Showing papers on "Contrast (vision) published in 1994"

Selective suppression of the magnocellular visual pathway during saccadic eye movements

Abstract: VISUAL scientists have long sought to explain why the world remains stable during saccades, the ballistic eye-movements that continually displace the retinal image at fast but resolvable1 velocities. An early suggestion was that vision may be actively suppressed during saccades2, but experimental support has been variable3–5. Here we present evidence that saccadic suppression does occur, but that it is selective for patterns modulated in luminance at low spatial frequencies. Patterns of higher spatial frequency, and equiluminant patterns (modulated only in colour) at all spatial frequencies were not suppressed during saccades, but actually enhanced. The selectivity of the suppression suggests that it is confined to the colour-blind magnocellular stream (which provides the dominant input to motion centres and areas involved with attention), where it could dull the otherwise disturbing sense of fast low-spatial-frequency image motion. Masking studies suggest that the suppression precedes the site of contrast masking and may therefore occur early in visual processing, possibly as early as the lateral geniculate nucleus.

Visual impairment and disability in older adults

Abstract: Background Previous studies have reported that reduced visual acuity is associated with functional dependence in older persons The purpose of this study was to determine whether components of vision impairment besides reduced acuity contribute to reduced functional independence Methods Community dwelling adults aged 65 years and older were administered a battery of vision tests including distance acuity, letter contrast sensitivity, disability glare, and stereoacuity A physical function questionnaire assessed self-reported difficulty with activities of daily living (ADL's), Instrumental activities of daily living (IADL's), and mobility activities A visual function questionnaire assessed self-reported difficulty with a wide variety of everyday visual activities Results Multiple logistic regression analysis showed that reduced acuity and reduced contrast sensitivity were independently associated with an overall vision disability score Acuity was associated with difficulty in tasks requiring good resolution and adaption to changing light conditions, whereas contrast sensitivity was associated with difficulty in tasks requiring distance judgments, night driving, and mobility Glare and stereoacuity were not associated with self-reported disability Conclusions Reduced acuity and contrast sensitivity are significant risk factors for self-reported disability

Test-retest reliability of the CSV-1000 contrast test and its relationship to glaucoma therapy.

Abstract: Purpose. To determine the relationship between changes in contrast sensitivity, if any, after glaucoma therapy and the test-retest reliability of the CSV-1000 contrast sensitivity test. Methods. Patients with primary open angle glaucoma (N = 16) were retrospectively evaluated to determine changes in visual function, as measured by contrast sensitivity, after beta-blocker therapy. A control group (N = 24) of normally sighted patients was tested and retested on contrast sensitivity. For the control group, the coefficients of repeatability (95% confidence interval for test-retest variability [COR]) were calculated for each spatial frequency. The CORs were compared to the changes in vision found after therapy in the patients with glaucoma. Results. The group with glaucoma showed a significant improvement (P < .01) in contrast sensitivity at all spatial frequencies. The test-retest variance for normals, as measured by the COR, was smaller than the mean differences in contrast sensitivity before and after therapy at all spatial frequencies, expect 18 cyc/deg. Conclusions. Visual function in patients with glaucoma, as measured by contrast sensitivity, does improve after beta-blocker therapy. Further, the CSV-1000 is a clinically reliable tool for monitoring these changes. Invest Ophthalmol Vis Sci. 1994;35:3357—3361.

Chromatic properties of neurons in macaque MT.

Abstract: We have studied the responses of MT neurons to moving gratings, spatially modulated in luminance and chromaticity. Most MT neurons responded briskly and with high contrast sensitivity to targets whose luminance was modulated, with or without added chromatic contrast. When luminance modulation was removed and only chromatic stimulation was used, the responses of air MT neurons were attenuated. Most were completely unresponsive to stimulation with targets whose modulation fell within a ''null'' plane in color space; these null planes varied from neuron to neuron, but all lay close to the plane of constant photometric luminance. For about a third of the neurons, there was no color direction in which responses were completely abolished; almost all of these neurons had a definite minimum response for chromatic modulation near the isoluminant plane. MT neurons that responded to isoluminant targets did so inconsistently and with poor contrast sensitivity, so that only intensely modulated targets were effective. Whereas the best thresholds of MT neurons for luminance targets are close to behavioral contrast threshold, the thresholds for isoluminant targets lie considerably above behavioral contrast threshold. Therefore, although some MT neurons do give responses to isoluminant targets, they are unlikely to be the source of the chromatic motion signals revealed behaviorally.

Contrast enhancement of medical images using multiscale edge representation

Abstract: Experience suggests the existence of a connection between the contrast of a gray-scale image and the gradient magnitude of intensity edges in the neighborhood where the contrast is measured. This observation motivates the development of edge-based contrast enhancement techniques. We present a simple and effective method for image contrast enhancement based on the multiscale edge representation of images. The contrast of an image can be enhanced simply by stretching or upscaling the multiscale gradient maxima of the image. This method offers flexibility to selectively enhance features of different sizes and ability to control noise magnification. We present some experimental results from enhancing medical images and discuss the advantages of this wavelet approach over other edge-based techniques.

Image-dependent sharpness enhancement

Abstract: A method of altering the sharpness in the reproduction of an electronically encoded natural scene image, including process steps of: a) converting the signals to a luminance-chrominance signal format, wherein at least one signal represents overall image intensity; b) determining the maximum local contrast within the image; c) determining a filter function applicable to the image which increases maximum local contrast to a predetermined target value, and all other contrast to an amount proportional thereto; and d) applying the determined filter function to the image to increase sharpness thereof.

Evaluation of myocardial motion tracking with cine-phase contrast magnetic resonance imaging.

Abstract: RATIONALE AND OBJECTIVES. The accuracy of myocardial motion measurements, computed from cine-phase contrast (cine-PC) magnetic resonance (MR) velocity data, was compared with directly visualized motion of ME signal voids caused by implanted tantalum markers in anesthetized dogs. METHODS. Magnetic resonance imaging (MRI) data were electrocardiogram-gated and divided into 16 phases per cardiac cycle. Myocardial trajectories as a function of time in the cardiac cycle were measured using both methods for four to seven markers in each of eight animals. RESULTS. The peak observed in-plane excursion was 4.0 ±2.1 mm. The average deviation between displacements derived from velocity data versus displacements visualized directly was 1.1 ± 0.7 mm (27.5% of the peak displacement). The difference was less if three separate MR scans were used to measure each velocity component in the cine-PC method. This improvement is probably caused by improved temporal resolution. CONCLUSIONS. Cine-PC MRI offers a noninvasive method for accurate quantification of myocardial motion.

Matching color images: the effects of axial chromatic aberration

Abstract: We show how to compute and to use the wavelength-dependent optical transfer function (OTF) to create color matches between spatially patterned images. We model the human OTF as a defocused optical system with a circular aperture. In our model the defocus arises from axial chromatic aberration and wavelength-independent aberrations. From the computed OTF it is apparent that high-spatial-frequency components of the image can play little role in contrast and color appearance and that in the spatial-frequency range from 5 to 20 cycles/deg the visual system is dichromatic, because there is no contrast in the short-wavelength receptor signal. We show how to use the wavelength-dependent OTF to match color images across displays by setting matches in corresponding spatial-frequency bands. Because chromatic aberration so affects the OTF, this new procedure is a significant improvement over the conventional procedure of setting matches point by point.

Population encoding of spatial frequency, orientation, and color in macaque V1

Abstract: 1. We recorded local field potentials in the parafoveal representation in the primary visual cortex of anesthetized and paralyzed macaque monkeys with a multicontact electrode that provided for sampling of neural activity at 16 sites along a vertical penetration. Differential recordings at adjacent contacts were transformed into an estimate of current source density (CSD), to provide a measure of local neural activity. 2. We used m-sequence stimuli to map the region of visual space that provided input to the recording site. The local field potential recorded in macaque V1 has a population receptive field (PRF) size of approximately 2 deg2. 3. We assessed spatial tuning by the responses to two-dimensional Gaussian noise, spatially filtered to retain power only within one octave. Responses to achromatic band-limited noise stimuli revealed a prominent band-pass spatial tuning in the upper layers, but a more low-pass spatial tuning in lower layers. 4. We assessed orientation tuning by the responses to band-limited noise whose spectrum was further restricted to lie within 45 degrees wedges. The local field potential showed evidence of orientation tuning at most sites. Orientation tuning in upper and lower layers was manifest by systematic variations not only in response size but also in response dynamics. 5. We assessed chromatic tuning by the responses to isotropic band-limited noise modulated in a variety of directions in tristimulus space. Some lower-layer locations showed a nulling of response under near-isoluminant conditions. However, response dynamics in upper and lower layers depended not only on luminance contrast, but also on chromatic inputs. 6. Responses to near-isoluminant stimuli and to low-contrast luminance modulation were shifted to lower spatial frequencies. 7. We determined the extent to which various temporal frequencies in the response conveyed information concerning spatial frequency, orientation, and color under the steady-state conditions used in these studies. In each case, information is distributed in the response dynamics across a broad temporal frequency range, beginning at 4 Hz (the lowest frequency used). For spatial frequency the information rate remains significant up to at least 25 Hz. For orientation tuning and chromatic tuning, the information rate is lower overall and remains significant up to 13 Hz. In contrast, for texture discrimination, information is shifted to lower temporal frequencies.

Envelope-responsive neurons in areas 17 and 18 of cat.

Abstract: 1. Single cortical neurons are known to respond to visual stimuli containing Fourier components only in a narrow range of spatial frequencies. This investigation demonstrates that some neurons in cat area 17 and 18 can also respond to certain stimuli that have no Fourier components inside the cell's luminance spatial frequency passband. 2. To study such “non-Fourier” responses, we used envelope stimuli that consisted of a high-spatial-frequency sinusoidal luminance grating (carrier) whose contrast was modulated by a low-spatial frequency sine wave (envelope). There was no Fourier component at the apparent periodicity of the envelope spatial frequency. However, some cells responded to such a “phantom” component of the envelope modulation when it fell inside the cell's luminance spatial frequency passband while all the real Fourier components in the stimuli were outside. 3. We conducted extensive control experiments to eliminate the possibility of producing artifactual responses to the envelope stimuli due to any small residual nonlinearity of the z-linearized CRT screen. The control experiments included 1) testing of screen linearity to ensure that the effect from the residual screen nonlinearity was no larger than the sensitivity level of visual responses and 2) comparing the responses to envelope stimuli with the responses to the equivalent contrast of the artifact produced by the screen nonlinearity. All these control experiments indicated that any effect of screen nonlinearity did not contribute significantly to the neural envelope responses. 4. We performed a statistical analysis to obtain an index of relative strength of envelope responses for each cell and to objectively classify cells as “envelope-responsive” or “non-envelope-responsive.”(ABSTRACT TRUNCATED AT 250 WORDS)

Brightness and contrast control for a digital pulse-width modulated display system

Abstract: A method and system for adjusting the brightness and contrast of a digital pulse-width modulated display without scaling the input image data. Brightness is adjusted by changing the duty cycle of a displayed pixel either by altering the bit display durations, or by turning the pixel on during blanking periods 36. The contrast ratio may be altered by changing the display duration of at least one of the MSBs differently than the display duration of at least one of the LSBs. Contrast may be increased by extending the MSB display periods 50 and shortening the LSB display periods 52. Contrast may be decreased by shortening the MSB display periods 56 and extending the LSB display periods 58. The color tint of the displayed image may be altered by individually changing the brightness of the constituent colors.

Procedure for monitoring contrast agent application in a CT imaging system

Abstract: In a CT imaging system a baseline image is acquired prior to the injection of a contrast agent into the subject and a series of monitoring images are acquired which enable the affects of the contrast agent to be observed in real time. Regions in the baseline image may be selected with a cursor and the contrast enhancement amounts in these regions are calculated and displayed in both graphic and numerical form. By observing the changes that occur in the displayed information, the operator can determine the moment to begin the image scan for optimal image contrast.

Doppler enhancement with SH U 508A in multiple vascular regions.

Abstract: PURPOSE: To determine the magnitude and duration of peripheral vascular and cardiac Doppler signal enhancement after intravenous administration of contrast agent SH U 508A. MATERIALS AND METHODS: Suboptimal cardiac or peripheral vascular Doppler examinations were evaluated. A total of 75 intravenous bolus injections were made in 30 patients. Spectral audio Doppler intensity was measured throughout the duration of contrast effect. RESULTS: No clinically relevant adverse effects were noted, and Doppler enhancement was apparent in all cases. The diagnostic confidence of the investigators when scored before and after Doppler enhancement improved from 35% to 91% (P < .05). Doppler intensity increased more than 16 dB in all vascular regions investigated (P < .05). The contrast effect lasted for more than 120 seconds in the peripheral vascular and cardiac groups at equivalent doses. CONCLUSION: Intravenously administered SH U 508A is effective in markedly increasing cardiac, femoral arterial, and transcranial (c...

New model of human luminance pattern vision mechanisms: analysis of the effects of pattern orientation, spatial phase, and temporal frequency

Abstract: Models of human pattern vision mechanisms are examined in light of new results in psychophysics and single-cell recording. Four experiments on simultaneous masking of Gabor patterns by sinewave gratings are described. In these experiments target contrast thresholds are measured as functions of masker contrast, orientation, spatial phase, and temporal frequency. The results are used to test the theory of simultaneous masking proposed by Legge and Foley that is based on mechanisms that sum excitation linearly over a receptive field and produce a response that is an s-shaped transform of this sum. The theory is shown to be inadequate. Recent single-cell-recording results from simple cells in the cat show that these cells receive a broadband divisive input as well as an input that is summed linearly over their receptive fields. A new theory of simultaneous masking based on mechanisms with similar properties is shown to describe the psychophysical results well. Target threshold vs masker contrast (TvC) functions for a set of target-masker pairs are used to estimate the parameters of the theory including the excitatory and inhibitory sensitivities of the mechanisms along the various pattern dimensions. The human luminance pattern vision mechanisms, unlike most of the cells, do not saturate at high contrast.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Direction identification thresholds for second-order motion in central and peripheral vision

Abstract: Evidence bearing on the question of whether first-order and second-order motion are detected by use of the same or different principles has been sought. This question was approached by measuring thresholds for correctly identifying the direction of motion of various second-order motion patterns. The patterns used were contrast-modulated noise patterns in which the contrast of a carrier was modulated sinusoidally in one dimension, and the modulating waveform drifted smoothly while the carrier itself remained stationary. The carrier used was in most cases static two-dimensional noise; other carriers gave similar results. Thresholds were measured in terms of amplitude of contrast modulation (modulation depth) for each of a range of envelope drift speeds and spatial frequencies in the fovea and at several viewing eccentricities. Along with direction-identification thresholds, thresholds for either simple detection of the modulation or for correctly identifying the orientation of the modulation were simultaneously measured. Thresholds for direction identification were generally somewhat higher than those for simple detection. However, they were in most cases very similar to thresholds for identification of orientation, as found for conventional luminance gratings. Contrary to some reports, sensitivity to contrast-modulated patterns declines with eccentricity at a similar rate to that found with luminance gratings. The results suggest that first-order and second-order motion are either detected by a common motion-detection mechanism or are detected by different mechanisms that use a common principle of motion detection.

Cortical Magnification Theory Fails to Predict Visual Recognition

Abstract: The sense of form is poor in indirect view. Yet the cortical magnification theory asserts that the disadvantage can be made up by scaling the image size according to the spatial variation in the mapping of the retina onto the cortex. It is thus assumed that all visual information passes through a functionally homogeneous neural circuitry, with the spatial sampling of input signals varying across the visual field. We challenge this notion by showing that character recognition in the visual field cannot be accommodated by any concept of sole size scaling but requires increasing both size and contrast of the target being viewed. This finding is formalized into a hyperbolic law which states that target size multiplied by log contrast is constant across the visual field. We conclude that the scalar cortical magnification theory fails for character recognition since the latter depends on multidimensional pattern representations in higher, i.e. striate and prestriate, cortical areas.

Foveal flicker sensitivity in healthy aging eyes. II. Cross-sectional aging trends from 18 through 77 years of age

Abstract: Foveal flicker contrast sensitivity was measured for healthy adults at temporal frequencies from 2.5 to 50 Hz. The first experiment compared two-interval forced-choice (2IFC) and yes-no detection (Y–N) testing procedures for younger (19–33-year-old) and older (67–73-year-old) observers. The 2IFC technique resulted in higher absolute estimates of sensitivity. However, within a method, relative differences were similar. Therefore the two methods gave similar estimates of temporal contrast-sensitivity change with age. Experiment 2 compared 89 observers from 18 through 77 years of age to explore the effect of the time course of aging on flicker sensitivity. The 2IFC procedure was used, and retinal illuminance changes with age were controlled. Significant overall losses in contrast sensitivity were found for the 45–54, 55–64, and 65–77-year-old age groups. Overall sensitivities for the 35–44-year-old group were comparable with or (not significantly) higher than those for the 18–24- and 25–34-year-old groups. The results suggested that (1) foveal temporal contrast sensitivity does not decline until after 44 years, (2) losses after 44 years are in amplitude but not in temporal resolution of the visual response, and (3) the mean rate of loss is ∼0.78 decilog per decade after 44 years. These results are consistent with the existence of three phases of development of temporal contrast sensitivity over the life span. The results also emphasize the importance of including healthy-eyed age-matched controls in studies of flicker sensitivity in visual dysfunctions that affect mainly older adults.

Phase-contrast microscope

Abstract: A phase contrast microscope including an illuminating optical system, a ring slit arranged at a pupil of the illuminating optical system, an imaging optical system, and a phase plate arranged at a pupil of the imaging optical system which is conjugate with the pupil of the illuminating optical system with respect to a specimen plane on which a specimen under inspection is placed. The phase plate is formed by a liquid crystal and a pair of transparent ring-shaped electrodes arranged on both sides of the liquid crystal. A voltage applied across the liquid crystal via the electrodes is changed such that a phase difference introduced by the phase plate is set to ±π/2 to obtain a dark contrast image and a bright contrast image. A difference between these dark and bright contrast images is derived to obtain a phase contrast image having high contrast and resolution.