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PubMed1999

Orientation and luminance polarity tuning of the transient-vergence system.

Previously, Edwards, Pope & Schoor, Vision Research, 38, 705-717, demonstrated that transient disparity vergence appears to be mediated by a system that employs a single low-pass sensitive spatial channel whose performance is not reduced by dichoptic mixed contrasts (no contrast paradox) or dichoptic mixed spatial frequencies. This broadband tuning to both contrast and spatial frequency may be indicative of a second-order or non-linear envelope extraction system. The current study tests for lack of tuning to orientation and luminance polarity which are typically taken as evidence of a second-order system. We found that when the transient vergence system was simultaneously presented with both convergent and divergent disparities, there was a small but distinct bias in favor of responding in the direction defined by matched orientations or luminance polarities over unmatched pairs. Although less frequent, responses to orthogonal carriers or opposite luminance polarities were possible. The vergence system could match a horizontal with a vertical carrier, or a light gaussian with a dark gaussian. The degree of orientation or luminance polarity tuning varied inversely with the disparity magnitude over the range of 2.5-5 degrees, and the orientation tuning peaked at a spatial frequency about 2 cpd. At all disparities tested, however, the tuning was very broad, and other candidate features for mediating transient-vergence need to be investigated.

Contrast Sensitivity↗
PubMed1991

Coarse spatial scales constrain the range of binocular fusion on fine scales.

To examine interactions among spatial scales in disparity processing, we have measured the upper disparity limit for binocular single vision (the diplopia threshold) for high-spatial-frequency test stimuli in the presence of cosine gratings of lower spatial frequency that defined a surface in depth. When the frequency of this grating surface was 2.0 octaves below that of the test, the test fusion range was reduced by a factor of 3-4 relative to the condition in which no grating surface was present. However, gratings 4.0 octaves below the test frequency had no effect, and the test and grating were seen transparently at different depths. Further experiments indicate that the effect is orientation specific and that high-frequency gratings do not affect low-frequency tests. Finally, experiments using grating surfaces tilted in depth indicate that fusion at high spatial frequencies is constrained to a range centered on the local disparity of the surface defined by the lower frequency. These results are important for computational models for stereopsis that are based on coarse-to-fine matching strategies.

Depth Perception↗
PubMed2001

Accommodation, occlusion, and disparity matching are used to guide reaching: a comparison of actual versus virtual environments.

The authors used a virtual environment to investigate visual control of reaching and monocular and binocular perception of egocentric distance, size, and shape. With binocular vision, the results suggested use of disparity matching. This was tested and confirmed in the virtual environment by eliminating other information about contact of hand and target. Elimination of occlusion of hand by target destabilized monocular but not binocular performance. Because the virtual environment entails accommodation of an image beyond reach, the authors predicted overestimation of egocentric distances in the virtual relative to actual environment. This was confirmed. The authors used -2 diopter glasses to reduce the focal distance in the virtual environment. Overestimates were reduced by half. The authors conclude that calibration of perception is required for accurate feedforward reaching and that disparity matching is optimal visual information for calibration.

Accommodation, Ocular↗
PubMed1988

Disparity-induced accommodation in late-onset myopia.

The effect of disparity-vergence on the accommodation response (AR) was measured in 10 late-onset myopes (LOMs) and 10 emmetropic subjects during the course of a 10 minute near-vision task (working distance 33 cm). Disparity was induced by the introduction of 3 and 6 delta base-out prisms placed before the right eye, and the resulting AR for the left eye was measured at 1 min intervals using an objective infra-red optometer. LOMs showed a significant increase in AR with increasing disparity whereas emmetropes showed no significant difference in AR for all disparity conditions. The implications of these findings are discussed with reference to the aetiology of late-onset myopia.

Accommodation, Ocular↗
PubMed1992

Effect of induced fixation disparity on binocular visual acuity.

Fixation disparities were artificially created for distance vision by prisms, and the monocular and binocular visual acuities were measured. The normal approximate 10% improvement in binocular visual acuity compared to monocular visual acuity, deteriorated in proportion to the amount of fixation disparity created by the prisms. This was true in both eso- and exo-disparity, although not to the same extent.

Adult↗
PubMedDate not supplied

A comparison of neural computations underlying stereo vision and sound localization.

Stereo vision and sound localization are behaviors that help organisms to orient in space. Stereo vision provides information about the distance of an object. In sound localization mainly directional signals are analyzed. Two specific cues, binocular disparities underlying stereo vision, and the interaural time difference, suited to represent azimuth in sound localization, have many computational problems in common, although they are generated in two different modalities. The extraction of both cues requires a comparison of signals that arise from two independent sensors, called two-sensor comparison here. This two-sensor comparison is achieved by algorithms similar to summation, half-wave rectification and multiplication. Since the underlying neurons are band-pass filters, the two-sensor comparison results in ambiguities. These are removed in a hierarchical way in several computational steps, involving squaring, inhibition and across-frequency integration.

Animals↗
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↗
PubMed1982

[Psychophysical and vergence responses of normal and stereoanomalous observers to pulse-disparity stimulus].

The relations between psychophysical and oculo-motor responses to retinal disparity were examined in two experiments. Experiment I examined the subject's discriminability of the depth produced by 2 degrees crossed, 2 degrees uncrossed, and zero disparities with the durations of 100 and 1000 ms. As a result, six stereonormal, one crossed stereoanomalous, and one stereoblind observers were identified. In Experiment II, eye movement responses of three subjects with the different types of stereoscopic vision to the disparity pulses were monitored by the photo-electric method. The results of Experiment II indicated that the normal observer showed normal vergence, the crossed stereoanomalous observer showed anomalous convergence, and the stereoblind observer did not show any vergences but saccadic movements. Several theoretical implications of the results are discussed.

Adult↗
PubMedDate not supplied

Augmenting surgery for sensory esotropia with near/distance disparity with a medial rectus posterior fixation suture.

Ten esotropic patients with near/distance disparity and diminished vision in one eye had monocular horizontal strabismus surgery combined with a posterior fixation suture placed on the medial rectus. Before surgery, all patients were at least 20 prism diopters (delta) more esotropic at near than at distance fixation. The disparity in the near/distance deviation was reduced by 18 delta.

Adolescent↗
PubMed1994

Clinical implications of the erroneous use of the Verhoeff Stereopter: a case report.

BACKGROUND: The Verhoeff Stereopter test is currently used by some branches of the military to evaluate depth perception for career placement. A 22-year-old male presented for consultation after failing the Verhoeff stereopter test during an examination for Marine flight training school. METHODS: A complete vision examination was conducted. Stereopsis was evaluated using the Verhoeff Stereopter, Randot Stereotest and the Multi-stereo test. RESULTS: A complete visual analysis revealed a mild refractive error, no ocular pathology, and normal binocularity. Stereopsis testing with the Verhoeff stereopter, Randot stereotest, and Multi-Stereo tests were normal. Subsequent investigation revealed that the original military tester did not follow proper testing protocol. When the erroneous testing condition was analyzed, we found that the stimulus horizontal disparity was eliminated. This lack of horizontal disparity makes it impossible to pass the test. We confirmed our findings by presenting the correct testing conditions and the erroneous testing conditions to five subjects with normal stereopsis. All subjects passed the Verhoeff test under standard protocols but failed under the erroneous condition. The critical factor that causes the elimination of horizontal disparity is identified. CONCLUSION: Although this instrument is not popular among vision care practitioners, it is being utilized by various organizations. An understanding of the theoretical basis for the instrument would therefore be beneficial.

Adult↗
PubMed2006

Neural bases of stereopsis across visual field of the alert macaque monkey.

Left and right retinal images of an object seen by the 2 eyes can occupy slightly disparate horizontal and/or vertical locations. The role of horizontal disparity (HD) in stereoscopic vision is well established, but the functional contribution of vertical disparity (VD) remains unclear. Various psychophysical studies have shown that HD and VD are used differently by the visual system depending on their location in the visual field, whether near the center of gaze or more peripheral. We show this horizontal/vertical distinction at the cellular level in monkey primary visual cortex (area V1). The range of VD encoding is reduced in central but not in the peripheral representation of the visual field. Moreover, neurons respond selectively to particular combinations of both types of disparities depending on the coded orientation as predicted by the disparity energy model. The preferred orientations of neurons near the fovea present a vertical bias that is well suited for stereopsis based on HD selectivity alone. In the periphery, instead, preferred orientations are radially biased, which allows a peripheral detector to convey the same depth signal based on either HD or VD. Such an organization has functional implications in both the perceptual and oculomotor domains.

Animals↗
PubMed1991

Alternating hyperphoria. Dissociated vertical deviation (DVD) 'occlusion hyperphoria'.

Alternating hyperphoria (synonyms: dissociated vertical deviation (DVD) or occlusion hyperphoria) and variants like 'unilateral patching hyperphoria' ('periodic vertical squint') and monocular vertical nystagmus, which may arise after strabismus operations or loss of the function of one of the eyes, have dynamic properties which differ from those of the vertical vergences or fusional movements in normal binocular vision. Alternating hyperphoria is the result of an early intense disturbance of binocular vision, leading to the absence of vertical fusional vergence or the detection of disparity necessary for this. Vertical disparity vergence is essentially a stabilizing and adaptive system.

Adolescent↗
PubMed1982

Detection of vertical disparties.

Psychometric curves determined for the detection of differences between vertical disparities as a function of the reference disparity for three presentation times (20 msec, 160 msec and 2 sec) and for two eccentricites of the stimulus (0.5 degree and 4 degrees) are presented. The results show that the detectability of disparity differences first increases and then decreases when the reference disparity is increased from zero. It further appears that the only effect of the stimulus presentation time and eccentricity on the psychometric curves and their dependency on the reference disparity is that the effective disparities observed are multiplied by a constant factor. The experimental data can be described by a signal detection model with two special features around zero disparity where binocular single vision occurs, viz. Loss of information about the sign of disparity and presence of intrinsic noise at a level up to 25% higher than the minimum level found at intermediate disparities.

Differential Threshold↗
PubMed1998

Stereoscopic transparency: a test for binocular vision's disambiguating power.

It has been suggested that to resolve ambiguities implicit in binocular perception of complex visual scenes, the brain adopts a continuity constraint assuming that disparities change smoothly with eccentricity. Stereoscopic transparency is characterized by abrupt changes of binocular disparity across retinal locations. The focus of the present study is how the brain uses the continuity constraint in the perception of stereoscopic transparency despite the presence of abrupt disparity changes. Observers viewed random-dot stereograms of overlapping transparent plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strictly controlled depth fixation. Surprisingly, maximal dot density of the transparent plane at which perception is still veridical dramatically decreases as depth separation between the surfaces grows. Persistence of this relationship, when binocular matching processes at each surface are separated to on and off brightness channels, suggests at least two stages in the underlying computation binocular matching and inter-surface interactions. We show that these phenomena cannot be accounted for by either higher severity of matching with high dot densities or the ability of the denser surface to pull vergence to its depth. We also measure contrast sensitivity and near-far symmetry of the underlying mechanism and propose a model of competitive interactions between dissimilar disparities.

Computer Graphics↗
PubMed2000

Exploring sequential stereopsis for co-planarity tasks.

We used the sequential stereopsis paradigm and apparatus described by Enright (Vision Research, 36, (1996) 307-312). The observer's task was to set targets to equidistance in Experiments 1-3, and to make them co-planar in Experiment 4. However, it is argued that in all experiments observers exploited a co-planarity setting strategy. Sequential stereopsis produced good performance throughout in terms of low disparity thresholds when head position was varied by rotations around three axes: vertical (azimuth condition); horizontal (elevation); and midline (tilt). It also produced good performance when the targets were shifted in position so that they both lay on one side of the median plane of the head. These results cannot be accounted for by Enright's isovergence hypothesis unless it is extended to incorporate other information about eye positions. Performance was better but not greatly so in control simultaneous stereopsis conditions, nor did it deteriorate much when the observer's view was restricted solely to the targets by removing visibility of the room in which the apparatus was located. Target settings were typically located on a concave arc centred on the median plane. This effect was quantitatively modelled using disparity correction for a relief task of co-planarity (Gârding, Porrill, Mayhew, & Frisby. Vision Research, 35 (1995) 703-722). This modelling indicated over-estimations of c.10-20 cm in fixation distance for target distances in the range 71.5-112.5 cm.

Adolescent↗
PubMed1979

Stereopsis and the random element in the organization of the striate cortex.

Receptive field position and orientation disparities are both properties of binocularly discharged striate neurons. Receptive field position desparities have been used as a key element in the neural theory for binocular depth discrimination. Since most striate cells in the cat are binocular, these position disparities require that cells immediately adjacent to one another in the cortex should show a random scatter in their monocular receptive field positions. Superimposed on the progressive topographical representation of the visual field on the striate cortex there is experimental evidence for a localized monocular receptive field position scatter. The suggestion is examined that the binocular position disparities are built up out of the two monocular position scatters. An examination of receptive field orientation disparities and their relation to the random variation in the monocular preferred orientations of immediately adjacent striate neurons also leads to the conclusion that binocular orientation disparities are a consequence of the two monocular scatters. As for receptive field position, the local scatter in preferred orientation is superimposed on a progressive representation of orientation over larger areas of the cortex. The representation in the striate cortex of visual field position and of stimulus orientation is examined in relation to the correlation between the disparities in receptive field position and preferred orientation. The role of orientation disparities in binocular vision is reviewed.

Animals↗
PubMed1986

Effect of nearpoint stress on the horizontal forced vergence fixation disparity curve.

The effect of nearpoint stress on the forced vergence fixation disparity curve was investigated directly. Data gathered from asymptomatic subjects with normal binocular vision showed that fixation disparity was not influenced by 25 min of continuous reading. Further investigation using symptomatic subjects with large fixation disparities or steep forced vergence curves showed that the nearpoint reading task had a significant effect on the curves of these subjects. Clinical implications and a possible physiological substrate are presented.

Adolescent↗