Maximum Contrast Sensitivity Research Articles

Spatial contrast sensitivity and grating acuity of barn owls

The eyes of barn owls (Tyto alba pratincola) display very little aberrations, and have thus excellent optical quality. In a series of behavioral experiments, we tested whether this presumably beneficial feature is also reflected at a perceptual level in this species. As fundamental indicators for visual performance, the spatial contrast sensitivity function (CSF) and grating acuity were measured in two barn owls with psychophysical techniques. Stimulus luminance was 2.7 cd/m(2). The CSF found here renders the typical band-limited, inverted U-shaped function, with a low maximum contrast sensitivity of 8-19 at a spatial frequency of 1 cyc/deg. Grating acuity was estimated from the CSF high frequency cut-off and yielded 3.0-3.7 cyc/deg. In a second experiment, in which contrast was held constant and spatial frequency was varied, grating acuity was measured directly (2.6-4.0 cyc/deg). These results put barn owls at the very low end of the visual acuity spectrum of birds, and demonstrate that visual resolution and sensitivity cannot be predicted by optical considerations alone.

Measuring Contrast Sensitivity Under Different Lighting Conditions: Comparison of Three Tests

The purpose of this study was to evaluate three psychophysical tests for the measurement of contrast sensitivity (CS) and disability glare (DG) at different luminance levels. In 60 eyes of 60 individuals (group 1: 20 healthy eyes of young individuals; group 2: 20 healthy eyes of elderly subjects; group 3: 20 eyes with nuclear cataract), CS with best correction was measured twice with the Frankfurt-Freiburg Contrast and Acuity Test System (FF-CATS) and the Functional Acuity Contrast Test (FACT, 1.5 cycles per degree [cpd]) at 167 cd/m2 and 0.167 cd/m2, and with the Pelli-Robson Chart (PRC) at 100 cd/m2 with and without glare. Repeatability of test and retest, and discriminative ability between the different subgroups, were assessed for CS values. Maximum CS values varied across tests. In all groups, highest CS values were obtained with the photopic FF-CATS. For FACT scores at 1.5 cpd, there was a ceiling effect for young subjects. CS scores obtained with the PRC were the lowest. The PRC had the best test-retest repeatability of all tests. Under mesopic conditions with glare, reliability was generally lower; the FF-CATS had the highest repeatability of the mesopic tests. The FF-CATS discriminated best between the different groups for all conditions. There are large discrepancies in the test results between CS testing methods, especially under different lighting conditions. Results from different CS tests are not interchangeable.

Optimization of detector pixel size for stent visualization in x‐ray fluoroscopy

Pixel size is of great interest in the flat-panel detector design because of its potential impact on image quality. In the particular case of angiographic x-ray fluoroscopy, small pixels are required in order to adequately visualize interventional devices such as guidewires and stents which have wire diameters as small as 200 and 50 microm, respectively. We used quantitative experimental and modeling techniques to investigate the optimal pixel size for imaging stents. Image quality was evaluated by the ability of subjects to perform two tasks: detect the presence of a stent and discriminate a partially deployed stent from a fully deployed one in synthetic images. With measurements at 50, 100, 200, and 300 microm, the 100 microm pixel size gave the maximum contrast sensitivity for the detection experiment with the idealized direct detector. For an idealized indirect detector with a scintillating layer, an optimal pixel size was obtained at 200 microm pixel size. A channelized human observer model predicted a peak at 150 and 170 microm, for the idealized direct and indirect detectors, respectively. With regard to the stent deployment task for both detector types, smaller pixel sizes are favored and there is a steep drop in performance with larger pixels. In general, with the increasing exposures, the model and measurements give the enhanced contrast sensitivities and a smaller optimal pixel size. The effects of electronic noise and fill factor were investigated using the model. We believe that the experimental results and human observer model predications can help guide the flat-panel detector design. In addition, the human observer model should work on the similar images and be applicable to the future model and actual flat-panel implementations.

Spatial contrast sensitivity of birds

Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. In previous studies, using a narrow range of species, birds have been reported to have low contrast detection thresholds relative to mammals and fishes. This was an unexpected finding because birds had been traditionally reported to have excellent visual acuity and color vision. This study reports CS in six species of birds that represent a range of visual adaptations to varying environments. The species studied were American kestrels (Falco sparverius), barn owls (Tyto alba), Japanese quail (Coturnix coturnix japonica), white Carneaux pigeons (Columba livia), starlings (Sturnus vulgaris), and red-bellied woodpeckers (Melanerpes carolinus). Contrast sensitivity functions (CSFs) were obtained from these birds using the pattern electroretinogram and compared with CSFs from the literature when possible. All of these species exhibited low CS relative to humans and most mammals, which suggests that low CS is a general characteristic of birds. Their low maximum CS may represent a trade-off of contrast detection for some other ecologically vital capacity such as UV detection or other aspects of their unique color vision.

Contrast sensitivity and reading in subjects with age-related macular degeneration

Thirteen subjects with age-related macular degeneration (AMD) participated in a study designed to investigate the predictive value of acuity and contrast sensitivity for reading. Compared with the normally sighted individuals, the spatial contrast sensitivity of all these AMD subjects was markedly reduced, particularly at the higher spatial frequencies. The difference was smaller at frequencies below 1 c/deg. For about half the subjects, maximum monocular contrast sensitivity was better than or equal to binocular sensitivity, indicating binocular inhibition or failure of summation. For the 11 subjects who were able to read at a luminance of 40cd/m², reading speed was correlated with various measures of contrast sensitivity and acuity. Of all the visual functions tested, maximum monocular contrast sensitivity and integrated monocular contrast sensitivity showed the best correlation with monocular reading speed. Binocular contrast sensitivity functions, monocular grating acuity, and monocular letter acuity ...

Assessment of contrast sensitivity in children aged 3 years 9 months – 6 years with normal vision, visual impairment due to ocular disease and strabismic amblyopia

The contrast sensitivity function was assessed in children and adults with normal vision, visual impairment due to ocular disease, and strabismic amblyopia. The methods used were: the LH contrast vision test and the ability to detect raisins, puffed rice and white and black sugar strands against a white and black background. The aim of the study was to see whether any of the tests could be used in the detection of subnormal vision, and to compare the results between the different groups of patients. The following parameters of contrast sensitivity function (CSF) were measured with the LH-test: the maximum contrast sensitivity, the total area of CSF, and the area of 8 spatial frequency bands. Difficulty in detecting an object with low versus high contrast was measured with the sugar strands. Subjects with subnormal vision due to ocular disease and strabismic amblyopia had lower contrast sensitivity than subjects with normal vision measured as the total area of the CSF or the area of a specific frequency band. However, the maximum contrast sensitivity value that could be measured with the test was not significantly different between the groups. No subject had difficulties in the detection of raisins and puffed rice on a white and black background. Difficulties in detecting sugar strands on a white background were not seen in the normal chil- dren, but approximately 65% of the children with visual impairment had difficulties, as did approximately 25% of the children and adults with strabismus. Most of these subjects also had low visual acuity, but there was no correlation between the level of reduction of visual acuity and the difficulty in detecting sugar strands.

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.

A comparison between different contrast sensitivity tests in the detection of amblyopia

Twenty adult patients with strabismic amblyopia were examined with seven different contrast sensitivity tests in order to determine which of these tests is the most sensitive in the detection of amblyopia. Both grating tests (Vistech, Arden, Cadwell, and Cambridge) and optotype tests (LH, Regan, and Pelli-Robson) were used. The following parameters of the contrast sensitivity function (CSF) were measured: maximum contrast sensitivity, total area of CSF and the area of 8 different spatial frequency bands in the CSF. Based on non-parametric tests, it was found that separations of the normal from the amblyopic eye by means of CSF could be done with:- maximum contrast sensitivity using the Vistech test;- total CSF area using all tests except the Cadwell test;- area of frequency bands (within 1–16 cycles per degree) using Vistech, Pelli-Robson, and the Arden tests;- maximum spatial frequency for tests with a fixed contrast using the LH and Regan tests.

Night driving: effects of glare from vehicle headlights on motion perception

Elderly drivers often experience disability glare at night from the headlights of oncoming vehicles. To assess the effect of glare from vehicle headlights on visual performance for seeing moving targets, experiments were performed at night on a dimly lit road with observers seated in a stationary motor car viewing a computer-generated stimulus display at a distance of 23 m (the stopping distance for 50 kph). The display was set 2 m to the side of a second stationary car whose position on the road was that of an oncoming vehicle with respect to the observer. The headlights of the observer's car were on low-beam while those of those of the opposing car were switched off (contro condition), on lpw-beam or on high-beam. Experiments were performed using mean display luminances of 50 cd/m2 and 0.5 cd/m2. Spatial contrast sensitivity functions for the directional discrimination of drifting (8 Hz sinusoidal gratings were measured using three different viewing conditions: normal vision (binocular visual acuity (BVA) = 6/6); blurred vision (BVA = 6/9-); and simulated intraocular lens opacities (BVA = 6/6-). The data were fitted with an exponential function, which was extrapolated to 100% contrast to estimate dynamic visual acuity. The results show that simulated lens opacities, which have little or no effect on standard day time measures of visual acuity, have a marked effect on night-time measures of contrast sensitivity for moving targets. Taking into account the average luminance of objects lit by road lighting, we estimate that high-beam glare reduces maximum contrast sensitivity by an order of magnitude in persons affected by mild lens opacities, giving a dynamic acuity of 1.0 c/deg (6/180 Snellen equivalent) or less. From this and other studies we argue that there is now a strong case for the introduction of vehicle-licensing sight re-testing at regular intervals in the UK. In addition, we suggest that vehicle-licensing authorities consider the feasibility of introducing sight tests under night-time driving conditions.

Modelling contrast sensitivity as a function of retinal illuminance and grating area

We extended the contrast detection model of human vision [Rovamo, Luntinen & Näsänen (1993b) Vision Research, 33, 2773-2788] to low light levels by taking into account the effect of light-dependent quantal noise. The extended model comprises (i) low-pass filtering due to the optical modulation transfer function of the eye, (ii) addition of light-dependent noise at the event of quantal absorption, (iii) high-pass filtering of neural origin (lateral inhibition), (iv) addition of internal neural noise, and (v) detection by a local matched filter whose efficiency decreases with increasing grating area. To test the model we measured foveal contrast sensitivity as a function of retinal illuminance and grating area at spatial frequencies of 0.125-32 c/deg. In agreement with the model, monocular contrast sensitivity at all grating areas increased in proportion to I when retinal illuminance (I) was smaller than critical illuminance. Thereafter the increase saturated and contrast sensitivity became independent of retinal illuminance. Similarly, at all levels of retinal illuminance contrast sensitivity increased in proportion to A when grating area (A) was smaller than critical area. Thereafter the increase saturated and contrast sensitivity became independent of area. Critical level of retinal illuminance increased in proportion to the spatial frequency squared. Critical area marking the saturation of spatial integration was constant at low spatial frequencies but decreased in inverse proportion to spatial frequency squared at medium and high spatial frequencies. The maximum contrast sensitivity obtainable by spatial integration in bright light increased at low spatial frequencies in proportion to spatial frequency, was constant at medium spatial frequencies, and decreased in inverse proportion to spatial frequency cubed at high spatial frequencies. The increase was due to the neural modulation transfer function of the visual pathways whereas the decrease was due to the optical modulation transfer function of the eye. The model explained 91-99% of the total variance of our contrast sensitivity data at various spatial frequencies.

Modelling the dependence of contrast sensitivity on grating area and spatial frequency

We modelled the human foveal visual system in a detection task as a simple image processor comprising (i) low-pass filtering due to the optical transfer function of the eye, (ii) high-pass filtering of neural origin, (iii) addition of internal neural noise, and (iv) detection by a local matched filter. Its detection efficiency for gratings was constant up to a critical area but then decreased with increasing area. To test the model we measured Michelson contrast sensitivity as a function of grating area at spatial frequencies of 0.125-32 c/deg for simple vertical and circular cosine gratings. In circular gratings luminance was sinusoidally modulated as a function of the radius of the grating field. In agreement with the model, contrast sensitivity at all spatial frequencies increased in proportion to the square-root of grating area at small areas. When grating area exceeded critical area, the increase saturated and contrast sensitivity became independent of area at large grating areas. Spatial integration thus obeyed Piper's law at small grating areas. The critical area of spatial integration, marking the cessation of Piper's law, was constant in solid degrees at low spatial frequencies but inversely proportional to spatial frequency squared at medium and high spatial frequencies. At low spatial frequencies the maximum contrast sensitivity obtainable by spatial integration increased in proportion to spatial frequency but at high spatial frequencies it decreased in proportion to the cube of the increasing spatial frequency. The increase was due to high-pass filtering of neural origin (lateral inhibition) and the decrease was mainly due to the optical transfer function of the eye. Our model explained 95% of the total variance of the contrast sensitivity data.

On Equivalent Sphere Illumination

Under the present equivalent sphere illumination (ESI) system, the actual visibility of a visual task is not known. The author recommends that for every different task and viewing angle, the maximum contrast sensitivity of the task be published; thus, one can determine the ESI required for any given task in order to achieve a certain visibility, or one can obtain task visibility by knowing its ESI value. This broadens the application of the ESI system by enabling conversion between ESI and visibility for a specific task and also conversions among different tasks and viewing angles. Moreover, differences in individual contrast sensitivity response may result in different ESI and visibility from the same task, with the same viewing angle, at the same location under the same luminous environment.