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Biomedical subjects

E Peli

Publications and source records attributed to E Peli.

At least 55 records · Page 3Linked to original sources
PubMed1990

Contrast in complex images.

The physical contrast of simple images such as sinusoidal gratings or a single patch of light on a uniform background is well defined and agrees with the perceived contrast, but this is not so for complex images. Most definitions assign a single contrast value to the whole image, but perceived contrast may vary greatly across the image. Human contrast sensitivity is a function of spatial frequency; therefore the spatial frequency content of an image should be considered in the definition of contrast. In this paper a definition of local band-limited contrast in images is proposed that assigns a contrast value to every point in the image as a function of the spatial frequency band. For each frequency band, the contrast is defined as the ratio of the bandpass-filtered image at the frequency to the low-pass image filtered to an octave below the same frequency (local luminance mean). This definition raises important implications regarding the perception of contrast in complex images and is helpful in understanding the effects of image-processing algorithms on the perceived contrast. A pyramidal image-contrast structure based on this definition is useful in simulating nonlinear, threshold characteristics of spatial vision in both normal observers and the visually impaired.

Contrast Sensitivity↗
PubMed1988

Use of circularly polarized light in fundus and optic disc photography.

Circularly polarized light was used in fundus and optic disc photography to reduce reflected glare and photographic artifacts originating from the ocular media and the lenses of the fundus camera. A standard fundus camera was used, with circular polarizers placed in the illumination and observation paths. The quality of fundus photography was improved significantly in eyes with intraocular lenses, cataract, vitreous haze, or high myopia as well as in normal eyes, in fundus photography of the retinal periphery, and in optic disc photography.

Fundus Oculi↗
PubMed1987

Enhancement of fundus photographs taken through cataracts.

Image-enhancement techniques can aid the clinician in evaluating fundus pictures taken through cataracts. The degradation of the fundus image by cataracts has been described as low-pass filtering. Means to partially overcome this degradation using homomorphic filtering and adaptive enhancement are presented. The clinical value of these enhancement techniques is demonstrated with two cases of progressive glaucoma and cataracts.

Cataract↗
PubMed1987

Nerve fiber layer photography. A comparative study.

Observation of the retinal nerve fiber layer with red-free light is an important means of evaluating optic nerve integrity. Many investigators have attempted to increase the usefulness of the technique by improving photographic procedures. Most reports comparing various parameters neither indicate the method of comparison nor give quantitative measure of the improvement gained with different films, filters, cameras, or processing procedures. We have compared in a masked fashion the parameters most recently recommended in the literature. Twelve trained observers compared 12 masked pairs of nerve fiber layer photos taken from the same eye in the same manner on two cameras and preferred those taken with a Canon CF-60Z camera 69% of the time and those taken with a Zeiss FF-111 17% of the time (this difference was statistically significant, P less than 0.01). Fourteen observers compared 14 pairs of photographs and judged Panatomic-X film to be superior to Plus-X film 51% of the time and both films to be of the same quality 25% of the time (this difference was also significant P less than 0.05). Fifteen observers compared 12 pairs and preferred the Standard SE-40 blue filter 2:1 over the green Spectrotech 540 filter. This difference was not statistically significant, and the preference was found to be dependent on fundus pigmentation and clarity of the media. The blue filter gave better contrast for the nerve fiber layer in patients with lightly pigmented fundi, and use of the green filter resulted in less light scatter in patients with ocular media opacities.

Evaluation Studies as Topic↗
PubMed1987

Reading with a macular scotoma. II. Retinal locus for scanning text.

To elucidate how patients with macular scotomas use residual functional retina for inspecting visual detail and reading, we tested three patients with dense macular scotomas using a scanning laser ophthalmoscope that allows an examiner to view and record stimuli on the retina while the patient views them. Using standard psychophysical techniques, we determined the retinal position of scotomas, the retinal areas used for fixating and inspecting acuity targets, and the retinal area used for reading simple, three-letter, nonsense syllables. We found that each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma, for fixating, inspecting acuity targets, and scanning simple, nonsense-syllable text. This preferred retinal locus (PRL) was at different retinal eccentricities (relative to the foveola) for each patient and was not always as close as possible to the foveola. There appears to be no simple rule by which patients "select" a particular PRL. Plots of text placement on the retina revealed considerable differences in patients' abilities to execute an orderly text scan. Two patients read text more rapidly with a novel retinal area than with their PRL, suggesting that the PRL may not be optimal for text reading.

Eye Movements↗
PubMed1986

Drusen measurement from fundus photographs using computer image analysis.

Drusen are yellowish deposits at the level of the retinal pigment epithelium and are frequently associated with age-related maculopathy (ARM). Drusen often change in size and number over time and may be followed by atrophic or exudative macular degeneration. A quantitative method to measure the development of drusen is needed for controlled studies of the natural history, prognosis, and treatment of ARM. An objective method is described using computer image analysis of fundus photographs for the detection and measurement of drusen. This technique enables us to measure both the area of drusen in the macula and the changes in the drusen pattern over time. Evaluation of repeated photographs showed reproducibility of 6.1%, whereas the reproducibility of processing photographic duplicates was 2.3%. Digitization with a high-quality linear array solid state camera did not change reproducibility significantly.

Fundus Oculi↗
PubMed1986

Control of eye movement with peripheral vision: implications for training of eccentric viewing.

A review of peripheral visual control of eye movements suggests that such control may be easier if the direction of the extrafoveal locus used is orthogonal rather than radial to the direction of target motion. When the extrafoveal locus lies parallel (radial) to the direction of target or eye motion, artificial feedback is usually required to avoid reflexive foveation. These findings have important implications for training eccentric viewing in low vision patients with central scotomata.

Aged↗
PubMed1986

Blink vergence in an antimetropic patient.

A patient with uncorrected antimetropia was found to attain motor fusion through blinking. Although this patient was also able to attain motor fusion through saccadic vergence and slow fusional vergence, he usually relied on blink vergence. In this patient, blink vergence was an efficient alternative to slow fusional vergence.

Adult↗
PubMed1986

Computerized enhancement of retinal nerve fiber layer.

We investigated a number of image processing techniques for enhancing the retinal nerve fiber layer (NFL) in fundus photographs taken with red-free light. Wedge-type defects within the NFL were enhanced best by contrast enhancement algorithms such as histogram modification and extremum sharpening. Narrow, slitlike NFL defects can be enhanced by directional enhancement techniques. Normal NFL features were enhanced best by high-pass filtering techniques such as homomorphic filtering and adaptive enhancement. The enhanced normal NFL may be useful in the analysis of diffuse NFL loss. All of these successful enhancements of normal and defective features indicate that enhancing fundus photographs with digital image processing techniques should facilitate early detection of optic nerve damage from glaucoma and other diseases.

Computers↗
PubMed1986

Ophthalmic applications of circular polarizers.

Circular polarizing filters have recently been introduced in several ophthalmic applications. These applications include clinical instruments for ophthalmoscopy, slit-lamp specular microscopy, and fundus photography, as well as consumer products such as 3-D movie projection and viewing systems and glare-control screens for computer visual display terminals. The nature of circularly polarized light and mode of operation of the circular polarizing filters in various applications are discussed. In view of the advantages of circular polarizers in vectographic applications, the current linearly polarized vectographic systems used for binocular testing and visual training should be replaced with circular polarizing systems.

Computers↗
PubMed1986

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

Adult↗
PubMed1985

Circular polarizers enhance visibility of endothelium in specular reflection biomicroscopy.

A simple and inexpensive technique for enhancing the visibility of corneal endothelial cells in specular reflection biomicroscopy involves the insertion of a circular polarizer in front of the patient's eye, intersecting both the incident and reflected light beams. This filter significantly reduces the glare from the epithelium and enables more comfortable and clearer viewing of the endothelial cells.

Cornea↗
PubMed1985

Effect of contrast on fusional visual evoked potential (VEP): a model and experimental results.

A visual evoked potential (VEP) component that appears in the power spectrum only during binocular fusion has recently been discovered. When both eyes are stimulated with the same checkerboard but at different pattern reversal rates, this fusional component appears at a frequency intermediate between the two stimulus frequencies. We have proposed a model to explain the appearance of this intermediate component and have tested the model's predictions that the fusional component will remain constant independent of binocular or monocular changes in stimulus contrast. As predicted by the model, changes in contrast over the range of 10 to 90% produced no significant change in the power of the fusional component.

Evoked Potentials, Visual↗
PubMed1984

Smooth eye-movement control with secondary visual feedback.

A signal derived from continuous measurement of eye position is displayed on a visual frame of reference, thereby closing a secondary visual feedback (2VFB) loop. The distance between a target and the displayed gaze point provides the subject with an extra, artificial position error. Experiments show that subjects can use the 2VFB to generate smooth eye movements, even in the absence of any smoothly moving independent targets. Under these conditions, both direction and velocity can be brought under voluntary control by the subjects. As a control signal for the smooth-eye-movement mode, the 2VFB is robust and easily manipulated and provides an attractive means for the investigation of the smooth-movement control system in a variety of tasks and under different conditions.

Eye Movements↗
PubMed1984

Effects of stimulus contrast on binocular VER.

The effect of stimulus contrast changes on the binocular visual evoked response (VER) was investigated using pattern reversal VER, dichoptic stimulation, and Fast Fourier Transform. When the stimulus contrast was changed binocularly, the monocular components increased as the contrast increased. The binocular component first appeared at the level of 10% contrast; its magnitude was stable under all recording conditions. When the stimulus contrast was changed monocularly, the eye that received the higher contrast stimulation showed more power in the power spectrum. The binocular component appeared even when the difference in contrast between the two eyes was large. The magnitude of the binocular component was stable under all recording conditions. We concluded that the binocular and the monocular components differ in the magnitude of their responses to a contrast change and speculate that the pathways responsible for the two components are not identical.

Adult↗
PubMed1983

Dynamics of cover test eye movements.

The dynamics of unilateral cover test eye movements, never before systematically investigated with an objective recording system, are shown to be more complex than textbook accounts of them. We administered the cover test to nine heterophoric subjects by means of electromechanical occluders. Eye movements were recorded using the infrared photoelectric technique. Saccadic and vergence movements of the fixating eye were observed in almost all records when the occluded eye was uncovered. These movements were found in esophores and exophores and in both large and small phoria cases. Such movements were previously described in other asymmetric vergence tasks and appear to obey Hering's law of equal innervation. Uncovering the dominant eye, in cases of clear dominancy, resulted in shorter latency and larger amplitude saccades than did uncovering the nondominant eye. These large saccades were frequently of unequal amplitude in each eye. Trained subjects appear to use dynamic overshoots to increase this saccadic inequality and thereby attain vergence during saccades. Movements after the application of a cover to one eye, while grossly similar to textbook descriptions of them, are found to contain small vergence drifts and refixation (correcting) saccades in the nonoccluded eye.

Adult↗