Elizabeth Thorpe Davis

Maintaining Spatial Orientation during Travel in an Immersive Virtual Environment

An experiment is presented comparing the effects of various virtual travel techniques in an immersive virtual environment (VE) on the spatial orientation of users. The experiment was designed and implemented in the context of a formal framework for the design and evaluation of VE travel techniques. Subjects traveled through virtual corridors, noting the location of objects along the way, and were asked to point in the direction of one of these objects when the end of the corridor was reached. Results indicate that virtual travel techniques, in which users do not physically translate their bodies, can allow the maintenance of a users spatial orientation as measured by a pointing task. The experiment also replicates an earlier result, showing that path dimension significantly affects user performance. Finally, the strategies used by subjects to perform the task were shown to be significant, indicating that performance depends not only on the technique, environment, and task, but also on the sophistication of the user.

The perception of distance in simulated visual displays: A comparison of the effectiveness and accuracy of multiple depth cues across viewing distances

The ability effectively and accurately to simulate distance in virtual and augmented reality systems is a challenge currently facing R&D. To examine this issue, we separately tested each of seven visual depth cues (relative brightness, relative size, relative height, linear perspective, foreshortening, texture gradient, and stereopsis) as well as the condition in which all seven of these cues were present and simultaneously providing distance information in a simulated display. The viewing distances were 1 and 2 m. In developing simulated displays to convey distance and depth there are three questions that arise. First, which cues provide effective depth information (so that only a small change in the depth cue results in a perceived change in depth)? Second, which cues provide accurate depth information (so that the perceived distance of two equidistant objects perceptually matches)? Finally, how does the effectiveness and accuracy of these depth cues change as a function of the viewing distance? Ten college-aged subjects were tested with each depth-cue condition at both viewing distances. They were tested using a method of constant stimuli procedure and a modified Wheat-stone stereoscopic display. The perspective cues (linear perspective, foreshortening, and texture gradient) were found to be more effective than other depth cues, while effectiveness of relative brightness was vastly inferior. Moreover, relative brightness, relative height, and relative size all significantly decreased in effectiveness with an increase in viewing distance. The depth cues did not differ in terms of accuracy at either viewing distance. Finally, some subjects experienced difficulty in rapidly perceiving distance information provided by stereopsis, but no subjects had difficulty in effectively and accurately perceiving distance with the perspective information used in our experiment. A second experiment demonstrated that a previously stereo-anomalous subject could be trained to perceive stereoscopic depth in a binocular display. We conclude that the use of perspective cues in simulated displays may be more important than the other depth cues tested because these cues are the most effective and accurate cues at both viewing distances, can be easily perceived by all subjects, and can be readily incorporated into simpler, less complex displays (e.g., biocular HMDs) or more complex ones (e.g., binocular or see-through HMDs).

Geometric considerations for stereoscopic virtual environments

We examine the relationship among the different geometries implicit in a stereoscopic virtual environment. In particular, we examine in detail the relationship of retinal disparity, fixation point, binocular visual direction, and screen parallax. We introduce the concept of a volumetric spatial unit called a stereoscopic voxel. Due to the shape of stereoscopic voxels, apparent depth of points in space may be affected by their horizontal placement.

Relevant Cues for the Visual Perception of Depth: Is Where You See it Where it is?

We tested seven visual depth cues (relative brightness, relative size, relative height, linear perspective, foreshortening, texture gradient, and stereopsis) at viewing distances of one and two meters to answer two questions. First, which cues provide effective depth information (i.e., only a small change in the depth cue results in a noticeable change in perceived depth). Second, how does the effectiveness of these depth cues change as a function of the viewing distance? Six college-aged subjects were tested with each depth cue at both viewing distances. They were tested using a method of constant stimuli procedure and a modified Wheatstone stereoscopic display. Accuracies for perceptual match settings for all cues were very high (mean constant errors were near zero), and no cues were significantly more or less accurate than any others. Effectiveness of the perspective cues (linear perspective, foreshortening, and texture gradient) was superior to that of other depth cues, while effectiveness of relative brightness was vastly inferior. Moreover, stereopsis, among the more effective cues at one meter, was significantly less so at two meters. These results have theoretical implications for models of human spatial perception and practical implications for the design and development of 3D virtual environments.

Can Audio Enhance Visual Perception and Performance in a Virtual Environment

Does the addition of audio enhance visual perception and performance within a virtual environment? To address this issue we used both a questionnaire and an experimental test of the effect of audio on recall and recognition of visual objects within different rooms of a virtual environment. We tested 60 college-aged students who had normal visual acuity, color vision, and hearing. The between-participants factor was audio condition (none, low fidelity, and high fidelity). The questionnaire results showed that ambient sounds enhanced the sense of presence (or “being there”) and the subjective 3D quality of the visual display, but not the subjective dynamic interaction with the display. We also showed that audio can enhance recall and recognition of visual objects and their spatial locations within the virtual environment. These results have implications for the design and use of virtual environments, where audio sometimes can be used to compensate for the quality of the visual display.

Visual search and attention: an overview.

This special feature issue is devoted to attention and visual search. Attention is a central topic in psychology and visual search is both a versatile paradigm for the study of visual attention and a topic of study in itself. Visual search depends on sensory, perceptual, and cognitive processes. As a result, the search paradigm has been used to investigate a diverse range of phenomena. Manipulating the search task can vary the demands on attention. In turn, attention modulates visual search by selecting and limiting the information available at various levels of processing. Focusing on the intersection of attention and search provides a relatively structured window into the wide world of attentional phenomena. In particular, the effects of divided attention are illustrated by the effects of set size (the number of stimuli in a display) and the effects of selective attention are illustrated by cueing subsets of stimuli within the display. These two phenomena provide the starting point for the articles in this special issue. The articles are organized into four general topics to help structure the issues of attention and search.

Divided attention and visual search for simple versus complex features.

Under what search conditions does attention affect perceptual processes, resulting in capacity limitations, rather than affecting noisy decision-making processes? Does parallel or serial processing cause the capacity limitations? To address these issues, we varied stimulus complexity, set size, and whether distractors were mirror images of the target. Both target detection and localization produced similar patterns of results. Capacity limitations only occurred for complex stimuli used in within-object conjunction searches. Parallel processing, rather than serial processing, probably caused these capacity limitations. Moreover, although mirror-image symmetry adversely affected early visual processing, it did not place additional demands on attention.

Reducing Uncertainty in Visual Search through the Priming of Visual-Relatedness and the Manipulation of Observer Expectancy

We examined two means of reducing uncertainty in visual search: 1) visual-relatedness of a prime to a target and 2) expectancy (based on the proportion of validly primed trials). The two processes were decoupled using a short and a long inter-stimulus interval to examine their respective time courses in visual search. Twelve participants engaged in a discrimination task and a visual search task. The obtained results suggest that visual-relatedness affects search performance early, but its effects rapidly decay. They also suggest that expectancy requires time to accrue before it can affect visual search performance, but its effects are more long lasting than visual-relatedness. These results offer guidance for designing visual displays. In tasks that require split-second decisions designers should be encouraged to prime visual-relatedness. Moreover if the onset of the display and the required decision are separated by a longer interval, displays should inform users of likely outcomes (i.e. manipulate users expectations).

Reduction of foveal desensitization with blurred backgrounds

Photopic spatial desensitization and sensitization (Westheimer) functions were measured with sharp edged and blurred edged backgrounds. Data show that for 6-min backgrounds, desensitization is reduced for test flashes presented against blurred backgrounds compared to test flashes presented against backgrounds with sharp edges. We suggest that the transients from edges of retinal images, caused by high frequency fixational eye movements, contribute to the mechanisms of spatial desensitization and sensitization; the effect of transients is reduced by blurring the backgrounds.

Stereoscopic Depth Perception in Simulated Displays: What Helps and What Hurts?

We tested the effect of direction of retinal disparity and stimulus orientation on stereoscopic depth perception to answer three questions. First, are some directions of disparity more efficient than others in providing stereoscopic depth information? Second, does the orientation of an object affect perceived stereoscopic depth? Third, are there any interactions between these parameters? Subjects were tested using a psychophysical, method of constant stimuli procedure with a modified Wheatstone stereoscopic display. Disparity threshold measurements show a significant effect of direction of retinal disparity. Contrary to expectations however, no significant effect of orientation was found if vertical retinal disparities were excluded from the analyses. Stereoacuity thresholds were measurable with obliquely-oriented stimuli and vertical disparity, however, suggesting that vertical disparities can provide useful information. The implications of these results for the graphics, calibration, and design of stereoscopic displays (e.g., HMDs) are discussed.