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PubMed1983

Functional properties of neurons in middle temporal visual area of the macaque monkey. II. Binocular interactions and sensitivity to binocular disparity.

1. Electrophysiological recordings were made in the middle temporal visual area (MT) of five macaque monkeys. Binocularity and selectivity for disparity were examined using a computer-driven stimulator to activate each eye independently. Results were obtained from 91 single units in MT. 2. Most units in MT receive approximately balanced inputs from the two eyes, and very few could be driven through only one eye. 3. In one type of test for disparity selectivity, units were examined with stimuli that had different but fixed horizontal disparities, thereby simulating frontoparallel movements at different distances from the animal. About two-thirds of ;the units tested for fixed disparity selectivity (52/76) showed pronounced sensitivity to horizontal disparity. Most of these units could be grouped into the same four classes of disparity-tuned units that have previously been described in V1 and V2 of the macaque: near, far, tuned excitatory, and tuned inhibitory. 4. Twenty units were tested for sensitivity to vertical stimulus disparity, which does not normally contribute to stereopsis. Most were as sensitive to vertical disparities as to horizontal. 5. Units were also tested for selectivity for stimuli that moved with changing disparity, simulating trajectories with components of motion toward or away from the animal (motion in depth). No units were found to be truly selective for motion in depth. Units tuned for fixed disparity could appear to prefer motion in depth if tested only with trajectories whose common center point was far from the unit's optimal fixed disparity. However, we do not consider this to represent genuine selectivity for motion in depth, since 1) the responses ara adequately and more easily explained in terms of selectivity for fixed disparity and 2) the best overall response of these units is to frontoparallel motion at the optimal fixed disparity. This observation bears importantly on the interpretation of motion in depth selectivity in previous investigations. 6. The presence of a substantial degree of selectivity for fixed disparity in MT, together with previously demonstrated selectivities for direction and speed, indicates that MT is well suited for the analysis of motion in three-dimensional space.

Animals↗

PubMed2003

Short-latency ocular following in humans is dependent on absolute (rather than relative) binocular disparity.

A previous study showed that the initial ocular following responses elicited by sudden motion of a large random-dot pattern were only modestly attenuated when that whole pattern was shifted out of the plane of fixation by altering its horizontal binocular disparity, but the same disparity applied to a restricted region of the dots had a much more powerful effect [Vision Research 41 (2001) 3371]. Thus, if the dots were partitioned into horizontal bands, for example, and alternate bands were moved in opposite directions to the left or right then ocular following was very weak, but if the (conditioning) dots moving in one direction were all shifted out of the plane of fixation (by applying horizontal disparity to them) then strong ocular following was now seen in the direction of motion of the (test) dots in the plane of fixation, i.e., moving images became much less effective when they were given binocular disparity. We sought to determine if the greater impact of disparity with the partitioned images was because there were additional relative disparity cues. We used a similar partitioned display and found that the dependence of ocular following on the absolute disparity of the conditioning stimulus had a Gaussian form with an x-offset that was close to zero disparity and, importantly, this offset was almost unaffected by changing the absolute disparity of the test stimulus. We conclude from this that it is the absolute--rather than the relative--disparity that is important, and that ocular following has a strong preference for moving images whose absolute disparities are close to zero. This is consistent with the idea that ocular following selectively stabilizes the retinal images of objects in and around the plane of fixation and works in harmony with disparity vergence, which uses absolute disparity to bring objects of interest into the plane of fixation [Archives of Ophthalmology 55 (1956) 848].

Eye Movements↗

PubMed1996

Short-latency disparity vergence responses and their dependence on a prior saccadic eye movement.

1. A dichoptic viewing arrangement was used to study the initial vergence eye movements elicited by brief horizontal disparity steps applied to large textured patterns in three rhesus monkeys. Disconjugate steps (range, 0.2-10.9 degrees) were applied to the patterns at selected times (range, 13-303 ms) after 10 degrees leftward saccades into the center of the pattern. The horizontal and vertical positions of both eyes were recorded with the electromagnetic search coil technique. 2. Without training or reinforcement, disparity steps of suitable amplitude consistently elicited vergence responses at short latencies. For example, with 1.8 degrees crossed-disparity steps applied 26 ms after the centering saccade, the mean latency of onset of convergence for each of the three monkeys was 52.2 +/- 3.8 (SD) ms, 52.3 +/- 5.2 ms, and 53.4 +/- 4.1 ms. 3. Experiments in which the disparity step was confined to only one eye indicated that each eye was not simply tracking the apparent motion that is saw. For example, when crossed-disparity steps were confined to the right eye (which saw leftward steps), the result was (binocular) convergence in which the left eye moved to the right even though that eye had seen only a stationary scene. This movement of the left eye cannot have resulted from independent monocular tracking and indicates that the vergences here derived from the binocular misalignment of the two retinal images. 4. The initial vergence responses to crossed-disparity steps had the following main features. 1) They were always in the correct (i.e., convergent) direction over the full range of stimuli tested, the initial vergence acceleration increasing progressively with increases in disparity until reaching a peak with steps of 1.4-2.4 degrees and declining thereafter to a nonzero asymptote as steps exceeded 5-7 degrees. 2) They showed transient postsaccadic enhancement whereby steps applied in the immediate wake of a saccadic eye movement resulted in much higher initial vergence accelerations than the same steps applied some time later. The response decline in the wake of a saccade was roughly exponential with time constants of 67 +/- 5 (SD) ms, 35 +/- 2 ms, and 54 +/- 4 ms for the three animals. 3) That the postsaccadic enhancement might have resulted in part from the visual stimulation associated with the prior saccade was suggested by the finding that enhancement could also be observed when the disparity steps were applied in the wake of (conjugate) saccadelike shifts of the textured pattern. However, this visual enhancement did not reach a peak unit 17-37 ms after the end of the "simulated" saccade, and the peak enhancement averaged only 45% of that after a "real" saccade. 4) Qualitatively similar transient enhancements in the wake of real and simulated saccades have also been reported for initial ocular following responses elicited by conjugate drifts of the visual scene. We replicated the enhancement effects on ocular following to allow a direct comparison with the enhancement effects on disparity vergence using the same animals and visual stimulus patterns and, despite some clear quantitative differences, we suggest that the enhancement effects share a similar etiology. 5. Initial vergence responses to uncrossed-disparity steps had the following main features. 1) They were in the correct (i.e., divergent) direction only for very small steps (< 1.5-2.5 degrees), and then only when postsaccadic delays were small; when the magnitude of the steps was increased beyond these levels, responses declined to zero and thereafter reversed direction, eventually reaching a nonzero (convergent) asymptote similar to that seen with large crossed-disparity steps; convergent responses were also seen with larger vertical disparity steps, suggesting that they represent default responses to any disparity exceeding a few degrees. 2) As the postsaccadic delay was increased, responses to small steps (1.8 degrees) declined to zero and thereafter re

Animals↗

PubMed2001

Single-unit activity in cortical area MST associated with disparity-vergence eye movements: evidence for population coding.

Single-unit discharges were recorded in the medial superior temporal area (MST) of five behaving monkeys. Brief (230-ms) horizontal disparity steps were applied to large correlated or anticorrelated random-dot patterns (in which the dots had the same or opposite contrast, respectively, at the two eyes), eliciting vergence eye movements at short latencies [65.8 +/- 4.5 (SD) ms]. Disparity tuning curves, describing the dependence of the initial vergence responses (measured over the period 50-110 ms after the step) on the magnitude of the steps, resembled the derivative of a Gaussian, the curves obtained with correlated and anticorrelated patterns having opposite sign. Cells with disparity-related activity were isolated using correlated stimuli, and disparity tuning curves describing the dependence of these initial neuronal responses (measured over the period of 40-100 ms) on the magnitude of the disparity step were constructed (n = 102 cells). Using objective criteria and the fuzzy c-means clustering algorithm, disparity tuning curves were sorted into four groups based on their shapes. A post hoc comparison indicated that these four groups had features in common with four of the classes of disparity-selective neurons in striate cortex, but three of the four groups appeared to be part of a continuum. Most of the data were obtained from two monkeys, and when the disparity tuning curves of all the individual neurons recorded from either monkey were summed together, they fitted the disparity tuning curve for that same animal's vergence responses remarkably well (r(2): 0.93, 0.98). Fifty-six of the neurons recorded from these two monkeys were also tested with anticorrelated patterns, and all showed significant modulation of their activity (P < 0.005, 1-way ANOVA). Further, when all of the disparity tuning curves obtained with these patterns from either monkey were summed together, they too fitted the disparity tuning curve for that same animal's vergence responses very well (r(2): 0.95, 0.96). Indeed, the summed activity even reproduced idiosyncratic differences in the vergence responses of the two monkeys. Based on these and other observations on the temporal coding of events, we hypothesize that the magnitude, direction, and time course of the initial vergence velocity responses associated with disparity steps applied to large patterns are all encoded in the summed activity of the disparity-sensitive cells in MST. Latency data suggest that this activity in MST occurs early enough to play an active role in the generation of vergence eye movements at short latencies.

Algorithms↗

PubMedDate not supplied

A summary measure of health disparity.

OBJECTIVES: Eliminating health disparities is a goal of Healthy People 2010. In order to track progress toward this goal, we need improved methods for measuring disparity. The authors present the Index of Disparity (ID) as a summary measure of disparity. METHODS: The ID, a modified coefficient of variation, was used to measure disparity across populations defined on the basis of race/ethnicity, income, education, and gender. Disparity was also assessed for a diverse range of health indicators and over time to monitor trends. RESULTS: Disparity in cardiovascular disease deaths decreased based on gender from 1989 to 1998 but was largely unchanged based on race/ethnicity. The magnitude of disparities in cervical cancer and cholesterol screening, smoking, exercise, and health insurance ranged from 1.9% to 78.6%. The largest disparities for health indicators were not associated with any particular population classification, whether defined on the basis of race/ethnicity, education, or income. CONCLUSIONS: To eliminate disparities, we need a means to assess disparities across many types of health indicators. Furthermore, for a given health indicator, disparities may differ for populations defined on the basis of race/ethnicity, education, income, and so on. The ID is a simple method for summarizing disparities across groups within a population that can be applied across health indicators regardless of magnitude, over time to monitor trends, and across different populations.

Age Distribution↗

PubMedDate not supplied

Measuring relative disparities in terms of adverse events.

OBJECTIVES: To compare relative measures of disparity in rates of favorable events with those in rates of adverse events. METHODS: Relative measures of disparity are applied to four health indicators to demonstrate how the size of a disparity between groups, and changes in disparity over time, depend on whether indicators are expressed in terms of favorable or adverse events. RESULTS: The size of an absolute measure of disparity is the same whether favorable or adverse events are studied. The size of a relative measure of disparity depends on the rate for each group and the reference point from which the disparity is measured. The rates for each group and for the reference point depend on whether the indicator is expressed in terms of favorable or adverse events. Relative measures of disparity, and conclusions about changes in relative measures of disparity, depend on whether indicators are expressed in terms of favorable or adverse events. CONCLUSIONS: When relative measures of disparity are used to monitor changes in disparity over time or to compare disparities across different indicators, disparities should be measured in terms of adverse events.

Female↗

PubMed2002

Stereo transparency and the disparity gradient limit.

Several studies (Vision Research 15 (1975) 583; Perception 9 (1980) 671) have shown that binocular fusion is limited by the disparity gradient (disparity/distance) separating image points, rather than by their absolute disparity values. Points separated by a gradient >1 appear diplopic. These results are sometimes interpreted as a constraint on human stereo matching, rather than a constraint on fusion. Here we have used psychophysical measurements on stereo transparency to show that human stereo matching is not constrained by a gradient of 1. We created transparent surfaces composed of many pairs of dots, in which each member of a pair was assigned a disparity equal and opposite to the disparity of the other member. For example, each pair could be composed of one dot with a crossed disparity of 6' and the other with uncrossed disparity of 6', vertically separated by a parametrically varied distance. When the vertical separation between the paired dots was small, the disparity gradient for each pair was very steep. Nevertheless, these opponent-disparity dot pairs produced a striking appearance of two transparent surfaces for disparity gradients ranging between 0.5 and 3. The apparent depth separating the two transparent planes was correctly matched to an equivalent disparity defined by two opaque surfaces. A test target presented between the two transparent planes was easily detected, indicating robust segregation of the disparities associated with the paired dots into two transparent surfaces with few mismatches in the target plane. Our simulations using the Tsai-Victor model show that the response profiles produced by scaled disparity-energy mechanisms can account for many of our results on the transparency generated by steep gradients.

Female↗

PubMed2000

Global factors that determine the maximum disparity for seeing cyclopean surface shape.

A disparity gradient limit explains why the maximum amplitude of sinusoidal disparity gratings increases with decreasing disparity spatial frequency. It also explains why the largest disparity for binocular fusion (diplopia threshold) varies directly with stimulus element separation. Does a disparity gradient limit also apply to the detection of cyclopean shape? A previous study addressed this question and concluded that it does not. We examined this question by measuring the largest disparity amplitude (dmax) at which observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to dissociate the effects of disparity gradient and disparity spatial frequency. Gabor micropatterns were used to minimize potential scale-dependent interactions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involved at high disparity spatial frequencies. Combining the gradient limit hypothesis with lowpass disparity filtering describes the pattern of dmax for both smooth and discontinuous surface shapes.

Depth Perception↗

PubMed2004

Stereoscopic processing of absolute and relative disparity in human visual cortex.

Stereoscopic vision relies mainly on relative depth differences between objects rather than on their absolute distance in depth from where the eyes fixate. However, relative disparities are computed from absolute disparities, and it is not known where these two stages are represented in the human brain. Using functional MRI (fMRI), we assessed absolute and relative disparity selectivity with stereoscopic stimuli consisting of pairs of transparent planes in depth in which the absolute and relative disparity signals could be independently manipulated (at a local spatial scale). In experiment 1, relative disparity was kept constant, while absolute disparity was varied in one-half the blocks of trials ("mixed" blocks) and kept constant in the remaining one-half ("same" blocks), alternating between blocks. Because neuronal responses undergo adaptation and reduce their firing rate following repeated presentation of an effective stimulus, the fMRI signal reflecting activity of units selective for absolute disparity is expected to be smaller during "same" blocks as compared with "mixed" ones. Experiment 2 similarly manipulated relative disparity rather than absolute disparity. The results from both experiments were consistent with adaptation with differential effects across visual areas such that 1) dorsal areas (V3A, MT+/V5, V7) showed more adaptation to absolute than to relative disparity; 2) ventral areas (hV4, V8/V4alpha) showed an equal adaptation to both; and 3) early visual areas (V1, V2, V3) showed a small effect in both experiments. These results indicate that processing in dorsal areas may rely mostly on information about absolute disparities, while ventral areas split neural resources between the two types of stereoscopic information so as to maintain an important representation of relative disparity.

Depth Perception↗

PubMed2006

Does depth perception require vertical-disparity detectors?

Stereo depth perception depends on the fact that objects project to different positions in the two eyes. Because our eyes are offset horizontally, these retinal disparities are mainly horizontal, and horizontal disparity suffices to give an impression of depth. However, depending on eye position, there may also be small vertical disparities. These are significant because, given both vertical and horizontal disparities, the brain can deduce eye position from purely retinal information and, hence, derive the position of objects in space. However, we show here that, to achieve this, the brain need measure only the magnitude of vertical disparity; for physically possible stimuli, the sign then follows from the stereo geometry. The magnitude of vertical disparity--and hence eye position--can be deduced from the response of purely horizontal-disparity sensors because vertical disparity moves corresponding features off the receptive fields, reducing the effective binocular correlation. As proof, we demonstrate an algorithm that can accurately reconstruct gaze and vergence angles from the population activity of pure horizontal-disparity sensors and show that it is subject to the induced effect. Given that disparities experienced during natural viewing are overwhelmingly horizontal and that eye position measures require only horizontal-disparity sensors, this work raises two questions: Does the brain in fact contain sensors tuned to nonzero vertical disparities, and if so, why?

Algorithms↗

PubMed2006

Relationship between quality of care and racial disparities in Medicare health plans.

CONTEXT: Overall quality of care and racial disparities in quality are important and related problems in health care, but their relationship has not been well studied. In the Medicare managed care program, broad improvements in quality have been accompanied by reduced racial gaps in processes of care, but substantial disparities in outcomes have persisted. OBJECTIVES: To assess variations among Medicare health plans in overall quality and racial disparity in 4 Health Plan Employer and Data Information Set (HEDIS) outcome measures, to determine whether high-performing plans exhibit smaller racial disparities, and to identify plans with high quality and low disparity. DESIGN, SETTING, AND PATIENTS: We assessed the relationship between quality and racial disparity using multilevel multivariable regression models. The study sample included 431,573 individual-level observations in 151 Medicare health plans from 2002 to 2004. MAIN OUTCOME MEASURES: Hemoglobin A(1c) of less than 9.5% or less than 9.0% for enrollees with diabetes; low-density lipoprotein cholesterol level of less than 130 mg/dL for enrollees with diabetes or after a coronary event; and blood pressure of less than 140/90 mm Hg for enrollees with hypertension. RESULTS: Clinical performance on HEDIS outcome measures was 6.8% to 14.4% lower for black enrollees than for white enrollees (P<.001 for all). For each measure, more than 70% of this disparity was due to different outcomes for black and white individuals enrolled in the same health plan rather than selection of black enrollees into lower-performing plans. Health plans varied substantially in both overall quality and racial disparity on each of the 4 outcome measures. Adjusted correlations between overall quality and racial disparity were small and not statistically significant, ranging from 0.01 (blood pressure control) to -0.21 (cholesterol control in diabetes). Only 1 health plan achieved both high quality and low disparity on more than 1 measure. CONCLUSIONS: In Medicare health plans, disparities vary widely and are only weakly correlated with the overall quality of care. Therefore, plan-specific performance reports of racial disparities on outcome measures would provide useful information not currently conveyed by standard HEDIS reports.

Black or African American↗

PubMed1983

Disparity range for local stereopsis as a function of luminance spatial frequency.

The disparity range for stereo sensitivity was investigated with spatially filtered bars, tuned narrowly over a broad range of spatial frequencies. When measured with narrow (high spatial frequency) bars the disparity range for stereopsis exceeded two orders of magnitude. The range was reduced with broad (low spatial frequency) bars by an elevation of stereothreshold that increased according to a constant 6 deg phase disparity. The upper disparity limit also increased at broad spatial periods but at a lower rate. These size disparity correlations illustrate quantitative stereopsis along a continuum from fine to coarse disparities (+/- 2 deg). A disparity matching task revealed that greater amounts of uncrossed than crossed disparity were required to match suprathreshold disparities. The ratio of standard/matched disparity was lower in both directions for small suprathreshold disparities subtended by broad than by narrow spatial periods. This selective reduction of stereo-efficiency ratio illustrates tuning to coarse disparities subtended by broad (low spatial frequency) stimuli and accounts for the marked reduction of stereoacuity caused by spectacle blur.

Depth Perception↗

PubMed1985

Electrophysiological evidence for the existence of coarse and fine disparity mechanisms in human.

Visual sensitivity to stereoscopic disparity changes was measured both psychophysically and by means of evoked potentials. The binocular disparity of a dynamic random-dot stereogram portraying a single flat plane alternated between two values symmetrical about the plane of fixation. The threshold for disparity alternation of the stereoscopic plane was determined at alternation rates between 4 and 12 depth reversals per second (rps). Evoked potential and forced-choice psychophysical estimates of stereoscopic threshold at each reversal frequency agreed, with a mean discrepancy of only +/- 0.1 log units. Evoked potential amplitude was a linear function of log disparity up to about 15 arc min peak to peak disparity. For larger disparities, the evoked potential amplitude versus log disparity function was found to be nonmonotonic with a dip occurring at approximately 26 arc min disparity. Responses to fine disparities of less than 20 arc min lay close to one temporal phase while those evoked by coarse disparities greater than 40 arc min lay near a different phase. The data suggest that disparity processing mechanisms either undergo dynamic changes as disparity increases or that processing shifts between at least two independent mechanisms.

Depth Perception↗

PubMed2000

Disparity selectivity of neurons in monkey inferior temporal cortex.

The inferior temporal cortex (IT) of the monkey, a final stage in the ventral visual pathway, has been known to process information on two-dimensional (2-D) shape, color, and texture. On the other hand, the dorsal visual pathway leading to the posterior parietal cortex has been known to process information on location in space. Likewise, neurons selective for binocular disparity, which convey information on depth, have been found mainly in areas along the dorsal visual pathway. Here, we report that many neurons in the IT are also selective for binocular disparity. We recorded extracellular activity from IT neurons and found that more than half of the neurons changed their response depending on the disparity added. The change was not attributed to monocular responses or eye movements. Most neurons selective for disparity were "near" or "far" cells; they preferred either crossed or uncrossed disparity, and only a small population was tuned to zero disparity. Disparity-selective neurons were also selective for shape. Most preferred the same type of disparity irrespective of the shape presented. Disparity preference was also invariant for the fronto-parallel translation of the stimuli in most of the neurons. Finally, nearby neurons exhibited similar disparity selectivity, suggesting the existence of a functional module for processing of binocular disparity in the IT. From the above and our recent findings, we suggest that the IT integrates shape and binocular disparity information, and plays an important role in the reconstruction of three-dimensional (3-D) surfaces.

Animals↗

PubMed2004

A coarse-to-fine disparity energy model with both phase-shift and position-shift receptive field mechanisms.

Numerous studies suggest that the visual system uses both phase- and position-shift receptive field (RF) mechanisms for the processing of binocular disparity. Although the difference between these two mechanisms has been analyzed before, previous work mainly focused on disparity tuning curves instead of population responses. However, tuning curve and population response can exhibit different characteristics, and it is the latter that determines disparity estimation. Here we demonstrate, in the framework of the disparity energy model, that for relatively small disparities, the population response generated by the phase-shift mechanism is more reliable than that generated by the position-shift mechanism. This is true over a wide range of parameters, including the RF orientation. Since the phase model has its own drawbacks of underestimating large stimulus disparity and covering only a restricted range of disparity at a given scale, we propose a coarse-to-fine algorithm for disparity computation with a hybrid of phase-shift and position-shift components. In this algorithm, disparity at each scale is always estimated by the phase-shift mechanism to take advantage of its higher reliability. Since the phase-based estimation is most accurate at the smallest scale when the disparity is correspondingly small, the algorithm iteratively reduces the input disparity from coarse to fine scales by introducing a constant position-shift component to all cells for a given location in order to offset the stimulus disparity at that location. The model also incorporates orientation pooling and spatial pooling to further enhance reliability. We have tested the algorithm on both synthetic and natural stereo images and found that it often performs better than a simple scale-averaging procedure.

Algorithms↗

PubMed2005

Reduction in racial and ethnic disparities after enrollment in the State Children's Health Insurance Program.

BACKGROUND: Racial/ethnic disparities are associated with lack of health insurance. Although the State Children's Health Insurance Program (SCHIP) provides health insurance to low-income children, many of whom are members of racial/ethnic minority groups, little is known about whether SCHIP affects racial/ethnic disparities among children who enroll. OBJECTIVES: The objectives of this study were to (1) describe demographic characteristics and previous health insurance experiences of SCHIP enrollees by race, (2) compare racial/ethnic disparities in medical care access, continuity, and quality before and during SCHIP, and (3) determine whether disparities before or during SCHIP are explained by sociodemographic and health system factors. METHODS: Pre/post-parent telephone survey was conducted just after SCHIP enrollment and 1 year after enrollment of 2290 children who had an enrollment start date in New York State's SCHIP between November 2000 and March 2001, stratified by race/ethnicity (non-Hispanic white, non-Hispanic black, and Hispanic). The main outcome measures were usual source of care (USC), preventive care use, unmet needs, patterns of USC use, and parent-rated quality of care before versus during SCHIP. RESULTS: Children were white (25%), black (31%), or Hispanic (44%); 62% were uninsured > or =12 months before SCHIP. Before SCHIP, a greater proportion of white children had a USC compared with black or Hispanic children (95%, 86%, and 81%, respectively). Nearly all children had a USC during SCHIP (98%, 95%, and 98%, respectively). Before SCHIP, black children had significantly greater levels of unmet need relative to white children (38% vs 27%), whereas white and Hispanic children did not differ significantly (27% vs 29%). During SCHIP, racial/ethnic disparities in unmet need were eliminated, with unmet need at 19% for all 3 racial/ethnic groups. Before SCHIP, more white children made all/most visits to their USC relative to black or Hispanic children (61%, 54%, and 34%, respectively); all improved during SCHIP with no remaining disparities (87%, 86%, and 92%, respectively). Parent-rated visit quality improved for all groups, but preexisting racial/ethnic disparities remained during SCHIP, with improved yet relatively lower levels of satisfaction among parents of Hispanic children. Sociodemographic and health system factors did not explain disparities in either period. CONCLUSIONS: Enrollment in SCHIP was associated with (1) improvement in access, continuity, and quality of care for all racial/ethnic groups and (2) reduction in preexisting racial/ethnic disparities in access, unmet need, and continuity of care. Racial/ethnic disparities in quality of care remained, despite improvements for all racial groups. Sociodemographic and health system factors did not add to the understanding of racial/ethnic disparities. SCHIP improves care for vulnerable children and reduces preexisting racial/ethnic disparities in health care.

Adolescent↗

PubMed2005

Pooling signals from vertically and non-vertically orientation-tuned disparity mechanisms in human stereopsis.

To understand the role that orientation-tuned disparity-sensitive mechanisms play in the perception of stereoscopic depth, we measured stereothresholds using two sets of random-dot stimuli that produce identical stimulation of disparity mechanisms tuned to vertical orientation but dissimilar stimulation of disparity mechanisms tuned to non-vertical orientations. Either 1 or 1.5D of astigmatic blur was simulated in the random-dot images presented to both eyes, using two axis configurations. In the parallel-axis conditions, the axis of simulated astigmatic blur was same in the two eyes (0, 45 or 135 o[rientation]deg). In the orthogonal-axis conditions, the axes of astigmatic blur were orthogonal in the two eyes (LE: 180, RE: 90; LE: 90, RE: 180; LE: 45, RE: 135; and LE: 135, RE: 45). Whereas the stimulation of disparity mechanisms tuned to near-vertical orientations should be similar in the oblique parallel- and orthogonal-axis conditions, the stimulation of non-vertically tuned disparity mechanisms should be dissimilar. Measured stereothresholds were higher in the orthogonal compared to the parallel-axis condition by factors of approximately 2 and 5, for 1 and 1.5D of simulated oblique astigmatic blur, respectively. Further, for comparable magnitudes of simulated astigmatic blur, stereothresholds in the (LE: 180, RE: 90 and LE: 90, RE: 180) conditions were similar to those in the (LE: 45, RE: 135 and LE: 135, RE: 45) conditions. These results suggest that the computation of horizontal disparity includes substantial contributions from disparity mechanisms tuned to non-vertical orientations. Simulations using a modified version of a disparity-energy model [Qian, N., & Zhu, Y. (1997). Physiological computation of binocular disparity. Vision Research, 37, 1811-1827], show (1) that pooling across disparity mechanisms tuned to vertical and non-vertical orientations is required to account for our data and (2) that this pooling can provide the spatial resolution needed to encode spatially changing horizontal disparities.

Depth Perception↗

PubMed2003

Short-latency disparity-vergence eye movements in humans: sensitivity to simulated orthogonal tropias.

Small disparity stimuli applied to large random-dot patterns elicit machine-like vergence eye movements at short latency. We have examined the sensitivity of these eye movements to simulated orthogonal tropias in three normal subjects by recording (1) the effects of vertical disparities on the initial horizontal vergence responses elicited by 2 degrees crossed and uncrossed (horizontal) disparity stimuli, and (2) the effects of horizontal disparities on the initial vertical vergence responses elicited by 1.2 degrees left-hyper and 0.8 degrees right-hyper (vertical) disparity stimuli. Initial vergence responses were strongest when the orthogonal disparity was close to zero, and decreased to zero as the orthogonal disparity increased to 3 degrees -5 degrees, i.e., there was only a limited tolerance for orthogonal disparity. Tuning curves describing the dependence of the initial change in the vergence angle on the orthogonal disparity were well fit by a Gaussian function. An additional subject, who had an esotropia of approximately 10 degrees in our experimental setup, showed almost no horizontal vergence responses but did show vertical vergence responses to vertical disparity stimuli at short latency (albeit slightly longer than normal) despite the fact that her esotropia resulted in uncrossed disparities that would have totally disabled the vertical vergence mechanism of a normal subject, cf., anomalous retinal correspondence.

Convergence, Ocular↗