[The way of disparity encoding in visual system in stereopsis--position shift? Phase shift?].
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Animals↗
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Hyperacuity, superresolution and gap resolution in human stereopsis.
Different types of stereoscopic acuity were studied with tasks adapted from studies of visual direction acuity. Dynamic, random-element stereograms portraying multiple surfaces in depth and a temporal 2AFC procedure were used for all measurements. The three tasks required detection of a depth offset (Hyperacuity task), a depth-axis thickening (Superresolution task), and a depth-axis gap between surfaces (Gap Resolution task). Thresholds for the three tasks were on the order of 3 sec arc, 30 sec arc and 200 sec arc of retinal disparity, respectively. These results are comparable to those for the analogous visual direction tasks on which they were patterned, suggesting that the underlying judgments involved are similar. Results are used to estimate the intrinsic noise of horizontal disparity processing.
Depth Perception↗
Neural encoding of binocular disparity: energy models, position shifts and phase shifts.
Neurophysiological data support two models for the disparity selectivity of binocular simple and complex cells in primary visual cortex. These involve binocular combinations of monocular receptive fields that are shifted in retinal position (the position-shift model) or in phase (the phase-shift model) between the two eyes. This article presents a formal description and analysis of a binocular energy model with these forms of disparity selectivity. We propose how one might measure the relative contributions of phase and position shifts in simple and complex cells. The analysis also reveals ambiguities in disparity encoding that are inherent in these model neurons, suggesting a need for a second stage of processing. We propose that linear pooling of the binocular responses across orientations and scales (spatial frequency) is capable of producing an unambiguous representation of disparity.
Depth Perception↗
Size-disparity correlation in stereopsis at contrast threshold.
Contrast thresholds for 75% correct depth identification in narrow-band filtered random dot stereograms were determined for different center spatial frequencies and binocular disparities. Rigorous control over vergence was maintained during testing, and a forced-choice procedure was used. The resulting contrast sensitivity function for stereopsis revealed sensitivity over a greater range of disparities at low than at high spatial frequencies. Sensitivity peaked for large disparities at low spatial frequencies and for small disparities at high spatial frequencies. When disparities were converted to effective binocular phase differences, the variation of contrast sensitivity with phase followed a consistent pattern across spatial frequencies, with peak sensitivity occurring mainly for binocular phases of between 90 degrees and 180 degrees. These results have implications for the extent of spatial integration at the input to the disparity sensing mechanism. A model postulating a spread of positional disparities independent of the spatial frequency selectivity of disparity-sensitive units cannot account for the results. But the size-disparity correlation strongly evident in our data is predicted by certain models of stereopsis, such as phase disparity encoding. An ideal observer analysis is developed that demonstrates that our results were not forced by the nature of the stimulus employed; rather, the quantum efficiency for stereopsis at contrast threshold follows the size-disparity correlation.
Contrast Sensitivity↗
Wheatstone-Panum limiting case: occlusion, camouflage, and vergence-induced disparity cues.
We examined effects of binocular occlusion, binocular camouflage, and vergence-induced disparity cues on the perceived depth between two objects when two stimuli are presented to one eye and a single stimulus to the other (Wheatstone-Panum limiting case). The perceived order and magnitude of the depth were examined in two experimental conditions: (1) The stimulus was presented on the temporal side (occlusion condition) and (2) the nasal side (camouflage condition) of the stimulus pair on one retina so as to fuse with the single stimulus on the other retina. In both conditions, the separation between the stimulus pair presented to one eye was systematically varied. Experiment 1, with 16 observers, showed that the fused object was seen in front of the nonfused object in the occlusion condition and was seen at the same distance as the nonfused object in the camouflage condition. The perceived depth between the two objects was constant and did not depend on the separation of the stimulus pair presented to one eye. Experiment 2, with 45 observers, showed that the disparity induced by vergence mainly determined the perceived depth, and the depth magnitude increased as the separation of the stimulus pair was made wider. The results suggest that (1) occlusion provides depth-order information but not depth-magnitude information, (2) camouflage provides neither depth-order nor depth-magnitude information, and (3) vergence-induced disparity provides both order and magnitude information.
Cues↗
A normative study of step vergence in elementary schoolchildren.
The purpose of this study was to develop normative data for step vergence testing in children age 6-12. Such data are not currently available for this age group. Base-in and base-out step vergence testing with a prism bar was performed on 386 elementary schoolchildren who had all passed a modified clinical technique (MCT) screening. A new polarized target was developed which enabled us to detect suppression, as well as diplopia or blur. The findings suggest significant differences from normative data previously reported for adults. Because the data for 6-year-olds were different from the rest of the group, we suggest that lower norms be used for this age group. This study provides expected findings for step vergence testing outside the phoropter, which can be used clinically for the assessment of disparity vergence ranges in young children.
Child↗
Slant or occlusion: global factors resolve stereoscopic ambiguity in sets of horizontal lines.
Perceived slant was measured for horizontal lines aligned on one side and of varying lengths whose length disparity was either a constant linear amount for all lines (consistent with uniocular occlusion) or proportional to line length (consistent with global slant). Although the disparity of any line was ambiguous with respect to these two possibilities, slant of individual lines did not occur in the former case, but a subjective contour in depth was reported along the alignment. For proportional disparity of the set, global slant was seen. Adding a constant length to each line on the invalid eye for occlusion resulted in multiple slants. Smooth uniocular variations in alignment shape elicited subjective contours slanting or curving in depth. Global context can disambiguate the depth status of individual disparate lines.
Adolescent↗
Glass-pattern detection is tuned for stereo-depth.
We investigated the role of disparity information in the detection of global form. Glass patterns, which allow insight into processing at both local and global stages of form analysis, were used as stimuli. We determined how detection of concentric Glass patterns is affected by a disparity difference introduced between partner dots forming local dipoles (Experiment 1), and how detection is affected by the addition of randomly oriented dot-pairs (noise dots) at crossed and uncrossed disparities (Experiment 2). The first experiment showed that detection thresholds increased when partner dots were separated in depth at disparities greater than approximately 17 min arc; the second experiment showed that noise dots disrupted the detection of form if they were presented at disparities of between approximately +/-20 min arc from the Glass pattern's presentation depth plane. Our findings suggest that disparity information plays a role in the recovery of the image structure and, importantly, local and global form mechanisms were found to be selective for a small range of stereo-depths. We discuss the findings of our study in the light of current evidence indicating that a common neural substrate is responsible for the analysis of form and binocular disparity.
Attention↗
[Analysis of motor response with respect to the time course in cyclofusion].
Torsional eye movements during cyclofusional responses are recorded objectively and analyzed with respect to the time course of the eye movement until the subject perceives a fused image. Eight subjects whose age ranged from 24 to 35 years old were studied. Torsional disparity was presented stepwise by a synoptiscope. The Number of subjects whose motor response was stepped was less than that of the subjects whose motor response was not stepped. The time course of responses varied in individual. Moreover, the number of the subjects whose motor response showed cyclovergence was less than that of subjects whose motor response showed cycloversion. Fusional cyclovergence is reasonable to bring the disparate images within Panum's area efficiently. However our analysis shows that stepped response and cyclovergence is less in number and the motor response is variable in each individual. Therefore, we do not accept the explanation that the cyclofusional motor response simply brings the disparate images within Panum's area. We speculate that cyclofusional motor response is carried out by a feed-back loop through the cyclofusional sensory input to fuse the disparate images.
Adult↗
Interocular differences in contrast and spatial frequency: effects on stereopsis and fusion.
Anisometropia produces interocular differences in contrast and spatial frequency. The influence of these two parameters on Panum's fusional limit (PFL) and stereoscopic depth thresholds was investigated with sinusoidal gratings and one-dimensional band-pass-limited targets. Vertical fusion limits were unaffected by large interocular differences in contrast (40-10%) at two spatial frequencies (0.8 and 1.6 cpd). However, when tested with a low spatial frequency (0.8 cpd), stereothresholds increased 150% with an interocular difference in contrast as small as 50-25%. Stereoacuity was reduced less by differential contrast when tested with higher spatial frequencies (3.2 cpd). When tested with low spatial frequencies the stereothreshold was elevated more by reducing the contrast of one image than by equal contrast reductions of both ocular images. Stereothresholds appear to be elevated by binocular suppression evoked by interocular differences in contrast. Vertical as well as horizontal fusion limits decreased with increasing interocular size difference. Horizontal fusion limits fell off more gradually with increasing size difference than did vertical fusion limits, particularly at higher spatial frequencies (2.4 cpd). Similarly, stereothresholds increased with increasing interocular size differences. Changes in the fusion limit and stereothreshold that occur with interocular size differences are predicted from positional disparities between edge features rather than from differences in spatial frequency.
Contrast Sensitivity↗
Binocular coordination of eye movements during reading.
Binocular coordination of the eyes during reading was examined. Fixation disparity greater than one character occurred on 47% of fixations, with the disparity being predominantly uncrossed (39%), though a small proportion of fixations were crossed. The average magnitude of disparity, measured at the end of fixation, was 1.1 characters for all fixations. For the 47% of non-aligned fixations the average magnitude of disparity was 1.9 characters. Vergence movements that reduced fixation disparity occurred during fixations, and their magnitude was positively correlated with fixation duration. Finally, eye dominance did not modulate fixation disparity magnitude or the proportion of disparate fixations.
Dominance, Ocular↗
Geometric and induced effects in binocular stereopsis and motion parallax.
This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.
Depth Perception↗
Fixation disparity.
Fixation disparity may be a symptom of binocular stress or a purposeful error signal to drive vergence eye movements. Two similar units for detecting fixation disparity (Mallett and Sheedy units) are compared and evaluated. The Mallett unit was found to be more reliable, with similar readings in symptom-free subjects, whereas the Sheedy unit gave a larger spread of fixation disparity measurements and appears to be less useful than the Mallett unit for routine clinical use. The exact role of fixation disparity remains unresolved. Fixation disparity has two components in symptomatic subjects; the vergence signal component and an oculomotor imbalance component.
Adolescent↗
Size-disparity correlation in human binocular depth perception.
To use the small horizontal disparities between images projected to the eyes for the recovery of three-dimensional information, our visual system must first identify which feature in one eye's image corresponds with which in the other. The earliest level of disparity processing in primates (V1) contains cells that are spatial-frequency tuned. If such cells have a disparity range that covers only a single period of their mean tuning frequency, there will always be exactly one potential match within this range. Here, this 'size-disparity' hypothesis was tested by measuring the contrast sensitivity of stereopsis as a function of disparity for single bandpass-filtered items. It was found that thresholds were low and relatively constant up to disparities an order of magnitude larger than is predicted by this constraint. Furthermore, peak sensitivity was relatively independent of spatial frequency. A control experiment showed that binocular correlation of the carrier is necessary for this task. In a third experiment, the maximum disparity that supports threshold performance was compared for an isolated bandpass item and bandpass-filtered noise. This limit was found to be five times larger for the isolated stimuli. In summary, these findings show that the initial stage of disparity detection is not limited by the size-disparity constraint. For stimuli with multiple false targets, however, processes subsequent to this stage reduce the disparity range over which the correspondence problem can be solved.
Depth Perception↗
How is motion disparity integrated with binocular disparity in depth perception?
Two experiments presented motion disparity conflicting with binocular disparity to examine how these cues determined apparent depth order (convex, concave) and depth magnitude. In each experiment, 8 subjects estimated the depth order and depth magnitude. The first experiment showed the following. (1) The visual system used one of these cues exclusively in selecting a depth order for each display. (2) The visual system integrated the depth magnitude information from these cues by a weighted additive fashion if it selected the binocular disparity in depth order perception and if the depth magnitude specified by motion disparity was small relative to that specified by binocular disparity. (3) The visual system ignored the depth magnitude information of binocular disparity if it selected the motion disparity in depth order perception. The second experiment showed that these three points were consistent whether the subject's head movement or object movement generated motion disparity.
Adult↗
Influences of monocular image degradation on the monocular components of fixation disparity.
The monocular components of the forced vergence fixation disparity curve were evaluated for equal retinal images and for two kinds of monocular retinal image degradation. Computer-generated nonius lines were used to measure the fixation disparity curve components. Monocular defocus was obtained by placing convex lenses before one eye. In the second instance monocular image degradation was achieved by placing light-scattering filters before one eye. A small percentage of subjects displayed unequal distribution of the monocular components of fixation disparity with no image degradation. For both conditions of monocular image degradation, the distribution of the monocular components was found to be biased in favor of the clear image. However, defocus, which affects the high spatial frequencies relatively more than scatter, was found to have a greater effect on the distribution of the monocular components of fixation disparity than scatter.
Adult↗
Disparity coding in the cat: a comparison between areas 17-18 and area 19.
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Animals↗
[Can fixation disparity be detected reliably by measurement techniques according to H. J. Haase?].
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Convergence, Ocular↗