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PubMed2003

Stereoscopic depth perception from oblique phase disparities.

In order to understand the role of oblique retinal image disparities in the perception of stereoscopic depth, we measured the depth perceived from random dot stereograms in which phase disparities were introduced in a selected band of stimulus orientations. A band of orientation was defined by a center orientation that ranged from 7.5 (near vertical) to 82.5 o[rientation]deg and by a bandwidth that was defined as the difference between the highest and the lowest orientation in the band. The bandwidths tested were 15, 30 and 45 odeg. A constant phase disparity of 90 p[hase]deg was introduced in all of the oriented spatial frequency components within the orientation band and the perceived depth of each stimulus was matched using a small square binocular probe. For each bandwidth, perceived depth increased with an increase in the center orientation up to approximately 60 odeg. This suggests that the human stereovision system derives a large proportion of information about perceived stereoscopic depth from oblique phase disparities. Simulations using an energy model of stereoscopic depth perception indicate that oblique phase disparities are unlikely to be processed by neural mechanisms tuned to near-vertical orientations within the stimulus. Our results therefore suggest that oblique retinal disparities are initially detected as oblique phase disparities by binocular mechanisms tuned to oblique orientations. Because the perceived depth from oblique phase disparities is consistent with the trigonometrically determined equivalent horizontal disparities, we presume that the information from oblique phase disparities is included in the visual system's computation of the horizontal retinal disparity.

Analysis of Variance↗

PubMed2000

The effect of disparity on the very earliest ocular following responses and the initial neuronal activity in monkey cortical area MST.

Movement of the visual scene evokes tracking movement with the eyes (ocular following response, OFR) at short latency (Miles, F.A., Kawano, K., Optican, L.M., 1986. Short-latency ocular following responses of monkey. I. Dependence on temporospatial properties of visual input. J. Neurophysiol. 56, 1321-1354). We examined the effect of binocular disparity on the initial OFR. The dependence of the OFR on horizontal disparity steps was studied in three monkeys (Macaca fuscata), and the associated unit discharges in the medial superior temporal area (MST) were studied in two of these. Based on the changes in eye position over the period 50-83 ms (measured from stimulus onset), the initial OFR showed clear dependence on the disparity imposed during the preceding centering saccade. The disparity tuning curves were S-shaped with a peak at a small crossed disparity and a trough at uncrossed disparities. Based on the changes in discharge rate over the period 40-73 ms (measured from stimulus onset), almost all OFR-related MST neurons (80/83, 96.4%) showed significant dependence on the disparity step (Student's t-test, P < 0.05). The early neuronal responses of the majority of units (41/75, 55%) had disparity tuning curves resembling those for the OFR, which peaked at small crossed disparities. These findings suggest that the neurons in the MST contain information on binocular disparity in their short-latency discharges, and are involved in the neural basis of the OFR's dependence on horizontal disparity.

Action Potentials↗

PubMed1993

A proximity-contingent stereoscopic depth aftereffect: evidence for adaptation to disparity gradients.

Prolonged inspection of a surface slanted in the third dimension of visual space typically results in a negative aftereffect such that, after adaptation, a surface in the fronto-parallel plane will appear slanted in the opposite direction. Binocular disparity is not necessary to generate such effects, since they can be obtained monocularly, presumably via adaptation to texture gradient. Six experiments demonstrated durable stereoscopic depth aftereffects in the absence of a texture gradient--by using discrete disparate objects rather than slanted surfaces--and demonstrated that adaptation was to the interobject disparity gradient rather than to the relative disparity of the objects per se. The disparity required to null the obtained aftereffects was inversely proportional to the horizontal separation of elements, for a constant disparity, and directly proportional to the separation of subsequently presented probes. When elements differed in depth (disparity), but were not laterally separated, nulling disparity was significant but invariant with changes in the horizontal separation of probe elements. In that case, adaptation was (i) either to the disparity gradient generated by the vertical separation of probe elements (of which the relative disparity component was tapped); or (ii) to relative disparity per se.

Adult↗

PubMed1989

Vertical disparity, egocentric distance and stereoscopic depth constancy: a new interpretation.

There has long been a problem concerning the presence in the visual cortex of binocularly activated cells that are selective for vertical stimulus disparities because it is generally believed that only horizontal disparities contribute to stereoscopic depth perception. The accepted view is that stereoscopic depth estimates are only relative to the fixation point and that independent information from an extraretinal source is needed to scale for absolute or egocentric distance. Recently, however, theoretical computations have shown that egocentric distance can be estimated directly from vertical disparities without recourse to extraretinal sources. There has been little impetus to follow up these computations with experimental observations, because the vertical disparities that normally occur between the images in the two eyes have always been regarded as being too small to be of significance for visual perception and because experiments have consistently shown that our conscious appreciation of egocentric distance is rather crude and unreliable. Nevertheless, the veridicality of stereoscopic depth constancy indicates that accurate distance information is available to the visual system and that the information about egocentric distance and horizontal disparity are processed together so as to continually recalibrate the horizontal disparity values for different absolute distances. Computations show that the recalibration can be based directly on vertical disparities without the need for any intervening estimates of absolute distance. This may partly explain the relative crudity of our conscious appreciation of egocentric distance. From published data it has been possible to calculate the magnitude of the vertical disparities that the human visual system must be able to discriminate in order for depth constancy to have the observed level of veridicality. From published data on the induced effect it has also been possible to calculate the threshold values for the detection of vertical disparities by the visual system. These threshold values are smaller than those needed to provide for the recalibration of the horizontal disparities in the interests of veridical depth constancy. An outline is given of the known properties of the binocularly activated cells in the striate cortex that are able to discriminate and assess the vertical disparities. Experiments are proposed that should validate, or otherwise, the concepts put forward in this paper.

Depth Perception↗

PubMed2002

Quantitative analysis of the responses of V1 neurons to horizontal disparity in dynamic random-dot stereograms.

Horizontal disparity tuning for dynamic random-dot stereograms was investigated for a large population of neurons (n = 787) in V1 of the awake macaque. Disparity sensitivity was quantified using a measure of the discriminability of the maximum and minimum points on the disparity tuning curve. This measure and others revealed a continuum of selectivity rather than separate populations of disparity- and nondisparity-sensitive neurons. Although disparity sensitivity was correlated with the degree of direction tuning, it was not correlated with other significant neuronal properties, including preferred orientation and ocular dominance. In accordance with the Gabor energy model, tuning curves for horizontal disparity were adequately described by Gabor functions when the neuron's orientation preference was near vertical. For neurons with orientation preferences near to horizontal, a Gaussian function was more frequently sufficient. The spatial frequency of the Gabor function that described the disparity tuning was weakly correlated with measurements of the spatial frequency and orientation preference of the neuron for drifting sinusoidal gratings. Energy models make several predictions about the relationship between the response rates to monocular and binocular dot patterns. Few of the predictions were fulfilled exactly, although the observations can be reconciled with the energy model by simple modifications. These same modifications also provide an account of the observed continuum in strength of disparity selectivity. A weak correlation between the disparity sensitivity of simultaneously recorded single- and multiunit data were revealed as well as a weak tendency to show similar disparity preferences. This is compatible with a degree of local clustering for disparity sensitivity in V1, although this is much weaker than that reported in area MT.

Animals↗

PubMed2001

Short-latency disparity vergence in humans.

Eye movement recordings from humans indicated that brief exposures (200 ms) to horizontal disparity steps applied to large random-dot patterns elicit horizontal vergence at short latencies (80.9 +/- 3.9 ms, mean +/- SD; n = 7). Disparity tuning curves, describing the dependence of the initial vergence responses (measured over the period 90-157 ms after the step) on the magnitude of the steps, resembled the derivative of a Gaussian, with nonzero asymptotes and a roughly linear servo region that extended only a degree or two on either side of zero disparity. Responses showed transient postsaccadic enhancement: disparity steps applied in the immediate wake of saccadic eye movements yielded higher vergence accelerations than did the same steps applied some time later (mean time constant of the decay, 200 ms). This enhancement seemed to be dependent, at least in part, on the visual reafference associated with the prior saccade because similar enhancement was observed when the disparity steps were applied in the wake of saccadelike shifts of the textured pattern. Vertical vergence responses to vertical disparity steps were qualitatively similar: latencies were longer (on average, by 3 ms), disparity tuning curves had the same general form but were narrower (by approximately 20%), and their peak-to-peak amplitudes were smaller (by approximately 70%). Initial vergence responses usually had directional errors (orthogonal components) with a very systematic dependence on step size that often approximated an exponential decay to a nonzero asymptote (mean space constant +/- SD, 1.18 +/- 0.66 degrees ). Based on the asymptotes of these orthogonal responses, horizontal errors (with vertical steps) were on average more than three times greater than vertical errors (with horizontal steps). Disparity steps >7 degrees generated "default" responses that were independent of the direction of the step, idiosyncratic, and generally had both horizontal and vertical components. We suggest that the responses depend on detectors that sense local disparity matches, and that orthogonal and "default" responses result from globally "false" matches. Recordings from three monkeys, using identical disparity stimuli, confirmed that monkeys also show short-latency disparity vergence responses (latency approximately 25 ms shorter than that of humans), and further indicated that these responses show all of the major features seen in humans, the differences between the two species being solely quantitative. Based on these data and those of others implying that foveal images normally take precedence, we suggest that the mechanisms under study here ordinarily serve to correct small vergence errors, automatically, especially after saccades.

Adult↗

PubMed1994

Size-disparity correlation in stereopsis at contrast threshold.

Contrast thresholds for 75% correct depth identification in narrow-band filtered random dot stereograms were determined for different center spatial frequencies and binocular disparities. Rigorous control over vergence was maintained during testing, and a forced-choice procedure was used. The resulting contrast sensitivity function for stereopsis revealed sensitivity over a greater range of disparities at low than at high spatial frequencies. Sensitivity peaked for large disparities at low spatial frequencies and for small disparities at high spatial frequencies. When disparities were converted to effective binocular phase differences, the variation of contrast sensitivity with phase followed a consistent pattern across spatial frequencies, with peak sensitivity occurring mainly for binocular phases of between 90 degrees and 180 degrees. These results have implications for the extent of spatial integration at the input to the disparity sensing mechanism. A model postulating a spread of positional disparities independent of the spatial frequency selectivity of disparity-sensitive units cannot account for the results. But the size-disparity correlation strongly evident in our data is predicted by certain models of stereopsis, such as phase disparity encoding. An ideal observer analysis is developed that demonstrates that our results were not forced by the nature of the stimulus employed; rather, the quantum efficiency for stereopsis at contrast threshold follows the size-disparity correlation.

Contrast Sensitivity↗

PubMed1997

Discrimination of the spatial derivatives of horizontal binocular disparity.

Observers discriminated the relative disparity, disparity gradient, and disparity curvature of surfaces defined by horizontal binocular disparity in random-dot stereograms. In experiment 1, thresholds for discriminating the depth of sinusoidal corrugations were very similar for different corrugation frequencies, despite large differences in disparity gradient and disparity curvature. Thus observers used a relative disparity cue in preference to a slant or curvature cue. Experiment 2 isolated the spatial derivatives of disparity by jittering the other available cues, using surfaces with square-wave, triangle-wave, and parabolic-wave profiles. Weber fractions were 4%-10% for relative disparity, 6%-12% for disparity gradient, and 15%-30% for disparity curvature. Experiment 3 confirmed this result for larger surfaces. The study supports the view that human stereoscopic vision aims to represent the local scene relative to the observer, at the expense of computing intrinsic properties of objects, such as curvature.

Depth Perception↗

PubMed1987

What causes stereoscopic tilt from spatial frequency disparity.

A controversy still exists concerning whether the tilt created with interocular spatial frequency disparity arises from a computation of spatial frequency differences or from cumulative positional disparity. In a first experiment, we examined the influence of positional disparity on tilt created with frequency disparity, reasoning that if tilt were computed from spatial frequency differences, the perceived angle should remain unaltered since adding a positional disparity does not change the harmonic content of the stimulus. The results indicated that positional disparity weakened perceived tilt. In a second experiment, we tested the idea that tilt results from the calculation of increasing positional disparity across the display, arguing if local matches of features in the two eyes are made in computing tilt, then the solution to binocular correspondence may be less ambiguous if the same number of cycles was displayed for both spatial frequencies. Perceived tilt increased when the number of cycles was equal, although the angle of tilt still decreased with positional disparity. In Experiment 3, we further reduced potential sources of ambiguity for the binocular matching process by employing D10s (the tenth derivative of a Gaussian) instead of grating patterns. Positional disparity exerted essentially no influence on the perceived angle of tilt of the D10s. Taken together, the results of these experiments suggest that tilt from frequency disparity can be explained solely on the basis of positional disparity.

Depth Perception↗

PubMed1998

Gene conversion disparity in yeast: its extent, multiple origins, and effects on allele frequencies.

The extent of disparity in gene conversion direction in yeast (Saccharomyces cerevisiae) is important for recombination mechanisms and for effects of conversion on allele frequencies in populations. An analysis of published and unpublished data demonstrates that yeast frequently shows significant and extensive conversion disparity, contrary to many published statements. All types of mutation--base-substitutions, frameshifts and longer deletions and additions--can show significant 6:2/2:6 and/or 5:3/3:5 disparity. There was little correlation between the occurrence of 6:2/2:6 and 5:3/3:5 disparities; when both were significant, they were more often in opposite directions than in the same direction. Surprisingly, there was little correlation between a mutation's molecular nature and its disparity properties, which generally seem unpredictable. Disparity in yeast has multiple origins. From the equations discussed, all disparity types can be explained by one or more of: correction direction disparity, chromatid invasion disparity (including cases caused by different frequencies of double-strand breaks or gaps in nonsister homologous chromatids), strand invasion disparity, and different correction frequencies for the two types of mispair for a heterozygous mutation. Levels of overall disparity and of conversion frequency mean that conversion must often change allele frequencies in sexually reproducing yeast populations.

Alleles↗

PubMedDate not supplied

Entamoeba dispar: ultrastructure, surface properties and cytopathic effect.

The cytological features of Entamoeba dispar, recently recognized by biochemical and molecular biology criteria as a distinct species, were compared to those of Entamoeba histolytica When cultured under axenic conditions, living trophozoites of E. dispar strain SAW 76ORR clone A were more elongated in form, had a single frontal pseudopodium, and showed a noticeable uroid. In sections of E. dispar trophozoites stained with Toluidine blue, characteristic areas of cytoplasmic metachromasia were seen due to the presence of large deposits of glycogen, seldom found in E. histolytica strain HM1:IMSS. Under the light microscope the periphery of the nucleus in E. dispar was, lined by finer, more regularly distributed dense granules. With transmission electron microscopy the surface coat of E. dispar was noticeable thinner. In addition. E. dispar had a lower sensitivity to agglutinate with concanavalin A and a higher negative surface charge, measured by cellular microelectrophoresis. The cytopathic effect of E. dispar was much slower, analyzed by the gradual loss of transmural electrical resistance of MDCK epithelial cell monolayers mounted in Ussing chambers. Whereas in E. histolytica phagocytosis of epithelial cells plays an important role in its cytopathic effect. E. dispar trophozoites placed in contact with MDCK cells showed only rare evidence of phagocytosis. The results demonstrate that the morphology of E. dispar is different to that of E. histolytica, both at the light microscopical and the ultrastructural levels. In addition they show that E. dispar in axenic culture has a moderate cytopathic effect on epithelia] cell monoLayers. However, when compared to E. histolytica, the in vitro lytic capacity of E. dispar is much slower and less intense.

Agglutination↗

PubMed1992

Interaction of Mycoplasma dispar with bovine alveolar macrophages.

The capacity to avoid phagocytosis and the activation of bovine alveolar macrophages (BAM) by encapsulated Mycoplasma dispar or purified M. dispar capsule was investigated. Encapsulated and unencapsulated M. dispar were cocultured with BAM in the presence or absence of antisera prepared against unencapsulated M. dispar or purified capsule antiserum. Unopsonized mycoplasmas resisted phagocytosis, while only anti-capsule antibodies enhanced the phagocytosis of encapsulated mycoplasmas. BAM were cultured in the presence of purified M. dispar capsule or either live or heat-killed encapsulated or unencapsulated M. dispar. These BAM were then activated with Escherichia coli endotoxin or left without further activation. The supernatants of these cultures were assayed for tumor necrosis factor, interleukin 1, and glucose consumption as indicators of macrophage activation. Tumor necrosis factor and interleukin 1 were produced by BAM stimulated with unencapsulated M. dispar but not when encapsulated M. dispar or its purified capsule was used. Similarly, glucose consumption was increased in the presence of unencapsulated M. dispar, but not when BAM were cocultured with encapsulated M. dispar or purified capsule. When BAM were treated with purified capsule or encapsulated mycoplasmas, they could not be subsequently activated by endotoxin. These results indicate that encapsulated M. dispar or purified capsule exerts an inhibitory effect on the activity of BAM and prevents the activation of these cells.

Animals↗

PubMed2005

Racial and ethnic disparities in care: the perspectives of cardiologists.

BACKGROUND: Despite extensive documentation of racial and ethnic disparities in care, provider awareness of disparities has been thought to be low. To be effective, educational efforts for physicians must consider providers' knowledge and beliefs about what causes disparities and what can be done about them. METHODS AND RESULTS: We conducted a Web-based survey of 344 cardiologists to determine their level of awareness of disparities and views of underlying causes. Responses were assessed by means of 5-point Likert scales. Thirty-four percent of cardiologists agreed that disparities existed in care overall in the US healthcare system, and 33% agreed that disparities existed in cardiovascular care. Only 12% felt disparities existed in their own hospital setting, and even fewer, 5%, thought disparities existed in the care of their own patients. Despite this, most respondents rated the strength of the evidence about disparities as "very strong" or "strong." Respondents identified many potential causes for disparities in care but were more likely to endorse patient and system level factors (eg, insurance status or adherence) rather than provider level factors. CONCLUSIONS: Cardiologists' awareness of disparities in care remains low, and awareness is inversely proportional to proximity to their own practice setting.

Adult↗

PubMed2003

Relations of perceived ethnic disparity to involvement in ethnic and host cultures.

Among 124 single foreign students residing in the USA, this study examined the relations between perceived ethnic disparity, consisting of physical, cultural. and linguistic dimensions, and ethnic and host culture involvement. The Ethnic Disparity Perception Inventory, a 44-item Likert scale devised to measure perceived ethnic disparity, showed perceived physical disparity (r = .60) and cultural disparity (r = .51) were positively related to ethnic involvement. Perceived cultural disparity (r = -.22) and linguistic disparity (r = -.27) were negatively associated with involvement in the host culture. Multiple regression analysis indicated that, although the findings held for non-White respondents, for White respondents (n = 39) perceived physical disparity (beta = .54), not cultural disparity, was a significant predictor of ethnic involvement (Adjusted R2 = .268) and perceived cultural disparity (beta = -.37), not linguistic disparity, was a significant (negative) indicator of host culture involvement (Adjusted R2 = .110).

Adult↗

PubMed2003

The role of race and ethnicity in the State Children's Health Insurance Program (SCHIP) in four states: are there baseline disparities, and what do they mean for SCHIP?

BACKGROUND: Elimination of racial and ethnic disparities in health has become a major national goal. The State Children's Health Insurance Program (SCHIP) has the potential to reduce disparities among the children who enroll if they exhibit the same disparities that have been documented in previous studies of low-income children. To determine the potential impact of SCHIP on racial and ethnic disparities, it is critical to assess baseline levels of health disparities among children enrolling in SCHIP. OBJECTIVE: To use data from the Child Health Insurance Research Initiative (CHIRI) to 1) describe the sociodemographic profile of new enrollees in SCHIP in Alabama, Florida, Kansas, and New York; 2) determine if there were differences in health insurance and health care experiences among white, black, and Hispanic SCHIP enrollees before enrollment in SCHIP; and 3) explore whether race or ethnicity, controlled for other factors, affected pre-SCHIP access to health coverage and health care. SETTING: SCHIP programs in Alabama, Florida, Kansas, and New York, which together include 26% of SCHIP enrollees nationwide. DESIGN: Telephone interview (mailed survey in Alabama) about the child's health, health insurance, and health care experiences conducted shortly after SCHIP enrollment to assess experience during the time period before SCHIP. SAMPLE: New SCHIP enrollees (0-17.9 years old in Alabama, Kansas, and New York and 11.5-17.9 years old in Florida). Stratified sampling was performed in Kansas and New York, with results weighted to reflect statewide populations of new SCHIP enrollees. MEASURES: Sociodemographic characteristics including income, education, employment, and other characteristics of the child and the family, race and ethnicity (white non-Hispanic, black non-Hispanic, and Hispanic [any race]), prior health insurance, health care access and utilization, and health status. ANALYSES: Bivariate analyses were used to compare baseline measures upon enrollment for white, black, and Hispanic SCHIP enrollees. Multivariate analyses were performed to assess health status and health care access measures (prior insurance, presence of a usual source of care (USC), and use of preventive care), controlling for demographic factors described above. Weighted analyses (where appropriate) were performed by using SPSS, STATA, or SUDAAN. RESULTS: Racial and ethnic composition varied across the SCHIP cohorts studied, with black and Hispanic children comprising the following proportion of enrollees, respectively: Alabama, 33% and <1%; Florida, 16% and 26%; Kansas, 12% and 15%; and New York, 24% and 36%. Black and Hispanic children were more likely to reside in single-parent and lower-income families. With some variation by state, children from minority groups were more likely to report poorer health status than were white children. Relative to white children, children from minority groups in Florida and New York were more likely to have been uninsured for the entire year before SCHIP enrollment. In all states, children from minority groups who had prior coverage were more likely to have previously been enrolled in Medicaid than in private health insurance and were less likely to have had employer-sponsored coverage compared with white children. Except in Alabama, there was a difference in having a USC, with children from minority groups less likely to have had a USC before SCHIP enrollment compared with white children. No consistent pattern of health care utilization before SCHIP was noted across states with respect to race or ethnicity. Findings from multivariate analyses, controlling for sociodemographic factors, generally confirmed that black and Hispanic children were more likely to have lacked insurance or a USC before enrollment in SCHIP and to have poorer health status compared with white children. CONCLUSIONS: SCHIP is enrolling substantial numbers of racial and ethnic minority children. There are baseline racial and ethnic disparities among new enrollees in SCHIP, with black and Hispanic children faring worse than white children on many sociodemographic and health system measures, and there are differences among states in the prevalence and magnitude of these disparities. After controlling for sociodemographic factors, these disparities persisted. IMPLICATIONS FOR MONITORING AND IMPROVING SCHIP: SCHIP has the potential to play a critical role in efforts to eliminate racial and ethnic disparities in health among the children it serves. However, study findings indicate that programmatic efforts are necessary to ensure that disparities are not perpetuated. Program effectiveness and outcomes should be monitored by race and ethnicity to ensure equity in access, use, and outcomes across all racial and ethnic groups. Assessing the health characteristics and needs of new SCHIP enrollees can provide a benchmark for evaluating the program's impact on eliminating racial and ethnic disparities in health and inform service delivery enhancements.

Adolescent↗

PubMed2005

Disparities and discrimination in health care: an introduction.

Racial disparities in health care and health outcomes are a disturbing feature of the American health care system. Efforts to reduce or ameliorate these disparities must be informed by an understanding of the factors that underlie and contribute to them. The papers in this issue are based on a recent conference that was held at the University of Chicago to address this problem. Socioeconomic status is an important determinant of health, and socioeconomic disparities are major determinants of the racial disparities in health. These socioeconomic disparities are complicated by access to health insurance, geographic factors, and unhealthy behaviors. Geographic disparities, both regional and local, also contribute to racial disparities in health. Moreover, current disparities in the health of adult populations may reflect socioeconomic disparities that prevailed during their intrauterine or early infant development. There seems little evidence that either overt or unconscious discrimination on the part of physicians is an important cause of racial disparities in health; blaming physicians for this problem is counterproductive. Improving the quality of medical care holds the promise not only of improving health for all Americans, but of decreasing the racial disparities in health care that are so troubling today.

Health Behavior↗

PubMed1993

Stereoscopic cooperation between the fovea of one eye and the periphery of the other eye at large disparities. Implications for anomalous retinal correspondence in strabismus.

In normal human observers we searched for the largest amount of visual disparity that can still provide depth information; we compared (1) crossed and uncrossed disparities and (2) symmetrical and asymmetrical locations of disparate stimuli. A pair of 3 degrees discs projected for 100 ms served as targets. Symmetrical stimuli were projected on temporal or nasal retinal loci in both eyes; asymmetrical stimuli were projected on the fovea of one eye and on the nasal or temporal periphery of the other eye. Thresholds were determined using a two-alternative forced choice procedure. Subjects had to distinguish binocular disparate images from monocular double images of identical angular separation. Among six subjects, crossed disparities were recognized by one up to 6 degrees, by three up to 9 degrees, by one up to 18 degrees, and by one up to 21 degrees. Uncrossed disparities were recognized by two at 3 degrees, by two up to 6 degrees and by two up to 9 degrees. Hence, crossed disparities could be recognized up to higher angles than uncrossed. No consistent difference was found between symmetrical and asymmetrical stimuli. Stimuli with crossed disparity appeared smaller and with uncrossed disparity larger than monocular stimuli of the same objective size, suggesting that the size-constancy mechanism operates when disparity stimuli are presented as briefly as 100 ms, i.e., without simultaneous vergence eye movements. We speculate that the far-reaching interocular connections demonstrated in normal subjects might also be utilized in the case of strabismus: these interocular connections could form the basis for anomalous retinal correspondence.

Adult↗

PubMed2001

Strength of depth effects induced by three types of vertical disparity.

The goal of the present study is to compare the strengths of depth effects induced by different types of vertical disparity. We use a nulling task, in which the depth effects induced by vertical disparity are nulled by horizontal disparity. The advantage of this method is that it prevents cue conflicts from arising between disparity and other depth cues. The ratios between horizontal and vertical disparity that evoke the percept of a fronto-parallel stimulus vary per type of vertical disparity. The ratios determined for vertical scale and vertical quadratic mix (vertical scale with a horizontal gradient) vary strongly across subjects. The ratios for vertical shear are constant, since all subjects needed the same amount of horizontal and vertical shear to perceive a fronto-parallel plane. In these experiments, one conflict cannot be avoided, namely the conflict between vertical disparity and oculomotor signals. This conflict may cause differential weighting of vertical disparity and oculomotor signals, which could explain the individual differences. The different ratios for different types of vertical disparity suggest that weighting is specific for each type of vertical disparity and the associated oculomotor signal.

Adolescent↗