Biomedical subjects

Paul B Hibbard

Publications and source records attributed to Paul B Hibbard.

11 recordsLinked to original sources

PubMed2006

Surface orientation, modulation frequency and the detection and perception of depth defined by binocular disparity and motion parallax.

Binocular disparity and motion parallax provide information about the spatial structure and layout of the world. Descriptive similarities between the two cues have often been noted which have been taken as evidence of a close relationship between them. Here, we report two experiments which investigate the effect of surface orientation and modulation frequency on (i) a threshold detection task and (ii) a supra-threshold depth-matching task using sinusoidally corrugated surfaces defined by binocular disparity or motion parallax. For low frequency corrugations, an orientation anisotropy was observed in both domains, with sensitivity decreasing as surface orientation was varied from horizontal to vertical. In the depth-matching task, for surfaces defined by binocular disparity the greatest depth was seen for oblique orientations. For surfaces defined by motion parallax, perceived depth was found to increase as surface orientation was varied from horizontal to vertical. In neither case was perceived depth for supra-threshold surfaces related to threshold performance in any simple manner. These results reveal clear differences between the perception of depth from binocular disparity or motion parallax, and between perception at threshold and supra-threshold levels of performance.

Cues↗

PubMed2006

2-D tilt and 3-D slant illusions in perception and action tasks.

There is now a well established dissociation between perception and action based primarily on neuropsychological evidence [Milner and Goodale, 1995 The Visual Brain in Action (Oxford: Oxford University Press)]. Although equivocal, an important source of evidence from normal observers is that 'perceptual illusions' may affect the systems differently. We investigated the relative effects of 2-D tilt and 3-D slant illusions in the two domains, using similar tasks to those employed originally by Milner and Goodale. Subjects were required to either post a card through, or set a paddle to match the orientation of, a plane that was presented in two conditions: surrounded by a striped surface tilted between +90 degrees and -90 degrees (2-D tilt contrast), or surrounded by a disparity defined surface slanted in depth between +60 degrees and -60 degrees (3-D depth contrast). For 2-D tilt, action and perception were equally affected by the illusion, whereas in the 3-D condition they were not. Here, the illusion appeared greater in the posting than in the perceptual task. We conclude that, although no qualitative differences exist, there were quantitative differences between perception and action tasks in the binocular condition.

Cues↗

PubMed2005

The orientation bandwidth of cyclopean channels.

Orientation bandwidths of cyclopean channels were estimated using a notched noise technique. Observers were presented with random dot stereograms depicting a horizontal or vertical target sinusoidal depth modulation and a mask consisting of sinusoidal depth modulations whose orientations flanked that of the target. Masking reduced as the orientation difference between signal and mask increased. The orientation bandwidth of the masking effect was similar to that found for stimuli defined by luminance contrast, and showed no systematic difference for horizontal and vertical targets. These results suggest that the elongated summation found by Tyler, C. W., and Kontsevich, L. L. (2001). Stereoprocessing of cyclopean depth images: Horizontally elongated summation fields. Vision Research, 41, 2235-2243, for horizontal stimuli occurs after a processing non-linearity.

Depth Perception↗

PubMed2005

Stereoscopic correspondence for ambiguous targets is affected by elevation and fixation distance.

Binocular correspondence must be determined if disparity is to be used to provide information about three-dimensional shape. The current study investigated whether knowledge of the statistical distribution of disparities in the natural environment is employed in this process. A simple model, which produces distributions of distances similar to those found in the natural environment, was used to predict the distribution of disparities in natural images. This model predicts that crossed disparities will be more likely as (i) stimulus elevation decreases below fixation and (ii) fixation distance increases. To determine whether these factors influence binocular correspondence for human observers, ambiguous stereograms were presented to observers, as stimulus elevation and fixation distance were manipulated. Clear biases were observed in the depth perceived in these stereograms, which were more likely to be seen as closer than fixation (i) for stimuli presented below fixation and (ii) as fixation distance increased. These results suggest that binocular correspondence is determined in a manner consistent with the distributions of disparities expected in natural scenes.

Depth Perception↗

PubMed2004

Binocular cues and the control of prehension.

The present study was designed to assess the importance of binocular information (i.e. binocular disparity and angle of convergence) in the control of prehension. Previous studies which have addressed this question have typically used the same experimental manipulation: comparing prehensile movements executed either under binocular conditions to those executed when one eye was occluded (monocular). However this may not be the correct comparison as in addition to depriving the subject of binocular depth cues. it also deprives the subject of any visual information in one eye. Therefore we determined the prehensile performance when the subject viewed the target object and scene with either (i) two different views (binocular), (ii) two identical views (bi-ocular), or (iii) one view only (monocular). Overall, the qualitative and quantitative performance in the bi-ocular and monocular control conditions was very similar on all the main measures (and different from the performance in the binocular condition). We conclude that the deficits in performance observed found for 'monocular' reaches should be attributed to the lack of local depth information specified by the binocular cues. In addition we speculate that convergence angle and binocular disparity, although involved in both the pre-movement and movement-execution phases of the reach, the cues may be weighted differently in both phases of a prehension movement depending on the behavioural strategy involved.

Adult↗

PubMed2003

Attention affects the stereoscopic depth aftereffect.

'Preattentive' vision is typically considered to include several low-level processes, including the perception of depth from binocular disparity and motion parallax. However, doubt was cast on this model when it was shown that a secondary attentional task can modulate the motion aftereffect (Chaudhuri, 1990 Nature 344 60-62). Here we investigate whether attention can also affect the depth aftereffect (Blakemore and Julesz, 1971 Science 171 286-288). Subjects adapted to stationary or moving random-dot patterns segmented into depth planes while attention was manipulated with a secondary task (character processing at parametrically varied rates). We found that the duration of the depth aftereffect can be affected by attentional manipulations, and both its duration and that of the motion aftereffect varied with the difficulty of the secondary task. The results are discussed in the context of dynamic feedback models of vision, and support the penetrability of low-level sensory processes by attentional mechanisms.

Adaptation, Ocular↗

PubMed2002

Reaching for virtual objects: binocular disparity and the control of prehension.

Although, in principle, binocular cues provide veridical information about the three-dimensional shape of objects, our perception on the basis of these cues is distorted systematically. The consequences of these distortions may be less serious than they first appear, however, since in everyday life we rarely are required to judge the absolute shape, size or distance of objects. An important exception to this is in the control of prehension, where veridical information about an object to be grasped is required to plan the transport of the hand and to select the most appropriate grip. Here we investigate whether binocular cues provide accurate depth information for the control of prehension using disparity-defined, virtual objects and report that whilst binocular disparity can support prehensile movements, the kinematic indices, which reflect distance-reached and perceived size, show clear biases. These results suggest that accurate metric depth information for the control of prehension is not available from binocular cues in isolation.

Analysis of Variance↗

PubMed2002

The stereoscopic anisotropy: individual differences and underlying mechanisms.

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms.

Anisotropy↗

PubMed2002

The stereoscopic anisotropy affects manual pointing.

Although binocular disparity can in principle provide absolute depth information, perceived stereoscopic depth depends on the relative disparities between points and their spatial arrangement. An example of this is the stereoscopic anisotropy--observers typically perceive less depth for stereoscopic surfaces when depth varies in the horizontal direction than in the vertical direction. We investigated whether this anisotropy also affects manual pointing. Participants were presented with stereograms depicting surfaces that were slanted in depth about either a horizontal axis (inclination) or a vertical axis (slant), and were asked either to point to the edge of a surface, or to estimate its inclination or slant. For both tasks, a clear anisotropy was observed, with participants perceiving greater depth, and also pointing out steeper surfaces, for inclined surfaces than for slanted surfaces. We conclude that both perception and the control of action are subject to a similar stereoscopic anisotropy, and that performance on the two tasks relies on similar depth processing mechanisms.

Anisotropy↗

PubMed2002

Isotropic integration of binocular disparity and relative motion in the perception of three-dimensional shape.

Richards (1985) showed that veridical three-dimensional shape may be recovered from the integration of binocular disparity and retinal motion information, but proposed that this integration may only occur for horizontal retinal motion. Psychophysical evidence supporting the combination of stereo and motion information is limited to the case of horizontal motion (Johnston et al., 1994), and has been criticised on the grounds of potential object boundary cues to shape present in the stimuli. We investigated whether veridical shape can be recovered under more general conditions. Observers viewed cylinders that were defined by binocular disparity, two-frame motion or a combination of disparity and motion, presented at simulated distances of 30 cm, 90 cm or 150 cm. Horizontally and vertically oriented cylinders were rotated about vertical and horizontal axes. When rotation was about the cylinder's own axis, no boundary cues to shape were introduced. Settings were biased for the disparity and two-frame motion stimuli, while more veridical shape judgements were made under all conditions for combined cue stimuli. These results demonstrate that the improved perception of three-dimensional shape in these stimuli is not a consequence of the presence of object boundary cues, and that the combination of disparity and motion is not restricted to horizontal image motion.

Cues↗

PubMed2002

Perceptual latencies to discriminate surface orientation in stereopsis.

The difference in sensitivity to stereoscopic surfaces oriented horizontally or vertically (the stereoscopic orientation anisotropy) can be redescribed as a difference in sensitivity to shear or compression transformations that relate the binocular images. The present experiment was designed to test this by dissociating the image transformation from the orientation of the surface. Surfaces were presented in isolation or in the presence of a surrounding frame that formed step and gradient discontinuities in the disparity field. Without discontinuities, observers required considerably more time to discriminate between surfaces differing in compression than between those differing in shear, irrespective of surface orientation. Disparity discontinuities facilitated the perception of the disparity gradients; minimum stimulus durations were reduced by over an order of magnitude when the reference frame was present. These results support the hypothesis that the disparity field is decomposed into different primitives during the recovery of depth and surface structure.

Anisotropy↗