Laurie M. Wilcox

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.

A reevaluation of the tolerance to vertical misalignment in stereopsis

The stereoscopic system tolerates some vertical misalignment of the images in the eyes. However, the reported tolerance for an isolated line stimulus (approximately 4 degrees) is greater than for a random-dot stereogram (RDS, approximately 45 arcmin). We hypothesized that the greater tolerance can be attributed to monoptic depth signals (E. Hering, 1861; M. Kaye, 1978; L. M. Wilcox, J. M. Harris, & S. P. McKee, 2007). We manipulated the vertical misalignment of a pair of isolated stereoscopic dots to assess the contribution of each depth signal separately. For the monoptic stimuli, where only one half-image was present, equivalent horizontal and vertical offsets were imposed instead of disparity. Judgments of apparent depth were well above chance, though there was no conventional disparity signal. For the stereoscopic stimuli, one element was positioned at the midline where monoptic depth perception falls to chance but conventional disparity remains. Subjects lost the depth percept at a vertical misalignment of between 44 and 88 arcmin, which is much smaller than the limit found when both signals were provided. This tolerance for isolated stimuli is comparable to the reported tolerance for RDS. We conclude that previous reports of the greater tolerance to vertical misalignment for isolated stimuli arose from the use of monoptic depth signals.

The Effect of Crosstalk on the Perceived Depth From Disparity and Monocular Occlusions

Crosstalk in stereoscopic displays is defined as the leakage of one eyes image into the image of the other eye. All popular commercial stereoscopic systems suffer from crosstalk to some extent. Studies show that crosstalk causes distortions, reduces image quality and visual comfort, and increases perceived workload. Moreover, there is evidence that crosstalk effects depth perception from disparity. In the present paper we present two experiments. The first addresses the effect of crosstalk on the perceived magnitude of depth from disparity. The second examines the effect of crosstalk on the magnitude of depth perceived from monocular occlusions. Our data show that crosstalk has a detrimental effect on depth perceived from both cues, but it has a stronger effect on depth from monocular occlusions. Our findings taken together with previous results suggest that crosstalk, even in modest amounts, noticeably degrades the quality of stereoscopic images.

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.

Coarse-fine dichotomies in human stereopsis

There is a long history of research into depth percepts from very large disparities, beyond the fusion limit. Such diplopic stimuli have repeatedly been shown to provide reliable depth percepts. A number of researchers have pointed to differences between the processing of small and large disparities, arguing that they are subserved by distinct neural mechanisms. Other studies have pointed to a dichotomy between the processing of 1st- and 2nd-order stimuli. Here we review literature on the full range of disparity processing to determine how well different proposed dichotomies map onto one another, and to identify unresolved issues.

The role of monocularly visible regions in depth and surface perception

The mainstream of binocular vision research has long been focused on understanding how binocular disparity is used for depth perception. In recent years, researchers have begun to explore how monocular regions in binocularly viewed scenes contribute to our perception of the three-dimensional world. Here we review the field as it currently stands, with a focus on understanding the extent to which the role of monocular regions in depth perception can be understood using extant theories of binocular vision.

Personal space in virtual reality

Improving the sense of “presence” is a common goal of three-dimensional (3D) display technology for film, television, and virtual reality. However, there are instances in which 3D presentations may elicit unanticipated negative responses. For example, it is well established that violations of interpersonal space cause discomfort in real-world situations. Here we ask if people respond similarly when viewing life-sized stereoscopic images. Observers rated their level of comfort in response to animate and inanimate objects in live and virtual (stereoscopic projection) viewing conditions. Electrodermal activity was also recorded to monitor their physiological response to these stimuli. Observers exhibited significant negative reactions to violations of interpersonal space in stereoscopic 3D displays, which were equivalent to those experienced in the natural environment. These data have important implications for the creation of 3D media and the use of virtual reality systems.

Stereoscopic transparency: constraints on the perception of multiple surfaces.

Stereo-transparency is an intriguing, but not well-understood, phenomenon. In the present experiment, we simultaneously manipulated the number of overlaid planes, density of elements, and depth separation between the planes in random dot stereograms to evaluate the constraints on stereoscopic transparency. We used a novel task involving identification of patterned planes among the planes constituting the stimulus. Our data show that observers are capable of segregating up to six simultaneous overlaid surfaces. Increases in element density or number of planes have a detrimental effect on the transparency percept. The effect of increasing the inter-plane disparity is strongly influenced by other stimulus parameters. This latter result can explain a difference in the literature concerning the role of inter-plane disparity in perception of stereo-transparency. We argue that the effects of stimuli parameters on the transparency percept can be accounted for not only by inhibitory interactions, as has been suggested, but also by the inherent properties of disparity detectors.

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.

On the typical development of stereopsis: Fine and coarse processing

Stereoscopic depth perception may be obtained from small retinal disparities that can be fused for single vision (fine stereopsis), but reliable depth information is also obtained from larger disparities that produce double vision (coarse stereopsis). While there is some evidence that stereoacuity improves with age, little is known about the development and maturation of coarse stereopsis. Here we address this gap by assessing the maturation of stereoscopic depth perception in children (4-14 years) and adults over a large range of disparities from fused (fine) to diplopic (coarse). The observers task was to indicate whether a stereoscopic cartoon character was nearer or farther away than a zero-disparity reference frame. The test disparities were grouped into fine (0.02, 0.08, 0.17, 0.33, 0.68, 1.0 deg) and coarse (2.0, 2.5, 3.0, 3.5 deg) ranges based on an initial determination of the diplopia threshold for each observer. Next, percent correct depth direction was determined as a function of disparity. In the coarse range, accuracy decreased slightly with disparity and there were no differences as a function of age. In the fine range, accuracy was constant across all disparities in adults and increased with disparity in children of all ages. Performance was immature in all children at the finest disparity tested. We conclude that stereopsis in the coarse range is mature at 4 years of age, but stereopsis in the fine range, at least for small disparities, continues to mature into the school-age years.