Assessment Of Contrast Sensitivity Research Articles

Differences in the legibility of letters at contrast threshold using the Pelli‐Robson chart

One disadvantage of using high-contrast letters as test objects when measuring visual acuity is the fact that they are not of equal legibility. A number of charts are now commercially available that assess contrast sensitivity using letter targets. This study attempted to assess the legibility of letters at contrast threshold on the Pelli-Robson letter contrast sensitivity chart by determining the percentage of correct responses for each of the ten Sloan letters at contrast threshold. Results of 493 contrast sensitivity measurements taken in optometric practice indicated that there is a definite difference in legibility between letters at contrast threshold as for letters at acuity threshold. The data suggest that the probability of correctly identifying two out of a group of three letters at threshold on the Pelli-Robson chart varies between 67% and 97% due to letter type alone. Because of the very regular and pronounced miscalling of the letter C as an O, we suggest that this should be accepted as a correct call during threshold measurements on the Pelli-Robson chart. This helps to balance the legibility of different groups of letters.

Clinical assessment of contrast sensitivity

Visual system transfers information at both high and low contrastlevels. It has been customary to assess visual function by measuring visual acuity although it has been well documented for over ten years' time that measurement of contrast sensitivity may reveal loss of visual function that is not detected by measurement of visual acuity.There are several commercially available clinical tests that are used in Scandinavian countries for assessment of visual transfer function in low contrast domain. We have chosen six of them for comparison: Cambridge Low Contrast Gratings, Vistech's contrast sensitivity tests for distance and near, PelliåRobson's low contrast letter chart, Vistest's LH‐5 contrast visual acuity test, and Mentor's computer based device for measurement of both contrast sensitivity and low contrast visual acuity.Each of the commercial tests measures low contrast vision in slightly different way. In order to have an overview of the function of these tests we have assessed vision of visually impaired patients and normally sighted subjects using all tests during one visit. In general the grating tests result inslightly higher contrast sensitivity values than the optotype tests. Contrast sensitivity values measured with grating tests are difficult to assess because the size of the stimulus affects the values.Based on our results, we recommend assessment of low contrast visual transfer using an optotype test as the basic test and a grating test in order to assess vision for orientation and mobility. The grating test should be used at two different distances resulting in approximately two and ten degree stimuli.

Contrast sensitivity in multiple sclerosis measured by Cambridge Low Contrast Gratings: a useful clinical test?

The Cambridge Low Contrast Gratings (CLCG) were used to assess contrast sensitivity (CS) in 39 patients with clinically definite multiple sclerosis and 60 control subjects. CS was determined in both horizontal and vertical orientations and compared with visual evoked responses (VERs) in the same populations. Only 33% of patients had abnormal CS whereas 82% had abnormal VERs. There was no correlation between the degree of abnormalities in CS and VERs. There was no significant difference between CS determined in the vertical and horizontal orientation; however, examination in the vertical orientation increased the number of abnormalities detected by 2 (5%). We conclude that although CS, using the CLCG, is abnormal in a proportion of multiple sclerosis patients its detection is not clinically useful and is insensitive as a measure of subclinical optic neuropathy in multiple sclerosis.

Contrast sensitivity changes in ocular hypertension and early glaucoma (summary)

The visual system is organized to distinguish spatial and temporal elements at various levels of contrast. This ability depends on the neural subsystems including the small X and larger Y ganglion cells of the retina. X cells have high spatial contrast sensitivity, but low temporal contrast sensitivity. Just the opposite is true for Y cells. Patients with glaucoma show reductions in both spatial and temporal contrast sensitivity, which suggests that both the X and Y systems are damaged by this disease. Static and dynamic tests are used to assess contrast sensitivity. Static contrast sensitivity tests measure the threshold contrast at which gratings of varying spatial frequency become visible. Dynamic or temporally modulated tests utilizing diffuse and counterphase flicker measure the threshold of visibility of a stimulus whose contrast varies cyclically, alternating between light and dark. Age, refractive error, pupil and stimulus size, and retinal eccentricity all affect the outcome of contrast sensitivity testing. The values obtained are also dependent on the psychophysical technique used to determine sensitivity. Early studies of contrast sensitivity in patients with chronic simple glaucoma (CSG) and ocular hypertension (OHT) have demonstrated spatial and temporal sensitivity losses.

Assessment of contrast sensitivity of patients with macular disease using reduced contrast near visual acuity charts.

A set of near Bailey-Lovie logMAR letter charts that varied in contrast from 0.40 dB (C = 0.95) to 58.0 dB (C = 0.001) were used to measure the middle and high spatial frequency range of the contrast sensitivity function (CSF) of 15 subjects with age-related maculopathy (ARM) and 15 age-matched normal subjects. The letter charts were shown to have good test-retest reliability. Compared with measures made using CRT generated square wave gratings of variable contrast and spatial frequency they were also shown to provide a valid measure of CSF in the mid to high spatial frequency range. The 20.0 dB letter chart alone was shown to be a good screening device for macular disease. The letter charts do not provide a measure of the peak of the CSF and a supplementary test of contrast sensitivity is needed to quantify contrast sensitivity at a low spatial frequency. Three measures are proposed as necessary to document foveal visual capability of patients with macular disease: distance logMAR visual acuity to measure high spatial frequency resolution, visual acuity with letter charts of 20 dB contrast to assess mid spatial frequency resolution; and contrast sensitivity for the detection of an edge to estimate contrast sensitivity for larger objects.

The objective assessment of visual contrast sensitivity by pattern reversal visual evoked potentials in diabetes

Recent studies have described abnormalities of visual evoked potentials and pattern electroretinography in diabetics without retinopathy. The visual contrast sensitivity, determined by psychophysical tests, has proved to be abnormal in diabetic patients with and without clinical retinopathy. In this study we evaluated contrast sensitivity function using both electrophysiologic and psychophysical methods. The objective assessment of functional visual contrast sensitivity was superior to psychophysical evaluation in the detection of contrast sensitivity alterations. No relationships were found between contrast sensitivity dysfunction and abnormalities of pattern electroretinography or fluorescein angiography. Our data suggest that functional visual deficits might precede background retinopathy and that the involvement of foveal function is early and very frequent in diabetic patients, even if they have normal visual acuity.

Rapid contrast sensitivity assessment in keratoconus.

Contrast sensitivity derangement may accompany keratoconus even in the presence of normal or near normal Snellen visual acuity. This has been demonstrated with computer-driven contrast sensitivity test devices. The purpose of this study was to evaluate the utility and efficacy of rapid screening devices for contrast sensitivity testing. However, such devices are often cumbersome and difficult to use clinically. We tested 12 patients with keratoconus on 2 simple chart systems designed to test contrast sensitivity in a rapid and clinically useful manner: the Vistech chart and the Regan multi-contrast visual acuity charts. Both devices detected the contrast sensitivity abnormalities present in early keratoconus, but some patients [with greater than or equal to 6/12 (20/40) visual acuity] were unable to respond to areas of the charts corresponding to high spatial frequency and/or low contrast tasks. Such simple wall charts may be useful in measuring the visual abnormalities in early keratoconus, in monitoring the progression of the disease, and in evaluating various treatment options.

Electrophysiological assessment of contrast sensitivity in human infants.

Contrast sensitivity functions (CSF's) were measured for a group of 6-month-old infants using the visual evoked potential (VEP). Sine-wave luminance gratings were counterphase modulated at 12 contrast reversals per s and simultaneously swept in contrast. Each contrast sweep lasted 10 s, spanning a range of 0.5 to 40% contrast in 19 equal logarithmic steps. The amplitude and phase of the response at the second harmonic were determined by a discrete Fourier transform. Contrast thresholds were estimated from a linear extrapolation to zero-amplitude of the VEP amplitude vs. log-contrast function. Contrast sensitivity was found to be nearly adult-like at 1 c/deg, but was not yet mature at higher spatial frequencies.

An anticipated threshold technique for measuring contrast sensitivity.

A new psychophysical procedure for assessing contrast sensitivity rapidly and reliably is reported. The technique consists of increasing the contrast of a conventional sine wave grating according to the parameters set in a computer implemented algorithm. During the first half of a trial, the contrast increases quickly to a point just below an expected threshold, and in the second stage the contrast increases slowly until a threshold is reached. Details and advantages of the algorithm are presented as well as a comparison of data generated by this and other techniques.

Contrast-coding in amblyopia. I. Differences in the neural basis of human amblyopia.

To shed light on the basis of normal contrast perception in general and its susceptibility during early visual development in particular the contrast-coding deficit of amblyopic eyes was investigated. This was accomplished by using two different but complementary paradigms, one involving equating the contrast sensations between the amblyopic and normal fellow eye and the other involving the assessment of incremental sensitivity at different contrast levels. Since human amblyopia is known to have three different forms, representatives of each were tested. These include strabismics, anisometropes and astigmats. The results of the contrast-matching approach suggest that (i) strabismic and anisometropic (including meridional) amblyopes show important differences in supra-threshold contrast matching (this difference, which occurs across different spatial frequencies, at different luminances and for different field sizes, suggests a different neural basis for the two main forms of human amblyopia); (ii) all forms of amblyopia share one common feature, that of large threshold losses relative to the extent of the suprathreshold anomaly; (iii) the accelerating growth in subjective contrast above the raised threshold shown by the matching procedure is not evident in the assessment of incremental contrast sensitivity. From these results it is argued that, although contrast perception (in the low contrast range) is disturbed in amblyopia, amblyopia cannot be adequately understood in these terms alone. This raises important questions for our understanding of the physiological basis of contrast-coding in normal vision and its perceptual importance.

The horse and zebras test in visual acuity and contrast sensitivity assessment

When horse outlines are filled in with square-wave gratings, they look like zebras when the grating is resolved by the eye. If, in front of the gratings, a progressively increasing number of veiling papers are put until the zebras seem to be horses, it is possible to draw a contrast sensitivity curve as a veiling resistance curve.

Contrast sensitivity of the human infant for moving and static patterns

A fixation preference method was used to assess contrast sensitivity of 1–3-month infants for stationary and drifting (3-Hz) sinusoidal gratings. The infants showed greater sensitivity for moving gratings at low spatial frequencies, but at their acuity limits there was no difference between static and moving stimuli. We conclude that the advantage of moving stimuli at low spatial frequencies reflects a differential sensitivity similar to that of the adult.

Interference filters in optics

The objective of this study was to use contrast sensitivity measurements to determine the effect of windshield salt film on driver vision. This effect simulates road salt spray that dries in place on a vehicle windshield during winter driving conditions.Ten emmetropic subjects were tested to assess contrast sensitivity functions through windshields that were clear compared with the same stimuli as viewed through windshields coated with salt film. Achromatic stimuli were generated by a VisionWorks system and were presented at 4 spatial frequencies (0.5, 3, 10, and 20 cycles per degree).A significant reduction in contrast sensitivity through the salt film was found at all spatial frequencies. In addition, reaction time to detect the stimuli was also found to be increased significantly at 2 of the 4 spatial frequencies through the saline-covered windshield.The results of this study indicate that windshield salt film reduces the ability to detect high and low contrast objects in a simulated driving environment as well as increases the subjects’ reaction times.

Liquid rheostats

The objective of this study was to use contrast sensitivity measurements to determine the effect of windshield salt film on driver vision. This effect simulates road salt spray that dries in place on a vehicle windshield during winter driving conditions.Ten emmetropic subjects were tested to assess contrast sensitivity functions through windshields that were clear compared with the same stimuli as viewed through windshields coated with salt film. Achromatic stimuli were generated by a VisionWorks system and were presented at 4 spatial frequencies (0.5, 3, 10, and 20 cycles per degree).A significant reduction in contrast sensitivity through the salt film was found at all spatial frequencies. In addition, reaction time to detect the stimuli was also found to be increased significantly at 2 of the 4 spatial frequencies through the saline-covered windshield.The results of this study indicate that windshield salt film reduces the ability to detect high and low contrast objects in a simulated driving environment as well as increases the subjects’ reaction times.