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PubMed1999

Aniseikonia in relation to strabismus, anisometropia and amblyopia.

PURPOSE: To study the interrelationships among these four entities which are critical to binocular vision and its precision. SUBJECTS AND METHODS: 102 selected patients (for their ability to have stereoscopic depth perception, a requisite for space eikonometry) were evaluated. Patient testing included stereoscopic testing, Essilor Projection Space Eikonometry, ultrasonic echographic axial length measurements and orthoptic evaluation. Aniseikonia was measured on the Essilor Projection Space Eikonometer. RESULTS: 1. Anisometropia alone was correlated with a marked increase in amblyopia, a moderate increase in aniseikonia and no noteworthy increase in strabismus. Statistical analysis (chi square ratio) showed that persons with elevated anisometropic values had a 4.4 fold increased risk of aniseikonia (p=.003). 2. Aniseikonia alone was not responsible for marked variations in strabismus. 3. Amblyopia was correlated with increases in anisometropia and aniseikonia. 4. Adding aniseikonia to anisometropia produced a possible increase in strabismus and a great increase in amblyopia (using Fisher's Exact Test, 2-tailed). 5. Spearman correlations of the "absolute values" (the mean of the mathematical difference between the two eyes of anisometropia and amblyopia) were as follows: anisometropia (abs) vs. aniseikonia r=.294, p=.006; anisometropia (abs) vs. amblyopia (abs) 4=.555, p=<.001; amblyopia (abs) vs. aniseikonia r=.234, p=.02. CONCLUSIONS: Aniseikonia per se does not appear to have a major causal role in amblyopia or strabismus, but anisometropia does for amblyopia. This role is greatly augmented by aniseikonia and this combination may then produce strabismus.

Amblyopia↗
PubMed2001

The relevance of stereopsis for motorists: a pilot study.

PURPOSE: To study the influence of stereoscopic depth perception on automobile driving performance. METHODS: Ten patients with strabismus and defective stereopsis were compared with ten healthy controls with respect to their performance in a series of automobile driving manoeuvres. The two groups were individually matched as to age, annual miles driven, years of licence holding and type of vehicle owned. After an ophthalmologic examination the subjects in each group performed the following series of driving tests: (1) stopping in front of an obstacle, (2) reversing into a parking space, (3) driving through a slalom course, (4) estimating the relative positions of two cars. All tests were performed binocularly and monocularly (with the non-dominant eye covered). RESULTS: Only in the slalom test did the normal subjects perform significantly better than the stereo-deficient subjects (odds ratio 10.5; P<0.01). In estimating position, normal subjects actually performed significantly worse (odds ratio 0.091; P<0.01). A significant distance ratio of 2.5 (95% CI 1.1-5.5; P=0.033) of the monocular with respect to the binocular performance of the normal subjects was found for the stopping task only, while the subjects with defective stereopsis showed no difference between their monocular and binocular performance. CONCLUSION: In this study, stereopsis had a positive effect on driving performance only in dynamic situations at intermediate distances.

Adult↗
PubMed2000

Weighted directional energy model of human stereo correspondence.

Previous work [Prince, S. J. D, & Eagle, R. A. (1999). Size-disparity correlation in human binocular depth perception. Proceedings of the Royal Society: Biological Sciences, 266, 1361-1365] has demonstrated that disparity sign discrimination performance in isolated bandpass patterns is supported at disparities much larger than a phase disparity model might predict. One possibility is that this extended performance relies on a separate second-order system [Hess, R. F., & Wilcox, L. M. (1994). Linear and non-linear filtering in stereopsis. Vision Research, 34, 2431-2438]. Here, a 'weighted directional energy' model is developed which explains a large body of crossed versus uncrossed disparity discrimination data with a single mechanism. This model assumes a population of binocular complex cells at every image point with a range of position disparity shifts. These cells sample a local energy function which is weighted so that energy at large disparities is relatively attenuated. Disparity sign is determined by summing and comparing energy at crossed and uncrossed disparities in the presence of noise. The model qualitatively predicts matching data for one-dimensional Gabor stimuli. This scheme also predicts DMax in Gabor stimuli and filtered noise. Moreover, a range of 'non-linear' phenomena, in which disparity is perceived from contrast envelope information alone, can be explained. The weighted directional energy model presents a biologically plausible, parsimonious explanation of matching behaviour in bandpass stimuli for both 'first-order' and 'second-order' stimuli which obviates the need for multiple mechanisms in stereo correspondence.

Depth Perception↗
PubMed2000

[Are cross-eyed persons worse drivers? The effect of stereoscopic disparity on driving skills].

BACKGROUND: The motorist obtains about 90% of the information required to drive a vehicle from the sense of vision. In road traffic, various situations with respect to size, distance, and position have to be evaluated at different speeds. Does depth perception based on disparity play an essential role in these situations? METHODS: Each of ten subjects with defective stereopsis due to strabismus was compared with an age matched healthy control, who was also matched in yearly mileage, type of vehicle and duration of driving experience. After ophthalmological examination including stereotests for both distance and near, all subjects performed the following driving tests: (1) Stopping as close as possible in front of an obstacle, (2) reversing into a parking position, (3) driving a slalom parcours at the speed of 40 km/h, (4) estimating the relative position of two cars within a limited time while approaching them with 40 km/h. All tests were performed both under monocular (the non-leading eye covered) and binocular conditions. The sequence of the tests was randomized in order to minimize training effects. RESULTS: Comparing the driving performance of both groups, the controls performed significantly better (p < 0.01; ANOVA) only in the "slalom task". In "estimation of position" the controls performed significantly worse (p < 0.01; ANOVA) compared to their counterparts with defective stereopsis. A significant difference between the monocular and binocular performance was found only in the "stopping task", exclusively for the group of normals, benefitting from the binocular performance (p < 0.05; ANOVA). CONCLUSION: Stereopsis improves the driving performance only in dynamic situations at intermediate distances.

Adult↗
PubMed2000

Stereo correspondence in one-dimensional Gabor stimuli.

Previous data [Prince, S.J.D., & Eagle, R.A., (1999). Size-disparity correlation in human binocular depth perception. Proceedings of the Royal Society of London B, 266, 1361-1365] have demonstrated that the upper disparity limit for stereopsis (DMax) is considerably smaller in filtered noise stereograms than in isolated Gabor patches of the same spatial frequency. This discrepancy is not currently understood. Here, the solution of the correspondence problem for bandpass stereograms was further examined. On each trial observers were presented with two one-dimensional Gabor stimuli containing disparities of equal magnitude but opposite sign. Subjects were required to indicate which interval contained the crossed disparity stimulus. It was found that matching behaviour changed as a function of Gabor envelope size. As a function of disparity magnitude, performance cycled between mostly correct and mostly incorrect at large envelope sizes but was always correct at small envelope sizes. At intermediate envelope sizes performance was cyclical at small disparities but always correct at large disparities. The critical envelope size at which performance changed from mostly correct to mostly incorrect at 270 degrees phase disparity was used as a measure of the matching performance as other parameters of the Gabor were varied. Both absolute and relative contrast were shown to influence the perceived sign of matches. Critical envelope size was also found to decrease as a function of spatial frequency, but more slowly than a phase-based limit would predict. These data cannot be predicted by current models of stereopsis, and can be used to constrain future models.

Contrast Sensitivity↗
PubMed2003

Computer-enhanced stereoscopic vision in a head-mounted operating binocular.

Based on the Varioscope, a commercially available head-mounted operating binocular, we have developed the Varioscope AR, a see through head-mounted display (HMD) for augmented reality visualization that seamlessly fits into the infrastructure of a surgical navigation system. We have assessed the extent to which stereoscopic visualization improves target localization in computer-aided surgery in a phantom study. In order to quantify the depth perception of a user aiming at a given target, we have designed a phantom simulating typical clinical situations in skull base surgery. Sixteen steel spheres were fixed at the base of a bony skull, and several typical craniotomies were applied. After having taken CT scans, the skull was filled with opaque jelly in order to simulate brain tissue. The positions of the spheres were registered using VISIT, a system for computer-aided surgical navigation. Then attempts were made to locate the steel spheres with a bayonet probe through the craniotomies using VISIT and the Varioscope AR as a stereoscopic display device. Localization of targets 4 mm in diameter using stereoscopic vision and additional visual cues indicating target proximity had a success rate (defined as a first-trial hit rate) of 87.5%. Using monoscopic vision and target proximity indication, the success rate was found to be 66.6%. Omission of visual hints on reaching a target yielded a success rate of 79.2% in the stereo case and 56.25% with monoscopic vision. Time requirements for localizing all 16 targets ranged from 7.5 min (stereo, with proximity cues) to 10 min (mono, without proximity cues). Navigation error is primarily governed by the accuracy of registration in the navigation system, whereas the HMD does not appear to influence localization significantly. We conclude that stereo vision is a valuable tool in augmented reality guided interventions.

Depth Perception↗
PubMed2001

Perceptually bistable three-dimensional figures evoke high choice probabilities in cortical area MT.

The role of the primate middle temporal area (MT) in depth perception was examined by considering the trial-to-trial correlations between neuronal activity and reported depth sensations. A set of moving random dots portrayed a cylinder rotating about its principal axis. In this structure-from-motion stimulus, the direction of rotation is ambiguous and the resulting percept undergoes spontaneous fluctuations. The stimulus can be rendered unambiguous by the addition of binocular disparities. We trained monkeys to report the direction of rotation in a set of these stimuli, one of which had zero disparity. Many disparity-selective neurons in area MT are selective for the direction of rotation defined by disparity. Across repeated presentations of the ambiguous (zero-disparity) stimulus, there was a correlation between neuronal firing and the reported direction of rotation, as found by Bradley et al. (1998). Quantification of this effect using choice probabilities (Britten et al., 1996) allowed us to demonstrate that the correlation cannot be explained by eye movements, behavioral biases, or attention to spatial location. MT neurons therefore appear to be involved in the perceptual decision process. The mean choice probability (0.67) was substantially larger than that reported for MT neurons in a direction discrimination task (Britten et al., 1996). This implies that MT neurons make a different contribution to the two tasks. For the depth task, either the pool of neurons used is smaller or the correlation between neurons in the pool is larger.

Action Potentials↗
PubMed1993

Long-term dysfunctions of neural stereoscopic mechanisms after unilateral extraocular muscle proprioceptive deafferentation.

1. Neural correlates of the permanent deficits in depth perception that occur when extraocular muscle proprioceptive (EMP) afferents are interrupted unilaterally in kittens were investigated by performing extracellular recordings in the primary visual cortex (area 17) in adulthood. Unilateral section of the ophthalmic branch of the trigeminal nerve (V1 nerve) were performed in 11 cats when they were between 5 and 12 weeks of age (uni-V1 group). Electrophysiological results were compared with those obtained in 17 normal adult cats (control group). 2. Binocular interactions were assessed by testing the sensitivity of cortical neurons to dichoptic presentations of moving sine-wave gratings whose interocular positional phase relationship was randomly varied. The amplitude modulation between the minimum and the maximum binocular responses defined the dynamic range. The degree of binocular suppression or facilitation was assessed by comparing these binocular response limits with the optimal monocular responses evoked through either eye at the best spatial frequency. The variability of both monocular and binocular responses was estimated by using the variation coefficient. 3. In uni-V1 cats, both the dynamic range and the degree of binocular suppression were significantly less pronounced than in controls, whereas binocular facilitation was not affected. The variability of the binocular responses was significantly increased, unlike monocular responses, whose variability was similar to control values. 4. From Fourier analysis of the poststimulus time histograms, two clear-cut categories of cells emerged that were differentially affected in the uni-V1 group. The "modulated" cells showed significantly less binocular suppression than in controls, and the "unmodulated" cells had binocular responses that were significantly more variable than in controls. Results from "simple" cells were similar to those of modulated cells, and results from "complex" cells were similar to those of unmodulated cells. However, in the unmodulated population, which was composed of both simple and complex cells, it was shown that the increase of variability was due to that of complex cells. 5. A nonparametric statistical test was applied on the interocular phase shift tuning curves to determine the minimum stimulus change necessary to elicit a significant change in the neural response. Two categories of cells were determined: the "discriminative" cells (80% in controls but 45% in uni-V1 cats) combined pronounced binocular suppression and dynamic range with relatively low variability. The reverse was true in the case of "nondiscriminative" cells (20% in controls and 55% in uni-V1 cats). 6. In uni-V1 cats, about half of the cells were monocularly activated.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1998

Perception and action in depth.

Little is known about distance processing in patients with posterior brain damage. Although many investigators have claimed that distance estimates are normal or abnormal in some of these patients, many of these observations were made informally and the examiners often asked for relative, and not absolute, distance estimates. The present investigation served two purposes. First, we wanted to contrast the use of distance information in peripersonal space for perceptual report as opposed to visuomotor control in our visual form agnosic patient, DF. Second, we wanted to see to what extent her abilities to process distance cues were dependent on binocular vision, in light of Milner et al.'s (1991) observations of preserved stereopsis in DF, and Dijkerman et al.'s (1996) and Marotta et al.'s (1997) observations that her visual guidance of grasping may be particularly dependent on binocular vision of the target. We hypothesized that DF's visuomotor responses would show normal sensitivity to target distance, while her perceptual estimates would not. In the first experiment, we required DF and two age- and sex-matched control subjects to reach out and grasp black cubes placed at varying distances, or to estimate the distance of the cubes from the hand starting position without making a reaching movement. In the second experiment, we required DF and two age-matched control subjects to point as rapidly and accurately as possible to small LED targets which differed in spatial location, under binocular and monocular conditions. The results showed that, relative to the control subjects, DF's grasping movements produced normal peak velocity-distance scaling-when she reached for blocks which varied in depth or pointed to LED targets which were presented at different distances in depth. In contrast, in the cube experiment, her verbal estimates of object distance were poorly scaled, although they improved slightly under the binocular conditions. The results are discussed in terms of current theories of processing streams in extrastriate visual cortex and the distinction between categorical and coordinate spatial processing.

Agnosia↗
PubMed2002

Absolute distance perception during in-depth head movement: calibrating optic flow with extra-retinal information.

We investigated the ability of monocular human observer to scale absolute distance during sagittal head motion in the presence of pure optic flow information. Subjects were presented at eye-level computer-generated spheres (covered with randomly distributed dots) placed at several distances. We compared the condition of self-motion (SM) versus object-motion (OM) using equivalent optic flow field. When the amplitude of head movement was relatively constant, subjects estimated absolute distance rather accurately in both the SM and OM conditions. However, when the amplitude changed on a trial-to-trial basis, subjects' performance deteriorated only in the OM condition. We found that distance judgment in OM condition correlated strongly with optic flow divergence, and that non-visual cues served as important factors for scaling distances in SM condition. Absolute distance also seemed to be better scaled with sagittal head movement when compared with lateral head translation.

Adolescent↗
PubMed1993

The perception of movement and depth in moiré patterns.

Moiré patterns can produce striking movement effects and in more complex stimuli can induce vivid stereoscopic depth. The physical rules underlying these phenomena are reviewed and their relationship to psychophysics is discussed. First, it is shown how moirés in 'optical line interference' patterns are created by superimposing periodic visual stimuli, eg gratings, and shifting them relative to each other. When two gratings are presented in this manner, small differences in spatial frequency, orientation, and speed are magnified. This magnification has prompted the use of moiré patterns both in industry and in art where their enhanced sensitivity to misalignment and spatial distortion has been widely exploited. Next, it is demonstrated how enhanced depth in 'stereoscopic interference' patterns is produced by presenting grating stimuli in two (or more) depth planes. The perceived depth effect in the resulting moiré pattern can be elicited similarly by binocular disparity and motion parallax. Finally, it is described how perceived movements occurring in different directions and at different depths are the basis for the perceptual 'irritations' that fascinate observers in complex moiré patterns. The use of moirés for the noninvasive examination of the human retina by aliasing is discussed.

Contrast Sensitivity↗
PubMed1975

The relation between discrimination and sensitivity in the perception of motion in depth.

1. Binocular discrimination of the direction of a target's motion in depth was measured in terms of the smallest angular difference in direction that could be detected with a probability 50% better than chance. Directional discrimination was measured for targets moving along 16 different trajectories directed to the left and right of the nose. 2. The relative velocities of the retinal images in the left and right eyes gave a sensitive cue to the direction of the target's motion in depth. 3. The direction of motion was bets discriminated when the target moved along a line directed close to the nose. A change in direction of only 0.2 degrees from this direction of motion could be detected. Discrimination showed two other maxima, one on each side of the central maximum. Discrimination fell to about 0.6-0.8 degrees when the target's direction was changed by only 6 degrees to either side of the nose. 4. The curve of sensitivity to movement in depth had a generally inverse shape to the directional discrimination curve: sensitivity was minimal for trajectories directed near the nose and increased for trajectories directed so as to miss the head. 5. The directional discrimination curve can be related to the sensitivity curves of the four postulated neural mechanisms tuned to different directions of motion in depth; there are three discrimination maxima and, correspondingly, three trajectories for which the slopes of adjacent sensitivity curves differ maximally. This suggests that binocular psychophysical judgements of the direction along which a target moves in depth are to some extent mediated by neural mechanisms that compare (e.g. subtract) the outputs of directionally tuned movement detectors. One function of such neural comparators might be to enhance psychophysical sensitivity to the direction along which a target moves in depth, and thus to provide a physiological basis for precisely judging whether or not an object will hit the head. 6. We suggest that the neural basis for judging the direction of moving objects has an analogy in colour vision where opponent-colour mechanisms enhance sensitivity to wave-length differences in such a way that wave-lengths are more easily discriminated in those parts of the spectrum where the slopes of the pigment action spectra differ maximally.

Adaptation, Ocular↗