were on average 4.2 cells deep (range: 2-5) in one case and for the other the plugs were 8.78 cells deep (range: 1.5-11.5) and 2.72 cells wide (range: 2-4). In the unsuccessful case the plugs were on average 6.89 cells deep (range: 5-11) and 8.56 cells wide (range: 4-13). The average epithelial thickness, measured 200 microm on either side away from the plug, was 47.11 microm and 55.09 microm for the successful cases, and 27.52 microm for the unsuccessful case. Degenerate cells were often observed within the plug and along the overlying surface. Lamellae cut during surgery remained severed in all corneas observed. The stroma adjacent to the incision contained an elevated number of stromal cells (8%-78%). CONCLUSIONS: Healing did not include repair of anterior limiting lamina or stromal lamellae, which all remained severed in all incisions observed. Epithelial plugs were invariably present in all incisions regardless of clinical outcome, thus, increasing the distance a basal cell travels to reach the epithelial surface by 2 to 4 times. Since the epithelial cell has a limited life, it is hypothesized that cells originating in the plug may die before reaching the surface, thus, leading to intermittent loss of surface integrity. The loss of the epithelial barrier allows invasion of microorganisms and the delayed onset of keratitis. The larger the plug the greater the possibility of surface defects and resultant infection.     

Hemianopsias in cortical and subcortical lesions of the visual system: what is the difference?

The neuronal correlate of perception in the visual area remains unimpaired in lesions of the optic tracts or the radiatio optica. Hemianopsias due to lesions of these pathways are perceived immediately. On the contrary, hemianopsias due to a lesion of the visual cortex are not perceived or are only indirectly realized. It has not been established if a lesion of the primary visual area alone is sufficient to make the patient unaware of a hemianopsia or if additional impairments of the peristriate visual areas are required. Data processing within the cortical visual projection chain makes it likely, however, that a lesion of the striate cortex is enough. Under physiological conditions an uniformly illuminated field is represented in the neuronal activity by the borders of the field. Neurons within the field are not activated. The higher visual cortical areas can so not discriminate if an input missing from V1 is a result of the physiological processing or of a lesion. Scotomas are therefore not perceived. By analogy this argument should also hold for a hemianopic "scotoma".     

Glaucoma in China: how big is the problem?

AIMS: To derive preliminary estimates for the number of adults in China suffering from glaucoma, and project the burden of visual morbidity attributable to primary and secondary glaucoma. METHODS: Age and sex specific data from two population surveys were applied to US Census Bureau population estimates for urban and rural China. It was assumed that data from Singapore were representative of urban China, and those from Mongolia were representative of rural China. RESULTS: It was estimated that 9.4 million people aged 40 years and older in China have glaucomatous optic neuropathy. Of this number, 5.2 million (55%) are blind in at least one eye and 1.7 million (18.1%) are blind in both eyes. Primary angle closure glaucoma (PACG) is responsible for the vast majority (91%) of bilateral glaucoma blindness in China. The number of people with the anatomical trait predisposing to PACG (an "occludable" drainage angle) is in the region of 28.2 million, and of these 9.1 million have significant angle closure, indicated by peripheral anterior synechiae or raised intraocular pressure. CONCLUSIONS: This extrapolation of data from two east Asian countries gives an approximate number of people in China suffering from glaucoma. It is unlikely that this crude statistical model is entirely accurate. However, the authors believe the visual morbidity from glaucoma in China is considerable. PACG is probably the leading cause of glaucoma blindness in both eyes, and warrants detailed investigation of strategies for prevention.     

What is the signal in noise?

Visual perception is limited by both the strength of the neural signals, and by the noise in the visual nervous system; however, little is known about what aspects of the input noise the human visual system is sensitive to, i.e., what is the signal in noise? In order to investigate this question we asked observers to discriminate differences in the strength of one-dimensional white noise. We measured their response consistency and classification images and compared the results with an ideal energy detector. Our results and modelling show that discrimination of noise is limited by the observer's template (i.e., the weighted combination of energy in each stimulus component) plus higher order nonlinearities (systematic noise), and by sources of random internal noise. We found that systematic noise is present only near detection threshold. Surprisingly, we found that the human template is "adaptive"-- its shape depends on the spatial frequency band of the noise-suggesting that sensitivity to spatial noise is not simply determined via passive filtering.     

Carbonic anhydrase XIV identified as the membrane CA in mouse retina: strong expression in Müller cells and the RPE

The presence of carbonic anhydrase (CA) activity in the neural retina has been known for several decades. CA-II, a soluble cytoplasmic isoform expressed by Müller cells and a subset of amacrine cells, was thought to be the sole source of CA activity in the neural retina. However, CA-II deficient mice retain CA activity in the neural retina, which implies that another isoform must be present in that tissue. Recently CA-XIV, an integral membrane protein, was cloned and characterized. We, therefore, sought to determine whether CA-XIV is expressed in the neural retina, and hence is responsible for the CA activity observed in CA-II null animals. Immunohistochemical analyses of histological sections from CA-II null, CA-XIV null, and control mice were performed to localize the CA-XIV isoform, as well as other known retinal markers. Immunoblotting and real-time RT-PCR analyses were also performed to test for CA-XIV expression in retina and other mouse tissues. We determined herein that CA-XIV, a approximately 45kDa membrane protein, is expressed in retina, as it is in kidney. In the retina, CA-XIV is expressed on the plasma membrane of Müller cells. CA-XIV is also found on both the apical and basal membranes of the retinal pigmented epithelium. The data presented here indicate that like CA-II, CA-XIV is highly expressed in the neural retina and, like CA-II, more specifically by the Müller cells. The cellular compartmentalization of the two isoforms in the Müller cell-one cytoplasmic and the other on the plasma membrane-suggest that the two enzymes have specific and unique functions. Future studies will be necessary to assign functions to CA-II and CA-XIV in the mouse neural retina.     

Diabetic retinopathy screening in the public sector in India: What is needed?

India has been witnessing an epidemic of diabetes for several years now. A large proportion of patients with undiagnosed and poorly controlled diabetes are at great risk of developing diabetic retinopathy (DR) and irreversible blindness. The goal of DR screening is to identify people with sight-threatening DR early so that prompt treatment can be initiated, and blindness can be prevented. Systematic DR screening is essential to identify disease early, and a national effort for the same is required. We adopt a health system approach to outline the actions that need to take place for effective DR screening in the public sector in India. We discuss the role of national leadership, needs assessment, finalization of DR screening and referral pathway, trainings, strategies to improve the uptake, allocation of roles and responsibilities, public-private partnerships, quality control, and financing.     

Retinopathy in non diabetics,diabetic retinopathy and oxidative stress: a new phenotype in Central Africa?

AIM: To evaluate the rates of retinopathy without diabetes and diabetic retinopathy (DR), associated with some markers of oxidative stress, antioxidants and cardiometabolic risk factors.METHODS: We determined the prevalence of DR in 150 type 2 diabetes mellitus (T2DM) patients, that of retinopathy in 50 non diabetics, the levels of body mass index (BMI), waist circumference (WC), blood pressure, lipids, 8-isoprostane, 8-hydroxydeoxyguanosine (8-OHdG), gamma-glutamyl transferase GT (GGT), oxidized low-density lipoprotein (OxLDL), thiobarbituric acid reacting substances (TBARS), reduced glutathione (GSH), superoxide dismutase (SOD), uric acid, creatinine, albumin, total antioxidant status (TAOS), zinc, selenium, magnesium, vitamin C, vitamin D, vitamin E, glucose, apolipoprotein B (ApoB).RESULTS: The prevalences of DR at 53y and Rtp at 62y were 44% (n=66) and 10% (n=5), respectively. The highest levels of 8-isoprostane, 8-OHdG, TBARS, SOD, and OxLDL were in DR. The lowest levels of vitamin D, vitamin C, TAOS, and vitamin E were in DR. In the case-control study discriminant analysis, the levels of vitamin C, vitamin D, ApoB, 8-OHdG, creatinine, Zn, vitamin E, and WC distinguished significantly non-diabetics without DR (controls), T2DM patients without DR and T2DM patients with DR.CONCLUSION: Anticipation of DR onset is significantly associated with the exageration of oxidative stress biomarkers or decrease of antioxidants in African type 2 diabetics. Prevention of oxidative stress and abdominal obesity is needed. Supplementation in vitamin C, D, and E should be recommended as complement therapies of T2DM.     

Thrombospondin-1 Expression in RPE and Choroidal Neovascular Membranes

Introduction Age-related macular degeneration (AMD) s the leading cause of blindness in the West or individuals more than 50 years of age[1-3].Severe visual loss in the late stages of AMD most commonly results from choroidal neovas- cularization (CNV), a process characterized by the growth of new vessels from the choriocapil- laris through Bruch′s membrane. These new vessels are prone to leakage and bleeding and may be associated with detachment of the retinal pigment epithelium (RPE). …     

How large is the optic disc? Systematic errors in fundus cameras and topographers

PURPOSE: To determine whether or not there are systematic differences in the areas of optic discs as measured by different machines using different measurement algorithms and whether racial or gender differences exist in optic disc area measurements. METHODS: We examined the results of twenty-three published studies on the size of normal optic discs of various patient populations. Studies differed in the type of instrument and method used to measure optic disc area, and the number, age, race and gender of subjects examined. Noticing that different machines exhibited statistically significant systematic differences in optic disc sizes of comparable populations, we computed a "normalization" factor for each machine based on these mean differences. Applying this normalization factor to the results, we then re-examined the differences between racial and gender groups. RESULTS: By comparing the results of mean optic disc areas of different racial groups made with different machines, and normalizing results according to those of the Zeiss fundus camera, we found the normalization factors for the following machines to be, Zeiss fundus camera: 1 (by definition), Rodenstock Optic Disc Analyzer (RODA): 1.51, Topcon fundus camera: 1.04, Heidelberg Retina Tomograph (HRT): 1.15 and TopSS scanning laser ophthalmoscope: 1.29. That is, to bring the results of area measurements made with a RODA machine in line with those made with a Zeiss fundus camera, one should multiply the former by the factor 1.51. Using the normalized results, we confirmed the findings of previous authors that the optic disc areas of black subjects were statistically significantly larger than those of white subjects (n-weighted mean effect = 0.556 +/- 0.142 S.E., n = 5). Further, the meta-analysis of various racial populations from five studies shows that males have significantly larger discs than females (n-weighted mean effect = 0.151 +/- .055 S.E., n = 9). CONCLUSION: Different machines and techniques give different results when populations of similar racial composition are measured. We recommend applying the above normalizing factors when comparing studies that employ different instruments.