Showing papers on "Contrast (vision) published in 1986"

Functions of the ON and OFF channels of the visual system.

Abstract: In the mammalian eye, the ON-centre and OFF-centre retinal ganglion cells form two major pathways projecting to central visual structures from the retina. These two pathways originate at the bipolar cell level: one class of bipolar cells becomes hyperpolarized in response to light, as do all photoreceptor cells, and the other class becomes depolarized on exposure to light, thereby inverting the receptor signal. It has recently become possible to examine the functional role of the ON-pathway in vision by selectively blocking it at the bipolar cell level using the glutamate neurotransmitter analogue 2-amino-4-phosphonobutyrate (APB)1. APB application to monkey, cat and rabbit retinas abolishes ON responses in retinal ganglion cells, the lateral geniculate nucleus and the visual cortex but has no effect on the centre-surround antagonism of OFF cells or the orientation and direction selectivities in the cortex2-5. These and related findings6-11 suggest that the ON and OFF pathways remain largely separate through the lateral geniculate nucleus and that in the cortex, contrary to some hypotheses, they are not directly involved in mechanisms giving rise to orientation and direction selectivities. We have examined the roles of the ON and OFF channels in vision in rhesus monkeys trained to do visual detection and discrimination tasks. We report here that the ON channel is reversibly blocked by injection of APB into the vitreous. Detection of light increment but not of light decrement is severely impaired, and there is a pronounced loss in contrast sensitivity. The perception of shape, colour, flicker, movement and stereo images is only mildly impaired, but longer times are required for their discrimination. Our results suggest that two reasons that the mammalian visual system has both ON and OFF channels is to yield equal sensitivity and rapid information transfer for both incremental and decremental light stimuli and to facilitate high contrast sensitivity.

Mach bands are phase dependent

Abstract: The prevailing explanation of Mach bands, the paradoxical bands of light and dark seen where luminance gradients meet plateaux, is that they are due to lateral inhibition in the visual system1–4. This explanation equates Mach bands with distortions in a processed luminance distribution due to selective attenuation of low frequency components. But square waveforms exhibit no Mach bands2,5,6, although they should also be distorted after processing. Measurements of the contrast required to see Mach bands in trapezoidal waveforms and manipulations of their spectra lead us to conclude that phase relationships between Fourier components are important to the structure we perceive. A model based on the odd and even symmetry of visual receptive fields explains our results.

New look at Bloch's law for contrast.

Abstract: It has been commonly reported that the temporal integration of grating contrast proceeds more slowly as spatial frequency is increased. Such results have been based on the critical duration for sensitivity to contrast pulses varying in duration, but the analyses have not assumed full integration at short durations and have neglected the effects of probability summation over time. To take such effects into account, we discuss a class of analytical models based on nonlinear temporal integration. On the assumption that the temporal impulse response of the visual system determines contrast integration over time, we develop both a high-threshold model and a signal-detection approach involving multiple and independent nonlinear signal detectors with a time-limited integration span. The redefined critical durations predicted by the models and verified by the data are about 35 msec and vary by no more than 10 msec across spatial frequency. This variation is entirely attributable to a change in the strength of inhibition with spatial frequency, and the analysis implies that the excitatory component is constant at all spatial frequencies, contrary to previous accounts.

Impairment of contrast sensitivity function (CSF) as a measure of disability glare.

Abstract: A method for quantitative measurements of disability glare in clinical practice is presented. Glare is induced by a circular fluorescent tube which surrounds a sinusoidal grating displayed on a monitor. The threshold contrast that is needed for detection of the grating is measured with and without presence of the glare light. This is repeated for several different spatial frequencies. The discrepancy between the contrast sensitivity function (CSF) obtained with and without glare light was used to calculate the glare score (n) as a measure of disability glare. This was done for normals and cataract patients. The results show that an increasing glare score is related to an increase in turbidity of the optic media, while visual acuity had a weak correlation to the glare score. We also studied the relation glare score versus luminance and found that normals had a glare score that was almost independent of luminance level, while the cataract patients had a marked decrease in glare sensitivity when the luminance decreased. Most patients had a glare score that corresponded to their glare problems. These findings indicate a potential for using this psychophysical disability glare test method in industry, transport, and clinical ophthalmology. Some sources of methodological error inherent in the test method are evaluated and discussed.

A Theory of Insect Vision: Velocity Parallax

Abstract: Many insects show by their behaviour that they detect visually the existence of separate objects. The experimental material to analyse how they perceive objects is provided by an insect that walks to the end of a stick; then, because it has no alternative, it reaches with a foreleg towards a neighbouring object that it perceives to be within range. Some insects make horizontal peering movements as an aid to vision. The peering motion is exactly appropriate for generating an apparent velocity of nearby objects relative to the background. These experiments, when put together with the known properties of optic lobe neurons, suggest that a mechanism based on velocity parallax projected to the horizontal plane accounts for much insect visual behavour. Velocity parallax is defined as the discrepancy seen at the edge of an object against a distant background when the eye moves laterally. On this theory, perception of an object is inseparable from the local detection of velocity differences. The background may not be `perceived' at all when an object occurs in the foreground. The postulated mechanism is a two- or three-stage feedback, in which the perceived velocity (or, more accurately, the spatially correlated contrast frequency) in small-field motion-perception units is reduced by the averaged contrast frequency in larger fields, which feed back upon them. Contrast frequency is defined as the frequency of the flicker that is generated by a pattern moving across the eye. An alternative mechanism to the feedback of the velocity signal with lateral spread is adaptation to the local average background velocity, while sensitivity to a smaller local change in velocity is retained. That idea comes from recent work on the H1 neuron in the fly optic lobe, and could be the basis of a primitive form vision that, if present in medium-field neurons, is adequate for the whole of the normal visual behaviour of a freely moving insect. These speculations invite a variety of experimental tests, ranging from visual discrimination tests with bees that are shown the velocity parallax situation, to appropriate stimulation of optic lobe neurons, to simulation of a visual processing system that relies on velocity parallax cues to detect objects.

Colour and luminance vision in human optic neuritis

Abstract: A comparison of sensitivities to chromatic and luminance stimuli has been carried out in patients with a past history of optic neuritis Patients were selected with differing degrees of stable residual visual deficits, and with marked interocular differences in sensitivity. Threshold contrast sensitivity was measured to sinusoidal luminance gratings and to chromatic red/green and blue/yellow gratings, all with the same spatial frequency of 1 cycle per degree. A psychophysical criterion was used to ensure that detection of the chromatic grating was determined only by its colour differences. When the difference between the sensitivity to luminance and chromatic gratings was compared between the more and less severely affected eyes of each subject, it was found that, overall, chromatic sensitivity was more severely impaired than luminance sensitivity in the disorder Sensitivities to the red/green and the blue/yellow stimuli were found to be affected equally.

Form from motion parallax and form from luminance contrast: vernier discrimination.

Abstract: Some objects are perfectly camouflaged when stationary, but are clearly visible when moving; the boundaries of such an object are defined entirely by motion parallax. Little is known about the eye's ability to make spatial discriminations between motion-defined objects. In this study, subjects viewed a pseudorandom pattern of dots within which a camouflaged bar was made visible by relative motion of dots. Vernier acuity for the motion-defined bar was 27-45 sec arc for three subjects, much less than the interdot separation of 360 sec arc, much less than the 2 deg receptive field size for motion, and comparable with the foveal intercone separation of 30 sec arc. It is proposed that an opponent-orientation process and an opponent-position process can both contribute to vernier judgements for motion-defined objects. Real-world motion contrast commonly confounds the following cues for figure-ground segregation: (1) different texture velocities on either side of the figure's boundary; (2) in any given time interval, texture in figure and ground moves different distances; and (3) texture continually appears and disappears along the figure's boundary. When cues (2) and (3) were eliminated, thus ensuring figure-ground segregation was achieved entirely by motion-sensitive neural elements, vernier acuity was 44 ± 5 sec arc compared with 36 ± 8 sec arc for a dotted bar defined by luminance contrast. Conclusion: Vernier acuity for a dotted bar whose boundary was defined entirely by motion-sensitive neural elements was similar to vernier acuity for a dotted bar whose boundary was defined by luminance contrast.

Contrast sensitivity versus visual acuity in retinal disease.

Abstract: A large group of individuals with retinal disease were tested prospectively for contrast sensitivity by means of Arden gratings. A subgroup of 19 were also tested with the Nicolet automated television system. Individuals with macular or peripheral dystrophy showed a general reduction in contrast sensitivity as visual acuity decreased. The loss of contrast sensitivity was more prominent for high spatial frequencies (6.4 cycles per degree) than for low ones (0.2 cycles per degree). Similar results were obtained for patients with achromatopsia and congenital stationary night blindness. Patients with functional complaints, but no organic basis for decreased acuity, showed greater scatter in their test scores. The Nicolet results showed somewhat smoother curves, but were no more specific in separating normality from abnormality. Contrast sensitivity testing was not specific for the retinal disease entities considered, but may be useful in recording a degree of retinal damage and a degree of functional visual disability. Contrast sensitivity appeared to be reduced whenever acuity was reduced, so that a distinction could be made between patients having a loss of contrast beyond the expectations for their level of acuity and those in whom a loss of contrast simply corroborates the reduction of acuity.

Vanishing optotypes. New type of acuity test letters.

Abstract: Acuity test letters generated on a television monitor were studied with regard to detection and resolution thresholds, both for the native configurations and after high-pass spatial frequency filtering. Comparisons were made with printed acuity charts of different contrast. Filtering diminished the interval between detection and resolution thresholds. Under some circumstances, a small (1 dB, about 26%) increment in size could make a previously invisible optotype clearly visible, or vice versa. The term vanishing optotype is suggested for acuity targets with this peculiar property. Vanishing optotypes do not seem useful for routine clinical acuity testing, but they may be advantageous, for example, in preferential looking-acuity tests for infants and visual field examinations. Like the present tests, such novel applications can be generated relatively easily on personal computers.

What is psychophysically perfect image stabilization? Do perfectly stabilized images always disappear?

Abstract: High-contrast luminance gratings stabilized on the retina with a Purkinje image eyetracker do not disappear completely. This could be due to small errors of stabilization, or the visual system could include mechanisms capable of responding to temporally constant images. We examined the visual system’s sensitivity to small movements of gratings. We (1) replicated previous measurements of contrast sensitivity for gratings with controlled retinal-drift velocities, (2) developed a method for calculating sensitivity to small oscillations of gratings using thresholds for flickering stabilized gratings, and (3) examined the calculations empirically. We calculated that movements of only 8 sec of arc peak to peak produce detectable temporal changes. Since existent stabilization systems cannot eliminate movements this small, residual stabilized-grating detectability does not require detectors sensitive to temporally constant images.

Temporary loss of foveal contrast sensitivity associated with panretinal photocoagulation.

Abstract: • Snellen visual acuity and spatial contrast sensitivity measurements were used to monitor changes in foveal vision in two patients undergoing argon laser panretinal photocoagulation (PRP) in both eyes for proliferative diabetic retinopathy. Foveal edema developed in one of the four eyes. The remaining three eyes showed no signs of foveal edema, but developed temporary losses in high spatial frequency contrast sensitivity during the closely spaced PRP treatments. Since Snellen visual acuity remained stable at the prelaser level, these results indicate the need for more sensitive measures of visual resolution to monitor foveal integrity in patients undergoing PRP.

Suprathreshold stereo-depth matches as a function of contrast and spatial frequency.

Abstract: Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg−1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies.Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15–9.6 cycles deg−1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10–100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of cros...

Electrophysiological assessment of contrast sensitivity in human infants.

Abstract: 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, span

Spatial and temporal properties of human rod vision in the achromat.

Abstract: The spatial and temporal properties of rod vision were measured for stimuli at and above the detection threshold in an achromat whose spectral sensitivity, dark adaptation, spatial and temporal thresholds and Stiles-Crawford effect suggest the presence of only a normally functioning rod system. The properties of rod and cone vision were compared at illuminances where their respective sensitivities were optimum. The threshold spatial sensitivity of the rod mechanism under optimum illumination (180 scotopic trolands) exhibits bandpass properties with a peak sensitivity of around 80 at 0.5 cycles/deg and a spatial acuity of 6-7 cycles/deg. The threshold temporal sensitivity also exhibits bandpass properties under these conditions with a peak sensitivity of around 80 at 5 Hz and a temporal acuity of 30 Hz. For stimuli of low spatial frequency (less than 0.3 cycles/deg) and low temporal frequency, the threshold sensitivities of rod- and cone-mediated vision are identical. Rod- and cone-mediated vision display comparable spatial and temporal discrimination for targets of equal suprathreshold contrast over the low to mid spatial and temporal range that they share. Rod-mediated discriminations fall below those of cone vision above 1 cycle/deg for spatial judgements and above 15 Hz for temporal judgements. The number of discriminable steps in spatial frequency and temporal frequency at threshold is similar for rod and cone vision over the spatio-temporal frequency range that they share. Over this range rod- and cone-mediated vision can discriminate four steps in spatial frequency and one step in temporal frequency. These results suggest that rod vision shows comparable spatio-temporal discrimination performance to cone vision and that it is subserved by at least five spatial and two temporal labelled detectors. The response of the highest spatial frequency filter subserving rod vision extends from 0.5 to 6 cycles/deg.

High myopia does not affect contrast sensitivity.

Abstract: The axial elongation of high myopia is known to induce tangential stretching forces on the retina. Such forces might be expected to compromise retinal circuitry and thus affect the temporal and spatial contrast sensitivity function. In fact, we find that simple high myopes have normal contrast sensitivity for stationary gratings, moving gratings, and uniform field flicker. This suggests that, unlike many other retinal disease processes, the high myope's retina retains its normal integrity until the outer retina is compromised.

Glare and contrast: Indications for cataract surgery

Abstract: The present method of using Snellen visual acuity to evaluate the vision of cataract patients is inadequate because it may grossly overestimate the nature of their visual world. Many of these patients have poor visual functioning as a result of glare disability and loss of contrast. The loss of visual contrast on the retina may be quantified using devices currently available and applicable to a clinical setting in the office. These are described. A study of glare disability in pseudophaldc eyes following phacoemulsification or planned extracapsular cataract extraction with intact capsules is made and compared with a previous report of eyes with a primary posterior capsulotomy. It is recommended that glare disability be measured in eyes of cataract patients and be used as an indication for cataract surgery.

Orientation discrimination in amblyopia.

Abstract: Using extended sinusoidal gratings to avoid potential problems of eccentric fixation, the authors have studied orientation discrimination in amblyopia. For all subjects, elevated orientation discrimination thresholds at high spatial frequencies were found. However, raised thresholds decrease with decreasing spatial frequency, and can be normal at low frequencies. Orientation discrimination thresholds for both amblyopic and non-amblyopic eyes are independent of contrast over most of the visible range. Therefore, amblyopic orientation discrimination thresholds cannot be mimicked in non-amblyopic eyes by reducing contrast. Control experiments show that the orientation discrimination deficits are not restricted to vertical stimuli and that they are not a result of exaggerated cyclotorsional eye movements.

Alterations of visual contrast sensitivity in Parkinson's disease.

Abstract: Contrast sensitivity functions were determined in a population of 18 patients suffering from Parkinson's disease, and compared with the data obtained in an age-matched group of healthy controls. The controls were more sensitive at all spatial frequencies tested than the patients. The statistical comparisons were highly significant, indicating general differences between the PD patients and the controls not related to individual spatial frequency channels. When comparing the sensitivity loss between low and high spatial frequencies no significant differences were found suggesting that the decrease in contrast sensitivity is a global effect. We controlled for effects of age and cerebral atrophy, and our findings cannot be accounted for by these factors. In addition, the amount of contrast sensitivity loss was not correlated with the severity of the disease. These global functional alterations appear to be related to the reduction of dopamine at various sites of the visual system.

Visual evoked potential latency and contrast sensitivity in patients with posterior chamber intraocular lens implants.

Abstract: An electrophysiological investigation of visual evoked potential (VEP) latency and contrast sensitivity was performed in a group of 13 patients who had undergone extracapsular cataract surgery with posterior chamber lens implantation. In spite of good postoperative visual acuity, abnormalities were detected in nine of the group (69%). This study suggests that, even with successfully implanted lenses, there may be a reduction in visual function which could be the result of altered transmission through the plastic lenticulus or fibrosis of the posterior lens capsule, and/or subtle changes in retinal architecture, not observed ophthalmoscopically.

Low contrast charts effectively differentiate between types of blur.

Abstract: Test charts with three different contrast levels were used to measure acuity of six subjects at distance and near. Reduction in measured acuity as a result of refractive defocus was shown to be independent of contrast level. Reduction in measured acuity with diffusive blur was greater on the medium and low contrast charts than on the high contrast chart. We advocate the use of charts with two different contrast levels to help differentiate between causes of visual degradation.

Some Tests of the Marr-Ullman Model of Movement Detection:

Abstract: Several psychophysical experiments are described which test and uphold predictions derived from the Marr–Ullman model of movement detection. First, we demonstrate the existence of adaptation which is specific not merely to the direction of movement of an edge, but also to its contrast polarity. Second, it is shown that adaptation to a spatially homogeneous field whose luminance is modulated according to a temporal sawtooth waveform produces predictable changes in sensitivity to the movement of an edge; these changes, too, are specific to particular conjunctions of direction and edge polarity. Third, similar changes in sensitivity are demonstrated to occur when the luminance of an edge is physically perturbed at the moment of its displacement. Finally, it is shown that, as predicted, the sudden onset of an edge can itself give rise to a momentary impression of movement, the apparent direction of which depends upon the change in luminance that accompanies the onset of the edge.