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PubMed2002

[A clinical observation on variation of crossed disparity and uncrossed disparity in intermittent exotropia].

OBJECTIVE: To understand the clinical variety of crossed disparity and uncrossed disparity in intermittent exotropia. METHODS: Zero disparity stereo-acuity, crossed disparity and uncrossed disparity stereo-acuity of 55 cases with intermittent exotropia were examined with stereogram designed by Yan Shaoming before surgery. RESULTS: In group 1 consisting of 25 cases, the zero disparity stereo-acuity of normal persons was 48%, the values of crossed disparity and uncrossed disparity stereoacuity were not in the normal range, and the crossed disparity was larger than the uncrossed disparity stereo-acuity (t = 843.5, P < 0.000 1). In group 2 including 26 cases, the zero disparity stereo-acuity of normal person was 30.8%, the value of crossed disparity was abnormal, and the uncrossed disparity was not found. In group 3 consisting of 4 cases, only the abnormal zero disparity stereo-acuity was found. In the 55 cases, there was no coexistence of zero disparity and uncrossed disparity, no existence of only crossed disparity or only uncrossed disparity, and no coexistence of crossed and uncrossed disparity. CONCLUSIONS: (1) In the cases with intermittent exotropia, the zero disparity stereoacuity, the crossed disparity and uncrossed disparity stereoacuities are abnormal. (2) The sequential damage of stereopsis in intermittent exotropia is uncrossed disparity, crossed disparity and zero disparity. (3) It is proved in the clinic that the binocular temporal retinae correspond to the crossed disparity and the binocular nasal retinae correspond to the uncrossed disparity.

Adolescent↗
PubMed2000

Perceiving binocular depth with reference to a common surface.

A common surface is a spatial regularity of our terrestrial environment. For instance, we walk on the common ground surface, lay a variety of objects on the table top, and display our favorite paintings on the wall. It has been proposed that the visual system utilizes this regularity as a reference frame for coding objects' distances. Presumably, by treating the common surface as such--i.e. an anticipated constant--the visual system can reduce its coding redundancy, and divert its resources to representing other information. For intermediate-distance space perception, it has been found that absolute distance judgment is most accurate when a common ground surface is available. Here we explored if the common surface also serves as the reference frame for the processing of binocular-disparity information, which is a predominant cue for near-distance space perception. We capitalized on an established observation where the perceived slant of a surface with linear binocular-disparity gradient is underestimated. Clearly, if the visual system utilizes this incorrectly represented slant surface as a reference frame for coding the objects' locations, the perceived depth separation between the objects will be adversely affected. Our results confirm this, by showing that the depth judgment of objects (two laterally separated vertical lines) on, or in the vicinity of, the surface is underestimated. Furthermore, we show that the impact of the common surface on perceived depth separation most likely occurs at the surface-representation level where the visual surface has been explicitly delineated, rather than at the earlier disparity-processing level.

Cues↗
PubMed1989

Viewing distance affects stereoscopic tilt created with spatial frequency disparity.

A controversy exists as to whether stereoscopic tilt created with interocular differences in spatial frequency is based on perception of spatial frequency disparity or positional disparity. To determine which hypothesis is correct, we investigated the influence of viewing distance on perceived tilt. Tilt was induced by having observers view, at three viewing distances, dichoptic spatial frequency grating patterns differing in frequency by 25%. By appropriate physical scaling of the size of the patterns, their spatial frequency and angular width remained unchanged as distance varied. Under such conditions, the spatial frequency disparity hypothesis predicts no effect of distance, whereas the positional disparity hypothesis predicts a significant effect of distance (due to stereoscopic depth constancy) on the magnitude of tilt. The results showed that perceived tilt does covary with distance, a result consistent with only the positional disparity hypothesis.

Adolescent↗
PubMed1993

Vertical fixation disparity in learning disabled.

In this correlation study of visual parameters as related to learning disabilities, we considered several deficits: uncorrected refractive error, accommodative infacility, inaccurate pursuits, and vertical fixation disparity. Because vision is the primary sensory input involved in reading, it is nothing new to find a correlation between visual deficits and learning problems; however, vertical fixation disparity, unique in its relation to other sensory systems, deserves a great deal of attention. The high incidence of vertical fixation disparity measured in 5th and 6th graders with learning disabilities suggests that there is a plausible causal relation. Vertical oculomotor imbalance, vestibular problems, and learning disability are discussed.

Accommodation, Ocular↗
PubMed2000

Motion in depth based on inter-ocular velocity differences.

Two different binocular cues are known for detecting motion in depth. One is disparity change in time and the other is inter-ocular velocity difference. In contrast to the well known fact of the use of the disparity cues, no evidence of contribution of inter-ocular velocity differences for detecting motion in depth has been reported. We demonstrate that motion in depth can be seen based solely on inter-ocular velocity differences using binocularly uncorrelated random-dot kinematograms. This indicates that the visual system uses monocular velocity signals for processing motion in depth in addition to disparity change in time.

Depth Perception↗
PubMed2002

An unexpected specialization for horizontal disparity in primate primary visual cortex.

The horizontal separation of the eyes means that objects nearer or farther than the fixation point project to different locations on the two retinae, differing principally in their horizontal coordinates (horizontal binocular disparity). Disparity-selective neurons have generally been studied with disparities applied in only one direction (often horizontal), which cannot determine whether the encoding is specialized for processing disparities along the horizontal axis. It is therefore unclear if disparity selectivity represents a specialization for naturally occurring disparities. I used random dot stereograms to study disparity-selective neurons from the primary visual cortex (V1) of awake fixating monkeys. Many combinations of vertical and horizontal disparity were used, characterizing the surface of responses as a function of two-dimensional disparity. Here I report that the response surface usually showed elongation along the horizontal disparity axis, despite the isotropic stimulus. Thus these neurons modulated their firing rate over a wider range of horizontal disparity than vertical disparity. This demonstrates that disparity-selective cells are specialized for processing horizontal disparity, and that existing models of disparity selectivity require substantial revision.

Action Potentials↗
PubMed2004

Sensitivity to depth relief on slanted surfaces.

The finest stereoacuity is known to depend on the disparity of a target relative to other visible points. Here we show that a more important factor in determining sensitivity to displacement can be the disparity of a target relative to an invisible interpolation plane through other neighboring points. We tested the sensitivity of observers to displacements of the central column of a regular grid of dots that was either fronto-parallel or slanted about a vertical axis. We found that subjects' sensitivity to displacement was better predicted by a model based on the disparity of a target with respect to the grid plane than it was by a model based on disparity with respect to other reference points. In control conditions carried out on one subject, we found that this result did not depend on adaptation to the grid slant because it also occurred when the direction of grid slant varied from trial to trial. Nor did it depend on the perception of slant, because the data were similar for trials on which the grid was perceived as approximately fronto-parallel or markedly slanted. Our results indicate that sensitivity to the depth component of the target displacement is based on disparity relative to a local reference plane.

Cues↗
PubMed1996

Spatio-temporal requirements for binocular correlation in stereopsis.

We measured sensitivity to binocular correlation in dynamic random-dot stereograms that defined moving sinusoidal gratings-in-depth. At a range of spatial frequencies and drift rates we established sensitivity by adding Gaussian distributed disparity noise to the modulation of disparity that defined a cyclopean grating, and finding the noise amplitude that rendered the grating just detectable. This permitted correlation thresholds to be measured at a range of suprathreshold disparity amplitudes. Spatial requirements for binocular correlation depend little on temporal frequency, and vice versa. This suggests that binocular correlation mechanisms can be characterized by independent spatial and temporal sensitivity functions. The temporal frequency function has a low pass characteristic. Sensitivity declines above about 1 c/sec, reaching its limit at 4-8 c/sec. The spatial characteristic depends greatly on the amplitude of disparity modulation, changing from band pass at low amplitude to low pass at high amplitude. The maximum resolvable spatial frequency is 4-6 c/deg, but declines sharply for relatively high amplitudes. The interaction between amplitude and spatial frequency cannot be explained by fixed high or low limits on detectable disparity gradients.

Depth Perception↗
PubMed2001

Fixation disparity and accommodation for stimuli closer and more distant than oculomotor tonic positions.

Both the vergence and the accommodative system have individual tonic positions (also referred to as dark vergence and dark focus, respectively) where the static response may be expected to be most accurate. This was confirmed by measuring fixation disparity with nonius lines and accommodation with an autorefractometer for foveal stimuli at viewing distances of 460, 100, 60, 40, and 30 cm. Multiple regression analysis was used at each viewing distance to predict fixation disparity from dark vergence, dark focus, accommodative gain and accommodative convergence: these accommodative measures had little effect on the inter-individual variability of near fixation disparity nor on the linear slope of fixation disparity as a function viewing distance.

Accommodation, Ocular↗
PubMed1999

Depth aliasing by the transient-stereopsis system.

A fundamental problem in stereo-processing is determining which images in the two eyes correspond to the same object. This problem is particularly pronounced with periodic stimuli where it is theoretically possible to binocularly match a given feature in one eye with any of the identical features in the other eye. One way to minimise the likelihood of the occurrence of such aliasing is to restrict the upper-disparity limit that a particular binocular cell can process to one-half of the spatial period to which the cell is sensitive. While such a restriction would not be a major problem for the sustained stereo-system (which processes small disparities) it would be for the transient system (which is capable of processing disparities as large as 10 degrees). Large-field sinewave variations in luminance were used to compare the propensity of the sustained and transient systems to exhibit depth aliasing--that is to signal a depth sign that corresponds to a binocular match that is greater than the nearest-neighbour pairing. Results were that: depth aliasing was exhibited at short, but not at long durations; decreasing the disparity of the stimulus reduced the likelihood of depth-aliasing; and the critical disparity for this reduction in depth aliasing was dependent upon the spatial frequency of the stimulus, i.e. it was phase, not absolute disparity dependent. Based upon these results, we conclude that while the sustained system implements the half-cycle disparity-processing limit, the transient system does not.

Depth Perception↗
PubMed2002

Bi-stability in perceived slant when binocular disparity and monocular perspective specify different slants.

We examined how much depth we perceive when viewing a depiction of a slanted plane in which binocular disparity and monocular perspective provide different slant information. We exposed observers to a grid stimulus in which the monocular--and binocular-specified grid orientations were varied independently across stimulus presentations. The grids were slanted about the vertical axis and observers estimated the slant relative to the frontal plane. We were particularly interested in the metrical aspects of perceived slant for a broad spectrum of possible combinations of disparity--and perspective-specified slants. We found that observers perceived only one grid orientation when the two specified orientations were similar. More interestingly, when the monocular--and binocular-specified orientations were rather different, observers experienced perceptual bi-stability (they were able to select either a perspective--or a disparity-dominated percept).

Depth Perception↗
PubMed2006

Measurements of objective and subjective fixation disparity with and without a central fusion stimulus.

BACKGROUND: Fixation disparity is the condition in which the images of a binocularly fixated object are not imaged on exactly corresponding retinal points, but are still within Panum's fusional areas. Measurements of objective and subjective fixation disparity have indicated that subjective fixation disparity does not indicate the true eye position. However, the effect of the foveal fusional lock (FFL) on the relationship between objective and subjective fixation disparity is still not clear. MATERIAL/METHODS: This relationship was investigated in five subjects. The objective measurements were made using scleral search coils, and at the same time the Sheedy distance disparometer indicated the subjective fixation disparity. Measurements were obtained both with and without an FFL. RESULTS: It was found that without an FFL there was a significant difference (p<0.05) between subjective and objective fixation disparity, whereas when an FFL was added, the difference was no longer significant. Additionally, the absolute sizes of both SFD and OFD were found to be significantly smaller with the FFL. CONCLUSIONS: These results indicate that the presence of an FFL makes subjective fixation disparity a more accurate indicator of the objective eye position. An instrument with an FFL should therefore be used clinically. The difference between objective and subjective fixation disparity presumably indicates a change in correspondence.

Adult↗