Evidence of non-linear combination of stereo and motion information
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Current theories of depth-cue combination postulate independent modules in the visual system for processing separate cues to depth. Further, the outputs of these modules are associated with weights based on the reliabilities of the depth estimates, as described by the modified weak fusion (MWF) model (Landy et al., 1995). In this study, the MWF model was tested by asking observers to provide depth judgments from (i) stereo-alone, (ii) motion-alone, and (iii) stereo and motion combined. The observers' task was to judge whether the apparent elongation of a horizontally oriented cylinder was more or less deep than an apparently circular cylinder (ACC) — see Johnston et al. (1994). The MWF model predicts that the mean estimate of observers' settings in the combined-cue condition will be a linear combination of the depth magnitudes from the stereo-alone and motion-alone conditions, each weighted by a reliability measure computed from the variances of observers' judgments in the two conditions. Our results do not conform to the predictions of the MWF model: perceived depth judgements were overestimated in the combined-cue and motion- alone conditions whereas stereo-alone judgements were veridical, despite the greater variance (less reliability) of the motion-alone condition. On the other hand, the MWF model predicts that perceived depth in the combined-cue condition should be closer to the depth estimate based on the more reliable cue (stereo, in our investigation). The present results are predicted by a new theory (Domini, Vuong, and Caudek; Di Luca, Domini, and Caudek ) which postulates that the visual system takes into account natural covariation of depth cues.Stereopsis is the perception of depth generated from differences between the two eyes' views of a single image; this difference is termed disparity. When artists learn to draw, they are often instructed to close one eye; this defeats stereopsis and makes monocular depth cues—such as perspective, occlusion, and shading—more apparent. In this study, we tested the hypothesis that poor stereopsis might therefore be an asset to someone whose goal is to represent the three-dimensional world on two-dimensional surfaces (Livingstone & Conway, 2004).
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Monocular
Binocular disparity
Monocular vision
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Stereopsis, the perception of depth based on the disparity of the images projected to the retinas of the two eyes, is an important process in our three-dimensional world; however, 3–5% of the population is stereoblind or has seriously impaired stereovision. Here we provide evidence for the recovery of stereopsis through perceptual learning, the repetitive practice of a demanding visual task, in human adults long deprived of normal binocular vision. We used a training paradigm that combines monocular cues that were correlated perfectly with the disparity cues. Following perceptual learning (thousands of trials) with stereoscopic gratings, five adults who initially were stereoblind or stereoanomalous showed substantial recovery of stereopsis, both on psychophysical tests with stimuli that contained no monocular cues and on clinical testing. They reported that depth “popped out” in daily life, and enjoyed 3D movies for the first time. After training, stereo tests with dynamic random-dot stereograms and band-pass noise revealed the properties of the recovered stereopsis: It has reduced resolution and precision, although it is based on perceiving depth by detecting binocular disparity. We conclude that some human adults deprived of normal binocular vision can recover stereopsis at least partially.
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Binocular disparity
Perceptual Learning
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Stereopsis develops very early in life and is thought to be present in a normally developing child by six months of age. In order to develop stereopsis, multiple components of visual development must be intact including visual acuity and bifoveal fixation. Stereopsis is the most sensitive way to assess sensory fusion but can be unreliable in very young age groups due to difficulty understanding the test or instructions. It is best to choose an option with global stereopsis (high level cortical stereo), as local stereopsis may overestimate ability due to available monocular cues. Global is created using random dot stereograms (RDS) – computer-generated patterns to create a stereoscopic form, while local contains line stereograms which create horizontal retinal image disparity giving the perception of depth. Stereopsis can be affected by strabismus, amblyopia, and other binocular vision dysfunctions that interfere with visual efficiency (especially in school-age children). The chapter discusses the most commonly used clinical tests of global and local stereopsis.
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To determine the role of motion-in-depth perception and static stereopsis in strabismics, and factors associated with the perception of motion-in-depth.A total of 84 strabismic patients (including 57 intermittent exotropes, 12 constant exotropes and 15 esotropes) and 16 normal controls were recruited. Binocular fusion ability, static stereopsis and motion-in-depth perception were tested using the computer-generated stereoscopic stimuli. The correlations between these tests were analyzed.There was a significant correlation between motion-in-depth perception and static stereopsis in strabismics. Only patients with static stereopsis demonstrated the perception of motion-in-depth. A positive correlation was found between motion-in-depth perception thresholds and static stereopsis in intermittent exotropes. All participants in the control group had motion-in-depth perception and static stereopsis. The participants with stereopsis had significantly lower thresholds than those with strabismus (P<0.01). The perception of motion-in-depth varied with the type of strabismus: 87.7% of the intermittent exotropes exhibited motion-in-depth perception, whereas none of the constant strabismics were able to pass the motion-in-depth perception tests. The perception of motion-in-depth was correlated with the presence of binocular fusion.Motion-in-depth perception is correlated with static stereopsis in strabismics. The perception of motion-in-depth varies with the type of strabismus: only intermittent exotropes have the perception of motion-in-depth.
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An instrument is described which tests for the use of stereopsis by measuring how long it takes for depth perception to begin (its latency) after a transition from monocular to binocular vision. Starting at 1 s, the duration of binocular vision is reduced progressively toward a limit of 16 ms (or increased to a limit of 4 s) until the latency of stereopsis is found. The preliminary period of monocular vision acts as a probe for suppression of either eye and allows time for accommodation and vergence to stabilize before binocular vision begins. Typical results are presented.
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Vergence (optics)
Binocular disparity
Monocular vision
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Four experiments were conducted to examine the transition of depth selection by mutual inhibition among disparities. (a) With random-dot stereograms, the time required for stereoscopic depth disappearance under fixation was found to be an inverse function of the magnitude of disparity. (b) Measurement of the cumulative time for perception of ambiguous stereograms showed that the depth was perceived longer with relatively smaller disparity. (c) The perception of stereoscopic depth under fixation was more stable in random-dot target than in ambiguous target. (d) Stereoscopic depth reversal occurred more frequently under fixation than in free observation. These results suggest that the whole range of disparities might be detected simultaneously and that the inhibition mechanism might have some important role in stereopsis.
Binocular disparity
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Stereoscopic acuity
Monocular
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