An indirect measure of perceived distance from oculomotor cues

The sensitivity of an indirect method of measuring perceived distance was compared in two experiments with the direct procedure of eliciting verbal reports of distance. Perceived distance was varied by varying the oculomotor cues to object distance. The indirect method, called the “adjustable pivot method,” uses an apparatus that physically moves the stimulus object laterally concomitantly with the lateral motion of the head. The magnitude and direction of this concomitant motion determines the distance of the point around which the direction of gaze to the object rotates (the pivot distance) as the head is moved. The pivot distance at which the object appears stationary with head movement measures the apparent distance of the object. Both types of measures were found to vary systematically with the oculomotor distance of the object for points of light (Experiment 1) and extended objects (Experiment 2). A previous study has shown that the adjustable pivot method avoids cognitive errors that can distort verbal reports of distance. The present study, by demonstrating the discriminative capability of this method under conditions in which differences in perceived distance were expected to occur, provides clear evidence that the adjustable pivot method is a sensitive and useful procedure for measuring perceived distance.     

The sensing of retinal motion

The apparent relative motion of physically stationary objects that frequently occurs as the head is moved in a frontoparallel plane is almost always in the direction expected from the projection into the distal world of the relative motion of the images on the eye. It is hypothesized that this is the result of the perceptual underestimation of the depth between the objects. If a perceptual overestimation of the depth were produced, it was predicted that the apparent relative motion would be in a direction opposite to that expected from the projection of the retinal motions. This prediction was tested using the binocular disparity cue to produce perceptual overestimation of the slant (depth) of a luminous line. In this case, perceived slant was the indicator of perceived depth, and perceived rotation concomitant with the motion of the head was the indicator of perceived relative motion. The results support the prediction and also provide some support for a theoretically derived equation specifying the relation between these two perceptual variables.     

The common occurrence of errors of perceived distance

The effect on the perceived distance of a test object of fixating to a distance different from that of the test object was investigated using monocular observation and two methods for measuring perceived distance. One method, the size adjustment procedure, applying the size-distance invariance hypothesis, measured perceived distance by measuring perceived size. The results from this method were compared with those from a head-motion procedure which used the apparent concomitant motion resulting from head motion to measure perceived distance. The results from both procedures indicated that the apparent distance of the test object physically located at a constant distance varied directly as a function of the fixation distance. This occurred despite the presence of texture on the walls and floor of the visual alley. These and other perceptual effects are interpreted as demonstrating that errors in perceived distance (contrary to the theory of direct perception) are a common occurrence in ordinary visual fields.     

Absence of compensation and reasoning-like processes in the perception of orientation in depth.

When errors are present in the perceived depth between the parts of a physically stationary object, the object appears to rotate as the head is moved laterally (Gogel, 1980). This illusory rotation has been attributed either to compensation (Wallach, 1985, 1987) or to inferential-like processes (Rock, 1983). Alternatively, the perceived distances of and directions to the parts of the object are sufficient to explain the illusory perceived orientations and perceived rotations of the stimulus. This was examined in three experiments. In Experiment 1, a perceived illusory orientation of a stimulus object extended in depth was produced by misleading binocular disparity and was measured at two different lateral positions of the head under two conditions. In the static condition, the head was stationary at different times at each of the two measurement positions of the head. In the dynamic condition, continuous motion of the head occurred between these two positions. In Experiment 2, static and dynamic conditions of illusory stimulus orientation were observed with the head stationary. In Experiment 3, a perspective illusion instead of binocular disparity produced the errors in perceived depth. In no experiment did the perceived orientation of the object differ for the static and dynamic conditions. In the absence of head motion, neither compensatory nor inferential-like processes were available. It is concluded that these processes are not needed to explain either illusory or nonillusory perceptions of the orientation or rotation of stimuli viewed with a laterally moving head.     

Absence of compensation and reasoning-like processes in the perception of orientation in depth

When errors are present in the perceived depth between the parts of a physically stationary object, the object appears to rotate as the head is moved laterally (Gogel, 1980). This illusory rotation has been attributed either to compensation (Wallach, 1985, 1987) or to inferential-like processes (Rock, 1983). Alternatively, the perceived distances of and directions to the parts of the object are sufficient to explain the illusory perceived orientations and perceived rotations of the stimulus. This was examined in three experiments. In Experiment 1, a perceived illusory orientation of a stimulus object extended in depth was produced by misleading binocular disparity and was measured at two different lateral positions of the head under two conditions. In the static condition, the head was stationary at different times at each of the two measurement positions of the head. In the dynamic condition, continuous motion of the head occurred between these two positions. In Experiment 2, static and dynamic conditions of illusory stimulus orientation were observed with the head stationary. In Experiment 3, a perspective illusion instead of binocular disparity produced the errors in perceived depth. In no experiment did the perceived orientation of the object differ for the static and dynamic conditions. In the absence of head motion, neither compensatory nor inferential-like processes were available. It is concluded that these processes are not needed to explain either illusory or nonillusory perceptions of the orientation or rotation of stimuli viewed with a laterally moving head.     

The effect of perceived distance on induced movement

The magnitude of induced movement was measured as a function of the perceived depth between the test object and the plane of the induction object, with this perceived depth produced by stereoscopic cues. Three experiments were conducted. In each experiment, the induction object (a frame of constant physical size) was positioned at one of three distances with the test object (a point of light) placed successively at each of the three distances. Predictions of the magnitude of induction as a function of the depth separation of the test and induction object were made from the subject-relative and object-relative hypotheses of induced motion. It was expected, however, that neither of these hypotheses would predict the results independently of a factor described in the adjacency principle. This principle states that the effectiveness of whatever cues or processes determine the induced movement will decrease with increased depth between the test and induction object. The data indicate that the adjacency principle must be considered in explaining the results. The subject-relative rather than object-relative hypothesis as modified by the adjacency principle was most successful in predicting the results. Control conditions in which the frame was stationary and the point of light was physically moving were also used. Despite the fact that the relative displacement of the objects on the eye in the experimental and control conditions were the same, the results indicate that O could distinguish between these two kinds of conditions. Although the apparent movement was greater in the control conditions than in the experimental conditions, the reverse is true if the total perceived movement of the test and induction object are considered together.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head.

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in perceived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation-for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in pereived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

Analysis of the perception of motion concomitant with a lateral motion of the head

This study is concerned with two questions regarding the illusory motion of objects that occurs concomitantly with motion of the head. One is whether this illusory concomitant motion, unlike the perception of real motion, is paradoxical in the sense that, although the object appears to move, it does not appear to go anywhere. The second question is whether illusory concomitant motion can be explained by errors in convergence produced by a tendency for the convergence of the eyes to displace in the direction of the resting state of convergence. Both questions receive a negative answer. In Experiment 1, it is shown that the illusory motion perceptually can add to or subtract from apparent motion resulting from real motion. In Experiment 2, it is shown that, for a binocularly viewed object at a near distance, the error in convergence (fixation disparity) is far too small to be an explanation for the illusory object motion associated with a moving head. The results of both experiments support an interpretation of illusory concomitant motion in terms of errors in the apparent distance of the stimulus object and the veridical perception of its direction.     

Contribution of bottom-up and top-down motion processes to perceived position

Perceived position depends on many factors, including motion present in a visual scene. Convincing evidence shows that high-level motion perception--which is driven by top-down processes such as attentional tracking or inferred motion--can influence the perceived position of an object. Is high-level motion sufficient to influence perceived position, and is attention to or awareness of motion direction necessary to displace objects' perceived positions? Consistent with previous reports, the first experiment revealed that the perception of motion, even when no physical motion was present, was sufficient to shift perceived position. A second experiment showed that when subjects were unable to identify the direction of a physically present motion stimulus, the apparent locations of other objects were still influenced. Thus, motion influences perceived position by at least two distinct processes. The first involves a passive, preattentive mechanism that does not depend on perceptual awareness; the second, a top-down process that depends on the perceptual awareness of motion direction. Each contributes to perceived position, but independently of the other.     

Symmetry and elongation of objects influence perceived direction of translational motion

Five experiments were conducted to examine how perceived direction of motion is influenced by aspects of shape of a moving object such as symmetry and elongation. Random polygons moving obliquely were presented on a computer screen and perceived direction of motion was measured. Experiments 1 and 2 showed that a symmetric object moving off the axis of symmetry caused motion to be perceived as more aligned with the axis than it actually was. However, Experiment 3 showed that motion did not influence perceived orientation of symmetry axis. Experiment 4 revealed that symmetric shapes resulted in faster judgments on direction of motion than asymmetric shapes only when the motion is along the axis. Experiment 5 showed that elongation causes a bias in perceived direction of motion similar to effects of symmetry. Existence of such biases is consistent with the hypothesis that in the course of evolution, the visual system has been adapted to regularities of motion in the animate world.     

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.     

Aging and the perception of slant from optical texture, motion parallax, and binocular disparity

The ability of younger and older observers to perceive surface slant was investigated in four experiments. The surfaces possessed slants of 20°, 35°, 50°, and 65°, relative to the frontoparallel plane. The observers judged the slants using either a palm board (Experiments 1, 3, and 4) or magnitude estimation (Experiment 2). In Experiments 1–3, physically slanted surfaces were used (the surfaces possessed marble, granite, pebble, and circle textures), whereas computer-generated 3-D surfaces (defined by motion parallax and binocular disparity) were utilized in Experiment 4. The results showed that the younger and older observers' performance was essentially identical with regard to accuracy. The younger and older age groups, however, differed in terms of precision in Experiments 1 and 2: The judgments of the older observers were more variable across repeated trials. When taken as a whole, the results demonstrate that older observers (at least through the age of 83 years) can effectively extract information about slant in depth from optical patterns containing texture, motion parallax, or binocular disparity.     

Perceived causality and perceived force: Same or different?

Visually perceived interactions between objects, such as animated versions of billiard ball collisions, give rise to causal impressions, impressions that one object produces some effect in another, and force impressions, impressions that one object exerts a certain amount of force on another. In four experiments, evidence for strong divergence between these two impressions is reported. Manipulations of relative direction of motion and point of contact between the objects had different effects on the causal and force impressions (Experiment 1); delay between one object contacting another and the latter starting to move had a stronger effect on the causal impression than on the force impression (Experiment 2); a context of other moving objects significantly weakened the causal impression but not the force impression (Experiment 3); and there was an inverse relation between an impression of one object penetrating another and the amount of force the former was perceived as exerting on the latter (Experiment 4). These findings are explained in terms of differential effects of instructions on attention, and also in terms of differences in meaning between force and causality.     

Transient attention degrades perceived apparent motion

Transient spatial attention refers to the automatic selection of a location that is driven by the stimulus rather than a voluntary decision. Apparent motion is an illusory motion created by stationary stimuli that are presented successively at different locations. In this study we explored the effects of transient attention on apparent motion. The motion target presentation was preceded by either valid attentional cues that attract attention to the target location in advance (experiments 1-4), neutral cues that do not indicate a location (experiments 1, 3, and 4), or invalid cues that direct attention to a non-target location (experiment 2). Valid attentional cues usually improve performance in various tasks. Here, however, an attentional impairment was found. Observers' ability to discriminate the direction of motion diminished at the cued location. Analogous results were obtained regardless of cue type: singleton cue (experiment 1), central non-informative cue (experiment 2), or abrupt onset cue (experiment 3). Experiment 4 further demonstrated that reversed apparent motion is less likely with attention. This seemingly counterintuitive attentional degradation of perceived apparent motion is consistent with several recent findings, and together they suggest that transient attention facilitates spatial segregation and temporal integration but impairs spatial integration and temporal segregation.     

An indirect method of measuring perceived distance from familiar size

Two methods of measuring perceived distance as a function of familiar size were compared in five experiments. The method which uses the perception of motion concomitant with a motion of the head, unlike the method of verbal report, is considered to provide a measure of perceived distance that is unaffected by factors of cognitive distance. The results of the experiments indicate that although the perceived egocentric distance of an object can vary somewhat as a function of the cue of familiar size, the larger variation often found with verbal reports of distance is based upon cognitive, not perceptual, information. The cognitive information is interpreted as resulting from the perception of the object as off-sized and the observer’s assumption that the perceived size of an object will vary inversely with its physical distance.     

Apparent extended body motions in depth.

Five experiments were designed to investigate the influence of three-dimensional (3-D) orientation change on apparent motion. Projections of an orientation-specific 3-D object were sequentially flashed in different locations and at different orientations. Such an occurrence could be resolved by perceiving a rotational motion in depth around an axis external to the object. Consistent with this proposal, it was found that observers perceived curved paths in depth. Although the magnitude of perceived trajectory curvature often fell short of that required for rotational motions in depth (3-D circularity), judgments of the slant of the virtual plane on which apparent motions occurred were quite close to the predictions of a model that proposes circular paths in depth.     

Relative motion and the adjacency principle

The perception of motion of physically moving points of light was investigated in terms of the distinction between absolute and relative motion cues and the change in the effectiveness of the latter as a function of the frontoparallel separation between the points. In situations in which two competing relative motion cues were available to determine the perceived path of motion of a point of light, it was found that the relative motion cue between more adjacent points was more effective than the relative motion cue between more separated points. In situations in which only one relative motion cue was available to determine the perceived motion of a point it was found that the effectiveness of this cue as compared with the absolute motion cue decreased with increased separation. These results are predictable from the adjacency principle which states that the effectiveness of cues between objects is an inverse function of object separation. Some consequences of the study for the theory of motion perception are discussed.     

Visually perceived motion in depth resulting from proximal changes. II

According to a model for motion and form perception proposed by Johansson (1964). every two-dimensional change in a changing proximal stimulation is projected out as a motion in depth The model assumes that the amount of perceivedrelative motion (the fraction between the perceived amount of motion of the object and the perceived initial distance to the object) is determined only by the amount ofrelative change (the fraction between the absolute amount of change and the initial size). The aim of the present study was to test this hypothesis by studying the effect of some other variables on perceived relative motion in depth. As stimuli, continuously shrinking and growing squares were used. No effects were found when varying the absolute amount of change. Neither did the rate of change influence the perceived relative motion in any important way. The only variable that gave rise to strong and systematic effects on perceived relative motion was the initial distance to the perceived object. The greater the initial distance, the less relative motion was perceived.     

When texture takes precedence over motion in depth perception

Both texture and motion can be strong cues to depth, and estimating slant from texture cues can be considered analogous to calculating slant from motion parallax (Malik and Rosenholtz 1994, report UCB/CSD 93/775, University of California, Berkeley, CA). A series of experiments was conducted to determine the relative weight of texture and motion cues in the perception of planar-surface slant when both texture and motion convey similar information. Stimuli were monocularly viewed images of planar surfaces slanted in depth, defined by texture and motion information that could be varied independently. Slant discrimination biases and thresholds were measured by a method of single-stimuli binary-choice procedure. When the motion and texture cues depicted surfaces of identical slants, it was found that the depth-from-motion information neither reduced slant discrimination thresholds, nor altered slant discrimination bias, compared to texture cues presented alone. When there was a difference in the slant depicted by motion and by texture, perceived slant was determined almost entirely by the texture cue. The regularity of the texture pattern did not affect this weighting. Results are discussed in terms of models of cue combination and previous results with different types of texture and motion information.