Harold T. Nefs

Two independent mechanisms for motion-in-depth perception: evidence from individual differences.

Our forward-facing eyes allow us the advantage of binocular visual information: using the tiny differences between right and left eye views to learn about depth and location in three dimensions. Our visual systems also contain specialized mechanisms to detect motion-in-depth from binocular vision, but the nature of these mechanisms remains controversial. Binocular motion-in-depth perception could theoretically be based on first detecting binocular disparity and then monitoring how it changes over time. The alternative is to monitor the motion in the right and left eye separately and then compare these motion signals. Here we used an individual differences approach to test whether the two sources of information are processed via dissociated mechanisms, and to measure the relative importance of those mechanisms. Our results suggest the existence of two distinct mechanisms, each contributing to the perception of motion-in-depth in most observers. Additionally, for the first time, we demonstrate the relative prevalence of the two mechanisms within a normal population. In general, visual systems appear to rely mostly on the mechanism sensitive to changing binocular disparity, but perception of motion-in-depth is augmented by the presence of a less sensitive mechanism that uses interocular velocity differences. Occasionally, we find observers with the opposite pattern of sensitivity. More generally this work showcases the power of the individual differences approach in studying the functional organization of cognitive systems.

Effects of stereoscopic viewing on presence, anxiety, and cybersickness in a virtual reality environment for public speaking

In this study, we addressed the effect of stereoscopy on presence, anxiety, and cybersickness in a virtual public speaking world, and investigated the relationships between these three variables. Our results question the practical relevance of applying stereoscopy in head-mounted displays (HMDs) for virtual reality exposure therapy (VRET) in a virtual public speaking world. In VRET, feelings of presence improve the efficacy (B. K. Wiederhold & M. D. Wiederhold, 2005). There are reports of a relatively large group of dropouts during VRET at low levels of presence in the virtual environment (Krijn, Emmelkamp, Olafsson, & Biemond, 2004). Therefore, generating an adequate level of presence is essential for the success of VRET. In this study, 86 participants were recruited and they were immersed in the virtual public speaking world twice: once with stereoscopic rendering and once without stereoscopic rendering. The results showed that spatial presence was significantly improved by adding stereoscopy, but no difference for reported involvement or realism was found. The heart rate measurements also showed no difference between stereoscopic and nonstereoscopic viewing. Participants reported similar anxiety feelings about their talk and similar level of cybersickness in both viewing modes. Even though spatial presence was significantly improved, the size of statistical effect was relatively small. Our results therefore suggest that stereoscopic rendering may not be of practical importance for VRET in public speaking settings.

A meta-analysis on the relationship between self-reported presence and anxiety in virtual reality exposure therapy for anxiety disorders

In virtual reality exposure therapy (VRET) for anxiety disorders, sense of presence in the virtual environment is considered the principal mechanism that enables anxiety to be felt. Existing studies on the relation between sense of presence and level of anxiety, however, have yielded mixed results on the correlation between the two. In this meta-analysis, we reviewed publications on VRET for anxiety that included self-reported presence and anxiety. The comprehensive search of the literature identified 33 publications with a total of 1196 participants. The correlation between self-reported sense of presence and anxiety was extracted and meta-analyzed. Potential moderators such as technology characteristics, sample characteristics including age, gender and clinical status, disorder characteristics and study design characteristics such as measurements were also examined. The random effects analysis showed a medium effect size for the correlation between sense of presence and anxiety (r = .28; 95% CI: 0.18–0.38). Moderation analyses revealed that the effect size of the correlation differed across different anxiety disorders, with a large effect size for fear of animals (r = .50; 95% CI: 0.30–0.66) and a no to small effect size for social anxiety disorder (r = .001; 95% CI: −0.19–0.19). Further, the correlation between anxiety and presence was stronger in studies with participants who met criteria for an anxiety disorder than in studies with a non-clinical population. Trackers with six degrees of freedom and displays with a larger field of view resulted in higher effect sizes, compared to trackers with three degrees of freedom and displays with a smaller field of view. In addition, no difference in effect size was found for the type of presence measurement and the type of anxiety measurement. This meta-analysis confirms the positive relation between sense of presence and anxiety and demonstrates that this relation can be affected by various moderating factors.

Three-dimensional object shape from shading and contour disparities.

Both non-Lambertian shading, specularities in particular, and occluding contours have ill-matched binocular disparities. For example, the disparities of specularities depend not only on a surfaces position but also on its curvature. Shading and contour disparities do in general not specify a point on the surface. I investigated how shading and contours contribute to perceived shape in stereoscopic viewing. Observers adjusted surface attitude probes on a globular object. In Experiment 1, the object was either Lambertian or Lambertian with added specularities. In the next experiment, I removed the Lambertian part of the shading. In Experiment 3, I reduced the disparity of the contour to zero, and in Experiment 4, I removed both cues. There was little effect of shading condition in Experiment 1. Removing the Lambertian shading in Experiment 2 rendered the sign of the surface ambiguous (convex/concave) although all surfaces were perceived as curved. Results in Experiment 3 were similar to those in Experiment 1. Removing both cues in Experiment 4 made all surfaces appear flat for three observers and convex for one observer. I conclude that in the absence of Lambertian shading, observers have categorically different perceptions of the surface depending on whether disparate specular highlights and disparate contours are present or not.

What Visual Information is Used for Stereoscopic Depth Displacement Discrimination

There are two ways to detect a displacement in stereoscopic depth, namely by monitoring the change in disparity over time (CDOT) or by monitoring the interocular velocity difference (IOVD). Though previous studies have attempted to understand which cue is most significant for the visual system, none has designed stimuli that provide a comparison in terms of relative efficiency between them. Here we used two-frame motion and random-dot noise to deliver equivalent strengths of CDOT and IOVD information to the visual system. Using three kinds of random-dot stimuli, we were able to isolate CDOT or IOVD or deliver both simultaneously. The proportion of dots delivering CDOT or IOVD signals could be varied, and we defined the discrimination threshold as the proportion needed to detect the direction of displacement (towards or away). Thresholds were similar for stimuli containing CDOT only, and containing both CDOT and IOVD, but only one participant was able to consistently perceive the displacement for stimuli containing only IOVD. We also investigated the effect of disparity pedestals on discrimination. Performance was best when the displacement crossed the reference plane, but was not significantly different for stimuli containing CDOT only and those containing both CDOT and IOVD. When stimuli are specifically designed to provide equivalent two-frame motion or disparity-change, few participants can reliably detect displacement when IOVD is the only cue. This challenges the notion that IOVD is involved in the discrimination of direction of displacement in two-frame motion displays.

Vergence effects on the perception of motion-in-depth.

When the eyes follow a target that is moving directly towards the head they make a vergence eye movement. Accurate perception of the target’s motion requires adequate compensation for the movements of the eyes. The experiments in this paper address the issue of how well the visual system compensates for vergence eye movements when viewing moving targets. We show that there are small but consistent biases across observers: When the eyes follow a target that is moving in depth, it is typically perceived as slower than when the eyes are kept stationary. We also analysed the eye movements that were made by observers. We found that there are considerable differences between observers and between trials, but we did not find evidence that the gains and phase lags of the eye movements were related to psychophysical performance.

Induced motion in depth and the effects of vergence eye movements

Induced motion is the false impression that physically stationary objects move when in the presence of other objects that really move. In this study, we investigated this motion illusion in the depth dimension. We raised three related questions, as follows: (1) What cues in the stimulus are responsible for this motion illusion in depth? (2) Is the size of this illusion affected by vergence eye movements? And (3) are the effects of eye movements different for motion in depth and for motion in the frontoparallel plane? To answer these questions, we measured the point of subjective stationarity. Observers viewed an inducer target that oscillated in depth and a test target that was located directly above it. The test target moved in phase or out of phase with the inducer, but with a smaller amplitude. Observers had to indicate whether the test target and the inducer target moved in phase or out of phase with one another. They were asked to keep their eyes either on the test target or on the inducer. For motion in depth, created by binocular disparity and retinal size change or by binocular disparity alone, we found that when the eyes followed the inducer, subjective stationarity occurred at approximately 40-45% of the inducers amplitude. When the eyes were kept fixated on the test target, the bias decreased tenfold to around 4%. When size change was the only cue to motion in depth, there was no illusory motion. When the eyes were kept on an inducer moving in the frontoparallel plane, induced motion was of the same order as for induced motion in depth, namely, approximately 44%. When the induced motion was in the frontoparallel plane, we found that perceived stationarity occurred at approximately 23% of inducers amplitude when the eyes were kept on the test target.

Visual Discomfort and Depth-of-Field

Visual discomfort has been reported for certain visual stimuli and under particular viewing conditions, such as stereoscopic viewing. In stereoscopic viewing, visual discomfort can be caused by a conflict between accommodation and convergence cues that may specify different distances in depth. Earlier research has shown that depth-of-field, which is the distance range in depth in the scene that is perceived to be sharp, influences both the perception of egocentric distance to the focal plane, and the distance range in depth between objects in the scene. Because depth-of-field may also be in conflict with convergence and the accommodative state of the eyes, we raised the question of whether depth-of-field affects discomfort when viewing stereoscopic photographs. The first experiment assessed whether discomfort increases when depth-of-field is in conflict with coherent accommodation–convergence cues to distance in depth. The second experiment assessed whether depth-of-field influences discomfort from a pre-existing accommodation–convergence conflict. Results showed no effect of depth-of-field on visual discomfort. These results suggest therefore that depth-of-field can be used as a cue to depth without inducing discomfort in the viewer, even when cue conflicts are large.

Depth of Field Affects Perceived Depth in Stereographs

Although it has been reported that depth of field influences depth perception in nonstereo photographs, it remains unclear how depth of field affects depth perception under stereo viewing conditions. We showed participants stereo photographs with different depths of field using a Wheatstone stereoscope and a commercially available 3D TV. The depicted scene contained a floor, a background, and a measuring probe at different locations. Participants drew a floor plan of the depicted scene to scale. We found that perceived depth decreased with decreasing depth of field for shallow depths of field in scenes containing a height-in-the-field cue. For larger depths of field, different effects were found depending on the display system and the viewing distance. There was no effect on perceived depth using the 3D TV, but perceived depth decreased with increasing depth of field using the Wheatstone stereoscope. However, in the 3D TV case, we found that the perceived depth decreased with increasing depth of field in scenes in which the height-in-the-field cue was removed. This indicates that the effect of depth of field on perceived depth may be influenced by other depth cues in the scene, such as height-in-the-field cues.

1 – ON THE VISUAL APPEARANCE OF OBJECTS

Publisher Summary When looking around in the world, humans have to figure out what it is they see, what the shapes of things are, what kinds of materials they are made of, and what their potential uses or dangers are. In addition to meaning, aesthetic value or affordance, the visual system also extracts some more fundamental object properties. The position, that perceived object shape is determined exclusively by the objects’ physical shape, is therefore clearly untenable. Likewise, perceived material and perceived illumination characteristics are also not determined exclusively by their respective physical counterparts. The problem of what objects look like is certainly not a trivial problem. It determines whether people approach an object carefully or not, if and how they pick it up, etc. This chapter examines the difficulties that humans face when looking at the real world. It examines the perceived shape, material properties, and illumination of objects with special emphasis on the perceived shape. There are physical relationships between shape, material, and illumination once certain simple assumptions are made. This chapter provides the necessary theoretical background on the physical properties and relationships between shape, material, and illumination of objects in projection. It discusses the physical correlates for perceived object properties. It also discusses perceptual organization and whether perceived object properties conform to physical constraints, that is, whether they form a coherent set. In addition it discusses how additional cues can be used to form a coherent percept.