The development of depth perception from motion parallax in infancy

North Dakota State University, Fargo, North Dakota Little is known about infants' perception of depth from motion parallax, even though it is known that infants are sensitive both to motion and to depth-from-motion cues at an early age. The present experiment assesses whether infants are sensitive to the unambiguous depth specified by motion parallax and, if so, when this sensitivity first develops. Eleven infants were followed longitudinally from 8 to 29 weeks. Infants monocularly viewed a translating Rogers and Graham (1979) random-dot stimulus, which appears as a corrugated surface to adult observers. Using the infant-control habituation paradigm, looking time was recorded for each 10-sec trial until habituation, followed by two test trials: one using a depth-reversed and one using a flat stimulus. Dishabituation results indicate that infants may be sensitive to unambiguous depth from motion parallax by 16 weeks of age. Implications for the developmental sequence of depth from motion, stereopsis, and eye movements are discussed.     

Dynamic occlusion and motion parallax in depth perception

Random-dot techniques were used to examine the interactions between the depth cues of dynamic occlusion and motion parallax in the perception of three-dimensional (3-D) structures, in two different situations: (a) when an observer moved laterally with respect to a rigid 3-D structure, and (b) when surfaces at different distances moved with respect to a stationary observer. In condition (a), the extent of accretion/deletion (dynamic occlusion) and the amount of relative motion (motion parallax) were both linked to the motion of the observer. When the two cues specified opposite, and therefore contradictory, depth orders, the perceived order in depth of the simulated surfaces was dependent on the magnitude of the depth separation. For small depth separations, motion parallax determined the perceived order, whereas for large separations it was determined by dynamic occlusion. In condition (b), where the motion parallax cues for depth order were inherently ambiguous, depth order was determined principally by the unambiguous occlusion information.     

Relative distance cues contribute to scaling depth from motion parallax

The visual system scales motion parallax signals with information about absolute distance (M. E. Ono, Rivest, & H. Ono, 1986). The present study was designed to determine whether relative distance cues, which intrinsically provide information about relative distance, contribute to this scaling. In two experiments, two test stimuli, containing an equal extent of motion parallax, were presented simultaneously at a fixed viewing distance. The relative distance cues of dynamic occlusion and motion parallax in the areas surrounding the test stimuli (background motion parallax) and/or relative size were manipulated. The observers reported which of the two parallactic test stimuli appeared to have greater depth, and which appeared to be more distant. The results showed that the test stimulus specified, by the relative distance cues, as being more distant was perceived as having more depth and as being more distant. This indicates that relative distance cues contribute to scaling depth from motion parallax by modifying the information about the absolute distance of objects.     

Aging and the perception of depth and 3-D shape from motion parallax

The ability of younger and older observers to perceive 3-D shape and depth from motion parallax was investigated. In Experiment 1, the observers discriminated among differently curved 3-dimensional (3-D) surfaces in the presence of noise. In Experiment 2, the surfaces' shape was held constant and the amount of front-to-back depth was varied; the observers estimated the amount of depth they perceived. The effects of age were strongly task dependent. The younger observers' performance in Experiment 1 was almost 60% higher than that of the older observers. In contrast, no age effect was obtained in Experiment 2. Older observers can effectively perceive variations in depth from patterns of motion parallax, but their ability to discriminate 3-D shape is significantly compromised.     

Reversals of visual depth caused by motion parallax

The role of the velocity and direction of retinal movement in the determination of apparent depth from motion parallax was examined. Motion parallax was produced either by linking the movement of random-dots to head movement or by making this motion independent of the head movement. The results show that apparent depth was largely estimated from the velocity difference between the stimuli. The direction of retinal movement in the absence of head movement did not determine whether the pattern appeared to protrude or recede. Information about direction linked to head movement was able to stabilize protrusion/recession by providing a cue for the location of the fixation point. Depth reversal occurred less frequently in the presence than in the absence of head movement. When the fixation point shifted from the apparently protruding pattern to the apparently receding pattern, in both the presence and absence of head movement, depth reversal was readily observed.     

Disruption of eye movements by ethanol intoxication affects perception of depth from motion parallax

Motion parallax, the ability to recover depth from retinal motion generated by observer translation, is important for visual depth perception. Recent work indicates that the perception of depth from motion parallax relies on the slow eye movement system. It is well known that ethanol intoxication reduces the gain of this system, and this produces the horizontal gaze nystagmus that law enforcement's field sobriety test is intended to reveal. The current study demonstrates that because of its influence on the slow eye movement system, ethanol intoxication impairs the perception of depth from motion parallax. Thresholds in a motion parallax task were significantly increased by acute ethanol intoxication, whereas thresholds for an identical test relying on binocular disparity were unaffected. Perhaps a failure of motion parallax plays a role in alcohol-related driving accidents; because of the effects of alcohol on eye movements, intoxicated drivers may have inaccurate or inadequate information for judging the relative depth of obstacles from motion parallax.     

The observer-relative velocity field as the basis for effective motion parallax

Earlier studies of motion parallax found unambiguous relative depth perception when random dot patterns were systematically translated in accordance with either motion of the observer's head or motion of the display scope. The need for such relative motion between an observer and a flow field was examined by placing a flow field in a limited area (window) in a large scope and translating the window relative to the observer. Accuracy in judging surface orientation and quantitative depth estimates were determined by the velocity field relative to the observer and were not measurably affected by whether this field was produced with a stationary or a moving window. Accuracy was consistently higher for smaller ratios of maximum to minimum projected velocities, reaching 100% in one experiment with a 1.12:1 ratio. We conclude that fully effective motion parallax does not require relative motion between the observer's head and the contours of a flow field.     

The roles of convergence and apparent distance in depth constancy with motion parallax

The question of whether motion parallax is calibrated by convergence or by apparent distance for depth perception was addressed in three experiments. In Experiment 1, a random dot parallactic display was viewed monocularly at a distance of 80 cm, and the convergence angles were set for distances of 40, 60, and 80 cm. Averaged apparent depth was not different across conditions. In Experiment 2, a display consisting of one surface showing dollar bills and one surface showing random dots was viewed monocularly at a distance of 80 cm. It was presented at two different apparent distances, which were manipulated by varying the size of the dollar bills. In one condition, normally sized dollar bills were presented, and in another condition, the size was reduced by 30%. The averaged apparent depth associated with the small-bill display was larger than the depth associated with the normally sized bill display. In Experiment 3, a random dot display was viewed monocularly at 120 cm. In the primary condition, the random dot display was viewed with an induction screen at 80 cm, and it was moved from side to side such that it appeared stationary and close to the plane of the induction screen. In a comparison condition, the display was viewed without the induction screen and was moving from side to side at 120 cm. In another comparison condition, the display was again viewed without the induction screen but was stationary at 120 cm. Observers adjusted the extent of motion parallax so that apparent depth was 1 cm. The mean extent of parallax was larger in the primary conditio.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of depth perception mediated by motion parallax in unidimensional projections of rotation in depth.

Motion parallax is a composite of five transformations demonstrated to be effective in adult judgments of rotation direction in polar motion projections of a horizontal row of dots rotating in depth. The effectiveness of these transformations as a function of age was tested by presenting six such motion projections to first graders (age = 6 years), seventh graders (age = 13 years), and college students (age = 19 years). Identical age functions were obtained for judged rotation direction from the four motion projections representing (1) Velocity, corresponding to the traditional definition of motion parallax as differential velocity, (2) Velocity plus differences between ratios of instantaneous displacement to instantaneous acceleration for dots on the near and far sides of the rotation axis ($\text{D}\text{A}$ Difference), (3) Velocity, $\text{D}\text{A}$ Difference, and a gradient across the row of $\text{D}\text{A}$ ratios, and (4) all transformations. First graders, unable to use horizontal transformations, performed at chance on these four projections, while older students made correct judgments. Order, separated from Velocity for the first time, resulted in chance performance at all ages, while Direction, also separated from Velocity for the first time, resulted in veridical judgments in only 4 of 24 college students.     

Independence of sampling of motion parallax and binocular disparity in discrimination of depth1

Abstract: The sampling strategy of the visual system in binocular disparity and motion parallax to discriminate depth was investigated. Human observers were asked to discriminate between the depths of two surfaces defined by both cues. Gaussian noise was added to the depths represented by each cue, and the correlation in noise was manipulated. Human performance was compared with two types of likelihood models. The first was based on independent sampling, in which data from the two cues were gathered from independent sets of points in the display. The second was based on paired sampling, in which data from these cues were gathered from the same set of points. The former model yielded a better fit with human performance. This suggests that the visual system is more likely to adopt independent sampling.     

Detectability of motion as a factor in depth perception by monocular movement parallax

A display of two objects at different distances was presented to 10 observers, who were requested in two experiments to match the width of the more distant (comparison) object to the width of the nearer (standard) one under conditions permitting monocular observation and lateral head motion. The matched width of the comparison object was considered a measure of the effectiveness of movement parallax. The effectiveness of movement parallax decreases with increasing angular separation of the objects and with increasing background distance. A background without visible texture leads to a better perception of depth between two objects than a textured background The results can be explained by postulating that, whenever the detectability of motion is enhanced, i.e., the threshold for the detection of motion is lowered, the effectiveness of movement parallax as a cue to depth is increased.     

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.     

Effects of static and motion parallax depth information on perception of size in children and adults

The effects of static and kinetic information for depth on judgments of the relative size of objects placed at different distances was studied in 3- and 7-yr-old children and adults. Subjects viewed either a pair of objects placed on the floor of a textured alley or a projected slide of the identical scene. The presence of motion parallax information for depth was also manipulated. All subjects showed a clear sensitivity to static pictorial depth information in judging objects placed so they projected equal retinal areas. When the retinal size of objects was very different, however, children tended to respond to retinal rather than physical size. Motion parallax information increased responsiveness to depth when a 3-dimensional scene was being viewed, but decreased responsiveness with 2-dimensional projections. The decrease was greater in children than adults.     

The interaction of binocular disparity and motion parallax in determining perceived depth and perceived size.

Although binocular disparity and motion parallax are powerful cues for depth, neither, in isolation, can specify information about both object size and depth. It has been shown that information from both cues can be combined to specify the size, depth, and distance of an object in a scene (Richards, 1985 Journal of the Optical Society of America A 2 343-349). Experiments are reported in which natural viewing and physical stimuli have been used to investigate the nature of size and depth perception on the basis of disparity and parallax presented separately and together at a range of viewing distances. Observers adjusted the relative position of three bright LEDs, which were constrained to form a triangle in plan view with the apex pointing toward the observer, so its dimensions matched that of a standard held by the subject. With static monocular viewing, depth settings were inaccurate and erratic. When both cues were present together accuracy increased and the perceptual outcome was consistent with an averaging of the information provided by both cues. When an apparent bias evident in the observers' responses (the tendency to under-estimate the size of the standard) was taken into account, accuracy was high and size and depth constancy were close to 100%. In addition, given this assumption, the same estimate of viewing distance was used to scale size and depth estimates.     

Absolute motion parallax and the specific distance tendency

In the absence of definitive cues’to distance, the perceived distance of an object will be in error in the direction of the object appearing at a distance of about 2 m from O. This tendency to perceive an object at a relatively near distance is termed the specific distance tendency (Gogel. 1969). Also, it has been found that an error in perceiving the distance of an object will result in an apparent movement of the object when the head is moved (Hay & Sawyer. 1969; Wallach, Yablick. & Smith. 1972). From these two results, it was expected that the direction of trie apparent movement of a stationary point of light resulting from head movement would vary predictably as a function of the physical distance of the point of light from O. This expectation was confirmed in an experiment in which both the perceived motion and perceived distance of the point of light were measured. The consequences of the study for the role of motion parallax in the perception of distance and for the reafference principle in the perception of object motion with head motion are discussed     

Surface discontinuity is critical in a moving observer's perception of objects' depth order and relative motion from retinal image motion.

The visual system perceptually decomposes retinal image motion into three basic components that are ecologically significant for the human observer: object depth, object motion, and self motion. Using this conceptual framework, we explored the relationship between them by examining perception of objects' depth order and relative motion during self motion. We found that the visual system obeyed what we call the parallax-sign constraint, but in different ways depending on whether the retinal image motion contained velocity discontinuity or not. When velocity discontinuity existed (e.g. in dynamic occlusion, transparent motion), the subject perceptually interpreted image motion as relative motion between surfaces with stable depth order. When velocity discontinuity did not exist, he/she perceived depth-order reversal but no relative motion. The results suggest that the existence of surface discontinuity or of multiple surfaces indexed by velocity discontinuity inhibits the reversal of global depth order.     

Scene layout from ground contact,occlusion, and motion parallax

An important factor in determining perceived scene layout is optical contact between objects and the ground surface (“ground contact”). The relation between ground contact and occlusion was investigated in four experiments with stationary and motion parallax scenes containing a pole that occluded or was occluded by one or more cylinders. Some judgements were inconsistent with occlusion in stationary scenes, but judgements were consistent with occlusion in motion parallax scenes. When the range of object positions consistent with occlusion did not include the positions specified by ground contract or by motion parallax, the ground contact and motion parallax information still had a quantitative effect on the perceived position of the object within the range consistent with occlusion. These results demonstrate a cooperative interaction between ground contact, occlusion, and motion parallax in determining perceived layout in 3-D scenes.