Showing papers on "Contrast (vision) published in 1996"

Ultrasonic diagnostic imaging with contrast agents

Abstract: Apparatus and methods are disclosed for the detection and imaging of ultrasonic contrast agents. Ultrasonic apparatus is provided for coherent imaging of ultrasonic contrast agents, and for detecting harmonic contrast agents. The inventive apparatus includes a dual display for simultaneously viewing a real time image which displays the location of the contrast agent and a triggered contrast image. Methods of contrast agent detection and imaging include the measurement of perfusion rate characteristics, multizone contrast imaging, multifrequency contrast imaging, tissue perfusion display, and high PRF contrast image artifact elimination.

Multineuronal codes in retinal signaling

Abstract: The visual world is presented to the brain through patterns of action potentials in the population of optic nerve fibers. Single-neuron recordings show that each retinal ganglion cell has a spatially restricted receptive field, a limited integration time, and a characteristic spectral sensitivity. Collectively, these response properties define the visual message conveyed by that neuron's action potentials. Since the size of the optic nerve is strictly constrained, one expects the retina to generate a highly efficient representation of the visual scene. By contrast, the receptive fields of nearby ganglion cells often overlap, suggesting great redundancy among the retinal output signals. Recent multineuron recordings may help resolve this paradox. They reveal concerted firing patterns among ganglion cells, in which small groups of nearby neurons fire synchronously with delays of only a few milliseconds. As there are many more such firing patterns than ganglion cells, such a distributed code might allow the retina to compress a large number of distinct visual messages into a small number of optic nerve fibers. This paper will review the evidence for a distributed coding scheme in the retinal output. The performance limits of such codes are analyzed with simple examples, illustrating that they allow a powerful trade-off between spatial and temporal resolution.

Visual performance after photorefractive keratectomy. A prospective study.

Abstract: Objective: To prospectively examine the effect of excimer laser photorefractive keratectomy (PRK) on best-corrected visual performance using psychophysical tests that were likely to be more sensitive to image degradation than high-contrast Snellen visual acuity. Design: Prospective cases series. Patients: A cohort of 18 subjects with an average of -5.08 diopters (D) of myopia (SD=±1.63 D) was tested before PRK and at 3, 6, and 12 months after PRK. Intervention: Photorefractive keratectomy was performed using a laser (Excimed UV200, Summit Technology, Waltham, Mass) and a polymethylmethacrylate mask; a 5-mm ablation zone was used. Main Outcome Measures: Best-corrected highcontrast visual acuity, best-corrected low-contrast visual acuity (18% Weber contrast), and best-corrected letter-contrast sensitivity. Measurements were repeated with dilated pupils and in the presence of a glare source. Results: One year after PRK, the mean best-corrected high-contrast visual acuity was reduced by half a line (P=.01), and the mean best-corrected low-contrast visual acuity was reduced by 11/2 lines (P=.002). The losses were somewhat greater when the subject's pupils were dilated and a glare source was used. The reduction in dilated low-contrast visual acuity was positively correlated with the decentration of the ablation zone (r=0.47), providing evidence of an association between corneal topography and the functional outcome of PRK. Conclusion: Low-contrast visual acuity losses after PRK are notably greater than high-contrast visual acuity losses for best-corrected vision. Low-contrast visual acuity is a sensitive measure for gauging the outcome success and safety of refractive surgery.

Impairment of the perception of second order motion but not first order motion in a patient with unilateral focal brain damage.

Abstract: Unlike first order motion, which is based on spatiotemporal variations in luminance, second-order motion relies on spatiotemporal variation of attributes derived from luminance, such as contrast. Here we show that a patient with a small unilateral cortical lesion adjacent to human cortical area MT (V5) has an apparently permanent disorder in perceiving several forms of second-order but not first-order motion in his contralateral visual field. This result indicates that separate pathways for motion perception exist, either as divergent pathways from area MT or even from primary visual cortex, or as separate pathways from subcortical areas to extrastriate visual areas.

Retinal and cortical evoked responses to chromatic contrast stimuli. Specific losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis.

Abstract: Summary // is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits (Ishihara). Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/ (red+green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and greenblack). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg~') modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However, the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast, suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour, suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.

Polarization contrast vision in Octopus.

Abstract: While the ability to analyze polarized light is widespread among animals, its contribution to form vision has not yet been documented. We tested the hypothesis that polarization vision can be used for object discrimination, by training octopuses to distinguish between targets on the basis of the presence or absence of a pattern produced by a 90 degrees polarization contrast within the target. Octopuses recognized a 90 degrees contrast pattern within a single target, when presented either on a horizontal/vertical axis or on a 45 degrees/135 degrees axis. They were able to transfer their learning to new situations and to detect a polarization contrast when the orientations of the e-vector of light passing through the target center and background differed by as little as 20 degrees. Polarization vision may provide information similar to that available from color vision and thus serve to enhance the detection and recognition of objects.

Rapid colour-specific detection of motion in human vision

Abstract: The human visual system is much better at analysing the motion of luminance (black and white) patterns than it is at analysing the motion of colour patterns, especially if the pattern is presented very briefly or moves rapidly. We report here that observers reliably distinguish the direction of motion of a colour pattern presented for only 17 milliseconds, provided that the contrast is several times the threshold value (the contrast needed to detect the presence of the pattern). A control experiment, in which a static luminance 'mask' is added to the moving colour pattern, proves that discrimination of the direction of motion of these brief stimuli is colour-specific. The mask drastically impairs discrimination of the direction of motion of a luminance pattern, but it has little effect on a colour pattern. We conclude that the human visual system contains colour-specific motion-detection mechanisms that are capable of analysing very brief signals.

Effect of a cataract simulation on clinical and real world vision.

Abstract: AIMS/BACKGROUND: Many reports have indicated that some patients with cataract can retain good visual acuity but complain of significant visual problems. This is the first in a series of papers trying to determine what causes these symptoms and whether other clinical tests can predict the real world vision loss. METHODS: The effect of a cataract simulation with a similar angular distribution of light scatter as real cataract on clinical (visual acuity, contrast sensitivity, and disability glare) and real world vision (face recognition, reading speed, and mobility orientation) was investigated. RESULTS: The simulation had a relatively small effect on visual acuity (6/6 with the simulation), but much larger effects on contrast sensitivity and low contrast acuity with and without glare. The simulation had no effect on high luminance and high contrast real world tasks, such as mobility orientation in room light and optimal reading speed. A small, but significant deterioration was found for the slightly lower contrast task of face and expression recognition. However, under low luminance conditions, substantial defects in mobility orientation were obtained (despite 6/6 acuity). CONCLUSIONS: Although the relative effect of the cataract simulation on acuity and contrast tasks is not typical of the average cataract, it can be found in those cataract patients with visual problems despite good visual acuity. This corroborates the suggestion that it is large amounts of wide angle light scatter (forward and/or backward) which are at least partly responsible for visual disability in cataract patients with good visual acuity. A patient's reported visual disability may depend on the percentage of time he or she spends under low contrast and/or low luminance conditions, such as walking or reading in dim illumination, and walking or driving at night, in fog, or heavy rain.

Sources of contrast in confocal reflectance imaging.

Abstract: The relationship between optical properties and image contrast in confocal imaging is investigated A Monte Carlo simulation has been developed to analyze the effects of changes in scattering, index of refraction, and absorption in a three-layer medium Contrast was calculated from the computed signal-to-background ratios for changes in tissue optical properties Results show that the largest source of contrast is changes in refractive index

Spatial properties of envelope-responsive cells in area 17 and 18 neurons of the cat

Abstract: 1. Many neurons in areas 17 and 18 respond to spatial contrast envelope stimuli whose Fourier components fall outside the cell's spatial-frequency-selective range. The spatial properties of such envelope responses are investigated here and compared with responses to conventional luminance-defined gratings to explore the underlying receptive-field mechanism. 2. Three spatial properties of envelope responses are reported more extensively in this paper. First, the envelope responses were selective to the carrier spatial frequency in a narrow range of frequencies higher than a given cell's luminance spatial frequency selective range (luminance passband). Second, a given cell's dependence on envelope spatial frequency often differed from its luminance passband. Last, the optimal carrier spatial frequency did not shift systematically with the envelope spatial frequency, supporting the hypothesis that the carrier and envelope spatial-frequency dependencies were mediated by distinct mechanisms. 3. In contrast to the direction selectivity to the envelope motion in many envelope-responsive cells, no direction preference to carrier motion was found for envelope responses. The direction of carrier motion did not alter the direction selectivity for envelope motion, further supporting the hypothesis that the carrier and envelope temporal properties were mediated by separate mechanisms. 4. The distributions of the optimal carrier and luminance spatial frequencies among envelope-responsive cells were analyzed. The optimal carrier spatial frequencies were randomly distributed from five times the cell's optimal luminance spatial frequency to the upper resolution limit of the X-retinal ganglion cells at the same retinal eccentricity, suggesting that the selective ranges of envelope responses and luminance responses are not strongly correlated over the population of envelope-responsive cells. 5. Our data support a "two-stream" receptive-field model for envelope-responsive cells. One stream is a conventional, spatially linear receptive-field mechanism, mediating luminance responses for the cell; the other mediates envelope responses and consists of a two-stage processing: a set of spatially small and distributed nonlinear neural subunits whose outputs are spatially pooled at the second stage. 6. In conclusion, this study indicates that envelope responses in area 17 and 18 neurons cannot be due to a nonlinearity that is common to all visual stimuli before narrowband spatial-frequency-selective filtering; instead, a specialized processing stream, parallel to the conventional luminance response stream, is needed to supplement the traditional luminance processing stream in these cells. This specialized stream responds to the envelope stimuli and is selective to their carrier and envelope spatial frequencies. The distributions of the optimal luminance and carrier spatial frequencies indicate a rich variety of possible integration between luminance and envelope information.

Increased ultrasound contrast and decreased microbubble destruction rates with triggered ultrasound imaging.

Abstract: Although transient myocardial contrast imaging has been able to produce visually evident myocardial contrast in animals and humans with very low intravenous doses of perfluorocarbon-exposed sonicated dextrose albumin (PESDA) microbubbles, the mechanism for improved contrast remains unclear. In this study we devised a flow chamber that measured the concentration of PESDA microbubbles that remained after exposure to diagnostic ultrasound pressures of 0.9 to 1.9 MPa and frequencies of 2.0, 2.5, and 3.5 MHz (first and second harmonic for 2.0 MHz), which were delivered at either 30 Hz (frames per second), 0.5 to 1.0 Hz, or without any ultrasound transmission. The videointensity within the flow chamber was also measured at 0, 20, 40, and 100 ml/min flow rates with the flow loop closed (i.e., constant microbubble concentration) with both triggered (0.5 to 1.0 Hz) and conventional (30 Hz) frame rates. The effluent microbubble concentration was significantly larger when PESDA was exposed to either no ultrasound or 0.5 to 1.0 Hz ultrasound. Furthermore, the videointensity of a constant number of microbubbles was significantly greater with 0.5 to 1.0 Hz (triggered) compared with 30 Hz (conventional) frame rates at each transmit frequency. The greatest difference was noted with the lower 2.0 MHz transmit frequency and the 20 ml/min flow rate, especially when a second harmonic receiving frequency was used. We conclude that the mechanism for improved contrast with triggered ultrasound imaging is because of both less microbubble destruction and increased videointensity from a constant number of microbubbles. Lower transducer frequencies and lower flow rates result in the greatest improvement in videointensity with triggered ultrasound transmission.

Visual search for size is influenced by a background texture gradient.

Abstract: Research on the perception of texture gradients has relied heavily on the subjective reports of observers engaged in free-viewing. We asked whether these findings generalized to speeded performance. Experiment 1 showed that an important aspect of subjective perception—sizeconstancy scaling with perceived distance—also predicted the speed of pop-out visual search for cylinders viewed against a texture gradient. Experiment 2 showed that this finding could not be attributed to the local contrast between search items and the background texture. Experiment 3 assessed the relative contributions of 2 separable dimensions of texture gradients—perspective (radial spreading) and compression (foreshortening)—finding them to be independent in the more rapid search conditions (long target among shorter distractors) but combined in their influence in the slower conditions (short target among longer distractors). When observers view the texture gradient shown in Figure 1A they usually report seeing a flat surface recede into the distance, despite the fact that a two-dimensional (2-D) image alone cannot specify the three-dimensional (3-D) surface that gave rise to the projection. This study asked whether the factors influencing the perceived slant of such texture gradients also influences rapid visual search for objects placed on their surface. Although a large number of previous studies have exam

Visual information processing in primate cone pathways. I. A model

Abstract: At the retinal level, the strategies utilized by biological visual systems allow them to outperform machine vision systems, serving to motivate the design of electronic or "smart" sensors based on similar principles. Design of such sensors in silicon first requires a model of retinal information processing which captures the essential features exhibited by biological retinas. In this paper, a simple retinal model is presented, which qualitatively accounts for the achromatic information processing in the primate cone system. The computer retina model exhibits many of the properties found in biological retinas such as data reduction through nonuniform sampling, adaptation to a large dynamic range of illumination levels, variation of visual acuity with illumination level, and enhancement of spatiotemporal contrast information. The main emphasis of the model presented here is to demonstrate how different adaptation mechanisms play a role in extending the operating range of the primate retina.

Risk factors associated with contrast sensitivity loss in diabetic patients

Abstract: • Background: Psychophysical tests in patients with diabetes mellitus reveal deficits of central vision before the development of overt retinopathy. We evaluated the contrast sensitivity thresholds in 30 patients with type 11 diabetes mellitus and without retinopathy, taking into account the crystalline lens density. Risk factors for contrast sensitivity deficits were investigated. • Methods: Contrast sensitivity was compared in 30 aretinopathic diabetic patients and age-matched controls. Contrast thresholds were determined for stationary gratings at three spatial frequencies (6, 15, and 27 cycles/deg) and for mesopic (5 cd/m2) and low photopic (85 cd/m2) vision. Lens density was measured using a IntraOptics opacity lensmeter.

Dielectric versus topographic contrast in near-field microscopy

Abstract: Using a fully vectorial three-dimensional numerical approach (generalized field propagator, based on Green’s tensor technique), we investigate the near-field images produced by subwavelength objects buried in a dielectric surface. We study the influence of the object index, size, and depth on the near field. We emphasize the similarity between the near field spawned by an object buried in the surface (dielectric contrast) and that spawned by a protrusion on the surface (topographic contrast). We show that a buried object with a negative dielectric contrast (i.e., with a smaller index than its surrounding medium) produces a near-field image that is reversed from that of an object with a positive contrast.

Environmental variables and mobility performance in adults with low vision.

Abstract: Background Surveys and empirical studies of visually impaired (VI) persons suggest that avoidance of objects while walking may depend on type of vision loss and be influenced by light level, object contrast, and object type. Methods Mobility performance in 88 VI adults was assessed on an indoor obstacle course under photopic and mesopic lighting conditions. Subjects were divided into three categories: primarily an acuity loss, primarily a peripheral field restriction, and a combination of the two. Performance measures were time to walk the course and total number of contacts with objects in the course. Results Decreasing light level from photopic to mesopic resulted in a significant increase (roughly double) in the time required to complete the course and in the total number of contacts, regardless of type of vision loss. Under photopic illumination, subjects with acuity loss took less time on average to complete the course and contacted fewer objects than the other two groups. At mesopic levels, acuity loss subjects performed better than those in the other categories. Low contrast obstacles were contacted more frequently than high contrast ones at both light levels. Finally, floor level walk-around objects were contacted significantly less often than either step-over or head level objects, regardless of type of vision loss and light level. Conclusions The ability of VI persons to avoid obstacles is significantly impaired under mesopic illumination. Object contrast and location are significant factors in determining the success of VI persons in avoiding obstacles in the travel path.

Visual stabilization of posture in persons with central visual field loss.

Abstract: Purpose To determine whether people with central visual field loss (CFL) show a smaller visual contribution to posture stabilization than people with normal vision and to determine the visual factors that predict the magnitude of visual stabilization in people with central visual field loss. Methods Posture information was recorded in 19 subjects with CFL and in 20 subjects with normal vision. Data were collected as the subject stood in a dark environment and also as he or she viewed a stationary visual display. In both conditions, somatosensory feedback was concurrently altered. The central visual fields of the subjects with CFL were measured by static perimetry with the confocal scanning laser ophthalmoscope. Binocular visual acuity and contrast sensitivity were measured on all subjects using the ETDRS and Pelli-Robson charts, respectively. Image-displacement thresholds were measured in a subset of the subjects. Results On average, subjects with central field loss showed a smaller visual contribution to posture stabilization than subjects with normal vision. The reduction in sway caused by visual stimuli was only 29% for the subjects with CFL compared to 41% for the subjects with normal vision. Displacement thresholds accounted for 45% of the variance in the visual stabilization magnitude of the subjects with CFL. No other visual factor significantly increased the coefficient of determination. Conclusions The visual self-motion cues generated by small body oscillations may be undetectable and, thus, unusable as cues to postural sway by people with central field loss.

Vision in dementia: contrast effects.

Abstract: 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.

The role of light scatter in the residual visual sensitivity of patients with complete cerebral hemispherectomy.

Abstract: Various residual visual capacities have been reported for the phenomenally blind field of hemispherectomized patients, providing evidence for the relative roles of cortical and subcortical pathways in vision. We attempted to characterize these functions by examining the ability of five patients to detect, localize, and discriminate high-contrast flashed, flickering and moving targets. Dependent measures were verbal, manual, and oculomotor responses. As a control for light scatter, intensity thresholds for monocular detection of targets in the hemianopic field were compared with thresholds obtained when using an additional half eyepatch to occlude the blind hemiretina of the tested eye. One unilaterally destriate patient was tested on the same tasks. In photopic conditions, none of the hemispherectomized patients could respond to visual cues in their impaired fields, whereas the destriate patient could detect, discriminate, and point to targets, and appreciate the apparent motion of stimuli across his midline. Under reduced lighting, the threshold luminance required by hemispherectomized patients to detect stimuli presented monocularly was similar to that required for their detection when all visual information was occluded in the blind field, and only available to the visual system indirectly via light scatter. In contrast, the destriate patient's monocular threshold in his blind field was substantially lower than that for stimuli directly occluded in the blind field. As we found no range of stimuli which the hemispherectomized patients could detect or discriminate that was not also associated with discriminable scattered light, we conclude that the subcortical pathways which survive hemispherectomy cannot mediate voluntary behavioural responses to visual information in the hemianopic field.

Lightness contrast in CRT and paper-and-illuminant displays

Abstract: The increased use of CRT monitors for displaying and controlling stimuli in studies of surface color poses problems of comparability with data obtained with traditional paper-and-illuminant methods. A review of comparable studies using the two methodologies revealed that CRT studies tend to report larger contrast effects. To investigate factors that may be responsible for this difference, simultaneous lightness contrast was measured using both CRT and paper-and-illuminant presentations. The spatial distribution of luminance in the whole field of view and the visual angles subtended by the displays were controlled. The CRT presentation yielded contrast effects twice as big as those measured for a paper surface in a homogeneously illuminated room. However, a paper display under Gelb lighting yielded almost exactly the same effect size as that measured in the CRT presentation. These results demonstrate that contrast effects in both modes of presentation are affected by the spatial distribution of luminance beyond the basic experimental stimuli.