Background surface and horizon effects in the perception of relative size and distance

The projected height of an object in a scene relative to a ground surface influences its perceived size and distance, but the effect of height should change when the object is moved above the horizon. In four experiments, observers judged relative size or relative distance for pairs of objects varying in height with respect to the horizon. Higher objects equal in projected size were judged larger below the horizon, but the relative size effect was reversed either when one object was on the horizon and one was above the horizon or when both objects were above the horizon. With the real horizon not explicitly present in the display, relative size judgements were affected both by the boundary of the visible surface and the vanishing point implied by the converging lines. For relative distance judgements, the higher object was judged more distant regardless of the height of the objects relative to the perceptual horizon, resulting in a reversal of the relation between size and distance judgements for objects above the horizon.     

Contribution of extraretinal signals to the scaling of object distance during self-motion

We investigated the role of extraretinal information in the perception of absolute distance. In a computer-simulated environment, monocular observers judged the distance of objects positioned at different locations in depth while performing frontoparallel movements of the head. The objects were spheres covered with random dots subtending three different visual angles. Observers viewed the objects ateye level, either in isolation or superimposed on a ground floor. The distance and size of the spheres were covaried to suppress relative size information. Hence, the main cues to distance were the motion parallax and the extraretinal signals. In three experiments, we found evidence that (1) perceived distance is correlated with simulated distance in terms of precision and accuracy, (2) the accuracy in the distance estimate is slightly improved by the presence of a ground-floor surface, (3) the perceived distance is not altered significantly when the visual field size increases, and (4) the absolute distance is estimated correctly during self-motion. Conversely, stationary subjects failed to report absolute distance when they passively observed a moving object producing the same retinal stimulation, unless they could rely on knowledge of the three-dimensional movements.     

The horizon-ratio relation as information for relative size in pictures

The horizon-ratio relation was found to be an effective source of information for relative size in pictures under some conditions: when the difference in image size of depicted “same real size” objects was not too great (Experiment 1), and when the horizon line was not too high or too low in the picture (Experiment 2). The latter finding seemed to be linked to the observers’ identification of the horizontal line as the horizon (and not as the edge of a finite surface). In addition, individual patterns of response were remarkably systematic even in the absence of a horizon, or any other pictorial information, (Experiment 3). It is suggested that in this case observers imposed a horizon on the picture on which to base their relative size judgments, possibly based on the observer’s own eye level or on the content of the picture. It is concluded that although the horizon-ratio relation provides the same kind of information as that available in the optic arrays from real scenes, pictorial information requires the satisfaction of additional constraints in order to be fully effective.     

Surface lightness influences perceived room height

Surprisingly little scientific research has been conducted on the effects of colour and lightness on the perception of spaciousness. Practitioners and architects typically suggest that a room's ceiling appears higher when it is painted lighter than the walls, while darker ceilings appear lower. Employing a virtual reality setting, we studied the effects of the lightness of different room surfaces on perceived height in two psychophysical experiments. Observers judged the height of rooms varying in physical height as well as in the lightness of ceiling, floor, and walls. Experiment 1 showed the expected increase of perceived height with increases in ceiling lightness. Unexpectedly, the perceived height additionally increased with wall lightness, and the effects of wall lightness and ceiling lightness were roughly additive, incompatible with a simple effect of the lightness contrast between the ceiling and the walls. Experiment 2 demonstrated that the floor lightness has no significant effect on perceived height, and that the total brightness of the room is not the critical factor influencing the perceived height. Neither can the results be explained by previously reported effects of brightness on apparent depth or perceived distance.     

Relative Distance Perception Through Expanding and Contracting Motion and the Role of Propiospecific Information in Walking

Two experiments using a new device that correlates simulated optic flow with forward and backward head motions are reported. The first experiment tested the effectiveness of the rate of optical expansion/contraction as a cue for relative distance perception; the second experiment examined the role of propriospecific information in determining whether or not a simulated wall was perceived to moving relative to the ground. In walking along the line of sight in a stationary environment, the image of a nearer object expands/contracts more than the image of objects farther away. In Experiment 1, observers' abilities to judge which of two walls was nearer, according to expanding/contracting patterns, were tested. The results show that both walking and stationary observers can detect the order of depth from expansion patterns but not from the contraction patterns. Experiment 2 assessed the role of propriospecific information for specifying the motion or nonmotion of simulated 'wall' relative to the ground. The results show the importance of synchrony between expansion/contraction patterns and head motion. Whether or not an observer is obtaining information actively does not seem to matter for perceiving relative distance but it does matter in perceiving object motion.     

Apparent depth of pictures reflected by a mirror: The plastic effect

We investigated the plastic effect in picture perception, in which the apparent depth of a picture is increased when it is reflected by a mirror. The plastic effect was well known in the mid-18th century, but very few studies have elucidated its nature. In Experiment 1, we examined how often the plastic effect occurs in different ocular conditions. A group of 22 observers compared directly observed pictures and their mirror-reflected images in each of free-binocular, free-monocular, and restrictive-monocular conditions. When the observers were forced to choose the picture that appeared greater in depth, 73?% of them chose the reflected pictures, regardless of oculomotor condition. In Experiment 2, we examined how often the plastic effect is detected as a function of observation time. When 22 observers compared a directly watched movie and its mirror-reflected movie for 5?min, the number of observers who judged the reflected movie to be greater in depth was about 55?% at the onset of the trial but was 86?% at the end. In Experiment 3, we examined transfer of the plastic effect. Ten observers judged the change in apparent depth of directly observed pictures after prolonged exposure to the same reflected or actual pictures. Transfer was confirmed and was greater for pictures that represented greater depth (r = .88). We suggested that the plastic effect is mainly induced by the double apparent locations of a reflected picture. From the long incubation time and the transfer to real pictures, we also suggested that it involves perceptual learning regarding visual skill.     

Depth perception as a function of motion parallax and absolute-distance information

The results of three experiments demonstrated that the visual system calibrates motion parallax according to absolute-distance information in processing depth. The parallax was created by yoking the relative movement of random dots displayed on a cathode-ray tube to the movements of the head. In Experiment 1, at viewing distances of 40 cm and 80 cm, observers reported the apparent depth produced by motion parallax equivalent to a binocular disparity of 0.47 degree. The mean apparent depth at 80 cm was 2.6 times larger than at 40 cm. In Experiment 2, again at viewing distances of 40 cm and 80 cm, observers adjusted the extent of parallax so that the apparent depth was 7.0 cm. The mean extent of parallax at 80 cm was 31% of that at 40 cm. In Experiment 3, distances ranged from 40 cm to 320 cm, and a wide range of parallax was used. As distance and parallax increased, the perception of a rigid three-dimensional surface was accompanied by rocking motion; perception of depth was replaced by perception of motion in some trials at 320 cm. Moreover, the mean apparent depths were proportional to the viewing distance at 40 cm and 80 cm but not at 160 cm and 320 cm.     

Object boundaries influence toddlers' performance in a search task

Previous research has shown that young children have difficulty searching for a hidden object whose location depends on the position of a partly visible physical barrier. Across four experiments, we tested whether children's search errors are affected by two variables that influence adults' object-directed attention: object boundaries and proximity relations. Toddlers searched for a car that rolled down a ramp behind an occluding panel and stopped on contact with a barrier. The car's location on each trial depended on the placement of the barrier behind one of two doors in the panel. In Experiment 1, when a part of the car (a pompom on an antenna) was visible at the same distance from the object as the barrier wall in past research, search performance was above chance but below ceiling. In Experiments 2 and 3, when the visible part was close to the hidden body of the car and could be seen through one of two windows in the doors of the occluding panel, performance was near ceiling. In Experiment 4, when only the barrier was visible through one of the same windows, performance was at chance. Toddlers' search for a hidden object therefore is affected by the proximity of a visible part of the object, though not by the proximity of a separate visible landmark. These findings suggest a parallel between the object representations of young children and those of adults, whose attention is directed to objects and spreads in a gradient-like fashion within an object.     

Perceived depth of 3-D objects in 3-D scenes

Effects of information specifying the position of an object in a 3-D scene were investigated in two experiments with twelve observers. To separate the effects of the change in scene position from the changes in the projection that occur with increased distance from the observer, the same projections were produced by simulating (a) a constant object at different scene positions and (b) different objects at the same scene position. The simulated scene consisted of a ground plane, a ceiling plane, and a cylinder on a pole attached to both planes. Motion-parallax scenes were studied in one experiment; texture-gradient scenes were studied in the other. Observers adjusted a line to match the perceived internal depth of the cylinder. Judged depth for objects matched in simulated size decreased as simulated distance from the observer increased. Judged depth decreased at a faster rate for the same projections shown at a constant scene position. Adding object-centered depth information (object rotation) increased judged depth for the motion-parallax displays. These results demonstrate that the judged internal depth of an object is reduced by the change in projection that occurs with increased distance, but this effect is diminished if information for change in scene position accompanies the change in projection.     

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.     

Discriminating direction of motion trajectories from angular speed and background information

The effects of a background scene on the perception of the trajectory of an approaching object and its relation to changes in angular speed and angular size were examined in five experiments. Observers judged the direction (upward or downward) of two sequentially presented motion trajectories simulating a sphere traveling toward the observer at a constant 3-D speed from a fixed distance. In Experiments 1–4, we examined the effects of changes in angular speed and the presence of a scene background, with changes in angular size based either on the trajectories being discriminated or on an intermediate trajectory. In Experiment 5, we examined the effects of changes in angular speed and scene background, with angular size either constant or consistent with an intermediate 3-D trajectory. Overall, we found that (1) observers were able to judge the direction of object motion trajectories from angular speed changes; (2) observers were more accurate with a 3-D scene background, as compared with a uniform background, suggesting that scene information is important for recovering object motion trajectories; and (3) observers were more accurate in judging motion trajectories based on angular speed when the angular size function was consistent with motion in depth than when the angular size was constant.     

Flipping and spinning: Spatial transformation procedures in the identification of rotated natural objects

The proposal that identification of inverted objects is accomplished by either a relatively slow rotation in the picture plane or a faster rotation in the depth plane about the horizontal axis was tested. In Experiment 1, subjects decided whether objects at 0° or 180° corresponded to previously learned normal views of the upright objects, or were mirror images. Instructions to mentally flip an inverted object in the depth plane to the upright produced faster decision times than did instructions to mentally spin the object in the picture plane. In Experiment 2, the effects of orientation were compared across an object-naming task and a normal-mirror task for six orientations from 0° to 300°. In the normal-mirror task, objects at 180° were cued for rotation in the picture plane or in the depth plane in equal numbers. The naming function for one group of subjects did not differ from the normalmirror function where inverted objects had been mentally rotated to the upright. For both functions, response time (RT) increased linearly from 0° to 180° and the slopes did not differ. The naming function for a second group of subjects did not differ from the normal-mirror function where inverted objects had been mentally flipped to the upright. For both functions, RT increased linearly at a similar rate from 0° to 120°, but decreased from 120° to 180°. The results are discussed in terms of theories of orientation-specific identification.     

Monocular without stereopsis with and head movement

Random dots moving with various velocity gradients were presented to observers; the motion was yoked to head movement in one condition and to no head movement in another. In Experiment 1, 12 observers were shown motion gradients with sine, triangle, sawtooth, and square waveforms with amplitudes (equivalent disparities) of 12′ and 1° 53′. In Experiment 2, 48 observers were shown only the sinewave or square-wave gradient of 1° 53′ disparity either with or without head movement so that the observers’ expectation to see depth in one condition did not transfer to another. The main findings were: (1) with 12′ disparity, the head-movement condition produced perceived depth but almost no perceived motion, whereas the no-head-movement condition produced both perceived depth and perceived motion; (2) with 1° 53′ disparity, both conditions produced perceived depth and perceived motion; and (3) when the expectation to see depth was removed, the no-head-movement condition with the square-wave gradient produced no perceived depth, only motion. We suggest that monocular stereopsis with head movement can be achieved without perception of motion but monocular stereopsis without head movement requires perception of motion.     

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).     

Virtual and drawing structures for the Müller-Lyer illusions

This research assessed the relative contribution of 3-D virtual structure that generated the stimulus drawings (scene-based and picture-based theories) and 2-D structure of the drawings (object-based theories). Virtual structures were right-angle convex and concave corners in front of and behind the picture plane, respectively. Virtual corner size was manipulated directly (Experiment 1) and indirectly by manipulating drawing station point distance (Experiments 2 and 3), corner depth (Experiment 4), and corner distance from the picture plane (Experiments 5 and 6). Experiments 2 and 4 held the size of the projected corner edge (interior target line) constant, causing virtual corner size to vary, whereas Experiments 3, 4, 5, and 6 held size of the virtual corners constant, causing size of the projected corner edge or interior target line to vary. Subjects reproduced the length of the projected corner edge (interior target line). The illusions (difference between reproduced size of the projected corner edge and T-junction control) were generally well fit by the weighted sum of virtual corner size and size of the projected corner edge, but the projected distance between boundary line terminations (intertip distance) appeared as an additional contributing factor in Experiments 5 and 6. The implications of this methodological approach are discussed for theories of the illusions.     

The importance of being upright: Use of environmental and viewer-centered reference frames in shape discriminations of novel three-dimensional objects

We investigated the frame of reference that people use to make shape discriminations when their heads are either upright or tilted. Observers madesame-different judgments of pairs of novel threedimensional objects that were aligned along their length within the frontal-parallel plane and rotated in depth around an axis parallel to their own axes of elongation. The aligned objects were displayed vertically, tilted 45°, or horizontally with respect to the environmental upright, but the distance of each pair from the upright was irrelevant to resolving the angular disparity between the stimuli for thesame-different judgment. Nevertheless, when observers’ heads were upright, the time to encode the stimuli was a linear function of the distance of the stimuli from the environmental upright, whereas when observers’ heads were tilted 45°, encoding times for tilted and vertical stimuli did not differ and were faster than the times to encode horizontal stimuli. We interpreted these data to mean that observers either rotate or reference the top of an object to the environmental upright, and they can use either a gravitational or retinal reference frame to do so when either they or the objects are not upright.     

Monocular stereopsis with and without head movement

Random dots moving with various velocity gradients were presented to observers; the motion was yoked to head movement in one condition and to no head movement in another. In Experiment 1, 12 observers were shown motion gradients with sine, triangle, sawtooth, and square waveforms with amplitudes (equivalent disparities) of 12' and 1 degrees 53'. In Experiment 2, 48 observers were shown only the sinewave or square-wave gradient of 1 degrees 53' disparity either with or without head movement so that the observers' expectation to see depth in one condition did not transfer to another. The main findings were: (1) with 12' disparity, the head-movement condition produced perceived depth but almost no perceived motion, whereas the no-head-movement condition produced both perceived depth and perceived motion; (2) with 1 degrees 53' disparity, both conditions produced perceived depth and perceived motion; and (3) when the expectation to see depth was removed, the no-head-movement condition with the square-wave gradient produced no perceived depth, only motion. We suggest that monocular stereopsis with head movement can be achieved without perception of motion but monocular stereopsis without head movement requires perception of motion.     

Information about three-dimensional shape and direction of illumination in a square-wave grating

A homogeneous grey picture and a 'Mondrian' type of picture were illuminated by a projector with square-wave gratings of thirty different contrast values used as slides. Ten observers reported whether the picture appeared three-dimensional (3-D) (pleated) or flat. 3-D responses in this situation indicate colour constancy 'at the cost of' nonveridical depth perception. The frequencies of 3-D responses were significantly higher for the structured picture than for the homogeneous grey one. In reports of the direction from which the apparent 3-D object appeared to be illuminated there was a significant preference for responses "from above" when the grating was horizontally oriented. With vertical orientation there was no preference for "from the left" or "from the right". The results from the first experiment contradict traditional cue theories of depth perception since the projection of the borders between the fields of the structured picture was invariant and expected to inform about the flatness of the picture. They are, however, in line with a model for perceptual analysis of reflected light into common and relative components proposed earlier by Bergstr?m. The difference in perceived direction of illumination between horizontally and vertically orientated gratings is discussed in connection with human ecology.     

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.     

The perception of depth and slant from texture in three-dimensional scenes.

The perception of depth and slant in three-dimensional scenes specified by texture was investigated in five experiments. Subjects were presented with computer-generated scenes of a ground and ceiling plane receding in depth. Compression, convergence, and grid textures were examined. The effect of the presence or absence of a gap in the center of the display was also assessed. Under some conditions perceived slant and depth from compression were greater than those found with convergence. The relative effectiveness of compression in specifying surface slant was greater for surfaces closer to ground planes (80 degrees slant) than for surfaces closer to frontal parallel planes (40 degrees slant). The usefulness of compression was also observed with single-plane displays and with displays with surfaces oriented to reduce information regarding the horizon.