Perceiving depth and motion in depth from successive occlusion.

Occlusion, or interposition, is one of the strongest and best-known pictorial cues to depth. Furthermore, the successive occlusions of previous objects by newly presented objects produces an impression of increasing depth. Although the perceived motion associated with this illusion has been studied, the depth percept has not. To investigate, participants were presented with two piles of disks with one always static and the other either a static pile or a stacking pile where a new disk was added every 200 ms. We found static piles with equal number of disks appeared equal in height. In contrast, the successive presentation of disks in the stacking condition appeared to enhance the perceived height of the stack-fewer disks were needed to match the static pile. Surprisingly, participants were also more precise when comparing stacking versus static piles of disks. Reversing the stacking by removing rather than adding disks reversed the bias and degraded precision. In follow-up experiments, we used nonoverlapping static and dynamic configurations to show that the effects are not due to simple differences in perceived numerosity. In sum, our results show that successive occlusions generate a greater sense of height than occlusion alone, and we posit that dynamic occlusion may be an underappreciated source of depth information.

Differences in virtual and physical head orientation predict sickness during active head-mounted display-based virtual reality.

During head-mounted display (HMD)-based virtual reality (VR), head movements and motion-to-photon-based display lag generate differences in our virtual and physical head pose (referred to as DVP). We propose that large-amplitude, time-varying patterns of DVP serve as the primary trigger for cybersickness under such conditions. We test this hypothesis by measuring the sickness and estimating the DVP experienced under different levels of experimentally imposed display lag (ranging from 0 to 222 ms on top of the VR system's ~ 4 ms baseline lag). On each trial, seated participants made continuous, oscillatory head rotations in yaw, pitch or roll while viewing a large virtual room with an Oculus Rift CV1 HMD (head movements were timed to a computer-generated metronome set at either 1.0 or 0.5 Hz). After the experiment, their head-tracking data were used to objectively estimate the DVP during each trial. The mean, peak, and standard deviation of these DVP data were then compared to the participant's cybersickness ratings for that trial. Irrespective of the axis, or the speed, of the participant's head movements, the severity of their cybersickness was found to increase with each of these three DVP summary measures. In line with our DVP hypothesis, cybersickness consistently increased with the amplitude and the variability of our participants' DVP. DVP similarly predicted their conscious experiences during HMD VR-such as the strength of their feelings of spatial presence and their perception of the virtual scene's stability.

Shape judgments in natural scenes: Convexity biases versus stereopsis.

Determining the relief of upcoming terrain is critical to locomotion over rough or uneven ground. Given the significant contribution of stereopsis to perceived surface shape, it should play a crucial role in determining the shape of ground surfaces. The aim of this series of experiments was to evaluate the relative contribution of monocular and binocular depth cues to judgments of ground relief. To accomplish this goal, we simulated a depth discrimination task using naturalistic imagery. Stimuli consisted of a stereoscopically rendered grassy terrain with a central mound or a dip with varying height. We measured thresholds for discrimination of the direction of the depth offset. To determine the relationship between relief discrimination and measures of stereopsis, we used two stereoacuity tasks performed under the same viewing conditions. To assess the impact of ambiguous two-dimensional shading cues on depth judgments in our terrain task, we manipulated the intensity of the shading (low and high). Our results show that observers reliably discriminated ground reliefs as small as 20 cm at a viewing distance of 9.1 m. As the shading was intensified, a large proportion of observers (30%) exhibited a strong convexity bias, even when stereopsis indicated a concave depression. This finding suggests that there are significant individual differences in the reliance on assumptions of surface curvature that must be considered in experimental conditions. In impoverished viewing environments with limiting depth cues, these convexity biases could persist in judgments of ground relief, especially when shading cues are highly salient.

Sensors for Fire and Smoke Monitoring.

Mastery of fire is intimately linked to advances in human civilization, culture and technology [...].

Contributions of Stereopsis and Aviation Experience to Simulated Rotary Wing Altitude Estimation.

OBJECTIVE: We examined the contribution of binocular vision and experience to performance on a simulated helicopter flight task. BACKGROUND: Although there is a long history of research on the role of binocular vision and stereopsis in aviation, there is no consensus on its operational relevance. This work addresses this using a naturalistic task in a virtual environment. METHOD: Four high-resolution stereoscopic terrain types were viewed monocularly and binocularly. In separate experiments, we evaluated performance of undergraduate students and military aircrew on a simulated low hover altitude judgment task. Observers were asked to judge the distance between a virtual helicopter skid and the ground plane. RESULTS: Our results show that for both groups, altitude judgments are more accurate in the binocular viewing condition than in the monocular condition. However, in the monocular condition, aircrew were more accurate than undergraduate observers in estimating height of the skid above the ground. CONCLUSION: At simulated altitudes of 5 ft (1.5 m) or less, binocular vision provides a significant advantage for estimation of the depth separation between the landing skid and the ground, regardless of relevant operational experience. However, when binocular cues are unavailable aircrew outperform undergraduate observers, a result that likely reflects the impact of training on the ability to interpret monocular depth cues.

Monovision: Consequences for depth perception from large disparities.

Recent studies have confirmed that monovision treatment degrades stereopsis but it is not clear if these effects are limited to fine disparity processing, or how they are affected by viewing distance or age. Given the link between stereopsis and postural stability, it is important that we have full understanding of the impact of monovision on binocular function. In this study we assessed the short-term effects of optically induced monovision on a depth-discrimination task for young and older (presbyopic) adults. In separate sessions, the upper limits of stereopsis were assessed with participants' best optical correction and with monovision (-1D and +1D lenses in front of the dominant and non-dominant eyes respectively), at both near (62 cm) and far (300 cm) viewing distances. Monovision viewing resulted in significant reductions in the upper limit of stereopsis or more generally in discrimination performance at large disparities, in both age groups at a viewing distance of 300 cm. Dynamic photorefraction performed on a sample of four young observers revealed that they tended to accommodate to minimize blur in one eye at the expense of blur in the other. Older participants would have experienced roughly equivalent blur in the two eyes. Despite this difference, both groups displayed similar detrimental effects of monovision. In addition, we find that discrimination accuracy was worse with monovision at the 3 m viewing distance which involves fixation distances that are typical during walking. These data suggest that stability during locomotion may be compromised, a factor that is of concern for our older participants.

Smoothness of stimulus motion can affect vection strength.

We examined the effect of the smoothness of motion on vection strength. The smoothness of stimulus motion was modulated by varying the number of frames comprising the movement. In this study, a horizontal grating translated through 360° of phase in 1 s divided into steps of 3, 4, 6, 12, 20, 30, or 60 frames. We hypothesized that smoother motion should induce stronger vection because the smoother stimulus is more natural and contains more motion energy. We examined this effect of frame number on vection for both downward (Experiment 1) and expanding (Experiment 2) optical flow. The results clearly showed that vection strength increased with increasing frame rate, however, the rates of increase in the vection strength with frame rate are not constant, but rapidly increase in the low frame-rate range and appear to asymptote in the high range. The strength estimates saturated at lower frame rates for expanding flow than for downward flow. This might be related to the fact that to process expanding flow it is necessary to integrate motion signals across the visual field. We conclude that the smoothness of the motion stimulus highly affects vection induction.

Airborne Optical and Thermal Remote Sensing for Wildfire Detection and Monitoring.

For decades detection and monitoring of forest and other wildland fires has relied heavily on aircraft (and satellites). Technical advances and improved affordability of both sensors and sensor platforms promise to revolutionize the way aircraft detect, monitor and help suppress wildfires. Sensor systems like hyperspectral cameras, image intensifiers and thermal cameras that have previously been limited in use due to cost or technology considerations are now becoming widely available and affordable. Similarly, new airborne sensor platforms, particularly small, unmanned aircraft or drones, are enabling new applications for airborne fire sensing. In this review we outline the state of the art in direct, semi-automated and automated fire detection from both manned and unmanned aerial platforms. We discuss the operational constraints and opportunities provided by these sensor systems including a discussion of the objective evaluation of these systems in a realistic context.

Vergence eye movements are not essential for stereoscopic depth.

The brain receives disparate retinal input owing to the separation of the eyes, yet we usually perceive a single fused world. This is because of complex interactions between sensory and oculomotor processes that quickly act to reduce excessive retinal disparity. This implies a strong link between depth perception and fusion, but it is well established that stereoscopic depth percepts are also obtained from stimuli that produce double images. Surprisingly, the nature of depth percepts from such diplopic stimuli remains poorly understood. Specifically, despite long-standing debate it is unclear whether depth under diplopia is owing to the retinal disparity (directly), or whether the brain interprets signals from fusional vergence responses to large disparities (indirectly). Here, we addressed this question using stereoscopic afterimages, for which fusional vergence cannot provide retinal feedback about depth. We showed that observers could reliably recover depth sign and magnitude from diplopic afterimages. In addition, measuring vergence responses to large disparity stimuli revealed that that the sign and magnitude of vergence responses are not systematically related to the target disparity, thus ruling out an indirect explanation of our results. Taken together, our research provides the first conclusive evidence that stereopsis is a direct process, even for diplopic targets.

Aftereffect of motion-in-depth based on binocular cues: Effects of adaptation duration, interocular correlation, and temporal correlation.

There are at least two possible binocular cues to motion-in-depth, namely disparity change over time and interocular velocity differences. There has been significant controversy about their relative contributions to the perception of motion-in-depth. In the present study, we used the technique of selective adaptation to address this question. In Experiment 1, we found that adaptation to motion-in-depth depicted by temporally correlated random-dot stereograms, which contained coherent interocular velocity difference, produced motion aftereffect in the depth direction irrespective of the adaptors' interocular correlation for any adaptation duration tested. This suggests that coherent changing disparity does not contribute to motion-in-depth adaptation. Because the aftereffect duration did not saturate in the tested range of adaptation duration, it is unlikely that the lack of the effect of changing disparity was due to a ceiling effect. In Experiment 2, we used a novel adaptor that contained a unidirectional coherent interocular velocity difference signal and a bidirectional changing disparity signal that should not induce a motion aftereffect in depth. Following the adaptation, motion aftereffect in depth occurred in the opposite direction to the adaptor's motion-in-depth based on interocular velocity difference. Experiment 3 demonstrated that these results generalized in 12 untrained subjects. These experiments suggest that the contribution of interocular velocity difference to the perception of motion-in-depth is substantial, while that of changing disparity is very limited (if any), at least at the stages of direction-selective mechanisms subject to an aftereffect phenomenon.

Interactions between cues to visual motion in depth.

Information about the motion in depth of an object along the midline of a stationary observer is provided by changes in image size (looming), changes in vergence produced by changes in binocular disparity of the images of the object, and changes in relative disparity between the moving object and a stationary object. Each of these cues was independently varied in the dichoptiscope, which is described in Howard, Fukuda, and Allison (2013). The stimuli were a small central dot and a textured surface moving to and fro in depth along the midline. Observers tracked the motion with the unseen hand. Image looming was normal or absent. The change in vergence was absent, normal, more than normal, or reversed relative to normal. Changing relative disparity between the moving stimulus and a stationary surface was present or absent. Changing vergence alone produced no motion in depth for the textured surface but it produced some motion of the dot. Looming alone produced strong motion in depth for the texture but not for the dot. When the direction of motion indicated by looming was opposite that indicated by changing relative disparity observers could use either cue. The cues dissociated rather than combined.

Binocular contributions to linear vertical vection.

Compelling illusions of self-motion, known as vection, can be produced in a stationary observer by visual stimulation alone. The role of binocular vision and stereopsis in these illusions was explored in a series of three experiments. Previous research had provided evidence of stereoscopic enhancements for linear vection in depth (e.g., Palmisano, 1996, 2002). Here we examined for the first time the effects of binocular vision and stereopsis on linear vertical vection. Vertical vection was induced by the upward or downward translation of large stereoscopic surfaces. These surfaces were horizontally oriented depth corrugations produced by disparity modulation of patterns of persistent or short lifetime dot elements. We found that binocular viewing of such surfaces significantly increased the magnitudes and decreased the onset delays of vertical vection. Experiments utilizing short lifetime dot stereograms demonstrated that these particular binocular enhancements of vection were due to the motion of stereoscopically defined features.

A computational theory of da Vinci stereopsis.

In binocular vision, occlusion of one object by another gives rise to monocular occlusions­regions visible only in one eye. Although binocular disparities cannot be computed for these regions, monocular occlusions can be precisely localized in depth and can induce the perception of illusory occluding surfaces. The phenomenon of depth perception from monocular occlusions, known as da Vinci stereopsis, is intriguing, but its mechanisms are not well understood. We first propose a theory of the mechanisms underlying da Vinci stereopsis that is based on the psychophysical and computational literature on monocular occlusions. It postulates, among other principles, that monocular areas are detected explicitly, and depth from occlusions is calculated based on constraints imposed by occlusion geometry. Next, we describe a biologically inspired computational model based on this theory that successfully reconstructs depth in a large range of stimuli and produces results similar to those described in the psychophysical literature. These results demonstrate that the proposed neural architecture could underpin da Vinci stereopsis and other stereoscopic percepts.

The effects of long-term exposure to microgravity and body orientation relative to gravity on perceived traveled distance.

Self-motion perception is a multi-sensory process that involves visual, vestibular, and other cues. When perception of self-motion is induced using only visual motion, vestibular cues indicate that the body remains stationary, which may bias an observer's perception. When lowering the precision of the vestibular cue by for example, lying down or by adapting to microgravity, these biases may decrease, accompanied by a decrease in precision. To test this hypothesis, we used a move-to-target task in virtual reality. Astronauts and Earth-based controls were shown a target at a range of simulated distances. After the target disappeared, forward self-motion was induced by optic flow. Participants indicated when they thought they had arrived at the target's previously seen location. Astronauts completed the task on Earth (supine and sitting upright) prior to space travel, early and late in space, and early and late after landing. Controls completed the experiment on Earth using a similar regime with a supine posture used to simulate being in space. While variability was similar across all conditions, the supine posture led to significantly higher gains (target distance/perceived travel distance) than the sitting posture for the astronauts pre-flight and early post-flight but not late post-flight. No difference was detected between the astronauts' performance on Earth and onboard the ISS, indicating that judgments of traveled distance were largely unaffected by long-term exposure to microgravity. Overall, this constitutes mixed evidence as to whether non-visual cues to travel distance are integrated with relevant visual cues when self-motion is simulated using optic flow alone.

The dichoptiscope: an instrument for investigating cues to motion in depth.

A stereoscope displays 2-D images with binocular disparities (stereograms), which fuse to form a 3-D stereoscopic object. But a stereoscopic object creates a conflict between vergence and accommodation. Also, motion in depth of a stereoscopic object simulated solely from change in target vergence produces anomalous motion parallax and anomalous changes in perspective. We describe a new instrument, which overcomes these problems. We call it the dichoptiscope. It resembles a mirror stereoscope, but instead of stereograms, it displays identical 2-D or 3-D physical objects to each eye. When a pair of the physical, monocular objects is fused, they create a dichoptic object that is visually identical to a real object. There is no conflict between vergence and accommodation, and motion parallax is normal. When the monocular objects move in real depth, the dichoptic object also moves in depth. The instrument allows the experimenter to control independently each of several cues to motion in depth. These cues include changes in the size of the images, changes in the vergence of the eyes, changes in binocular disparity within the moving object, and changes in the relative disparity between the moving object and a stationary object.

Vection underwater illustrates the limitations of neutral buoyancy as a microgravity analog.

Neutral buoyancy has been used as an analog for microgravity from the earliest days of human spaceflight. Compared to other options on Earth, neutral buoyancy is relatively inexpensive and presents little danger to astronauts while simulating some aspects of microgravity. Neutral buoyancy removes somatosensory cues to the direction of gravity but leaves vestibular cues intact. Removal of both somatosensory and direction of gravity cues while floating in microgravity or using virtual reality to establish conflicts between them has been shown to affect the perception of distance traveled in response to visual motion (vection) and the perception of distance. Does removal of somatosensory cues alone by neutral buoyancy similarly impact these perceptions? During neutral buoyancy we found no significant difference in either perceived distance traveled nor perceived size relative to Earth-normal conditions. This contrasts with differences in linear vection reported between short- and long-duration microgravity and Earth-normal conditions. These results indicate that neutral buoyancy is not an effective analog for microgravity for these perceptual effects.

Perception of smooth and perturbed vection in short-duration microgravity.

Successful adaptation to the microgravity environment of space and readaptation to gravity on earth requires recalibration of visual and vestibular signals. Recently, we have shown that adding simulated viewpoint oscillation to visual self-motion displays produces more compelling vection (despite the expected increase in visual-vestibular conflict experienced by stationary observers). Currently, it is unclear what role adaptation to gravity might play in this oscillation-based vection advantage. The vection elicited by optic flow displays simulating either smooth forward motion or forward motion perturbed by viewpoint oscillation was assessed before, during and after microgravity exposure in parabolic flight. During normal 1-g conditions subjects experienced significantly stronger vection for oscillating compared to smooth radial optic flow. The magnitude of this oscillation enhancement was reduced during short-term microgravity exposure, more so for simulated interaural (as opposed to spinal) axis viewpoint oscillation. We also noted a small overall reduction in vection sensitivity post-flight. A supplementary experiment found that 1-g vection responses did not vary significantly across multiple testing sessions. These findings: (i) demonstrate that the oscillation advantage for vection is very stable and repeatable during 1-g conditions and (ii) imply that adaptation or conditioned responses played a role in the post-flight vection reductions. The effects observed in microgravity are discussed in terms of the ecology of terrestrial locomotion and the nature of movement in microgravity.

da Vinci decoded: does da Vinci stereopsis rely on disparity?

In conventional stereopsis, the depth between two objects is computed based on the retinal disparity in the position of matching points in the two eyes. When an object is occluded by another object in the scene, so that it is visible only in one eye, its retinal disparity cannot be computed. Nakayama and Shimojo (1990) found that a precept of quantitative depth between the two objects could still be established for such stimuli and proposed that this precept is based on the constraints imposed by occlusion geometry. They named this and other occlusion-based depth phenomena "da Vinci stereopsis." Subsequent research found quantitative depth based on occlusion geometry in several other classes of stimuli grouped under the term da Vinci stereopsis. However, Nakayama and Shimojo's findings were later brought into question by Gillam, Cook, and Blackburn (2003), who suggested that quantitative depth in their stimuli was perceived based on conventional disparity. In order to understand whether da Vinci stereopsis relies on one type of mechanism or whether its function is stimulus dependent we examine the nature and source of depth in the class of stimuli used by Nakayama and Shimojo (1990). We use three different psychophysical and computational methods to show that the most likely source for depth in these stimuli is occlusion geometry. Based on these experiments and previous data we discuss the potential mechanisms responsible for processing depth from monocular features in da Vinci stereopsis.

Motion aftereffect in depth based on binocular information.

We examined whether a negative motion aftereffect occurs in the depth direction following adaptation to motion in depth based on changing disparity and/or interocular velocity differences. To dissociate these cues, we used three types of adapters: random-element stereograms that were correlated (1) temporally and binocularly, (2) temporally but not binocularly, and (3) binocularly but not temporally. Only the temporally correlated adapters contained coherent interocular velocity differences while only the binocularly correlated adapters contained coherent changing disparity. A motion aftereffect in depth occurred after adaptation to the temporally correlated stereograms while little or no aftereffect occurred following adaptation to the temporally uncorrelated stereograms. Interestingly, a monocular test pattern also showed a comparable motion aftereffect in a diagonal direction in depth after adaptation to the temporally correlated stereograms. The lack of the aftereffect following adaptation to pure changing disparity was also confirmed using spatially separated random-dot patterns. These results are consistent with the existence of a mechanism sensitive to interocular velocity differences, which is adaptable (at least in part) at binocular stages of motion-in-depth processing. We did not find any evidence for the existence of an "adaptable" mechanism specialized to see motion in depth based on changing disparity.

The relative contributions of radial and laminar optic flow to the perception of linear self-motion.

When illusory self-motion is induced in a stationary observer by optic flow, the perceived distance traveled is generally overestimated relative to the distance of a remembered target (Redlick, Harris, & Jenkin, 2001): subjects feel they have gone further than the simulated distance and indicate that they have arrived at a target's previously seen location too early. In this article we assess how the radial and laminar components of translational optic flow contribute to the perceived distance traveled. Subjects monocularly viewed a target presented in a virtual hallway wallpapered with stripes that periodically changed color to prevent tracking. The target was then extinguished and the visible area of the hallway shrunk to an oval region 40° (h) × 24° (v). Subjects either continued to look centrally or shifted their gaze eccentrically, thus varying the relative amounts of radial and laminar flow visible. They were then presented with visual motion compatible with moving down the hallway toward the target and pressed a button when they perceived that they had reached the target's remembered position. Data were modeled by the output of a leaky spatial integrator (Lappe, Jenkin, & Harris, 2007). The sensory gain varied systematically with viewing eccentricity while the leak constant was independent of viewing eccentricity. Results were modeled as the linear sum of separate mechanisms sensitive to radial and laminar optic flow. Results are compatible with independent channels for processing the radial and laminar flow components of optic flow that add linearly to produce large but predictable errors in perceived distance traveled.