Effects of collimation on perceived layout in 3-D scenes.

The effect of varying information for overall depth in a simulated 3-D scene on the perceived layout of objects in the scene was investigated in two experiments. Subjects were presented with displays simulating textured surfaces receded in depth. Pairs of markers were positioned at equal intervals within the scenes. The subject's task was to judge the depth between the intervals. Overall scene depth was varied by viewing through either a collimating lens or a glass disk. Judged depth for equal depth intervals decreased with increasing distance of the interval from the front of the scene. Judged depth was greater for collimated than for non-collimated viewing. Interestingly, collimated viewing resulted in a uniform rescaling of the perceived depth intervals.     

Propagation of depth information from local regions in 3-D scenes

The effects of regions with local linear perspective on judgments of the depth separation between two objects in a scene were investigated for scenes consisting of a ground plane, a quadrilateral region, and two poles separated in depth. The poles were either inside or outside the region. Two types of displays were used: motion-parallax dot displays, and a still photograph of a real scene on which computer-generated regions and objects were superimposed. Judged depth separations were greater for regions with greater linear perspective, both for objects inside and outside the region. In most cases, the effect of the region's shape was reduced for objects outside the region. Some systematic differences were found between the two types of displays. For example, adding a region with any shape increased judged depth in motion-parallax displays, but only high-perspective regions increased judged depth in real-scene displays. We conclude that depth information present in local regions affects perceived depth within the region, and that these effects propagate, to a lesser degree, outside the region.     

What's Scene and Not Seen: Influences of Movement and Task Upon What We See

Studies concerning the processing of natural scenes using eye movement equipment have revealed that observers retain surprisingly little information from one fixation to the next. Other studies, in which fixation remained constant while elements within the scene were changed, have shown that, even without refixation, objects within a scene are surprisingly poorly represented. Although this effect has been studied in some detail in static scenes, there has been relatively little work on scenes as we would normally experience them, namely dynamic and ever changing. This paper describes a comparable form of change blindness in dynamic scenes, in which detection is performed in the presence of simulated observer motion. The study also describes how change blindness is affected by the manner in which the observer interacts with the environment, by comparing detection performance of an observer as the passenger or driver of a car. The experiments show that observer motion reduces the detection of orientation and location changes, and that the task of driving causes a concentration of object analysis on or near the line of motion, relative to passive viewing of the same scene.     

Recovery of 3-D shape from binocular disparity and structure from motion

Four experiments were conducted to examine the integration of depth information from binocular stereopsis and structure from motion (SFM), using stereograms simulating transparent cylindrical objects. We found that the judged depth increased when either rotational or translational motion was added to a display, but the increase was greater for rotating (SFM) displays. Judged depth decreased as texture element density increased for static and translating stereo displays, but it stayed relatively constant for rotating displays. This result indicates that SFM may facilitate stereo processing by helping to resolve the stereo correspondence problem. Overall, the results from these experiments provide evidence for a cooperative relationship between. SFM and binocular disparity in the recovery of 3-D relationships from 2-D images. These findings indicate that the processing of depth information from SFM and binocular disparity is not strictly modular, and thus theories of combining visual information that assume strong modularity-or-independence cannot accurately characterize all instances of depth perception from multiple sources.     

Distance perception from motion parallax and ground contact

Meng and Sedgwick (2001, 2002) found that the perceived distance of an object in a stationary scene was determined by the position at which it contacted the ground in the image, or by nested contact relations among intermediate surfaces. Three experiments investigated whether motion parallax would allow observers to determine the distance of a floating object without intermediate contact relations. The displays consisted of one or more computer-generated textured cylinders inserted into a motion picture or still image of an actual 3-D scene. In the motion displays, both the cylinders and the scene translated horizontally. Judged distance for a single cylinder floating above the ground was determined primarily by the location at which the object contacted the ground in the projected image (“optical contact”), but was altered in the direction indicated by motion parallax. When more than one cylinder was present and observers were asked to judge the distance of the top cylinder, judged distance moved closer to that indicated by motion parallax, almost matching that value with three cylinders. These results indicate that judged distance in a dynamic scene is affected both by optical contact and motion parallax, with motion parallax more effective when multiple objects are present.     

Do object-category selective regions in the ventral visual stream represent perceived distance information?

It is well established that scenes and objects elicit a highly selective response in specific brain regions in the ventral visual cortex. An inherent difference between these categories that has not been explored yet is their perceived distance from the observer (i.e. scenes are distal whereas objects are proximal). The current study aimed to test the extent to which scene and object selective areas are sensitive to perceived distance information independently from their category-selectivity and retinotopic location. We conducted two studies that used a distance illusion (i.e., the Ponzo lines) and showed that scene regions (the parahippocampal place area, PPA, and transverse occipital sulcus, TOS) are biased toward perceived distal stimuli, whereas the lateral occipital (LO) object region is biased toward perceived proximal stimuli. These results suggest that the ventral visual cortex plays a role in representing distance information, extending recent findings on the sensitivity of these regions to location information. More broadly, our findings imply that distance information is inherent to object recognition.     

Overestimation of the projected size of objects on the surface of mirrors and windows

Four experiments investigated judgments of the size of projections of objects on the glass surface of mirrors and windows. The authors tested different ways of explaining the task to overcome the difficulty that people had in understanding what the projection was, and they varied the distance of the observer and the object to the mirror or window and varied the size of the mirror. The authors compared estimations of projected size with estimations of the physical size of the object that produced the projection. For both mirrors and windows, observers accurately judged the physical size of objects but greatly overestimated the projected size of the same objects. Indeed, judgments of projected size were more similar to physical than to projected size. People were also questioned verbally about their knowledge of projected size relative to physical size. The errors produced for these conceptual questions were similar to those found in the perceptual estimation tasks. Together, these results suggest that projections of objects on mirrors and windows are treated in the same way and that observers cannot perceive such projections as distal objects.     

The Role of Fixation Position in Detecting Scene Changes Across Saccades

Target objects presented within color images of naturalistic scenes were deleted or rotated during a saccade to or from the target object or to a control region of the scene. Despite instructions to memorize the details of the scenes and to monitor for object changes, viewers frequently failed to notice the changes. However, the failure to detect change was mediated by three other important factors: First, accuracy generally increased as the distance between the changing region and the fixation immediately before or after the change decreased. Second, changes were sometimes initially missed, but subsequently noticed when the changed region was later refixated. Third, when an object disappeared from a scene, detection of that disappearance was greatly improved when the deletion occurred during the saccade toward that object. These results suggest that fixation position and saccade direction play an important role in determining whether changes will be detected. It appears that more information can be retained across views than has been suggested by previous studies.     

The spatial and temporal characteristics of perceiving 3-D structure from motion

In four experiments, a scalar judgment of perceived depth was used to examine the spatial and temporal characteristics of the perceptual buildup of three-dimensional (3-D) structure from optical motion as a function of the depth in the simulated object, the speed of motion, the number of elements defining the object, the smoothness of the optic flow field, and the type of motion. In most of the experiments, the objects were polar projections of simulated half-ellipsoids under-going a curvilinear translation about the screen center. It was found that the buildup of 3-D structure was: (1) jointly dependent on the speed at which an object moved and on the range through which the object moved; (2) more rapid for deep simulated objects than for shallow objects; (3) unaffected by the number of points defining the object, including the maximum apparent depth within each simulated object-depth condition; (4) not disrupted by nonsmooth optic flow fields; and (5) more rapid for rotating objects than for curvilinearly translating objects.     

The spatial and temporal characteristics of perceiving 3-D structure from motion.

In four experiments, a scalar judgment of perceived depth was used to examine the spatial and temporal characteristics of the perceptual buildup of three-dimensional (3-D) structure from optical motion as a function of the depth in the simulated object, the speed of motion, the number of elements defining the object, the smoothness of the optic flow field, and the type of motion. In most of the experiments, the objects were polar projections of simulated half-ellipsoids undergoing a curvilinear translation about the screen center. It was found that the buildup of 3-D structure was: (1) jointly dependent on the speed at which an object moved and on the range through which the object moved; (2) more rapid for deep simulated objects than for shallow objects; (3) unaffected by the number of points defining the object, including the maximum apparent depth within each simulated object-depth condition; (4) not disrupted by nonsmooth optic flow fields; and (5) more rapid for rotating objects than for curvilinearly translating objects.     

Scene and position specificity in visual memory for objects

This study investigated whether and how visual representations of individual objects are bound in memory to scene context. Participants viewed a series of naturalistic scenes, and memory for the visual form of a target object in each scene was examined in a 2-alternative forced-choice test, with the distractor object either a different object token or the target object rotated in depth. In Experiments 1 and 2, object memory performance was more accurate when the test object alternatives were displayed within the original scene than when they were displayed in isolation, demonstrating object-to-scene binding. Experiment 3 tested the hypothesis that episodic scene representations are formed through the binding of object representations to scene locations. Consistent with this hypothesis, memory performance was more accurate when the test alternatives were displayed within the scene at the same position originally occupied by the target than when they were displayed at a different position.     

Object-position binding in visual memory for natural scenes and object arrays

Nine experiments examined the means by which visual memory for individual objects is structured into a larger representation of a scene. Participants viewed images of natural scenes or object arrays in a change detection task requiring memory for the visual form of a single target object. In the test image, 2 properties of the stimulus were independently manipulated: the position of the target object and the spatial properties of the larger scene or array context. Memory performance was higher when the target object position remained the same from study to test. This same-position advantage was reduced or eliminated following contextual changes that disrupted the relative spatial relationships among contextual objects (context deletion, scrambling, and binding change) but was preserved following contextual change that did not disrupt relative spatial relationships (translation). Thus, episodic scene representations are formed through the binding of objects to scene locations, and object position is defined relative to a larger spatial representation coding the relative locations of contextual objects.     

Human heading judgments in the presence of moving objects

When moving toward a stationary scene, people judge their heading quite well from visual information alone. Much experimental and modeling work has been presented to analyze how people judge their heading for stationary scenes. However, in everyday life, we often move through scenes that contain moving objects. Most models have difficulty computing heading when moving objects are in the scene, and few studies have examined how well humans perform in the presence of moving objects. In this study, we tested how well people judge their heading in the presence of moving objects. We found that people perform remarkably well under a variety of conditions. The only condition that affects an observer’s ability to judge heading accurately consists of a large moving object crossing the observer’s path. In this case, the presence of the object causes a small bias in the heading judgments. For objects moving horizontally with respect to the observer, this bias is in the object’s direction of motion. These results present a challenge for computational models.     

Distance perception as a function of photographic area of view

Distance perception of depicted objects was examined as a function of photographic area of view. Subjects viewed slides of natural outdoor scenes and directly estimated the distances to specified objects. Area of view was manipulated by means of photographing each scene with lenses of five different focal lengths: 135, 80, 48, 28, and 17 mm. Distance perception along the pictorial depth plane was systematically transformed through changing the photographic area of view: the shorter the focal length of the camera lens, the wider the area of view, and the greater the perceived distance. Linear functions for each subject's distance judgments revealed a very high goodness of fit. Both the y-intercepts and the slopes increased as focal length decreased. The increasing y-intercepts suggest that viewers place themselves farther away from the depicted scene as focal length decreases, compensating for the advancing proximal edge. The increasing slopes suggest that distance information throughout the pictorial depth plane appears to change with focal length. The subjects also made direct judgments of foreground truncation, revealing that foreground truncation decreased as focal length decreased, but that this decrease did not account for the considerable expansion in distance perception.     

Scene perception: Detecting and judging objects undergoing relational violations

Five classes of relations between an object and its setting can characterize the organization of objects into real-world scenes. The relations are (1) Interposition (objects interrupt their background), (2) Support (objects tend to rest on surfaces), (3) Probability (objects tend to be found in some scenes but not others), (4) Position (given an object is probable in a scene, it often is found in some positions and not others), and (5) familiar Size (objects have a limited set of size relations with other objects). In two experiments subjects viewed brief (150 msec) presentations of slides of scenes in which an object in a cued location in the scene was either in a normal relation to its background or violated from one to three of the relations. Such objects appear to (1) have the background pass through them, (2) float in air, (3) be unlikely in that particular scene, (4) be in an inappropriate position, and (5) be too large or too small relative to the other objects in the scene. In Experiment I, subjects attempted to determine whether the cued object corresponded to a target object which had been specified in advance by name. With the exception of the Interposition violation, violation costs were incurred in that the detection of objects undergoing violations was less accurate and slower than when those same objects were in normal relations to their setting. However, the detection of objects in normal relations to their setting (innocent bystanders) was unaffected by the presence of another object undergoing a violation in that same setting. This indicates that the violation costs were incurred not because of an unsuccessful elicitation of a frame or schema for the scene but because properly formed frames interfered with (or did not facilitate) the perceptibility of objects undergoing violations. As the number of violations increased, target detectability generally decreased. Thus, the relations were accessed from the results of a single fixation and were available sufficiently early during the time course of scene perception to affect the perception of the objects in the scene. Contrary to expectations from a bottom-up account of scene perception, violations of the pervasive physical relations of Support and Interposition were not more disruptive on object detection than the semantic violations of Probability, Position and Size. These are termed semantic because they require access to the referential meaning of the object. In Experiment II, subjects attempted to detect the presence of the violations themselves. Violations of the semantic relations were detected more accurately than violations of Interposition and at least as accurately as violations of Support. As the number of violations increased, the detectability of the incongruities between an object and its setting increased. These results provide converging evidence that semantic relations can be accessed from the results of a single fixation. In both experiments information about Position was accessed at least as quickly as information on Probability. Thus in Experiment I, the interference that resulted from placing a fire hydrant in a kitchen was not greater than the interference from placing it on top of a mail ? in a street scene. Similarly, violations of Probability in Experiment II were not more detectable than violations of Position. Thus, the semantic relations which were accessed included information about the detailed interactions among the objects—information which is more specific than what can be inferred from the general setting. Access to the semantic relations among the entities in a scene is not deferred until the completion of spatial and depth processing and object identification. Instead, an object's semantic relations are accessed simultaneously with its physical relations as well as with its own identification.     

Visual distance estimation in static compared to moving virtual scenes

Visual motion is used to control direction and speed of self-motion and time-to-contact with an obstacle. In earlier work, we found that human subjects can discriminate between the distances of different visually simulated self-motions in a virtual scene. Distance indication in terms of an exocentric interval adjustment task, however, revealed linear correlation between perceived and indicated distances but with a profound distance underestimation. One possible explanation for this underestimation is the perception of visual space in virtual environments. Humans perceive visual space in natural scenes as curved, and distances are increasingly underestimated with increasing distance from the observer. Such spatial compression may also exist in our virtual environment. We therefore surveyed perceived visual space in a static virtual scene. We asked observers to compare two horizontal depth intervals, similar to experiments performed in natural space. Subjects had to indicate the size of one depth interval relative to a second interval. Our observers perceived visual space in the virtual environment as compressed, similar to the perception found in natural scenes. However, the nonlinear depth function we found can not explain the observed distance underestimation of visual simulated self-motions in the same environment.     

Updating representations of learned scenes

Two experiments were designed to compare scene recognition reaction time (RT) and accuracy patterns following observer versus scene movement. In Experiment 1, participants memorized a scene from a single perspective. Then, either the scene was rotated or the participants moved (0°–360° in 36° increments) around the scene, and participants judged whether the objects’ positions had changed. Regardless of whether the scene was rotated or the observer moved, RT increased with greater angular distance between judged and encoded views. In Experiment 2, we varied the delay (0, 6, or 12 s) between scene encoding and locomotion. Regardless of the delay, however, accuracy decreased and RT increased with angular distance. Thus, our data show that observer movement does not necessarily update representations of spatial layouts and raise questions about the effects of duration limitations and encoding points of view on the automatic spatial updating of representations of scenes.     

Spatial updating according to a fixed reference direction of a briefly viewed layout

Three experiments examined the role of reference directions in spatial updating. Participants briefly viewed an array of five objects. A non-egocentric reference direction was primed by placing a stick under two objects in the array at the time of learning. After a short interval, participants detected which object had been moved at a novel view that was caused by table rotation or by their own locomotion. The stick was removed at test. The results showed that detection of position change was better when an object not on the stick was moved than when an object on the stick was moved. Furthermore change detection was better in the observer locomotion condition than in the table rotation condition only when an object on the stick was moved but not when an object not on the stick was moved. These results indicated that when the reference direction was not accurately indicated in the test scene, detection of position change was impaired but this impairment was less in the observer locomotion condition. These results suggest that people not only represent objects’ locations with respect to a fixed reference direction but also represent and update their orientation according to the same reference direction, which can be used to recover the accurate reference direction and facilitate detection of position change when no accurate reference direction is presented in the test scene.     

Viewpoint-dependent recognition of scenes

Three experiments investigated scene recognition across viewpoint changes, involving same/different judgements on scenes consisting of three objects on a desktop. On same trials, the comparison scene appeared either from the same viewpoint as the standard scene or from a different viewpoint with the desktop rotated about one or more axes. Different trials were created either by interchanging the locations of two or three of the objects (location change condition), or by rotating either one or all three of the objects around their vertical axes (orientation change condition). Response times and errors increased as a function of the angular distance between the standard and comparison views, but this effect was bigger for rotations around the vertical axis than for those about the line of sight or horizontal axis. Furthermore, the time to detect location changes was less than that to detect orientation changes, and this difference increased with increasing angular disparity between the standard and comparison scenes. Rotation times estimated in a double-axis rotation were no longer than other rotations in depth, indicating that alignment was not necessarily simpler around a "natural" axis of rotation. These results are consistent with the hypothesis that scenes, like many objects, may be represented in a viewpoint dependent manner and recognized by aligning standard and comparison views, but that the alignment of scenes is not a holistic process.     

The role of visual experience on the representation and updating of novel haptic scenes

We investigated the role of visual experience on the spatial representation and updating of haptic scenes by comparing recognition performance across sighted, congenitally and late blind participants. We first established that spatial updating occurs in sighted individuals to haptic scenes of novel objects. All participants were required to recognise a previously learned haptic scene of novel objects presented across the same or different orientation as learning whilst they either remained in the same position to moved to a new position relative to the scene. Scene rotation incurred a cost in recognition performance in all groups. However, overall haptic scene recognition performance was worse in the congenitally blind group. Moreover, unlike the late blind or sighted groups, the congenitally blind group were unable to compensate for the cost in scene rotation with observer motion. Our results suggest that vision plays an important role in representing and updating spatial information encoded through touch and have important implications for the role of vision in the development of neuronal areas involved in spatial cognition.