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PubMedDate not supplied

Current applications of the Corneal Modeling System.

The ability of the Corneal Modeling System to incorporate curvature and thickness data into a single mathematical construct of the cornea is an important achievement. As experience with this and similar instruments is gained, a number of important issues in corneal topography can be addressed. Snellen visual acuity, refraction, and keratometric changes do not fully reveal the effects of refractive surgical procedures. Detailed analysis of corneal topography will prove necessary to understanding the quality of vision resulting from surgery, as it can explain the disparity between uncorrected visual acuity and residual refractive error after radial keratotomy. Computer-assisted topographic measurement devices will undoubtedly play a particularly important role in the design and evaluation of current and future refractive procedures. Similarly, mild topographic changes (eg, subclinical keratoconus) can be detected that are not apparent with refraction, biomicroscopy, or keratometry. The author reviews his personal experience with one instrument.

Astigmatism↗
PubMed1996

Night vision testing in unoperated eyes.

BACKGROUND: We previously described a simple test which evaluates image degradation in post-excimer laser (PRK) patients under scotopic conditions. After refractive surgery, corneal haze, ablation zone decentration, ablation zone/pupillary diameter disparity, and under-correction each result in a characteristic pattern on the Night Vision Recording Chart. METHODS: Using the same method, further studies evaluated night vision image degradation in 118 un-operated emmetropic, myopic, hyperopic, and astigmatic eyes and in 26 contact lens wearers. RESULTS: Scotopic image degradation increases with myopic refractive error, image displacement increases with astigmatism, and contact lens wearers have more image degradation that with spectacle correction. CONCLUSION: Our Night Vision Recording Chart offers a simple, reproducible method to characterize image degradation under scotopic conditions.

Contact Lenses↗
PubMed1985

PMF: a stereo correspondence algorithm using a disparity gradient limit.

The advantages of solving the stereo correspondence problem by imposing a limit on the magnitude of allowable disparity gradients are examined. It is shown how the imposition of such a limit can provide a suitable balance between the twin requirements of disambiguating power and the ability to deal with a wide range of surfaces. Next, the design of a very simple stereo algorithm called PMF is described. In conjunction with certain other constraints used in many other stereo algorithms, PMF employs a limit on allowable disparity gradients of 1, a value that coincides with that reported for human stereoscopic vision. The excellent performance of PMF is illustrated on a series of natural and artificial stereograms. Finally, the differences between the theoretical justification for the use of disparity gradients for solving the stereo correspondence problems presented in the paper and others that exist in the stereo algorithm literature are discussed.

Depth Perception↗
PubMed1988

Stereoscopic evoked responses to crossed and uncrossed disparity accompanying simulated refractive error.

Evoked potentials were recorded to the occurrence of a disparate stimulus in dynamic random dot stereograms. Seven adult males, all of whom had vision which was normal or corrected to normal, participated in the experiment. Subjects viewed 100 ms duration stimuli which embodied 30 arc min of either crossed or uncrossed disparity under four conditions of spherical overcorrection: -0.25, +1.0, +2.0, +3.0 dioptres. The first condition, essentially normal refraction, yielded reliable behavioural reports of the stimulus and clear evoked potentials to both crossed and uncrossed disparity. With increasing overcorrection the behavioural reports became less reliable, and the evoked potentials were degraded for both conditions of disparity. The responses to the crossed disparity condition, however, showed significantly less degradation in both behavioural and electrophysiological measures. The implications of this finding may be that there are separate cortical subsystems for the processing of crossed and uncrossed disparity and that the former is more robust under non-ideal viewing conditions.

Adult↗
PubMed2000

[Are cross-eyed persons worse drivers? The effect of stereoscopic disparity on driving skills].

BACKGROUND: The motorist obtains about 90% of the information required to drive a vehicle from the sense of vision. In road traffic, various situations with respect to size, distance, and position have to be evaluated at different speeds. Does depth perception based on disparity play an essential role in these situations? METHODS: Each of ten subjects with defective stereopsis due to strabismus was compared with an age matched healthy control, who was also matched in yearly mileage, type of vehicle and duration of driving experience. After ophthalmological examination including stereotests for both distance and near, all subjects performed the following driving tests: (1) Stopping as close as possible in front of an obstacle, (2) reversing into a parking position, (3) driving a slalom parcours at the speed of 40 km/h, (4) estimating the relative position of two cars within a limited time while approaching them with 40 km/h. All tests were performed both under monocular (the non-leading eye covered) and binocular conditions. The sequence of the tests was randomized in order to minimize training effects. RESULTS: Comparing the driving performance of both groups, the controls performed significantly better (p < 0.01; ANOVA) only in the "slalom task". In "estimation of position" the controls performed significantly worse (p < 0.01; ANOVA) compared to their counterparts with defective stereopsis. A significant difference between the monocular and binocular performance was found only in the "stopping task", exclusively for the group of normals, benefitting from the binocular performance (p < 0.05; ANOVA). CONCLUSION: Stereopsis improves the driving performance only in dynamic situations at intermediate distances.

Adult↗
PubMedDate not supplied

Vertical disparity pooling and the induced effect.

If the image received by one eye is vertically magnified by a small amount then an illusory tilt is perceived around the vertical axis through the fixation point. This is known as the induced effect and it can be explained by a recent computational theory of binocular vision which treats it as a side-effect of the use of vertical disparities to recover information about the distance to the fixation point and the angle of gaze. We have investigated the consequences of introducing vertical magnifications of some parts of a scene and not others and report here that there is a simple linear relationship between the size of the induced effect and the average vertical magnification. This suggests that a pooling strategy is adopted in the measurement of vertical disparities, a result which fits in well with expectations of the theory.

Computers↗
PubMed1982

The dependence of the induced effect on orientation and a hypothesis concerning disparity computations in general.

The induced size effect is an apparent rotation about a vertical axis that results from binocularly viewing a target in which one half-image is vertically magnified. A previous paper (Arditi et al., 1981, Vision Res. 21, 755-764) described a theory of this effect in terms of horizontal disparities that exist between vertically magnified images of oblique features and their unmagnified counterparts. The present studies test two aspects of that theory: the requirement of oblique features in stimuli eliciting the induced effect, and the assumption that binocular associations (inputs to disparity computations) are made across horizontal meridians. The former aspect was confirmed in a stereo discrimination experiment in which the direction of rotation (tilt) for crossed line patterns of varying orientation was judged, for a fixed vertical magnification of one half-image. The latter aspect was rejected on the basis of the results of that experiment, and of two experiments in which observers matched the apparent tilt of the lines with a horizontal adjustment line which could be stereoscopically rotated in depth. The data and some associated demonstrations suggest that stereoacuity and apparent depth of oblique lines vertically magnified in one half-image are determined by the horizontal separation between binocular points which are nearest in a fixed binocular coordinate map, rather than by purely horizontal point-matchings. This "nearest neighbor hypothesis" seems to be operative in classic measures of stereoacuity as well as in the induced effect.

Eye Movements↗
PubMed1981

A neurophysiological model for anomalous correspondence based on mechanisms of sensory fusion.

Normal retinal correspondence is not stable. The arguments for the plasticity of correspondence in normal binocular vision have been given into previous papers (Nelson, 1975, 1977). In this paper, both laboratory research and the clinical strabismus literature are reviewed to show similarities between normal and abnormal binocular vision. In particular, it is argued that sensory fusion (Panum's areas) and anomalous retinal correspondence (AC) obey similar principles, and so a sensory fusional model of AC may be developed. Recent advances in the neurophysiology of binocular vision are reviewed, but current laboratory knowledge cannot account for many phenomena known clinically unless certain postulates are made. Two hypothesized intracortical interactions among binocular disparity detectors, termed disparity domain inhibition and spatial domain facilitation, play key roles in extending the neurophysiology of binocular vision to an account of both normally - and clinically - observed plasticities of correspondence. The fusional model of retinal correspondence developed here from postulated domain interactions contrasts with the older concept of fixed corresponding points, an approach which has failed to provide a unified foundation for the treatment of normal and abnormal binocular vision.

Animals↗
PubMed2000

Stereoscopic vision: what's the first step?

Neurons in primary visual cortex respond to binocular disparity, the raw material of stereoscopic depth perception. Although these neurons are probably essential to depth perception, a recent study has shown that they are unable to compute depth itself.

Animals↗
PubMed1982

Visual evoked potentials to binocular stimulation with disparate patterns.

Experiments with VEP to checkerboard stimulation in 4 subjects with normal stereoscopic vision showed that binocular interaction was the same for fused images and for linear pattern displacement induced by horizontal and vertical prisms, as long as the patterns were projected on the macular areas of both eyes. Also in a stereo-blind subject the impaired binocular VEP interaction remained unchanged during linear displacement of the patterns. Thus the previously reported differences in binocular VEP interaction between stereo-normal stereoblind subjects cannot be explained by binocular stimulation with unfused patterns in the latter group. With patterns displaced to different hemifields in the 2 eyes, no binocular interaction was recorded. Experiments with black-and-white gratings of different orientations to the 2 eyes showed that binocular VEP reactions remained the same even with patterns perpendicular to each other. However, the binocular interaction was disrupted when the subject viewed the perpendicular patterns through coloured filters.

Adolescent↗
PubMed2005

Non-linear mixed effects modeling - from methodology and software development to driving implementation in drug development science.

Few scientific contributions have made significant impact unless there was a champion who had the vision to see the potential for its use in seemingly disparate areas-and who then drove active implementation. In this paper, we present a historical summary of the development of non-linear mixed effects (NLME) modeling up to the more recent extensions of this statistical methodology. The paper places strong emphasis on the pivotal role played by Lewis B. Sheiner (1940-2004), who used this statistical methodology to elucidate solutions to real problems identified in clinical practice and in medical research and on how he drove implementation of the proposed solutions. A succinct overview of the evolution of the NLME modeling methodology is presented as well as ideas on how its expansion helped to provide guidance for a more scientific view of (model-based) drug development that reduces empiricism in favor of critical quantitative thinking and decision making.

Algorithms↗
PubMed1996

Rapid eye movement sleep deprivation in kittens amplifies LGN cell-size disparity induced by monocular deprivation.

The abundance of rapid eye movement (REM) sleep in the neonatal mammal and its subsequent decline in the course of development, as well as the dramatic and widespread enhancement of CNS activity during REM sleep, led us to propose that this state plays a functional role in the normative physiological and structural maturation of the brain [54]. When, after 1 week of monocular deprivation (MD), a second week of MD was coupled with behavioral deprivation of REM sleep, the structural alteration in the visual system provoked by MD alone (interlaminar relay cell-size disparity in the lateral geniculate nucleus (LGN) was amplified. With the addition of REM deprivation during MD, the LGN cells connected to the surgically patched eye, which are smaller than normal after MD, became even smaller, whereas the LGN cells receiving input from the seeing eye, which display compensatory hypertrophy after MD, grew even larger. We believe that the interlaminar disparity effect widened because during REM deprivation, the already vision-compromised LGN cells associated with the patched eye also lose the ascending brainstem activation reaching them during the REM state. Loss of the two main sources of 'afference' by these LGN cells permits their seeing-eye LGN counterparts to gain even greater advantage in the competition for synaptic connections in cortex, which is reflected in the relative soma sizes of the LGN relay cells. It is likely that the relatively abundant REM state in early maturation provides symmetric stimulation to all LGN relay cells, irrespective of eye of innervation. The symmetric activation propagated from brainstem to LGN acts to 'buffer' abnormal, asymmetric visual input and, thereby diminishes the extreme, asymmetric structural alteration that results from MD in the absence of REM sleep. We conclude that REM sleep-generated CNS discharge in development has the effect of 'protecting' the CNS against excessive plasticity changes. This is consistent with the possibility that REM sleep plays a role in the genetically programmed processes that direct normative brain development.

Animals↗
PubMed1999

The "wider view": André Helleger's passionate, integrating intellect and the creation of bioethics.

This article provides an account of how André Hellegers, founder and first Director of the Kennedy Institute of Ethics at Georgetown University, laid medicine open to bioethics. Helleger's approach to bioethics, as to morality generally and also to medicine and biomedical science, involved taking the "wider view" -- a value-filled vision that integrated and gave meaning to what otherwise was disparate, precarious, and conflicting. This article shows how Helleger's wider view of bioethics was shaped by events in his own life, his resultant sense of the precariousness of life and health, his commitment to religious inclusiveness, his research in fetal medicine, his clinical experience in obstetrics, his role in the struggle to change the teaching of the Roman Catholic Church on fertility control, and his developing concepts of health and disease. Hellegers was committed to and worked toward bioethics as a self-consciously interdisciplinary field in which the contributing disciplines adapt to each other -- rather than sustain themselves as autonomous disciplines -- to create a dynamic and complex intellectual, clinical, and social activity.

Academies and Institutes↗
PubMedDate not supplied

Engendering health disparities.

How is gender implicated in our exploration of health disparities in Canada? Set against the backdrop of federal government policy, this review paper examines the ways in which gender intersects with other health determinants to produce disparate health outcomes. An overview of salient issues including the impact of gender roles, environmental exposures, gender violence, workplace hazards, economic disparities, the costs of poverty, social marginalization and racism, aging, health conditions, interactions with health services, and health behaviours are considered. This review suggests health is detrimentally affected by gender roles and statuses as they intersect with economic disparities, cultural, sexual, physical and historical marginalization as well as the strains of domestic and paid labour. These conditions result in an unfair health burden borne in particular by women whose access to health determinants is--in various degrees--limited. While progress has certainly been made on some fronts, the persistence of health disparities among diverse populations of women and men suggests a postponement of the vision of a just society with health for all that was articulated in the Federal Plan on Gender Equality. Commitment, creativity and collaboration from stakeholders ranging from various levels of government, communities, academics, non-governmental agencies and health professionals will be required to reduce and eliminate health disparities between and among all members of our society.

Adult↗
PubMed2006

Legislative issues facing pediatric ophthalmology in 2006.

PURPOSE OF REVIEW: There has been a surge in legislative activity concerning children's vision in recent years. This review will summarize and compare newly enacted state statutes and review legislation that has been introduced at both the state and federal level. Detailed reference sites are provided and may be a useful resource for those involved in legislative advocacy. RECENT FINDINGS: The legislative arena has often mirrored the disparate views of ophthalmology and optometry regarding the most appropriate methods for identifying vision problems in young children. While state legislation mandating comprehensive eye examinations for all asymptomatic and risk free children was passed in Kentucky in 2000, no other state since has successfully enacted and implemented similar legislation. In contrast, numerous states, have enacted mandatory preschool vision screening legislation. Federal legislation, introduced separately by ophthalmology and optometry, would provide funds for uninsured children's eye exams, but the two bills have important differences in eligibility requirements. Issues such as eye safety and retinoblastoma detection have also been addressed through the legislative process. SUMMARY: There has been a myriad of state and federal legislative activity in the area of children's vision. The momentum is likely to continue as additional states file new legislation.

Child↗
PubMed1987

Line correspondence in binocular vision.

The mathematical analysis of binocular vision introduced by Helmholtz is applied to the problem of the use of disparity information to position a stimulus in depth. It is shown that matching the images from the left and right eyes along radial directions is an alternative to matching images along the horizontal direction only.

Attention↗