Low Contrast Levels Research Articles

Effect of Environmental Conditions on Performance of Image Recognition-Based Lane Departure Warning System

Lane departure warning systems (LDWS) provide an effective counter-measure against road departure crashes. Much commercial development of these systems has concentrated on image processing-based systems. Because these systems depend on the detection and tracking of lane markings, it is logical to hypothesize that the visibility of the markings under different weather and lighting conditions may significantly influence the performance of LDWS. Work was done to determine the effects of environmental conditions such as rain and lighting on LDWS performance with typical lane-marking installations on limited-access facilities. The main performance measure used in this study is the efficacy rate (ER). This is defined as the ratio of the number of instances in which the lane departure warning device is able to provide lane crossing alarms in all instances in which an LDWS-equipped vehicle crosses the lane markings. The results indicate that in most cases the ER of the LDWS under dry and light rain conditions is 100%. However, it was found that performance of LDWS of lane markings was affected significantly by heavy rain conditions at night for typical lane-marking installations. With heavy-rain night conditions, most of the observed ER values were between 0% and 30%. It appears that the ER associated with night rain conditions increases with increased lane-marking retroreflectivity. Dusk conditions were found to drop the ER of LDWS by 15% to 18%. At a few locations that had yellow lane markings near the end of their service lives or had yellow markings on concrete pavements, LDWS had difficulty detecting the markings because of the low level of contrast between the yellow markings and the pavement.

Contrast Acuity in Cataracts of Different Morphology and Association to Self-Reported Visual Function

To evaluate the relationship between contrast acuity at declining contrast levels and the type and density of lens opacity in cataract. Contrast acuity at declining contrast levels was determined with the Holladay Contrast Acuity Test, in relation to the type and density of age-related cataract in 180 patients with bilateral cataract and 20 control subjects with normal macular function. Cataracts were graded according to the Lens Opacities Classification System (LOCS) III of nuclear color (NC), nuclear opalescence (NO), cortical (C), and posterior subcapsular (P) cataract. Best-corrected visual acuity and near contrast acuity were determined in randomized order monocularly in both eyes. Visual difficulties in everyday life were evaluated, using the VF-14 questionnaire and the Cataract Symptom Score. The contrast-dependent effect of cataract on contrast acuity was statistically significant (P < 0.001; two-way ANOVA). In the comparison of early, intermediate, and advanced nuclear, nuclear-cortical, and posterior subcapsular cataracts (PSCs), significantly reduced contrast acuity scores were found for the PSC groups (P < 0.001). Comparison of nuclear and nuclear-cortical cataracts showed the contrast acuity scores to be comparable at all contrast levels (P > 0.05). High correlation coefficients were found between the LOCS III P score and the contrast acuity measurements (r = 0.77-0.84; P < 0.001). In contrast, the correlation coefficients of the NO, NC, and C scores were considerably lower (r = 0.45-0.66; P < 0.001). High correlation coefficients were also found between the contrast acuity measurements and self-reported functional vision. The statistically significant, contrast-dependent effect of cataract on contrast acuity supports the clinical relevance of recording visual acuity at low contrast levels in patients with age-related cataract. Particularly, the severity of PSC has a strong influence on the impairment of contrast acuity. Contrast acuity corresponded closely to the self-reported visual difficulties in everyday life.

Enhancing retinal image by the Contourlet transform

The evaluation of retinal images is widely used to help doctors diagnose many diseases, such as diabetes or hypertension. Due to the acquisition process, retinal images often have low grey level contrast and dynamic range. This problem may seriously affect the diagnostic procedure and its results. Here we present a new multi-scale method for retinal image contrast enhancement based on the Contourlet transform. The Contourlet transform has better performance in representing edges than wavelets for its anisotropy and directionality, and is therefore well-suited for multi-scale edge enhancement. We modify the Contourlet coefficients in corresponding subbands via a nonlinear function and take the noise into account for more precise reconstruction and better visualization. We compare this approach with enhancement based on the Wavelet transform, Histogram Equalization, Local Normalization and Linear Unsharp Masking. The application of this method on images from the DRIVE database showed that the proposed approach outperforms other enhancement methods on low contrast and dynamic range images, with an encouraging improvement, and might be helpful for vessel segmentation.

Depiction of Low-Contrast Detail in Digital Radiography

We sought to evaluate the low-contrast performance of a newly developed needle image plate/line scanner (NIP) computed radiography system in comparison with a standard powder image plate/flying-spot scanner (PIP) system. A total of 36 images of a CDRAD phantom, simulating low-contrast structures with different drill holes of different diameters, were obtained with both imaging systems using 9 different exposure variables. All images had window and level set to generate consistent density and contrast. In addition, multiscale contrast-dependent contrast amplification was applied to some of the images. All images obtained were printed and presented to a total of 10 observers (5 radiologists, 5 engineers/physicists), who were blinded to both the image plate and parameter setting used. The smallest detectable drill hole depth (= contrast) correctly identified was recorded for each diameter. The median values observed were calculated and tested for statistical differences between PIP and NIP using Student t test for matched pairs (level of significance P < or = 0.05). At all but 2 settings of the variables, NIP images depicted significantly lower contrast levels (= lower depth of drill holes) compared with PIP images. The 2 settings also showed a trend towards better low contrast depiction with NIP. In no case was low contrast performance better using PIP images. Images obtained with needle image plate/line scanner provide superior low contrast performance compared with the images obtained with powder image plate/flying-spot scanner.

Mesopic radial frequency contrast sensitivity function for young and older adults

The objective of the present study was to determine contrast sensitivity curves of concentric circular patterns with radial frequencies of 0.25, 0.5, 1.0, 2.0, and 4.0 cycles per degree in young and older adult volunteers. These parameters were also compared with sensitivity contrasts for sine-wave gratings. All participants had normal acuity vision and were free of identifiable ocular illness. Contrast sensitivity was measured in 6 young adults aged 19 to 23 years and 6 older adults aged 60 to 69 years using the psychophysical forced-choice method. In this paradigm the volunteers had to decide which of two stimuli contained the above radial frequencies at low contrast levels. The other neutral stimulus was gray with homogeneous luminance. We detected a decline in contrast sensitivity for older adults at all radial frequencies compared to young adults. Also, contrast sensitivity for sine-wave gratings at all measured frequencies was better, as predicted, for all young adults. Maximum sensitivities in the radial frequency contrast sensitivity function and contrast sensitivity function occurred at 0.25 and 0.5 cycles per degree, respectively, for both young and older adults. These results suggest age-related changes in the contrast sensitivity function for concentric symmetrical stimuli.

Induced gamma-band oscillations correlate with awareness in hemianopic patient GY

In normal vision gamma oscillations are involved in object perception, are modulated by attention, and have been linked to awareness by way of their putative role in perceptual integration, arguably as a mechanism for synchronizing activity in separate neural assemblies. We tested the hypothesis that the presence of gamma oscillations (∼30–80 Hz) signal the entry of a neural representation into awareness (as indexed by direct report), while attempting to control for other measures of neural information processing such as discrimination accuracy and reaction time. Hemianopic patient GY sometimes reports an awareness “that something happened” in his blind visual hemifield, in response to stimuli of sufficiently high contrast, although he may deny “seeing” anything. At lower contrast levels GY denies any awareness, but may continue to exhibit greater-than-chance accuracy (blindsight). Using a near-threshold level of contrast offers a unique way to test hypotheses concerning correlates of perceptual awareness, since GY's accuracy on certain tasks is independent of awareness. We tested GY on an orientation–discrimination task using stationary stimuli at a fixed near-threshold level of contrast, to which GY sometimes responded “aware” and sometimes “unaware”. We recorded brain activity using magnetoencephalography (MEG) in order to determine the relationship between local induced gamma-band oscillations and awareness. GY's accuracy was significantly greater than expected by chance and no different whether or not he reported awareness of the stimulus. Oscillatory activity in the gamma band (44–66 Hz) over the left occipito-parietal region correlated significantly with awareness (but not accuracy), whereas activity in the alpha band (8–12 Hz) did not.

Radial frequency stimuli and sine-wave gratings seem to be processed by distinct contrast brain mechanisms

An assumption commonly made in the study of visual perception is that the lower the contrast threshold for a given stimulus, the more sensitive and selective will be the mechanism that processes it. On the basis of this consideration, we investigated contrast thresholds for two classes of stimuli: sine-wave gratings and radial frequency stimuli (i.e., j0 targets or stimuli modulated by spherical Bessel functions). Employing a suprathreshold summation method, we measured the selectivity of spatial and radial frequency filters using either sine-wave gratings or j0 target contrast profiles at either 1 or 4 cycles per degree of visual angle (cpd), as the test frequencies. Thus, in a forced-choice trial, observers chose between a background spatial (or radial) frequency alone and the given background stimulus plus the test frequency (1 or 4 cpd sine-wave grating or radial frequency). Contrary to our expectations, the results showed elevated thresholds (i.e., inhibition) for sine-wave gratings and decreased thresholds (i.e., summation) for radial frequencies when background and test frequencies were identical. This was true for both 1- and 4-cpd test frequencies. This finding suggests that sine-wave gratings and radial frequency stimuli are processed by different quasi-linear systems, one working at low luminance and contrast level (sine-wave gratings) and the other at high luminance and contrast levels (radial frequency stimuli). We think that this interpretation is consistent with distinct foveal only and foveal-parafoveal mechanisms involving striate and/or other higher visual areas (i.e., V2 and V4).

Visual Evoked Potentials and Reaction Time Measurements to Motion-reversal Luminance- and Texture-defined Stimuli

Previous studies have suggested that compared to first-order (FO) motion stimuli, second-order (SO) motion stimuli required more cortical time to be processed. The purpose of this study was: 1- to verify this claim with Visual Evoked Potential (VEP) and eye-hand Reaction Time (RT) measurements and 2- examine if the VEP and RT responses are similarly modulated by the same trigger features of the stimuli. The VEPs and eye-hand RT for motion-reversal luminance- (first-order) and texture-defined (second-order) stimuli were recorded from ten normal human subjects. VEPs and RTs were measured for each motion class at eight different modulation depths (from 3 to 100%). Our results reveal that for stimuli of low contrast levels, the SO-FO timing differences are approximately 100 ms (RT) or 20 ms (VEP), while for contrasts >or= 15-20% (VEP) or >or= 50% (RT), the SO-FO difference is no longer significant (p < 0.007), suggesting either that the brain can no longer distinguish SO from FO stimuli or that in spite of the added complexity of SO stimuli the brain takes equal time to process both. Interestingly, the above contrast discrepancy in SO-FO resolution threshold suggests that, compared to the VEP, the more psychophysical RT measurement can process and thus distinguish a larger spectrum of motion stimuli, thus further confirming the latter measure of the retinocortical processing time as a valid alternative to the VEP.

Intracellular measurements of spatial integration and the MAX operation in complex cells of the cat primary visual cortex.

We have examined the spatial integration properties of complex cells to determine whether some of their responses can be described by a maximum operation (MAX)-like computation, as suggested by Riesenhuber and Poggio's model of object recognition. Membrane potential was recorded from anesthetized cats while optimally oriented bars were presented, either alone or in pairs, in different parts of the cells' receptive field. In most cells, the membrane potential response to two bars presented simultaneously could not be predicted by the sum of the responses to individual bars. In many cells, however, the responses closely approximated a MAX-like model. That is, the response of the cell to two bars was similar to the larger of the two individual responses ("soft-MAX"). The degree of nonlinear summation varied from cell to cell and varied within single cells from one stimulus configuration to another but on average fit most closely to the MAX model. The firing response of the cells was also well predicted by the MAX-like model. The MAX-like behavior was independent of the distance between the bars (orthogonal to the preferred orientation), independent of the relative amplitude of the responses, and slightly less pronounced at low levels of contrast. This MAX-like behavior of a subset of complex cells may play an important role in invariant object recognition in clutter.

The effect of trough colour on the voluntary food intake of postweaned pigs—a short communication

The aim of this experiment was to assess the effect of trough colour on the voluntary food intake of weaned pigs. Pigs were weaned at 28 days of age and offered feed in either a red, yellow or black trough. The trough colours were chosen as they represented a high (yellow), intermediate (red) or low (black) level of contrast with the pelleted feed. It was hypothesised that by increasing the contrast between the trough and the solid diet, the visual stimulus provided to the pigs would encourage feed intake. There were, however, no significant effects of trough colour on feed intake, daily liveweight gain or FCR during the 4-week period following weaning. Therefore, it was concluded that trough colour does not affect voluntary feed intake in the postweaned pig.

Practical assessment of vision in the elderly.

In this article we have demonstrated the significant visual impairments in elders with good vision by standard measures when the viewing conditions are less than ideal--low contrast, low or changing light level, or glare. Much of this impairment can be accounted for by the known age-related changes in the eye. We have emphasized that two people of the same age with the same standard acuity can have dramatically different vision under these nonideal conditions. Because one cannot predict vision under these conditions on the basis of standard acuity, it is important to use these additional measures. These other vision measures are more closely related to task performance (reading, face recognition) than standard acuity. We find that persons who perform poorly on these low-contrast measures are more likely to have significant losses of standard acuity in the future. Hence, these measures help identify persons who should be more closely monitored. Each of the vision functions shows significant decline across age. It is important to recognize that a 60-year-old with 20/30 acuity is different from an 80-year-old with the same acuity. The older person is more likely to be impaired under the conditions of daily life. Measurement and appreciation of visual function should allow better understanding of subjective visual symptoms and allow the clinician to offer appropriate advice for mitigating these impairments. Often simple interventions, such as correcting refractive error, improving lighting, avoiding large changes in light level, increasing contrast, and avoiding glare (perhaps through cataract surgery), can greatly improve not only vision function but quality of life. Many studies have reported that good vision allows for independent living, which is the goal for most elders.

Testing Visual Search Performance Using Retinal Light Scanning as a Future Wearable Low Vision Aid

For the goal of designing a wearable low vision aid, aspects of both head-mounted display (HMD) design and performance evaluation were integrated into a single study of scanning ability. A head-mounted version of a novel retinal light scanning display known as the virtual retinal display was fabricated for this study. A remote head CCD (charge coupled device) attached approximately at the user's line of sight was used as the input source. Scanning ability was quantified as the time to identify a target in a wide field of distractors while using the HMD design in four different display interface modes (DIMs). Each DIM was tested with respect to their corresponding controls: A (augmented, see-through) and CO (center occluded) DIMs were compared to the augmented control (augmented, retinal display turned off), and CPO (center and periphery occluded) and PO (periphery occluded) DIMs were compared to the periphery occluded control (periphery obstructed, retinal display turned off). Each DIM was tested at high, medium, and low contrast levels. Five subjects were tested without optical correction (visual acuity worse than 20/200), which accurately represented low vision subjects. Results showed that for each DIM, scanning performance decreased as the contrast level decreased. At the lowest contrast level, the PO DIM provided the greatest and most significant improvement in scanning performance. At the medium and highest contrast levels, all DIMs provided equal or worse performance than their controls. Additionally, the subjects' natural visual acuity showed no correlation with scan time. All subjects reported a higher acuity with the retinal display, and showed a diverse range of preference for the most helpful DIM. Based on these results, we realized that low vision applications have unique design requirements unlike those for normally sighted users, and therefore customization of a wearable low vision aid may be an optimal design strategy.

Contrast letter acuity as a measure of visual dysfunction in patients with Friedreich ataxia.

Friedreich ataxia is a progressive neurodegenerative disorder affecting afferent cerebellar pathways and other neuronal systems, including afferent visual pathways. A systematic clinical outcome measure for examination of visual dysfunction in Friedreich ataxia has not been identified. We sought to identify a simple, reliable method for assessing clinical and subclinical visual dysfunction in patients with Friedreich ataxia. Contrast letter acuity was measured binocularly in Friedreich ataxia patients and age-matched visually asymptomatic volunteers (control group) using the Low-contrast Sloan Letter Charts at three different low-contrast levels (5.0%, 1.25%, and 0.6%). Binocular high-contrast visual acuity (100% level) was also determined for each participant. Despite equal median binocular high-contrast visual acuities between the two groups, patients with Friedreich ataxia had significantly lower (worse) Low-contrast Sloan Letter Chart scores compared with controls, particularly at the lowest contrast levels (1.25% and 0.6%). Ambulation status significantly predicted Low-contrast Sloan Letter Charts scores in linear regression models accounting for patient age, suggesting a potential complementary role for Low-contrast Sloan Letter Chart testing in the assessment of disease status as well as visual function in Friedreich ataxia. This study demonstrates that Low-contrast Sloan Letter Chart testing may provide a useful clinical outcome measure for Friedreich ataxia and other neuro-ophthalmologic disorders.

Foveal and eccentric acuity in one-eyed observers

The purpose of these two experiments was to examine the effects of enucleation on the foveal and near-periphery acuity of human observers. In Experiment 1, using high contrast illiterate-E optotypes, we measured foveal and 7-degree eccentric (nasal and temporal) acuity in the remaining eye of unilaterally enucleated children and adults. The data were compared to those of the fellow or ‘good eye’ of age-matched non-amblyopic and amblyopic strabismic observers—a different kind of monocular deprivation—and binocularly normal subjects. The main finding was that the 7-degree eccentric acuity of enucleated observers and normal controls was best, followed by that of the non-amblyopic and lastly, the amblyopic subjects. There were no naso-temporal asymmetries. In Experiment 2, foveal and 7-degree eccentric acuities of enucleated and binocularly normal subjects were tested at 96, 13.5 and 4.7% contrast levels using the same kind of stimuli. The foveal acuity of the enucleated observers was better than the monocular acuity of controls at all contrasts, and equivalent to their binocular acuity. The eccentric acuity of the enucleated observers was better at the two low contrast levels. No naso-temporal asymmetries were found in either group at any contrast level. These findings suggest that, in humans, visual deprivation from unilateral enucleation does not result in a naso-temporal asymmetry in the near periphery.

Word acuity threshold as a function of contrast and retinal eccentricity.

We determined word acuity thresholds as a function of contrast and retinal eccentricity to determine the rate of threshold alteration in the normal retinal periphery. Subjects identified words presented foveally (0 degrees eccentricity) or above the point of fixation at retinal eccentricities of 0.5, 1, 2, 3, 6, and 8 degrees for three contrast levels of 10, 45, and 85%. A descending method of limits was used to determine thresholds for random four-letter words flashed for 90 ms at the different retinal eccentricities. For high-contrast letters, word acuity displayed threshold elevation in the periphery similar to previous reports and similar threshold elevation to those reported for vernier acuity. Lower contrast levels displayed different threshold change as a function of eccentricity, approaching levels reported for grating acuity. When comparing the relative elevation of word acuity thresholds for the different contrast levels (85 vs. 45% and 85 vs. 10%), both comparisons showed that the most rapid decline in word acuity threshold occurs within 2 degrees of the fovea. The peripheral retina displays a reduction in word acuity threshold that is dependent on letter contrast and shows a change similar to those reported for higher cortical functions such as vernier thresholds. The greatest word threshold elevation occurs within the central 2 degrees of the fovea.

The influence of level and polarity of figure-ground contrast on vision.

To document the effect upon human foveal vision of changes in the level and polarity of figure-ground contrast under photopic controlled test conditions, with particular emphasis on performance at low contrast levels. Using a forced-choice psychophysical paradigm, threshold acuity estimates were derived at 9 discrete levels over a near-3 octave contrast range for Landolt ring-type stimuli of either positive or negative polarity. Data were obtained under binocular conditions from 10 young adults, each wearing their optimum low myopic spectacle correction. Visual acuity declined linearly with reducing stimulus contrast, the deterioration increasing substantially at <10% figure-ground contrast regardless of stimulus polarity. Performance was slightly (but not statistically significantly) better for positive contrast stimuli. Irrespective of contrast polarity, a reduction in stimulus figure-ground contrast <10% produces an accelerated decrement in photopic foveal vision compared to the performance at levels >10%. Some clinical and practical implications of this outcome are considered with regard to the examination of patients with normal and compromised visual function.

Contrast matching across spatial frequencies for isoluminant chromatic gratings

Contrast matching was performed with isoluminant red–green and s-cone gratings at spatial frequencies ranging from 0.5 to 8 c/deg. Contrast threshold curves were low-pass in shape, in agreement with previous findings. Contrast matching functions resembled threshold curves at low contrast levels, but became flat and independent of spatial frequency at high contrasts. Thus, isoluminant chromatic gratings exhibited contrast constancy at suprathreshold contrast levels in a similar manner as has been demonstrated for achromatic gratings.

Experimental determination of practical peak voltage.

In diagnostic radiology the practical peak voltage was initially derived by postulating that, for a given combination of X-ray tube and contrast geometry, a constant X-ray tube voltage should produce the same low level contrast as an arbitrarily pulsating X-ray tube voltage. It has been shown previously that the practical peak voltage can be properly defined as a weighted average of the X-ray tube voltage. Up to now the concept of practical peak voltage was based entirely on the results of calculations. The present paper describes the experimental investigations for measuring and comparing the contrast-equivalent X-ray tube voltage and practical peak voltage derived from an invasive measurement of the time-dependent X-ray tube voltage. Within the experimental uncertainties, the results demonstrate the mutual equivalence of the practical peak voltage and the contrast-equivalent X-ray tube voltage.

Binocular function after bilateral implantation of monofocal and refractive multifocal intraocular lenses

Purpose: To evaluate binocular function after bilateral implantation of a refractive multifocal intraocular lens (IOL). Methods: Contrast sensitivity, glare, depth discrimination, and distance and near, visual acuity were examined in 21 patients with a multifocal IOL and 15 patients with a monofocal IOL. Results: There was no significant difference in binocular distance visual acuity,between the 2 groups. In near visual acuity with distance correction, the multifocal group had a significant advantage in monocular, and binocular vision ,( P < ,05). High contrast with Regan contrast letter acuity charts did not differ significantly between the groups, However, at a contrast of 11%, sensitivity was significantly lower monocularly in the multifocal group. Contrast sensitivity in this group was not significantly lower binocularly. The brightness acuity testdr revealed no between group difference in glare. With the Titmus stereotest, depth discrimination was significantly better with the multifocal IOL with distance correction. Conclusion: Bilateral implantation of a refractive multifocal IOL provided goodbinocular function and alleviated the well-known disadvantage of reduced contrast sensitivity at low contrast levels.

Distortion-invariant filter for nonoverlapping noise

A new heuristic filter based on the optimum filter for disjoint noise developed by Javidi and Wang [J. Opt. Soc. Am. A 11, 2604 (1995)] is presented. In this new filter a number of optimum filters built from single training images are combined linearly by use of the synthetic discriminant function (SDF) approach into a distortion-invariant filter for disjoint noise. Like the traditional SDF approach, this summation technique makes it possible to control the height of the correlation peak easily, for example, if a uniform filter response is needed. The filter is compared with the distortion-invariant version of the optimum filter on images with low contrast and high levels of nonoverlapping clutter. The new filter shows good results, demonstrating that it is, with very simple heuristic methods, possible to improve the performance of distortion-invariant filters for nonoverlapping noise.