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PubMed1998

Anomalous correspondence--the cause or consequence of strabismus?

I hypothesize that the functional role of binocular correspondence, whether normal or anomalous, is that of detecting retinal disparity stimuli. Normal correspondence involves disparity stimulus detection that is quantitatively normal and stable, whereas anomalous correspondence involves disparity detection that is quantitatively abnormal and variable. Normal disparity stimuli typically produce motor fusion between normally corresponding biretinal areas. The result is orthotropia, which permits normal single binocular vision and acute stereopsis. In contrast, abnormal disparity stimuli typically produce motor fusion between anomalously corresponding areas. The result is strabismus, usually coupled with anomalous single binocular vision (harmonious anomalous correspondence). Accordingly, corrective prism or muscle surgery only produces vergence movements that restore anomalous motor fusion, rather than eliminating the heterotropia. The inference is that anomalous correspondence involves a neurological disorder in the disparity detection mechanism, either on the convergent side (estropia with anomalous correspondence) or divergent side (exotropia with anomalous correspondence).

Brain Diseases↗
PubMed2000

Perceived speed of motion in depth is reduced in the periphery.

The perceived speed of motion in depth (MID) for a monocularly visible target was measured in central and peripheral vision using a 2AFC speed discrimination task. Only binocular cues to MID were available: changing disparity and interocular velocity difference (IOVD). Perceived speed for monocular lateral motion and perceived depth for static disparity were also assessed, again in both central and peripheral vision. The purpose of the experiment was to assess the relative contributions of changing disparity and IOVD cues to the perceived speed of stereomotion. Although peripheral stimuli appeared to lie at approximately the same depth as their central counterparts, their apparent speed was reduced. Monocular/lateral and binocular/MID speeds were reduced to a similar extent. It seems that reduced apparent monocular speed leads to reduced perceived MID speed, despite the fact that the disparity system appears to be unaffected. These results suggest that the IOVD cue makes a significant contribution to MID speed perception.

Adult↗
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↗
PubMed1990

On the neurophysiological organization of binocular vision.

The considerable mixing in the visual cortex, of signals from left and right eyes, provides an abundant population of binocularly activated neurons. Based on this and on the fact that cortical cells respond best to different ranges of retinal disparities, it has been proposed that these neurons form the physiological substrate of stereoscopic depth discrimination. We outline reasons here for addressing first the more fundamental issue of the rules of convergence in the visual cortex, for input from the two eyes. We show that most of this convergence may be described by a linear summation process. However, there is a nonlinear mechanism that maintains binocular interaction regardless of large differences in stimulus strength between the eyes. This finding suggests that a cell which appears to be dominated by one eye, when monocular tests are conducted, may respond equally under binocular conditions. In this case, binocular processing for all cortical cells could be uniform and independent of the ocular dominance values determined monocularly. With respect to a neural mechanism for the processing of information concerning different depths in space, we propose an alternative to the conventional notion. First, we identify fundamental problems with the current view. Second, we describe a procedure which allows us to distinguish between the conventional view and our alternative proposal. Standard receptive field mapping techniques are not adequate for determining phase-disparity relationships of the type we require. Therefore, we have employed a reverse correlation procedure which enables efficient and detailed mapping of receptive field structure. Third, we describe preliminary data concerning the physiological mechanism of stereoscopic depth discrimination.

Action Potentials↗
PubMed1999

The role of vergence adaptation in recovery of binocular single vision (BSV) following sensory strabismus.

Vergence adaptation is an important element of comfortable binocular single vision and probably contributes to the high incidence of orthophoria or small angles of heterophoria in the normal population. Where binocular single vision has been absent for a period of time, restoration of good visual acuity appears to enable the vergence adaptation mechanism to become active again. A case is presented in which a moderate to large angle of deviation rapidly 'disappeared' once good visual acuity was restored and the disparate images could be fused. Known factors concerning vergence adaptation are discussed in relation to such clinical cases.

Adaptation, Ocular↗
PubMedDate not supplied

Mechanisms of stereopsis in monkey visual cortex.

A substantial proportion of neurons in the striate and prestriate cortex of monkeys have stereoscopic properties; that is, they respond differentially to binocular stimuli that are known in humans to provide cues for stereoscopic depth perception. Stereoscopic neurons, as these cells may be called, are selective for horizontal positional disparity (i.e., display disparity selectivity) and for the textural correlation between images over their receptive fields (i.e., they show correlation selectivity). Many neurons have tuned disparity response profiles that collectively cover the entire range of physiological disparities. Neurons with peak responses at or about the zero disparity ("tuned zero neurons," excitatory or inhibitory) have narrow and symmetrical profiles. Neurons that are tuned to larger disparities, either crossed ("tuned near neurons") or uncrossed ("tuned far neurons"), have broader excitatory profiles that are asymmetrically wider toward the smaller disparities, and commonly include an inhibitory component about the zero disparity. Other stereoscopic neurons have reciprocal profiles ("near" or "far" neurons, respectively) in the sense that they respond with excitation to crossed or uncrossed disparities, and with suppression to disparities of opposite sign. Stereoscopic neurons can also signal the textural correlation between paired retinal images by giving different responses to random-dot patterns that have, and to those that do not have, the same dot distribution over the neuron's left and right receptive fields. Tuned-zero excitatory neurons characteristically respond to uncorrelation with suppression; tuned-zero inhibitory neurons, with excitation; and both types give the opposite responses to correlated stereopatterns. Neurons selective for nonzero disparities, both tuned and reciprocal, also give excitatory responses to uncorrelated stimuli, but these responses are smaller and more variable than those evoked by correlated patterns at the effective disparities. These findings suggest that stereoscopic neurons in the visual cortex of the macaque comprise three operational systems: (1) a zero-disparity system that is involved in fine depth discrimination with the obligatory singleness of vision, and the maintenance of vergence; and (2) a near-, and (3) a far-disparity system that together signal qualitative estimates of depth with double vision, and vergence responses to large disparities.

Animals↗
PubMed2004

An integrative model of binocular vision: a stereo model utilizing interocularly unpaired points produces both depth and binocular rivalry.

Half-occluded points (visible only in one eye) are perceived at a certain depth behind the occluding surface without binocular rivalry, even though no disparity is defined at such points. Here we propose a stereo model that reconstructs 3D structures not only from disparity information of interocularly paired points but also from unpaired points. Starting with an array of depth detection cells, we introduce cells that detect unpaired points visible only in the left eye or the right eye (left and right unpaired point detection cells). They interact cooperatively with each other based on optogeometrical constraints (such as uniqueness, cohesiveness, occlusion) to recover the depth and the border of 3D objects. Since it is contradictory for monocularly visible regions to be visible in both eyes, we introduce mutual inhibition between left and right unpaired point detection cells. When input images satisfy occlusion geometry, the model outputs the depth of unpaired points properly. An interesting finding is that when we input two unmatched images, the model shows an unstable output that alternates between interpretations of monocularly visible regions for the left and the right eyes, thereby reproducing binocular rivalry. The results suggest that binocular rivalry arises from the erroneous output of a stereo mechanism that estimates the depth of half-occluded unpaired points. In this sense, our model integrates stereopsis and binocular rivalry, which are usually treated separately, into a single framework of binocular vision. There are two general theories for what the "rivals" are during binocular rivalry: the two eyes, or representations of two stimulus patterns. We propose a new hypothesis that bridges these two conflicting hypotheses: interocular inhibition between representations of monocularly visible regions causes binocular rivalry. Unlike the traditional eye theory, the level of the interocular inhibition introduced here is after binocular convergence at the stage solving the correspondence problem, and thus open to pattern-specific mechanisms.

Depth Perception↗
PubMed2005

Two eyes in action.

Do relative binocular disparities guide our movements in depth? In order to find out we asked subjects to move a 'cursor' to a target within a simulated horizontal plane at eye height. They did so by moving a computer mouse. We determined how quickly subjects responded to the target jumping in depth. We found that it took subjects about 200 ms to respond to changes in binocular disparity. Subjects responded just as quickly if the cursor was temporarily only visible to one eye near the time that the target jumped in depth, and less vigorously, though just as quickly, if the cursor jumped rather than the target, so the fastest binocular responses cannot be based directly on the relative retinal disparity between the target and the cursor. Subjects reacted faster to changes in the target's height in the visual field than to changes in binocular disparity, but did not react faster to changes in image size. These results suggest that binocular vision mainly improves people's everyday movements by giving them a better sense of the distances of relevant objects, rather than by relative retinal disparities being used to directly guide the movement. We propose that relative disparities only guide parts of very slow movements that require extreme precision.

Adult↗
PubMed1999

Stereopsis based on monocular gaps: metrical encoding of depth and slant without matching contours.

It is often the case in binocular vision that one eye can see between two objects lying at different distances but the other eye cannot. We have found that the visual system is able to correctly interpret images produced this way in which a single solid rectangle in one eye is fused with two half-sized rectangles in the other eye separated by a vertical gap comprising the background. Two rectangles in depth are seen. It is as if the solid rectangle is treated as two components which each match one of the physically separated rectangles in the contralateral eye. The sign of the depth depends on which eye's view has the gap and its magnitude increases with gap width. Measured depth is found to be equivalent to real stereoscopic depth with a relative disparity equal to the monocular gap. If overall disparity differences are eliminated, between the left and the right images, variations in perceived slant of the two rectangles are still seen with increasing gap size. That two surfaces can be seen in metric binocular depth despite complete camouflage of their separation in one eye's view, suggests that stereopsis be regarded as a broad process of surface recovery not necessarily requiring image disparity at the location of the depth step.

Depth Perception↗
PubMed1999

'Coarse-to-fine' cyclopean processing.

Previously (Popple et al, 1998 Vision Research 38 319-326) we found, using random-dot stereograms, that initial vergence increases with the size of a cyclopean disc. A corresponding improvement in stereoacuity within the disc was predicted, because disparities in the disc would be brought closer to the plane of current fixation. In the present experiment, we looked at the effect of the spatial extent of a briefly presented (< or = 500 ms) cyclopean depth pedestal on stereoacuity thresholds. Observers were required to judge the depth of a small, 1.7 deg, central disc relative to a larger surrounding disc in a random-pattern stereogram. The larger disc was set, initially, at a pedestal disparity of +/- 24 min of arc against a fixation-plane surround. The size of the larger disc was varied from 2.6 to 8.0 deg. As predicted, stereoacuity thresholds fell significantly with increasing pedestal disc size. Next, the disparity of the pedestal disc was varied. When pedestal disparity was reduced to +/- 2.4 min of arc, a disparity too small to demand vergence, the size effect disappeared except when the pedestal boundary was within 30 min of arc of the test disc boundary. We argue from this result that the effect was largely due to vergence and not cyclopean integration alone. However, the effect of pedestal size was found to persist with stimuli too brief to permit vergence (< or = 100 ms) suggesting that factors other than vergence may also play a role.

Depth Perception↗
PubMed2003

Binocular vision with primary microstrabismus.

PURPOSE: Patients with primary microstrabismus have a high degree of binocularity, which suggests that their ocular misalignment may have a sensory rather than an oculomotor origin, as in large-angle strabismus. The purpose of these experiments was to determine whether microstrabismic subjects have sensory abnormalities that could give rise to a small angle of strabismus. METHODS: The binocular disparity response functions for sensory and motor processes were compared in seven orthotropic subjects and six strabismic subjects (four with primary microstrabismus and two with infantile esotropia). Binocularity was assessed by disparity vergence (central and peripheral stimuli) and depth discrimination (relative and absolute disparities) measures. Motor and sensory disparity response functions were both determined by psychophysical methods: vergence responses by dichoptic nonius alignment and sensory responses by forced-choice depth discrimination. RESULTS: All the strabismic subjects demonstrated normal retinal correspondence with peripheral binocular stimuli and anomalous retinal correspondence with central fusion stimuli. The microstrabismic subjects' disparity vergence responses with peripheral fusion stimuli were centered on disparities relative to their angle of strabismus. However, with central fusion stimuli, the disparity vergence responses were relative to the subjective angle of strabismus. The microstrabismic subjects' stereoacuities were substantially reduced, but their discrimination responses did not show an asymmetry indicative of an unrepresented population of disparity-selective mechanisms. CONCLUSIONS: The data do not support a sensory abnormality as the primary cause of microstrabismus. The results are not compatible with an oculomotor adaptation to an inherent anomalous correspondence or with a strabismus caused by an absence of a class of disparity-selective mechanisms. Thus, just as in large-angle strabismus, the anomalous retinal correspondence and defective stereopsis of microstrabismus appear to be consequences of abnormal visual experience caused by an interocular deviation.

Child↗
PubMed2006

Orientation-specific computation in stereoscopic vision.

The left and right eyes receive subtly different images from a visual scene. Binocular disparities of retinal image locations are correlated with variation in the depth of objects in the scene and make stereoscopic depth perception possible. Disparity stereoscopically specifies a stimulus; changing the stimulus in a way that conserves its disparity leaves the stimulus stereoscopically unchanged. Therefore, a person's ability to use stereo to see the depth separating any two objects should depend only on the disparities of the objects, which in turn depend on where the objects are, not what they are. However, I find that the disparity difference between two stimuli by itself predicts neither stereoacuity nor perceived depth. Human stereo vision is shown here to be most sensitive at detecting the relative depth of two gratings when they are parallel. Rotating one grating by as little as 10 degrees lowers sensitivity. The rotation can make a perceptible depth separation invisible, although it changes neither the relative nor absolute disparities of the gratings, only their relative orientations. The effect of relative orientation is not confined to stimuli that, like gratings, vary along one dimension or to stimuli perceived to have a dominant orientation. Rather, it is the relative orientation of the one-dimensional components of stimuli, even broadband stimuli, that matters. This limit on stereoscopic depth perception appears to be intrinsic to the visual system's computation of disparity; by taking place within orientation bands, the computation renders the coding of disparity inseparable from the coding of orientation.

Depth Perception↗
PubMed2003

Errors of binocular fixation are common in normal subjects during natural conditions.

PURPOSE: To investigate the accuracy of fixation after symmetrical vergence eye movements along the midline during natural full-field viewing conditions using a video method of eye position measurement. METHODS: The accuracy of binocular fixation after symmetrical vergence eye movements during natural conditions was measured on 29 young adults using a precise head-mounted video eye movement measuring system. All subjects had normal binocular vision and good visual acuity. Measurements were taken for both near and far fixation after vergence changes of 5 degrees, 10 degrees, and 15 degrees using three rates of change, approximately 0.25, 0.5, and 1 Hz. RESULTS: The amplitude of the vergence movement tended to be hypometric, resulting in underconvergence for near fixations, and overconvergence for distance fixation. For far fixations, most errors (82%) were from -120 to +120 min arc, and for near, most errors (85%) were from -30 to +120 min arc. For far fixations, there was a significant effect of the size of vergence change (F1,28 = 61.8; p < 0.001), the rate of change (F1,28 = 7.08; p = 0.013), and the interaction between these two factors (F1,28 = 7.17; p = 0.012) on resulting errors, with the eyes showing greater overconvergence on the target for the larger and faster fixation changes. For near fixations, there was a significant effect (F1,28 = 15.9; p < 0.001) for the angle of change with the faster vergence changes producing relatively more convergence, thus reducing the mean vergence error. No subject reported diplopia during any conditions despite our measures showing vergence errors of up to 5 degrees. CONCLUSIONS: Vergence errors of up to +/-2 degrees, without diplopia, were common in subjects with normal binocular single vision. Errors of 5 degrees were rare but present. In all, the largest number of errors occurred as a failure of divergence for far fixations, consistent with previous studies that have suggested differences in the neural control of pathways for convergence and divergence, or possibly caused by differences in the anatomical properties of the medial and lateral rectus muscles and their associated fascia. The absence of diplopia during the period of fixation could only be partly associated with the visual suppression associated with vergence eye movements that has been reported by others because it was still present after the vergence movement was completed. The natural viewing conditions in this study that included a full visual field and multiple disparities may have contributed to this effect.

Adolescent↗
PubMed1995

Spatial vision in adults and infants: a tribute to Russell Harter.

To be recognized, the retinal image of an object must first be segregated from the retinal image of its surroundings. Luminance contrast is only one way by which an object can be rendered visible. Motion contrast alone, texture contrast alone, colour contrast alone and disparity contrast alone can also render an object visible. This paper notes that psychophysical studies of patients with brain lesions show that motion-defined form and luminance-defined form are processed separately and describes how magnetic (MEG) or electrical (VEP) recording can be used to compare brain responses to luminance-defined form, motion-defined form, colour-defined form, texture-defined form and disparity-defined form. At an early stage in visual processing, the retinal images of luminance-defined objects pass through neural filters, each of which is selectively sensitive to a limited range of orientations, spatial frequencies and temporal frequencies. The properties of individual filters can be isolated and quantified by means of the two-sinewave VEP approach. The two-sinewave method also allows binocular function to be tested objectively and its normality assessed quantitatively even when one or both eyes have low acuity. Evoked potential studies of visual development in human infants and studies of labile phenomena in adults are both facilitated by the speed of our VEP sweep method.

Adult↗
PubMed1998

Interaction of stereo vision and optic flow processing revealed by an illusory stimulus.

The influence of stereoscopic vision on the perception of optic flow fields was investigated in experiments based on a recently described illusion. In this illusion, subjects perceive a shift of the center of an expanding optic flow field when it is transparently superimposed by a unidirectional motion pattern. This illusory shift can be explained by the visual system taking the presented flow pattern as a certain self-motion flow field. Here we examined the dependence of the illusory transformation on differences in depth between the two superimposed motion patterns. Presenting them with different relative binocular disparities, we found a strong variation in the magnitude of the illusory shift. Especially when translation was in front of expansion, a highly significant decrease of the illusory shift occurred, down to 25% of its magnitude at zero disparity. These findings confirm the assumption that the motion pattern is interpreted as a self-motion flow field. In a further experiment we presented monocular depth cues by changing dot size and dot density. This caused a reduction of the illusory shift which is distinctly smaller than under stereoscopic presentation. We conclude that the illusory optic flow transformation is modified by depth information, especially by binocular disparity. The findings are linked to the phenomenon of induced motion and are related to neurophysiology.

Adult↗
PubMed2005

Eye preference within the context of binocular functions.

BACKGROUND: Eye preference refers to an asymmetric use of the two eyes, but it does not imply a unitary asymmetry between the eyes. Many different methods are used to assess eye preference, including eyedness questionnaires and sighting tasks that require binocular and monocular alignment of a target through a hole in the middle of a card or funnel. The results of these coarse accounts of eye preference are useful as a first screening, but do not allow for graded quantification of the manifested asymmetry in binocular vision. Moreover, they often concern only a rather selective range of binocular functions. The aim of the present study was to further differentiate eye preference within the context of other binocular functions as measured in standard optometric tests, and to validate their relation to questionnaire data of eyedness. METHODS: Conventional accounts of eye preference (German adaptation of Coren's questionnaire and a sighting task) were compared with various optometric tests of binocular function within a sample of 103 subjects. Examination included visual acuity and accommodation in each eye, stereoscopic prevalence, suppression due to binocular rivalry, fixation disparity (Mallett test). RESULTS: Sighting dominance was leftward in 32% and rightward in 68% of the cases and was highly correlated (Kendall's tau(b)=0.70) with eyedness. Further significant associations were restricted to stereoscopic prevalence which correlated with sighting dominance (tau(b)=0.55), eyedness (tau(b)=0.50), and rivalry dominance (tau(b)=0.28). CONCLUSION: Eye preference seems to be essentially reflected by eyedness, sighting dominance, and stereoscopic prevalence, but largely unrelated to fixation disparity, accommodation, and visual acuity.

Accommodation, Ocular↗
PubMed1994

Independent encoding of surface orientation and surface curvature.

Marr [(1982) Vision, San Francisco, Calif.: Freeman] proposed that we represent surface geometry in terms of a viewer-centred description of surface orientation and distance. This description is computed by a range of independent processing systems which take as input particular kinds of information present in images, like surface texture, shading, retinal disparity and motion parallax. The outputs of these modules are integrated in order to provide a unitary representation of the layout of visible surfaces. Higher order properties of surface geometry, like surface curvature, might be computed from this symbolic representation or might be encoded independently from the visual information available at the retinae. We measured surface slant and surface curvature discrimination thresholds for surface patches defined by shading, texture and retinal disparity as a function of the elevation of the illumination. We found that observers judgements about the curvature of local surface patches were too precise to be based on a symbolic representation of surface orientation and we conclude that surface curvature is computed directly from depth cues present in the retinal images.

Depth Perception↗
PubMed2002

Depth generalization from stereo to motion parallax in the owl.

Although many sources of three-dimensional information have been isolated and demonstrated to contribute independently, to depth vision in animal studies, it is not clear whether these distinct cues are perceived to be perceptually equivalent. Such ability is observed in humans and would seem to be advantageous for animals as well in coping with the often co-varying (or ambiguous) information about the layout of physical space. We introduce the expression primary-depth-cue equivalence to refer to the ability to perceive mutually consistent information about differences in depth from either stereopsis or motion-parallax. We found that owls trained to detect relative depth as a perceptual category (objects versus holes) when specified by binocular disparity alone (stereopsis), immediately transferred this discrimination to novel stimuli where the equivalent depth categories were available only through differences in motion information produced by head movements (observer-produced motion-parallax). Motion-parallax discrimination did occur under monocular viewing conditions and reliable performance depended heavily on the amplitude of side-to-side head movements. The presence of primary-depth-cue equivalence in the visual system of the owl provides further conformation of the hypothesis that neural systems evolved to detect differences in either disparity or motion information are likely to share similar processing mechanisms.

Animals↗