Showing papers on "Foveal published in 1988"

Idiopathic senile macular hole. Its early stages and pathogenesis.

Abstract: Evidence is presented that idiopathic senile macular hole is caused by focal shrinkage of the vitreous cortex in the foveal area. The most reliable biomicroscopic signs of impending hole formation (stage 1) are the development of a yellow spot or ring in the center of the fovea, loss of the foveal depression, and no evidence of separation of the vitreous from the foveal retina. Although in a majority of eyes with stage 1 changes there is a progression to hole formation, spontaneous separation of the vitreous without hole formation may occur in some cases (44%) and cause characteristic biomicroscopic changes, including foveal reattachment, disappearance of the yellow spot or ring, and, in some cases, a pseudo-operculum, with one or more lamellar holes or facets. A prospective collaborative study is recommended to confirm these findings and to test the clinical value of surgical peeling of the vitreous cortex in eyes with stage 1 changes as a means of preventing hole formation.

Optical and photoreceptor immaturities limit the spatial and chromatic vision of human neonates.

Abstract: We examine the contributions of preneural mechanisms, i.e., the optics of the eye and the aperture, spacing, and efficiency of foveal cones, to poor spatial and chromatic vision in human neonates. We do so by comparing the performances of ideal observers incorporating the characteristics of the optics and the foveal cones of adults and neonates. Our analyses show that many, but not all, of the differences between neonatal and adult contrast sensitivities and grating acuities can be explained by age-related changes in these factors. The analyses also predict differing growth curves for vernier and grating acuities. Finally, we demonstrate that preneural mechanisms constrain chromatic discrimination in human neonates and that discrimination failures may reflect poor visual efficiency rather than immature chromatic mechanisms per se.

Eccentric fixation with macular scotoma

Abstract: People with macular scotoma tend to read and visually scan more slowly than others with equivalently reduced visual acuity but intact central fields We measured fixation eye movements and considered the contribution of fixation variability and centripetal eye drift to poor visual performance These factors might confound efforts to consistently use an optimum retinal locus outside of the macula We measured monocular horizontal and vertical eye movements using a search coil eyetracker while subjects with naturally occurring central scotomata or control subjects with simulated scotomata eccentrically fixated a single character that was sized to their visual acuity Motivated subjects with long-standing stable maculopathies were chosen to estimate attainable performance limits During attempts to eccentrically fixate, an ubiquitous foveal pursuit or centripetal drift tendency was not found; rather a pattern of drift was idiosyncratic from subject to subject This finding was confirmed by an analysis of eye drift of 32 eyes with long-standing bilateral macular scotomata Moreover, the eye drift speeds (15-200 minarc/sec) were too low to be of functional significance Drift speeds during eccentric fixation with a visible target were not significantly different than those after the target was extinguished; however, drift speeds were greater than during foveal fixation This suggests that the fovea has a specialized control of slow eye movements Fixation variability increased with scotoma size for both simulated and real scotomata, with an abrupt rise when scotomata diameters exceeded 20 degrees C A significant minority of subjects (39%) adopted two or more distinct preferred retinal loci (PRL) during fixation Multiple PRL were also more likely if scotoma size exceeded 20 degrees C Reasonably steady fixation is thus attainable when central scotoma sizes are smaller than approximately 20 degrees C

Birefringence of the human foveal area assessed in vivo with Mueller-matrix ellipsometry

Abstract: We have assessed retinal birefringence in the foveal and parafoveal regions by applying Mueller matrix ellipsometry on the human eye in vivo. Basically, a light beam passed the ocular media twice and was scattered at the fundus intermediately. Keeping the entry and exit positions on the cornea constant and varying the retinal location along a circle around the foveal center enabled us to separate the corneal and retinal components of the measured retardation. We conclude that the retina within the outer margin of the parafovea behaves as a uniaxial crystal, with its slow axis radially oriented from the fovea and a retardation of about 16 deg (to 70 deg in the corneal center). We believe that Henle’s fiber layer causes retardation in this specific configuration of entrance and exit beams. The outer segments of the photoreceptors, although birefringent, have their optic axes aligned with these beams.

Tunnel vision or general interference? Cognitive load and attentional bias are both important

Abstract: Accuracy of identification of single letters presented in the near periphery (2.2 degrees, 3.3 degrees, or 4.4 degrees) was adversely affected when subjects were given a simultaneous foveal visual task as part of the same display. The level of processing required by the foveal task as well as the attentional allocation instructions were both found to be important. Tunnel vision as opposed to general interference can be induced by a combination of high foveal cognitive load, a focused attention strategy, and speed stress.

Two-dimensional spatial structure of receptive fields in monkey striate cortex

Abstract: Measurements of the spatial contrast sensitivity function and orientation selectivity of visual neurons in the foveal striate cortex (V1) of primates were interpreted within the context of a model of the two-dimensional spatial structure of their receptive fields Estimates of the spatial dimensions of the receptive fields along the axis of preferred orientation were derived from the application of the model and were compared with estimates of the smallest spatial subunit in the dimension orthogonal to the preferred orientation Some measure of agreement was found with corresponding estimates of parameters for psychophysical channels in human foveal vision

Cueing spatial attention during processing of words and letter strings in normals

Abstract: Work with patients has shown that lesions of the right posterior cortex produce a deficit that affects ability to report letters on the left side of a foveal nonsense string but has little effect on foveal words We have proposed that this is the result of a deficit in visual spatial attention The current studies use cues on the left and right of foveally centred letter strings to bias visual spatial attention in normals The studies show that the cues serve to bias report to the cued side very strongly for nonword letter strings and are less effective the more wordlike the string becomes These results show that covert attention controls access of letters to consciousness in those cases where spatial attention is used to organise input

Focal cone electroretinograms: aging and macular disease.

Abstract: Focal cone electroretinograms were obtained with a 3-degree flickering stimulus from 100 normal eyes and 134 eyes with known macular disease. Responses were obtained during direct visualization of the fundus with a hand-held stimulator-ophthalmoscope. Mean foveal cone amplitude for 100 normal eyes was 0.31 μV, with 95% of all amplitudes greater than 0.18 μV. There was a significant inverse correlation between amplitude and age for responses obtained from the fovea (r = -0.91; p < 0.001) but not for responses obtained from the parafovea (midway between fovea and disk, r = -0.53; not significant). In eyes with known maculopathy, mean foveal cone amplitude was correlated with Snellen acuity. Even after correcting for normal decreases in amplitude with age, responses were significantly reduced in 88/94 (94%) of all eyes with 20/40 or poorer acuity, suggesting that the focal electroretinogram is a sensitive test for detecting macular disease.

Effects of foveal task load on visual-spatial attention: Event-related brain potentials and performance

Abstract: The effects of foveal task difficulty on the processing of events in the visual periphery were investigated through an analysis of event-related brain potentials and performance measures. Subjects performed a foveally presented continuous monitoring task both separately and together with an arrow discrimination task that was presented at three different retinal eccentricities. The subjects detected occasional failures in the monitoring task while also responding to designated targets in the left and right visual fields. The analysis of the event-related brain potentials elicited by discrete events in the arrow discrimination task indicated that the amplitude of the N190 and P300 components decreased with both the introduction of the foveal task and an increase in its difficulty. The N160 component was sensitive to the distribution of attention within a task but was uninfluenced by dual task demands. These findings suggest that the N160 reflects the distribution of attention to different spatial locations within a task while the N190 may index the distribution of general purpose perceptual resources. P300 appears to index the allocation of perceptual/central processing resources. The implications of the results for models of resource allocation and attentional gradients are discussed.

Target crowding in foveal and peripheral stereoacuity.

Abstract: Disparity thresholds were obtained for a single point target when surrounded by a hexagonal array of comparison point targets. The experiment was carried out in two observers at the fovea and at retinal eccentricities of 3, 6, and 9 degrees. In each case the array diameter for best stereoacuity was determined. Care was taken that the data represented the optimum performance of the observer for the level of training in peripheral stereo tasks and also for the distance from the horopter. The results show a steady rise with eccentricity both for the stereothreshold and for the minimum target separations needed for uncrowded stereo performance. Both increase by a factor of about 10 between the fovea and the 9 degrees periphery, where a clear zone of at least 2 degrees diameter between test and comparison targets is needed for best stereoacuity.

Microholes of the fovea centralis.

Abstract: Foveal microholes are a possible cause of unilateral or bilateral small central visual defects in the absence of other macular pathology. In 18 eyes of 17 patients we noted the following features: small, dark-reddish holes in the centre of the fovea ranging from 50 to 150 micron in size; normal adjacent retinal pigment epithelium and neuroepithelium; unilateral in most cases; males and females are equally affected; mean age 40 years; insidious onset; no history of direct eye trauma or sungazing; non-progressive; relatively favourable visual outcome; no relationship with regular macular holes; aetiology unknown. These distinctive features justify reporting these patients as an important separate group.

Apparatus and method for the precision evaluation of visual function in the fovea centralis (macula) area of the retina

Abstract: A foveal vision evaluation instrument and procedure enables a precise evaluation of the visual function of the fovea centralis (macula) area of the retina. A foveal profile is developed to document the presence and state of foveal degeneration manifested by foveal segmentation and incremental changes and abnormalities in foveal vision, such as are caused by age related macular degeneration. Beam source optics 10, beam polarization optics 20 and image selection system 30 provide a polarized projection beam PPB that transmits an image array obtained by directing one component of polarized projection beam PPB through an image transparency IT. Image element blanking optics 40 enables the individual elements of the image array to be selectively blanked using a multi-cell electro-optic shutter 45. Zoom projection optics 50 projects the image array onto a screen within the foveal field of vision, and allows the size of the projected image elements to be incrementally changed through selective control of zoom magnification with compensating brightness (contrast) adjustment through automatic control of iris setting.

A re-examination of the principal retinae ofPhidippus johnsoni andPlexippus validus (Araneae: Salticidae): implications for optical modelling

Abstract: 1. The anatomy of the principal retina of one of the two species of Californian salticid spider studied by Land (1969 a, b),Phidippus johnsoni, is re-examined, using semithin sections of resin-embedded material and electron microscopy. 2. The retina conforms to the architecture disclosed by subsequent studies of phylogenetically advanced jumping spiders. In particular, foveal Layer I rhabdomeres, farthest from the dioptrics, are constructed as light guides, and are longer than Land estimated (ca. 43 μn compared to an earlier estimate of ca. 23 μn). 3. The interface between the retinal matrix and the materials that fill the retinal tube distal to it constitutes the diverging component of a miniature Galilean telescope, as in other salticids, magnifying an image received from the corneal lens by an estimated factor of 1.2. 4. The distal tips of foveal Layer I receptive segments are ‘staircased’ along a horizontal line, and centre-to-centre spacings are some 2.5 μn at the fovea. From geometrical optics, adopting the assumptions of Williams and McIntyre (1980), inter-receptor angles are around 8 arc min, compared to the 9 arc min estimated by Land from the earlier optical model. 5. Foveal receptive segments of Layers II–IV are all to various degrees contiguous such that there must be substantial optical pooling between adjacent rhabdomeres. The punctate mosaics mapped by Land are shown to be artefacts generated by traditional techniques of histological fixation for light microscopy and subsequent embedding in paraffin wax. 6. Foveal Layer I rhabdomeres are shown to be aligned to the exit pupil of the eye both inPhidippus, and inPlexippus for which we have more accurate optical data. This provides direct evidence for the optical power of the pit (previously inferred from different evidence), and, less directly, for the light-guiding properties of the rhabdomeres.

Speed Estimation through Restricted Visual Field during Driving in Day and Night: Naso-Temporal Hemifield Differences

Abstract: The peripheral visual field plays an important role in visually guided behavior such as driving. The role of the peripheral visual field, which occupies more than 90% of the total visual field, appears to be to detect the rate of expanding flow motion field associated with changing speed rather than analyzing fine details of the static scene, which subserves the central foveal visual field. Flow patterns generated in the peripheral due to the driver’s vehicle movement appear to be the critical cue for his speed estimation. It appears that when a driver is moving forward the peripheral retinal image contains an expanding flow field, and the local center of the expanding flow field coincides with the driver’s direction of the eye. As the gazing point changes so does the rate of expansion on the peripheral flow of field changes.

The Effect of Monocular Pattern Deprivation and Open-Loop Stimulation on Optokinetic Nystagmus in Squirrel Monkeys (Saimiri sciureus)

Abstract: Under normal visual conditions the optokinetic reflex (OKR) mainly serves the purpose of minimizing retinal image movement during head or body movements. The gaze-stabilizing OKR of primates is evoked by the movement-induced visual flow of foveal and extrafoveal stimuli (including parallax movement). It supports the foveal fixation mechanism (saccades and attentive gaze pursuit system) and the compensatory eye movements evoked by the vestibulo-ocular reflex. Optokinetic stimulation by means of a large continuously or sinusoidally rotating visual pattern (e.g., an optokinetic drum) activates in untrained monkeys the extrafoveal OKR predominantly, thus evoking binocularly conjugate optokinetic stare-nystagmus (OKN). Barring special instructions to the human observer, his OKN under similar conditions also belongs to this class of stare-nystagmus (ter Braak 1936).

Motivational influences on word recognition: I. Foveal and parafoveal viewing

Abstract: The effects of motivation (hunger vs. satiation) on tachistoscopic word recognition were investigated. Overall procedures were identical for foveal viewing (Experiment 1) and parafoveal viewing (Experiment 2). Results with foveal viewing confirmed earlier findings (Ferguson, 1983) that hunger facilitated word recognition and no need-relevance effects were evident. Under foveal viewing, words were recognized significantly earlier under conditions of hunger, but overt responding was not significantly faster. Under parafoveal viewing, no significant motivation effects occurred. It is possible that different processing mechanisms operate under parafoveal and foveal viewing conditions.

Visual after-effects associated with the use of high-resolution visual display units

Abstract: A long-lasting visual after-effect associated with the use of a high-resolution green visual display unit (VDU) is described in eight normal volunteers. Its duration to a standard VDU exposure is measured. It is related to the McCollough effect and is thought to be either a fatiguing of green colour-coded edge-detectors or an associative phenomenon. A reduction or loss of the usual foveal peak of the critical flicker frequency (CFF) to red is also described.