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Neurons in early visual areas respond to horizontal disparity in images that do not give rise to stereopsis. False binocular matches, however, are discarded at the apex of the visual pathway: the activity of neurons in the primate inferior temporal cortex correlates directly with conscious depth perception.
When counterphase spatio-temporal flicker is presented to the left and right eye continuous directional motion can be perceived. Here, we investigate whether this type of dichoptic motion can be observed at different depth planes. Four observers indicated direction of motion for dichoptic motion stimuli, presented in a context containing crossed and uncrossed disparity information in different conditions. Our results show that despite the presence of disparity cues in the stimulus, discrimination of motion direction remained maximal at interocular phase offsets that correspond to binocular motion perception at zero disparity. This constraint brings into question perception of dichoptic motion as the result of an early binocular motion system. We compared our results with predictions of a computational stereo-motion model [Qian, N. (1994). Computing stereo disparity and motion with known binocular cell properties. Neural Computations, 6, 390-404; Qian, N., & Andersen, R. A. (1997). A physiological model for motion-stereo integration and a unified explanation of Pulfrich-like phenomena. Vision Research, 37, 1683-1698]. In contrast to our empirical results, simulations of cell activation in this hybrid energy model predict maximal activation at non-zero disparities. It is concluded that perception of dichoptic motion is a by-product of early interocular combination at low contrasts rather than the result of a dedicated stereo-motion system.
Observers generally fail to recover three-dimensional shape accurately from binocular disparity. Typically, depth is overestimated at near distances and underestimated at far distances [Johnston, E. B. (1991). Systematic distortions of shape from stereopsis. Vision Research, 31, 1351-1360]. A simple prediction from this is that disparity-defined objects should appear to expand in depth when moving towards the observer, and compress in depth when moving away. However, additional information is provided when an object moves from which 3D Euclidean shape can be recovered, be this through the addition of structure from motion information [Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America A, 2, 343-349], or the use of non-generic strategies [Todd, J. T., & Norman, J. F. (2003). The visual perception of 3-D shape from multiple cues: Are observers capable of perceiving metric structure? Perception and Psychophysics, 65, 31-47]. Here, we investigated shape constancy for objects moving in depth. We found that to be perceived as constant in shape, objects needed to contract in depth when moving toward the observer, and expand in depth when moving away, countering the effects of incorrect distance scaling (Johnston, 1991). This is a striking example of the failure of shape constancy, but one that is predicted if observers neither accurately estimate object distance in order to recover Euclidean shape, nor are able to base their responses on a simpler processing strategy.
Recently, Enright described an unexpected association between disparity-induced vertical vergence and cycloversion (conjugate eye torsion) [Enright (1992) Vision Research, 415, 279]. The present experiments were performed to verify these findings and investigate the nature of this association. We presented subjects with a dichoptic image of concentric circles in which a step in vertical disparity of 1 deg was introduced. After 4 sec the disparity was eliminated. Eye movements were measured with scleral coils. We confirmed Enright's findings in that a left-over right vertical vergence was associated with levo-cycloversion (upper poles towards left shoulder) and vice versa. The size of the cycloversion and the vertical vergence were in the same range. In addition we found that part of the cycloversion response was in the form of a torsional nystagmus and that the relative contribution of the left and right eyes was independent of the horizontal gaze angle. These additional findings are in conflict with the hypothesis, offered by Enright, that the association is caused by a bilateral activity of the superior oblique muscles.
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BACKGROUND: The vertical forced vergence fixation disparity (VFD) curve represents the amount of vertical fixation disparity, the steady-state vertical bifixation error of the eyes, at various levels of vertical vergence demand. The main aim of the present study was to examine the effects of vertical vergence training on the slope of the VFD curve in a normal, young adult population. METHODS: Forty-five subjects with normal vision and binocular function underwent vertical vergence training for 1 week. The training was done using a vertical prism bar, and the vertical fixation disparity was measured using the Disparometer. RESULTS: The mean slope of the VFD curve in a normal, young adult population was 1.103 min arc/delta. The slope of the VFD curve decreased significantly after the training and remained flattened for at least 3 months. There was no evidence to support the idea that the decrease in the VFD slope was related to the increase of vertical fusional amplitude. CONCLUSIONS: Vertical prism bar training provided a long-term effect, both increasing the vertical fusional amplitude and flattening the slope of the VFD curve. The decrease in the slope of the VFD curve was thought to be independent of the increase of vertical fusional amplitude.
PURPOSE: It has been reported that changes in visual direction and retinal correspondence may limit the validity of nonius lines as a subjective measure of vergence, at least in particular viewing conditions, e.g., dynamic or forced vergence. Nonius lines may be valid at larger spatial separation between fusion contour and nonius lines. Therefore, we measured fixation disparity varying the amount of a vertical gap between nonius lines. METHODS: A static central fusion stimulus was given at a 100-cm viewing distance. The nonius lines were presented with various vertical gaps either continuously (and adjusted to subjective alignment) or flashed in a series of trials (in adaptive testing). Twelve subjects with normal binocular vision were tested repeatedly to identify individual vergence characteristics. RESULTS: When the vertical gap between nonius lines was increased up to 7.9 deg, the amount of subjective fixation disparity tended to increase in part of the subjects, predominantly in those with an exo fixation disparity. Most subjective fixation disparity measures were correlated with each other and with tonic vergence (i.e., dark vergence tested subjectively without fusion stimulus); however, flashed nonius lines at larger nonius gaps were an exception. CONCLUSIONS: We found physiologically plausible measures of idiosyncratic fixation disparity with continuous nonius lines at any amount of nonius gap or with flashed nonius lines at small gaps. In these conditions, the intersubject variability of fixation disparity was much larger than effects of the spatial separation between fusion stimulus and nonius lines.
Investigations on macaque monkeys have provided much of our knowledge of the neural mechanisms of binocular vision, but there is little psychophysical data on the accuracy of vergence responses or the precision of stereoscopic depth perception in these primates. We have conducted comparative behavioral studies of binocular disparity processing in rhesus monkeys and humans via measurements of prism-induced fixation disparities (disparity vergence) and relative depth discrimination for spatially localized stimuli (local stereopsis). The results of these studies demonstrated a remarkable similarity in both the oculomotor and the sensory aspects of binocular vision in the two species when the stimulus dimensions were specified in visual angles, which were independent of interocular separation. The disparity vergence functions for the two species revealed fusion responses over the same range of prism-induced vergence and comparable vergence errors for stimuli near their fusional limits. Disparity vergence responses were independent of the spatial frequency of the binocular fusion stimulus. Stereothresholds as a function of the spatial frequency of the difference-of-Gaussian stimuli were of the same form, with equivalent stereoacuities, in monkey and human observers. The presence of substantial vergence errors had only a small effect on the precision of stereoscopic depth perception. We conclude that, after compensation for the differences in the lateral separation of their eyes, the operating characteristics of disparity vergence and stereoscopic vision are virtually identical in rhesus monkeys and humans and, consequently, the performance limits for these visual functions must be determined by anatomical and/or neural constraints that are similar in both species.
When an observer views a moving scene binocularly, both motion parallax and binocular disparity provide depth information. In Experiments 1A-1C, we measured sensitivity to surface curvature when these depth cues were available either individually or simultaneously. When the depth cues yielded comparable sensitivity to surface curvature, we found that curvature detection was easier with the cues present simultaneously, rather than individually. For 2 of the 6 subjects, this effect was stronger when the component of frontal translation of the surface was vertical, rather than horizontal. No such anisotropy was found for the 4 other subjects. If a moving object is observed binocularly, the patterns of optic flow are different on the left and right retinae. We have suggested elsewhere (Cornilleau-Pérès & Droulez, in press) that this motion disparity might be used as a visual cue for the perception of a 3-D structure. Our model consisted in deriving binocular disparity from the left and right distributions of vertical velocities, rather than from luminous intensities, as has been done in classical studies on stereoscopic vision. The model led to some predictions concerning the detection of surface curvature from motion disparity in the presence or absence of intensity-based disparity (classically termed binocular disparity). In a second set of experiments, we attempted to test these predictions, and we failed to validate our theoretical scheme from a physiological point of view.
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Vergence has transient components that are stimulated by brief presentations of stimuli at large disparities (up to several degrees). The question that we have addressed is what stimulus features are encoded by this system. A competition paradigm [Jones & Kerr, (1972)]. Vision Research, 12, 1425-1430) was used in which three gabors were presented. A single Gabor was presented to the fovea of one eye and two gabors, 2.5 deg to either side of the fovea, to the other; one of which, when paired with the single Gabor defined a convergent direction, the other a divergent direction. First we determined if increasing the luminance contrast of the Gabor pair whose disparity was opposite to the observer's response-bias direction (variable-contrast pair) relative to the remaining Gabor (reference) could alter the observer's response direction. Secondly, we determined if the contrast required for such a change in response was affected by the relative spatial frequency of the convergent and divergent Gabors. The reference Gabor was held at 2 cpd and the variable Gabor pair was varied between 5.6 and 0 (a gaussian) cpd. Results demonstrated that increasing the luminance contrast of the variable pair relative to the reference Gabor could alter the observer's response direction, even when the contrast of only one of the variable-pair Gabors was increased. The luminance contrast required for this change to occur was directly related to the spatial frequency of the variable pair over the entire frequency range tested. Vergence responses were preferentially made to lower spatial frequencies, even when a low spatial frequency was pared with a high one. We conclude that transient-vergence responses are not reduced by mixed contrasts (i.e. no contrast-paradox effect) and appear to be mediated by a system that employs a single lowpass sensitive channel.
A neural model is developed to probe how corticogeniculate feedback may contribute to the dynamics of binocular vision. Feedforward and feedback interactions among retinal, lateral geniculate, and cortical simple and complex cells are used to simulate psychophysical and neurobiological data concerning the dynamics of binocular disparity processing, including correct registration of disparity in response to dynamically changing stimuli, binocular summation of weak stimuli, and fusion of anticorrelated stimuli when they are delayed, but not when they are simultaneous. The model exploits dynamic rebounds between opponent ON and OFF cells that are due to imbalances in habituative transmitter gates. It shows how corticogeniculate feedback can carry out a top-down matching process that inhibits incorrect disparity responses and reduces persistence of previously correct responses to dynamically changing displays.
Fixation disparity or vergence noise produce instantaneous vergence errors. These errors are analogous to the imposition of a pedestal disparity, which is known to elevate stereothresholds. In this study, stereothresholds were measured as a function of induced vergence errors in subjects with normal binocular vision. Stereo half-images were viewed in the dark through a custom mirror haploscope. Vergence constant error and vergence variability were induced by introducing horizontal disconjugate position offsets in a pair of moveable mirrors within the haploscope, resulting in forced vergence demands of 6(Delta) base-in to 12(Delta) base-out. In addition, vergence variability was simulated by producing oscillatory disconjugate retinal image motion via motion of the moveable mirrors. The motion of the mirrors was either sinusoidal (frequency=2-6 Hz) or random, with peak-to-peak amplitudes of 0 degrees -0.5 degrees per eye. Stereopsis worsened systematically with induced or simulated VV that exceeded approximately 1.5' disparity. The results were similar regardless of whether the vergence error was induced by forced vergence or was simulated by periodic or random disconjugate retinal image motion. Stereothresholds were invariant with the frequency of disconjugate oscillation, within the range of frequencies and amplitudes tested. Hence, the simulated vergence velocity is not the essential factor that limits stereopsis within Panum's fusional area. The results indicate that the stereothreshold is elevated if the vergence error exceeds a critical value.
Fixation disparity has been taken as a sign of stress on binocular vision because it is established that prism stress creates fixation disparity. This paper looks at the effect on fixation disparity of the stress caused by requiring subjects to read in inadequate illumination. It is found that the reduction in illumination does not in itself immediately change the magnitude of the fixation disparity. There is, however, an increase in the mean slope of the central part of the fixation disparity curve which suggests that when the effect of reduced illumination is added to prism stress, fixation disparity is increased. The stress created by asking subjects to read in reduced illumination for half an hour resulted in the mean associated heterophoria being increased, and over half the subjects reported symptoms of stress. It is concluded that fixation disparity is changed by this type of visual stress in some subjects, and in near vision is increased to a more marked degree of exo-disparity. Most of this increase occurs in the first ten minutes.
UNLABELLED: Ocular dominance manifests itself in tests that contain stereo-objects with a disparity beyond Panum's area, e.g. in pointing a finger. These tests force subjects to decide in favour of one or the other eye. In contrast, ocular prevalence is determined using stereo-targets imaged within Panum's areas. These tests allow a graded quantification of the balance between the eyes. Here we present the computer-based Freiburg Ocular Prevalence Test in which stereo-disparate targets have to be aligned, and compare it with the Haase Stereo-balance Test that requires an estimation of the horizontal distance between stationary stereo-disparate objects. In addition, we compare ocular prevalence with ocular dominance. METHODS: (1) We measured the influence of a neutral-grey filter in front of one eye to assess the suitability of the Freiburg and the Haase Tests in revealing graded amounts of ocular prevalence. (2) About 20 subjects with equal vision of their two eyes underwent the Freiburg and the Haase Tests for ocular prevalence, and Parson's Monoptoscope Test for ocular dominance. RESULTS: (1) In both the Freiburg and the Haase Tests, the neutral-grey filter shifted ocular prevalence by about 50%. (2) An ocular prevalence of more than 10% occurred in 13 of the 20 subjects using the Freiburg, and in 14 using the Haase Test. On average, the ocular prevalence was 24.1+/-3.8% in the Freiburg and 32.0+/-8.2% in the Haase Test. The dominant eye coincided with the prevalent eye in 15 of the 20 subjects. DISCUSSION: The effect of the neutral-grey filter indicated that both the Freiburg and the Haase Tests can be used to measure fractions of ocular prevalence, although the Freiburg Test carries a higher reproducibility. Spontaneous ocular prevalence occurs frequently in persons with equal vision of their two eyes. This suggests that ocular prevalence does not represent a condition that requires treatment. Rather, partial suppression of one eye, the correlate of ocular prevalence, may play a physiological role in that it helps to disregard double images at stereo-disparities close to the limits of Panum's area.
An object moving in depth produces retinal images that change in position over time by different amounts in the two eyes. This allows stereoscopic perception of motion in depth to be based on either one or both of two different visual signals: inter-ocular velocity differences, and binocular disparity change over time. Disparity change over time can produce the perception of motion in depth. However, demonstrating the same for inter-ocular velocity differences has proved elusive because of the difficulty of isolating this cue from disparity change (the inverse can easily be done). No physiological data are available, and existing psychophysical data are inconclusive as to whether inter-ocular velocity differences are used in primate vision. Here, we use motion adaptation to assess the contribution of inter-ocular velocity differences to the perception of motion in depth. If inter-ocular velocity differences contribute to motion in depth, we would expect that discriminability of direction of motion in depth should be improved after adaptation to frontoparallel motion. This is because an inter-ocular velocity difference is a comparison between two monocular frontoparallel motion signals, and because frontoparallel speed discrimination improves after motion adaptation. We show that adapting to frontoparallel motion does improve both frontoparallel speed discrimination and motion-in-depth direction discrimination. No improvement would be expected if only disparity change over time contributes to motion in depth. Furthermore, we found that frontoparallel motion adaptation diminishes discrimination of both speed and direction of motion in depth in dynamic random dot stereograms, in which changing disparity is the only cue available. The results provide strong evidence that inter-ocular velocity differences contribute to the perception of motion in depth and thus that the human visual system contains mechanisms for detecting differences in velocity between the two eyes' retinal images.
PURPOSE: In cases of esotropia combined with a high AC/A ratio, partly accommodative convergence excess or with nonaccommodative convergence excess, two methods of surgical therapy are possible: recession of the medial recti (Parks) and retroequatorial myopexy (Cüppers). Our aim was to answer the question whether retroequatorial myopexy alone and in combination with bimedial recession are appropriate methods of reducing the strabismic angle at near and distant fixation to values under 10 PD and near-distance disparity to less than 10 PD to form the basis for single binocular vision without bifocals. PATIENTS AND METHODS: Eighty-three patients, aged 2-14 years (39 boys and 44 girls), were included in the study: 37 children (group A) with early-onset near-distance esotropia of 14-48 PD for distance and 26-65 PD for near objects and 46 children (group B) with acquired near-distance esotropia of 8-45 PD for distance and 26-70 PD for near objects. Inclusion criteria were a near-distanced disparity of at least 10 PD (range 10-33 PD), a follow-up of at least 3 months (median 5 months, range 3-69 months), retinoscopy in cycloplegia and full refractive correction. The amount of conventional surgery was chiefly based on the distance angle of esotropia. The myopexy was placed 12, 13 and 14 mm behind the insertion of the medial rectus. RESULTS: In 73 of 83 patients (88%) we were able to reduce the strabismic angle for distance and near fixation to less than 10 PD and in 73 cases even under 5 PD. In 77 of 83 patients (93%) the near-distance disparity was reduced to less than 10 PD and in 72 patients (87%) even under 5 PD. Postoperatively, 9 children had bifocals, but 3 of them have meanwhile discarded them. Two cases were slightly ocvercorrected and 1 case undercorrected. Seventy children (84%) attained grade of binocularity. CONCLUSION: Retroequatorial myopexy (fadenoperation) alone and combined with bimedial recession is an effective procedure in treating esotropia with abnormal near-distance disparity.
Disparity-selective neurons in striate cortex (V1) probably implement the initial processing that supports binocular vision. Recently, much progress has been made in understanding the computations that these neurons perform on retinal inputs. The binocular energy model has been highly successful in providing a simple theory of these computations. A key feature of the energy model is that it is linear until after inputs from the two eyes are combined. Recently, however, a modified version of the energy model, incorporating threshold nonlinearities before binocular combination, has been proposed to account for the weaker disparity tuning observed with anticorrelated stimuli. In this study, we present new data needed for a critical assessment of these two models. We compare two key predictions of the models with responses of disparity-selective neurons recorded from V1 of awake fixating monkeys. We find that the original energy model, and a family of generalizations retaining linear binocular combination, are quantitatively inconsistent with the response of V1 neurons. In contrast, the modified version incorporating threshold nonlinearities can explain both sets of observations. We conclude that the energy model can be reconciled with experimental observations by adding a threshold before binocular combination. This gives us the clearest picture yet of the computation being carried out by disparity-selective V1 neurons.