Showing papers on "Foveal published in 1993"

Oxygen distribution in the macaque retina.

Abstract: Purpose Oxygen distribution was characterized in the macaque retina, which is more like the human retina than others studied previously. Methods Profiles of oxygen tension (PO2) as a function of distance were recorded in a parafoveal region about halfway between the fovea and optic disk, and from the fovea in one animal. A one-dimensional diffusion model was used to determine photoreceptor oxygen consumption (QO2). Results In the parafovea, the PO2 decreased as the electrode was withdrawn from the choroid toward the inner retina, reaching a minimum value during dark adaptation of about 9 mmHg at about 70% retinal depth, and then increasing more proximally. Approximately 90% of the oxygen requirement of the photoreceptors was supplied by the choroidal circulation and 10% by the retinal circulation. In light adaptation, there was a monotonic PO2 gradient from the choroid to the inner retina, indicating that all of the oxygen used by photoreceptors was supplied by the choroid. In the fovea, the choroid supplied almost all the oxygen in both dark and light adaptation, with a minor supply from the vitreous humor. Dark-adapted foveal oxygen consumption was lower than parafoveal oxygen consumption. Light reduced the oxygen consumption of the photoreceptors, in both regions studied, by 16-36%. Conclusions The results show that oxygenation of the parafoveal monkey retina is similar to that previously observed in the cat area centralis. In the fovea, the oxygen distribution differs as expected considering the thinner retina and the absence of inner retinal neurons and retinal circulation.

The modular organization of projections from areas V1 and V2 to areas V4 and TEO in macaques

Abstract: In addition to the major anatomical pathways from V1 into the temporal lobe, there are other smaller, “bypass” routes that are poorly understood. To investigate the direct projection from V1 to V4 (bypassing V2) and from V2 to TEO (bypassing V4), we injected the foveal and parafoveal representations of V4 and TEO with different retrograde tracers in five hemispheres of four macaques and analyzed the distributions of labeled neurons in V1 and V2 using flattened preparations of the cortex. In V1, labeled neurons were seen after injections in V4 but not TEO. The V4-projecting neurons were located in the foveal representation of V1, in both cytochrome oxidase (CO)-rich blobs and CO-poor interblob regions. In V2, TEO-projecting neurons were intermingled with V4-projecting neurons, although the former were far sparser than the latter. Across the cases, 6–19% of the TEO-projecting neurons were double labeled, that is, also projected to area V4. Both V4- and TEO-projecting neurons formed bands that ran orthogonal to the V1/V2 border, and both were located in CO-rich thin stripes and CO-poor interstripe regions. In some cases, a continuous band of V4-projecting neurons was also found along the V1/V2 border in the foveal representation of V2. The results indicate that the pathways from V1 to V4 and from V2 to TEO involve anatomical subcompartments thought to be concerned with both color and form. These “bypass” routes may allow coarse information about color and form to arrive rapidly in the temporal lobe. The bypass route from V2 to TEO might explain the partial sparing of color and form vision that is seen after lesions of V4. By analogy, given the bypass route from the foveal representation of V1 to V4, lesions of V2 affecting the foveal visual field would also be insufficient to block color and form vision.

Promising directions in active vision

Abstract: Active vision systems have mechanisms that can actively control camera parameters such as position, orientation, focus, zoom, aperture, and vergence (in a two-camera system) in response to the requirements of the task and external stimuli. They may also have features such as spatially variant (foveal) sensors. More broadly, active vision encompassesattention, selective sensing in space, resolution, and time, whether it is achieved by modifying physical camera parameters or the way data is processed after leaving the camera. In the active-vision paradigm, the basic components of the visual system are visual behaviors tightly integrated with the actions they support; these behaviors may not require elaborate categorical representations of the 3-D world. Because the cost of generating and updating a complete, detailed model of most environments is too high, this approach to vision is vital for achieving robust, real-time perception in the real world. In addition, active control of imaging parameters has been shown to simplify scene interpretation by eliminating the ambiguity present in single images. Important research areas in active vision include attention, foveal sensing, gaze control, eye-hand coordination, and integration with robot architectures.

Modulation transfer of the human eye as a function of retinal eccentricity

Abstract: We measured the monochromatic image quality of the eye across a wide visual field (120°), with natural pupil (4 mm) and accommodation (3 diopters). The method is based on the acquisition and the posterior processing of double-pass aerial images of a point source imaged on the retina, which was kept at a fixed distance from the eye at all retinal eccentricities. The two-dimensional modulation transfer functions (MTF’s) computed from the aerial images show that astigmatism is the dominant monochromatic aberration in both the fovea and the periphery and is also the major cause of variability among individuals. We found a slower decline in optical quality with eccentricity than had been found by previous measurements. Our foveal results are in close agreement with those of Campbell and Gubisch [ J. Physiol. (London)186, 558– 578 ( 1966)], but off-axis optical quality is much better than found previously by Jennings and Charman [ Am. J. Optom. Physiol. Opt.55, 582– 590 ( 1978);Vision Res.21, 445– 454 ( 1981)]. The optical system of the eye seems to follow a wide-angle lens design: the optical quality in the center (fovea) is not particularly good (it is far from the diffraction limit at this pupil size), but the modulation transfer function remains roughly constant for a wide visual field.

Reactions to peripheral image motion using a head/eye platform

Abstract: The authors demonstrate four real-time reactive responses to movement in everyday scenes using an active head/eye platform. They first describe the design and realization of a high-bandwidth four-degree-of-freedom head/eye platform and visual feedback loop for the exploration of motion processing within active vision. The vision system divides processing into two scales and two broad functions. At a coarse, quasi-peripheral scale, detection and segmentation of new motion occurs across the whole image, and at fine scale, tracking of already detected motion takes place within a foveal region. Several simple coarse scale motion sensors which run concurrently at 25 Hz with latencies around 100 ms are detailed. The use of these sensors are discussed to drive the following real-time responses: (1) head/eye saccades to moving regions of interest; (2) a panic response to looming motion; (3) an opto-kinetic response to continuous motion across the image and (4) smooth pursuit of a moving target using motion alone. >

Viewing behavior : ocular and attentional disengagement

Abstract: In five experiments, we examined the role of the ocular and attentional systems in determining saccadic latencies. Prior to making a saccade to a target stimulus, subjects were required to direct their attention to a foveal stimulus or to an eccentric stimulus. Either stimulus could be extinguished before the onset of the target. Saccadic latencies were shortest when the foveal stimulus was extinguished, regardless of whether it was attended or not. Control experiments showed that subjects were able to attend properly and that warning, arising from turning off a stimulus before target onset, could not completely account for the results. The results were discussed in terms of ocular disengagement, attentional disengagement, and joint ocular-attentional disengagement. It was concluded that an explanation emphasizing ocular disengagement provided the best account of the data.

Morphometric analysis of human retinal pigment epithelium: correlation with age and location.

Abstract: Flat bleached preparations of retinal pigment epithelial (RPE) cells from foveal, temporal posterior pole, and temporal equatorial fundus locations of 20 normal human eyes from donors age ranked from 13 to 96 years were analyzed for density, and 10 were analyzed for area, hexagonality and polymegathism. These factors were related to fundus locations and age. Foveal RPE cells were significantly more dense and more hexagonal in eyes from younger donors. Loss of hexagonality in the fovea, but not elsewhere, was correlated with increasing age. As the eye ages, foveal RPE cells selectively lose unique morphologic characteristics and resemble nonfoveal cells. These morphologic changes resemble other pathologic features of age-related degeneration in their proclivity for the fovea and posterior pole.

Effects of cognitive foveal load on a peripheral single-target detection task.

Abstract: This experiment investigated the effects of foveal cognitive load on a primary peripheral single-target detection task. Four levels of foveal task with cognitive loads involving identification and summation of numerals were used. Number of correct targets detected seemed unaffected by the foveal load in the near periphery but a decrement occurred beyond 7.7°. Response times for correct responses showed large dispersion compared with that for correct locations. At a low cognitive load, foveal task performance showed no deterioration for all eccentricities tested, but at a higher cognitive load performance declined gradually across eccentricities. Mild evidence of runnel vision was obtained as indicated by the significant interaction of cognitive loads × eccentricities. Resources theory accounted well for the results.

Effects of foveal stimulation on peripheral visual processing and laterality in deaf and hearing subjects.

Abstract: This research examines visual field differences in the detection and identification of a peripheral stimulus for deaf and hearing subjects, as a function of concurrent foveal stimulation. Deaf and hearing subjects were presented with peripheral target stimuli (simple geometric shapes) presented tachistoscopically to the left or right visual fields under four conditions of foveal stimulation: (a) no stimulus; (b) simple geometric shapes; (c) pictorial shapes (outline drawings); and (d) orthographic letters. Dependent measures were detection response latency and peripheral shape recognition (errors). With error data, hearing subjects showed a right field advantage under foveal conditions of no stimulus and simple shape stimulus, but a left field advantage with pictorial and letter foveal stimuli. Deaf subjects showed the opposite effect, with a left field advantage under foveal conditions of no stimulus and simple shape stimulus, but a right field advantage with pictorial and letter foveal stimuli. Latency data revealed the same pattern of results for hearing subjects, but no significant visual field differences for deaf subjects. Results are interpreted in terms of differences in hemispheric visual processing used by deaf and hearing subjects, as affected by varying conditions of foveal load.

Modeling fish-eye lenses

Abstract: The human visual system can be characterized as a variable-resolution system: foveal information is processed at very high spatial resolution whereas peripheral information is processed at low spatial resolution. Various transforms have been proposed to model spatially varying resolution. Unfortunately, special sensors need to be designed to acquire images according to existing transforms. In this work, two models of fish-eye transform are presented. The validity of the transformations is demonstrated by fitting the alternative models to a real fish-eye lens.

Eye Movements in Reading: Recent Developments

Abstract: When we read, our eyes do not move smoothly across the page of text as our phenomenological im pressions imply. Rather, we make a series of eye movements (referred to as saccades) separated by periods of time when the eyes are relatively still (fixations). Eye movements are nec essary during reading because of acuity limitations in the visual sys tem. A line of text extending around the point of fixation can be divided into three regions: foveal, parafoveal, and peripheral. The foveal region is the center of vision (extending 1? of visual angle to the left and right of fixation), where acu ity is sharpest, enabling us to resolve the letters in the text easily. In the parafoveal region (extending out to 5? of visual angle on either side of fixation) and the peripheral region (everything on the line beyond the parafoveal region), acuity drops off markedly so that our ability to iden tify letters is not very good even in the near parafovea. The purpose of eye movements in reading is there fore to place the foveal region on that part of the text we wish to pro cess next. The basic facts about eye move ments during reading have been known for some time. The typical saccade is about eight to nine letter spaces; this value is not affected by the size of the print as long as it is not too small or too large. The ap propriate metric to use when dis cussing eye movements during read ing is therefore letter spaces, and not visual angle; generally, three to four letter spaces is equivalent to 1? of visual angle. Because of the high ve locity of the saccade, no useful in formation is acquired while the eyes are moving; readers acquire infor mation from the text during fixa tions. The average fixation duration is about a quarter of a second (250 ms). The other primary characteristic of eye movements during reading is that about 10% to 15% of the time, we move our eyes back in the text (regressions) to look at material that we have already read. It is also important to note that as text difficulty increases, fixation du ration increases, saccade length de creases, and regression frequency increases. More important, the val ues just presented are averages, and there is considerable variability in the measures, both between and within readers. Thus, although the average fixation duration for a given reader might be 250 ms, for others it could be on the order of 200, 235, 280, or 300 ms. The average sac cade length and regression fre quency also vary across readers. As far as within-subject variability is concerned, although a reader's av erage fixation duration might be 250 ms, the range could be from under 100 ms to over 500 ms within a pas sage of text. Likewise, although the average saccade length for a given reader might be 8 letter spaces, the range could be from 1 letter to over 15 (though such long saccades typi cally follow regressions). A great deal of recent research has docu mented that the variability in these measures is related to the ease or dif ficulty associated with processing the text. Since the mid-1970s, there has been a resurgence of interest in us ing eye movement data to study the reading process.1 Researchers have typically been interested in eye movements during reading for two reasons: (1) to describe the charac teristics of the eye movements per se and (2) to use eye movement data to infer something about perceptual and cognitive processes during read ing. Because most of the work in my laboratory has focused on the latter issue, the major focus of this article is on processing during reading.

Foveal cone ERGs in fellow eyes of patients with unilateral neovascular age-related macular degeneration.

Abstract: PURPOSE To determine whether fellow eyes with normal visual acuity of patients with unilateral neovascular age-related macular degeneration (AMD) have retinal malfunction. METHODS Foveal cone electroretinograms (ERGs) were recorded from fellow eyes with visual acuities of 20/25 or better of 73 patients with unilateral neovascular AMD and from 28 normal volunteers of comparable age. Responses were elicited with a 4 degrees stimulus flickering at 42 Hz presented by a stimulator-ophthalmoscope. RESULTS The study eyes of the patients were found to have foveal cone ERGs that, on average, were normal in amplitude but delayed in implicit (peak) time after adjustment of the data for age, sex, iris pigmentation, and refractive error by multiple linear regression. Based on all subjects, amplitude declined with increasing age and was smaller in eyes with darker irides; implicit time increased with increasing age. CONCLUSIONS These findings suggest that fellow eyes with normal visual acuity of patients with unilateral neovascular AMD tend to have foveal cones that are normal in number but that function abnormally. In addition, foveal cone ERG amplitude should be adjusted for both age and iris pigmentation and implicit time should be adjusted for age when assessing retinal function of elderly patients.

The problem of macular sparing after unilateral occipital lesions.

Abstract: Whether or not unilateral occipital damage produces sparing of central vision, namely macular sparing, is controversial. We tested two subjects with left occipital lesions by means of fundus perimetry combined with fundus image analysis. This method made it possible to measure the distance of the stimulus projected on the retina from the foveal centre defined as the centre of the foveal reflex. The results indicated that macular sparing, if it exists, must be less than 0.4° wide. Two of the four eyes during the stimulus presentation often but not always showed eccentric fixation of a small magnitude, whose mean was less than 0.6° from the foveal centre in the right hemiretina.

Detection of Visual Information

Abstract: The purpose of this chapter is to briefly review some of the early theories of vision, to discuss some simple visual experiments which vividly demonstrate properties of the eye, and to discuss several current models of vision.

A morphometric and stereologic analysis of ganglion cells of the central human retina.

Abstract: Unbiased estimates of the cell density of the ganglion cell layer (GCL) were made by the Disector method, and the topography of the nerve fibre layer (NFL) between the centre of the fovea and the optic disc was studied, in human retina under both normal and pathological conditions Three human retinae were examined, the eyes having been removed in the course of cancer surgery Before operation two of the patients had visual acuity within normal limits The third patient had impaired vision In the two normal retinae, the cell densities of the GCL in the foveal wall were ≈60 000/ mm2 Estimates were also made outside the fovea and these showed a progressive reduction in cell density from the foveal wall towards the optic disc The NFL simultaneously increased in thickness to approximately 70–100 μm at the peripapillary border At 4 deg eccentricity nasal to the fovea the majority of axons had a mean diameter of less than 042 μm Closer to the optic disc, the number of large axons increased in the inner parts of the NFL In the third eye with long- and short-term visual impairment there was a substantial cell loss in the GCL, with only some 15 000 cells/mm2 in the foveal wall In spite of cell/axon loss the thickness of the NFL was increased due to axonal swelling

Models of the Visual Cortex Based on Visual Evoked Potentials

Abstract: Publisher Summary The cortical surface operates as a dipole sheet in a volume conducting medium. Theoretical models, whether simplified or complex, indicate that the most effectively placed tangential dipole in a fissure can only provide some 20% of the amplitude of a neighbouring radial generator on a gyral crest. However, it should be expected that it would frequently extend to both sides of the fissure resulting in cancellation. Therefore, the effect might be much less than the estimated 20% of a radial dipole and often below the noise level of the large signals from neighbouring gyral generators. When the foveal and lower macular hemifields are stimulated, the scalp potential map is largely determined by the gyral crests in the striate and circumstriate areas, which lie on the contralateral occipital–temporal surface in the majority of brains. The rescaled versions of the foveal circumstriate areas of the macaque, with various amounts of V2 and V4 concealed because of the intrusion of the lunate and inferior occipital sulci, fit onto a human brain cast with very little distortion, enabling the proposal of an interim model of the exposed portions of the occipital areas.

Foveal l and m cone ratio and topography estimated with chromatic hyperacuity stimuli

Abstract: FOVEAL LAND M CONE RATIO AND TOPOGRAPHY ESTIMATED Wl'rH CHROMATIC HYPERAClnTY STIMULI P. D. G9wc1y. C. M 1 Cicerone, and S. Otake University of California, Irvine, CA. pymos~. The high foveal cone density and the optical characteristics of the human eye are barriers to psychophysical measurements of the separate topographies of L and M cones in fovea centralis. The aims of this study, for central fovea, were (1) to devise a method allowing the estimation of the topographies of L - and M-cones, and (2) to provide a novel method for estimating the relative numbers of L~ and M-cones. Methods. Monochromatic (620, 560, and 520nm) vernier dot stimuli (1' of arc squares with 3' of arc separation, 200ms dw-ation) were presented with target (top) displacements ranging from O to 70 of arc to the left and right of the reference (lower) upon selective, cone- suppressing background fields. The task was to indicate whether the target was displaced to the left or right of the reference dot in each self-presented trial. The data generated from computer simulations, talting into account optical scatter, eye movements, and different configurations of L- and M-cone mosaics, were matched to the experimental data to estimate the L- and M-cone ratio and their topographies for each observer. Results. Obtained L- and M-cone ratios were in close agreement with previous estimates based on a small-spot detection task (Cicerone and Nerger, 1989; Otake and Cicerone, 1992). The simulation-gen.e rated mosaic that best matched the data specified each observer's underlying cone mosaic. Copclusions. We show that this novel method can be used to estimate cone ratios as well as the separate topographies of L- and M-cones in the densely-packed photoreceptor array of fovea centralis. Supported by NSF-BNS 8819874 and NIH-EY 08200 to CMC. None.

Models of the Visual Cortex on the Basis of Psychophysical Observations

Abstract: Publisher Summary The visual field is represented topographically in many visual areas of the human cortex. The extent of visual cortex devoted to a given angle of visual field, in general, decreases from central vision towards the periphery. Human visual performance also tends to decrease from the point of fixation. The thresholds in various spatial visual tasks are the linear functions of eccentricity within 0–10°. The rate at which the threshold increases with eccentricity gives a direct indication of how fast performance declines in the task under consideration. It is usually defined by stating its inverse, that is, the E2 value that represents the eccentricity at which the foveal threshold in a spatial visual task doubles. When the isoeccentric paradigm is used, there is no direct estimate of the scaling factor for an eccentricity of zero. However, the linear regression can be extrapolated to an eccentricity of zero to find the foveal scaling factor.

Foveal topography in the optic nerve and primary visual centers in Falconiforms.

Abstract: The topography of the retinal nasal and temporal foveal projections upon the optic nerve and primary visual centers was studied in diurnal bifoveate birds of prey by means of restricted tritiated proline intraocular injection. According to the degree of retinotopy, this study reveals that a single injection of tracer in the nasal or temporal fovea produces a well-defined and complementary pattern of projections in the following contralateral nuclei: lateral anterior thalamus, lateroventral geniculate nucleus (glv), superficial synencephalic (ss), tectal grey (gt), and optic tectum. In the thalamic nucleus dorsolateral anterior, the nasal foveal projections are seen mainly in the lateral and rostrolateral subdivision, while temproal projections are seen mainly in the magnocellular subdivision. In the external and ectomammillary nuclei there is some evidence of retinotopic innervation. Finally, a discrete field of projection from the nasal or temporal fovea is detected in lateral hypothalamus, ventrolateral thalamus, lateral geniculate intercalated nucleus, and pretectal optic area. The nasotemporal axis of the retina is ventrodorsally oriented in the optic nerve with ganglion cell axons of the temporal fovea more dorsally placed than the nasal ones. In the primary visual centers this retinal axis is mediolaterally rpresented in the nuclei glv, ss, and gt, and dorsoventrally oriented in the optic tectum. © 1993 Wiley-Liss, Inc.

Focal attention and the perception of pattern structure in extrafoveal vision.

Abstract: Several studies have shown that visual attention enhances the encoding of the positional relationships between pattern elements, i.e., the perception of pattern structure. This theoretical note discusses the possibility that the inferiority of extrafoveal vision in the perception of pattern structure results, at least partially, from less effective focal attention in the periphery. Hence, there may exist higher level differences between foveal and extrafoveal vision in information processing which cannot be explained by the spatial inhomogeneity of the retino-striate system.