Effect of viewing plane on perceived distances in real and virtual environments

Three experiments examined perceived absolute distance in a head-mounted display virtual environment (HMD-VE) and a matched real-world environment, as a function of the type and orientation of the distance viewed. In Experiment 1, participants turned and walked, without vision, a distance to match the viewed interval for both egocentric (viewer-to-target) and exocentric (target-to-target) extents. Egocentric distances were underestimated in the HMD-VE while exocentric distances were estimated similarly across environments. Since egocentric distances were displayed in the depth plane and exocentric distances in the frontal plane, the pattern of results could have been related to the orientation of the distance or to the type of distance. Experiments 2 and 3 tested these alternatives. Participants estimated exocentric distances presented along the depth or frontal plane either by turning and walking (Experiment 2) or by turning and throwing a beanbag to indicate the perceived extent (Experiment 3). For both Experiments 2 and 3, depth intervals were underestimated in the HMD-VE compared to the real world. However, frontal intervals were estimated similarly across environments. The findings suggest anisotropy in HMD-VE distance perception such that distance underestimation in the HMD-VE generalizes to intervals in the depth plane, but not to intervals in the frontal plane. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Berkeleian Principles in Ecological Realism: An Ontological Analysis

Three experiments are reported, which examined the relation between the percep- tion of the distance reachable with a hand-held rod that can be wielded but not seen and the rod's resistance to having its rotational speed changed by application of a torque. In these experiments, subjects wielded any given rod about an axis intermediate between its endpoints. The subject's task was to adjust a visible, movable surface to coincide with where he or she could reach with the given rod if allowed to hold it at its proximal end. Two experiments considered the effects of wielding a rod at different orientations to the pull of gravity. Rods were wielded either within a plane roughly perpendicular to the ground or within a plane roughly parallel to the ground. Plane of wielding did not affect the patterning of perceived reachable distances as a function of the various conditions, which included variations in the positioning of grasp and the positioning of a mass affixed to the rods. The patterning of the moments of inertia associated with the various conditions determined the patterning of perceived reachable distances. The third experiment restricted wielding to a plane roughly parallel to the ground and varied how the rods were grasped, either overhand or underhand. The variation in grasp amounted to a variation in the neuromuscular patterning associated with the wielding of any given rod. Perceived reachable distances proved to be indifferent to the overhand versus underhand contrast.     

Dissociation between visual perception of allocentric distance and visually directed walking of its extent

Walking without vision to previously viewed targets was compared with visual perception of allocentric distance in two experiments. Experimental evidence had shown that physically equal distances in a sagittal plane on the ground were perceptually underestimated as compared with those in a frontoparallel plane, even under full-cue conditions. In spite of this perceptual anisotropy of space, Loomis et al (1992 Journal of Experimental Psychology. Human Perception and Performance 18 906-921) found that subjects could match both types of distances in a blind-walking task. In experiment 1 of the present study, subjects were required to reproduce the extent of allocentric distance between two targets by either walking towards the targets, or by walking in a direction incompatible with the locations of the targets. The latter condition required subjects to derive an accurate allocentric distance from information based on the perceived locations of the two targets. The walked distance in the two conditions was almost identical whether the two targets were presented in depth (depth-presentation condition) or in the frontoparallel plane (width-presentation condition). The results of a perceptual-matching task showed that the depth distances had to be much greater than the width distances in order to be judged to be equal in length (depth compression). In experiment 2, subjects were required to reproduce the extent of allocentric distance from the viewing point by blindly walking in a direction other than toward the targets. The walked distance in the depth-presentation condition was shorter than that in the width-presentation condition. This anisotropy in motor responses, however, was mainly caused by apparent overestimation of length oriented in width, not by depth compression. In addition, the walked distances were much better scaled than those in experiment 1. These results suggest that the perceptual and motor systems share a common representation of the location of targets, whereas a dissociation in allocentric distance exists between the two systems in full-cue conditions.     

An airplane illusion: apparent velocity determined by apparent distance

When a small drone plane appears to be a normal-sized airplane, it appears to be very far away and moving too fast. This is the airplane illusion. In the illusory situation, familiar size determines the apparent size and distance of the plane. It sets the depth for the frontal-plane component of the perceived motion and the relative depth difference for the motion-in-depth component. Because these perceived distances are very large, the perceived velocities are very large in the respective directions. Cognition can override familiarity and produce a veridical perception of the drone.     

Distance perception within near visual space

We investigated the perception of distance of visual targets with constant size and luminance presented between 20 and 120 cm from subjects' eyes. When retinal disparity cues were present, the subjects could reproduce very accurately the distance of a seen reference in this area. When only extraretinal information was available, distance perception was still correct for distances of 40 cm or less. However, distances beyond 60 cm were underestimated. When forced to evaluate the distance between a reference and themselves, e.g. when evaluating the absolute distance or half the distance or twice the distance of a reference, subjects used an egocentric plane of reference located on average 10.4 cm in front of their eyes. Measurements of binocular eye movements indicated a clear relationship between vergence angle and target distance. The egocentric plane of reference at 10.4 cm also corresponds to the maximum achievable vergence. These results suggest that ocular convergence can be used as a reliable cue for distance within the arm's reaching space.     

The importance of a visual horizon for distance judgments under severely degraded vision

In two experiments we examined the role of visual horizon information on absolute egocentric distance judgments to on-ground targets. Sedgwick [1983, in Human and Machine Vision (New York: Academic Press) pp 425-458] suggested that the visual system may utilize the angle of declination from a horizontal line of sight to the target location (horizon distance relation) to determine absolute distances on infinite ground surfaces. While studies have supported this hypothesis, less is known about the specific cues (vestibular, visual) used to determine horizontal line of sight. We investigated this question by requiring observers to judge distances under degraded vision given an unaltered or raised visual horizon. The results suggest that visual horizon information does influence perception of absolute distances as evident through two different action-based measures: walking or throwing without vision to previously viewed targets. Distances were judged as shorter in the presence of a raised visual horizon. The results are discussed with respect to how the visual system accurately determines absolute distance to objects on a finite ground plane and for their implications for understanding space perception in low-vision individuals.     

Research note: a multidimensional scaling comparison of color metrics for response times and rated dissimilarities

Individual-differences multidimensional scaling was applied to a set of proximity data for equiluminant lights (Paramei & Cavonius, 1999) to explore any differences between two data collection procedures (rated dissimilarities, and same/different response times [RTs]), as well as between color-normal and abnormal observers. Two conclusions emerged: (1) The pattern of similarities from observers with anomalous color vision can be understood in terms of a compressed color plane (the weighted Euclidean model of individual differences); and (2) there is evidence that the color "plane" is either curved or governed by a non-Euclidean distance function. When color-normal observers are examined in the weighted-Euclidean framework, minor differences emerge between RT and rating data. But the main distinguishing feature of RT data is a pattern of decreasing accuracy for larger color distances. This must be taken into account, since it can itself induce curvature.     

Some principles governing changes in the apparent lightness of test surfaces isolated from their normal backgrounds

When smooth matt test surfaces are suspended by fine threads in a strong illumination gradient and in isolation from any ordinary background, lightness constancy fails. It may be restored by introducing other objects into the field in the same plane as the test surfaces. Using a large number of subjects making independent judgements by a comparison scale method, the present investigation attempts to discover the principles underlying the restoration of lightness constancy by the introduction of inducing surfaces of varied size and reflectance at varied distances from the test surface.     

Perceptual alternations in stereokinesis

When a flat ellipse is slowly rotated in the frontoparallel plane, two different 3-D percepts can be obtained: (i) a rigid circular disc tilting back and forth in 3-D space, and (ii) an elongated egg, slanted into 3-D space, whose end parts seem to be located at different distances from the observer and describe a circular trajectory with respect to the frontal plane. Under prolonged observation, the two 3-D percepts alternate in time, separated by brief intervals in which either the rotation of a rigid flat ellipse in the frontal plane or an amoeba-like distortion of a 2-D shape can be perceived. An experiment is reported in which the sequence of perceptual alternations was investigated. Results show that the 3-D disc is mostly preceded by impressions of elastic amoeba-like deformations, whereas the 3-D egg is mostly preceded by the percept of a rotating flat ellipse. Direct transitions from egg to disc are not as frequent as transitions from disc to egg. Results are discussed in relation to Braunstein and Andersen's hypothesis that phenomena of this sort might result from the stimulation of automatic mechanisms for perceiving size change (changing-size neutral channels).     

Perceived size and perceived direction: the interplay of the two descriptors of visual space

The stimulus in the outdoor psychophysical experiment was formed by two rods placed on the ground plane over a range of possible distances and orientations. Observers estimated its size and direction by positioning the third rod in the neighbouring space to form an evenly spaced collinear triple of rods. The data revealed interesting similarities between the profiles of the deviations in both judgments: for size judgments, the variability of the responses was least when the targets were at a frontal orientation and gradually increased as the orientation approached the medial plane. For direction judgments, on the other hand, the variability of the responses was least when the stimuli were aligned with the observer's line of sight and gradually increased as the orientation approached the frontoparallel plane. The finding of inverse relationship between the precision of size and direction judgments is interpreted as a consequence of the unequal precision in localisation between the frontal and in-depth dimensions of visual space. The question of the best parameterisation of the observers' responses is discussed.     

Distance perception in photographic displays of natural settings

Distance perception of depicted objects was examined as a function of photographic perspective. 24 subjects viewed slides of outdoor scenes and directly estimated the distances to specified objects. Perspective was manipulated by photographing each scene with lenses of four different focal lengths: 48 mm, 28 mm, 24 mm, and 17 mm. Distance perception along the pictorial depth plane (z-axis) was systematically transformed by changing the photographic perspective: the shorter the focal length of the camera lens, the greater the perceived distance. Perceived distance between objects along the lateral plane (x-axis) was unaffected by changes in lens focal length.     

Absolute distance perception to locations off the ground plane

Using vision, humans can accurately judge distances to locations on the ground surface up to distances of at least 20 m. Most theories of depth perception assume that this ability is associated with the fact that we live in a terrestrial world in which locations of interest often appear on the ground and for which feedback about distance is often available from nonvisual sources such as walking. Much less is known about the ability of humans to judge absolute distances to locations other than on or supported by the ground plane beyond a few meters, at which point binocular stereo provides at best limited information about distance scaling. We show that one commonly used action measure for probing absolute distance perception exhibits accurate performance, even for targets located on the ceiling of a large room. We follow this with evidence that distance to ceiling locations is recovered with a mechanism that depends, at least in part, on the angle from the line of sight to the target location and a gravity-based frame of reference.     

Fooling the eyes: trompe l'oeil and reverse perspective

Trompe l'oeil pictures have been produced for hundreds of years. They attempt to create the impression of a surface that has different three-dimensional structure to the work; successful examples of trompe l'oeil typically constrain the observer's viewpoint and require use of a single eye. The works of Patrick Hughes are in relief but are painted to appear like conventional flat pictures; those parts that protrude from the picture plane are pictorially distant, or in reverse perspective. Movements of the observer result in fluid distortions of the pictorial image. These distortions occur with binocular observation and over a wide range of viewing distances.     

Perceived distance of targets in convex mirrors

We investigated the perceived distance of targets in convex and plane mirrors. In Experiment 1, 20 subjects matched the distance of targets in a real scene to the distance of a virtual target in different mirrors. The matched distances were much larger for convex mirrors than for a plane mirror. In Experiment 2, 20 subjects viewed two targets in a mirror and adjusted their own positions so that the distance to the closer target was perceived to equal the distance between the targets. The mean distance to the closer target was smaller for the convex mirrors than for the plane mirror. In Experiment 3, 20 subjects adjusted the position of a target so that the distance to it in a mirror was perceived to equal the distance designated by the experimenter. The best-fitting power functions showed that the scaling factors were larger for the convex mirrors than for the plane mirror, but the exponents were smaller for the convex mirrors than for the plane mirror. It is suggested that distance in the convex mirrors was perceived to be larger than in the plane mirror, and that the growth of perceived distance in the convex mirrors was slower than in the plane mirror.     

Catching and matching bars with different orientations

The hypothesis that perception enslaves action is examined by assessing whether systematic distortions in perceptual judgments are reflected by inaccuracies in catching. In the first experiment, participants had to align manually the orientation of a reference bar placed at different distances in the frontoparallel plane. In the second experiment participants had to catch differently orientated moving bars, which became invisible at different distances from the interception point. In the matching experiment, systematic errors in the alignment of orientation were found in particular for oblique orientations, the magnitude of which increased with increasing distance of the reference bar. The inaccuracies in the final hand orientation during the catching task, however, did not mirror this pattern of deviations. The findings are interpreted to be more consistent with recent views that vision for perception (i.e., matching) and vision for action (i.e., catching) are dissociated than with the view that perception enslaves action.     

Speed of aiming movements

Fitts' Law describes human movement time in the horizontal and frontal planes, but its application to movement in the sagittal plane has been questioned by Beggs and Howarth. To evaluate the Law in this plane, eight subjects performed a forearm action, similar to dart-throwing without release, moving a stylus through the sagittal plane from a back contact plate forward to a target. Four movement distances, 8, 12, 16 and 20 in, were combined with four target widths, 1/4, 1/2, 1 and 2 in, in 16 experimental conditions. Following practice all subjects were tested in each condition, with visual knowledge of results. The plot of movement time against log 2A/W was essentially linear, accounting for 92.6% of the variance.     

Analyzing distances in a frontal plane

In an exploration of the process whereby horizontal and vertical components are extracted from distances between pairs of points, participants made speeded absolute distance judgments—deciding whether horizontal distances between pairs of points in a frontal plane exceeded a criterion distance. Judgments reflected the horizontal and vertical distances between the points. Three accounts of the results were considered. (1) Only the overall distance between two points was directly available to judgment processes; the horizontal or vertical distance between the points was available only to the extent that the horizontal and vertical positions of the points were differentially weighted prior to the assessment of overall distance. (2) The perceptual effects of positions on the horizontal and vertical dimensions were collapsed onto a composite dimension. (3) The decision criterion for the distance judgment considered both the horizontal and vertical distances between the points. In support of the overall distance account, (1) performance in the distance judgment task was facilitated by repetition of the same overall distance from trial to trial, but not by repetition of the distance on the composite dimension or by repetition of horizontal or vertical distance; (2) differential weighting that the overall distance account predicted for the absolute distance judgments was reflected in concurrent relative distance judgments pitting horizontal against vertical distance, counter to the composite dimension account, which sees the horizontal and vertical dimensions as collapsing onto the composite dimension in balanced, symmetrical fashion, and counter the decisional account, given that no criterion is required for the relative judgment.     

One-dimensional and multi-dimensional studies of the exocentric distance estimates in frontoparallel plane, virtual space, and outdoor open field

The aim of this study is twofold: on the one hand, to determine how visual space, as assessed by exocentric distance estimates, is related to physical space. On the other hand, to determine the structure of visual space as assessed by exocentric distance estimates. Visual space was measured in three environments: (a) points located in a 2-D frontoparallel plane, covering a range of distances of 20 cm; (b) stakes placed in a 3-D virtual space (range = 330 mm); and (c) stakes in a 3-D outdoors open field (range = 45 m). Observers made matching judgments of distances between all possible pairs of stimuli, obtained from 16 stimuli (in a regular squared 4 x 4 matrix). Two parameters from Stevens' power law informed us about the distortion of visual space: its exponent and its coefficient of determination (R2). The results showed a ranking of the magnitude of the distortions found in each experimental environment, and also provided information about the efficacy of available visual cues of spatial layout. Furthermore, our data are in agreement with previous findings showing systematic perceptual errors, such as the further the stimuli, the larger the distortion of the area subtended by perceived distances between stimuli. Additionally, we measured the magnitude of distortion of visual space relative to physical space by a parameter of multidimensional scaling analyses, the RMSE. From these results, the magnitude of such distortions can be ranked, and the utility or efficacy of the available visual cues informing about the space layout can also be inferred.