Perceiving in Depth, Volume 2: Stereoscopic Vision

Magnitude, precision, and realism of depth perception in stereoscopic vision
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Progress in retinal and eye research 18 6 , , Handbook of perception and human performance, Human spatial orientation. Third line shows individual and combined visual direction codes for the three types of neurons.

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Motion-dependent filling-in of spatiotemporal information at the blind spot. The most drastic version of this is pseudoscopy , in which the half-images of stereograms are swapped between the eyes, reversing the binocular disparity. Under conditions of three-dimensional perception, the average value of the analogous ratio is 1. EEG alpha power and creative ideation. Perception as Bayesian inference.

Now consider the aggregate of neuronal responses as stimuli to the two eyes are presented on corresponding points and then moved gradually away in disparity until fusion between the two images breaks and diplopia is perceived. When the stimuli are at corresponding points, the three classes binocular corresponding neurons and monocular right and left neurons derived from a single visual direction label are not in conflict, and the stimulus, encoded as the sum of all neurons responding, is seen as single. When a small disparity is introduced some binocular disparate neurons are stimulated, and the binocular corresponding neurons should cease responding.

But now the monocular right and monocular left neurons each are stimulated for a visual direction slightly to either side of the mean visual direction for the binocular disparate neurons see Fig. Thus, the two monocular visual directions, which would be discriminately different if presented singly, are integrated with a third set of responses from the binocular disparate neurons. There should, therefore, be a range of small disparities for which the binocular response gives a unitary perception of a fused stimulus.

Finally, a point is reached at which the disparity is increased beyond the range in which the binocular response can be integrated with the two monocular responses. Now each monocular response is associated with a different visual direction; therefore, two separate stimuli are perceived in diplopia.

What happens to the visual direction associated with the binocular disparate neurons? No ghost image is seen between the diplopic images when the disparity is large. It is possible that no visual direction was assigned in the first place, or that there is suppression of the visual direction of the disparate neurons by the monocular excitatory neurons. However, there is a more likely explanation. As discussed more fully under the Stereopsis heading, the binocular disparate neurons probably operate in a small range, essentially only in the region of fusion.

Larger disparities do not stimulate these neurons, so that the question of their visual direction would not arise when the disparities are beyond Panum's area.

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The variation in fusion limit as a function of eccentricity 79 is shown in Figure 17A. Variation in Panum's area of binocular fusion with retinal eccentricity. The field of geometric disparities of a flat plane viewed at 20 cm and slightly in front of the fixation point. This shows that relatively large disparities can occur in peripheral regions under conditions that might occur while reading or writing. Ogle KN: On the limits of stereoscopic vision. J Exp Psychol , ; B. Nakayama K: Geometric and physiological aspects of depth perception. Proc SPIE , This increase in fusion limit is adaptive from three standpoints.

One is that the size of retinal receptive fields and, hence, visual acuity both show a corresponding proportional change with eccentricity.

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It is appropriate for the size of Panum's area to be matched to the monocular grain of the retina at that point. The increase in Panum's area also is adaptive in terms of the binocular environment.

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The three-volume work Perceiving in Depth is a sequel to Binocular Vision and Stereopsis and to Seeing in Depth, both by Ian P. Howard and Brian J. Rogers. Editorial Reviews. About the Author. Ian P. Howard is Professor emeritus in the Centre for Vision Research at York University in Toronto. He is the co-author of.

Figure 17B shows the disparities produced by binocular viewing of a plane optimally slanted at the angle of the vertical horopter at a distance of 20 cm. This situation might be approximated by a person reading a book or looking at a flat-screen monitor at a comfortable distance. The disparities present at large distances from fixation are substantial and increase roughly in proportion to degree of eccentricity. A corresponding increase in Panum's area, therefore, allows a much larger region of such a plane to appear fused than would otherwise be the case.

The third reason why it is helpful to have fusion increasing with eccentricity is that it allows a degree of sensory cyclofusion. If Panum's area remained constant at all eccentricities, then the maximum interocular orientation difference between two lines that would remain fused would be only about 4 arc minutes for a line across the full extent of the retina, such as the horizon. As it is, the increase in Panum's area at large eccentricities allows fusion of orientation differences of as much as 2 degrees.

FUSION HOROPTER As an application of this idea of the range of sensory fusion, one can measure the range of fusion around the horopter of corresponding points to show the total region of space before the observer within which point stimuli will appear fused. This empirical fusion horopter is depicted in Figure 18 for the special case of symmetric fixation in the visual plane A and the general case of asymmetric fixation of the visual plane B.

The case for asymmetric fixation see Fig. The narrowing of Panum's area near fixation produces the thinning of the fusion horopter in this region. These rather strange forms represent the only regions of space that produce fused visual images of point sources of light under the selected conditions of fixation. It is also evident that blurred images will show a greater fusional range than sharply focused images.

In this manner, fusion depends on the spatial extent of the stimulus. More systematically, Tyler 76 has examined fusion as a function of size of the waves in a sinusoidal line stimulus. A sinusoidal wavy line was presented to one eye to be fused with a straight line in the other. When the stimuli were horizontal, the threshold for fusion remained reasonably constant Fig.

The maximum retinal disparity could be as much as 1 degree when the waves had a period of 30 degrees per cycle. These variations all occurred with the stimulus passing through the fovea.

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For horizontal disparity upper inset , fusion limit increases for stimuli with large cycles and decreases for stimuli with very small cycles. For vertical disparities lower inset , the fusion limit remains much more constant.

Perceiving in Depth, Volume 2: Stereoscopic Vision

Tyler CW: The spatial organization of binocular disparity sensitivity. Thus, the traditional concept of Panum's area as a fixed property or a particular retinal region must be replaced by the awareness that the fusional extent is strongly dependent on the stimulus used to measure it. Hence, the fusional horopter presented in the previous section is not a fixed range around the point horopter; the depictions in Figure 18 provide only an indication of the fusional range in the real world, which will expand and contract according to the objects present in the field and the optical characteristics of the eyes viewing them.

Von Helmholtz had experimented with fusion in stereograms illuminated by a microsecond electric spark. Woo examined the effect of duration systematically and found that fusion appeared to be complete by approximately 30 msec. This duration is probably the same as the luminance integration time under his conditions, so the speed of simple fusion seems to be limited mainly by the rate of integration of luminance. However, the fusion of complex targets is a very different matter. It is possible to generate fields of dynamically changing random dots that are identical in the two eyes and can be perceived as fused or dots whose positions are spatially uncorrelated between the two eyes, which are perceived as entirely unfused.

Fusion will persist even though the dots are rapidly changing, providing they always occupy instantaneous corresponding positions in the two eyes. Such a stimulus provides the opportunity to examine the speed of fusion and defusion in complex stimuli. A change from correlation correspondence to complementation between the eyes is not visible to either eye alone when the random dots are dynamically changing.

Julesz and Tyler used this paradigm to show that the minimum time required for fusion between two periods of unfused stimuli complemented fields was an average of 30 msec. But when they measured the time required to detect a break in fusion immediately followed by a return to the fused stimulus identical fields , the duration was about ten times shorter. This kind of temporal anisotropy was found to be a particular property of the fusion mechanism, and no equivalent effect occurred for a comparable stereoscopic task. Some extreme nonlinearity of binocular temporal processing would be required to account for this bias between the two types of binocular correlation threshold.

These fall into the categories of binocular summation, binocular rivalry and suppression, and stereopsis, each of which is considered separately. VEP amplitude shows partial binocular summation under most conditions of binocular corresponding stimulation, — whether the stimulus is a flickering field or an alternating pattern of some kind. Here complete summation is defined such that the binocular response is the algebraic sum of the two monocular responses, or the stimulus contrast required to produce a given response is half as great for binocular stimulation as compared with a monocular condition.

In fact, most of the cited studies report partial binocular summation on the order of 1. However, these studies all involved transient VEPs measured at a single peak. An earlier study by Spekreijse had used sinusoidal flicker of a uniform field for the stimulus.

This study revealed that saturation of the VEP occurred in many circumstances high-amplitude stimulation, which could eliminate any appearance of binocular summation. Often an appropriate choice of contrast and field size would reveal full 2. More detailed work using sinusoidal patterns flickering in counterphase at high rates e. Such binocular facilitation is presumed to reflect the activity of stereoscopic neurons. The first two classes are dealt with in the section on stereoscopic vision. They are not generally referred to as dichoptic, because the two retinal patterns are sufficiently similar as to be combined into a unified impression particularly for fused stereopsis.

The latter four classes are clearly dichoptic. There is not much to be said about diplopia, except as an indicator of the failure of fusion. As such, it has been included in the previous section. This section on dichoptic stimulation therefore covers the remaining topics: dichoptic fusion, binocular rivalry, binocular suppression, and binocular luster. Liu and associates found that orthogonal dichoptic gratings show complete perceptual summation for periods of up to 30 seconds after stimulus onset. The dichoptic summation lasts the longest for high spatial frequencies and near threshold contrasts, but at medium spatial frequencies e.

In a given region of retina, the image in one eye predominates while the other is suppressed and suddenly the suppressed image emerges into perception and dominates the region Fig. The opposite contrasts in each eye tend to switch at random between perception of the light and dark phases. They also exhibit a lustrous, shimmering quality. Note that there is a disparity between the center pentagons in each eye that may be perceived as a relative depth signal even though the contours are of opposite contrast at all points in the figure.

Perceiving in Depth, Volume 1 v. 1 Basic Mechanisms | Βιβλία Public

Binocular rivalry fluctuations are similar in many respects to fluctuations of attention and are widely supposed to be under voluntary control. Actually, a number of studies — has found that there is little voluntary control over which eye dominates at any given time. In fact, the fluctuations in rivalry are well described by a sequentially independent random variable with no periodicities, as though the arrival of each change in dominance had no effect on the occurrence of subsequent changes. A series of studies by Fox and co-workers on the characteristics of binocular rivalry has made some headway in localizing the site in the visual pathway at which rivalry operates.

Even though the localization is derived by inference from psychophysical evidence, the result is quite significant in determining the processes of binocular cooperation and their breakdown in pathologic conditions. Interocular suppression has a number of interesting characteristics. The suppression state is inhibitory. Test stimuli presented during suppression are attenuated relative to the same stimuli presented during dominance or during nonrivalry conditions.

This attenuation occurs for a variety of test probes and testing procedures, including forced-choice detection of incremental light flashes, forced-choice recognition of letter forms, and reaction time for detection of targets set into motion during suppression. The magnitude of the inhibitory effect varies among subjects and with stimulus conditions but is generally approximately a factor of three, a value frequently observed in studies of saccadic suppression and visual masking.

The inhibitory effect of suppression endures throughout the duration of the suppression phase, and the magnitude of the inhibition remains constant. The inhibitory suppression state acts nonselectively on all classes of test stimuli independent of their similarity to the rivalry stimulus.

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Evidence of nonselectivity is the attenuation of several different kinds of test probe stimuli. More systematic evidence of nonselectivity is provided by experiments that use a spatial frequency grating as a rivalry stimulus and then change either frequency or orientation of the grating during suppression while keeping mean luminance and contrast constant.

Changes in orientation of 45 degrees and of a factor of two or more in frequency remain undetected. These studies suggest that rivalry is a process that is independent of monocular pattern recognition but is triggered by a binocular mismatch and then continues with its own characteristics independent of most stimulus parameters. However, one factor that is very important is the stimulus effectiveness in each eye. The higher the stimulus strength in terms of luminance, contrast, or movement in one eye, the greater the suppression of the other eye. If the stimulus strength is increased in both eyes equally, the rate of alternation between the two increases.

Finally, two interesting experiments have explored the relationship between aftereffects of visual adaptation and rivalry suppression. Examples of visual aftereffects are the perceived motion obtained as an aftereffect of adaptation to a moving display, threshold elevation and perceived spatial frequency shift after adaptation to a grating. Perceptual occlusion of the stimulus during binocular rivalry did not affect the strength of these aftereffects, whereas equivalent physical occlusion of the stimulus reduced the aftereffect dramatically. Because these aftereffects are almost certainly cortical, binocular rivalry must be occurring at a higher level in the cortex.

Cobb and colleagues used a stimulus with vertical bars to the left eye and horizontal bars to the right eye, with pattern reversals at 12 Hz, degrees out of phase for the two eyes. The response changes from the phase appropriate to each eye were well correlated with the subjective responses, indicating changes in perceptual dominance at any given moment. No correlation was found between rivalry suppression and the amplitude of potentials evoked solely by luminance changes.

Similarly, Van der Tweel and co-workers found that perceptual suppression of a flickering pattern presented to one eye by a static pattern presented to the other eye was accompanied by almost complete suppression of the VEP from the stimulated eye. How do the VEP rivalry data accord with neurophysiology? The two are in conflict because the known physiology would suggest that during rivalry the monocular neurons for both eyes would be stimulated, whereas the VEP reflects the subjective suppression of one eye at a time. It therefore appears that binocular rivalry operates before the site at which the pattern VEP is generated at least for low frequencies of alternation.

The rivalry process must then inhibit the response of one set of monocular neurons at a time, producing the reduction in the VEP. It occurs in areas of uniform illumination in which the luminance or color of the reflected light is different for the two eyes. It was described by von Helmholtz and Panum as a kind of lustrous or shimmering surface of indeterminate depth see Fig. The lustrous appearance of surfaces like a waxed tabletop or a car body is largely attributable to binocular luster.

It results from the different position of partially reflected objects in the surface by virtue of the different position of the two eyes. This kind of lustrous appearance is distinct from both the shininess of a surface as seen by reflected highlights and from the clear depth image seen in a mirror, both of which may be depicted in a photograph corresponding to the image by a single eye. Viewed binocularly, the lustrous surface appears to have a translucent quality of depth due to diffusion from the surface as well as the partial reflected providing a fixation plane at which the partially reflected image usually has a large disparity and, hence, areas of binocular luminance difference.

That the phenomenon of binocular luster has been largely ignored except as an incidental observation is surprising in view of the fact that it is qualitatively different from depth, diplopia, or rivalry. The lustrous region is not localizable in depth, but it seems unitary and does not fluctuate in the manner of binocular rivalry. Binocular luster may also be observed in static and dynamic random-dot stereograms in which all the elements have opposite contrast in the two eyes. This remarkable performance is the most powerful yet demonstrated for an exclusively binocular cyclopean task and suggests that binocular luster is a phenomenon worth further study.

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