Low Contrast Sensitivity Research Articles

Electron Microscopic Tomography of Cellular Organelles: Chemical Fixation Vs. Cryo-Substitution of Rat-Liver Mitochondria

Abstract Electron microscopic tomography is proving to be a valuable tool for investigating the 3D structure and organization of cellular organelles. Important progress is being made in the application of the technique to frozen-hydrated material, but it is likely that success with thick specimens will be limited by the low contrast and beam sensitivity of naked biological material. Thus, optimizing procedures for fixing, embedding, staining, and selectively labelling cells for 3D electron microscopy remains a priority. Tomography of chemically fixed and plastic-embedded rat-liver tissue and isolated mitochondria has shown that the cristae (the invaginations of the mitochondrial inner membrane) are pleiomorphic and connected to each other and to the surface of the inner membrane by tubular regions 30-40 nm in diameter. This basic design feature has important implications for the microcompartmentation of ions and molecules within this organelle.

Do older drivers with visual and cognitive impairments drive less?

To determine whether older drivers with poorer cognitive and/or visual function drive fewer miles or avoid driving in situations that pose higher crash risks, such as at nighttime, in rush hour traffic, or when weather conditions are bad. A cross-sectional data analysis conducted as part of a larger prospective study. A total of 3238 drivers aged 65 and older applying for renewal of their driver's license at one of eight participating North Carolina driver's license offices. Subjects were administered a battery of brief tests of cognitive and visual function, which included the Trail Making Test Parts A and B, the Short Blessed Orientation-Memory-Concentration test, and measures of high and low contrast visual acuity, contrast sensitivity, and peripheral vision. Participants were also asked to complete a brief driving survey containing questions about the number of miles they drove and whether they avoided driving under certain conditions, such as after dark or on busy, multi-lane roadways. Driver age and gender were covariates in the analyses. Results of multivariate logistic regression models show a clear pattern of reduced driving exposure--lower annual miles and greater avoidance of high-risk driving situations--associated with lower levels of cognitive and visual function. In general, the prevalence odds of reduced driving exposure were higher for the cognitive function variables than for the visual function variables, and higher for males than for females. Men who scored in the lowest quartile of performance on one of the cognitive tests were six to seven times more likely to report driving fewer than 3000 miles a year than were men scoring in the highest quartile, and women with low scores were one-and-one-half to two times more likely to report driving less than 3000 miles than women with higher scores. While the findings of this study are reassuring, they do not guarantee that all drivers with cognitive and visual impairments are limiting their driving exposure appropriately, and geriatricians and other health professionals should be encouraged to evaluate their patients' cognitive and visual fitness for driving and provide counsel where indicated.

Assessment of contrast sensitivity in kittens after the critical developmental period.

Spatial contrast sensitivity was measured in kittens aged 6, 9, and 12 months and in adult cats. Cats had to open one of two small windows, which had a photograph of a grid, in order to obtain food reinforcement. The nonreinforced stimulus was a photograph of a uniform field of the same mean luminance. Visual acuity was constant in kittens aged 6 to 12 months. However, six-month-old kittens had low contrast sensitivity at low spatial frequencies (< 0.6 cycles/degree). At the age of nine months, contrast sensitivity over this range increased, though the level seen in adult cats was reached only at the age of 12 months. It is suggested that the increase in contrast sensitivity occurring after the critical developmental period in kittens reflects maturation of higher-order cortical fields involved in the process of recognition.

Utility of low mA 1.5 pitch helical versus conventional high mA abdominal ct

The objective of this study was to evaluate the utility of a low mA 1.5 pitch helical versus conventional high mA conventional technique in abdominal computed tomography (CT). Twenty-five patients who had both a conventional high mA (>300) and a 1.5 pitch low mA (80–125) helical CT within 3 months were selected for inclusion in the study. Patients were excluded who had a significant change in pathology between the two studies. The other parameters (injection rate, contrast type and volume, and filming window/level) were constant. The studies were randomized and blinded to five separate experienced readers who graded the studies by a variety of normal anatomical structures and pathological criteria. Overview questions also assessed noise, resolution, contrast, and overall quality. The abdominal wall/retroperitoneum and hiatal hernias were statistically better visualized on the conventional high mA studies. However, for all other normal anatomical and pathological sites, there was equivalent or better visualization on the helical versus the conventional CT examinations. The resolution of the helical studies was graded statistically better than the high mA conventional CT scans as was the amount of noise present on the images. While there was some advantage for conventional high mA CT with respect to contrast enhancement and low contrast sensitivity, these differences were not statistically significant. It appears from the data of this study that a low mA technique in evaluating the abdomen may be a useful option in performing routine abdominal CT. The radiation dose savings to the patient is significant and there appears to be little degradation of image quality using a low mA 1.5 helical versus mA conventional CT technique.

Extended pitch thoracic helical ct 47

The objective of this study was to test whether extended 1.5 pitch helical computed tomography (CT) can be used for routine thoracic CT without a significant loss of clinical scan quality. Thirty consecutive patients presenting for contrast thoracic CT were computer randomized into one of three groups: conventional, 1.0 pitch helical, and 1.5 pitch helical. All other variables, including kV, mA, slice thickness and reconstruction interval, and contrast administration, were kept constant. The studies were randomized to five independent, blinded, experienced radiologists who rated visualization 25 normal structures, and up to five pathologic findings per patient. In addition, each reader evaluated the studies' contrast enhancement, low contrast sensitivity, linear resolution, motion artifact, noise, and overall quality. The visualization score for all normal and overall for pathological lesions did not vary between groups. The three groups were not equivalent for several individual pathologic categories. However, these differences were not consistently in favor of one technique over the other two. The overall score for scan quality was not significantly different between the three groups. Extended 1.5 pitch thoracic helical CT provides equivalent quality versus either 1.0 pitch helical or conventional CT. The use of 1.5 pitch helical thoracic CT allows faster scanning, greater patient coverage, and the use of reduced amounts of intravenous contrast.

The separation of parallel visual systems by disease processes.

to investigate psychophysics and electrophysiology in disease states with altered retinal function. determination of colour and luminance contrast sensitivity functions using computer controlled VDUs: recording of flash and pattern ERGs to short and long flashes of coloured lights under differing conditions of light adaptation: recording of pattern ERG. In melanoma-associated retinopathy (MAR) low spatial frequency temporal flicker loss occurs for achromatic Gaussians, but colour Gaussians are seen normally. Low spatial frequency luminance contrast sensitivity and motion losses are severe while red-green gratings are seen normally. In Cuban Tropical Amblyopia, achromatic luminance contrast sensitivity may be normal, in the presence of considerable losses of colour vision, to spatial frequencies as high as 32 c/degree. There are supernormal cone ERGs. these 2 conditions represent highly selective loss of 'M' and 'P' pathways respectively. Almost the entire 'bell shaped curve' normally represents M activity. In CTA, there may be a selective loss of the receptive field surrounds of P ganglion cells.

Improvements in clinical and functional vision and perceived visual disability after first and second eye cataract surgery

AIMSTo determine the improvements in clinical and functional vision and perceived visual disability after first and second eye cataract surgery.METHODSClinical vision (monocular and binocular high and low contrast visual acuity,...

A comprehensive assessment of visual impairment in a population of older Americans: the SEE study.

Results. Visual function decreased linearly with age for the acuity, contrast sensitivity, glare, and visual field tests. Stereoacuity remained constant into the mid-70s and declined at an accelerating rate thereafter. Black participants had lower contrast sensitivity, reduced stereoacuity, and worse visual fields, at all ages compared to white participants; however, white participants were more sensitive to glare. The overall prevalence of visual acuity impairment in blacks was 5.6% versus 3.0% for whites, using the traditional United States definition (worse than 20/40 to better than 20/200) and 3.3% for blacks versus 1.6% for whites, using the World Health Organization definition (worse than 20/60 to 20/400). Acuity was correlated moderately with contrast sensitivity, stereoacuity, and visual fields (Spearman rho = 0.50, 0.35, and 0.34, respectively). The correlation between acuity and glare sensitivity was low (rho = 0.12).

Nationwide survey of fluoroscopy: radiation dose and image quality.

To determine the average abdominal entrance air kerma, low-contrast sensitivity, and spatial resolution in upper gastrointestinal tract fluoroscopy in the United States. A random sample of fluoroscopic facilities was selected to be surveyed for the Nationwide Evaluation of X-ray Trends program. Measurements were performed by using a newly developed fluoroscopic phantom. The surveys were conducted by state radiation control personnel. Average air kerma rates 1 cm above the tabletop, free in air, were 43 mGy/min (n = 340). The rate increased to 64 mGy/min when a 1.6-mm-thick copper filter, which simulated the use of barium contrast medium, was added to increase attenuation. The average entrance air kerma, free in air, for radiographs was 3.4 mGy, and an average of 12 radiographs were obtained per examination. Of 352 facilities surveyed, 306 (87%) were able to resolve wire mesh with 20 or more lines per inch. Of 339 facilities for which percentage contrast could be calculated, 192 (57%) had minimum percentage contrast values of 4% or more. Spatial resolution for fluoroscopy is adequate for most of the facilities surveyed, but a substantial proportion of facilities could not visualize low-contrast test objects, which strongly suggests image quality problems.

Development of contrast sensitivity and temporal-frequency selectivity in primate lateral geniculate nucleus.

We studied the development of spatial contrast-sensitivity and temporal-frequency selectivity for neurons in the monkey lateral geniculate nucleus. During postnatal week 1, the spatial properties of P-cells and M-cells are hardly distinguishable, with low contrast-sensitivity, sluggish responses, and poor spatial resolution. The acuity of P-cells improves progressively until at least 8 months, but there is no obvious increase in their maximum contrast-sensitivity with age. The contrast sensitivity of M-cells is already clearly higher than that of P-cells by 2 months, and at 8 months of age this characteristic difference between M- and P-cells approaches the adult pattern. There is a major increase in responsiveness during the first 2 postnatal months, especially for M-cells, the peak firing rate of which rises fivefold, on average, between birth and 2 months. Many P-cells in the neonatal and 2-month-old animals did not give statistically reliable responses to achromatic gratings, even at the highest contrasts: this unresponsiveness of P-cells might result from low gain and/or chromatic opponency. The upper limit of temporal resolution in the neonate is low--about one-third of that in the adult. Among M-cells, the improvement in temporal resolution, like that in contrast sensitivity, is rapid over the first 2 months, followed by a slower change approaching the adult value by 8 months of age. The development of contrast sensitivity, responsiveness and temporal tuning are little affected, if at all, by binocular deprivation of pattern vision from birth for even a prolonged period.

The analysis of the transient PERGs to contrast variation

Black and white checkerboards (56′) of varying contrast levels were presented in the transient reversal mode (2 Hz). The transient Pattern Electroretinograms were recorded on four subjects with normal vision using DTL fibres. Across the contrast range 9–80%, the amplitudes of the response were linearly sensitive to contrast variation without saturation (P &lt; 0.001). Two potentials on the major negative potential (N95) divided by a notch had different contrast sensitivities and contrast gains: the potential after the notch (a‐Nw) was strikingly less sensitive to contrast variation and had a lower contrast gain than the potential before the notch (Nw). a‐Nw had a linearity at contrast level over 23%. Nw showed a high similarity to P50 in both contrast versus amplitude function and contrast gain.The evident difference of the contrast gains between Nw and a‐Nw suggests that there are possibly some corresponding differences in their retinal origins. It is assumed that the major negative potential after the notch is functioned by the inner retinal neurones with low contrast sensitivity, which includes the ganglion cells, being possibly predominated by the retinal magnocellular cells; the potential Nw has some common retinal mechanisms with P50, probably representing the macular function.

The analysis of the transient PERGs to contrast variation

Black and white checkerboards (56′) of varying contrast levels were presented in the transient reversal mode (2 Hz). The transient Pattern Electroretinograms were recorded on four subjects with normal vision using DTL fibres. Across the contrast range 9–80%, the amplitudes of the response were linearly sensitive to contrast variation without saturation (P < 0.001). Two potentials on the major negative potential (N95) divided by a notch had different contrast sensitivities and contrast gains: the potential after the notch (a-Nw) was strikingly less sensitive to contrast variation and had a lower contrast gain than the potential before the notch (Nw). a-Nw had a linearity at contrast level over 23%. Nw showed a high similarity to P50 in both contrast versus amplitude function and contrast gain. The evident difference of the contrast gains between Nw and a-Nw suggests that there are possibly some corresponding differences in their retinal origins. It is assumed that the major negative potential after the notch is functioned by the inner retinal neurones with low contrast sensitivity, which includes the ganglion cells, being possibly predominated by the retinal magnocellular cells; the potential Nw has some common retinal mechanisms with P50, probably representing the macular function.

Vision in dementia: Contrast effects

Contrast sensitivity has been shown to be affected in Alzheimer's disease (Ad). We investigated low contrast acuity and contrast sensitivity using clinical test charts in this patient population. Additionally, we tested patients with vascular dementia (vd) and mixed dementia (md), (Alzheimer' with vascular dementia). Contrast sensitivity was assessed using the Vistech VCTS 6500 test chart. Low contrast acuity was measured using the Regan charts at four contrast levels (96%, 50%, 25% and 11%). The patient population consisted of 19 Ad patients, 9 vd patients and 10 md patients. Reduction in acuity was found with contrast level in all cases. Regression lines were fit to the data and statistical analysis was performed. We did not find a statistically significant difference between the Ad and vd or md groups. We did, however, find a difference between the vd and md groups. We did find reduction in contrast sensitivity at all spatial frequencies when compared to the elderly normal. Correspondingly, we found a significant difference in acuity when compared with normal data at the four contrast levels tested. Acuity is reduced with contrast in all patient groups. Our contrast sensitivity results are similar to those reported in the literature. This study points out the importance of using simple clinical test charts and further underscores the idea that there is a primary visual deficit in Ad.

Retinal illuminance and contrast sensitivity in human infants

Several investigators have related infants' low contrast sensitivity to immaturities in the optics and receptor lattice of the immature eye. A critical element in the modeling is how much the lower photon catch of the immature retina reduces sensitivity; the assumptions vary from square-root to Weber's law and lead to very different modeling outcomes. We measured the relationship between retinal illuminance and contrast sensitivity at different spatial frequencies. The sweep visual-evoked potential was used to measure thresholds in 2- and 3-month olds and adults over a 2.5-log-unit range of illuminances. The contrast threshold vs illuminance functions were fit by power functions. The best-fitting exponents for adults were about −0.5 at higher spatial frequencies (consistent with square-root law) and lower at lower frequencies. The best-fitting exponents for 2- and 3-month olds were −0.2 to −0.35 which indicates that threshold is less affected by changes in illuminance than is the case in adults. These results suggest that none of the models relating optical and receptoral immaturities to infants' spatial vision has assumed an appropriate relationship between lower photon catch and contrast sensitivity. Once the models are modified to incorporate the relationship obtained in the present experiment, the predictions fall well short of explaining 2-month old's low contrast sensitivity.

Sensory impairments and physical disability in aged women living at home.

Studies of the impact of visual or hearing impairments on physical disability in older people have provided conflicting results. Furthermore, the consequences of the loss of such visual functions as depth perception or contrast sensitivity have rarely been studied. We examined the relationship of visual acuity, depth perception, contrast sensitivity, and hearing difficulty to the ability of older women living at home to accomplish instrumental activities of daily living independently. Data on self-reported physical disability and hearing impairment, as well as objective measures of functional vision and physical ability were collected from a sample of 1210 community-dwelling women aged 75 years and older. Multivariate logistic regression modelling was used to assess the strength of the association between physical disability and sensory impairments, controlling for age, education level, motor limitations, and prevalent chronic diseases. Women with low visual acuity or low contrast sensitivity were significantly more likely to be physically dependent than women with good vision. Contrast sensitivity was, however, a better predictor than functional acuity, after controlling for age, education level, motor limitations and chronic medical conditions (adjusted odds ratio: 5.1; 95% confidence interval: 2.0-12.9). Depth perception was not related to physical disability. Women with serious hearing difficulty had significantly increased odds of dependency (4.1; 1. 4-12.1). Severe sensory impairments are strongly related to physical dependency in older women. It may be useful to add a test of contrast sensitivity to the traditional acuity test to predict better which elderly individuals may have difficulty carrying out routine daily activities.

Contrast Sensitivity and Glare Disability in Different IOL Types After Clear-corneal Cataract Surgery

OBJECTIVE: The effect of one PMMA and, two different silicone IOLs on contrast sensitivity and glare disability was evaluated in a selected series of patients (best cases) following clearcorneal incision phacoemulsification. STUDY DESIGN: Prospective clinical study of visual function 6 months postoperatively. SETTING: Department of Ophthalmology, University of Giessen, Germany. PATIENTS: Forty-two patients with visual acuity (VA) of 0.8 (logMAR unit) or better. MAIN OUTCOME MEASURES: Contrast sensitivity (CS) under photopic conditions using a sinusoidal grating pattern wall chart (Vistech Consultants, Dayton, Ohio); CS and glare disability under mesopic conditions using the Mesotest (Oculus, Wetzlar, Germany). RESULTS: No significant difference in CS and glare disability between the different IOL groups can be demonstrated. Differentiating according to age showed that patients in the 70-and-over age group had significantly lower CS values compared with patients in the 70-and-below age group. No difference in glare disability between the age groups could be determined. CONCLUSIONS: Examination of visual function by means of contrast sensitivity and glare disability testing after successful clear-corneal cataract surgery shows no difference between the different silicone IOLs and PMMA IOL, respectively. Patient age is a more important factor in CS than IOL-type and material in good VA and absence of other ocular pathology.

Isolation and characteristics of a steady-state visually-evoked potential in humans related to the motion of a stimulus

We have examined the visual potential evoked by two motion stimuli. In the first stimulus (termed coherent motion) a random-dot pattern oscillated between phases of coherent and incoherent (“snowstorm”) motion, and in the second a random-dot pattern alternated in direction of motion (termed direction change). We found that the response to the coherent motion stimulus is low-pass with respect to speed, has low contrast sensitivity and increases steadily with the contrast of the stimuli. The direction change visually-evoked potential (VEP) is band-pass with respect to speed, has high contrast sensitivity but then saturates and even reduces as the stimulus contrast is raised above 0.1. The behaviour of the direction change VEP is similar in nature to results from psychophysical experiments of motion perception and to the known properties of directionally selective cells of the cortex. On the other hand the behaviour of the coherent motion VEP suggests this may not be mediated by a mechanism specific to motion.

Contrast Sensitivity Differences Between Proficient and Disabled Readers Using Colored Lenses

Colored overlays or lenses (e.g., Irlen lenses) have been used in attempts to remediate reading difficulties. The present study included four middle-socioeconomic status (SES) adults and four middle-SES children with reading disabilities as well as an equal number of nondisabled readers of the same age groups and SES. Examined were (a) the relationship of wavelength (lens color) to visual grating performance, (b) the effect of reading disability on performance with each lens-color/luminosity-grating combination, (c) group performances on a visual detection task with the clear and chromatic lenses, and (d) peripheral retinal brightness thresholds. The spatial frequency of the gratings (and not the lens color) permitted subjects with reading disabilities to be differentiated from the proficient readers. Subjects with reading disabilities displayed significantly lower contrast sensitivity when tested with sine-wave gratings, as well as displaying higher brightness thresholds in the peripheral retina.

Test-Retest Variability and Correlations between Tests of Texture Processing, Motion Processing, Visual Acuity, and Contrast Sensitivity

To compare the test-retest variability (reliability) and the relations among clinical tests of texture and motion processing, visual acuity for high- and low-contrast letters, and the Pelli-Robson contrast sensitivity test. In 20 normally sighted subjects, monocular visual acuity for letters of 96% and 11% contrast, Pelli-Robson contrast sensitivity, and motion-defined and texture-defined letter recognition thresholds were measured on each of two different days. Test-retest correlation coefficients were 0.75, 0.91, 0.61, 0.90, and 0.84 and bivariate test-retest regression slopes were 1.0, 1.1, 0.8, 1.0, and 1.2 for high- and low-contrast acuity, contrast sensitivity, and motion and texture processing, respectively. The inter-test correlations with both test and retest significant were as follows: visual acuity for high-contrast letters vs. visual acuity for low-contrast letters; and recognition threshold for texture-defined letters vs. acuity for letters of both high and low contrast. Test-retest variability for the tests of motion and texture processing were at least as low as for established clinical tests of high and low contrast acuity and contrast sensitivity. We conclude that these new tests offer a reliable means of obtaining clinical information which complements that provided by conventional tests with luminance-defined letters.

Visual Changes in Human Immuno-Deficiency Virus Infection

Many Human Immuno-Deficiency Virus (HIV) patients develop ocular involvement during the course of the disease, including HIV retinopathy and cytomegalovirus (CMV) retinitis. It is well established that contrast sensitivity and color vision may be affected in other retinal diseases, such as diabetes, before obvious signs and symptoms. We therefore examined patients with early HIV disease for visual involvement. Subjects consisted of 19 HIV-positive patients and 15 controls. None of the HIV-positive patients showed any signs of HIV retinopathy. High and low contrast visual acuity, luminance contrast sensitivity (CS), short wavelength cone (S-cone) CS, and color vision were assessed in the right eye of each subject. S-cone CS was significantly reduced in the HIV-positive group (HIV mean = 0.91 +/- 0.15 log CS; normal mean = 1.10 +/- 0.09 log CS, t = 4.19, p < 0.001). Consistent with this finding, four of the HIV-positive patients demonstrated tritanopic D-15 results. High and low contrast visual acuity and CS were not significantly different in the HIV-positive and control groups. These findings indicate that HIV-positive patients can have S-cone/tritanopic abnormalities despite normal appearing fundi and that there may be damage to the visual system early in HIV infection.