Showing papers on "Foveal published in 1973"

Light-difference threshold and subjective brightness in the periphery of the visual field

Abstract: Light-difference thresholds were measured in the center and periphery of the visual field at photopic and scotopic levels. Under photopic conditions the fovea has the lowest light-difference threshold. From the fovea to 10 degrees eccentricity threshold gradually increases. It remains constant up to approximately 35 degrees eccentricity in the temporal visual field (nasal retina). Beyond the edge of this plateau of constant light-difference threshold, it again increases to the limit of the visual field. Under scotopic conditions the extent of the plateau of constant light-difference threshold remains the same as under photopic conditions. The fovea itself, however, and its immediate environment are less sensitive than the plateau area. Subjective brightness of a supra-threshold target is not dependent on its position in the visual field. A target with a given luminance will elicit the same brightness sensation at all retinal positions. As a consequence of this brightness constancy throughout the visual field, peripheral targets at threshold appear brighter than foveal targets at threshold because a peripheral target at threshold has more luminance than a foveal target at threshold.

Normal fixation of eccentric targets.

Abstract: Two-dimensional eye movements were recorded by a contact-lens optical lever while two experienced subjects attempted to maintain fixation at the center of a 1°.3-diam disk, at the center of two- and four-disk arrays separated by 10°.0 to 29°.5, or to maintain the same eye position after the disk was removed from view. Fixation stability was better with the foveal disk than when the target was presented in the near periphery. Fixation stability deteriorated slowly as target separation increased, but fixation stability with the most peripheral target was better than that with no target at all. This deterioration of fixation stability was associated with increases of the size of both saccades and intersaccadic drifts, but the frequency of saccades was not influenced systematically.

Directional Sensitivity of the Foveal and Parafoveal Retina

Abstract: Photopic directional sensitivity was measured at the fovea and at 2°, 3.75°, and 10° from fixation (referred to the center of the entrance pupil of the eye; nonfoveal tests were conducted in the nasal field projected onto temporal retina). These data confirm and extend an earlier experiment performed by Westheimer. The central fovea exhibited significantly less directionality than parafoveal test points. Differences in directionality between the three parafoveal test loci were not significant. Possible factors giving rise to the foveal-parafoveal difference in directionality are reviewed. Evidence corroborating our earlier observations that receptors across the retina are aligned with the point approximating the center of the exit pupil of the eye is also presented.

Interferometric resolution determinations in the fovea and parafovea.

Abstract: Foveal and extrafoveal interferometric determinations of retinal resolution were made on two observers. Resolution fell with increasing decentration of fixation. For example, the two subjects exhibited retinal resolution ‘equivalent’ to Snellen 20/40 respectively at 6.3° and 3.75° from fixation. The test criterion was the ability to distinguish the vertical line (fringe) interferometric pattern formed on the retina. One must conclude that comparable interferometric determinations (resulting in equivalent resolution measurements) do not imply that a patient has central fixation. This finding means that we must use care when interpreting the results of such tests on patients. Of course, if test parameters are altered, or if media are hazy or cloudy, the result may be somewhat different.

Visual acuity in the pigeon. II. Effects of target distance and retinal lesions.

Abstract: Visual acuity thresholds for grating targets were determined for three pigeons at target distances ranging from 13 to 73 cm. These measurements were made both while the birds were wearing goggles restricting vision to the frontal field of view and when vision was unrestricted. Using a slightly different method, performance was also compared for target distances of 6 and 13 cm while the goggles were in place. For a second group of three pigeons, acuity data were obtained before and after laser lesions of the retina's foveal region. The findings suggested that acuity was relatively poor for targets at the intermediate distances and that it improved as distance both increased and decreased from these intermediate values. The acuity improvement with increasing distance did not occur, however, when the birds were wearing frontal goggles. The data appear to be consistent with Catania's 1964 suggestion that the pigeon has separate frontal and lateral visual systems that differ in their refractive characteristics. Foveal lesions did not appear to affect acuity for distant targets, and it was concluded that, while the fovea serves the lateral field of view, its presence is not necessary to the static acuity characteristics of this system.

Macular-foveal capillaries in human retina.

Abstract: Summary The occurrence of atypical vascularization of the macular-foveal zone in 20 eyes of 17 patients is described. The significance of this fluorescein stereoangiographic finding is discussed.

Continuous hue estimation of brief flashes by deuteranomalous observers.

Abstract: The continuous hue-estimation technique with restricted categories was used to determine the chromatic response functions of six deuteranomalous observers T ce stimuli, 12 min in diameter, were presented as 12-msec foveal flashes The computed response functions indicate that the subjects did not always need four chromatic response categories to describe the spectrum: at low luminances, some did not need yellow; at high luminances, some did not need green, replacing it with white

A method for producing foveal retinal exposures in an awake, task-oriented, rhesus monkey*

Abstract: A method for producing retinal exposures from a laser source in an awake, task-oriented S is presented. The technique allows for the measurement of rhesus monkey visual acuity immediately following exposures.

Information processing in the visual periphery

Abstract: The information processing capacity of the visual periphery was investigated in two experiments using stimuli of known relative information content. The results of Experiment I showed redundant patterns to be easiest to identify at all peripheral angles tested (5, 10, 20, and 30 deg) as compared to intermediate and nonredundant patterns. Performance on all patterns decreased as peripheral angle increased. In Experiment II, simultaneous foveal and peripheral vision was required in a discrimination task. Performance was above chance only when the peripheral (7-deg) pattern was redundant. The foveal pattern had no effect on discrimination.

Independence of sensitivity on different foveal areas

Abstract: Evidence suggests that for close retinal areas a correlation in sensitivity exists between the areas. Correlations as a function of distance from fixation and between areas were studied. In Experiment I, both forms were equidistant from fixation and five different distances apart. In Experiment II, both forms fell on an imaginary line through fixation. The forms usually did not fall on equally sensitive areas as in Experiment I. Both experiments showed that accuracy was lower for two- than for one-form displays. Closer forms had the lowest accuracy, suggesting perhaps mutual contour masking. However, in the sense of an intratrial correlation, no significant relationships were found. These two types of independence were discussed in terms of contour masking and varying sensitivity.

A distractive effect of peripheral attention on foveal trigram recognition.

Abstract: The recognizability of tachistoscopically presented foveal trigrams was investigated as a function of paying attention, or not paying attention, to simultaneously exposed parafoveal trigrams. The same set of stimuli was presented in both conditions. All six observers made a significant number of errors at 50 ms stimulus duration in the attentive condition. This occurred even though they could not correctly report the parafoveal stimulus. Performance reached 95% correct at 200 ms stimulus duration, just when parafoveal trigrams were occasionally identified. At 50 ms stimulus duration in the attentive condition, errors were more marked for non-wordlike center trigrams. When the parafoveal trigram was non-wordlike, its distractive effect was greater than when it was wordlike. Neither distance nor position of the parafoveal trigrams were significant factors. The distractive effect of peripheral attention on recognition of letter stimuli suggests that information from parafoveal stimuli is available through at...

Experimental Investigation of Flashblindness Parameters

Abstract: : An automated flashblindness apparatus was constructed and is described. Various experiments designed to investigate the effect of flashblindness on foveal vision were conducted and are presented. These experiments included: correlation of flashblindness recovery data with fundus reflectometry data; the effect of preadaptation to high ambient light levels of flashblindness recovery time; foveal recovery time; foveal recovery time as a function of location and area of flash source image; intensity x time relationships; foveal dark adaptation; variability of interindividual flashblindness recovery times and the effect of flash source intensity and duration on recovery times, afterimage brightness and ratio of photopigment bleached.