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PubMed2005

Stereoscopic correspondence for ambiguous targets is affected by elevation and fixation distance.

Binocular correspondence must be determined if disparity is to be used to provide information about three-dimensional shape. The current study investigated whether knowledge of the statistical distribution of disparities in the natural environment is employed in this process. A simple model, which produces distributions of distances similar to those found in the natural environment, was used to predict the distribution of disparities in natural images. This model predicts that crossed disparities will be more likely as (i) stimulus elevation decreases below fixation and (ii) fixation distance increases. To determine whether these factors influence binocular correspondence for human observers, ambiguous stereograms were presented to observers, as stimulus elevation and fixation distance were manipulated. Clear biases were observed in the depth perceived in these stereograms, which were more likely to be seen as closer than fixation (i) for stimuli presented below fixation and (ii) as fixation distance increased. These results suggest that binocular correspondence is determined in a manner consistent with the distributions of disparities expected in natural scenes.

Depth Perception↗

PubMed2004

What is the depth of a sinusoidal grating?

Stereo matching of a textured surface is, in principle, ambiguous because of the quasi-repetitive nature of texture. Here, we used a perfectly repetitive texture, namely a sinusoidal grating, to examine human stereo matching for repetitive patterns. Observers matched the depth of a vertical grating segment, 6-deg wide and presented in a rectangular envelope at or near the disparity of the segment edges. The interocular phase of the carrier also influenced stereo matching, producing shifts in depth arrayed around the plane specified by the edges. The limiting disparity for the edge matches was 40-60 arcmin, independent of the spatial frequency of the carrier. One explanation for these results is that first-order disparity energy mechanisms, tuned to lower spatial frequencies, respond to the edge disparities, while showing little response to the interocular phase of the carrier. In principle, these first-order low frequency mechanisms could account for edge-based stereo matching at high contrasts. But, edge matching is also observed at carrier contrasts as low as 5%, where these low frequency mechanisms are unlikely to detect the grating stimulus. This result suggests that edge matching for gratings depends on coarse-scale second-order stereo mechanisms, similar to the second-order mechanisms that have been proposed for encoding two-dimensional texture. We conclude that stereo matching of gratings (or any other texture) depends on a combination of responses in both coarse-scale second-order and fine-scale first-order disparity mechanisms.

Contrast Sensitivity↗

PubMed1997

Racial variation in retinopathy of prematurity. The Cryotherapy for Retinopathy of Prematurity Cooperative Group.

OBJECTIVES: To determine and compare the incidence of severe, vision-threatening retinopathy of prematurity (ROP) in black and white low-birth-weight infants. DESIGN: Prospective cohort study. SETTING: Seventy neonatal intensive care units in 23 US participating centers in the Multicenter Trial of Cryotherapy for Retinopathy of Prematurity. PATIENTS: A total of 4099 premature infants weighing less than 1251 g at birth were enrolled to evaluate the natural history of ROP. This 'Natural History' cohort included 2158 white infants and 1584 black infants who were followed up prospectively according to a Natural History protocol. MAIN OUTCOME MEASURES: Incidence and severity of acute ROP. RESULTS: While ROP occurred with similar frequency in all racial subgroups, severe ROP was less common in black infants. One hundred sixty (7.4%) of 2158 white infants reached threshold ROP (defined as at least 5 contiguous or 8 cumulative clock-hours of stage 3 retinopathy in zone 1 or zone 2 in the presence of "plus disease" [dilation and tortuosity of the posterior pole blood vessels]), but only 51 (3.2%) of 1584 black infants progressed to threshold ROP. Using multiple logistic regression analysis, race emerged as a highly significant factor (P < .001) in the development of threshold disease, even when birth weight, gestational age status at delivery, sex, multiple births, and transport status were considered. CONCLUSIONS: Severe, vision-threatening ROP occurs with greater frequency in low-birth-weight white infants than in low-birth-weight black infants who are seemingly at equivalent risk. The reason for this disparity is unknown. We speculate that differences in retinal pigmentation may confer relative protection against free radical-mediated phototoxic injury in black infants.

Birth Weight↗

PubMed2006

Medical education for a changing world: moving beyond cultural competence into transnational competence.

Given rapidly changing global demographic dynamics and the unimpressive evidence regarding health outcomes attributable to cultural competence (CC) education, it is time to consider a fresh and unencumbered approach to preparing physicians to reduce health disparities and care for ethnoculturally and socially diverse patients, including migrants. Transnational competence (TC) education offers a comprehensive set of core skills derived from international relations, cross-cultural psychology, and intercultural communication that are also applicable for medical education. The authors discuss five limitations (conceptual, vision, action, alliance, and pedagogical) of current CC approaches and explain how an educational model based on TC would address each problem area.The authors then identify and discuss the skill domains, core principles, and reinforcing pedagogy of TC education. The five skill domains of TC are analytic, emotional, creative, communicative, and functional; core principles include a comprehensive and consistent framework, patient-centered learning, and competency assessment. A central component of TC pedagogy is having students prepare a "miniethnography" for each patient that addresses not only issues related to physical and mental health, but also experiences related to dislocation and adaptation to unfamiliar settings. The TC approach promotes advances in preparing medical students to reduce health disparities among patients with multiple and diverse backgrounds, health conditions, and health care beliefs and practices. Perhaps most important, TC consistently directs attention to the policy and social factors, as well as the individual considerations, that can alleviate suffering and enhance health and well-being in a globalizing world.

Clinical Competence↗

PubMed1985

Visual motion, binocular correspondence and binocular rivalry.

Human observers dichoptically viewed displays consisting of isotropic random dots, with the dots in each eye's view all moving in a given direction or appearing stationary. When the interocular difference in direction of motion was less than 30 deg, a stable, fused percept resulted. Once this interocular difference was exceeded, binocular rivalry ensued. Rivalry was also obtained when dots seen by the two eyes moved in identical directions but at different velocities. Under this condition, the proportion of time that rivalry was experienced increased with the interocular difference in velocity. Moving dots predominated over stationary ones, and when both sets of dots moved there was no clear advantage of one speed of motion over the other. Contrary to some earlier reports, these results show that motion is not immune to binocular rivalry. Evidently the process responsible for establishing binocular correspondence between images received by the two eyes is sensitive to disparities in direction and velocity of motion.

Functional Laterality↗

PubMed1995

Integration of motion and stereopsis in middle temporal cortical area of macaques.

The primate visual system incorporates a highly specialized subsystem for the analysis of motion in the visual field. A key element of this subsystem is the middle temporal (MT) cortical area, which contains a majority of direction-selective neurons. MT neurons are also selective for binocular disparity (depth), which is perplexing given that they are not sensitive to motion through depth. What is the role of disparity in MT? Our data suggest an important link between disparity and transparent motion detection. Motion signals in different directions tend to inhibit each other within a given MT receptive field. This inhibition has an averaging effect which minimizes MT responses to random motion signals created by light intensity changes and other non-motion stimuli (motion noise). But, in the absence of disparity cues, inhibition may also occur between surfaces moving in different directions through the same part of the visual field (transparent motion), thus impairing the detection of either surface. Here we show that inhibition in MT occurs mainly between motion signals with similar disparities. Transparent surface movements at different depths are thus represented independently in MT (that is, without inhibiting each other) whereas spurious motion signals from a given surface tend to cancel out. To our knowledge, these results provide the first evidence for a functional integration of motion and disparity in MT.

Animals↗

PubMed1999

[Disturbances of binocular vision in macular heterotopia].

BACKGROUND: Tractive translocation of the macula (secondary macular heterotopia) may result in disturbance of binocular vision. The report of a case shall discuss the sensorial problems of these patients. HISTORY AND SIGNS: We report of a 40-years old male who had decreased visual acuity and loss of binocular vision for several years due to episodes of uveitis with intravitreous hemorrhage and cataract formation. After bilateral vitrectomy and cataract extraction a good visual acuity was restored in both eyes. Postoperatively, the patient monocularly complained about disturbed egocentric localization (tilting of the visual environment, "past-pointing") and metamorphopsia. Binocularly he was confused by doubled vision with tilted images. Both maculae showed a tractive translocation of 15 degrees downward. Measurements of binocular alignment with the tangent screen showed an excyclotropia of 8 degrees and an exotropia of 7 degrees in all directions of gaze. Haploscopic examination with fusion images demonstrated that sensorial fusion was not possible even with perfect ocular alignment due to disturbed relative retinal localization (obligate fixation disparity). THERAPY AND OUTCOME: Initially, full time occlusion of the left eye was required. After improvement of symptoms occlusion therapy was slowly tapered. Within one year the patient had learned to suppress the image of his left eye and reported only minor residual visual disturbances even without occlusion of his left eye. CONCLUSIONS: Secondary translocation of the macula monocularly results in a disturbance of egocentric localization and in metamorphopsia. Binocularly doubled vision with tilted images and a loss of sensorial fusion are seen. With monocular vision, perceptual adapting to the aberration in egocentric localisation is possible within weeks by reallocation of the retinal meridians in the central nervous system. Binocular improvement of symptoms is limited to the learning of suppression. Improvement of binocular symptoms by adaptation of retinal correspondence does not occur.

Adult↗

PubMed1980

Heine's principle in vergence stimulation.

When left and right pairs of vertical parallel lines are presented stereoscopically with some disparity, they fuse so that they are perceived as a single pair of lines separated in depth. Heine showed that when such line pairs are rotated about the lines of sight in the same direction and by equal amounts from the vertical, their perceived separation in depth increases, an effect which results from the increase on horizontal disparity. This report describes how certain aspects of Heine's principle can be adapted to manipulate vergence stimulation. The technique can be implemented with very simple equipment. Possible applications of this method in binocular vision management are suggested.

Accommodation, Ocular↗

PubMed2004

A feedback model of visual attention.

Feedback connections are a prominent feature of cortical anatomy and are likely to have a significant functional role in neural information processing. We present a neural network model of cortical feedback that successfully simulates neurophysiological data associated with attention. In this domain, our model can be considered a more detailed, and biologically plausible, implementation of the biased competition model of attention. However, our model is more general as it can also explain a variety of other top-down processes in vision, such as figure/ground segmentation and contextual cueing. This model thus suggests that a common mechanism, involving cortical feedback pathways, is responsible for a range of phenomena and provides a unified account of currently disparate areas of research.

Algorithms↗

PubMed1996

Comparison of photorefractive keratectomy for myopia using 5 mm and 6 mm diameter ablation zones.

BACKGROUND: We compared the 5 mm and 6 mm ablation zones of the Summit Omnimed in the treatment of myopia in 2 eyes of the same patient. METHOD: One hundred and twenty-four consecutive patients with myopia less than 6 diopters (D) has one eye treated with a 5 mm ablation zone and the other eye with a 6 mm ablation zone. Minimum follow up was 6 months. RESULTS: Follow-up was achieved in 101 patients of the total 124. In the 5 mm group 79% achieved 20/30 uncorrected visual acuity while 68% of the 6 mm group achieved 20/30 acuity. Ninety-seven percent of all eyes in both groups achieved 20/40. Some of the disparity between the 20/30 acuity in the two groups is that initial corrections with the 6 mm zones were conservative. Corneal haze was less in the 6 mm group; subjectively they had better night vision. CONCLUSIONS: Both lasers with the different ablation zones gave reasonably predictable correction of myopia of up to -6 D. Overcorrection and corneal haze were less in the 6 mm group and night vision was subjectively better.

Corneal Opacity↗

PubMed2005

The National Library of Medicine's 2004 "Symposium on community-based health information outreach". Introduction.

OBJECTIVES: This paper introduces the special supplement to the Journal of the Medical Library Association (JMLA) that documents the proceedings of the "Symposium on Community-based Health Information Outreach" held on December 2 and 3, 2004, at the National Library of Medicine (NLM). The goal of the symposium was to explore new models of health information outreach that are emerging as technology dramatically changes the abilities of medical and health services libraries to provide resources and services beyond their traditional institutional boundaries, with particular concern for consumer health information outreach through community-based organizations. The symposium's primary objectives were to learn about successful and promising work that had already been done as well as to develop a vision for the future that could inform the NLM's next National Network of Libraries of Medicine (NN/LM) contract. Another objective was to review and assess the NLM's Strategic Plan to Reduce Health Disparities with special emphasis on Native Americans. METHOD: The paper describes the background events and rationale that led to the NLM's decision to convene the symposium and summarizes the supplement's ten other papers, some of which were presented at the symposium and some of which were written afterward to capture the symposium's working sessions. RESULTS: The symposium convened approximately 150 invited participants with a wide variety of perspectives and experience. Sessions were held to present exemplary outreach projects, to review the NLM's Strategic Plan to Reduce Health Disparities, to summarize the research underpinnings for evaluating outreach projects, and to provide a futurist's perspective. A panel of community representatives gave voice to the participants in outreach projects, and sixteen posters describing outreach projects were available, many of them with community representatives on hand to explain the work. IMPLICATIONS: This JMLA supplement provides a comprehensive summary of the state of the art in community-based outreach and a jumping-off point for future outreach efforts.

Community-Institutional Relations↗

PubMed2006

Stereoscopic depth discrimination with contrast windowed stimuli.

Depth discrimination with a shifted contrast window was compared to that with a fixed contrast window. Stereoscopic performance with the fixed window was limited to small disparities and varied with spatial frequency. Performance with the shifted window extended to larger disparities and was more similar for low and high spatial frequencies. The results depended upon window shape, indicating that edge blur is an important factor. Stereoscopic performance with shifted patterns was supported at disparities larger than a phase disparity model might predict, suggesting that a combination of position and phase disparity computations are used for the perception of stereoscopic depth.

Contrast Sensitivity↗

PubMed1997

Integrating contours within and through depth.

To better understand the role of disparity in contour integration we compared detection performance of "paths" composed of elements confined either to a single depth plane, or spanning multiple depth planes. In both cases paths defined by alignment of elements were embedded in a noise background-field made up of similar, but randomly positioned, elements covering the same depth range as the path elements. We show that a systematic disparity cue can enhance the detectability of paths which traverse depth, but that this detectability is weak compared to paths made up of elements of the same disparity. These results suggest that the outputs of disparity detectors tuned to different disparities can be linked to define contours.

Depth Perception↗

PubMed2005

The effect of monocularly and binocularly induced astigmatic blur on depth discrimination is orientation dependent.

PURPOSE: Naturally occurring astigmatism varies according to the age of the person. Although uncorrected astigmatism may be associated with meridional amblyopia, there is little information of its effect on stereopsis. The purpose of this study was to determine the effect of astigmatism on depth discrimination and whether this was dependent on the axis of the astigmatism. METHODS: Astigmatic blur was induced in four healthy subjects (mean age, 31.5 years; range, 22 to 42 years) using plain cylinders (-8.75 D to +11.5 D) for orientation control and Jackson cross-cylinders (0 to 12 D) for spherical neutrality. Horizontal, vertical, and oblique astigmatism was induced with five monocular and three binocular axis steps. Depth discrimination was recorded at near using Frisby, TNO, and Titmus stereoacuity tests and at distance (4 m) using the variable distance stereoacuity test. Visual acuity was recorded at 0.4 m and 4 m. RESULTS: Visual acuity and depth discrimination degraded with increasing astigmatic blur. The effect of monocular astigmatic blur on depth discrimination and visual acuity was not dependent on the axis of orientation. For binocular astigmatic blur, the reduction in depth discrimination was dependent on the axis of the induced astigmatism (p < 0.01). The maximum effect occurred with orthogonal-oblique orientations (x45 left; x135 right), followed by against-the-rule (ATR) astigmatism; with-the-rule (WTR) astigmatism had the least effect (p < 0.001). CONCLUSIONS: The lesser effect of WTR compared with ATR astigmatic blur on depth discrimination may reflect the contribution of horizontal compared with nonhorizontal disparity processing in stereopsis. The pronounced effect of oblique astigmatic blur may be because of the effects on horizontal and nonhorizontal disparity and interocular differential image blur.

Adult↗

PubMed1994

The role that binocular vision and stereopsis have in evaluating fundus features.

PURPOSE: We report two experiments that address the role of stereopsis in viewing the fundus. METHODS: Thirteen observers viewed stereo slides obtained by lateral displacement of a fundus camera. We quantified the capacity of seven clinicians (normal stereopsis) to resolve depth in a model eye viewed monocularly and binocularly. We also compared the ability of the same seven and six additional clinicians (with reduced stereopsis) to determine the relative position of the cup shown in normal and reverse disparity as well as answering a questionnaire. We determined optimal performance based on a schematic eye. RESULTS: Monocular viewing gives better than expected results with binocularity being of greatest benefit for small disparities. Clinicians perform much worse in stereoscopic judgments than predicted by calculation. Few clinicians report making stereo depth judgments in practice. All amblyopic observers report using hyperacuity cues. CONCLUSION: This study shows that binocularity and stereopsis are beneficial for judging small disparities under limited circumstances. The level of stereopsis achieved in practice fails to approach the expected limit. We believe that this constriction arises from a known limit to stereo acuity associated with extended fields.

Adult↗

PubMed2006

Childhood poverty: specific associations with neurocognitive development.

Growing up in poverty is associated with reduced cognitive achievement as measured by standardized intelligence tests, but little is known about the underlying neurocognitive systems responsible for this effect. We administered a battery of tasks designed to tax-specific neurocognitive systems to healthy low and middle SES children screened for medical history and matched for age, gender and ethnicity. Higher SES was associated with better performance on the tasks, as expected, but the SES disparity was significantly nonuniform across neurocognitive systems. Pronounced differences were found in Left perisylvian/Language and Medial temporal/Memory systems, along with significant differences in Lateral/Prefrontal/Working memory and Anterior cingulate/Cognitive control and smaller, nonsignificant differences in Occipitotemporal/Pattern vision and Parietal/Spatial cognition.

Adolescent↗

PubMed2006

The influence of depth segmentation on colour constancy.

In real scenes, surfaces in different depth planes often differ in the luminance and chromatic content of their illumination. Scene segmentation is therefore an important issue when considering the compensation of illumination changes in our visual perception (lightness and colour constancy). Chromatic adaptation is an important sensory component of colour constancy and has been shown to be linked to the two-dimensional spatial structure of a scene (Werner, 2003 Vision Research 43 1611 - 1623). Here, the question is posed whether this cooperation also extends to the organisation of a scene in depth. The influence of depth on colour constancy was tested by introducing stereo disparity, whereby the test patch and background were perceived in either the same or one of five different depth planes (1.9-57 min of arc). There were no additional cues to depth such as shadows or specular highlights. For consistent illumination changes, colour constancy was reduced when the test patch and background were separated in depth, indicating a reduction of contextual influences. An interaction was found between the influences of stereo depth and spatial frequency on colour constancy. In the case of an inconsistent illumination change, colour constancy was reduced if the test patch and background were in the same depth plane (2-D condition), but not if they were separated in depth (3-D condition). Furthermore, colour constancy was slightly better in the 3-D inconsistent condition than in the 2-D inconsistent condition. It is concluded that depth segmentation supports colour constancy in scenes with inconsistent illumination changes. Processes of depth segmentation are implemented at an early sensory stage of colour constancy, and they define visual regions within which the effects of illuminant changes are discounted for separately. The results support recent models that posit such implementation of scene segmentation in colour constancy.

Adaptation, Ocular↗

PubMed1998

Stereoscopic depth perception in the owl.

It is unclear whether the neural algorithm that underlies stereoscopic vision in birds incorporates both low level (camouflage breaking) and high level (depth ordering) comparisons of information available to each of the eyes. Both visual functions were successfully tested by examining transitive inference performance in two barn owls trained to discriminate static Julesz random dot stereograms, thus demonstrating a capacity to detect relative depth using fine retinal disparity as the sole cue for discrimination. Our behavioral tests provide strong evidence that the barn owl possesses global stereopsis comparable to that found in the macaque monkey--the owl's best stereoacuity was 2 min of arc.

Algorithms↗