Showing papers on "Contrast (vision) published in 1983"

Low Contrast Detectability and Contrast/Detail Analysis in Medical Ultrasound

Abstract: Abstruct-The first- and second-order statistics of envelope detected ultrasound (US)B-mode images for the case of a scattering phantom with many scatteiers per resolution cell have been previously derived. These characteristics are integrated over the region of a simulated focal (disk) lesion and the signal-to-noise ratio (SNR) for lesion detectability is obtained. This SNR requires the average number of independent speckle cells over the lesion area (analogous to the number of x-ray photons over the lesion area in incoherent light or x-ray imaging). This number is obtained from our autocorrelation analysis (second-order statistics). By setting the SNR expression equal to the threshold value SNRT required to detect a lesion in the presence of speckle noise, the dependence of lesion contrast on lesion diameter at threshold is found, i.e., the contrast/detail function. This is a simple inverse relation for ideal observers of US B-scans. It is also found that the contrast/detail results for envelope detectionin diagnostic ultrasound are almost identical with the results for square law detection (the usual laser case) with the latter serving as an upper limit for performance in lesion detection. Finally, the results of human observer performance using a contrast/ detail phantom are compared with the predictions for optimal or ideal 2cm Fig. 1. Schematic of contrast/detail phantom for diagnostic ultrasound. performance. The results are comparable with results for photon imaging systems, with valuesof the SNR at threshold in the neighborhood of 2-3.

The quantized geometry of visual space: The coherent computation of depth, form, and lightness

Abstract: A theory is presented of how global visual interactions between depth, length, lightness, and form percepts can occur. The theory suggests how quantized activity patterns which reflect these visual properties can coherently fill-in, or complete, visually ambiguous regions starting with visually informative data features. Phenomena such as the Cornsweet and Craik-O'Brien effects, phantoms and subjective contours, binocular brightness summation, the equidistance tendency, Emmert's law, allelotropia, multiple spatial frequency scaling and edge detection, figure-ground completion, coexistence of depth and binocular rivalry, reflectance rivalry, Fechner's paradox, decrease of threshold contrast with increased number of cycles in a grating pattern, hysteresis, adaptation level tuning, Weber law modulation, shift of sensitivity with background luminance, and the finite capacity of visual short term memory are discussed in terms of a small set of concepts and mechanisms. Limitations of alternative visual theories which depend upon Fourier analysis, Laplacians, zero-crossings, and cooperative depth planes are described. Relationships between monocular and binocular processing of the same visual patterns are noted, and a shift in emphasis from edge and disparity computations toward the characterization of resonant activity-scaling correlations across multiple spatial scales is recommended. This recommendation follows from the theory's distinction between the concept of a structural spatial scale, which is determined by local receptive field properties, and a functional spatial scale, which is defined by the interaction between global properties of a visual scene and the network as a whole. Functional spatial scales, but not structural spatial scales, embody the quantization of network activity that reflects a scene's global visual representation. A functional scale is generated by a filling-in resonant exchange, or FIRE, which can be ignited by an exchange of feedback signals among the binocular cells where monocular patterns are binocularly matched.

Changes in differential threshold in patients with glaucoma during prolonged perimetry.

Abstract: Patients with chronic open-angle glaucoma and others suspected of having glaucoma were subjected to continuous contrast threshold measurement by automatic and manual perimetric techniques. The results show that an increase of contrast threshold over time is a frequent finding in glaucoma patients and that this increment can be large, especially at points close to existing field defects. it is reproducible and independent of background illumination and can be demonstrated with both automatic and manual testing, but it is larger in the automatic than in the manual mode of testing. The results may have some important implications for visual field testing in glaucoma.

Combining attributes in rapid serial visual presentation tasks.

Abstract: A series of three exploratory experiments sought to compare and contrast two broad classes of theory pertaining to visual information pick-up. One theoretical approach, exemplified by Keele and Nei...

The 20/20 eye in multiple sclerosis.

Abstract: Using clinical and electrophysiologic measures, we evaluated the visual pathway of patients who had multiple sclerosis, 20/20 Snellen acuity, and no history of optic neuritis. Delayed latencies were found in the transient visual evoked potentials (VEPs) of 38% of the patients, and interocular latency differences were abnormal in 67%. Contrast VEPs were abnormal in 4.6%. Psychophysical determinations of contrast sensitivity were abnormal in 78%. Only 17% of the patients had dyschrornatopsia, 36% had afferent pupillary abnormalities, and 59% had optic nerve pallor or nerve fiber layer loss. Psychophysical contrast evaluations and VEP studies were superior to other clinical evaluations in demonstrating visual dysfunction in these patients.

Comparison of three methods for rapid determination of threshold contrast sensitivity.

Abstract: Threshold contrast sensitivity functions for 13 subjects were determined on 5 consecutive days using three psychophysical methods Each method was implemented on a microprocessor controlled display system and considered as a possible candidate for a rapid visual screening test The methods used were Bekesy tracking, method of adjustment, and method of increasing contrast The method of increasing contrast proved superior to the other methods in repeatability, speed, and the preservation of individual differences in contrast sensitivity over the testing period

Age, the eye, and the visual channels: contrast sensitivity and response speed.

Abstract: Age differences in spatial vision were examined by establishing contrast sensitivity functions for young and old adults. In addition, differences in the speed of operation of the different visual channels and their contribution to age differences in reaction time were assessed in a comparison of response speed to sinusoidal gratings of varied spatial frequency. An age-related loss in contrast sensitivity was observed primarily for stimuli of intermediate and high spatial frequency, an effect apparently due mostly to age-related optical differences. Reaction time varied markedly as a function of spatial frequency for the older participants, an outcome only partly attributable to age differences in target visibility. These data indicate a slowing in the speed of operations of individual visual channels and/or a change in the relationship between visual channel classes. These findings also raise a question in regard to the meaning and comparability of any studies that have used measures of response latency to the figural aspects of stimuli.

Contrast thresholds for sine gratings of children with amblyopia.

Abstract: Contrast threshold functions were measured on 26 amblyopic children before and after minimal occlusion therapy. On initial presentation the relative contrast sensitivity deficit the amblyopic eye was in every case much less than that predicted from the deficit in visual acuity for letters. In fact, in seven of the children the contrast sensitivity functions for the amblyopic and fellow nonamblyopic eye were indistinguishable despite the expression of substantial amblyopia on letter charts. Only four children exhibited a substantial contrast sensitive loss in the amblyopic eye with a cut-off spatial frequency below 30 cycles/degree. The majority of children who showed deficits in contrast sensitivity did so only at medium and high spatial frequencies. On the basis of these findings it appears that measurements of contrast thresholds for single sinusoidal gratings do not probe fully the deficits of spatial resolution in amblyopia. Finally, among the few children who exhibited a sizeable initial deficit, only two showed obvious improvement in contrast sensitivity in response to minimal occlusion therapy. Invest Ophthalmol Vis Sci 24:782-787, 1983 Measurements of contrast sensitivity functions in amblyopia associated with either strabismus or anisometropia usually reveal deficits for gratings of high spatial frequency but deficits at low spatial frequencies appear to be less common. 1 " 5 Previous studies conducted on relatively small numbers of adult amblyopes suggest that the deficits in contrast sensitivity are considerably less than might be expected on the basis of their visual acuity for letters. Virtually all such measurements have been made on adult amblyopes but the nature of the deficits in children is less well documented. Accordingly, as part of a controlled clinical trial of a new treatment 6 for amblyopia described in previous papers, 78 measurements were made of the contrast threshold functions for sine gratings in 26 children (6-14 years of age) with strabismic or anisometropic amblyopia. In this paper the contrast threshold functions and grating acuities measured before and after treatment are compared with the visual acuities measured on letter charts.

Visual fields described by contrast sensitivity, by acuity, and by relative sensitivity to different orientations.

Abstract: Sinewave grating contrast sensitivity was measured as a function of eccentricity and azimuthal angle for four orientations of gratings whose spatial frequencies ranged from 2 to 20 c/deg. Visual fields for cutoff spatial frequency were also mapped. Log contrast sensitivity fell off approximately linearly with eccentricity for all azimuths. Orientational differences in contrast sensitivity varied irregularly over the visual field and, though small for central vision, could reach as high as 25 dB in localized patches at eccentricities greater than about 12 degrees.

Canine visual acuity: retinal and cortical field potentials evoked by pattern stimulation.

Abstract: The visual acuities (i.e., visual thresholds for pattern detection) of four dogs under neuromuscular block were measured using visually evoked cortical potentials (VECP) and/or pattern-evoked retinal potentials (PERR). Stimuli were phase-reversing square-wave gratings with a mean luminance of 86 cd/m2 and 70% contrast. The mean of the VECP thresholds of two dogs tested was 12.59 cycles per degree (cycles/deg). The mean of the PERR thresholds of four dogs tested was 11.61 cycles/deg. The difference between VECP and PERR thresholds was not statistically significant. VECP acuities appear to be determined at or before the last stage of retinal processing (PERR). Our estimates of canine acuity are 1.3–2 times those reported for cats and 0.2–0.4 times those reported for primates when tested under comparable luminance and contrast conditions.

Suprathreshold contrast perception in functional amblyopia.

Abstract: Three experiments were conducted to investigate suprathreshold contrast perception of a group of strabismic and anisometropic amblyopes. In the first experiment, simple reaction-time was measured as a function of the contrast of sinusoidal gratings. At all contrast levels, the amblyopic eyes showed prolonged reaction-times compared to the non-amblyopic eyes. In the second experiment, the perceived contrast of suprathreshold sinusoidal gratings was measured using an interocular successive matching paradigm. The third experiment compared perceived contrast of the amblyopic and non-amblyopic eye by contrast magnitude estimation. The results of the matching and magnitude estimation experiments showed that while amblyopic observers show large losses of contrast sensitivity at threshold, at suprathreshold contrast levels perceived contrast is essentially normal or near normal in the amblyopic eye. Taken together, these experiments suggest that: 1) the amblyopic eye has a higher contrast gain than the non-amblyopic eye, and 2) the amblyopic eye shows prolonged response latencies. Similar results can be obtained in normal observers under conditions of peripheral viewing and noise masking.

Effects of luminance on contrast sensitivity in senile macular degeneration.

Abstract: Patients with senile macular degeneration (SMD) often have difficulties in adapting to changing luminance levels. They may complain of problems at both high and low luminance levels, although some patients have their vision function markedly improved at high luminance levels. We examined the effect of luminance on the contrast sensitivity function in six patients with SMD and five normals of approximately the same age. The contrast sensitivity of the SMD patients compared to the control subjects was reduced at 72 cd/m2 but was comparable to that of the controls at 0.007 cd/m2. The peak of the contrast sensitivity function was moved to lower spatial frequencies at all luminances in the SMD group, and the slope of the function relating contrast sensitivity to luminance was reduced. These data suggest that adaptation mechanisms which optimize contrast detection are disrupted in SMD. Further disturbances of function might be expected in the dynamics of the adaptation process.

Contrast Sensitivity Predicts Target Detection Field Performance of Pilots

Abstract: Our recent research has shown that contrast sensitivity, not visual acuity, can predict a pilot's ability to detect an air-to-ground target in a flight simulator. Here we report a further study that shows a similar predictive power under actual field conditions. Eighty-four Air Force pilots, seated at the end of a runway, typically in groups of ten per week, reported the detection of an approaching T-39 jet aircraft under visibility conditions varying from 0.5 to over 15 miles for 10 field trials. The pilots' detection ranges were correlated with their individual contrast sensitivities and standard visual acuities. Contrast sensitivity, not visual acuity, was found to be a good predictor of detection range. These results have strong implications for the creation of performance-related vision standards.

Transient visually evoked potentials to sinusoidal gratings in optic neuritis.

Abstract: Transient visually evoked potentials (VEPs) to sinusoidal gratings over a range of spatial frequencies have been recorded in cases of optic neuritis. The use of the response to pattern onset in addition to the response to pattern reversal extended the range to higher spatial frequencies by up to two octaves. There was an increase in VEP delay and a greater degree of discrimination from a control group at higher spatial frequencies. This finding is discussed in the light of previous reports of luminance and checkerboard VEPs in demyelinating optic nerve disease. An attempt is made to relate amplitude changes in various VEP components to contrast sensitivity measurements in this group of patients.

Lightness constancy through a veiling luminance.

Abstract: Observers were asked to select samples from a Munsell chart to match the lightness of seven identified surfaces in an outdoor scene they were shown. A separate group that was given the same task but viewed the same scene covered with a veiling luminance equal in intensity to the highest luminance in the scene selected almost the same matches. The same lightness constancy results were obtained using an abstract laboratory display to rule out memory color. These results challenge ratio and contrast theories because a veiling luminance, by adding a constant luminance to every poing in the image, dramatically alters luminance ratios. Lightness constancy was not obtained, however, when these three-dimensional real-world-type displays were replaced by a flat, Mondrian-type display consisting of surface grays from white to black, whether or not colored regions were present in the display; lightness matches were consistent with ratio predictions both with and without the veil.

Contrast enhanced photolithography

Abstract: This paper introduces a new method, contrast enhancement, which improves the imaging properties of conventional photoresist through the use of a photobleachable layer applied to the resist surface. Contrast enhancement allows high contrast photopatterns to be formed even when low contrast illumination is used. A short discussion of the theory and an experimental demonstration of the method are presented.

Two results concerning ambiguity in shape from shading

Abstract: Two shape from shading problems are considered, one involving an image of a plane and the other an image of a hemisphere. The former is shown to be ambiguous because it can be generated by an infinite number of ruled surfaces. The latter, in contrast, is shown to have only the hemisphere and its reversal as solutions, although some subregions of the image are shown to be infinitely ambiguous.

Spatio-temporal interaction in neurones of the cat's dorsal lateral geniculate nucleus.

Abstract: Temporal contrast sensitivities in the range 0.33-42 Hz for optimum spatial frequency grating stimuli were measured for large populations of XG and YG neurones. For fewer cells, complete spatial contrast sensitivity profiles were measured at a series of temporal frequencies and, in some cases, at a range of levels of retinal illumination too. Contrast sensitivities were measured from responses of cells reliably different from their maintained discharges. The cells' discharges were recorded extracellularly from the binocular segment of the A laminae of the cat's dorsal lateral geniculate nucleus. At their respective optimum spatial frequencies, YG cell were more sensitive on average than XG cells for most temporal frequencies, though the average temporal contrast sensitivity profiles of both cell classes had similar shapes. The optimum temporal frequency for both cell types was around 5 Hz. XG and YG cells seem to be relatively less sensitive to low temporal frequencies than their ganglion cell counterparts. At a retinal illumination of 230 cd/m2 (pupil, 3 mm2), increasing temporal frequency in the range 0.65-21 Hz produced a relative improvement in low spatial frequency contrast sensitivity in most XG and all YG cells studied. There were some XG cells, though, which showed little or no effect of temporal frequency on their spatial contrast sensitivity curves. At all temporal frequencies, the shapes of spatial contrast sensitivity curves and the cells' temporal contrast sensitivity profiles were not markedly dependent on the criterion level set to measure 'threshold' contrast. Reducing the level of retinal illumination in the range 230-0.007 cd/m2 (pupil, 3 mm2) produced a fall in contrast sensitivities for both XG and YG cells. The loss in sensitivity was more marked at high spatial and high temporal frequencies. The similar shapes of the temporal contrast sensitivity curves of XG and YG cells weakens the suggestion that the human counterparts of these cells would provide a suitable physiological substrate for the psychophysical sustained and transient channels. Although the behaviour of XG and YG cells parallels quite closely changes in cat and human psychophysical spatial contrast sensitivities with temporal frequency and retinal illumination, many problems remain for equating results from the two fields.

Photoreceptor contributions to contrast sensitivity: Applications in radiological diagnosis

Abstract: Electrophysiological and psychophysical observations are described which appear to have important implications in diagnostic image interpretation. While the interpretation of a complex radiographic image depends upon processing by higher-order neural centers, this interpretation process is influenced by the sensitivity of the visual system. Attention is given to two mechanisms which operate at the input stages of the visual system and which directly affect visual sensitivity: (1) the effects of background illumination on cone photoreceptor sensitivity, and (2) the image-forming properties of the eye (including small involuntary eye movements). The authors have constructed a model of the preprocessing occurring in the eye, by which light patterns viewed by a subject are transformed into patterns of neural activity of the cone photoreceptors in the subject's retina. The desensitization of cone photoreceptors by steady background illumination was studied using intracellular techniques. Background intensities desensitize both cone photoreceptors and the photopic visual system in a similar manner. This finding is consistent with the hypothesis that photopic visual threshold may be set at the very input to the visual system by the sensitivity of the cone photoreceptors. These two mechanisms appear to account for much of the visual system desensitizations caused by both uniform and stylized nonuniform backgrounds.

Evidence against saturation of contrast adaptation in the human visual system.

Abstract: In many experiments on visual pattern perception, it has been assumed that the effects of adaptation to contrast saturate within a few minutes. However, we show here that such saturation does not necessarily occur. Instead, both the buildup and recovery of the threshold elevation aftereffect fit closely a power function.

Vision evaluation using contrast sensitivity functions.

Abstract: The use of Contrast Sensitivity Functions (CSF's) in vision evaluation has been explored for several years. Although theoretically superior to Snellen acuity and similar measures, the use of CSF is not widespread. A review of some of the clinical conditions in which CSF's have been used is followed by a discussion of the problems involved in their measurement and interpretation.