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At least 235 records · Page 13Linked to original sources

PubMed1999

An orientation anisotropy in the effects of scaling vertical disparities.

Gårding et al. (Vis Res 1995;35:703-722) proposed a two-stage theory of stereopsis. The first uses horizontal disparities for relief computations after they have been subjected to a process called disparity correction that utilises vertical disparities. The second stage, termed disparity normalisation, is concerned with computing metric representations from the output of stage one. It uses vertical disparities to a much lesser extent, if at all, for small field stimuli. We report two psychophysical experiments that tested whether human vision implements this two-stage theory. They tested the prediction that scaling vertical disparities to simulate different viewing distances to the fixation point should affect the perceived amplitudes of vertically but not horizontally oriented ridges. The first used elliptical half-cylinders and the 'apparently circular cylinder' judgement task of Johnston (Vis Res 1991;31:1351-1360). The second experiment used parabolic ridges and the amplitude judgement task of Buckley and Frisby (Vis Res 1993;33:919-934). Both studies broadly confirmed the anisotropy prediction by finding that large scalings of vertical disparities simulating near distances had a strong effect on the perceived amplitudes of the vertically oriented stimuli but little effect on the horizontal ones. When distances > 25 cm were simulated there were no significant differential effects and various methodological reasons are offered for this departure from expectations.

Adult↗

PubMed1999

Sensitivity to horizontal and vertical corrugations defined by binocular disparity.

Sensitivity to corrugations defined by binocular disparity differs as a function of the modulation frequency. Such functions have proved to be useful descriptive and analytical tools in the study of the mechanisms involved in disparity processing. Indeed, given certain assumptions, these sensitivity functions can be used to predict certain perceptual outcomes. Given their importance, it is surprising that there is no comprehensive data set of disparity sensitivity functions (DSF) for a range of observers over a broad range of spatial frequencies and orientations. Here we report DSFs for six observers over an eight octave range of sinusoidal corrugations in disparity (0.0125-3.2 cpd). Multi-cycle, low frequency surfaces were used to assess the degree to which the fall-off in sensitivity at low corrugation frequencies is attributable to the decreasing number of cycles displayed. The data was found to form a continuous function despite the different number of cycles displayed. We conclude that the fall off in sensitivity is due to the spatial interactions in disparity processing. We also determined DSFs for the same observers to both vertically and horizontally oriented sinusoidal disparity corrugations in order to characterise the extent of the stereoscopic anisotropy. In general, the best thresholds for detecting vertically oriented disparity corrugations were higher (approximately 4 arc sec) than for horizontally oriented corrugations (approximately 2 arc sec). Moreover, the functions were shifted toward the high spatial frequency end of the spectrum.

Humans↗

PubMedDate not supplied

Disparity-tuned channels of the human visual system.

Traditionally, it has been thought that the processing of binocular disparity for the perception of stereoscopic depth is accomplished via three types of disparity-selective channels--"near," "far," and "tuned." More recent evidence challenges this notion. We have derived disparity-tuning functions psychophysically using a subthreshold summation (i.e. low-level masking) technique. We measured correlation-detection thresholds for dynamic random-element stereograms containing either one or two surfaces in depth. The resulting disparity-tuning functions show an opponent-type profile, indicating the presence of inhibition between disparity-tuned units in the visual system. Moreover, there is clear inhibition between disparities of the same sign, obviating a strict adherence to near-far opponency. These results compare favorably with tuning functions derived psychophysically using an adaptation technique, and with the tuning profiles from published single-unit recordings. Our results suggests a continuum of overlapping disparity-tuned channels, which is consistent with recent physiological evidence as well as models based on other psychophysical data.

Depth Perception↗

PubMedDate not supplied

Reliable disparity estimation through selective integration.

A network model of disparity estimation was developed based on disparity-selective neurons, such as those found in the early stages of processing in the visual cortex. The model accurately estimated multiple disparities in regions, which may be caused by transparency or occlusion. The selective integration of reliable local estimates enabled the network to generate accurate disparity estimates on normal and transparent random-dot stereograms. The model was consistent with human psychophysical results on the effects of spatial-frequency filtering on disparity sensitivity. The responses of neurons in macaque area V2 to random-dot stereograms are consistent with the prediction of the model that a subset of neurons responsible for disparity selection should be sensitive to disparity gradients.

Animals↗

PubMed2006

Disparity and shading cues cooperate for surface interpolation.

In two experiments, we tested whether disparity and shading cues cooperated for surface interpolation. Observers adjusted a probe dot to/lie on a surface specified either by a sparse disparity field, a continuous stereo shading or monocular shading gradient, or both cues. Observers' adjustments were very consistent with disparity information but their adjustments were much more variable with shading information. However, observers significantly improved their precision when both cues were present, relative to when only disparity information was present. These results cannot be explained by assuming that separate modules analyze disparity and shading information, even if observers optimally combined these cues. Rather, we attribute this improvement to a process through which the shading gradient constrains the disparity field in regions where disparities cannot be directly measured. This cooperative process may be based on the natural covariation existing between these cues produced by the retinal projection of smooth surfaces.

Adult↗

PubMed1989

The appearance of surfaces specified by motion parallax and binocular disparity.

The experiments reported in this paper were designed to investigate how depth information from binocular disparity and motion parallax cues is integrated in the human visual system. Observers viewed simulated 3-D corrugated surfaces that translated to and fro across their line of sight. The depth of the corrugations was specified by either motion parallax, or binocular disparities, or some combination of the two. The amount of perceived depth in the corrugations was measured using a matching technique. A monocularly viewed surface specified by parallax alone was seen as a rigid, corrugated surface translating along a fronto-parallel path. The perceived depth of the corrugations increased monotonically with the amount of parallax motion, just as if observers were viewing an equivalent real surface that produced the same parallax transformation. With binocular viewing and zero disparities between the images seen by the two eyes, the perceived depth was only about half of that predicted by the monocular cue. In addition, this binocularly viewed surface appeared to rotate about a vertical axis as it translated to and fro. With other combinations of motion parallax and binocular disparity, parallax only affected the perceived depth when the disparity gradients of the corrugations were shallow. The discrepancy between the parallax and disparity signals was typically resolved by an apparent rotation of the surface as it translated to and fro. The results are consistent with the idea that the visual system attempts to minimize the discrepancies between (1) the depth signalled by disparity and that required by a particular interpretation of the parallax transformation and (2) the amount of rotation required by that interpretation and the amount of rotation signalled by other cues in the display.

Attention↗

PubMed1997

Encoding of binocular disparity by complex cells in the cat's visual cortex.

To examine the roles that complex cells play in stereopsis, we have recorded extracellularly from isolated single neurons in the striate cortex of anesthetized paralyzed cats. We measured binocular responses of complex cells using a comprehensive stimulus set that encompasses all possible combinations of positions over the receptive fields for the two eyes. For a given position combination, stimulus contrast could be the same for the two eyes (2 bright or 2 dark bars) or opposite (1 bright and 1 dark). These measurements provide a binocular receptive field (RF) profile that completely characterizes complex cell responses in a joint domain of left and right stimulus positions. Complex cells typically exhibit a strong selectivity for binocular disparity, but are only broadly selective for stimulus position. For most cells, selectivity for disparity is more than twice as narrow as that for position. These characteristics are highly desirable if we assume that a disparity sensor should exhibit position invariance while encoding small changes in stimulus depth. Complex cells have nearly identical binocular RFs for bright and dark stimuli as long as the sign of stimulus contrast is the same for the two eyes. When stimulus contrast is opposite, the binocular RF also is inverted such that excitatory subregions become suppressive. We have developed a disparity energy model that accounts for the behavior of disparity-sensitive complex cells. This is a hierarchical model that incorporates specific constraints on the selection of simple cells from which a complex cell receives input. Experimental data are used to examine quantitatively predictions of the model. Responses of complex cells generally agree well with predictions of the disparity energy model. However, various types of deviations from the predictions also are found, including a highly elongated excitatory region beyond that supported by a single energy mechanism. Complex cells in the visual cortex appear to provide a next level of abstraction in encoding information for stereopsis based on the activity of a group of simple-type subunits. In addition to exhibiting narrow disparity tuning and position invariance, these cells seem to provide a partial solution to the stereo correspondence problem that arises in complex natural scenes. Based on their binocular response properties, these cells provide a substantial reduction in the complexity of the correspondence problem.

Animals↗

PubMed2000

Disparity sensitivity of frontal eye field neurons.

Information about depth is necessary to generate saccades to visual stimuli located in three-dimensional space. To determine whether monkey frontal eye field (FEF) neurons play a role in the visuo-motor processes underlying this behavior, we studied their visual responses to stimuli at different disparities. Disparity sensitivity was tested from 3 degrees of crossed disparity (near) to 3 degrees degrees of uncrossed disparity (far). The responses of about two thirds of FEF visual and visuo-movement neurons were sensitive to disparity and showed a broad tuning in depth for near or far disparities. Early phasic and late tonic visual responses often displayed different disparity sensitivity. These findings provide evidence of depth-related signals in FEF and suggest a role for FEF in the control of disconjugate as well as conjugate eye movements.

Animals↗

PubMed2003

Eye movements facilitate stereo-slant discrimination when horizontal disparity is noisy.

Conditions in which saccadic gaze shifts within planar surfaces facilitate stereo-slant discrimination for slant about the horizontal and vertical axis were investigated. When horizontal disparity noise was added, large gaze shifts in the direction of the slant lowered stereo-slant discrimination thresholds compared to thresholds measured with steady central fixation, whereas eye movements orthogonal to the slant orientation did not lower slant-discrimination thresholds. When no horizontal noise was added, performance was the same with and without gaze shifts. These results suggest that slant is recovered from depth differences between target edges when horizontal disparity signals are variable and that foveal fixation improves the measures of disparity. Eye movements did not lower slant thresholds by providing multiple foveal samples of slant at different target locations that were averaged to reduce disparity noise levels, because eye movements only lowered the thresholds when there was a depth difference between the fixation points. To study which signals for azimuth are used when slant is recovered from the difference in depth between target edges, vertical disparity noise was added and stimulus height was reduced. Both methods elevated slant-discrimination thresholds when horizontal disparity noise was present, suggesting that vertical disparity is used as a cue for azimuth.

Depth Perception↗

PubMed2005

Gain of human torsional optokinetic nystagmus depends on horizontal disparity.

PURPOSE: To investigate the effects of horizontal disparity on torsional optokinetic nystagmus (tOKN) in humans. METHODS: Ten healthy human subjects were selected for this experiment. Monocular eye movements were recorded three-dimensionally using dual-search coil methods. Torsional OKN was induced by a rotating random-dot pattern (22 degrees in diameter, constant angular velocity: +/-54 deg/s) projected on the virtual screen of the optical see-through, head-mounted display (HMD). The optical distance of the HMD's virtual screen was 2 m. A red LED that could be fixated through the virtual screen of the HMD was located near the center of the rotating random-dot pattern. Horizontal disparity was induced by changing the distance between the fixated target and the subject systematically (1, 1.5, 2, or 3 m; five subjects) or by the prism (+1.5, +0.5, 0, or -0.5 prism-diopter [PD] in each eye; five subjects) in front of the HMD. RESULTS: The average gain with zero horizontal disparity (0.022 +/- 0.008/0.025 +/- 0.014, fixated target at 2 m/fixated target with the plain glass) was significantly higher than the gain with crossed disparity (0.017 +/- 0.003/0.019 +/- 0.008, target at 3 m/with the prism of -0.5 PD) and uncrossed disparity (0.017 +/- 0.002, target at 1 m/with the prism of +1.5 PD; one-way ANOVA, P <0.05). CONCLUSIONS: The horizontal disparity of optokinetic stimulus affects tOKN. Nonzero horizontal disparity decreases the gain of tOKN.

Adult↗

PubMed2005

Retinal correspondence of monocular receptive fields in disparity-sensitive complex cells from area V1 in the awake monkey.

PURPOSE: To explore the neural mechanisms underlying disparity sensitivity in complex cells of the macaque visual cortex, the relationship between interocular receptive field (RF) positional shift and disparity sensitivity was studied in area V1. METHODS: Single-unit recordings were made from area V1 of awake Macaca mulatta. Monocular RFs were mapped by means of a reverse cross-correlation technique, and their centers were determined after performing a bidimensional Gaussian function fitting. Interocular RF shifts were calculated for both bright and dark stimuli. Similarly, Gabor adjustments were obtained from disparity profiles to bright and dark dynamic random-dot stereograms (RDSs). RESULTS: Twenty-five complex cells were studied. The response profiles to disparity were similar for bright and dark RDSs. Interocular RF positional shift correlated significantly with both the peaks of Gabor fittings of disparity-sensitivity profiles and the peaks of the Gaussian envelopes of these Gabor fittings. Correlation between interocular RF positional shift and the peaks of the Gaussian envelopes was stronger than correlation between interocular RF positional shift and peaks of Gabor fittings. CONCLUSIONS: Interocular shift of monocular RFs is more related to the center of the range of disparities to which the cell is sensitive, than to the preferred disparity of the cell.

Animals↗

PubMed2006

Linking neural representation to function in stereoscopic depth perception: roles of the middle temporal area in coarse versus fine disparity discrimination.

Neurons selective for binocular disparity form the neural substrate for stereoscopic depth perception and are found in several areas of primate visual cortex. Presumably, multiple representations of disparity exist to serve different functions, but the specific contributions of different visual areas to depth perception remain poorly understood. We examine this issue by comparing the contributions of the middle temporal (MT) area to performance of two depth discrimination tasks: a "coarse" task that involves discrimination between absolute disparities in the presence of noise, and a "fine" task that involves discrimination of very small differences in relative disparity between two stimuli in the absence of noise. In the fine task, we find that electrical microstimulation of MT does not affect perceptual decisions, although many individual MT neurons have sufficient sensitivity to account for behavioral performance. In contrast, microstimulation at the same recording sites does bias depth percepts in the coarse task. We hypothesized that these results may be explained by the fact that MT neurons do not represent relative disparity signals that are thought to be essential for the fine task. This hypothesis was supported by single-unit recordings that show that MT neurons signal absolute, but not relative, disparities in a stimulus configuration similar to that used in the fine task. This work establishes a link between the neural representation of disparity in MT and the functional contributions of this area to depth perception.

Action Potentials↗

PubMed1997

Relationship between binocular disparity and motion parallax in surface detection.

The ability to detect surfaces was studied in a multiple-cue condition in which binocular disparity and motion parallax could specify independent depth configurations. On trials on which binocular disparity and motion parallax were presented together, either binocular disparity or motion parallax could indicate a surface in one of two intervals; in the other interval, both sources indicated a volume of random points. Surface detection when the two sources of information were present and compatible was not better than detection in baseline conditions, in which only one source of information was present. When binocular disparity and motion specified incompatible depths, observers' ability to detect a surface was severely impaired if motion indicated a surface but binocular disparity did not. Performance was not as severely degraded when binocular disparity indicated a surface and motion did not. This dominance of binocular disparity persisted in the presence of foreknowledge about which source of information would be relevant.

Humans↗

PubMed1998

Fixation disparity at different viewing distances and the preferred viewing distance in a laboratory near-vision task.

Previous research has shown that subjects with normal binocular vision differ reliably in the extent to which their fixation disparity changes in the exo direction when the viewing distance is shortened from 100 to 20 cm. Since fixation disparity can lead to asthenopic complaints, the present study investigates whether an exo fixation disparity induced by proximity may cause subjects to move to a longer viewing distance during a near-vision task in order to reduce exo fixation disparity. In two optometric sessions, fixation disparity and accommodation were tested at 60, 40, and 30 cm viewing distance. In a further session, subjects were required to begin a one-hour near-vision task at about 40 cm viewing distance, at which the text characters subtended a comfortable visual angle of 21 min arc. Later, the subjects were free to adopt any viewing distance. In the initial phase of the task, subjects moved back from the screen to a greater or lesser extent that was correlated with the amount of proximal exo fixation disparity: the more a subject's fixation disparity changed to exo when the viewing distance had been shortened from 60 to 30 cm the more he or she moved to longer viewing distances in the course of the near-vision task. Further, the more distant the resting position of vergence (dark vergence), the more visual complaints the subjects indicated after the task relative to before.

Accommodation, Ocular↗

PubMed2004

Genetic research and health disparities.

Alleviating health disparities in the United States is a goal with broad support. Medical research undertaken to achieve this goal typically adopts the well-established perspective that racial discrimination and poverty are the major contributors to unequal health status. However, the suggestion is increasingly made that genetic research also has a significant role to play in alleviating this problem, which likely overstates the importance of genetics as a factor in health disparities. Overemphasis on genetics as a major explanatory factor in health disparities could lead researchers to miss factors that contribute to disparities more substantially and may also reinforce racial stereotyping, which may contribute to disparities in the first place. Arguments that promote genetics research as a way to help alleviate health disparities are augmented by several factors, including research funding initiatives and the distinct demographic patterns of health disparities in the United States.

Genetic Research↗

PubMed2001

High prevalence of infection with Entamoeba dispar, but not E. histolytica, in captive macaques.

A total of 268 nonhuman primates (20 species) kept in the Primate Research Institute, Kyoto University, Japan, were surveyed for intestinal amebas. Total positive rates as based on the presence of cysts in the stool following formalin-ether sedimentation were as follows: Entamoeba histolytica/E. dispar, 53%; E. coli, 34%; E. hartmanni, 34%; Iodamoeba buetschlii, 25%; Endolimax nana, 8%; and E. chattoni, 3%. Positive rates were higher in Old World monkeys and lower in New World monkeys. All the 141 E. histolytica/E. dispar-positive animals were Macaca monkeys. The E. histolytica/E. dispar-positive samples were analyzed by polymerase chain reaction (PCR) for identification of E. histolytica and E. dispar. E. dispar DNA was detected in 137 samples, whereas no E. histolytica DNA was seen. Zymodeme analysis and reactivity to monoclonal antibodies of cultured trophozoites also supported the presence of E. dispar and the absence of E. histolytica. When the sera of 93 macaques were examined by an indirect fluorescent antibody test, only 3 animals proved to be positive for E. histolytica, showing the lowest titer. These results demonstrate that infection with E. dispar, but not E. histolytica, is predominant in macaques.

Animals↗

PubMed2005

Cancer disparities: developing a multidisciplinary research agenda - preface.

In this Special Issue, we present a series of papers concerned with developing a multidisciplinary research agenda to address social disparities in cancer. Motivating this Special Issue are two concerns: (1) despite major advances in knowledge during the 20th century about the extent, determinants, treatment, and prevention of cancer, social disparities in cancer, including within the United States, remain serious, persistent, and require redress; and (2) huge gaps in knowledge exist regarding the causes of and solutions to social disparities in cancer across the full cancer continuum, from prevention to occurrence to diagnosis to treatment and, all too often, to death. Consequently, critical research is critically needed, both to answer the unanswered questions and to ascertain why existing knowledge is not implemented to reduce and eliminate these disparities. To help advance work in this field, we share papers developed for a January 2004 workshop on cancer disparities held at the Dana Farber/Harvard Cancer Center (DF/HCC), in Boston, MA. Topics addressed include: the definition and magnitude of, and programmatic responses to, social disparities in cancer, plus development of a systematic approach to raising research questions to address cancer disparities, using the case examples of breast, cervix, colon, and prostate cancer.

Diagnosis, Differential↗

PubMed1982

Neurons in cat visual cortex tuned to the direction of motion in depth: effect of positional disparity.

We investigated sensitivity to the direction of stimulus motion in depth in neurons of cat visual cortex by using bar stimuli with different image velocities on the two retinae. These stimuli were presented at seven different retinal disparities (i.e. with different locations in depth). Approximately one-fourth of the neurons examined were sensitive to the direction of stimulus motion in depth. In general, the motion-in-depth tuning of these neurons was either unaffected by disparity or changed simply and systematically as a function of disparity, even when disparity was varied over the broad range of 12 degrees. In their relative indifference to disparity, the motion-in-depth neurons contrast with the units that are very selective to disparity and that respond best to sideways motion. Human equivalents to these two classes of units might provide a physiological basis for the distinction between binocularly-driven channels for motion in depth and for disparity (i.e. relative position in depth) that have been proposed on psychophysical grounds.

Animals↗