What is normal binocular vision?

The vergence position of the eyes is determined by the near fixation-accommodationmiosis synkinesis and the fusion mechanism. The contribution of both systems was analysed in 30 normal subjects and 16 subjects with abnormal binocular vision. Prism fixation disparity curves were determined in three different experimental situations: the routine method according to Ogle, a method to stimulate the synkinetic convergence (Experiment I, with one fixation point as sole binocular stimulus) and a method to stimulate the fusion mechanism (Experiment II, with random dot stereograms). Experiment I produced flat curves and Experiment II steep curves. The mean diameter of the horizontal Panum area was 5 minutes of arc in Experiment I and 2 in Experiment II. On the basis of these findings, it was postulated that the synkinetic system operates in the absence of fixation disparity and the fusion system in the presence of fixation disparity. In Experiment II, esodisparities of 100 minutes of arc occur in a number of normal subjects.The dividing line between normal and abnormal binocular vision therefore is blurred. Normal persons can display disparities, the order of magnitude of which is equal to that of the angle of squint in micro-strabismus.     

How well does color perimetry isolate responses from individual cone mechanisms?

PURPOSE: Over the past two decades there has been renewed interest in the use of color perimetry as a means of detecting early functional defects resulting from glaucomatous optic neuropathy and other forms of ophthalmic and neurologic pathology. The most popular form of color perimetry employs a colored background that selectively desensitizes two of the cone mechanisms, while the wavelength of the test target is selected to favor detection by the remaining, relatively unadapted, cone mechanism. While there are data to support the assertion that blue on yellow perimetry adequately isolates the short wavelength sensitive (SWS) cone mechanism, the only estimates of isolation of the other two cone mechanisms are for the region of the visual field corresponding to the fovea. The purpose of this experiment was thus to determine the amount of cone mechanism isolation that is afforded by automated perimetry when using yellow, blue, and magenta adapting backgrounds. METHODS: To estimate cone mechanism isolation, we determined spectral sensitivity for a range of narrow-band stimuli using a standard 30-2 program on a modified Humphrey perimeter. Targets were presented against three different backgrounds of different luminance; yellow at 2.1 log cd.m-2, magenta at 1.3 log cd.m-2, and blue at 1 log cd.m-2. Sensitivity values for each background at each stimulus position were plotted as a function of wavelength, normalized, and then fitted with cone sensitivity templates to determine the relative sensitivity of the three cone mechanisms. RESULTS: The maximum relative isolation of an individual cone mechanism was achieved with a yellow background, where there was an average relative isolation of 0.94 for the SWS cone mechanism; the blue background provided a relative isolation of 0.89 for the long wavelength sensitive (LWS) cone mechanism. The magenta background proved poor at isolating the medium wavelength sensitive (MWS) cone mechanism, where a relative isolation of only 0.51 was obtained. CONCLUSION: Although color perimetry is capable of isolating individual cone mechanisms, the magnitude of isolation in normal observers may be small under certain circumstances. Therefore, when the technique is used to examine pathologic states, it may be necessary to employ at least two target wavelengths to determine the cone mechanism that is performing target detection. Furthermore, we suggest that MWS cone mechanism isolation may be improved through the combination of the so-called silent substitution technique with that of selective adaptation.     

Can attention selectively bias bistable perception? Differences between binocular rivalry and ambiguous figures

It is debated whether different forms of bistable perception result from common or separate neural mechanisms. Binocular rivalry involves perceptual alternations between competing monocular images, whereas ambiguous figures such as the Necker cube lead to alternations between two possible pictorial interpretations. Previous studies have shown that observers can voluntarily control the alternation rate of both rivalry and Necker cube reversal, perhaps suggesting that bistable perception results from a common mechanism of top-down selection. However, according to the biased competition model of selective attention, attention should be able to enhance the attended percept and suppress the unattended percept. Here, we investigated selective attentional modulation of dominance durations in bistable perception. Observers consistently showed much weaker selective attentional control for rivalry than for Necker cube reversal, even for rivalry displays that maximized the opportunities for feature-, object-, or space-based attentional selection. In contrast, nonselective control of alternation rate was comparably strong for both forms of bistable perception and corresponded poorly with estimates of selective attentional control. Our results support the notion that binocular rivalry involves a more automatic, stimulus-driven form of visual competition than Necker cube reversal, and as a consequence, is less easily biased by selective attention.     

How does noise influence the estimation of speed?

Local motion signals have to be combined in space and time, to yield a coherent motion percept as it is involved in a variety of visual tasks. This combination necessarily means to trade-off between loosing spatio-temporal resolution by pooling local signals and maintaining perceptually significant segmentation between them. When signals are pooled to detect the presence of coherent motion in large amounts of random noise, the question raised is how the noise affects the perceived quality, in particular speed, of the coherent motion. Is there an analogy to the well-known reduction in the perceived speed of moving gratings at low contrast? Using a two-interval forced-choice procedure, we have investigated the assessment of speed in random-dot kinematograms containing different proportions of noise. Under the conditions investigated, there is no strong reduction of perceived speed with increasing noise, as long as coherence levels remain well above the thresholds for directional judgements. This basic result, which could suggest considerable but not perfect segregation of signal and noise motion components in the pooling process leading to speed estimation, is discussed in relation to a model that is designed to decode speed from a population of elementary motion detectors (EMDs) of the correlation type. A strategy to estimate speed from a set of EMDs with a variety of spatio-temporal tuning does not only provide a velocity predictor unambiguous with the spatial structure of the stimulus, but also is largely independent of noise.     

Center-surround antagonism in spatial vision: retinal or cortical locus?

Mach and Hering had early advanced a model of spatial visual processing featuring an antagonistic interaction between adjoining areas in the visual field. Spatial opponency was one of the first findings when single-unit studies of the retina were begun. Not long afterwards psychophysical experiments revealed a center-surround organization closely matching that found in the mammalian retina. It hinged on the demonstration of reduction of sensitivity in a small patch of the visual field when its surround was changed from dark to bright. Because such patterns inevitably produce borders, well-known phenomena of border interaction could be seen as providing alternative explanations, whose substrate would most likely be in the visual cortex. These competing viewpoints are discussed especially as they pertain to the recent demonstration of spatial differences in the center/surround organization between the normal and affected eyes of amblyopes. To the extent that most findings favor a retinal site for the psychophysically measured antagonism, and that evidence is accumulating for a direct effect on the mammalian retina of stimulus manipulation during visual development, the difference in spatial parameters of center/surround antagonism in amblyopia suggests that the dysfunction in amblyopia begins already in the retina.     

How does amniotic membrane work?

Transplantation of amniotic membrane as a temporary or permanent graft promotes epithelial wound healing and exerts potent anti-inflammatory and anti-scarring effects on the ocular surface. These actions depend on the killing of allogeneic amniotic cells and preservation of the cytokine-containing matrix during the preparation of the amniotic membrane. This review describes how these actions inherently operate in utero and how amniotic membrane transplantation aims to recreate such a fetal environment to exert these actions by insulating the surgical site from the host environment. These actions also render the amniotic membrane a unique niche capable of expanding both epithelial and mesenchymal progenitor cells ex vivo, while maintaining their normal cell phenotypes. As a result, the amniotic membrane becomes an ideal substrate for engineering different types of ocular surface tissues for transplantation. Further studies investigating the exact molecular mechanism by which the amniotic membrane works will undoubtedly unravel additional applications in reconstruction and engineering of both ocular and nonocular tissues in the burgeoning field of regenerative medicine.     

How does nonpenetrating surgery work?

The aqueous leaves the anterior segment via two different pathways, the classical and the uveoscleral routes. The main resistance to aqueous egress lies at the level of the juxtacanalicular trabeculum and the inner wall of Schlemm's canal. Impaired function of either one of these structures leads to a decrease in the outflow facility which eventually results in a significant increase in intraocular pressure. Success of non-penetrating surgery lies in the physical removal of these resisting elements without compromising of the structural integrity of the eye, thus preventing a sudden drop in the intraocular pressure. The selective filtration through the trabeculodescemetic membrane, the evacuation of the aqueous in the intrascleral space, the subconjunctival bleb and the subchoroidal space, and the final drainage into collector channels and drain veins are the key elements for a successful deep sclerectomy.     

How does the trabecular meshwork regulate outflow? Clues from the vascular endothelium.

Although it is well known that the trabecular meshwork plays a central role in regulating the outflow of aqueous humor from the eye, the cellular mechanisms and events responsible for this function are poorly understood. In contrast, the mechanisms by which vascular endothelial cells modulate the permeability of blood vessels have been more thoroughly investigated. It is hypothesized that the cells of the TM employ mechanisms similar to those observed in the vascular endothelium to modulate aqueous humor permeability. Specifically, it is hypothesized that the cells of the TM employ Na-K-Cl cotransport to modulate their intracellular volume and thus the volume of the paracellular pathways through which aqueous humor may travel. The current knowledge about the role of Na-K-Cl cotransport and volume regulation in the regulation of vascular permeability, and evidence that similar physiologic events occur in the TM are reviewed. In addition, the implications for further study of aqueous humor outflow physiology are discussed.     

Corneoplastique?: Art of vision surgery

Corneoplastique incorporates the entire spectrum of Vision corrective surgery including Lasik, premium cataract surgery, corneal surgery, ocular surface surgery and the full range of anterior segment surgery itself in manipulating the optics of every eye towards unaided emmetropia to define each and every eye surgeon as a “Vision Corrective Surgeon”. This concept of approaching each case individually and designing vision therewith enables surgeons to correct not only virgin eyes but also approach complex cases and complications with the goal of 20/20 vision. Armed with this holistic approach, eye surgeons can use minimally invasive, aesthetically pleasing and visually focused surgery in single or staged process aiming for each patient''s Best Vision Potential (BVP) raising eye surgery itself then to an Art!     

How does saccade adaptation affect visual perception?

Three signals are used to visually localize targets and stimulate saccades: (1) retinal location signals for intended saccade amplitude, (2) sensory-motor transform (SMT) of retinal signals to extra-ocular muscle innervation, and (3) estimates of eye position from extra-retinal signals. We investigated effects of adapting saccade amplitude to a double-step change in target location on perceived direction. In a flashed-pointing task, subjects pointed an unseen hand at a briefly displayed eccentric target without making a saccade. In a sustained-pointing task, subjects made a horizontal saccade to a double-step target. One second after the second step, they pointed an unseen hand at the final target position. After saccade-shortening adaptation, there was little change in hand-pointing azimuth toward the flashed target suggesting that most saccade adaptation was caused by changes in the SMT. After saccade-lengthening adaptation, there were small changes in hand-pointing azimuth to flashed targets, indicating that 1/3 of saccade adaptation was caused by changes in estimated retinal location signals and 2/3 by changes in the SMT. The sustained hand-pointing task indicated that estimates of eye position adapted inversely with changes of the SMT. Changes in perceived direction resulting from saccade adaptation are mainly influenced by extra-retinal factors with a small retinal component in the lengthening condition.     

How long range is contour integration in human color vision?

We quantified and compared the effect of element spacing on contour integration between the achromatic (Ach), red-green (RG), and blue-yellow (BY) mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, sigma = 0.17 deg), measured using a temporal 2AFC method. A contour of ten elements was pasted into a 10 x 10 cells array, a ndbackground elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean interelement distance between two and six times the period of the Gabor elements (lambda = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures, we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than does performance for the achromatic mechanism. Averaged critical element separations were 4.6 +/- 0.7, 3.6 +/- 0.4, and 2.9 +/- 0.2 deg for Ach, BY, and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the Ach mechanism. We found that the critical separation decreases linearly with the spatial frequency, from about 5 deg at low spatial frequency (larger elements) to about 1 deg at high spatial frequency (smaller elements) suggesting a scale invariance in contour integration. In both experiments we also found no differences between closed and open jagged contours detection in terms of element separation. The neuroanatomical implications of these findings relatively to area V1 are discussed.     

How does anisometropia affect the choroidal vascularity index?

Purpose:To investigate the choroidal vascularity index (CVI) and morphological features of the choroid in anisometropic amblyopia.Methods:In this prospective cross-sectional study, 39 patients with unilateral anisometropic amblyopic patients and 33 eyes of 33 healthy control participants were involved. These participants were examined in terms of axial length (AL), spherical equivalent (SE), central macular thickness (CMT), choroidal thickness (CT), total choroidal area (TCA), luminal area (LA), stromal area (SA), LA/SA ratio, and CVI. All parameters were compared between amblyopic eyes, healthy fellow eyes, and healthy control eyes. The Shapiro–Wilk tests, Chi-square test, the paired t-test, Wilcoxon signed-rank test, Mann–Whitney U test, Kruskal–Wallis test, and Pearson/Spearman correlation tests were used.Results:In the hyperopic patients; SE, subfoveal CT, nasal CT, temporal CT, TCA, LA, SA, and CMT were greater in amblyopic eyes than in healthy fellow eyes and control eyes (P < 0.001, P < 0.001, P < 0.001, P < 0.001, P < 0.001, P < 0.001, and P < 0.001, respectively), and CVI, LA/SA ratio, and AL were smaller in amblyopic eyes than in healthy fellow eyes and control eyes ([P < 0.001, P = 0.006], P < 0.001, and P < 0.001, respectively). In the myopic patients, subfoveal CT, nasal CT, temporal CT, TCA, LA, SA values were statistically smaller in amblyopic eyes than in healthy eyes and control eyes ([P < 0.001, P = 0.002), [P = 0.004, P = 0.012], [P = 0.012, P = 0.032], [P < 0.001, P = 0.013], [P < 0.001, P = 0.024], and [P < 0.001, P = 0.047], respectively). The differences in the AL and choroidal parameters were due to myopia and hyperopia.Conclusion:The choroidal structural parameters of the amblyopic eyes were different from that of the healthy eyes.     

How does the structure of extraocular muscles and their nerves affect their function?

The sensory and motor control of human extraocular muscles (EOMs) have been subjected to considerable speculation in ophthalmic literature, often related to infranuclear structures such as the unique complement of muscle fibres and their associated sensory organs. The intrafusal fibres do not resemble their somatic counterparts and their peculiar morphology has raised questions about their proprioceptive capacity. No Golgi tendon organs have so far been observed and the myotendinous nerve endings, previously assumed to convey sensory information, have recently been argued to merely represent constituents of the efferent innervation serving the multiply innervated muscles fibres. These observations raise questions about the overall capacity to monitor the activity created by the generous efferent nerve supply observed in these muscles. Furthermore, the argued independent activity of muscular layers and compartments suggest that the required feedback must be highly structured and more specific than previously assumed. Yet, uncertainty about the source of such information remains. The purpose of this paper is to provide a short review of neuromuscular properties of human extraocular muscles. Their functional implications and the most reputable sources of proprioception will also be discussed. The promoted views are based on pertinent literature and previous research undertaken by the authors.