Showing papers on "Contrast (vision) published in 1991"

Visual dysfunction in Alzheimer's disease: Relation to normal aging

Abstract: In patients with Alzheimer's disease (AD), compared with age-matched and young healthy control subjects, visual deficits in the following functions were observed: color, stereoacuity, contrast sensitivity, and backward masking (homogeneous and pattern). Critical flicker fusion thresholds were normal, relative to age-matched healthy subjects. For color, the majority of the errors were tritanomalous (blue axis). Color and stereoacuity deficits were unrelated to severity of dementia, in accordance with models of vision that describe these functions as modular rather than diffuse for cortical localization. Although contrast sensitivity was depressed throughout the frequency range in AD, more patients were impaired at low than at high spatial frequencies, contrasting with the observed normal aging pattern of high-frequency loss. Healthy elderly subjects showed depressed critical flicker fusion thresholds and reduced contrast sensitivity at high frequencies, relative to the young group; differences between these groups were not found for the other vision tests. A subset of the AD group received detailed neuro-ophthalmological examination, and no abnormalities were found. This finding, taken together with normal thresholds for critical flicker fusion, suggests that the widespread visual dysfunction reported here is more likely to be related to known pathological changes in primary visual and association cortex in AD than to changes in the retina or optic nerve.

The effects of parvocellular lateral geniculate lesions on the acuity and contrast sensitivity of macaque monkeys

Abstract: The effects of ablating the visual pathway that passes through the parvocellular (dorsal) LGN were tested in 2 macaque monkeys by measuring acuity and both luminance and chromatic contrast sensitivity. Thresholds were tested monocularly before and after ibotenic acid was used to lesion parvocellular layers 4 and 6 of the contralateral geniculate. The injections were centered at the representation of 6 degrees in the temporal field on the horizontal meridian, and vision was tested with localized stimuli at this location. In addition, in one of the monkeys, a lesion was made in magnocellular layer 1 of the opposite geniculate, and the same thresholds were tested. Physiological and anatomical reconstructions demonstrated complete destruction of the target layers in 1 parvocellular lesions and in the magnocellular lesion, and sparing of the nontarget layers in the tested region. Parvocellular lesions caused a 3–4-fold reduction in visual acuity within the affected part of the visual field, while the magnocellular lesion did not affect acuity. Both luminance and chromatic contrast sensitivity, tested with stationary gratings of 2 c/degree, were severely reduced by parvocellular lesions, but not affected by the magnocellular lesion. However, when luminance contrast sensitivity was tested with 1 c/degree gratings, counterphase modulated at 10 Hz, it was reduced by both parvocellular and magnocellular lesions. This study demonstrates that the parvocellular pathway dominates chromatic vision, acuity, and contrast detection at low temporal and high spatial frequencies, while the magnocellular pathway may mediate contrast detection at higher temporal and lower spatial frequencies.

Visual acuity and contrast sensitivity in relation to falls in an elderly population

Abstract: Visual acuity and contrast sensitivity were measured in 95 residents of a hostel for the aged (mean age = 83 years) using a dual-contrast letter chart and the Melbourne Edge Test (MET). Vision (as measured by visual acuity, the MET, low-contrast visual acuity, and difference between high- and low-contrast acuity) decreased significantly with age and all four measures were significantly correlated. Subjects with a clinical eye disorder had poorer vision than those without a disorder although the differences were not significant. Visual acuity and contrast sensitivity were not associated with body sway when subjects were standing on a firm base. However, when the subjects were placed in a situation which provided reduced support (standing on a compliant surface), body sway was associated with poor visual acuity and contrast sensitivity. There was also a difference in contrast sensitivity between those who fell one or more times in a year of follow-up and those who did not fall. It appears that reduced vision may be a predisposing factor to postural imbalance and falls in elderly persons.

Visual requirements for safety and mobility of older drivers.

Abstract: Efforts to assess visual deterioration with increasing age, coupled with new mechanisms proposed to limit the exposure of visually impaired drivers to driving risks, have emerged in response to the increase in older drivers. Visual functions discussed in this context include static acuity (photopic, mesopic, and in the presence of glare), dynamic visual acuity, visual field, contrast sensitivity, and motion perception. Exposure control mechanisms discussed include alternative periodic vision testing strategies, visual training, and environmental and vehicular modifications to accommodate the older driver. Finally, relevant research needs are addressed.

Relative visual performance: A basis for application:

Abstract: Visual performance is defined in terms of the speed and accuracy of processing visual information. To evaluate illuminated tasks in terms of visual performance, it is necessary to have a valid computational model that relates measurable, salient aspects of the visual environment (e.g. target contrast) to measurable human responses (e.g. visual response time). A model of visual performance should be independent of the influence of non-visual factors as they influence speed and accuracy. It is also important to have practical application tools that can measure the salient aspects of the visual environment and compute (predict) visual performance for real tasks. This paper describes the basis for a visual performance model and how it can be applied using a computer imaging device.

Apparent surface curvature affects lightness perception

Abstract: The human visual system has the remarkable capacity to perceive accurately the lightness, or relative reflectance, of surfaces, even though much of the variation in image luminance may be caused by other scene attributes, such as shape and illumination. Most physiological, and computational models of lightness perception invoke early sensory mechanisms that act independently of, or before, the estimation of other scene attributes. In contrast to the modularity of lightness perception assumed in these models are experiments that show that supposedly 'higher-order' percepts of planar surface attributes, such as orientation, depth and transparency, can influence perceived lightness. Here we show that perceived surface curvature can also affect perceived lightness. The results of the earlier experiments indicate that perceiving luminance edges as changes in surface attributes other than reflectance can influence lightness. These results suggest that the interpretation of smooth variations in luminance can also affect lightness percepts.

Contrast adaptation and contrast masking in human vision.

Abstract: After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.

Evolution of the eye and visual system

Abstract: Evolution of visual behaviour in animals evolution of the eye - from the light sensitive cephalic organ of nemertoid worms to the vertebrate eye the eye of Copelia - tectile and visual scanning as primitive means of form perception the response to light in prokaryotes and protozoa evolution of the compond eye and visual systems of arthropods evolution of the eye and visual system of molluscs evolution of visual pigments and photoreceptors evolution of the vertebrate retina (including - phylogenetic differences in the morphology and function of retinal ganglion and other nerve cells and their possible relationship to differences in visual behaviour) morphological and functional evolution of pretectum, dorsal thalamus and visual cortex nucleus isthmi or parabigeminal nucleus nucleus rotundus and pulvinar visual tracking and the cerebral control of gaze response to visual shadows. Lateral inhibition and contrast sensitivity and the subsequent development of movement and directional sensitivity the detection of visual form evolution of colour vision evolution of depth perception (binocular vision and stereopsis) the phylogenetic segregation of movement, colour and pattern systems within the brain.

Image enhancement for the visually impaired. Simulations and experimental results.

Abstract: Digital image enhancement has been proposed as an aid for the visually impaired The capability of two enhancement techniques to improve recognition of images by patients with central scotoma or cataracts was evaluated using image-processing simulations and direct patient testing Enhancements and simulations were based on measurements of contrast sensitivity loss for patients with macular disease Contrast sensitivity loss was measured using Gabor-type localized stimuli and paradigms that are appropriate for analyzing form perception The simulations using the contrast sensitivity data suggested that patients with moderate visual loss (20/70-20/200) may have difficulty recognizing faces and may benefit from enhancement by both of the techniques used Ability to recognize celebrities from enhanced images improved for 39 of the 46 patients tested The improvement was significant (P less than 005) for 16 of the 38 patients with central visual loss and for 3 of 8 patients with anterior segment media opacities tested The simulations suggest that the benefits of image enhancement may be similar or even greater for recognition of other types of images

Displays having improved contrast

Abstract: A display presenting an image having enhanced contrast has a screen which switches between a bright, image-presenting state and a dark, image-less state, to minimize glare from ambient light reflections. Alternatively, a shutter which switches between a substantially transparent state and a dark, light absorbing state is placed over a conventional screen.

Peripheral color contrast. A new screening test for preglaucomatous visual loss.

Abstract: A new test of peripheral color contrast is described. A high-definition color monitor driven by a personal computer with a graphics interface card displays an annulus subtending 25 degrees at the eye. The color contrast between the annulus and the background can be varied. Forty-five degrees of the annulus is randomly removed in one of four quadrants. Patients are asked to identify the position of the gap while fixating a central spot. The minimum color contrast between annulus and background at which the identification is possible is between 13-16% for the protan, deuteran, and tritan axis in normal subjects. This threshold value changes little with age, refractive error, or pupillary aperture, and test-retest variability is low. Testing one eye takes only 1-2 min. The test was applied to ocular-hypertensive and glaucomatous patients. All patients with glaucoma had thresholds greater than two standard deviations (SD) above the normal mean. In addition, 97% of glaucoma patients had thresholds greater than four SDs, and 95% had thresholds more than five SDs above the normal mean. Most patients with ocular hypertension and clinical signs indicating a low or medium risk of conversion to glaucoma had thresholds under the upper limit of normal. High-risk patients with ocular hypertension fell into two groups. One approximated to normal; the other had elevated thresholds, which in many cases were more than four SDs above the normal mean. The epidemiologic consequences of this test are discussed.

Contrast, resolution, and detectability in MR imaging.

Abstract: With the introduction of fast scan techniques and high field imagers, the ability to achieve very high resolution MR images in reasonable imaging times is now possible. Increased resolution allows for better detection of small, high contrast pathological features, but at some cost. Increasing resolution leads to a nonrecoverable decrease in signal-to-noise ratio per pixel and a loss of low contrast detectability for constant imaging time. This article examines the tradeoffs between image resolution, signal-to-noise ratio, and low contrast detectability in MR imaging. Contrast detail curves are presented for images collected in a constant imaging time, with constant field of view and bandwidth but at different resolutions, and these are compared with theoretical curves. The problem of measuring contrast levels in magnitude images, with different resolutions and receiver attenuation values, is discussed and a definition that accommodates these parameters developed. In addition, a clinical example is shown demonstrating a decrease in soft tissue differentiation with increasing resolution, again for fixed imaging time. The results indicate that moving to high resolution imaging matrices requires consideration be given to the sacrifice in low contrast detectability that occurs. Most importantly, it is shown that filtering a high resolution image to a lower resolution image, through nearest neighbor averaging, does not regain the detectability lost in initially collecting the high resolution image.

Visual activity in macaque area V4 depends on area 17 input.

Abstract: It is known that some direct projections from the lateral geniculate nucleus terminate in area V4 of the macaque monkey. Retinal information can also bypass area 17 and reach V4 through relays in the superior colliculus and pulvinar. This raises the question whether area V4 is visually responsive in the absence of input from area 17. We tested this possibility by recording in area V4 while inactivating a region of area 17 by cooling. This led to a complete abolition of the visual responses of practically all the neurons whose receptive fields were included in the visual field region coded by the inactivated zone in area 17. In contrast, neurons whose receptive fields were outside this region remained visually responsive.

Visual dysfunction among former microelectronics assembly workers.

Abstract: Although known neurotoxins with potential ophthalmotoxic properties are commonly used in microelectronics assembly, there has been no systematic study of visual disturbances among past or present workers in this industry. The objective of the present study was to compare visual functions, using a matched-pair design, between former workers from a microelectronics plant and a local reference population. From an initial population of 180 former workers and 157 potential referents, 54 pairs were matched for age (± 3 y), education (± 2 y), sex, ethnic origin, and number of children. Near and far visual acuity, chromatic discrimination, and near contrast sensitivity were assessed monocularly. Paired comparisons (Signed-rank Wilcoxon test) revealed that the former microelectronics workers had significantly lower contrast sensitivity, particularly in the intermediate frequencies, independently of near visual acuity loss. There were no differences for far visual acuity in both eyes. Even though near visu...

Effects of spherical and astigmatic defocus on acuity and contrast sensitivity : a comparison of three clinical charts

Abstract: Two contrast sensitivity charts (Vistech and Pelli-Robson) have become available to the eye care practitioner. Their value as clinical tools for the assessment of visual function may be enhanced because of either an insensitivity to the effects of optical focus or a hypersensitivity to defocus. We compared the sensitivity to defocus of these charts to the traditional Snellen chart by examining the effect of up to +/- 5.00 D of spherical and astigmatic defocus on performance with each chart. In order to simulate the two types of clinical examination scenarios, tests were performed both with and without mydriatic/cycloplegic agents. The Pelli-Robson contrast sensitivity chart was very resistant to the effects of all types of optical defocus. As predicted, the high spatial frequencies on the Vistech chart were sensitive to defocus. However, although contrast sensitivities for the low frequencies were affected less, optical defocus produced significant decreases in low frequency Vistech contrast sensitivity. In addition, the Vistech chart was very insensitive to axis 180 blurring lenses. There was no indication that either contrast sensitivity chart was more sensitive to defocus than the standard Snellen chart.

Limits of Vision

Abstract: Part 1: nature of limits in vision threshold (barriers), just noticeable differences measures of visual performance contrast. Part 2: basic limitations imposed by the structure and mechanisms in the visual system receptors of the retinal network - compression of signals intermediate processing stages and visual cortical areas hierarchical and parallel visual processing principles of overlap practical implications of multi-stage and multi-changed processing. Part 3: visual signs of very low (scotopic) intensities - detection of spatial and temporal changes.

The importance of measuring binocular contrast sensitivity in unilateral cataract.

Abstract: Monocular and binocular contrast sensitivities were measured in patients with uniocular cataract. The cataractous eye showed a greater monocular loss at higher spatial frequencies compared to lower spatial frequencies. Binocular contrast sensitivity depended on the contrast sensitivity differences between the two eyes. At low spatial frequencies, where the monocular sensitivity difference was minimal, binocular summation was obtained. As the sensitivity difference increased at higher spatial frequencies, the binocular contrast sensitivity decreased steadily until it reached a level below the sensitivity of the cataractous eye, demonstrating binocular inhibition. The clinical implications of binocular inhibition obtained with uniocular cataract are discussed.

Contrast adaptation and contrast gain control.

Abstract: Electrophysiological measurements have shown that contrast adaptation can increase the contrast gain of cortical cells of the cat and the monkey. This implies that adaptation could enhance the contrast discrimination sensitivity. Psychophysical contrast discrimination experiments were performed with and without contrast adaptation. The stimuli were spatially separated stationary Gabor patterns. The pedestal contrast was varied from 6 to 75 per cent. The spatial frequency was 1.5, 5.0 or 20 cpd. After adaptation the contrast detection thresholds are elevated and the subjective contrast is lowered. The contrast discrimination thresholds remain unchanged.

Recognition of motion-defined shapes in patients with multiple sclerosis and optic neuritis.

Abstract: We have developed a simple procedure for assessing the ability of the visual pathway to extract a two-dimensional shape from motion. The test requires a patient to read motion-defined (MD) letters. These letters differ physically from the familiar contrast-defined (CD) letters that are dimmer or brighter than their surroundings in that the boundaries of MD letters are rendered visible exclusively by a step in velocity while the boundaries of CD letters are rendered visible by a step in luminance. Subjects viewed a random pattern of bright dots containing a perfectly camouflaged letter. Then the letter was revealed by moving dots within and outside the letter at equal speeds in opposite directions. Letter reading scores for 50 eyes of 25 patients with multiple sclerosis (MS) or optic neuritis were compared with norms based on 50 control subjects. When tested with large (50 are min, i.e., 6/60) MD letters, 34/50 eyes of patients required abnormally high dot speeds to read letters, visual loss being sufficiently selective in 10 eyes that contrast sensitivity, Snellen acuity, 11%-contrast and 4%-contrast acuity were all spared. Four eyes were effectively motion blind in the sense that they could not read large letters even at our highest relative speed of 0.9 deg/s and the failure could not be attributed to reduced Snellen acuity. Our normal limit was 2.5 SD from the control mean and there were 1/50 false positives. Of the 34/50 eyes with elevated speed thresholds, 23 had normal Snellen acuities. The number of eyes abnormal for intermediate (11%) contrast CD letters, was 19/50 of which 8 had normal Snellen acuity, confirming our previous finding that MS can degrade the ability to see low-contrast objects while sparing Snellen acuity. We conclude that MD test letters can detect lesions that are not picked up by testing with CD test letters of high or low contrast. We suggest that the MD letter test can detect dysfunction in the human equivalent of a pathway in monkey brain that originates in large retinal ganglion cells, passes through the magnocellular layers of the lateral geniculate body, includes cortical area MT, and is involved in processing motion.

Contrast-sensitivity loss in a group of former microelectronics workers with normal visual acuity.

Abstract: The measurement of contrast sensitivity at varying grating frequencies is used increasingly to study visual and neural disorders. It provides more information than conventional acuity measures. Refractive errors initially affect high spatial frequencies, whereas lower spatial frequencies are affected only when these errors are pronounced. Neurophysiological alterations are reflected by depressed sensitivity to coarse gratings. Visual dysfunction has been associated with workplace exposures to a wide range of organic solvents. In microelectronics assembly where large quantities of organic solvents are used in many aspects of the work processes, visual deficits have been observed. The objective of the present study was to compare contrast sensitivity among former microelectronics assembly workers, with normal far and near visual acuity, and a reference group from the same region, with similar acuity. No significant differences were observed between scores at the two ends of the contrast sensitivity curves; however, at the intermediate spatial frequencies, the former microelectronics workers' scores were significantly lower (Student's t-test; p less than 0.05). For the microelectronics workers, no relation was observed between age and contrast sensitivity at any spatial frequency, whereas for the reference group, contrast sensitivity scores were progressively lower with age at spatial frequencies greater than or equal to 6.0 cpd (r2 = 0.15 at 6 cpd to r2 = 0.45 at 18.0 cpd), suggesting that for the former there is some form of interference with the expected contrast sensitivity loss with age. Lower contrast sensitivity scores in intermediate spatial frequencies, observed among the former microelectronics workers, possibly reflect neural alterations, which may have resulted from exposure to neurotoxic substances. These findings suggest the need for further studies on visual functions in microelectronics workers.