Showing papers by "Eli Peli published in 2012"

Perceived contrast in complex images

Abstract: To understand how different spatial frequencies contribute to the overall perceived contrast of complex, broadband photographic images, we adapted the classification image paradigm. Using natural images as stimuli, we randomly varied relative contrast amplitude at different spatial frequencies and had human subjects determine which images had higher contrast. Then, we determined how the random variations corresponded with the human judgments. We found that the overall contrast of an image is disproportionately determined by how much contrast is between 1 and 6 c/°, around the peak of the contrast sensitivity function (CSF). We then employed the basic components of contrast psychophysics modeling to show that the CSF alone is not enough to account for our results and that an increase in gain control strength toward low spatial frequencies is necessary. One important consequence of this is that contrast constancy, the apparent independence of suprathreshold perceived contrast and spatial frequency, will not hold during viewing of natural images. We also found that images with darker low-luminance regions tended to be judged as having higher overall contrast, which we interpret as the consequence of darker local backgrounds resulting in higher band-limited contrast response in the visual system.

Contour enhancement benefits older adults with simulated central field loss.

Abstract: Purpose. Age-related macular degeneration is the leading cause of vision loss among Americans aged 65 years. Currently, no effective treatment can reverse the central vision loss associated with most age-related macular degeneration. Digital image-processing techniques have been developed to improve image visibility for peripheral vision; however, both the selection and efficacy of such methods are limited. Progress has been difficult for two reasons: the exact nature of image enhancement that might benefit peripheral vision is not well understood, and efficient methods for testing such techniques have been elusive. The current study aims to develop both an effective image enhancement technique for peripheral vision and an efficient means for validating the technique. Methods. We used a novel contour-detection algorithm to locate shape-defining edges in images based on natural-image statistics. We then enhanced the scene by locally boosting the luminance contrast along such contours. Using a gaze-contingent display, we simulated central visual field loss in normally sighted young (aged 18–30 years) and older adults (aged 58–88 years). Visual search performance was measured as a function of contour enhancement strength [“Original” (unenhanced), “Medium,” and “High”]. For preference task, a separate group of subjects judged which image in a pair “would lead to better search performance.” Results. We found that although contour enhancement had no significant effect on search time and accuracy in young adults, Medium enhancement resulted in significantly shorter search time in older adults (about 13% reduction relative to Original). Both age-groups preferred images with Medium enhancement over Original (2–7 times). Furthermore, across age-groups, image content types, and enhancement strengths, there was a robust correlation between preference and performance. Conclusions. Our findings demonstrate a beneficial role of contour enhancement in peripheral vision for older adults. Our findings further suggest that task-specific preference judgments can be an efficient surrogate for performance testing. (Optom Vis Sci 2012;89:1374–1384)

Effects of contour enhancement on low-vision preference and visual search

Abstract: Most people with impaired central vision report difficulty viewing television and other video and electronic displays.1 Electronic image enhancement may be a viable vision rehabilitation approach. The increasing trend for electronic dissemination of information makes it important to develop and evaluate image enhancement techniques that can serve the needs of a growing population with vision impairments.2, 3 With rapid improvements in display and computer technologies and with the frequent releases of new consumer digital imaging products, ranging from hand held devices to high definition televisions (HDTVs), the application of this approach may be wide spread and simple once an effective method is identified. Image enhancement for vision rehabilitation was first proposed in the 1980s.4, 5 Since then, various approaches and enhancement algorithms have been applied to both static images6–8 and videos.9–14 Enhanced images were shown to be preferred to the originals by people with vision impairments for both static images6, 7 and videos.9, 12–14 Establishing preference for image enhancement may be the most important indication of its value, but it is also desirable to measure and, if possible, demonstrate functional (performance) benefits of this technology. While tasks with clearly defined performance metrics, such as reading speed, have been shown to have improved with the use of image enhancement,15–18 determining performance changes in passive viewing tasks is challenging.19 Some studies have demonstrated performance benefits for image enhancement of static images, in face recognition20, 21 and in facial expression recognition tasks.8 However, no performance benefit of the adaptive enhancement was found in the ability to describe the content of a TV program that was enhanced.22 In a recent study of preferences for adaptive image enhancement,23, 24 participants with normal vision had one of two types of preferences. One group of subjects (‘Sharp’) preferred higher enhancement than the original video for all images, while a majority of the subjects(‘Smooth’) preferred higher levels of enhancement only for videos that didn’t contain faces. For videos that predominantly contained faces, less (Sharp group) or no (Smooth group) enhancement was preferred compared to videos with other contents. Thus, both video content and individual differences determine how much enhancement is preferred. It could be expected that such variance in preferences could be found also amongst people with vision impairment. To investigate this effect in our study, we selected images with different contents (including faces). Perceived image quality as a function of image enhancement levels are non-monotonic, as there is a level above which the enhanced images begins to look worse,25 as found for people with normal sight.12, 14, 23, 24 In some studies of people with vision impairments,9, 12, 14 the relationship between preference and enhancement level was monotonic, suggesting that the optimal enhancement level was higher than that used in those studies. Thus, when evaluating image enhancement, it is important to ensure that the relationship between enhancement and benefit is explored over a sufficient range to identify the inflection in the response to the enhancement, that inflection possibly being the optimal level or just above it. The relationships between preferences and performances with image enhancement are not known. Performance could continue to improve above the inflection point in perceived image quality, as a high level of image enhancement which the viewer does not subjectively prefer may, nevertheless, produce better performance. Thus, one purpose of our study was to measure and compare preference and performance of subjects with impaired vision for wideband enhancement of the same static images. Wideband enhancement consists of finding edges or contours in images and modifying those features to enhance their contrast.26 Preference for images with modest levels of enhanced edges were demonstrated for static images 27 and for video segments13 among people with vision impairments. We measured performance using a visual search. A task with which we recently found improved performance 28 using a different enhancement method that we also found to be preferred (JPEG based enhancement 9, 12). However, we did not compare the performance and preference in the same study or with the same subjects, as we report in the present manuscript. Image contrast was enhanced along semantically-relevant object contours using a local edge detection method based on trained natural edge statistics.

Hazard Detection by Drivers with Paracentral Homonymous Field Loss: A Small Case Series.

Abstract: Introduction: Stroke frequently causes homonymous visual field loss. We previously found in a driving simulator that patients with complete homonymous hemianopia had difficulty detecting potential hazards on the side of the field loss. Here we measured the effects of limited paracentral homonymous field loss on detection performance. Methods: Three patients with paracentral homonymous scotomas, yet meeting vision requirements for driving in the United States, performed a pedestrian detection task while driving in a simulator. Pedestrians appeared in a variety of potentially hazardous situations on both sides of the road. Three age- and gender-matched control participants with normal vision participated for comparison purposes. Results: Pedestrians appearing in the scotomatous side of the visual field were less likely to be detected, and when they were, reaction times were longer, frequently too late to respond safely. Conclusions: Although legally permitted to drive in the USA, and possibly in other countries, patients with paracentral homonymous field loss may have impaired hazard detection and may benefit from education about their deficit and a fitness-to-drive evaluation.

Peripheral prism glasses: effects of dominance, suppression, and background.

Abstract: Purpose. Unilateral peripheral prisms for homonymous hemianopia (HH) place different images on corresponding peripheral retinal points, a rivalrous situation in which local suppression of the prism image could occur and thus limit device functionality. Detection with peripheral prisms has primarily been evaluated using conventional perimetry, where binocular rivalry is unlikely to occur. We quantified detection over more visually complex backgrounds and examined the effects of ocular dominance. Methods. Detection rates of eight participants with HH or quadranopia and normal binocularity wearing unilateral peripheral prism glasses were determined for static perimetry targets briefly presented in the prism expansion area (in the blind hemifield) and the seeing hemifield, under monocular and binocular viewing, over uniform gray and more complex patterned backgrounds. Results. Participants with normal binocularity had mixed sensory ocular dominance, demonstrated no difference in detection rates when prisms were fitted on the side of the HH or the opposite side (p 0.2), and had detection rates in the expansion area that were not different for monocular and binocular viewing over both backgrounds (p 0.4). However, two participants with abnormal binocularity and strong ocular dominance demonstrated reduced detection in the expansion area when prisms were fitted in front of the non-dominant eye. Conclusions. We found little evidence of local suppression of the peripheral prism image for HH patients with normal binocularity. However, in cases of strong ocular dominance, consideration should be given to fitting prisms before the dominant eye. Although these results are promising, further testing in more realistic conditions including image motion is needed. (Optom Vis Sci 2012;89:1343–1352)

Torsional anomalous retinal correspondence effectively expands the visual field in hemianopia.

Abstract: Homonymous hemianopia can be acquired or congenital1, 2 Congenital homonymous hemianopia is frequently diagnosed only in early adulthood, with the patient and family having no prior knowledge of the visual field defect3–5 In some cases of congenital or presumed congenital homonymous hemianopia, exotropia of the eye ipsilateral to the visual field loss has been reported6–11 For example, in right homonymous hemianopia with right exotropia, the right eye covers more of the right field of view (Figure 1b) than in orthotropia (Figure 1a) A left esotropia would provide a similar field expansion effect, but with some reduction of the left temporal visual field (Figure 1c) Reports of esotropia in congenital homonymous hemianopia are relatively unknown We found only one such probable occurrence in the literature12 Figure 1 Schematic binocular (dichoptic) visual field diagrams in right homonymous hemianopia illustrating the (a) Binocular field in orthotropia (both eyes fixating) (b) Binocular field in right exotropia (left eye fixating) (c) Binocular field in left esotropia As a result of strabismus, patients may experience diplopia (two images of the same object with different visual directions) and visual confusion (two different objects seen in the same visual direction)13 Usually only diplopia is reported spontaneously The field expansion illustrated in Figure 1 may be less beneficial if accompanied by diplopia, since central diplopia can be very disturbing and ill-tolerated It is possible to correct the diplopia with prisms or surgery,14, 15 or avoid it by patching one eye However, these treatments, if successful, also eliminate the field expansion benefit of the strabismus For this reason some strabismus surgeons consider congenital hemianopia a contraindication for strabismus surgery6 in spite of the disagreement if the strabismus is a purposeful adaptation to congenital hemianopia or simply an epiphenomenon10, 16 Prisms are frequently fitted as an optical treatment for hemianopia17, 18 If a prism sector is fitted unilaterally (on one lens only) and when the patient’s gaze is directed through the prism towards the field loss, central diplopia occurs,19 similar to acquired strabismus The strabismus (exotropia, if the prism is fitted base-out) is optically induced by the prism20 Again, if the diplopia is resolved either by fusing the images through the prisms or by suppression, the field expansion effect is eliminated21 Sensory adaptations such as suppression or anomalous retinal correspondence (ARC) may develop in the presence of strabismus occurring in the early years of development22–24 Suppression may be total, affects the whole field, or may be partial, affects only the fovea of the deviating eye or peripheral locus in the deviating eye directed in the same direction as the fovea of the fixating eye25 If suppression is total, the exotropia may not provide an effective field expansion for patients with hemianopia, as the potentially expanded field is suppressed If suppression is partial there may be some field expansion peripherally but not centrally21 We are not aware of any reports of measured suppression either total or central, as an adaptation in hemianopia with strabismus or with prismatic correction for hemianopia ARC is a sensory adaptation wherein the correspondence of the two eyes gets remapped in such a way that a non-foveal region in the deviating eye has the same visual direction as the fovea of the fixating eye, thus angle of remapping is equal to the angle of the deviation13, 27 enabling the fixated object to be perceived single, despite the strabismus This results in harmonious ARC (HARC) HARC eliminates both diplopia and confusion (but provides only poor stereopsis)26 If the exotropia is accompanied by HARC in patients with hemianopia the panoramic visual field expansion is considered to be fully functional and provides veridical visual direction across the whole field, including the expanded field The adaptation may be a combination of suppression and HARC If central suppression is to be combined with HARC only peripherally the adaptation may be of intermediate benefit, less effective than full field HARC, but more functional than central suppression without HARC Torsion or cyclorotation is the rotation of the eye around its visual axis28–32 Torsion without a known cause is called anatomic torsion33 Torsional strabismus frequently goes unnoticed, especially when congenital This is due to patients’ lack of reporting symptoms of torsional diplopia presumed to be due to sensory-motor adaptations29 Detection of anatomic torsion depends mainly on objective measurement by indirect ophthalmoscopy, fundus photography, or perimetry (relative rotational shift between the physiological blind spot and the fovea)29, 33, 34 With fundus imaging and with conventional perimeters, torsion can only be measured monocualrly Thus it may represent cyclophoria that should disappear under binocular viewing condition Cyclophoria should result in perceived relative rotation of the two eyes’ images under dissociated binocular condition (ie, in the synoptophore or using the double Maddox rod test) In torsional strabismus or cyclotropia the eyes will remain counter rotated under binocular viewing condition This should result in constant torsional diplopia (as happens with acquired torsional strabismus) or single binocular vision under either full suppression or torsional HARC, a phenomenon demonstrated here Hemianopia in a patient with torsional strabismus provides an opportunity to record the cyclorotated visual fields (counter rotated vertical meridians) under both monocular and binocular perimetry along with the rotated blind spot of one eye in monocular perimetry (Figure 2) Torsional strabismus in conjunction with hemianopia also provides field expansion For a right hemianope with intorsion, a superior field expansion results from the left eye’s intorsion while the right eye contributes a field expansion inferiorly (Figure 2a) The field expansion effects due to both lateral and torsional deviation add up providing a wider overall field expansion (Figure 2b & c) In the presence of HARC such expansion would be most useful Functional field expansion due to torsional deviation has not been reported before Figure 2 Schematic binocular visual field diagrams illustrating the visual field expansion in right homonymous hemianopia with torsional strabismus Field expansion is evident to the right of the vertical midline Areas seen by both eyes (white) are diplopic unless In the presence of strabismus, the monocular and binocular visual fields could differ considerably If a visual field expansion is recorded under binocular conditions it is difficult to ascertain whether the fixating or the deviating eye (in the absence of suppression) detected the target With a dichoptic perimeter the patient can be provided with a binocular fixation target (or even a complex background seen binocularly) while the monocular visual fields are measured with dichoptic stimuli35, 36 We used our dichoptic perimeter (see Methods) to investigate the retinal correspondence and visual fields of patients with homonymous hemianopia and strabismus

Volume perimetry: measurement in depth of visual field loss.

Abstract: Purpose. Volume scotomas are three-dimensional regions of space that are not visible to the observer. Volume perimetry maps volume scotomas. Volume scotomas predicted from combining monocular visual fields assume known fixation locus (mainly foveal). However, fixation loci are not always known, especially with central field loss (CFL). Here we demonstrate methods for measuring and calculating volume scotomas and discuss their practical implications. Methods. Three patients (bitemporal hemianopia, binasal scotoma, and CFL) were evaluated. Slices through the volume scotomas were measured at three distances: at the plane of fixation, at a plane anterior to fixation (representing anterior volume perimetry), and at a plane posterior to fixation (representing posterior volume perimetry). For anterior volume perimetry, patients fixated on a screen 100 cm away through a beamsplitter that reflected the perimetric stimulus (at 50 cm). For posterior volume perimetry, patients fixated on a near target (50 cm), while perimetric stimuli were presented on a screen 150 cm beyond fixation. At the plane of fixation, monocular visual fields under binocular viewing conditions were measured using a computerized dichoptic perimeter. Results. Posterior and anterior volume scotomas were documented in patients with bitemporal hemianopia and binasal scotomas, respectively. The CFL patient demonstrated both anterior and posterior volume scotomas. Scotoma magnitude was considered to determine its effect on visual function. Conclusions. Direct measurement of volume scotomas can be performed. Anterior and posterior volume visual fields can vary substantially from conventional binocular perimetry measured at the fixation plane, revealing blind areas not otherwise identified. These volume scotomas are likely to impair functional vision such as driving (for bitemporal hemianopes) and near work with small hand tools (for binasal scotomas). Patients with CFL will have impaired functional vision for both distance and near tasks. Consideration of volume scotomas can help provide more effective vision rehabilitation and counseling. (Optom Vis Sci 2012;89:E1265–E1275)

Methods for Automated Identification of Informative Behaviors in Natural Bioptic Driving

Abstract: Visually impaired people may legally drive if wearing bioptic telescopes in some developed countries. To address the controversial safety issue of the practice, we have developed a low-cost in-car recording system that can be installed in study participants' own vehicles to record their daily driving activities. We also developed a set of automated identification techniques of informative behaviors to facilitate efficient manual review of important segments submerged in the vast amount of uncontrolled data. Here, we present the methods and quantitative results of the detection performance for six types of driving maneuvers and behaviors that are important for bioptic driving: bioptic telescope use, turns, curves, intersections, weaving, and rapid stops. The testing data were collected from one normally sighted and two visually impaired subjects across multiple days. The detection rates ranged from 82% up to 100%, and the false discovery rates ranged from 0% to 13%. In addition, two human observers were able to interpret about 80% of targets viewed through the telescope. These results indicate that with appropriate data processing the low-cost system is able to provide reliable data for natural bioptic driving studies.

Complexities of Complex Contrast

Abstract: For the visual system, luminance contrast is a fundamental property of images, and is one of the main inputs of any simulation of visual processing. Many models intended to evaluate visual properties such as image discriminability compute perceived contrast by using contrast sensitivity functions derived from studies of human spatial vision. Such use is of questionable validity even for such applications (i.e. full-reference image quality metrics), but it is usually inappropriate for no-reference image quality measures. In this paper, we outline why the contrast sensitivity functions commonly used are not appropriate in such applications, and why weighting suprathreshold contrasts by any sensitivity function can be misleading. We propose that rather than weighting image contrasts (or contrast differences) by some assumed sensitivity function, it would be more useful for most purposes requiring estimates of perceived contrast or quality to develop an estimate of efficiency: how much of an image is making it past the relevant thresholds. 1. CONTRAST 1.1 Primacy For the visual system, luminance contrast is the fundamental carrier of information about images. Motion is perceived through temporal changes in luminance contrast; the most initial sensations of depth are formed from binocular combinations of monocularly sensed contrasts; chromatic variation is almost always correlated with luminance changes. All of these qualities (motion, depth, color) are important parts of normal visual experience, and thus of any full-quality representation of it; but a monochromatic, cyclopean still-image is a perfectly acceptable visual representation of a scene. Here we argue that in estimating the visual quality of an image, contrast thresholds are of principal importance; perceived (suprathreshold) contrast magnitudes although noticeable in side-by-side comparison are relatively less important; and that the specific sensitivity functions commonly used in standard practice to estimate perceived contrast and quality may be misapplied or inappropriate. 1.2 Measurement Given the primary importance to vision of luminance contrast, it is of great practical and theoretical importance to have operational measures of it (1). The simplest summary measures will usually fail in characterizing the apparent contrast of an image, and thus are not used except for the simplest of patterns. Michelson contrast, the absolute range of luminances in a pattern, is thus not widely used except for periodic grating patterns - it errs by ignoring too much of an image's spatial variation. A much more common measure of a complex image's contrast is the standard deviation of luminances in an image (RMS contrast), a measure of the average deviation in luminance from the image mean over a specified spatial area. This measure is less susceptible to extreme values in an image, and thus tracks better with perceptual appearance of image contrast. Still, RMS contrast is a relatively poor predictor of perceived contrast - it errs by equally weighting all of the image's spatial variation and has been shown repeatedly to fail in predicting perceived image quality. 1.3 Perception The visual system responds to images through a system of overlapping neural networks, repeated across the visual field, which are sensitive to different spatial scales, and perceived contrast is related to the response of these networks. The

Electronic magnification and perceived contrast of video.

Abstract: Electronic magnification of an image results in a decrease in its perceived contrast. The decrease in perceived contrast could be due to a perceived blur or to limited sampling of the range of contrasts in the original image. We measured the effect on perceived contrast of magnification in two contexts: either a small video was enlarged to fill a larger area, or a portion of a larger video was enlarged to fill the same area as the original. Subjects attenuated the source video contrast to match the perceived contrast of the magnified videos, with the effect increasing with magnification and decreasing with viewing distance. These effects are consistent with expectations based on both the contrast statistics of natural images and the contrast sensitivity of the human visual system. We demonstrate that local regions within videos usually have lower physical contrast than the whole, and that this difference accounts for a minor part of the perceived differences. Instead, visibility of ‘missing content’ (blur) in a video is misinterpreted as a decrease in contrast. We detail how the effects of magnification on perceived contrast can be measured while avoiding confounding factors.

Scanning And Detection Of Static And Moving Pedestrians By Drivers With Hemianopia In A Simulator

Abstract: PURPOSE: In our previous simulator study of drivers with hemianopia (Bowers et al., 2009), we reported large detection deficits for stationary pedestrians that appeared in the blind hemifield. Using a more realistic hazard, we are now evaluating detection of pedestrians that move on a collision course toward the car’s heading direction. We predicted that blindside detection rates would be higher for moving pedestrians (as they maintain an approximately constant eccentricity with respect to the car) than for static pedestrians (eccentricity of the pedestrian increases rapidly as the car approaches, thus moving the hazard further into the blind hemifield). In addition, we are evaluating the relationship between gaze behaviors and detection performance.