Binocular disparity and motion parallax are powerful cues to the relative depth between objects. However to recover absolute depth, either additional scaling parameters are required to calibrate the information provided by each cue, or it can be recovered through the combination of information from both cues (Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America, 2, 343 -- 349). However, not all tasks necessarily require a full specification of the absolute depth structure of a scene and so psychophysical performance may vary depending on the amount of information available, and the degree to which absolute depth structure is required. The experiments reported here used three different tasks that varied in the type of geometric information required in order for them to be completed successfully. These included a depth nulling task, a depth-matching task, and an absolute depth judgement (shape) task. Real world stimuli were viewed (i) monocularly with head movements, (ii) binocularly and static, or (iii) binocularly with head movements. No effect of viewing condition was found whereas there was a large effect of task. Performance was accurate on the matching and nulling tasks and much less accurate on the shape task. The fact that the same perceptual distortions were not evident in all tasks suggests that the visual system can switch strategy according to the demands of the particular task. No evidence was found to suggest that the visual system could exploit the simultaneous presence of disparity and motion parallax. 2000 Elsevier Science Ltd. All rights reserved.

In this investigation of monocular perception of egocentric distance, the authors advocate the necessity of a perception-action approach because calibration is intrinsic to definite distance perception. A helmet-mounted camera and display were used to isolate optic flow generated by participants ' head movements toward a target, and participants ' reaches to place a stylus either in a target hole (Experiments 1, 2, and 4) or aligned under a target surface (Experiment 3) were analyzed. Conclusions are that binocular distance perception is accurate, monocular distance perception yields compression that is not eliminated by feedback, but feedback is used to eliminate underestimation generated by restriction of the size of the visual field. The study of definite distance perception requires a perception-action approach. As we argue, the reason is twofold. First, definite distance perception entails calibration and, therefore, a task-specific action that provides both feedback and a standard of accuracy. Calibration is complete once measurements are within a task-specific tolerance. The tolerance is determined by error variability and task requirements,

The relationship between simulated and judged depth separations for pairs of probe dots on planar surface patches was examined in a series of 6 experiments. The simulated slant of the patches was varied without varying the simulated depth separation of the probe dots by varying the depth gradient orthogonal to the direction determined by the probe dots on the image plane. Judged depth separation varied with mean slant for constant simulated depth separations. When observers judged depth separations along a closed path, the integral of the signed depths did not sum to zero, as would be required in Euclidean geometry. These results are inconsistent with the view that the mapping between simulated and perceived 3-D structure is alfme and indicate that, in general, the perceived structure cannot be represented in either a Euclidean space or an affine space. Moreover, these results are consistent with a first-order temporal analysis of the optic flow. A pattern of moving two-dimensional (2-D) features on a flat screen can give rise to a compelling impression of three-dimensionality. This phenomenon has been called the kinetic depth effect (Wallach & O'Connell, 1953) or structure

Contents 6. Seeing with the mind's eye 1: The puzzle of mental Imagery..6-2 6.1 What is the puzzle about mental imagery?.............................................................................................................6-2 6.2 Content, form and substance of representations ....................................................................................................6-6 6.3 What is responsible for the pattern of results obtained in imagery studies? ..............................................................6-7 6.3.1 Cognitive architecture or tacit knowledge ....................................................................................................6-7 6.3.2 Problem-solving by "mental simulation": Some additional examples ............................................................. 6-11 6.3.3 A Note concerning tacit knowledge and the criterion of cognitive penetrability.......................................... 6-20 6.3.4 Summary of so

this article (although this distinction is the subject of extensive discussion in Pylyshyn, 1984a, Chapter 7). This informal characterization and the following example will have to do for present purposes. To make this point in a more concrete way, I invented a somewhat frivolous but revealing example, involving a certain mystery box of unknown construction whose pattern of behavior has been assiduously recorded (Pylyshyn, 1984a). This box is known to emit long and short pulses with a reliable recurring pattern. The pattern (illustrated in Figure 6-1) can be described as follows: pairs of short pulses usually precede single short pulses, except when a pair of long-short pulses occurs first. In this example it turns out that the observed regularity, though completely regular when the box is in its "ecological niche," is not due to the nature of the box (to how it is constructed) but to an entirely extrinsic reason. These two sorts of "reasons" for the observed pattern (intrinsic or extrinsic) are analogous to the architecture versus tacit knowledge distinction and is crucial to understanding why the box works the way it does, as well as to why certain patterns of cognition occur. 6-9 Figure 6-1. Pattern of blips observed from a box in its typical mode of operation. The question is: Why does it exhibit this pattern of behavior? What does this behavior tell us about how it works? The reason why this particular pattern of behavior occurs in this case can only be appreciated if we know that the pulses are codes, and the pattern is due to a pattern in what they represent, in particular that the pulses represent English words spelled out in International Morse Code. The observed pattern does not reflect how the box is wired or its functional architecture -- it is due entirel...

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