Binocular integration of partially occluded surfaces

Stereoscopic depth perception through foliage

Both humans and computational methods struggle to discriminate the depths of objects hidden beneath foliage. However, such discrimination becomes feasible when we combine computational optical synthetic aperture sensing with the human ability to fuse stereoscopic images. For object identification tasks, as required in search and rescue, wildlife observation, surveillance, and early wildfire detection, depth assists in differentiating true from false findings, such as people, animals, or vehicles vs. sun-heated patches at the ground level or in the tree crowns, or ground fires vs. tree trunks. We used video captured by a drone above dense woodland to test users' ability to discriminate depth. We found that this is impossible when viewing monoscopic video and relying on motion parallax. The same was true with stereoscopic video because of the occlusions caused by foliage. However, when synthetic aperture sensing was used to reduce occlusions and disparity-scaled stereoscopic video was presented, whereas computational (stereoscopic matching) methods were unsuccessful, human observers successfully discriminated depth. This shows the potential of systems which exploit the synergy between computational methods and human vision to perform tasks that neither can perform alone.

Contradictory influence of context on predominance during binocular rivalry

BACKGROUND

Binocular rivalry is a complex process characterised by alternations in perceptual suppression and dominance that result when two different images are presented simultaneously to the left and right eyes. It has been reported recently that the addition of contextual cues will promote the predominance of the context consistent rivalry target. In contrast to Levelt's second proposition (1965), this effect has been found to result exclusively from an increase in the dominance phase duration, while the suppression phase duration remains unaffected.

METHODS

Human subjects were simultaneously presented with a small (2 degrees ) disc consisting of gratings (four cycles per degree) of different orientations to the two eyes. Four experiments were conducted to ascertain the effects of background gratings and contextual colour information on target predominance and phase duration. For each of the four experimental conditions, the orientation and colour of the target gratings and surrounding contextual background were systematically manipulated.

RESULTS

In this study, we report an effect opposite to that of Levelt. Contradictory contextual information increases target predominance and phase duration during binocular rivalry. Our results demonstrate that it is possible to promote the dominance of the context contradictory percept with co-linearity, co-chromaticity and orientation cues. In line with previous studies involving context, we find that this effect on predominance is due to an increase in the duration of the dominance rather than the suppression phase.

DISCUSSION

We discuss our findings in respect to those from previous studies and consider high- and low-level processes that may be responsible for these apparently 'contradictory' roles of context on binocular rivalry. In addition, we discuss how the apparent 'anti-Levelt' effect of context can be reinterpreted in a manner that brings it back in line with Levelt's second proposition and raises the question of whether 'suppressability' plays a disproportionately large role in determining the duration of perceptual phases in binocular rivalry.

Natural statistics of depth edges modulate perceptual stability

Binocular fusion relies on matching points in the two eyes that correspond to the same physical feature in the world; however, not all world features are binocularly visible. Near depth edges, some regions of a scene are often visible to only one eye (so-called half occlusions). Accurate detection of these monocularly visible regions is likely to be important for stable visual perception. If monocular regions are not detected as such, the visual system may attempt to binocularly fuse non-corresponding points, which can result in unstable percepts. We investigated the hypothesis that the visual system capitalizes on statistical regularities associated with depth edges in natural scenes to aid binocular fusion and facilitate perceptual stability. By sampling from a large set of stereoscopic natural images with co-registered distance information, we found evidence that monocularly visible regions near depth edges primarily result from background occlusions. Accordingly, monocular regions tended to be more visually similar to the adjacent binocularly visible background region than to the adjacent binocularly visible foreground. Consistent with our hypothesis, perceptual experiments showed that perception tended to be more stable when the image properties of the depth edge were statistically more likely given the probability of occurrence in natural scenes (i.e., when monocular regions were more visually similar to the binocular background). The generality of these results was supported by a parametric study with simulated environments. Exploiting regularities in natural environments may allow the visual system to facilitate fusion and perceptual stability when both binocular and monocular regions are visible.

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.

"X-ray vision" and the evolution of forward-facing eyes

Why do our eyes face forward, and why do many mammals have eyes facing sideways? Here, we describe results suggesting that the degree of binocular convergence is selected to maximize how much the mammal can see in its environment. Mammals in non-cluttered environments can see the most around them with panoramic, laterally directed eyes. Mammals in cluttered environments, however, can see best when their eyes face forward, for binocularity has the power of "seeing through" clutter out in the world. Evidence across mammals closely fits the predictions of this "X-ray" hypothesis.

Depth from binocular half-occlusions in stereoscopic images of natural scenes

Over the past two decades psychophysical experiments have firmly established that binocular half-occlusions are useful sources of information for the human visual system. The existing literature has focused on simplified stimuli that have no additional cues to depth, apart from stereopsis. From this large body of work we can be confident that the visual system is able to exploit binocular half-occlusions to aid depth perception; however, we do not know if this signal has any influence on perception when observers view complex stereoscopic stimuli with multiple sources of depth information. This issue is addressed here with the use of stereoscopic images of natural scenes, some of which have been digitally altered to manipulate a major half-occlusion signal. Our results show that depth-ordering judgments for these relatively complex stimuli are significantly affected by the nature of the half-occlusion signal, but only when highly textured surfaces are viewed. Under such conditions, the replacement of a binocular half-occlusion with background texture slows reaction time relative to performance when the occluded region is consistent with the foreground object. This result is specific to conditions when the depth ordering is correct (ie not reversed) and depends upon the size of the half-occlusion. The influence of the half-occlusion information in the presence of potent depth cues such as perspective, texture gradient, shading, and interposition is convincing evidence that this information plays a significant role in human depth perception.

Staying focused: a functional account of perceptual suppression during binocular rivalry

Presenting different images to either eye can induce perceptual switching, with alternating disappearances of each image--a phenomenon called binocular rivalry. We believe that disappearances during binocular rivalry can be driven by a process that facilitates visibility near the point of fixation. As the point of fixation is tied neither to a particular stimulus nor to a specific eye, indifference to both would be an essential characteristic for the process we envisage. Many factors that influence disappearances during binocular rivalry scale with distance in depth from fixation. Of these, here we use blur. We break the links between this cue and both eye of origin and stimulus type. We find that perceptual dominance can track a better focused image as it is swapped between the eyes and that perceptual switches can be driven by alternating the focus of images fixed in each eye. This implies that, as a determinant of suppression selectivity, blur is functionally independent from both eye of origin and stimulus type. Our data and theoretical account suggest that binocular rivalry is not an irrelevant laboratory curiosity but, rather, that it is a product of a functional adaptation that promotes visibility in cluttered environments.

Monocular transparency and unpaired stereopsis

Howard and Duke [Howard, I. P. & Duke, P. A. (2003). Monocular transparency generates quantitative depth. Vision Research, 43, 2615-2621] recently proposed a new source of binocular information they claim is used to recover depth in stereoscopic displays. They argued that these displays lack conventional disparity and that the metrical depth experienced results from transparency rather than occlusion relations. Using a variety of modified versions of their stimuli, we show here that the conditions for transparency are not required to elicit the depth experienced in their stereograms. We demonstrate that quantitative and precise depth depended not on the presence of transparency but horizontal contours of the same contrast polarity. Depth was attenuated, particularly at larger target offsets, when horizontal contours had opposite contrast polarity for at least a portion of their length. We also show that a demonstration they used to control for the role of horizontal contours can be understood with previously identified mechanisms involved in the computations associated with stereoscopic occlusion. These results imply that the findings reported by Howard and Duke can be understood with mechanisms responsible for the computation of binocular disparity and stereoscopic occlusion.

Monocular transparency and unpaired stereopsis

Howard and Duke [Howard, I. P. & Duke, P. A. (2003). Monocular transparency generates quantitative depth. Vision Research, 43, 2615-2621] recently proposed a new source of binocular information they claim is used to recover depth in stereoscopic displays. They argued that these displays lack conventional disparity and that the metrical depth experienced results from transparency rather than occlusion relations. Using a variety of modified versions of their stimuli, we show here that the conditions for transparency are not required to elicit the depth experienced in their stereograms. We demonstrate that quantitative and precise depth depended not on the presence of transparency but horizontal contours of the same contrast polarity. Depth was attenuated, particularly at larger target offsets, when horizontal contours had opposite contrast polarity for at least a portion of their length. We also show that a demonstration they used to control for the role of horizontal contours can be understood with previously identified mechanisms involved in the computations associated with stereoscopic occlusion. These results imply that the findings reported by Howard and Duke can be understood with mechanisms responsible for the computation of binocular disparity and stereoscopic occlusion.

Binocular interaction and performance of visual tasks

Binocular vision implies the fusion of the right and left retinal images to perceive a single image. For this, interocular interaction is required. We measured the reaction times to carry out a visual fixation task in order to determine whether binocular interaction influences performance. Several combinations of test and distraction stimuli were monocularly and binocularly presented to one monkey and three human subjects. The overall median reaction times were 340 ms for the animal and 308, 342 and 381 for human subjects 1, 2 and 3 respectively. Reaction time was shorter when the test stimulus was presented binocularly. Moreover, we observed that the presence of a distraction stimulus increased the reaction time and that a correlated distraction stimulus had a greater influence on this increase than an uncorrelated distraction stimulus. These findings indicate that with binocular vision a more rapid performance of a visual task occurs.