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.

The visual system must generate a reference frame to relate retinal images in spite of head and eye movements. We show how a reference frame for storing the visual direction and depth of points can be composed from the angles and changes in angles between pairs and triples of points. The representation has no unique origin in 3-D space nor a unique set of cardinal directions (basis vectors). We show how this relative representation could be built up over a series of fixations and for different directions of translation of the observer. Maintaining gaze on a point as the observer translates helps in building up this representation. In our model, retinal flow is divided into changes in eccentricity and changes in meridional angle. The latter, called `polar angle disparities' for binocular viewing (Weinshall, 1990. Computer Vision Graphics and Image Processing, 49 222 -- 241), can be used to recover the relief structure of the scene in a series of stages up to full Euclidean structure. We show how the direction of heading can be recovered by a similar series of stages. 2001 Elsevier Science Ltd. All rights reserved.

Abstract. In an immersive virtual environment, observers fail to notice the expansion of a room around them and consequently make gross errors when comparing the size of objects. This result is difficult to explain if the visual system continuously generates a 3-D model of the scene based on known baseline information from interocular separation or proprioception as the observer walks. An alternative is that observers use view-based methods to guide their actions and to represent the spatial layout of the scene. In this case, they may have an expectation of the images they will receive but be insensitive to the rate at which images arrive as they walk. We describe the way in which the eye movement strategy of animals simplifies motion processing if their goal is to move towards a desired image and discuss dorsal and ventral stream processing of moving images in that context. Although many questions about view-based approaches to scene representation remain unanswered, the solutions are likely to be highly relevant to understanding biological 3-D vision.

The target article is based on the assumption that our senses' ultimate purpose is to provide us with perfect information about the outside world. We argue that it is often more important to have information quickly, than for it to be perfect. Consequently our nervous system processes different aspects of information about our surrounding as separately as possible. The separation is not between the senses, but between separate aspects of our surrounding. This results in inconsistencies between judgements, sometimes because different frames of reference are used. However, such inconsistencies are fundamental to the way the information is picked up, and therefore cannot be avoided with clearer instructions to the subjects. Since the target article deals with human interactions with the environment it is impossible to ignore the physiology involved. Once one considers the physiology it becomes evident that in practice there can be no `specification' of the kind described in the target a...

Abstract not found

The difference between the way in which binocular disparity scales with viewing distance and the way in which motion parallax scales with viewing distance introduces a potential indirect cue for viewing distance: the viewing distance is the only distance at which disparity and motion specify the same depth. The present study examines whether this information is used. Two simulated ellipsoids were presented on a computer screen in complete darkness. The two ellipsoids were 6 to the left and right of straight ahead. Subjects set the width and depth of each ellipsoid to match a tennis ball, and set the distance of the one on the right to half that of the one on the left. The distance of the left ellipsoid varied between trials. On half of the trials it was static. On the other half it was rotating up and down around its frontal horizontal axis. Rotating the left ellipsoid influenced its set depth: rotating ellipsoids were set to be much more spherical. There was no influence on the set depth of the other ellipsoid, or on the set width of either. The set distance of the right ellipsoid was also unaffected. We conclude that subjects do not combine binocular disparity and motion parallax to obtain more veridical information about viewing distance. 1999 Elsevier Science Ltd. All rights reserved.

Stereoacuity thresholds have been shown to depend on the disparity of a point with respect to a slanted reference plane through neighbouring points [Curr. Biol. 12 (2002) 825]. Here we explored a wider range of conditions, including slanting the reference points about a horizontal axis and varying the spacing of the reference dots, allowing alternative hypotheses for the effect to be distinguished. The stimulus consisted of three dots;the outer two defined a line that was slanted in depth. Observers judged in which of two intervals the third, central dot was displaced from the location midway between the outer reference dots. The displacement consisted of both a disparity and a shift in the fronto-parallel plane. We compared performance for pairs of conditions in which the disparity was the same but the fronto-parallel shifts were in opposite directions. Models based purely on relative disparity predict that performance should be the same for these conditions. We found consistent differences: performance was always better when the target had a greater disparity with respect to the line joining the reference dots. The other stimulus parameters varied were: target disparity (concave/convex), stimulus size (large/small), slant sign (sky/ground) and axis (vertical/horizontal). The results suggest that either (a) disparity with respect to the line drawn through the outer reference dots or (b) difference in disparity gradients on either side of the target determines the depth discrimination threshold for these stimuli.