Showing papers on "Foveal published in 1992"

Spatial zones of binocular rivalry in central and peripheral vision

Abstract: This paper presents results from psychophysical experiments on human binocular rivalry in central and peripheral vision. Results show that the incidence of periods of exclusive visibility of a given eye's rival target increased with decreasing target size, and for a given sized target exclusive visibility increased with retinal eccentricity. Control measures confirmed that these results were not attributable solely to reduced peripheral acuity, to Troxler's effect, or to spatial frequency. We computed the minimum-sized stimulus that would lead to a criterion level of exclusive visibility of one or the other eye; this we term the spatial zone of binocular rivalry. The change in estimated size of spatial zones of rivalry with eccentricity compares favorably with estimates of human cortical magnification. We propose a model that assumes concentrically organized zones of rivalry. These zones do not function independently, but instead exhibit a high degree of mutual excitatory cooperativity. The model has multiple solutions for the foveal zone size, but the best fits predict a diameter of 5.3 or 7.3 min of visual angle; these values dovetail nicely with our empirical estimates of the foveal zone size.

Simulation of a phosphene-based visual field: Visual acuity in a pixelized vision system

Abstract: A visual prosthesis for the blind using electrical stimulation of the visual cortex will require the development of an array of electrodes Passage of current through these electrodes is expected to create a visual image made up of a matrix of discrete phosphenes The quality of the visual sense thus provided will be a function of many parameters, particularly the number of electrodes and their spacing We are conducting a series of psychophysical experiments with a portable “phosphene” simulator to obtain estimates of suitable values for electrode number and spacing The simulator consists of a small video camera and monitor worn by a normally sighted human subject To simulate a discrete phosphene field, the monitor is masked by an opaque perforated film The visual angle subtended by images from the masked monitor is 17° or less, depending on the mask, and falls within the fovea of the subject In the study presented here, we measured visual acuity as a function of the number of pixels and their spacing in the mask Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image We conclude that 625 electrodes implanted in a 1 cm by 1 cm area near the foveal representation of the visual cortex should produce a phosphene image with a visual acuity of approximately 20/30 Such an acuity could provide useful restoration of functional vision for the profoundly blind

Reading speed with a pixelized vision system.

Abstract: A visual prosthesis based on electrical stimulation of the visual cortex with an array of penetrating electrodes is expected to produce pixelized visual images consisting of punctate spots of light (phosphenes). We measured reading speed in subjects viewing text with optically simulated phosphene fields in order to obtain estimates of the following design parameters for such an electrode array: pixel number, pixel spacing, and visual-field size. Comparisons were made between scanning the text with eye movements and scanning the text with head movements. The results indicate that a 25 × 25 array of pixels representing four letters of text projected on a visual field of 1.7° is sufficient to provide reading rates near 170 words/min with scrolled text and near foveal 100 words/min with fixed text.

A morphological comparison of foveal development in man and monkey.

Abstract: The fovea can first be identified in both monkey and human retina at 26-30% gestation as a region containing all adult retinal layers and only cone photoreceptors. A shallow foveal pit and cone outer segments appear by 63-65% gestation in both species. Prenatal development continues rapidly in the monkey, so that by birth a single layer of inner retinal neurons are present in the fovea, cones are three cells deep, inner segments are elongated, and outer segments are up to 50% of inner segment length. In contrast, human fovea does not reach a similar stage until several months after birth. The fovea is adult-like in monkey at 12 weeks and in human at 11-15 months, although human will mature further up to four to five years. This study shows that human fovea is less mature at birth than monkey but develops rapidly in infancy, suggesting that it may be even more susceptible to postnatal environmental influences than the commonly-used monkey model.

The dorsomedial frontal cortex of the macaca monkey: fixation and saccade-related activity.

Abstract: The activity of 249 neurons in the dorsomedial frontal cortex was studied in two macaque monkeys. The animals were trained to release a bar when a visual stimulus changed color in order to receive reward. An acoustic cue signaled the start of a series of trials to the animal, which was then free to begin each trial at will. The monkeys tended to fixate the visual stimuli and to make saccades when the stimuli moved. The monkeys were neither rewarded for making proper eye movements nor punished for making extraneous ones. We found neurons whose discharge was related to various movements including those of the eye, neck, and arm. In this report, we describe the properties of neurons that showed activity related to visual fixation and saccadic eye movement. Fixation neurons discharged during active fixation with the eye in a given position in the orbit, but did not discharge when the eye occupied the same orbital positions during nonactive fixation. These neurons showed neither a classic nor a complex visual receptive field, nor a foveal receptive visual field. Electrical stimulation at the site of the fixation neurons often drove the eye to the orbital position associated with maximal activity of the cell. Several different kinds of neurons were found to discharge before saccades: 1) checking-saccade neurons, which discharged when the monkeys made self-generated saccades to extinguish LED's; 2) novelty-detection saccade neurons, which discharged before the first saccade made to a new visual target but whose activity waned with successive presentations of the same target. These results suggest that the dorsomedial frontal cortex is involved in attentive fixation. We hypothesize that the fixation neurons may be involved in codifying the saccade toward a target. We propose that their involvement in arm-eye-head motor-planning rests primarily in targeting the goal of the movement. The fact that saccaderelated neurons discharge when the saccades are self initiated, implies that this area of the cortex may share the control of voluntary saccades with the frontal eye fields and that the activation is involved in intentional motor processes.

Position of the fovea centralis with respect to the optic nerve head.

Abstract: Examination of the ocular fundus via indirect ophthalmoscopy gives the clinician an impression of foveal position relative to the nerve head. In some patients, the fovea appears to be in an unusual position (i.e., ectopic): it may appear to be higher or lower than expected, or closer to or farther from the nerve head. There is little published quantitative information on this subject. The purpose of this study was to examine foveal position in a group of normal adult eyes, so that clinicians and other researchers will be able to determine on a more objective basis whether or not a given patient shows foveal ectopia. Using ocular fundus photographs for 446 normal adult eyes, we found the foveal center to be, on average, 6.11 degrees +/- 3.32 degrees below a horizontal line bisecting the nerve head. For a smaller sample of 66 eyes, we found the average distance between the nerve head and foveal centers to be 4.93 +/- 0.33 mm (right eye) and 4.88 +/- 0.36 mm (left eye). Correlations of these data for right and left eyes are also examined. Nerve head data for the group of 66 right eyes were also analyzed to yield dimensions of a best-fitting ellipse: the mean minor axis was 1.75 +/- 0.2 mm; the mean major axis 1.95 +/- 0.2 mm. Ectopia (heterotopia) of the fovea has been found in association with chorioretinitis, fibrous traction bands, and/or colobomas of the choroid and optic nerve (including anomalous insertion of the optic nerve), microcephalus, and microphthalmia. A number of separate cases with anomalous nerve heads and/or foveal positions are discussed in this paper.(ABSTRACT TRUNCATED AT 250 WORDS)

Foveal flicker sensitivity discriminates ARM-risk from healthy eyes.

Abstract: The "good" eyes of 13 patients with monocular exudative ARM were compared with age-matched healthy eyes of 19 subjects. Membership in the two study groups was based upon careful clinical evaluation of the tested eye as well as upon status of the fellow eye. We asked whether temporal contrast sensitivity for a long-wavelength, low spatial frequency stimulus can be used to identify the group in which a given eye belongs. Using step-wise discriminant analysis, we found that the ARM-risk and healthy eyes could be classified with 78% accuracy on the basis of foveal flicker sensitivity at two temporal frequencies--14 and 10 Hz (in order of estimated weight.)

Viewing-distance invariance of movement detection.

Abstract: Since visual movement information is often presented in electronic displays or films it is amazing that there is a paucity of research on the influence of viewing distance on motion detection in cinematograms. We report a relatively high degree of detection constancy with changing viewing distance for coherent motion in random-pixel cinematograms. A constant performance irrespective of viewing-distance is called 'distance-invariance' and for motion detection it proves to hold reasonably well for a relatively wide range of viewing distances both for foveal and eccentric vision. The limits of this viewing-distance invariance are explored as a function of screen velocity. Detection performance is quantified by a threshold signal-to-noise-ratio (SNR-) value, S, which is determined as a function of velocity for a range of viewing distances from 53 to 13,476 mm for foveal vision and from 60 to 1925 mm at 24 degrees eccentricity on the nasal horizontal meridian of the right eye's retina. The data can be explained, at least qualitatively, by a model in which a spatial-resolution stack has a stack of velocity-tuned motion detectors at every resolution layer. Such a 'stack-of-stacks' model is in line with proposals for contrast-detection stack-models, but it suggests that the usual hypothesis that motion perception is based on the activity of two separate systems, the short-range and the long-range system, might be superfluous. This two-systems distinction was largely based on the different performance found for moving random dot patterns and moving form-defined stimuli. A moving random pixel array viewed at very close range (e.g. 6 cm) presents the subject with relatively large almost square 'blobs', which are less dissimilar from the phi-stimuli used in classic motion perception studies than random dot stimuli at the usual medium to large viewing distances. It leads to maximum displacement threshold (Dm-) values that are not untypical of the 'long-range' system, but by gradually increasing the viewing-distance and thus decreasing the pixel-size a continuous change is found from typical long-range to typical short-range values of Dm. The two-systems distinction for motion detection appears to refer to the stimulus rather than to the visual system: The motion-detection system might be forced into a local or a global 'mode of operation' by the choice of stimulus.

Effect on grating identification of sampling with degenerate arrays.

Abstract: We have measured observer performance in identifying gratings that were sampled by degenerate-sampling arrays Grating identification for high-spatial-frequency stimuli fell to threshold levels only after more than 88% of the sampling elements were subtracted Although several studies with normal observers have shown that foveal visual acuity corresponds to estimates of interphotoreceptor spacing derived from anatomical photoreceptor density, a simple transformation of sampling-element density to mean spacing does not describe the results in our simulation of degenerate-sampling arrays To the extent that our model simulates the anatomical changes of retinal pathology, our results suggest that grating acuity reflects the normal high-spatial-sampling rate of small regions of preserved anatomy in the degenerate retina rather than the mean density or spacing of foveal receptors after massive degeneration

Simultaneous measurement of two-point-spread functions at different locations across the human fovea

Abstract: An experimental system for measuring simultaneously the retinal images of two-point tests has been developed. In particular we present one experiment in which one of the points is located at the center of the fovea and the other one is at 1 deg of eccentricity. At these two foveal locations the optical image quality is expected to be approximately the same, while the structure of the retina is known to be quite different. Our results of aerial images show small but systematic differences between the two-pointspread functions that are measured at 0 and 1 deg of eccentricity. The image quality is always slightly better in the center of the fovea with the differences more marked in the nasal and inferior orientations. That could be explained by a small but noticeable contribution of the retinal thickness to the optical aberrations of the eye. The possible increment of scattering caused by the increase in retinal thickness at 1 deg was barely measurable in our experiment. An indirect consequence is that retinal reflection has little practical influence on our particular double-pass measurements of the eye's image quality.

Contrast thresholds for letter identification in retinitis pigmentosa.

Abstract: To assess mechanisms of foveal vision loss in retinitis pigmentosa (RP), contrast thresholds were measured for the identification of Sloan letters at four adapting field luminances (0.4, 1.4, 2.4, and 3.4 log td) in a group of 16 patients with RP who had best-corrected Snellen visual acuities of 20/30 or better, minimal or no posterior subcapsular cataracts, and no atrophic or cystic-appearing foveal lesions. Letter contrast sensitivities of the patients with RP were reduced below those of a group of ten subjects with normal vision for all letter sizes and at all adapting field luminances. The overall pattern of these results indicated that neither a reduced quantal absorption by foveal cones nor spatial undersampling from a loss of foveal cones accounted for the reductions in letter contrast sensitivities. The findings were most consistent with a uniform increase in intercone spacing in the foveas of this group of patients with RP and mild visual acuity loss.

Problems of deep foveas.

Abstract: Deep or convexiclivate foveas occur in some birds, including raptors, some lizards and certain deep-sea fishes. Theories on their function are reviewed. Common to raptor and deep-sea fish foveas is a radial fibre lining, dark staining so probably optically dense, adjacent to the less refractile vitreous. The foveal curvatures and size are remarkably similar in a wide taxonomic and size range of birds and fishes. Ray plotting through traced foveal outlines suggests that sharp images will be formed beneath the centre and shoulders, with the centre image enlarged enough to account for the high acuity of raptors. Deep foveas will also exaggerate eccentricity of off-centre images of a point source, such as deep-sea fishes may meet. Despite similarities in foveal shape, the receptors differ widely between species. Raptors, and notosudid fishes, have short cones. Searsid fishes have long single rods. Howella and Bajacalifornia have multiple bank rods, more in the fovea than the periphery. Those of Howella are shown to be multiple inner-outer segment complexes rather than interrupted single rods. Implications of foveal and receptor features are discussed.

Saccadic object recognition with an active vision system

Abstract: Proposes an active vision system for saccadic camera gaze shifts and explorative scene analysis as a new integral approach to image understanding. The model consists of two sensory subsystems: preattentive peripheral feature detection and high resolution foveal image identification based on a hypercolumnar representation. Visual objects are non-explicitly stored in two sparsely coded associative memories separating fixation locations for identities of foveal views. An egocentric interest map integrates bottom-up and top-down information sources and decides when to generate a camera movement. A selective masking of preattentive processes supports a cooperation with cognitive object recognition. The system is easily extendible, copes with occlusions and distortions and can be driven in different modes for exploration tasks. This model is able to perform visual search and reproduce findings in the human visual system. >

Foveal cone electroretinograms in patients with central visual loss of unexplained etiology.

Abstract: • Foveal cone electroretinograms (ERGs) were recorded in five patients (aged 24 to 66 years) referred because of central visual loss of unexplained etiology. These patients had no family history of visual loss and no diagnostic fundus abnormalities seen on ophthalmoscopy or fluorescein angiography. Foveal cone ERGs were elicited with a 4° white stimulus flickering at 42 Hz centered within a steady 10° white surround presented through the dilated pupil to the fovea by a hand-held, dual-beam stimulatorophthalmoscope. All five patients showed reduced foveal cone ERG amplitudes. Their abnormal responses were similar to those previously reported in cases of clinically apparent macular degeneration and contrasted with previously reported normal responses in patients with optic atrophy or strabismic amblyopia. The progressive loss of central vision as determined by history and these subnormal foveal cone ERG amplitudes suggest that these patients have a form of macular degeneration.

Spatial Memory and Integration Across Saccadic Eye Movements

Abstract: Many aspects of visual perception can be treated in terms of the information available in a single fixation, or frame. However, the eyes, head, and body are constantly moving, and from this time-varying input on the retina we must construct a representation of a scene in coordinates that are fixed to the world and not to the body. Most research on this problem has focused on the consequences of saccadic eye movements. These high-velocity movements, which occur several times a second, move the high-acuity foveal region around the scene and impose a frame-like structure on the visual input. That is, we can think of the visual input as a sequence of stationary frames sampling different parts of the scene, interposed by brief periods of blur. To explain our perception of a stable world it is generally assumed that the perceptual system constructs a representation of the scene that is integrated across these successive views in an external coordinate frame. If this is so, we can identify three aspects of the problem, as follows:

Method and apparatus for enhanced visual training

Abstract: A vision training device, useful for improving hand-eye coordination activities, takes the form of eyewear having two red colored, translucent lenses, each lens containing a clear target sight positioned and dimensioned to allow a trainee to focus the image of an object onto the foveal vision areas of his eyes through the apertures while simultaneously stimulating the rod vision of the eye by exposure to light in the red spectrum.

Recovery from pulsed-dye laser retinal injury.

Abstract: The increased use of lasers in industry, science, and medicine has led to a growing case report file dealing with accidental exposure of the eye to laser radiation 1-3 Laser-induced injury of the foveal region is of greatest concern since visual prognosis is expected to be very poor Report of a Case —A 29-year-old male research scientist sustained a foveal hemorrhage from exposure to one or two 055-MJ, 620-nm, 20-nanosecond laser pulses from orange laser light emitted by a neodymiumYAG pumped-dye laser He was at arm's length from a beam-positioning mirror and looked directly at the mirror when the beam position was not apparent He experienced a brilliant orange flash of light in his right eye and then saw only black Visual acuity was counting fingers at 180 cm in the right eye, with an 8° central scotoma A preretinal foveal hemorrhage with surrounding radiating folds was noted (Fig 1),

Low resolution cues for guiding saccadic eye movements

Abstract: The high-resolution field of view of the human eye only covers a tiny fraction of the total field of view, which allows for great economy in computational resources but forces the visual system to solve other problems that would not exist with uniformly high resolution. One of these is how to determine where to redirect the fovea, given only the low-resolution information available in the periphery. The advent of spatially variant receptor arrays for cameras has made it imperative that computational solutions to this problem be found. Color has been traditionally associated with foveal vision, but it is shown that color cues are well preserved under low resolution, and an algorithm for locating objects based on color histograms that is both effective under low resolution and computationally efficient is illustrated. >

Central Visual Field, Visual Acuity, and Sudden Visual Loss After Glaucoma Surgery

Abstract: The relationship between Snellen visual acuity and central visual field loss as determined by the Humphrey 10-degree test was examined in 96 glaucomatous eyes of 79 patients. The severity of the field defect was determined by the number of affected quadrants, defined as a sensitivity loss of at least 10 decibels from normal at the most central point of 1.4-degree eccentricity. The decrease in median acuity was gradual, one-half line per quadrant for one and two affected quadrants, and an additional one and one-half line to two lines for three and four quadrants. Loss of acuity was disproportionate when both temporal quadrants were affected. The major source of error was difficulty with fixation. One type, a prolonged fixation shift, was not related to short-term fixation losses. There was a significant correlation between acuity and foveal threshold over a wide range of both variables. Sudden visual loss from additional optic nerve damage was studied retrospectively in 96 eyes with advanced glaucoma and a central field defect. A frequency of 3% for any loss of visual acuity and a frequency of 1% for a loss to 20/200 or worse were noted. There were no apparent predictive factors.

Grating, vernier, and letter acuity in retinitis pigmentosa.

Abstract: Grating, vernier, and letter acuities were compared in 25 patients with retinitis pigmentosa (RP), whose Snellen visual acuities were better than 20/40, to address the mechanism of visual acuity loss. For these patients with RP, all three types of visual acuity were reduced to an equivalent degree from those of a control group of 10 age-similar, visually normal subjects. The findings indicate that the visual acuity losses of these subjects with RP did not result from cone spatial undersampling (due, for example, to a random loss of foveal cones), from cone sampling irregularities (due to random alterations in foveal cone position), or from a selective loss of sensitivity to high spatial frequencies (as might result from changes in media transmission characteristics or a gain reduction in high spatial frequency mechanisms). In addition, previous studies have indicated that acuity losses in such patients with RP do not result from reductions in the quantum-catching ability of foveal cones. The most likely explanation for the equivalent losses in all three acuity types in these patients with RP appears to be an alteration in foveal spatial scale, consistent with a generalized increase in foveal intercone spacing.

Central function and visual field damage in glaucoma.

Abstract: 82 eyes of 82 patients with different types of glaucoma were examined with various psychophysical tests assessing central and paracentral function, including foveal temporal contrast sensitivity function, FM 100-Hue test, and foveal and parafoveal blue-on-yellow-sensitivity. For all eyes visual field tests were performed with the Humphrey-Field-Analyzer, program 30-2. Global visual field indices were calculated as follows: Mean Sensitivity MS, Mean Deviation MD, and Corrected Pattern Standard Deviation CPSD. Linear regression analysis and multiple regression analysis correcting for a possible influence of age between the central and paracentral criteria and the global indices was performed. For the entire study population highly significant correlations are present between foveal and parafoveal blue-on-yellow-sensitivity and MS, MD and CPSD. Whereas in the Normal Tension Glaucoma subgroup (19/82 eyes) no significant correlations are found, the subgroup of 35/82 eyes with markedly elevated intraocular pressure (> or = 30 mmHg) shows highly statistically significant correlations between the low- and high-frequency end of the foveal temporal contrast sensitivity function and foveal and parafoveal blue-on-yellow-sensitivity and the global field indices. The results of the present study support the idea that there are two different mechanisms of glaucomatous damage, one which is pressure-dependent and one which may be pressure-independent. The pressure-dependent mechanism is responsible for deficits of central or paracentral function which are correlated to overall visual field damage.

Low-resolution cues for guiding saccadic eye movements

Abstract: The high resolution field of view of the human eye only covers a tiny fraction of the total field of view. While this arrangement allows for great economy in computational resources, it forces the visual system to solve others that would not exist with a wide field of uniform high resolution. One of these problems is how to determine where to redirect the fovea given only the low-resolution information available in the periphery. The advent of spatially-variant receptor arrays for cameras has made it imperative that computational solutions to this problem be found. While the use of motion in this role is well accepted, color has been associated strongly with foveal vision. We show that color cues are well preserved under low resolution and illustrate an algorithm for locating objects based on color histograms that is both effective under low resolution and computationally efficient.

Diffuse field loss and central visual function in glaucoma.

Abstract: Eighty-two eyes of 82 patients with different types of glaucoma were examined with program 30-2 of the Humphrey Field Analyzer. Forty-six of 82 eyes (56.1%) had diffuse field loss and 36/82 (43.9%) eyes had no diffuse field loss according to the cumulative defect curves. Various foveal threshold criteria including temporal transfer and color vision were tested in all eyes. All criteria showed a loss of sensitivity in the presence of diffuse field loss. The loss of sensitivity was statistically significant for the high-frequency end of the foveal temporal contrast-sensitivity function (P = 0.0498), for foveal flicker-fusion frequency (P = 0.0275) and for foveal and parafoveal blue-on-yellow sensitivity (fovea: P = 0.0009; 4 parafoveal points: P = 0.0001; 16 points in the central 10 degrees: P < 0.0001). The loss in sensitivity was not statistically significant for the low and intermediate temporal frequencies of the foveal temporal contrast-sensitivity function and for the FM 100-Hue loss score. All temporal threshold criteria showed a low sensitivity (25-37%) and a low negative predictive value (48-50%) for the presence of diffuse field loss. Their specificity (75-94%) and positive predictive value (65-83%) are high, however. Thus, whenever one of the foveal temporal threshold criteria as used in the present study is abnormal, the probability is very high that there is some diffuse field loss.

Foveal flicker-fusion frequencies: a simple, new apparatus (4F).

Abstract: While spatial resolution may be easily assessed with contrast-sensitivity frequency (CSF) data, determination of foveal flicker-fusion frequency (4F) is a simple method of providing the ophthalmic practitioner with accurate information about temporal resolution. The authors describe a simple apparatus that controls luminance, light intensity, light intervals, stimulus size, homogeneity of background illumination, and binocular gaze, especially when using two wavelengths, allowing an increaed number of applications with reliable and comparable results. It may prove to be a useful tool for screening, clinical and subclinical assessment, and for evaluating visual acuity, also during follow-up.

Acute laser lesion effects on acuity sweep VEPs.

Abstract: Q-switched neodymium-YAG (infrared) laser lesions at energies up to and including retinal hemorrhages were placed under visual control in the parafovea and the fovea of anesthetized monkeys. Visual-evoked potential (VEP) data were obtained by parallel analog (vector voltmeter) techniques from scalp electrodes in response to high luminance counterphasing sine wave gratings. The gratings were swept downward in spatial frequency to determine an acuity estimate by recording of the VEP magnitude increase onset. Acuity estimates were determined immediately post-exposure and at 15 sec intervals up to 12 min. These were analyzed as a function of laser exposure site and retinal lesion produced. Significant delays in VEP lock-in were demonstrated in subjects that had parafoveal burns or parafoveal subretinal hemorrhages. Foveal burns caused severe short-term fluctuations before a sustained decrease in acuity. Contained foveal hemorrhages produced sustained acuity losses. Foveal exposures that did not produce an immediately visible lesion did not produce measurable changes in VEP response lock-in time. These results probably are independent of visible flash effects and indicate that there may be a transient neural shock effect from parafoveal lesions that can affect the fovea.