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Bias and sensitivity of stereo judgements in the presence of a slanted reference plane.

The perceived depth of features is known to be affected by the presence of a slanted reference plane. Mitchison and Westheimer ((1984). Vision Research, 24, 1063-1070) reported that two lines appear to be at the same depth when they lie in a plane approximately parallel with the reference plane. We measured the perceived depth of two lines presented in front of a regular grid of dots that was either fronto-parallel or slanted about a vertical axis. The effect of the slanted grid on perceived depth diminished as the grid was moved further in disparity from the lines. We also found that the slanted grid affected the sensitivity to differences in disparity. The minimum threshold for detecting changes in disparity is normally lowest in the fixation plane and rises systematically with increasing pedestal disparity. In the presence of a slanted reference plane, the minimum threshold is at or close to the plane of equal perceived depth and rises with increasing disparity from this plane.

Depth Perception↗

Fixation disparity analysis of oculomotor imbalance.

A device and method for measuring forced vergence fixation disparity curves in clinical situations are described. Phoria, vergence, and fixation disparity are measures of different functions of the oculomotor system. In decreasing order of diagnostic effectiveness, four features of the fixation disparity curve are curve type, slope, Y-intercept, and X-intercept. Patients with exophoria and non-Type I curves are likely to have ocular discomfort. The steeper the curve, the more likely the patient is to have symptoms. However, Type IV curves are usually flat, and many of these patients do have symptoms. Patients with Type I curves respond well to visual training, but those with Type II curves generally need lenses or prisms to obtain relief. The Y-intercept gives information that is particularly useful in exophoric patients. A complete analysis of the oculomotor system requires measurement of both phoria/vergence and fixation disparity.

Eyeglasses↗

Vertical fixation disparity correction: effect on the horizontal forced-vergence fixation disparity curve.

Following a suggestion made by Percival in regard to dissociated phorias, we corrected vertical associated phoria in several patients who had disparities in both vertical and horizontal fixation. The principal objective result of this correction was a flattening of the slope of the type I horizontal forced-vergence curve. This result may be significant particularly because the slope has been identified as being a good prognosticator of patients likely to be symptomatic. Attention to a concurrent vertical component may offer a convenient way to normalize a steep slope on horizontal fixation disparity curves.

Adult↗

Reducing disparity in behavioral health services: a report from the American College of Mental Health Administration.

UNLABELLED: The 2003 AMCHA Summit was an initial step. It served to provide a broad outline of the socio-political context and key issues involved in reducing disparities, and it provided some momentum for change. However, much more work remains to be done. The summit clearly demonstrated that the reduction of disparities requires a multi-level approach and multi-disciplinary leaders. As a neutral convener, AMCHA is in a unique position to help advance the debate and lead the field. The membership includes researchers, administrators, clinicians, and policy makers from all levels of the behavioral health system. As noted, a change agenda needs to include efforts at national, state, and local levels involving consumers, providers, purchasers, oversight organizations, and researchers. ACMHA is committed to advancing the field and helping the national effort to reduce disparities. Examples of potential projects include the following: Training: Much has been done to develop effective cultural-competency training modules and to guide states in its implementation. No one should reinvent the wheel at this time. Funding should be targeted to provide incentives to states for dissemination of existing training curricula and the documentation of effectiveness to all providers and administrators. DATA: Nationally, the field will benefit from data standards for the collection of and reporting on system disparities. This will facilitate interstate comparisons and provide baseline data for change efforts. Conducting surveys of providers, health plans, and public behavioral health systems on the availability and current uses of data by race and ethnicity is one example of a useful first step in this process of setting data standards. RESEARCH: Further research on the nature and causes of disparity is needed. There should be systematic research on factors influencing access, treatment, and outcomes for people of different cultures. Initially, because of the difficulties in deciding on standardized outcome measures, the encounter and claims data will provide the most useful information for analysis. Later, as standardized outcome measures are more widely utilized and the data collected, it may be possible to look for racial and ethnic differences in outcomes. The research agenda needs to be developed with a focus on services and health systems research data. Demonstrations: Demonstration efforts are urgently needed, similar to Connecticut's initiative, that integrate data on disparities with provider reporting, performance contracting, and system-wide interventions. These best practices need to be shared with the field. Coordination: The Summit showed that many are eager to learn from others in this area. As we move from further research to demonstration initiatives, AMCHA can play a role in coordinating these projects, particularly at the state and perhaps local levels. State efforts can benefit from best-practice presentations from other states and by an improved understanding of the nature and scope of the change required at a programmatic and local level. Local efforts need to clearly incorporate the views and perspectives of members of the community and consumers. The 2003 ACMHA Summit provided a foundation and a framework for work to proceed at all levels of the behavioral health delivery system. To accomplish meaningful change, we challenge SAMHSA, and the other federal agencies to provide the leadership to (1) develop common and core-performance measures focused on the reduction of disparities, (2) coordinate the research agenda, and (3) facilitate the use of new information technologies to collect and review these data. This is completely consistent with the vision of federal "leadership by example" that has been outlined by the Institute of Medicine (2003b) for the implementation of the "Crossing the Quality Chasm" report. We need to facilitate the efforts of the states and the federal government to identify and reduce disparities and provide a forum for states to share the results of their efforts, to benchmark their performance, and seek technical assistance. Over the next several years, we also expect that states will expand their efforts to implement evidence-based practices. However, we urge these states to implement existing evidence-based practices cautiously, especially with culturally diverse populations, due to the limited representation of ethnically diverse subjects in the research evidence on current practices. We strongly recommend collecting data on practice-based evidence-where effective interventions are routinely identified from existing practice and shared with the field, particularly those practices that seem effective with minority populations.

California↗

A Bayesian model of stereopsis depth and motion direction discrimination.

The extraction of stereoscopic depth from retinal disparity, and motion direction from two-frame kinematograms, requires the solution of a correspondence problem. In previous psychophysical work [Read and Eagle (2000) Vision Res 40: 3345-3358], we compared the performance of the human stereopsis and motion systems with correlated and anti-correlated stimuli. We found that, although the two systems performed similarly for narrow-band stimuli, broadband anti-correlated kinematograms produced a strong perception of reversed motion, whereas the stereograms appeared merely rivalrous. I now model these psychophysical data with a computational model of the correspondence problem based on the known properties of visual cortical cells. Noisy retinal images are filtered through a set of Fourier channels tuned to different spatial frequencies and orientations. Within each channel, a Bayesian analysis incorporating a prior preference for small disparities is used to assess the probability of each possible match. Finally, information from the different channels is combined to arrive at a judgement of stimulus disparity. Each model system--stereopsis and motion--has two free parameters: the amount of noise they are subject to, and the strength of their preference for small disparities. By adjusting these parameters independently for each system, qualitative matches are produced to psychophysical data, for both correlated and anti-correlated stimuli, across a range of spatial frequency and orientation bandwidths. The motion model is found to require much higher noise levels and a weaker preference for small disparities. This makes the motion model more tolerant of poor-quality reverse-direction false matches encountered with anti-correlated stimuli, matching the strong perception of reversed motion that humans experience with these stimuli. In contrast, the lower noise level and tighter prior preference used with the stereopsis model means that it performs close to chance with anti-correlated stimuli, in accordance with human psychophysics. Thus, the key features of the experimental data can be reproduced assuming that the motion system experiences more effective noise than the stereoscopy system and imposes a less stringent preference for small disparities.

Animals↗

Complementary spatial locations, width, and disparity.

An alternative analysis is offered for human depth perception in addition to the depth cue of disparity. The new analysis considers locations both proximal and distal to the fixation point and offers an explanation as to why a stimulus presented at one disparity sign may be mistakenly considered to possess the opposite sign. Three descriptions of applications, the Pulfrich phenomenon, an interpretation of the Hornbostel effect (the three-dimensional Necker cube), and the determination of the limits of stereoscopic vision, are discussed. In addition the new analysis discloses a particular advantage of binocular over monocular vision which had not been appreciated formerly. The new analysis offers a powerful analytical tool of simple mathematical form. The means of conversion from the new analysis to and from disparity is included. In addition the similarity between the new approach and the classical lens equation is examined.

Depth Perception↗

Spatial perception in normal and strabismic subjects: role of stereopsis and monocular clues.

PURPOSE: To evaluate the roles of binocular vision, monocular clues, and experience in spatial perception in a prospective, nonrandomized, case-control study. METHODS: A test was created consisting of three wood blocks arranged along the frontal plane inside Panum area. To produce some degrees of horizontal disparity, blocks were moved along a sagittal plane. Thirteen normal subjects (control group) and 13 nonamblyopic strabismic subjects (study group) were asked to identify, both under binocular and monocular vision, the position of the blocks in a series of 12 randomized presentations (phase 1). In phase II of the trial, a letter E in three different sizes, acting as a monocular clue, marked the three blocks. In both phases, the number of correct answers to the test during binocular and monocular vision was recorded. RESULTS: Binocular normal responses appeared greater than monocular normal responses and strabismic binocular responses in both phases (p < 0.001). Binocular strabismic responses and monocular answers of both groups appeared similar. CONCLUSIONS: In our experimental model, spatial orientation of strabismic subjects in binocular vision is substantially identical to that of normal subjects in monocular vision. Monocular clues were used in the same manner in both groups of subjects, and experience seemed to play no role in spatial localization of visual objects.

Adolescent↗

Advantages of binocular vision for the control of reaching and grasping.

Theoretical considerations suggest that binocular information should provide advantages, compared to monocular viewing, for the planning and execution of natural reaching and grasping actions, but empirical support for this is quite equivocal. We have examined these predictions on a simple prehension task in which normal subjects reached, grasped and lifted isolated cylindrical household objects (two sizes, four locations) in a well-lit environment, using binocular vision or with one eye occluded. Various kinematic measures reflecting the programming and on-line control of the movements were quantified, in combination with analyses of different types of error occurring in the velocity, spatial path and grip aperture profiles of each trial. There was little consistent effect of viewing condition on the early phase of the reach, up to and including the peak deceleration, but all other aspects of performance were superior under binocular control. Subjects adopted a cautious approach when binocular information was unavailable: they extended the end phase of the reach and pre-shaped their hand with a wider grip aperture further away from the object. Despite these precautions, initial grip application was poorly coordinated with target contact and was inaccurately scaled to the objects' dimensions, with the subsequent post-contact phase of the grasp significantly more prolonged, error-prone and variable compared to binocular performance. These effects were obtained in two separate experiments in which the participants' performed the task under randomized or more predictable (blocked) viewing conditions. Our data suggest that binocular vision offers particular advantages for controlling the terminal reach and the grasp. We argue that these benefits derive from binocular disparity processing linked to changes in relative hand-target distance, and that this depth information is independently used to regulate the progress of the approaching hand and to guide the digits to the (pre-selected) contact points on the object, thereby ensuring that the grip is securely applied.

Adult↗

Analysis of tonic and accommodative vergence disorders of binocular vision.

Tonic and accommodative vergence disorders of binocular vision are analyzed with separate methods. The center of symmetry and the associated phoria of forced duction fixation disparity curves are two criteria used for prismatic correction of tonic vergence disorders. The associated phoria is corrected physiologically when patients who have tonic disorders are able to develop prism adaptation with orthoptic exercises. Graphical methods are used to analyze accommodative disorders resulting from mutual interactions between accommodation and convergence. Graphic analysis illustrates that patients with high AC/A ratios have greater demands upon fusional divergence than other patients with similar ocular deviations who have normal or low AC/A ratios. Prism adaptation, which has no direct influence upon the open-loop accommodative response, replaces corrective fusional vergence innervation and thereby eliminates the main stimulus to convergence accommodation and its exaggeration of accommodative convergence. Because prism adaptation has a response time of 15 to 30 sec, abnormal accommodative convergence interactions exaggerated by convergence accommodation can be transient in patients with normal prism adaptation. Accommodative convergence disorders in patients who lack prism adaptation are classified as sustained. Orthoptic exercises which develop fast fusional vergence are recommended for the transient disorders and additional exercises to develop prism adaptation are recommended for the sustained disorders of accommodative vergence interactions.

Accommodation, Ocular↗

The interactive processes of accommodation and vergence.

A near target generates two different, though related stimuli: image disparity and image blur. Fixation of that near target evokes three motor responses: the so-called oculomotor "near triad". It has long been known that both disparity and blur stimuli are each capable of independently generating all three responses, and a recent theory of near triad control (the Dual Interactive Theory) describes how these stimulus components normally work together in the aid of near vision. However, this theory also indicates that when the system becomes unbalanced, as in high AC/A ratios of some accommodative esotropes, the two components will become antagonistic. In this situation, the interaction between the blur and disparity driven components exaggerates the imbalance created in the vergence motor output. Conversely, there is enhanced restoration when the AC/A ratio is effectively reduced surgically.

Accommodation, Ocular↗

Beyond prenatal care: a comprehensive vision of reproductive health.

The United States has lagged behind other developed nations with respect to infant and maternal mortality. In the past decade, the disparities between races and socioeconomic groups has actually widened. In response, local initiatives have sought to decrease infant mortality by enrolling women into prenatal care programs. These programs have focused solely on pregnancy, despite evidence that access to family planning, abortion, and gynecologic care is necessary to reduce both infant and maternal mortality. These prenatal care initiatives thus miss the opportunity to have an impact on some of the major public health issues facing women.

Female↗

Interactions between chromatic- and luminance-contrast-sensitive stereopsis mechanisms.

It is well known that chromatic information can assist in solving the stereo correspondence problem. It has also been suggested that there are two independent first-order stereopsis mechanisms, one sensitive to chromatic contrast and the other sensitive to luminance contrast (Vision Research 37 (1997) 1271). Could the effect of chromatic information on stereo correspondence be subserved by interactions between these mechanisms? To address this question, disparity thresholds (1/stereoacuity) were measured using 0.5 cpd Gabor patches. The stimuli possessed different relative amounts of chromatic and luminance contrast which could be correlated or anti-correlated between the eyes. Stereoscopic performance with these compound stimuli was compared to that with purely isoluminant and isochromatic stimuli at different contrasts. It was found that anti-correlated chromatic contrast severely disrupted stereopsis with achromatic stimuli and that anti-correlated luminance contrast severely disrupted stereopsis with chromatic stimuli. Less dramatic, but still significant, was the improvement in stereoacuity obtained using correlated colour and luminance contrast. These data are consistent with there being positive and negative interactions between chromatic and achromatic stereopsis mechanisms that take place after the initial encoding of disparity information, but before the extraction of stereoscopic depth. These interactions can be modelled satisfactorily assuming probability summation of depth sign information between independent mechanisms.

Color Perception↗

Functional division of the retina and binocular correspondence.

Spatial vision relies on a spatial coding of object directions sensed by the retinal photoreceptors. An empirically measured constant error in this coding of perceived space is described classically as a barrel distortion involving long horizontal lines. A Cartesian retinal coordinate system is constructed to describe the transformation from object space to perceived space. This system establishes a subjective horizontal and vertical (isoelevation and isoazimuth) grid for each eye. The retinal entities that mediate subjective sense of constant elevation (horizontal) and constant azimuth (vertical) constitute the functional division of the retina, and they are described by a model with a single parameter zp, which determines the curvatures of a set of hyperbolic isopters. Empirical isoelevation positions were measured, which were described by this model. The best-fitting zp values appeared to be independent of viewing distance, similar for the two eyes of the same observer but differing significantly among observers. The functional-retinal-division model and horizontal retinal shear, as described by Helmholtz, were used to predict the spatial distributions of binocular corresponding points. The disparity field on a frontoparallel plane at a finite viewing distance showed marked asymmetry between the upper and the lower visual fields. The hyperbolic isoelevation and isoazimuth curves dramatically exaggerated this asymmetry. This theoretical disparity field provides a guideline for experiments that attempt to show the effect of binocular disparity in the periphery.

Humans↗

An alternative three-dimensional interpretation of Hering's equal-innervation law for version and vergence eye movements.

In the context of Hering's equal-innervation law, this paper discusses the problem of how the three-dimensional positions of the two eyes, each expressed by a rotation vector, can be separated into contributions of the version and vergence system. As proposed by Van Rijn and Van den Berg [(1993) Vision Research, 33, 691-708], this can be done by taking the sum and difference of the position rotation vectors of each eye. In our alternative procedure the vergence signal is defined as the rotation which transforms the left eye position into the right eye position and the version signal is the common factor in both eye positions that remains after removing the vergence signal. The version and vergence contributions, defined in this way, can be interpreted straightforwardly as rotations. When Van Rijn and Van den Berg applied their definitions to their own data, they obtained the interesting result that the reconstructed version and vergence contributions were effectively limited to two dimensions (2D). The version signal was confined to Listing's plane (no torsion) whereas the vergence signal remained within a horizontal-torsional plane (no vertical vergence). They showed theoretically that a model based on 2D version/2D vergence control will indeed produce the torsional eye positions in near fixations found in their experiments. This model cannot account for a second set of data in the literature [Mok, Ro, Cadera, Crawford & Vilis (1992) Vision Research, 32, 2055-2064]. With our definitions, we found that the simple 2D version/2D vergence control strategy cannot account for the Van Rijn and Van den Berg (1993) data but is nicely compatible with the considerably smaller amount of cyclotorsion in the data collected by Mok et al. (1992). We also show that, in such a system, having 2D vergence control is compatible with minimization of torsional disparity and provides the cyclovergence signals suitable for stabilizing the eyes in the non-Listing positions caused by a vertical saccade in near vision.

Convergence, Ocular↗

Fixation disparity vs. binocular fixation misalignment.

Conventional fixation disparity was compared with the corresponding foveal fixation misalignment measured by a recently developed method based on the effect of retinal eccentricity on the width of the border enhancement band. Unlike procedures using binocular nonius alignment, the method is not subject to adaptive changes in perceived direction. Forced convergence and divergence stimulation served as the independent variable. It was found that conventional fixation disparity represents only a small, constant fraction of the misalignment through the greater portion of the vergence range. However, when the misalignment is restricted to a small, central portion of the foveola, this relation changes drastically, and the fixation disparity can then be much larger than the fixation misalignment. During perfect fixation alignment of the foveal centers, there is always a significant amount of fixation disparity present. It is suggested that the large differences between fixation disparity and fixation misalignment are due to a fundamental, perceptual mechanism that is always at work during binocular vision, changing the relation between retinal location and perceived visual direction.

Convergence, Ocular↗