Showing papers on "Foveal published in 1987"

Directional pursuit deficits following lesions of the foveal representation within the superior temporal sulcus of the macaque monkey.

Abstract: Ibotenic acid lesions of the middle temporal visual area (MT) have previously been shown to impair a monkey's ability to initiate smooth pursuit eye movements to targets moving in the extrafoveal v...

Peripheral Vision in Persons with Dyslexia

Abstract: We compared persons with dyslexia and normal readers with respect to how well they identified letters and short strings of letters briefly presented in the peripheral visual field at the same time that a single letter was presented at the fixation point of gaze. We found that the dyslexic subjects had a markedly wider area in which correct identification occurred in the peripheral field than did the normal readers. However, the dyslexie subjects had a "masking" between letters in the foveal field and letters in the near periphery. It appears that dyslexic persons learn to read outside the foveal field and, more generally, that there are different learned strategies for task-directed vision. Among such strategies are different mutual interactions between foveal and peripheral vision. (N Engl J Med 1987; 316: 1238–43.)

Physiological Evidence for Two Visual Subsystems

Abstract: Many studies, including some in this volume, describe two distinct types of higher functions in the visual system. One visual function is concerned primarily with evaluation using information about shape, color and pattern to identify or categorize objects. The other function is more involved with spatial considerations, making use of visual information to determine the position movements and spatial relationships among objects. The notion that the visual system performs two distinguishable types of higher functions is well established, and many pairs of terms have been applied in describing this dichotomy: evaluating/orienting, what/where, focal/ambient, examining/noticing, figural/spatial, foveal/ambient, and object/spatial. 1 Although these terminologies may not all describe precisely the same visual functions, they draw very similar distinctions between two qualitatively different aspects of vision.

Effects of Foveal Priming and Extrafoveal Preview on Object Identification

Abstract: The results of three different experiments suggested that the relation between an object in the fovea on fixation n and an object subsequently brought into the fovea on fixation n + 1 affects the time to identify the second object. In Experiment 1 we extended previous work by demonstrating that a previously seen related priming object speeded the time to name a target object even when a saccade intervened between the two objects. In Experiment 2 we replicated this result and further showed that the benefit on naming time was due to facilitation from the related object rather than inhibition from the unrelated object. In addition, naming of the target object was much slower in both experiments when there was not a peripheral preview of the target object on fixation n. However, because the effect of the foveal priming object was greater when the target was not present than when it was present, priming did not appear to make extraction of the extrafoveal information more efficient. In Experiment 3, fixation times were recorded while subjects looked at four objects in order to identify them. Fixation time on an object was shorter when a related object was fixated immediately before it, even though the four objects did not form a scene. The size of the facilitation was roughly comparable to that in several analogous experiments where scenes were used. The results suggest that the effects of a predictive scene context on object identification may be explainable in terms of an object-to-object or "intralevel" priming mechanism.

Calculation of the influence of lateral chromatic aberration on image quality across the visual field.

Abstract: The magnitude of lateral chromatic aberration and its effect on image contrast were computed for a modified, reduced-eye model of the human eye, using geometrical optics. The results indicate that lateral chromatic aberration is a major factor affecting image quality for obliquely incident rays of polychromatic light. Modulation transfer functions for white sinusoidal gratings decline monotonically with spatial frequency, with eccentricity of the stimulus in the peripheral visual field, with grating orientation relative to the visual meridian, and with decentering of the pupil. Image contrast is largely independent of the color temperature of white light over the range 2800 to 12,000 K, but it improves significantly for the polychromatic green light of the P-31 oscilloscope phosphor. Selective filtering by macular pigment increases image contrast by an amount that grows with spatial frequency to about a factor of 1.5 at the foveal resolution limit. Reduced contrast caused by lateral chromatic aberration accounts for most of the threefold loss of acuity that occurs for foveal viewing through a decentered pupil. The aberration probably has negligible effect on peripheral acuity but may act to limit aliasing of peripheral patterns.

Does cone positional disorder limit resolution

Abstract: We measure the center-to-center spacings and disorder in spacings between all pairs of cones in a strip of primate retina extending from the foveal center to approximately 5.75 deg of retinal eccentricity along the temporal horizontal meridian. The strip is partitioned into windows, and the positions of the cone centers in each lattice window are digitized for analysis of lattice structure and quality. We find a nearly monotonic increase in cone spacing with eccentricity. The cone mosaic is a high-quality hexagonal lattice near the foveal center, and cone positional disorder (jitter) relative to averaging spacing increases beyond about 1.5 deg. We estimate human acuity measured through the optics of the eye over a retinal region comparable with our lattice strip by pooling the results of previous investigators. When the monkey lattice is scaled to human foveal resolution, application of the sampling theorem to average cone spacing predicts these pooled visual-acuity data from the foveal center to about 1.5 deg and overestimates visual acuity more eccentrically. Orientation reversal, a new technique developed by Coletta and Williams [ J. Opt. Soc. Am.4, 1503 ( 1987)] for estimating the Nyquist limit, estimates Nyquist frequencies from the foveal edge to beyond 5 deg of retinal eccentricity that agree with the cutoff frequencies predicted on the basis of our average spacing measurements. We conclude that the sampling theorem based on average spacing alone predicts the Nyquist limit from the foveal center to about 5 deg when that limit is measured by using the new aliasing technique. The sampling theorem based on average spacing overestimates the pooled estimate of visual acuity from the foveal edge to about 5 deg, probably because of sampling noise caused. by orientation and spacing disorder combined with demodulation as a result of the optics of the eye.

Density of foveal cone pigments at older age.

Abstract: We investigated foveal cone photopigment kinetics by retinal densitometry in 34 eyes of 29 healthy subjects with clear optical media and good visual acuity, ranging in age from 39 to 79 years. Our aim was to assess possible senile disturbances of foveal cones. To assess the effects of ocular straylight, we measured not only in subjects with a clear crystalline lens, but also in pseudophakia and aphakia. In a limited number of subjects color vision was assessed with a Nagel anomaloscope; no systematic changes with age were found. A significant decrease in two-way density and in time constant of regeneration was found to occur only after age 60, with large individual variations. There was no indication that results for subjects with their natural crystalline lens, in aphakia, or in pseudophakia were different. We argue that a reduction in the number of cones with age, rather than an increase in ocular stray light is the most likely explanation of our findings.

The role of striate cortex in the guidance of eye movements in the monkey

Abstract: We studied the effect of unilateral striate cortical ablations on smooth pursuit and saccadic eye movements in the monkey. The monkeys made quite accurate saccades to stationary stimuli in the field contralateral to the lesion, and they readily pursued foveal targets moving in all directions. However, when visual stimuli were stepped into the field contralateral to the lesion and then began to move, thus insuring that the moving stimulus was confined to the impaired visual hemifield, several oculomotor abnormalities emerged. Saccades to moving stimuli presented in the impaired field consistently undershot targets that moved away from the central fixation point after the step, and overshot targets that moved back towards the central fixation point. There was little or no smooth pursuit eye velocity generated in any direction to moving stimuli in the impaired field, and the monkeys could not generate smooth pursuit to stimuli maintained a few degrees from the fovea in the impaired field, although they were able to pursue such stimuli held in the normal field. Ablation of striate cortex also affected the latencies of saccades. When step-ramp stimuli were presented in the normal field, the monkeys delayed the initiation of saccades to targets moving towards the central fixation point, and hastened the initiation of saccades to targets moving away from the central fixation point. By contrast, changes in the direction of target movement did not affect the latencies of saccades into the impaired field. The deficits seemed permanent, lasting as long as the monkeys were tested--over 2 years in one case--but they were not total. Each monkey could use stimuli moving into the affected field to develop some eye velocity, although this residual ability had a much longer latency and lower gain than that provided by the intact visual system. These results show that striate cortex is intimately involved in the estimation of stimulus velocity critical to the genesis of smooth pursuit and saccadic eye movements.

Isolated foveal hypoplasia.

Abstract: Fifteen patients presented with foveal hypoplasia as an isolated ocular finding. The characteristic findings associated with this entity are a visual acuity of 6/21 or worse, nystagmus, and a typical ophthalmoscopic appearance of the macular area, including absent or abnormal maculofoveal reflexes, unclear definition of the maculofoveal area, and capillaries running abnormally close to the presumed macular area, some of them even crossing the horizontal meridian. Fluorescein angiography revealed a variable and incomplete filtering effect of the choroidal fluorescence in the macular area, suggesting abnormalities in the amount and distribution of macular pigments. The fundal findings of isolated foveal hypoplasia, although typical, are very subtle and often difficult to detect, especially because of the accompanying nystagmus. For this reason we suspect that foveal hypoplasia may be more common than is generally believed.

Foveal cone optical density in retinitis pigmentosa.

Abstract: We have used a color matching technique to estimate the optical density of the foveal cone photopigments in a group of patients with retinitis pigmentosa. We find that foveal cone optical density is reduced in patients with retinitis pigmentosa. This reduction of density can occur early in the disease process and is found in patients with minimal visual field loss or 20/20 visual acuity. Foveal cone optical density is highly correlated with visual acuity and correlated with visual field area. Full-field ERG measurements are severely reduced early in the disease before significant foveal changes occur.

What is psychophysically perfect image stabilization? Do perfectly stabilized images always disappear?: comment.

Abstract: This Communication calls attention to an earlier paper [L. E. Arend and G. T. Timberlake, J. Opt. Soc. Am. A 3, 235 (1986)] in which direct measurements have shown that retinal-image movements less than 1 arc min are required for perfect stabilization. It also cites papers that show that stabilized images of targets that contain no sharp structure in the foveal region disappear completely and permanently. Other targets disappear and reappear as a whole or as fragments intermittently.

Adaptive optimization of eye-head coordination with degraded peripheral vision (*)

Abstract: In head-free visual target tracking, the eyes attain the target while the head is still moving. Visual discrimination may start as soon as gaze is stabilized. When peripheral vision is degraded by systems placed in front of the eyes and carried by the head such as spectacles with degraded peripheral optics, foveal discrimination will not be possible unless the head moves towards the target to recenter the eyes through the optical area of clear vision. Thus, an additional delay in visual discrimination is to be expected with degraded peripheral optics. We have investigated the adaptive changes in eye-head coordination resulting from continuous wearing of glasses which alter peripheral vision, in visual target discrimination tasks. The results show that adaptive processes develop which lead, within a few days to a few weeks, to partial recovery of visual motor performance. Early changes include, as predicted, a decrease of head movement delay and an increase of head velocity.

Scalp distribution of visual evoked potentials to foveal pattern and luminance stimuli

Abstract: Scalp potentials elicited by pattern reversal, pattern onset, and flash stimulation have been studied in normal subjects by means of a 16-channel brain mapping system. In two groups of experiments the topography of the major positive component in each response was compared with 4° full- and lateral half-field stimuli and, for more detailed exploration of the pattern components by M-scaled stimuli selected to activate 1 cm2-patches of striate cortex and associated extrastriate re-projections. The 4° stimuli were found to elicit scalp distributions for the pattern reversal P100 and the pattern onset C1 consistent with striate and extrastriate visual cortical origins respectively. Data on the flash P2 component suggest that stimulation was not localized accurately; this may have been due to lateral spread of neural activity in the retina. Predictably, the M-scaled stimuli showed extensive inter- and intra-individual variations depending on stimulus type and location relative to fixation. The manner in which, in some subjects, the region of maximal activity on the map appears to migrate with change of stimulus position possibly reflects local retinotopic order in visual evoked potential generator areas. However, data are neither extensive nor detailed enough to be conclusive.

Two-dot vernier discrimination within 2.0 degrees of the foveal center.

Abstract: Vernier acuity (delta v) as a function of two-dot separation (s) was measured at five retinal locations between the foveal center and 2.0 deg of eccentricity. We compare these results with average cone spacing at each of the corresponding retinal eccentricities and find that the angular dot separations at which the delta v versus s function intersects the angular cone spacing at each eccentricity remain nearly constant. Further, we define a vernier Weber fraction delta v/s and find that the average Weber fraction increases by nearly a factor of 2 from the fovea to 2.0 deg of retinal eccentricity. We suggest that both receptor factors and postreceptor factors contribute to limits of vernier acuity within 2 deg of the foveal center.

Comparisons Across Age of Selected Visual Functions

Abstract: Data from eyes having 20/20 or better best corrected visual acuity are compared across age for several measures or indices of foveal visual sensitivity. These measures and indices include blue cone sensitivity, sensitivity at absolute threshold to a long-wavelength test, color match area effect, and time constant of rate of recovery during dark adaptation.

An Eye Motion Dependent Interactive Display

Abstract: Using readily available equipment, we assembled a digital graphics system to investigate several facets of how the human visual system gathers information. Of particular interest is the relationship and interaction between central and peripheral visual processes, investigated by selectively prohibiting foveal or peripheral visual access to pertinent details in a display. This process requires realtime display modification in response to eye movements and is accomplished by utilizing the capabilities of a graphics array processor to manipulate the 512x512x8 bit images at TV line rates. An infra-red limbus reflection technique is used to track the observer's gaze. Our applications involved radiologists looking at x-ray images of the human chest. We describe both hardware and software aspects of our interactive visual display system and mention its applications in our laboratory.

Parafoveal preprocessing in bilinguals

Abstract: Using a bilingual lexical decision task, we investigated subjects' ability to process parafoveally presented words in their first and second language, and whether or not this ability depended upon expectancy. The sequence of events was as follows: a prime word was presented foveally, followed by a parafoveal stimulus, followed by a foveal target word. Results showed that, in both languages, the prior parafoveal presentation of foveal targets improved performance. At short delays between prime and parafoveal words, facilitation was observed only when the two words were related. At longer delays however, benefits were accrued from parafoveal words independently of primes.

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

Abstract: 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.

Some contributions of I.E. Purkynĕ to the visual physiology.

Abstract: In the research of binocular vision J E Purkynĕ pointed out the role of retinal corresponding areas He observed binocular interaction of pressure phosphenes elicited in them and the fusion of images projected on the corresponding foveal areas even in eyes with artificial divergency For the first time he proposed the examination of visual fields (perimetry) in several planes and he also introduced the colour perimetry Purkynĕ analyzed profoundly the different functions of direct (foveal) and indirect (peripheral) vision In indirect vision he anticipated the modern concept of the useful field of view Of fundamental importance was Purkynĕ's research of the strategy and different types of ocular movements In analysis of motion perception, he evolved the hypothesis of "general sense of space" which is very close to modern feed-back conception of "corollary discharge"

Juxta-Foveal Grid Treatment with Green Monochromatic Argon and with Red Krypton Lasers for Diabetic Macular EDEMA

Abstract: When diabetic macular edema (DME) affects vision it is due to its being a foveal disease, and any decline of central visual function is a result of the proximity of the edema to the fovea and the duration it is there (1,2). This was the rationale to carry out our present study, because it seemed that it might be reasonable to apply laser treatment as close to the fovea as possible.