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PubMed1991

Optimum technique of light brightness assessment in control subjects and patients with ocular hypertension and glaucoma.

The light brightness test is an investigation in which crossed polarizing lenses placed in front of each eye are rotated with respect to one another to ellicit any disparity in perception of a diffusely illuminated screen. A number of different strategies can be employed to achieve the aim of determining the degree of disparity which is perceived as equal by the patient. A cohort of 45 control subjects was assessed in order to determine the ranges of brightness disparity accepted as equal for six methods of assessment. The method which gave the optimal confidence limit was to preset one pair of lenses at 0 degrees (maximum transmission) and the other as a fixed reference at 45 degrees. Rotation of the lens set at 0 degrees was carried out until the brightness of an X-ray box is perceived as equal. The strategy was then reversed. A disparity in one or both recordings greater than 76.5% is outside 99% confidence limits for the whole population studied. One of 14 patients with ocular hypertension gave a consistently 'positive' results. For the optimal method, over 60% of patients with glaucoma fell outside 95% condidence limits for normal controls indicating that a disparity in brightness perception between eyes is a common feature in glaucoma, in which the disease process usually affects one eye more than the other. For the patients with chronic glaucoma there was a positive correlation between the difference in brightness perception and the difference in central visual field score, indicating that brightness perception may be subserved by the central 30 degrees of the retina. When combined with visual acuity assessment this easily performed test warrants evaluation as a potential screening test for chronic glaucoma.

Chronic Disease↗
PubMed1989

Occlusion and the solution to the aperture problem for motion.

The "aperture problem" indicates that a local reading of the velocity of an oriented contour is inherently ambiguous, insufficient by itself to recover the velocity of image points. In Wallach's "barber pole" display consisting of moving diagonal lines within an elongated rectangular aperture, it has been suggested that the unambiguous motion of edge-terminators along the longer edges of the aperture propagates towards the motion-ambiguous center part of drifting stripes. This results in the perception of a surface moving in the direction of the longer axis of the aperture. By manipulating the stereoscopic disparity of a striped pattern relative to the aperture plane, we found that the disambiguating effects of terminators could be abolished if the striped pattern was in uncrossed disparity relative to the aperture plane. Also, the motion in 3 separate horizontally oriented, and vertically aligned apertures which would otherwise be seen as moving horizontally, was seen as "linked" together and moving vertically. This occurred only when the horizontally oriented segments separating these apertures were stereoscopically coded so that they appeared as occluders in front. These findings suggest that accidental or "extrinsic" terminators created by occluding edges are treated differently from real or "intrinsic" terminators, and that the real-world constraint of occlusion is thus implemented in the ambiguity-solving processes for motion.

Depth Perception↗
PubMed2004

A laminar cortical model for 3D perception of slanted and curved surfaces and of 2D images: development, attention, and bistability.

A model of laminar visual cortical dynamics proposes how 3D boundary and surface representations arise from viewing slanted and curved 3D objects and 2D images. The 3D boundary representations emerge from non-classical receptive field interactions within intracortical and intercortical feedback circuits. Such non-classical interactions within cortical areas V1 and V2 contextually disambiguate classical receptive field responses to ambiguous visual cues using cells that are sensitive to colinear contours, angles, and disparity gradients. Remarkably, these cell types can all be explained as variants of a unified perceptual grouping circuit whose most familiar example is a 2D colinear bipole cell. Model simulations show how this circuit can develop cell selectivity to colinear contours and angles, how slanted surfaces can activate 3D boundary representations that are sensitive to angles and disparity gradients, how 3D filling-in occurs across slanted surfaces, how a 2D Necker cube image can be represented in 3D, and how bistable 3D Necker cube percepts occur. The model also explains data about slant aftereffects and 3D neon color spreading. It shows how chemical transmitters that habituate, or depress, in an activity-dependent way can help to control development and also to trigger bistable 3D percepts and slant aftereffects. Attention can influence which of these percepts is perceived by propagating selectively along object boundaries.

Attention↗
PubMed2005

Focus cues affect perceived depth.

Depth information from focus cues--accommodation and the gradient of retinal blur--is typically incorrect in three-dimensional (3-D) displays because the light comes from a planar display surface. If the visual system incorporates information from focus cues into its calculation of 3-D scene parameters, this could cause distortions in perceived depth even when the 2-D retinal images are geometrically correct. In Experiment 1 we measured the direct contribution of focus cues to perceived slant by varying independently the physical slant of the display surface and the slant of a simulated surface specified by binocular disparity (binocular viewing) or perspective/texture (monocular viewing). In the binocular condition, slant estimates were unaffected by display slant. In the monocular condition, display slant had a systematic effect on slant estimates. Estimates were consistent with a weighted average of slant from focus cues and slant from disparity/texture, where the cue weights are determined by the reliability of each cue. In Experiment 2, we examined whether focus cues also have an indirect effect on perceived slant via the distance estimate used in disparity scaling. We varied independently the simulated distance and the focal distance to a disparity-defined 3-D stimulus. Perceived slant was systematically affected by changes in focal distance. Accordingly, depth constancy (with respect to simulated distance) was significantly reduced when focal distance was held constant compared to when it varied appropriately with the simulated distance to the stimulus. The results of both experiments show that focus cues can contribute to estimates of 3-D scene parameters. Inappropriate focus cues in typical 3-D displays may therefore contribute to distortions in perceived space.

Accommodation, Ocular↗
PubMed2001

Spatial disparity sensitivity in area PMLS of the Siamese cat.

Previous studies of the visual system of Siamese cats have shown that binocular cells are scarce in areas 17, 18 and 19, yet significantly more abundant in suprasylvian areas such as the postero-medial lateral suprasylvian area (PMLS). The present study aims at evaluating the sensitivity to spatial disparity of PMLS binocular cells in paralyzed and anesthetized Siamese cats. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. Delays were introduced in the arrival of the luminous bars in the receptive fields so as to create the desired spatial disparities. Results indicate that approximately a third of PMLS units are binocular and that these binocular cells can detect spatial disparity cues. Indeed, although the sample was relatively small, cells of the tuned excitatory (14/34), tuned inhibitory (2/34), near (6/34) and far (1/34) types were identified. The spatial selectivity, as measured by the width at half height of the tuning curves of the excitatory and inhibitory cells and the slopes of the near and far cells, was similar to that obtained in PMLS of normal cats but not as precise as that found for primary visual areas in these animals. This suggests that these cells might serve as a substrate for coarse stereopsis.

Action Potentials↗
PubMed1994

Seeing depth in colour: more than just what meets the eyes.

Novel binocular depth illusions obtained from two-dimensional colour images are presented. It is demonstrated that the magnitude of these illusions is based on transverse chromatic aberration (TCA), however, the depth obtained cannot be observed unless specific conditions are met even if the TCA is present. Some form of perceptual organization occurring at and/or beyond the binocular fusion site of the brain, is required for some of these effects to occur. An example of a paradoxical finding leading to this conclusion is the observation that under some conditions the same colour can be perceived on separate depth planes while spatially adjacent colours from opposing ends of the visible spectrum (i.e. red and blue or green) can be perceived on the same depth plane simultaneously within the same image. Further, results show that some form of reference plane is required by the brain to use the colour induced disparity, without which, depth cannot be perceived even if the disparity information is present. This phenomenon is spatially tuned for medium to high frequency components and is still detectable under isoluminant conditions which would support the notion that it requires information from the parvocellular pathway. Binocular lustre and rivaldepth are ruled out as being significant factors in the effect. It is argued that this phenomenon represents an instance of global interactive processes induced by TCA while previous studies on chromostereopsis have concentrated on local aspects. Results of the present study may explain why under certain situations depth can be perceived in coloured images and not under other circumstances where TCA is still present.

Color Perception↗
PubMed1995

Binocular perception of slant about oblique axes relative to a visual frame of reference.

From the literature it is known that the processing of disparity for slant is different in the presence and in the absence of a visual frame of reference. The experimental finding that vertical disparity is not processed for slant perception in the presence of a visual reference is elaborated. This theoretical analysis results in a reduction of the three basic first-order transformations between the retinal half images (divergence, rotation, and deformation) to only two basic orthogonal transformations. The first of these, horizontal scale, results in slant perception about the vertical axis, whereas the second, horizontal shear, results in slant perception about the horizontal axis. These transformations are based primarily on horizontal disparity. It is shown experimentally that in the presence of a frame of reference the amount of vertical transformation that is added to the two basic transformations (horizontal scale and shear) of a random-dot stimulus is indeed irrelevant for slant perception. It is suggested that, in the presence of a visual reference, slant perception about oblique axes is based solely on linear combinations of the horizontal-scale and horizontal-shear transformations. Subjects are able to reproduce slants about oblique axes experimentally merely by combining horizontal scale and shear.

Adult↗
PubMedDate not supplied

Binocular depth perception following early experience with interocular torsional disparity.

The relationship between the behavioral and physiological consequences of rearing with optically induced cyclotropia was assessed. Beginning at the age of 4 weeks, kittens wore goggles that rotated the visual field in opposite directions in each eye for several hours each day over a period of several weeks. The amounts of interocular rotation were 0 deg (control), 16 deg, and 32 deg. Subsequently, they were tested to determine their monocular and binocular depth thresholds and, in some cases, visual acuity. In several kittens recordings were also made from the visual cortex. Binocular performance of all kittens in the 0-deg condition and three out of six kittens in the 16-deg condition was comparable to, although slightly lower than, that of normally reared kittens. In contrast, none of the 32-deg kittens showed any evidence of the binocular superiority that would suggest the presence of stereopsis. Extracellular unit recordings from the visual cortex confirmed our earlier results with goggle-reared kittens. In 16-deg kittens, the distribution of the cells' preferred interocular disparities (IOD) in receptive-field orientation showed a compensating shift so that the mean matched the experienced rotational disparity. In the 32-deg kittens, binocularity was greatly disrupted and there was no compensatory shift in the IOD distribution. Two 32-deg kittens were afforded 3 years of subsequent normal visual experience. Both the behavioral and the physiological findings were unaffected by normal visual exposure in adulthood. Control measurements of acuity indicated that any deficits in depth perception were not due to reduced spatial-resolution abilities. The data indicate that the kitten visual system is able to maintain functional binocularity sufficient to subserve a moderate level of stereoacuity with interocular rotations of up to at least 16 deg.

Animals↗
PubMed1991

Mechanism of anomalous retinal correspondence: maintenance of binocularity with alteration of receptive-field position in the lateral suprasylvian (LS) visual area of strabismic cats.

We have examined the effects of rearing kittens with a unilateral convergent strabismus, induced surgically at 3 weeks of age, on the binocularity (ocular dominance) and receptive-field position of neurons in the motion-sensitive lateral suprasylvian (LS) area of cat extrastriate cortex. Data were compared to those obtained from area 17 in the same animals, and from the two areas of cortex in normal adult cats. Interocular alignment of the operated cats was assessed in alert adults using corneal reflex photography and during recording from the positions of retinal landmarks under paralysis. The strabismus magnitude in each operated cat was calculated by comparison with equivalent data from the normal animals. Strabismus always caused a major loss of binocularity in area 17. The remaining binocular neurons had receptive-field (RF) pairs arising from positions of normal correspondence in the two retinae and would thus have been responsive to different regions of visual space through the misaligned eyes in the alert animal. In area LS, the effects were dependent on the strabismus magnitude. In the group of four cats with pronounced strabismus (18-30 deg crossed), a loss of binocularity occurred in area LS equivalent in severity to that in area 17. The majority of the remaining binocular LS neurons possessed RF pairs in normal retinal correspondence and would thus, in the alert animal, have received spatially disparate visual input through the two eyes. This also occurred in three other cats with more moderate strabismus (11-15 deg crossed), although only a small breakdown in the binocularity of area LS was apparent. The group of cats with mild strabismus (less than or equal to 10 deg crossed) had normal proportions of binocular neurons in area LS. In three of these cats, the maintenance of binocularity was accompanied by shifts in RF position, with visual inputs arising from anomalous retinal locations. These shifts compensated, in part, for the strabismus angle present in each cat, so that most of the binocular LS neurons would have received inputs from regions of visual correspondence through the misaligned eyes when the animal was alert. Similar mechanisms could afford a basis for the binocular visual compensations that occur in humans with small-angle strabismus of early onset. If so, anomalous retinal correspondence in such individuals would have as a locus areas of extrastriate cortex with a role in motion perception, and would involve alterations to the neural substrate underlying normal binocular vision.

Animals↗
PubMed2005

Visual discrimination of local surface structure: slant, tilt, and curvedness.

In four experiments, observers were required to discriminate interval or ordinal differences in slant, tilt, or curvedness between designated probe points on randomly shaped curved surfaces defined by shading, texture, and binocular disparity. The results reveal that discrimination thresholds for judgments of slant or tilt typically range between 4 degrees and 10 degrees; that judgments of one component are unaffected by simultaneous variations in the other; and that the individual thresholds for either the slant or tilt components of orientation are approximately equal to those obtained for judgments of the total orientation difference between two probed regions. Performance was much worse, however, for judgments of curvedness, and these judgments were significantly impaired when there were simultaneous variations in the shape index parameter of curvature.

Depth Perception↗
PubMed1994

Interocularly unpaired zones escape local binocular matching.

When a closer surface partially occludes a more distant surface, there exist image zones adjacent to the occluding edge on the rear surface which are visible to one eye and not the other. These half-occluded or interocularly unpaired zones do not carry explicit disparity information, yet their depth is perceived as a stable and continuous extension of the rear surface. Moreover, such zones escape binocular rivalry. In addition to these properties, we now report another special characteristic of this unpaired zone in comparison to normally paired regions. An unpaired probe dot added here escapes non-unique local Panum matching which would otherwise bestow it with a depth outside the surface. Thus paradoxically, depth of the probe is most stable in the unpaired zone. This finding indicates that what is considered to be one of the most fundamental processes for binocular depth perception, namely local matching, is subject to more global surface occlusion constraints.

Depth Perception↗
PubMed1996

Binocular correlation detection with oriented dynamic random-line stereograms.

Stereopsis relies principally on the extraction of horizontal retinal disparities. As such, we assume that the vertical contours (i.e., horizontal contrast energy) are of principle import for stereopsis. Yet there are theoretical reasons for believing that horizontal contours should be involved in binocular matching (if not stereopsis proper) as well. First, they would facilitate the computation of vertical disparities, which are necessary for the control of disjunctive eye movements and perhaps the computation of absolute depth. Second, the process of binocular matching is a two-dimensional one; its solution requires information along both principle orientations. In this study, we have measured the efficacy with which horizontal or vertical contours can be binocularly matched by measuring thresholds for the detection of interocular correlation for oriented dynamic random-line stereograms. We find that the slopes of the psychometric functions are almost a factor of two steeper when matching vertical contours, indicating a narrower noise distribution along the decision axis associated with these stimuli.

Fixation, Ocular↗
PubMed1995

The 'uniqueness constraint' and binocular masking.

In stereo-matching algorithms, the 'uniqueness constraint' requires that a feature in one stereo half-image be matched to, at most, one similar feature in the other half-image. Experiments are reported in which binocular contrast thresholds and depth-discrimination judgments have been used to determine whether the human stereo system makes unique matches. A single high-spatial-frequency target in the left eye was paired stereoscopically with two identical targets, presented near retinal correspondence (+/- 3.5 min of disparity), in the right eye. Contrast-increment thresholds were measured for each of the targets in the right eye, and it was found that the target in the left eye masked both. Indeed, the amount of binocular masking for each member of the double target nearly equaled the masking observed when only a single target was presented to the right eye. Depth judgments confirmed that the target in the left eye had been matched to both targets in the right eye. It is concluded that uniqueness is not an absolute constraint on human stereo matching.

Algorithms↗
PubMed2003

Spatio-temporal dynamics of depth propagation on uniform region.

The depth of each point on a binocularly presented untextured horizontal bar is physically ambiguous except for the two vertical edges at both ends, since the correspondence between left and right images is not unique on such a uniform region. These depths, however, are unambiguously perceived, and this suggests the existence of some mechanism that interpolates the depth information from the two ends toward the center. Temporal properties of this integration process were examined by a phase-matching task, which allowed us to measure the phase of the perceived depth at the center of a horizontal bar when disparities at the ends were sinusoidally oscillated. We found that the perceived depth at the center of the bar was slightly temporally delayed for 7-60 ms relative to the physical depth at the ends. The difference increased with the length of the bar, decreased as the vertical position of the bar became farther from the fixation point, and increased in the presence of occluders. This finding indicates that depth information is propagated over an object to solve this ambiguity by using a time-consuming process. Accordingly, we suggest that depth propagation is accomplished by spatially local diffusion-like interactions of locally represented depth information.

Analysis of Variance↗
PubMed1998

dmax for stereopsis and motion in random dot displays.

The upper displacement limit for motion was compared with the upper disparity limit for stereopsis using two-frame random dot kinematograms or briefly presented stereograms. dmax (the disparity/displacement at which subjects make 20% errors in a forced-choice paradigm) was found to be very similar for motion and stereo at all dot densities, and to fall with increasing dot density (0.006% or two dots to 50%) according to a power law (exponent -0.2). If dmax is limited by the spacing of false targets, this pattern of results suggests that the spatial primitives in the input to the correspondence process may be derived from multiple spatial scales. A model using MIRAGE centroids provides a good fit to the data.

Depth Perception↗
PubMed1989

Comparison of two fixation disparity determinantions.

Fixation disparity values derived from horopter measurements are compared with those determined using the disparometer on seven subjects wearing lateral prisms. Although the two measures are intrinsically different, five subjects showed similarity in the form and position of the two fixation disparity-forced vergence functions. Two subjects manifested fixation disparities that indicated the use of proximal or voluntary control of relative eye position for both base-out and base-in forms of lateral prism. Another subject showed a paradoxical response to base-out prism that could be attributed to recovery from previous application of base-in prism, adaptation to base-out prism, or both. An additional subject showed irregular alterations in vergence posture during nonius horopter measurements. The clinical implications of vergence responses other than those associated with fusion and accommodation are discussed and the effect of this form or eye movement control is related to the change in position and shape of the equidistance horopter when it is measured through lateral prism.

Fixation, Ocular↗
PubMed1997

Relative orientation of primary positions of the two eyes.

We evaluated the relative orientations of the displacement planes of the two eyes under various conditions: fixations of nearby targets, of far targets and targets presented dichoptically at optical infinity. We show that disparity driven vergence is not always required to rotate the primary positions. We find that eye orientation during fixation of far targets is idiosyncratic. We found a bimodal distribution ranging from null to about 30 deg of the relative exorotations of the two primary positions. By contrast, the difference of primary positions' orientation of the two eyes was, for targets at optical infinity, stable and similar across subjects. However, the displacement planes of the two eyes did not coincide, but were exorotated by 4.3 deg on average, even though horizontal vergence was close to zero. We discuss our results with reference to current models of binocular three-dimensional control.

Algorithms↗
PubMed2005

Different strategies for using motion-in-depth information in catching.

Previous studies on ball catching have had the limitation that the catcher was restricted to lateral hand movements. The authors investigated catching behavior in the more natural situation in which hand movements were unconstrained. Movements of the hand were tracked as participants tried to "catch" an approaching ball simulated with changing size and/or changing disparity. Participants used 1 of 2 distinct interception strategies: (a) a "cutting-off" strategy where time to passage (TTP) information was used to guide movements of the hand in depth such that the ball was caught farther in front of the face when the ball was approaching more slowly and (b) a "waiting" strategy where the hand was moved along a frontoparallel plane that was constant across ball trajectories and speeds. Cue dissociation and selective adaptation manipulations demonstrated that the catcher's estimates of TTP and crossing distance were based on a combination of binocular and monocular information.

Biomechanical Phenomena↗