Optical motions as information for unsigned depth

Optical motions and gradients of retinal flow have been assumed to be an important source of information for the perception of spatial layout. In the case of lateral parallax, however, the complicating effects of smooth eye movements on retinal flow fields and the known insensitivity of the visual system to absolute motion suggest that optical motions alone cannot provide the basis for accurate perception of the direction (sign) of depth relations. At most they can provide information for "unsigned" depth. Results of two experiments support the view that differential optical motions result in a strong impression of separation of objects in depth, but that the determination of near/far relations normally depends on other sources of information.     

Simple mechanisms for gathering social information

Social contexts are notoriously complex, yet decisions are nevertheless made by using simple strategies. We argue that the concept of fast and frugal heuristics provides a promising framework for understanding how we gather social information to make decisions in social environments. That is, we assume that under limitations of time, energy, and computational resources people use cognitively based shortcuts that rely on information from social environments to solve different types of real problems. We review three successful applications of heuristics to the social arena. We first introduce some commonly faced social inference problems (e.g., selecting a mate or deciding whether to cooperate with someone) and then discuss how individuals can use simple strategies to solve such problems. For each problem, we consider how social environments are structured, and how we take advantage of this structure when using heuristics to make inferences and decisions.     

Ratings of kinetic depth in multidot displays

Subjects saw kinetic depth displays whose shape (sphere or cylinder) was defined by luminous dots distributed randomly on the surface or in the volume of the object. Subjects rated perceived 3-D depth, rigidity, and coherence. Despite individual differences, all 3 ratings increased with the number of dots. Dots in the volume yielded ratings equal to or greater than surface dots. Each rating varied with 3 of 4 factors (shape, distribution, numerosity, and perspective), but the ratings either between trials or between conditions were often uncorrelated. Object shape affected rigidity but not depth ratings. Veridically perceived polar displays had slightly lower rigidity but higher depth ratings than parallel projection displays. (Reversed polar displays were always grossly nonrigid.) The interaction of ratings and stimulus parameters requires theories and experiments in which different KDE ratings are not treated interchangeably.     

Pictorial and motion-based information for depth perception.

When computer-generated objects approached the viewpoint in midair, a large far object appeared to be nearer than a small near object and appeared to hit the viewpoint before the small object, which was specified by time-to-contact information to arrive sooner. These judgements were consistent with relative size and occurred even when motion-based information was potentially above threshold. The effects of relative size persisted with higher resolution animated films of approaching objects, were weakened by ground-intercept information, and were not as robust with laterally translating objects. Although it is often asserted that the kinds of information that have traditionally been called static depth cues are superseded by motion-based depth information, this article attempts to show that the reverse also can occur in distance perception, as has been shown in form perception.     

Projective invariance and the kinetic depth effect.

Seven experiments test the assumption that, in the kinetic depth effect, observers have reliable and direct access to the equivalence of shapes in projective geometry. The assumption is implicit in 'inverse optics' approaches to visual form perception. Observers adjusted a comparison shape to match a standard shape; both standard and comparison were portrayed as in continuous rotation in space, using a graphics computer. The shapes were either plane quadrilaterals or solid prisms. The angular difference of the planes of the shapes was varied, as was the dot density of a texture in those planes. Departure from projective equivalence was measured in six studies by measuring the planar analogue of cross ratio, and in a seventh by measuring the cross ratio for points in space. Projective equivalence was not found to be perceived uniformly, except in one experiment that did not involve rotation in depth. Otherwise changes in orientation of up to 180 degrees about a single coordinate axis had no significant effect on matches in shape, while changes in orientation about more than one coordinate axis produced significant effects. The addition of texture and a change in rotation speed did not correct departures from projective equivalence.     

How to study the kinetic depth effect experimentally

Sperling, Landy, Dosher, and Perkins (1989) proposed an objective 3D shape identification task with 2D artifactual cues removed and with full feedback (FB) to the subjects to measure KDE and to circumvent algorithmically equivalent KDE-alternative computations and artifactual non-KDE processing. (1) The 2D velocity flow-field was necessary and sufficient for true KDE. (2) Only the first-order (Fourier-based) perceptual motion system could solve our task because the second-order (rectifying) system could not simultaneously process more than two locations. (3) To ensure first-order motion processing, KDE tasks must require simultaneous processing at more than two locations. (4) Practice with FB is essential to measure ultimate capacity (aptitude) and, thereby, to enable comparisons with ideal observers. Experiments without FB measure ecological achievement--the ability of subjects to extrapolate their past experience to the current stimuli.     

Early motion processes and the kinetic depth effect

It has been known for over 30 years that motion information alone is sufficient to yield a vivid impression of three-dimensional object structure. For example, a computer simulation of a transparent sphere, the surface of which is randomly speckled with dots, gives no impression of depth when presented as a stationary pattern on a visual display. As soon as the sphere is made to rotate in a series of discrete steps or frames, its 3-D structure becomes apparent. Three experiments are described which use this stimulus, and find that depth perception in these conditions depends crucially on the spatial and temporal properties of the display:

1. Depth is seen reliably only for between-frame rotations of less than 15°, using two-frame and four-frame sequences.

2. Parametric observations using a wide range of frame durations and inter-frame intervals reveal that depth is seen only for inter-frame intervals below 80 msec and is optimal when the stimulus can be sampled at intervals of about 40-60 msec.

3. Monoptic presentation of two frames of the stimulus is sufficient to yield depth, but the impression is destroyed by dichoptic presentation.

These data are in close agreement with the observed limits of direction perception in experiments using “short-range” stimuli. It is concluded that depth perception in the motion display used in these experiments depends on the outputs of low-level or “short-range” motion detectors.     

Seeing stereoscopic depth from disparity between kinetic edges

Traditionally, it is assumed that stereovision operates only on the positional difference (disparity) between luminance-defined features in the images in the left and the right eye. Here, I show that stereoscopic depth can be seen from disparity between edges created by relative motion of texture elements, and between edges created by correlated flicker of stationary texture elements. Luminance-based stereopsis was impossible since the texture was binocularly uncorrelated. Positional disparity of the centre of revolving patterns was not an efficient depth cue. Stereopsis from the stimuli presented here was possible even without binocular overlap of textured areas. The results provide evidence that positional disparity of kinetic edges, defined by correlated flicker or motion contrast alone, can be used as matching features to recover stereoscopic depth.     

Depth capture by kinetic depth and by stereopsis

The perceived depth of regions within a stereogram lacking explicit disparity information can be captured by the surface structure of regions where disparity is explicit: stereo capture. In two experiments, observers estimated surface curvature/depth of an untextured object (a 'ribbon') superimposed on a cylinder textured with dots, the cylinder curvature being defined by disparity (stereo depth) or by motion parallax (kinetic depth: KD). With the stereo-defined cylinder, depth capture was obtained under conditions where the disparity of the ribbon was ambiguous; with the KD, cylinder depth capture was obtained under conditions where the motion flow of the cylinder was in a direction parallel to that of the ribbon. These results demonstrate yet another similarity between KD and stereopsis.     

Stereoscopic depth cues can segment motion information

Can the motion system selectively process elements at a particular depth? We attempted to answer this question using global coherence tasks in which signal and noise elements could be given different disparities. In experiment 1 we found that, if all the signal elements had a disparity different from that of the noise elements, performance was far better than when they had the same disparity (at least for stereo-normal observers). In a second experiment we found that adding additional noise elements to the motion task had no effect if they had a different disparity (however, they had a marked effect for stereo-blind observers). We conclude that stereo disparity can be used as a segmentation cue by the motion system.     

Sensitivity to binocular depth information in infants

In order to study infants' sensitivity to binocular information for depth, 11 infants, 20 to 26 weeks of age, were presented with real and stereoscopically projected virtual objects at three distances, and the infants' reaching behavior was videotaped. When the virtual object was positioned out of reach, infants tended to lean further forward and to reach less frequently than when the virtual object was positioned within reach. In addition, the proportion of reaches in which the infants patted, closed their hands, or brought their hands together was greater when the virtual object was within reach. However, no difference in the terminal location of the infants' reaches was found as a function of the virtual object's position. Examination of reaches to a near real object revealed that infants frequently did not contact the object or show appropriate hand shape or orientation. The effectiveness of the cue of retinal size and of binocular information for the depth of an object is discussed. It is concluded that 5-month-old infants are sensitive to binocular information for depth.     

Rigid and non-rigid kinetic depth effect with rotating discrete helices

Summary To examine the conditions in which human observers fail to recover the rigid structure of a three-dimensional object in motion we used simulations of discrete helices with various pitches undergoing either pure rotation in depth (rigid stimuli) or rotation plus stretching (non-rigid stimuli). Subjects had either to rate stimuli on a rigidity scale (Experiments 1 and 2) or to judge the amount of rotation of the helices (Experiments 3 and 4). We found that perceived rigidity depended on the pitch of the helix rather than on objective non-rigidity. Furthermore, we found that helices with a large pitch/radius ratio were perceived as highly non-rigid and that their rotation was underestimated. Experiment 5 showed that the detection of a pair of dots rigidly related (located on. the helix) against a background of randomly moving dots is easier at small phases in which the change of orientation across frames is also small. We suggest that this is because at small phases the grouping of dots in virtual lines does occur and that this may be an important factor in the perceived nonrigidity of the helices.This research was supported by MPI 60% (1987, 1988) and CNR (1986), 1987, 1988) grants to Clara Casco and Sergio Roncato and Grant CNR 90.01603.PS93 to Giorgio Ganis.     

Development of sensitivity to static pictorial depth information

Sensitivity to the pictorial depth cues of shading, linear perspective, and position on the picture plane were investigated with children from 3 to 7 years of age as well as with adults. Using a discriminative learning method, all Ss were found to be sensitive to shading information for depth when the display was oriented vertically, but not when it was oriented horizontally. In addition, binocular view did not decrease sensitivity relative to monocular view. Linear perspective was found to be effective in controlling 3-year-olds’ size discriminations of equal-area figures, while position on the picture plane was ineffective in the absence of other information for depth.     

A note on "Optical motions as information for unsigned depth"

Farber and McConkie have questioned whether optical (or retinal) flows provide a basis for the accurate perception of relative distance (or filled depth, in Gibson, Gibson, Smith, & Flock's terms). Some of the analysis Farber and McConkie present is incorrect. In this article I identity some of the fallacies and discuss issues relevant to these fallacies. The critique does not concern the experiments presented by Farber and McConkie but rather the underlying analysis that culminated in the experiments.     

Stereokinetic effect and its relation to the kinetic depth effect.

The stereokinetic effect (SKE) has been defined and studied by nested circular patterns rotating on a turntable. Circles must appear not to rotate as they revolve, which in turn results in their appearing to translate relative to one another. A powerful illusion of object depth results even though the individual circles do not undergo an appropriate foreshortening consistent with their apparent changes in slant. It is suggested and tested that the SKE is based on the changing positions between the nested contours despite the absence of any change within each contour, whereas the kinetic depth effect (KDE) entails both kinds of change. It follows that a turntable method of presentation is not necessary, and between-contour transformations can be simulated by computer animation. Displays consisting of simple translations were shown to evoke robust depth impressions as were patterns consisting of contours of varying shapes. Comparisons of the depth, compellingness, and rigidity of matched SKE and KDE displays are reported. The SKE is taken to be paradigmatic for how the visual system perceives depth when observing small object rotations that occur in everyday situations.     

On the limits of perceptual complementarity in the kinetic depth effect

In a series of experiments, we have investigated the abilities of human observers to perceive geometric properties of moving three-dimensional objects as a function of their perspective and rotational complexities. The results indicate a decreasing ability of observers to extract metric, angular, and rigid motion as the perspectives and rotations depart from parallel projections and one-parameter central rotations. In this way, quantitative limits are suggested for the principle of perceptual complementarity suggested by Shepard (1981).     

Infants' coordination of auditory and visual depth information.

Using the preferential-looking procedure infants 5, 7, and 9 months of age were presented two videoimages side by side on separate monitors accompanied by a soundtrack that matched one of the images. Each infant was presented: (1) a stationary drum-beating toy paired with the same toy approaching and receding in depth, to assess infants' recognition both that changing sound amplitude is a property of an object that is moving in space and that constancy in amplitude is a property of a stationary object; (2) a drum-beating toy moving horizontally paired with one approaching and receding in depth, to assess infants' recognition that systematic increases and decreases in amplitude accompany object movement in a particular dimension, namely depth; and (3) two identical toys alternately approaching and receding in depth but out of phase (i.e., one approaching while the other is receding), to assess infants' recognition that increases and decreases in amplitude accompany a particular type of object movement in depth. Measures of mean duration of looking time indicated that the 5-month-olds looked reliably to the correct videoimage only for the stationary toy paired with the constant amplitude sound. The 7-month-olds recognized that changes in amplitude accompany object movement in depth but did not coordinate auditory with visual depth information as well as older infants. The 9-month-olds looked reliably to the correct videoimages in all conditions. Possible contributing factors to these developmental trends in performance are discussed.     

Infants' sensitivity to differential luminance as information about depth

To what degree are young infants able to perceive differential shadowing and to what degree are they able to utilize this stimulus parameter as information about depth? Two habituation experiments were performed. In Experiment 1, a group of 5-month-old infants were habituated to a low frequency, vertical, and approximately sinusoidal luminance grating superimposed on a flat colored surface. This display induced stable 3-D perception in adult subjects. After habituation, the infants viewed two test displays at alternating trials. One was made up of real half cylinders matching the light distribution of the habituation display and the other was made up of a square wave grating of the same spatial frequency as in the habituation one. Adults perceived the latter display as flat. Results showed that both test displays were treated as new ones by the infants habituated to the sinusoidal grating. Experiment 2 was identical to Experiment 1, except that the subjects were 3 1/2-month-old. These infants treated the half cylinders as familiar and the square wave grating as new. The results indicate that infants at both age levels (3 1/2 and 5 months of age) were sensitive to the difference between sharp and gradual change in luminance which is a prerequisite for perceiving form from luminance. However, neither age group seemed to utilize gradual change in luminance as information about space.