Foveal Sensitivity Research Articles

Peripheral field stimulation suppresses flicker but not pattern detection in foveal targets.

This report examines the effects of peripheral visual stimulation on foveal target thresholds. The foveal targets were luminance-modulated sine-wave gratings (subtending 2° arc), presented with or without a phase-alternation of 1 Hz. The peripheral stimulus was a 20° black-white radial or linear, square-wave grating. The radial grating contained a 2° blank central area and appeared either at rest or in radial motion at 5°/sec. The linear grating contained either a 2° or an 8° central blank aperture and appeared either at rest or in oscillation by one-half cycle at 4 hz. Subjects always viewed the foveal target with their left eyes. The peripheral stimulus was presented to the left eye for monocular viewing but to the right eye for dichoptic viewing. For dichoptic viewing, the radial but not the linear grating increased the thresholds for counterphase but not stationary target gratings. For monocular viewing, both peripheral stimuli had threshold elevating effects on counterphase but not stationary targets. Results are discussed in terms of the binocular control of foveal monocular sensitivity.

A study of the value of the central and peripheral isoptres in assessing visual field progression in the presence of paracentral scotoma measurements.

The visual field records of 45 eyes of 45 patients were evaluated to determine whether examination of the peripheral field of vision in patients with glaucoma and field defects adds useful information about the progression of visual field damage. The central scotoma mass, foveal sensitivity, and the central and peripheral mass of the visual field were quantified from measurements on the Tübinger perimeter. Most of the information on visual field progression was contained in the scotoma mass. When both scotoma mass and foveal sensitivity were known, central or peripheral isoptres did not add statistically significant information on progression. Evaluation of scotomas and assessment of central isoptres or of foveal sensitivity made perimetry of the peripheral isoptres redundant. The appearance of fresh peripheral scotomas or the quantification of changes in peripheral scotomas was not examined in the current study, and they would of course be important when they occurred.

Foveal sensitivity changes in retinitis pigmentosa

Loss in foveal sensitivity in retinitis pigmentosa (RP) has been attributed to a decrease in quantal catching ability. Using a psychophysical technique (the probe-flash paradigm), we previously found that the results obtained for six RP patients under light adapted conditions were not consistent with a quantal catch hypothesis. To test further this hypothesis twelve RP patients were examined in dark adapted conditions. Probe thresholds were normal for five patients and increased for seven patients. The decreased quantal catching hypothesis was rejected for six of the seven patients.

Foveal Cone Pigments and Sensitivity in Young Patients With Usher's Syndrome

Psychophysical and reflection retinal densitometric techniques were used to investigate foveal function of three patients (ages 14, 16, and 19 years) with Usher's syndrome who had good central visual acuity and, by ophthalmoscopic examination, normal foveal areas. During dark adaptation foveal sensitivity and its relation to proportion of cone pigment present were normal, but the kinetics of regeneration of the pigments were markedly abnormal. Measurements were made to consider the possibilities that the disorders of regeneration were associated with altered light-catching capacities of the patients' cone pigments, deranged transport of visual cycle substances between the retinal pigment epithelium and photoreceptors, or lingering photoproducts. None of the mechanisms fully accounted for the observed kinetics. The results suggested that abnormal kinetics of regeneration are among the earliest signs of malfunction of the foveal photoreceptor-pigment epithelial complex in some of the retinal degenerative disorders.

Macular pigment and reduced foveal short-wavelength sensitivity in retinitis pigmentosa

Some patients with retinitis pigmentosa (RP) show a reduced foveal short-wavelength sensitivity that cannot be attributed to a reduction in the sensitivity of the short-wavelength cone system. To determine whether an increased amount of macular pigment (xanthophyll) might account for this finding, we derived estimates of the two-way optical density of the macular pigment of five such RP patients as well as of five normals. The spectral reflectance of the foveal region of each subject was obtained from digitized images of the bleached fundus provided by a television-based reflectometer. The density spectra of the macular pigment, melanin, and oxygenated hemoglobin were fit by a least-squares procedure to the log of the ratio of parafoveal to foveal spectral reflectance in order to obtain a quantitative estimate of the contribution of each of these ocular pigments to foveal short-wavelength reflectance. By this analysis, the two-way densities of the macular pigment, melanin, and oxyhemoglobin of the RP patients were not significantly different from those of the normals. Therefore, the reduced foveal short-wavelength sensitivity of these patients was not due to an increased amount of macular pigment, but may result instead from morphological abnormalities in the foveal cones such that a normal amount of macular pigment screens the cones more effectively.

Foveal and non-foveal measures of short wavelength sensitive pathways in glaucoma and ocular hypertension

Abstract— Blue-sensitive cone thresholds (the ability to detect blue test lights presented on a bright yellow background) were measured to examine foveal and central vision field sensitivity in groups of 33 glaucoma, 31 ocular hypertension (OHT) and 24 age matched normal subjects. Results showed these thresholds were substantially reduced (in all areas tested) for the glaucoma group (˜ 0.8 log units) and less reduced for the OHT group (˜0.2 log units), when compared to the normal group threshold. Measures of red-sensitive and green-sensitive cone thresholds showed no difference for all three groups. Furthermore, 16% of the OHT group showed visual field abnormalities for the blue-sensitive cone threshold measure. We conclude that efforts to detect early function loss in glaucoma and OHT could be improved using these measures in clinical practice.

Optical density of human macular pigment

Foveal and parafoveal spectral sensitivities of the G cone mechanism were used to derive the difference spectrum for the macular pigment for 12 observers. Measurements of the peak density of the macular pigment were obtained with a shortened version of the same procedure for an additional 15 observers. The mean density for all 27 observers at 460 nm was0.77 ± 0.06 with a range of 0.21–1.22. The mean density is higher than that previously reported and, compared to the standard absorbance curve (Wyszecki and Stiles, 1967), the results indicate higher density values for wavelengths on the long wavelength side of the absorption maximum.

Temporary loss of foveal contrast sensitivity associated with panretinal photocoagulation.

Snellen visual acuity and spatial contrast sensitivity measurements were used to monitor changes in foveal vision in two patients undergoing argon laser panretinal photocoagulation (PRP) in both eyes for proliferative diabetic retinopathy. Foveal edema developed in one of the four eyes. The remaining three eyes showed no signs of foveal edema, but developed temporary losses in high spatial frequency contrast sensitivity during the closely spaced PRP treatments. Since Snellen visual acuity remained stable at the prelaser level, these results indicate the need for more sensitive measures of visual resolution to monitor foveal integrity in patients undergoing PRP.

Spatial extent of rod-cone and cone-cone interactions for flicker detection

Over a large range of light adaptation levels, sensitivity to 25 Hz flicker improves as the light level of the background increases. Using small background discs and annular surrounds, this effect was shown to be mediated by the surround and not the average luminance of the test region, in agreement with recent reports. The effect is due to two types of lateral interaction: at mesopic light Ievels (from 0. 1 to 1.0 td), cone-mediated flicker resolution is enhanced by the stimulation of surrounding rods; at photopic light levels (above 10 td), flicker sensitivity improves with light stimulation of adjacent cones. The spatial zone, or extent, over which the surround contributes to the flicker threshold was measured. The spatial area over which rods influence the cone 25 Hz flicker threshold is larger than the analogous spatial area of cone influence. In the parafovea, at 5 deg eccentricity, cone flicker sensitivity for a 20′ spot is influenced by cones in a 1 deg diameter area centered on the spot; the corresponding area of rod influence is about 3 deg. In the fovea, flicker sensitivity for a 10′ spot is influenced by cone stimulation in an area of about 20′ diameter. Rods which affect foveal flicker sensitivity appear to occupy an annular zone with about a 2 deg outer diameter and 1 deg inner diameter, centered on the fovea.

Cone Function and Cone Interaction in Hereditary Degenerations of the Central Retina

Spectral sensitivity functions and the transient decrease of sensitivity to short wavelengths after the offset of yellow light (transient tritanopia) were measured by increment threshold techniques in patients suffering from hereditary macular degenerations. Color vision defects were determined by arrangement tests and the anomaloscope. Central areolar choroidal dystrophy was found to produce a mild protan defect and to reduce foveal spectral sensitivity throughout the visible spectrum by a factor of 100; it also abolishes transient tritanopia. Electroretinogram (ERG) was normal, electrooculogram (EOG) subnormal. Stargardt's disease, despite numerous fluorescent macular spots, does not abolish transient tritanopia nor does it reduce spectral sensitivity, although scotopic matches were performed on the Nagel anomaloscope. Only in severe, advanced cases was transient tritanopia reduced and spectral sensitivity found to follow the absorption spectrum of rods. Routine ERGs and EOGs were normal. Vitelliform macular degeneration, despite the ophthalmoscopically pronounced dystrophic macula, produced only very small changes in spectral sensitivity and transient tritanopia, although a widened matching range on the Nagel anomaloscope and electrophysiological abnormalities were found. Apparently damage of the retinal circuit which connects long and short wavelength-sensitive cones, caused by hereditary conditions, is different from that caused by retinotoxic drugs.

Modification by fluoangiography of color vision in diabetic patients.

Fluoangiography causes in diabetics a temporary increase of the number of errors when reading Birch and SPP II pseudoisochromatic plates, especially in case of macular edema and for tritan plates. Other experiments show that this change is more due to an increased filter effect by the fluorescein accumulation than to the increase of glare by the flashes for photography. Measurements of the foveal spectral increment threshold sensitivity curves does not show impairment of the blue mechanism by fluoangiography; on the contrary the green and red mechanisms become less sensitive.

The Early Structural and Functional Disturbances of Chronic Open-angle Glaucoma: Robert N. Shaffer Lecture

The earliest psychophysical disturbances in color vision, foveal sensitivity, spatial and temporal contrast sensitivity precede nerve fiber bundle defects of the visual field in glaucoma. Optic nervehead changes such as enlargement of the physiological cup, as well as retinal nerve fiber layer losses, also precede visual field defects. Which of these is the earliest change has not yet been recognized. Evidence for more than one mechanism of damage in glaucoma is also presented.

Spectral sensitivity and wavelength discrimination of the human peripheral visual field.

Spectral sensitivity and wavelength discrimination are determined along the nasal horizontal meridian of the human peripheral retina. The target size as a function of eccentricity is varied according to a particular cortical magnification factor. Spectral sensitivity is measured by flicker photometry parameterized for the flicker frequency (10-20 Hz) and is found to be independent of the eccentricity (0-80 degrees) for 20-Hz flicker photometry after correction of the foveal spectral sensitivity for macular pigment absorption. This 20-Hz function is chosen as being representative for the peripheral luminous-efficiency function and is used in the wavelength-discrimination experiments. The peripheral retina can perform wavelength discrimination up to an eccentricity of 80 degrees. If field-size scaling according to the eccentricity-dependent cone density, the cortical magnification factor, or the reciprocal of the interganglion cell distance is applied, then wavelength-discrimination performance from 8 degrees to 80 degrees eccentricity is roughly the same. Foveal wavelength discrimination is considerably better than peripheral wavelength discrimination.

Development of temporal contrast sensitivity

A prolonged exposure to foveal defocus is well known to affect the visual functions in the fovea. However, the effects of peripheral blur adaptation on foveal vision, or vice versa, are still unclear. In this study, we therefore examined the changes in contrast sensitivity function from baseline, following blur adaptation to small as well as laterally extended stimuli in four subjects. The small field stimulus (7.5° visual field) was a 30 min video of forest scenery projected on a screen and the large field stimulus consisted of 7-tiles of the 7.5° stimulus stacked horizontally. Both stimuli were used for adaptation with optical blur (+2.00 D trial lens) as well as for clear control conditions. After small field blur adaptation foveal contrast sensitivity improved in the mid spatial frequency region. However, these changes neither spread to the periphery nor occurred for the large field blur adaptation. To conclude, visual performance after adaptation is dependent on the lateral extent of the adaptation stimulus.

Early Foveal Involvement and Generalized Depression of the Visual Field in Glaucoma

We selected for study 35 patients with glaucomatous visual field loss or cupping that was asymmetric between the two eyes, concentrating on cases in which the glaucomatous damage was mild. We found that 15 patients had a reduced foveal sensitivity usually associated with either a scotoma impinging on fixation or a depression in the central field. In addition, 19 of the 35 patients had a subtle reduction in visual acuity. Thus, the fovea is often mildly affected early in glaucoma, even though the point of fixation is typically among the last regions of visual field to have a severe loss of visual sensation. We also observed that there is often a diffuse visual field loss early in glaucoma, represented as a generalized depression, and this may occur before there are discrete nerve fiber-bundle defects (paracentral scotomas and the like).

The sensitivity of the peripheral visual system to amplitude-modulation and frequency-modulation of sine-wave patterns

The detectability of amplitude modulation and spatial frequency modulation of suprathreshold sine-wave gratings was measured at retinal eccentricities ranging from 0 to 30 deg. The main conclusions to be drawn from the experimental results are: (1) for this visual task it is not possible to bring peripheral sensitivity to the same level as foveal sensitivity by scaling the stimuli with the inverse of the cortical magnification factor M. (2) Increasing the number of cycles in the carrier grating improves modulation detectability. For gratings in the periphery this holds up to the largest stimulus size used (64 cycles). In earlier investigations it has been shown that for central vision the improvement ends at about 16 cycles.

The detection of motion in the peripheral visual field

To assess the sensitivity of the periphery to motion, we measured differential motion detection and velocity discrimination as a function of eccentricity in the lower visual field. The differential motion threshold, a measure of the ability to detect relative motion (shear) between adjacent visual stimuli, is smaller than the minimum angle of resolution at all retinal loci tested. The target size required to produce the lowest differential motion threshold is surprisingly large, ranging from [ deg in the fovea to about 20 deg at 40° eccentricity. When the peripheral thresholds for differential motion and for resolution are normalized against the fovea and plotted on linear axes, the eccentricity functions are linear. Velocity discrimination ( ΔV/V) is as precise in the periphery as it is in the fovea, amounting to about 6% for the optimum velocity range. In the fovea, the minimum Weber fraction is reached at velocities of 5deg/sec or faster. In the periphery this minimum is found for a faster range of velocities ( >30 deg/sec at 40° eccentricity). If target velocity is expressed in the resolution units/second appropriate to each tested eccentricity, the velocity discrimination functions coincide. Thus, while the spatial determinants of velocity discrimination follow the change in resolution found with eccentricity, peripheral temporal sensitivity must be nearly equal to foveal temporal sensitivity.

Three-Dimensional Topography of the Central Visual Field: Sparing of Foveal Sensitivity in Macular Disease

Threshold static perimetry was performed using test object patterns that covered contiguous areas of the central visual field. Computer imaging methods were used to display a three-dimensional surface that was interpolated between the sensitivity values at each of the test object locations. The examinations covered the area out to and including 10 degrees of eccentricity from the point of fixation, corresponding to the same area of the visual field covered by the Amsler grid. The normal visual field surface appears as a high plateau with a smoothly rising level of sensitivity forming a peak at the point of fixation. It was found that in a variety of macular diseases, including those caused by vascular, as well as primary degenerative disorders, central scotomas were characterized by relative sparing of visual sensitivity at the point of fixation. The pattern thus produced was one of a ring-shaped depression within the central 10 degrees of the visual field. This phenomenon was present in 20% of cases with central scotomas resulting from macular disease, but was not found in any eye of 64 patients suffering from central scotomas as a result of optic nerve disease. This pattern of visual field loss may be common, though not frequently recognized. It is proposed that the phenomenon of preservation of foveal sensitivity may be a marker for macular disease, as distinct from central visual field defects arising from optic nerve disease.

Foveal inhibition and facilitation caused by remote grating jerks: interaction between long-range and short-range effects.

Periodic oscillation of a luminance grating imaged upon the peripheral retina reduces the threshold visibility of a foveally presented test spot. This new effect has been named the "jerk effect". The present investigation is concerned with the effect of a single jerk of the remote grating on threshold sensitivity. Foveal sensitivity changes were measured for different delays between grating jerk and test spot presentation. For 0.38 degrees, 100 ms test spot, long-range transient inhibition was found for all delays, with a maximal effect between 0 and 30 ms delay. By combining the jerk effect with the Westheimer paradigm, both facilitatory and inhibitory long-range effects could be demonstrated. For facilitation to occur, it was necessary that the steady background extended into the sensitization zone of the Westheimer area. Inhibition was the only result for smaller backgrounds. Reduced visibility is consistent with the hypothesis that peripheral transient mechanisms inhibit foveal sustained mechanisms. Enhanced visibility indicates that thresholds depend on an interaction between foveal-sustained and foveal-transient units. Transient peripheral stimulation and steady backgrounds of increasing diameter change the balance of inhibitory and facilitatory processes between these units.

Vector model for normal and dichromatic color vision.

The inclusion of cone mechanisms in a slightly revised version of an earlier model allows accounts of phenomena that involve receptor effects as well as dichromatic color vision. Intensity-dependent parameters that stimulate the adaptation of receptors and opponent and nonopponent mechanisms are varied to predict a wide range of data for both normals and dichromats, including: (i) color matching; (ii) the approximate apparent hue and saturation of the spectrum; (iii) foveal spectral sensitivities obtained by flicker photometry and by detection in the dark and under conditions of achromatic or chromatic adaptation; (iv) heterochromatic additivity failures in the dark-adapted and chromatically adapted eye; (v) approximate differences between brightness and luminance; and, (vi) color and wavelength discrimination under varying adaptation conditions.