Representation and measurement of stereoscopic volumes.

Binocular disparity information provides the human visual system with a basis for the compelling perception of both three-dimensional (3-D) object shape, and of the 3-D space between objects. However, while an extensive body of research exists into the perception of disparity-defined surface shape, relatively little research has been conducted on the associated perception of disparity-defined volume. In this paper, we report three experiments that examine this aspect of binocular vision. Participants were asked to make judgments about the 3-D spread, location-in-depth, and 3-D shape of stereoscopic volumes. Volumes were comprised of random dots with disparities drawn from a uniform distribution, a Gaussian distribution, or a combination of both. These results were compared to two models: One of these made judgments about stereoscopic volumes using information about the distributions of disparities in each stimulus, while the other was limited to only maximum and minimum disparity information. Psychophysical results were best accounted for by the maximum-minimum decision rule model. This suggests that, although binocular vision affords a compelling phenomenal sense of 3-D volume, when required to make judgments about such volumes, the visual system's default strategies make only limited use of available binocular disparity signals.

The effects of blur and size on monocular and stereoscopic localization.

Monocular localization of non-abutting stimuli and stereoscopic localization of the same second-order targets are performed with the same precision (Wilcox, L.M. & Hess, R.F. (1996) Is the site of non-linear filtering in stereopsis before or after binocular combination? Vision Research, 36, 391-399). Further, both tasks show a similar dependence on the scale of the stimulus. Since prior studies used Gaussian-enveloped stimuli, modifications of stimulus scale produced concurrent changes in edge blur. The experiments reported here assess the relative contributions of size and blur to the observed dependence on envelope scale for both monocular localization and stereoacuity. Stereoacuity for first-order targets was found to be an order of magnitude better than stereoacuity for second-order targets and monocular acuity for both first- and second-order targets. Further, while first-order stereopsis was found to depend solely on blur, second-order stereoacuity and monocular acuity were affected by both size and blur. These results suggest that while stereoacuity for first-order stimuli may be determined by a correlative process limited by early additive noise, stereoacuity for second-order stimuli and monocular acuity for non-abutting targets are more likely limited by stimulus-dependent spatial subsampling.

First and second-order contributions to surface interpolation.

Comparisons of 1st- and 2nd-order stereopsis have typically employed isolated, or local, narrow-band targets. While these experiments have revealed a great deal about the distinction between these two types of processing, such stimuli are rare in the natural environment. Instead, local disparity signals are more likely to be part of extended surfaces that very smoothly in depth. The aim of the experiments presented here is to determine the relative contribution of 1st- and 2nd-order stereopsis to the perception of depth-modulated surfaces. Stereothresholds were measured under a range of conditions designed to isolate either 1st- or 2nd-order processing. The results demonstrate that while 2nd-order stereopsis provides local depth estimates for individual texture elements, 1st-order processing is essential to the global interpolation of those estimates across surfaces.

Comparison of motion and stereopsis: linear and nonlinear performance.

To address the issue of whether the luminance-dependent (linear) and contrast-dependent (nonlinear) processes in stereo and motion have a common computational basis, we compare both carrier-dependent and envelope-dependent performance for these two modalities by using the same stimulus and task: two-flash apparent motion/depth for a wide range of displacements. We do this for different densities, bandwidths, contrasts, spatial frequencies, and exposure durations. The results suggest that there is concordance not only between the luminance-dependent (linear) processes of motion and stereo but also between the envelope-dependent (nonlinear) processes of both modalities. Only one exception was found, but we show this to be amenable to an explanation based on a different contrast dependence for the nonlinear mechanisms of stereo and motion. This suggests that the computational basis of linear and nonlinear processes may be similar for stereopsis and motion.

When stereopsis does not improve with increasing contrast.

It is well known that stereoacuity for conventional (1st-order) stimuli improves with increasing contrast with an approximate slope of -0.5 on log-log axes (Halpern DL, Blake RR. Perception 1988;17:483-495; Legge GE, Gu Y. Vis Res 1989;29:989-1004). In the experiments reported here a variety of stimuli were used (Gabor patches, amplitude modulated stimuli and 1D noise patches) and tasks (stereoacuity and Dmax) to determine if 2nd-order stereopsis shows a similar square root dependence. The results consistently demonstrate that the effect of contrast on stereopsis is quite different for the 2nd-order stimuli. Increases in stimulus contrast have little effect on performance; the resulting slopes are very shallow. The pattern of results is similar when the interocular contrast ratio is varied, demonstrating that 2nd-order processing is more resilient to stimulus differences in the two eyes than 1st-order.

Scale selection for second-order (non-linear) stereopsis.

In addition to the conventional luminance spatial frequency-dependent, disparity processing mode, there is a second-order luminance spatial frequency-independent type of processing available to the stereoscopic system. Here we use gaussian-enveloped, amplitude-modulated grating patches to determine how the stereoscopic system responds to the presence of two sources of second-order disparity information at different scales when there is no disparity information available via the conventional luminance-based system. In the first experiment we show that the stereoscopic system uses the disparity signal provided by the stimulus envelope, even though it is at a coarser scale than that provided by the amplitude modulation (AM). We then demonstrate that if the stimulus envelope is degraded via blurring, or if it is fixed at zero disparity, then performance depends on the finer-scale AM disparity signal. To show that the stereoscopic system uses the disparity signal provided by the AM we extend the carrier grating outside the borders of the AM stimulus, thereby making the boundary of the patch less discernible. Results obtained using this stimulus suggest that when two sources of second-order disparity information are present within the same stimulus (i.e., with no reliable luminance-based disparity signal available), the disparity signal provided by the coarser-scale contrast envelope vetos the finer-scale disparity signal. The coarse-scale disparity information dominates as long at it provides an adequate disparity signal. When it is degraded, however, the finer-scale signal takes precedence.

The effect of dark and equiluminant occlusion on the interocular transfer of visual aftereffects.

Lehmkuhle and Fox [(1976) Vision Research, 16, 428-430] reported that interocular transfer (IOT) of a translational motion aftereffect (MAE) was greater if the non-adapting eye viewed an equiluminant field than if it viewed a dark field. They recommended equiluminant occlusion of the non-adapted eye when measuring IOT of aftereffects. We tested this proposal in three experiments. First, we assessed IOT with equiluminant and dark occlusion for three different classes of aftereffects. Although transfer was greater with equiluminant occlusion for the translational MAE, there was no significant difference in the amount of transfer for the tilt aftereffect or the contrast threshold elevation effect. Second, we tested the hypothesis that spuriously large IOT could be the result of an aftereffect from tracking eye movements in the non-adapting eye. When potential tracking movements were reduced by using rotating spokes, a rotating spiral or contracting concentric circles, there was a corresponding reduction in the occlusion-dependent transfer. Third, we found that luminance shifts had no influence on the amount of transfer when all contours were eliminated from the non-adapting eye. We conclude that the type of occlusion used for measuring IOT of the translational MAE is important only when visible contours in the non-adapting eye contribute to the adapting process.

Is the site of non-linear filtering in stereopsis before or after binocular combination?

There is recent evidence that both linear and non-linear filtering operations subserve stereoscopic localization. For example, for spatially band-pass stimuli, the overall Gaussian envelope, which is not explicitly represented by the output of linear filters, can provide coarse disparity information. Here we ask three questions about the nature of this non-linear processing in stereopsis. First, is the site of the non-linearity before or after binocular combination? Second, is the stimulus envelope extracted by orientation or non-orientation selective spatial filters? Finally, we ask whether the envelope-based 3-D localization performance is similar to that for monocular 2-D localization as would be the case if the localization of the monocular contrast envelope was common to both operations. Our results suggest that envelope extraction occurs before binocular combination and that the filters involved are orientation selective. Finally, we provide preliminary evidence that is compatible with the proposal that 3-D and 2-D localization use the same envelope extraction operations.

Dmax for stereopsis depends on size, not spatial frequency content.

Stereoacuity depends not only on the carrier frequency of Gabor stimuli, but also upon their size. To determine if this is also the case at large disparities, we have measured the upper limit for stereopsis, "Dmax", and assessed its dependence on carrier frequency and overall envelope size. The results differ markedly from the stereoacuity data. Dmax for stereopsis is primarily dependent on the size of the envelope of the Gabor patch, and is relatively independent of its carrier frequency. These results support the proposition that stereopsis is achieved at large disparities by way of non-linear processing (envelope extraction).

Linear and non-linear filtering in stereopsis.

To better understand the spatial filtering operations underlying stereopsis, and their relationship to those underlying monocular localization of the same stimuli, we examined the dependence of stereoacuity on carrier and envelope size of Gabor patches. For stimuli of broad spatial bandwidth, stereoacuity depends on the carrier spatial frequency whereas for stimuli of narrow bandwidth, stereoacuity depends on the modulation frequency. The dependence of stereoacuity on the separation of the reference elements differs for stimuli of broad and narrow spatial frequency bandwidths. These relationships suggests that stereopsis has access to two different types of information from the early filters which we term, linear and non-linear. This distinction is important not only for understanding the relationship between monocular and stereoscopic localization, but also for understanding the different filter operations underlying stereopsis.

On the contribution of a binocular 'AND' channel at contrast threshold.

Three experiments are reported in which an attempt was made to isolate the contribution of an AND channel by measuring aftereffects following alternating monocular adaptation. The first two were designed to test Wolf and Held's proposal that the binocular AND channel does not respond at contrast threshold. In the first experiment the relative sizes of monocular and binocular contrast threshold elevation were compared with the pattern of aftereffects obtained in a study of the suprathreshold tilt aftereffect. Identical patterns of results were obtained under the two adaptation conditions. In the second experiment, the monocular and binocular contrast-reduction aftereffect reported by Blakemore et al was measured over a wide range of reference contrasts. As in the previous experiment, the monocular effect was greater than the binocular effect. This occurred at all reference contrasts. These data support the conclusion that the AND channel contributes to visual performance in the same manner, irrespective of stimulus contrast. In the final experiment an alternative explanation for existing evidence against the existence of an AND channel was assessed.

Unilateral acute idiopathic maculopathy.

This is a report of nine patients who experienced sudden, severe, unilateral central vision loss following a flulike illness. Each patient had an exudative detachment of the macula. All patients experienced a spontaneous resolution of the acute macular manifestations with near-complete recovery of vision. A characteristic "bull's-eye" appearance in the macula persisted. The acute manifestations of the disorder did not recur in any of the patients during the period of follow-up. The constellation of findings was suggestive of an inflammatory disease of the retinal pigment epithelium, but a specific causative agent could not be identified. The acute clinical and angiographic features, the natural course, and the residual pigment epithelial derangement were not consistent with any previously described disorder.

Measurement of visual aftereffects and inferences about binocular mechanisms in human vision.

There is conflicting evidence concerning the characteristics of binocular channels in the human visual system with respect to the existence of a 'pure' binocular channel that responds only to simultaneous stimulation of both eyes. Four experiments were conducted to resolve these discrepancies and to evaluate the evidence for the existence of such an exclusive binocular channel. In the first three studies, tilt aftereffects were measured after monocular adaptation. The relative sizes of the direct, interocularly transferred, and binocular aftereffects were not influenced by the configuration of the adapting pattern (experiment 1), or by the eye used for adaptation (experiment 2). There were also consistent interobserver differences in the relative sizes of the aftereffect seen after monocular adaptation (experiment 3). Taken together, these data raise questions about the appropriateness of a monocular adaptation paradigm for evaluating the presence of a pure binocular channel in observers with normal binocular vision. In experiment 4, in which the paradigm of alternating monocular adaptation was used, data were obtained that are consistent with the presence of a pure binocular channel.

On the evidence for a 'pure' binocular process in human vision.

Wolfe (1986, Psychol. Rev. 93, 269-282) proposed a model of human binocular vision based on the assumption of two functionally distinct classes of binocular neuron. These neurons may be regarded as logical AND and OR gates. In the present paper we assess the evidence relevant to this assumption. We find that while both types of binocular neuron have been described in the cortex of cat and monkey, there is no indication that they form functionally separate populations. Critical analysis of the psychophysical evidence for AND and OR channels in human vision suggests that much of the data presented in favor of an AND channel is subject to alternative interpretations. We conclude that the available data are not consistent with the existence of separate channels as proposed by Wolfe.

Perfluoropropane gas in the management of proliferative vitreoretinopathy.

Perfluoropropane (C3F8), an expanding fluorinated hydrocarbon gas, was used as an adjunct to vitreous surgery in the management of 18 patients with retinal detachment complicated by proliferative vitreoretinopathy. Ten patients had reattached retinas six months after disappearance of the gas. The average intraocular longevity of the gas in successful cases was 92 days; that in failed cases was 62.7 days. This difference was not statistically significant. However, internal retinal tamponade by longer-lasting gases appears to increase the rate of retinal reattachment in the surgical management of proliferative vitreoretinopathy.

The perfluorocarbon gases in the treatment of retinal detachment.

Four straight chain perfluorocarbon gases that had been studied in animals were tried in 30 patients. The larger volumes injected into patients made for extended disappearance times. Two milliliters of CF4 diminished to half volume in six days and had a total disappearance time of 16 days; 0.8 ml of C2F6 diminished to half volume in 10 days and had a disappearance time of 40 days; 1 ml of C3F8 diminished to half volume in 35 days and disappeared in 70 days; 0.6 ml of C4F10 diminished to half volume in 45 days and disappeared in 120 days. The expansion potential was estimated to be the same as measured in the animal model.

Primate ocular blood flow following contiguous recti tenotomy.

Tenotomy of the medial and superior recti resulted in decreased blood flow to only the superior quadrant of the unit iris, ciliary body with processes (ICBCP). Blood flow to the medial, lateral, and inferior quadrants of the ICBCP was maintained. These data support the contention that tenotomy initiates a compensatory increase in medial quadrant blood flow, probably via the medial long posterior ciliary artery. The superior quadrant appears to be the least protected from ischemia immediately following recti tenotomy.