Showing papers on "Foveal published in 1991"

Contrast thresholds for identification of numeric characters in direct and eccentric view.

Abstract: Aubert and Foerster (1857) are frequently cited for having shown that the lower visual acuity of peripheral vision can be compensated for by increasing stimulus size. This result is seemingly consistent with the concept of cortical magnification, and it has been confirmed by many subsequent authors. Yet it is rarely noted that Aubert and Foerster also observed a loss of the “quality of form.” We have studied the recognition of numeric characters in foveal and eccentric vision by determining the contrast required for 67% correct identification. At each eccentricity, the lowest contrast threshold is achieved with a specific stimulus size. But the contrast thresholds for these optimal stimuli are not independent of retinal eccentricity as cortical magnification scaling would predict. With high-contrast targets, however, threshold target sizes were consistent with cortical magnification out to 6° eccentricity. Beyond 6°, threshold target sizes were larger than cortical magnification predicted. We also investigated recognition performance in the presence of neighboring characters (crowding phenomenon). Target character size, distance of flanking characters, and precision of focusing of attention all affect recognition. The influence of these parameters is different in the fovea and in the periphery. Our findings confirm Aubert and Foerster’s original observation of a qualitative difference between foveal and peripheral vision.

Recovery of visual responses in foveal V1 neurons following bilateral foveal lesions in adult monkey.

Abstract: Cells in the foveal representation of V1 cortex of adult primates became visually responsive after normal sensory input was removed. Immediately after foveae were lesioned bilaterally, a region was found where no cells' activity could be modulated by visual stimulation. Recordings made in that deafferented zone at > 2.5 months after lesions revealed that activity of over half of the cells could be modulated by visual stimuli presented to intact peripheral retina. Although response characteristics made cells with recovered driving quite unlike normal cells, the result suggests a level of visual cortical reorganization previously observed only in immature animals.

A cognitive-anatomical approach to attention in lexical access

Abstract: Recent PET studies have suggested a specific anatomy for feature identification, visual word forms and semantic associations. Our studies seek to explore the time course of access to these systems by use of reaction time and scalp electrical recording. Target detection times suggest that different forms of representation are involved in the detection of letter features, feature conjunctions (letters), and words. Feature search is fastest at the fovea and slows symmetrically with greater foveal eccentricity. It is not influenced by lexicality. Detecting a letter case (conjunction) shows a left to right search which differs between words and consonant strings. Analysis of scalp electrical distribution suggest an occipito-temporal distribution for the analysis of visual features (right sided) and for the visual word form discrimination (left sided). These fit with the PET results, and suggest that the feature related analysis begins within the first 100 millisec and the visual word form discriminates words from strings by about 200 msec. Lexical decision instructions can modify the computations found in both frontal and posterior areas.

Acuity-luminance and foveal increment threshold functions in retinitis pigmentosa.

Abstract: Acuity-luminance functions and foveal increment threshold functions were measured in 20 subjects with retinitis pigmentosa (RP) who had Snellen acuities of 20/40 or better, minimal or no posterior subcapsular cataracts, and no atrophic-appearing foveal lesions. Compared with the results from ten normal subjects, the visual acuities of the RP subjects were reduced at all luminance levels; the acuity deficits were more pronounced at low luminances. Foveal detection thresholds of the RP subjects showed the greatest elevation at low background luminances and approached normal values at high adapting levels. There was a statistically significant correlation (r = 0.79, P less than 0.01) between the visual acuities and absolute thresholds of the RP subjects. The overall pattern of results cannot be explained by a reduced quantal absorption in foveal cones, but it is consistent with the hypothesis that a reduced cone spatial density is the primary mechanism of foveal visual loss in this group of RP subjects.

Visual detection with search: an empirical model

Abstract: A visual detection model, including search, is constructed from empirical data on foveal and off-axis contrast thresholds, fixation times, saccadic sizes, and eye response times. The model includes one parameter to account for scene complexity and human preparedness, and its accounts quantitatively for the effects of clutter on this parameter and on other variables. The resulting algorithm correlates well with published laboratory results at light levels of 1 fL or more also gives intuitively satisfying predictions of field performance. >

Eye-head movement coordination: vestibulo-ocular reflex suppression with head-fixed target fixation

Abstract: To maintain clear vision, the images on the retina must remain reasonably stable. Head movements are generally dealt with successfully by counter-rotation of the eyes induced by the combined actions of the vestibulo-ocular reflex (VOR) and the optokinetic reflex. A problem of importance relates to the value of the so-called intrinsic gain of the VOR (VORG) in man, and how this gain is modulated to provide appropriate eye movements. We have studied these problems in two situations: 1. fixation of a stationary object of the visual space while the head moves; 2. fixation of an object moving with the head. These two situations were compared to a basic condition in which no visual target was allowed in order to induce "pure" VOR. Eye movements were recorded in seated subjects during stationary sinusoidal and transient rotations around the vertical axis. Subjects were in total darkness (DARK condition) and involved in mental arithmetic. Alternatively, they were provided with a small foveal target, either fixed with respect to earth (earth-fixed target: EFT condition), or moving with them (chair-fixed-target: CFT condition). The stationary rotation experiment was used as baseline for the ensuing experiment and yielded control data in agreement with the literature. In all 3 visual conditions, typical responses to transient rotations were rigorously identical during the first 200 ms. They showed, sequentially, a 16-ms delay of the eye behind the head and a rapid increase in eye velocity during 75 to 80 ms, after which the average VORG was 0.9 +/- 0.15. During the following 50 to 100 ms, the gain remained around 0.9 in all three conditions. Beyond 200 ms, the VORG remained around 0.9 in DARK and increased slowly towards 1 or decreased towards zero in the EFT and CFT conditions, respectively. The time-course of the later events suggests that visual tracking mechanisms came into play to reduce retinal slip through smooth pursuit, and position error through saccades. Our data also show that in total darkness VORG is set to 0.9 in man. Lower values reported in the literature essentially reflect predictive properties of the vestibulo-ocular mechanism, particularly evident when the input signal is a sinewave.

A new elementary feature of vision.

Abstract: Humans can discriminate among elementary features such as orientation, color, and depth in parallel. The author showed that discontinuities in lines, such as a vernier offset, can be detected in a flash presentation with subsequent mask, and an offset, even below the diameter of foveal photoreceptors, is identified among distractors. This result suggests that the visual system simultaneously detects misalignments or gradients of orientation in the hyperacuity range at many positions in the visual field. The direction of the vernier offset, however, cannot be classified simultaneously in several stimuli, but requires serial search.

The Computational Hoverfly; a Study in Computational Neuroethology

Abstract: Studies in computer vision have only recently realised the advantage of adding a behavioural component to vision systems, enabling them to make programmed ‘eye movements’ Such an animate vision capability allows the system to employ a nonuniform or foveal sampling strategy, with gaze-control mechanisms repositioning the limited high-resolution area of the visual field The hoverfly Syritta pipiens is an insect that exhibits foveal animate vision behaviour highly similar to the corresponding activity in humans This paper discusses a simulation model of Syritta created for studying the neural processes underlying such visually guided behaviour The approach differs from standard “neural network” modeling techniques in that the simulated Syritta exists within a elosed simulated environment, ie there is no need for human intervention: such an approach is an example of computational neuroethology

Changes in the visual field in diabetic retinopathy after grid laser coagulation of the posterior pole of the eye

Abstract: The influence of grid laser photocoagulation on the central visual field of 15 eyes with diffuse diabetic maculopathy was studied in a prospective clinical trial. Throughout the 3 month follow-up period, visual fields were monitored by static perimetry using a combination of 30-1 and 30-2 programs of the Humphrey Field Analyzer. Statistical analysis of all clinical parameters shows that visual acuity correlates very well with foveal light difference sensitivity. The overall sensitivity of the 30 degrees field, however, does not correlate significantly with either of them. During the follow-up period, patients' subjective vision improved significantly (p = 0.0234); for the overall sensitivity of the 30 degrees visual field there was a trend towards improvement (p = 0.0692). The sensitivities of the central 10 degrees and 3 degrees of the visual fields, of the fovea and visual acuity did not change. The results of the present study suggest that during the first 3 months after grid photocoagulation of diffuse diabetic macular edema, the treatment affects mainly the retina outside the macula. The improvement in patient vision may be due to these changes in the visual field.

Fixation-monitoring equipment for ophthalmic examination - uses rotary mark structure identifiable only on foveal fixation

Abstract: The fixation-monitoring equipment is for an opthalmic examination instrument. Means are provided to rotate a structured fixing mark (3) about a central axis (4). The mark has a structure and direction identifiable by the patient only on foveal fixation. The mark can be arranged to assume distinct orientations during rotation, and can be self-illuminating. ADVANTAGE - Allows functional and observable fixation stimulation.

Model of human preattentive visual detection of edge orientation anomalies

Abstract: Psychophysical studies provide evidence of preattentive visual processing characterized by parallel operations performed on a limited set of features. Since these operations extend well beyond the foveal or high-resolution area of the visual field, one may assume that they are based on lower-resolution features. Such parallelism and data reduction imply computationally efficient processing that could be emulated for machine vision pattern recognition purposes. Several models of preattentive texture segmentation have recently been presented in the computational vision literature. This paper presents a model of human preattentive visual detection of pattern anomalies. Operating on a low-frequency, band-pass filtered image, the model detects singularities by comparing local to global statistics of contrast and edge orientation. The model has been applied to simple schematic images. It successfully predicts the asymmetry in search latencies whereby a target characterized by a preattentively detectable feature 'pops out' of a field of distractors not containing the feature, but when target and distractors are switched, serial search is required to locate the 'odd man out.' The model has also been shown to detect pattern defects on periodic, multilevel integrated circuits.

The Role of Binocular Disparity Vergence Eye Movements in Disambiguating Superimposed Retinal Images

Abstract: In laboratory investigations of the disparity or fu-sional vergence mechanism, it is common to present the observer with isolated objects of regard having differing degrees of retinal disparity and to measure the resulting vergence motor response. However, under more naturalistic conditions, the retinal image may be composed from a complex, multilayered object space involving both overlapping and non-overlapping, spatially discrete object planes producing varying degrees of retinal disparity and defocus. Furthermore, these images frequently occupy an extended area of the retina well beyond the central foveal and macular regions. The task of disambiguating or separating these images in an optimal manner, with the requirement for accurate bifoveal fixation, is likely to represent a complex procedure involving multiple sensory and motor inputs.

Effects of small spot foveal exposure on spatial vision and ERG spectral sensitivity

Abstract: In previous experiments, we observed that small spot foveal laser exposure could produce morphological retinal damage while having only transient or delayed effects on spatial visual function. These effects are markedly different from those obtained with large spot foveal exposures, where permanent function change was observed at exposure levels below the retinal damage threshold. In this paper, we have examined functional effects below and above laser retinal damage thresholds for small spot foveal exposures. At subthreshold levels we have observed a dependency of recovery on spatial frequency of the acuity test target, and no significant difference in recovery from exposure levels below or significantly above retinal damage levels. At levels that produce frank retinal damage, long term recovery involves complex retinal receptor system reorganization and inner retinal neural change.

Modeling of Explorative Procedures for Remote Object Identification

Abstract: : This work addresses the issue of how humans manually explore remote objects using a telemanipulator. An understanding of how conceptual models are constructed is necessary since it will ultimately determine the efficiency of ROV's using telepresence. The representation of human search models is achieved by using the proprioceptive component of the haptic sensory system and the simulated foveal components of the visual system. Eventually it will allow multiple applications in remote sensing and superposition of sensory channels. The use of a force reflecting telemanipulator and computer simulated visual foveal component are the tools which offer the possibility of reconstructing these search patterns observed in different subjects under controlled laboratory conditions. The correlation between the two search strategies is explored and represented in code circles and strings which demonstrates the sequential nature of the two types of probing, as opposed of saccadic response in full vision. (Author)

Fine Structure of the Retina

Abstract: Figure 6 depicts radial light microscopic sections of the baboon retina at the regions examined. The area of the foveal rim is shown in Figure 6A. At the arrowhead at the bottom is the thickest part (455 µm) of the retina. The shape of the fovea can vary considerably among individuals (Dawson et al. 1986). In this specimen, which has a very steep foveal slope, the foveal center is approximately 0.1 mm toward the left from the arrowhead (Fig. 6A). The downward-pointing arrow at the right indicates the retinal level internal of which the tangential sections are cut to study the inner retinal layers connected to the fovea by Henle’s fibers (h). The tissue columns used for this purpose are taken from a region approximately 500 µm from the center. At the foveal rim the thickness of the inner nuclear layer is 114 µm. Approximately 200 µm from the center, it tapers down to 75 µm. This thickness does not change for the next 1 mm. The ganglion cell layer (g) is the thickest just in the region, from which the tangential section of the foveal inner layers are taken.

The effects of hemispheric differences on feature perturbations

Abstract: Hemispheric differences for feature perturbations were investigated in two experiments. Stimulus displays consisting of five small squares arranged in a single row were presented tachistoscopically, with the subject instructed to state in which square a horizontal tick mark was located. Ticks could occur in any of the three middle squares, with half of the ticks presented on the inside and half presented on the outside of the square in relation to the fovea. Experiment 1 presented each array of five squares to the right or left of fixation at one of three distances from the fovea. Experiment 2 manipulated the distance between the squares and kept foveal distance constant. In each experiment, fewer errors were made when stimuli were presented to the left visual field/right hemisphere than when they were presented to the right visual field/left hemisphere, when ticks migrated toward the fovea. Experiment 1 found that increasing the distance from the fovea increased the error rate, but did not change the hemispheric differences. Experiment 2 found that increasing the distance between the squares did not change hemispheric effects reliably. The data imply that hemispheric differences for perceptual processing begin very early during sensory analysis.

Effect of field size on the colour evoked potentials and the electroretinogram

Abstract: We have recorded colour evoked visual potentials (N87) and electroretino-grams (P60) with the stimulus diameter varied between 2° and 10°. VEPs tend to saturate at about 6°, while ERGs continue to increase in amplitude up to 10°. This different behavior is interpreted as a correlate of the cortical magnification of foveal vision.