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PubMed2005

Reducing and eliminating health disparities: a targeted approach.

Health disparities have dominated recent discourse among public health and medical researchers. Ever since the United States began to compile health statistics, differences in health status have been noted between majority and non-majority populations. Myriad approaches have been undertaken in an attempt to reduce or eliminate racial and ethnic disparities in health. However, the disparities continue to persist. We are at a point in our history where innovative strategies must be explored that will be more effective in addressing racial and ethnic disparities in health. In large part, health disparities exist as a result of inequitable distribution of goods, resources, services and power in America. We have learned that improvements in health cannot come about solely through primary and secondary interventions but rather through an examination of the availability of resources that would allow individuals to improve their health. The goal of this paper is to provide an overview of the contextual factors that affect health disparities, to integrate theory to address disparities and to provide recommendations to encourage systematic changes to eliminate health disparities. It is hoped that this paper will bring about a national discussion relating to addressing the real issues we face in reducing and ultimately eliminating health disparities.

Black or African American↗

PubMed2005

Methodological issues in measuring health disparities.

OBJECTIVES: This report discusses six issues that affect the measurement of disparities in health between groups in a population: Selecting a reference point from which to measure disparity. Measuring disparity in absolute or in relative terms. Measuring in terms of favorable or adverse events. Measuring in pair-wise or in summary fashion. Choosing whether to weight groups according to group size. Deciding whether to consider any inherent ordering of the groups. These issues represent choices that are made when disparities are measured. METHODS: Examples are used to highlight how these choices affect specific measures of disparity. RESULTS: These choices can affect the size and direction of disparities measured at a point in time and conclusions about the size and direction of changes in disparity over time. Eleven guidelines for measuring disparities are presented. CONCLUSIONS: Choices concerning the measurement of disparity should be made deliberately, recognizing that each choice will affect the results. When results are presented, the choices on which the measurements are based should be described clearly and justified appropriately.

Data Collection↗

PubMed2006

Monitoring racial/ethnic mortality disparities in Wisconsin: 1991-2000.

BACKGROUND: Wisconsin has a goal to eliminate health disparities by 2010, but there is no consistent standard used to evaluate progress. Methodological debates persist regarding using individual group change or relative comparisons to monitor disparities. OBJECTIVES: To examine mortality disparities among racial/ethnic populations in Wisconsin using statistically significant changes in individual population mortality rates and rate ratios as measures of disparity. These measures are proposed to monitor and evaluate progress in eliminating racial/ethnic health disparities. METHODS: The Wisconsin Interactive Statistics on Health database was queried to obtain Wisconsin all-cause mortality data by race and age for the 1991-1995 and 1996-2000 periods. Age-specific and age-adjusted rates were compared across 5 major racial/ethnic populations in Wisconsin. RESULTS: Age-adjusted mortality generally declined for all racial/ethnic populations in Wisconsin from 19911995 to 1996-2000. However, disparities increased significantly for African American infants, African Americans 45-64 years old, and Hispanics/Latinos 25-44 years old. Using non-Hispanic whites as a referent resulted in a paradoxical increase in disparities for Hispanics/Latinos despite a significant reduction in mortality in this group. CONCLUSION: A statistically significant percent change in mortality rates and rate ratios is a useful standard to monitor health disparities and foster communication and targeted action around Wisconsin's goal to eliminate racial/ethnic health disparities.

Black or African American↗

PubMed1990

Neural model of stereoacuity and depth interpolation based on a distributed representation of stereo disparity.

We have developed a model for the representation of stereo disparity by a population of neurons that is based on tuning curves similar in shape to those measured physiologically (Poggio and Fischer, 1977). Signal detection analysis was applied to the model to generate predictions of depth discrimination thresholds. Agreement between the model and human psychophysical data was possible in this model only when the population size representing disparity in a small patch of visual field was in the range of about 20-200 units. Interval encoding and rate encoding were found to be inconsistent with these data. Psychophysical data on stereo interpolation (Westheimer, 1986a) suggest that there are short-range excitatory and long-range inhibitory interactions between disparity-tuned units at nearby spatial locations. We extended our population model of disparity coding at a single spatial location to include such lateral interactions. When there was a small disparity gradient between stimuli at 2 locations, units in the intermediate, unstimulated position developed a pattern of activity corresponding to the average of the 2 lateral disparities. When there was a large disparity gradient, units at the intermediate position developed a pattern of activity corresponding to an independent superposition of the 2 lateral disparities, so that both disparities were represented simultaneously. This mixed population pattern may underlie the perception of depth discontinuities and transparent surfaces. Similar types of distributed representations may be applicable to other parameters, such as orientation, motion, stimulus size, and motor coordinates.

Depth Perception↗

PubMed1977

Fixation disparity and stereopsis.

The magnitude of fixation disparity has been proposed to be uncorrelated with stereoscopic sensitivity. Studies on this relationship have been confined to the effects of vergence-induced fixation disparity on stereoacuity: some subjects exhibited decreased stereoacuity with induced fixation disparity. The present study was designed to determine whether prismatic correction of naturally occurring horizontal fixation disparity would result in increased stereoacuity. Five subjects with eso and 5 with exo fixation disparity had their fixation disparity curves and stereothresholds evaluated before and after wearing corrective lenses and prisms that neutralized the fixation disparity. Five subjects with no fixation disparity served as controls. After 1 month most subjects showed no change in their fixation disparity curves and stereothresholds.

Depth Perception↗

PubMed2000

Motor execution is necessary to memorize disparity.

Binocular saccades in response to briefly flashed, memorized disparate targets (different for the two eyes) become disconjugate following repeated trials. After 15 min of such training, the disconjugacy persists, even when the target to memorize is no longer disparate. This study examines the hypothesis that disparity memorization has a motor basis. We report here three experiments in which subjects were trained for 15-min periods. In experiment 1, subjects made no saccade after target presentation (static training); in experiment 2 subjects intended to make a saccade, but they actually made a saccade in only 10% or 20% of the trials; in experiment 3 subjects made anti-saccades. For all three experiments, the flashed target was disparate and the memory delay for each trial was 1 s. To examine the effects of learning for all three experiments, before and after training, we recorded memory-guided saccades to non-disparate targets (monocular viewing). Experiments 1 and 2 produced inconsistent (before/after training) changes in the disconjugacy of saccades. Thus, the disparity of potential saccade targets had no lasting effect on the disconjugacy of saccades if a saccade was not made. In contrast, the anti-saccades in experiment 3 developed a disconjugacy opposite to the disparity of the remembered target. These findings indicate that the execution of the saccade is necessary to memorize disparity of the target.

Conditioning, Psychological↗

PubMed2004

Architecture of binocular disparity processing in monkey inferior temporal cortex.

Neurons in the inferior temporal (IT) cortex respond not only to the shape, color or texture of objects, but to the horizontal positional disparity of visual features in the right and left retinal images. IT neurons with similar shape selectivity cluster in columns. In this study, we examined how IT neurons are spatially arranged in the IT according to their selectivity for binocular disparity. With a single electrode, we simultaneously recorded extracellular action potentials from a single neuron and those from background multiple neurons at the same sites or recorded multineuronal responses at successive sites along electrode penetrations, while monkeys performed a fixation task. For neurons at each recording site, effective shapes were first determined from a set of 20 shapes presented at the zero-disparity plane. The most effective shape was then presented with varying amounts of disparity. Single neuron responses and background multiunit responses recorded at the same sites showed a similar ability of disparity discrimination and tended to share the preferred disparity, suggesting that neurons with similar disparity selectivity are clustered in the IT. We estimated from sequential recordings along electrode penetrations that the size of the neuronal clusters with similar disparity selectivity was smaller than the size of clusters with similar shape selectivity.

Action Potentials↗

PubMed2005

Illusory depth perception of oblique lines produced by overlaid vertical disparity.

Our visual system matches images from both eyes to establish a single view and stereo depth even when they contain a certain amount of vertical disparity. This paper demonstrates a new stereo effect showing an aspect of vertical disparity processing. When oblique lines without disparity are overlaid with sparse random dots with vertical disparity, the lines look closer or farther in depth. The characteristics of this stereo illusion were experimentally investigated. The results showed that the sign of the perceived depth of the oblique lines depended on the combination of the line orientation and the vertical disparity sign, and that the amount of perceived depth became larger as the line orientation became more horizontal. The depth illusion robustly existed even under conditions that ruled out eye movements (i.e., vertical vergence and cyclovergence) by local-parallel or brief presentations of the stereo figures. This phenomenon suggests that the visual system locally measures vertical disparity and is not simply tolerating a small amount of vertical disparity. Stereo capture of vertical disparity and horizontal matching after vertical image shifts were proposed as possible explanations for the depth illusion.

Depth Perception↗

PubMed2005

The structure of stereoscopic masking: position, disparity, and size tuning.

The masking effect of a Gaussian blob on detection of a Gaussian target was measured as a function of the position, disparity, width and polarity of the mask. The data reveal a large degree of disparity-specific masking that cannot be explained by the masking of its monocular constituents. At 5 degrees eccentricity, the masking range extends about +/-1 degrees around the lines of sight of the two eyes and 1-3 degrees in disparity, depending on the size of the test stimuli. The masking effects can be modeled as having three additive components, one that has a fixed disparity range and is polarity independent, one with a center/surround form keyed to both the disparity and the polarity of the mask, and one that derives from the monocular masking in each eye. Thus, the profound disparity interaction behavior is not limited to the simple monocular masking properties of the stimuli but reveals extensive connectivity across the disparity domain. Future models of disparity encoding will need to take these properties into account.

Adult↗

PubMed2001

Effects of disparity-perspective cue conflict on depth contrast.

The role of disparity-perspective cue conflict in depth contrast was examined. A central square and a surrounding frame were observed in a stereoscope. Five conditions were compared: (1) only disparity was introduced into either the centre or surround stimulus, (2) only perspective was introduced into the centre or surround, (3) concordant perspective and disparity were introduced into the centre or surround, (4) disparity was introduced into one stimulus and perspective into the other, and (5) only the centre stimulus was presented with horizontal shear disparity and perspective manipulated independently. The results show that individual differences in depth contrast were related to individual differences in the weighting of disparity and perspective in the single-stimulus conditions. We conclude that conflict between disparity and perspective contributes to depth contrast. However, significant depth contrast occurred when there was no disparity-perspective cue conflict, indicating that this cue conflict is not the sole mechanism producing depth contrast.

Adult↗

PubMed2001

The relative sensitivities of sensory and motor fusion to small binocular disparities.

Horizontal binocular disparity is the fundamental stimulus for both fusional vergence and stereopsis, but whether common disparity-sensitive mechanisms are involved in both responses is unknown. To determine whether the sensitivities of motor and sensory fusion are interdependent, we studied vergence eye movements and depth discrimination, using stimuli with haplopic binocular disparities, in subjects with normal stereopsis and in subjects with mild to severe stereoanomalies. Our results showed that the subjects' disparity discrimination functions varied from nearly perfect discrimination to chance performance for all of the experimental stimuli. Their sensory functions did not necessarily predict the shape of their motor fusion functions, but in most cases were correlated with the subject's fixation disparities. The results support the conclusion that the stereoanomalies and vergence anomalies that previously have been described for coarse binocular disparities also extend to the small, haplopic binocular disparities. The independence of the response properties of sensory and motor fusion suggests that neural pathways for sensory and motor fusion separate after the initial disparity-selective mechanisms in primary visual cortex.

Depth Perception↗

PubMed2002

Stereo disparity improves color constancy.

Binocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception.

Color Perception↗

PubMed2003

Reading a population code: a multi-scale neural model for representing binocular disparity.

Although binocular neurons in the primary visual cortex are sensitive to retinal disparity, their activity does not constitute an unambiguous disparity signal. A multi-spatial-scale neural model for disparity computation is developed to examine how population activity might be interpreted to overcome ambiguities at the single neuron level. The model incorporates a front end that encodes disparity by a family of complex cell-like energy units and a second stage that reads the population activity. Disparity is recovered by matching the population response to a set of canonical templates, derived from the mean response to white noise stimuli at a range of disparities. Model predictions are qualitatively consistent with a variety of psychophysical results in the literature, including the effects of spatial frequency on stereoacuity and bias in perceived depths, and the effect of standing disparity on increment thresholds. Model predictions are also consistent with data on qualitative appearance of complex stimuli, including depth averaging, transparency, and corrugation. The model also accounts for the non-linear interaction of disparities in compound grating stimuli. These results show that a template-match approach reduces ambiguities in individual and pooled neuronal responses, and allows for a broader range of percepts, consistent with psychophysics, than other models. Thus, the pattern of neural population activity across spatial scales is a better candidate for the neural correlate of depth perception than the activity of single neurons or the pooled activity of multiple neurons.

Depth Perception↗

PubMed1997

Spatial limitation of vertical-size disparity processing.

We investigated the upper limit of horizontal spatial modulation of vertical-size disparity in a textured surface for the perception of depth. In Experiment 1 subjects matched the appearance of a surface with modulated horizontal-size disparity to that of a surface with modulated vertical-size disparity. In Experiment 2 we determined the threshold amplitude of modulation of vertical-size disparity required for the perception of depth as a function of the spatial frequency of disparity modulation. The results indicate that sensations of depth are not elicited by modulations of vertical-size disparity of any amplitude at spatial frequencies higher than about 0.04 c/deg. We conclude that vertical disparities are averaged within about 20 deg-wide areas and suggest that this global measurement is used to scale local horizontal disparities for the perception of surface slant.

Depth Perception↗

PubMed1999

Fixation disparity and nonius bias.

Fixation disparity, i.e. the vergence error within Panum's area, can be measured psychophysically with two nonius (vernier) lines that are presented dichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates the amount of fixation disparity. The present experiments investigate the relation between fixation disparity and the nonius bias, which is the physical offset of the nonius lines that is adjusted by the observer in order to perceive them as aligned when both nonius lines are presented to both eyes (binocular nonius bias) or both to the left or both to the right eye (monocular nonius bias). It was found that (1) the fixation disparity is correlated with the binocular nonius bias in the horizontal and vertical meridian and (2) the binocular nonius bias can be predicted from the average of the right eye and left eye monocular nonius bias. To remove the influence of the nonius bias on measured fixation disparity it is possible to calculate the fixation disparity relative to the individual binocular nonius bias, rather than to the physical coincidence of the nonius lines. This procedure tends to increase the correlation between fixation disparity and the tonic resting position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compared to physical recordings of eye position.

Convergence, Ocular↗

PubMed2002

Phase- and position-disparity coding in the posteromedial lateral suprasylvian area of the cat.

The posteromedial lateral suprasylvian area of the cat is known to be involved in the analysis of motion and motion in depth. However, it remains unclear whether binocular cells in the posteromedial lateral suprasylvian area rely upon phase or positional offsets between their receptive fields in order to code binocular disparity. The present study aims at clarifying more precisely the neural mechanisms underlying stereoperception with two objectives in mind. First, to determine whether cells in the posteromedial lateral suprasylvian area code phase disparities. Secondly, to examine whether the cells sensitive to phase disparity are the same as those which code for position disparities or whether each group represent a different sub-population of disparity-sensitive neurons. We investigated this by testing both types of disparities on single neurons in this area. The results show that the vast majority of cells (74%), in the posteromedial lateral suprasylvian area, are sensitive to relative interocular phase disparities. These cells showed mostly facilitation (95%) and a few (5%) summation interactions. Moreover, most cells (81%) were sensitive to both position and phase disparities. The results of this study show that most binocular cells in the posteromedial lateral suprasylvian area are sensitive to both positional and phase offsets which demonstrate the importance of this area in stereopsis.

Action Potentials↗

PubMed1991

Orientation disparity, deformation, and stereoscopic slant perception.

Koenderink and van Doorn's theory, that the basis of stereoscopic slant perception is the deformation component of the disparity, field, was tested for slant around a horizontal axis, which produces images with a vertical ramp of horizontal disparity (horizontal shear) characterised by a global orientation disparity at the vertical meridian. The disparity field in this case can be parsed into two components, deformation and curl, which each contribute half of the orientation disparity. This case was compared with similar random-dot stimuli in which the deformation component was doubled and the curl component eliminated or vice versa. All three types of stimuli had identical orientation disparity at the vertical meridian. A condition in which there was no such orientation disparity, but deformation was present, was also included. It was found that perceived slant was not related to the deformation present, as Koenderink and van Doorn's theory would predict, but was predictable from the orientation disparity at the vertical meridian per se.

Depth Perception↗

PubMed2004

Stimulus dependence of disparity coding in primate visual area V4.

Disparity tuning in visual cortex has been shown using a variety of stimulus types that contain stereoscopic depth cues. It is not known whether different stimuli yield similar disparity tuning curves. We studied whether cells in visual area V4 of the macaque show similar disparity tuning profiles when the same set of disparity values were tested using bars or dynamic random dot stereograms, which are among the most commonly used stimuli for this purpose. In a majority of V4 cells (61%), the shape of the disparity tuning profile differed significantly for the two stimulus types. The two sets of stimuli yielded statistically indistinguishable disparity tuning profiles for only a small minority (6%) of V4 cells. These results indicate that disparity tuning in V4 is stimulus-dependent. Given the fact that bar stimuli contain two-dimensional (2-D) shape cues, and the random dot stereograms do not, our results also indicate that V4 cells represent 2-D shape and binocular disparity in an interdependent fashion, revealing an unexpected complexity in the analysis of depth and three-dimensional shape.

Analysis of Variance↗