Biomedical subjects

M A Hogervorst

Publications and source records attributed to M A Hogervorst.

7 recordsLinked to original sources

PubMed2000

Spatial frequency tuning for 3-D corrugations from motion parallax.

We provide evidence for the existence of multiple channels tuned to the spatial frequency of depth modulations defined by motion parallax. By linking the distortion of a random dot pattern to the horizontal position of an observer's head horizontally oriented 3-D corrugations were simulated in which the depth function consisted of a range of frequencies. In a baseline experiment thresholds were obtained for detecting depth modulations of single sinewaves for a range of spatial frequencies. In a masking experiment threshold signal strength was determined for detecting a signal frequency in the presence of noise with frequencies restricted to two bands around the signal component ('notched noise'). Threshold elevation was found to decrease with an increase in the spectral difference between signal and noise. By determining thresholds at various noise levels it was further established that the channel responded linearly in the tested range. Estimates of the bandwidth for spatial frequencies of 0.33 and 0.87 cycles/deg were both found to be 1.4 octaves. The results show that motion parallax processing is mediated by a series of narrowly tuned channels with bandwidths similar to those found for processing depth modulations defined by binocular disparity.

Depth Perception↗

PubMed2000

The role of perspective effects and accelerations in perceived three-dimensional structure-from-motion.

It has been suggested that perceived three-dimensional (3D) structure-from-motion can be accounted for by a 2-frame orthographic approximation of the flow field. This study investigated the extent to which higher order cues (perspective and acceleration) are used in addition to first-order flow. Participants matched the 3D dihedral angle of a hinged plane (probe) defined by multiple-depth cues to one defined by motion only, for stimulus sizes of 8 and 33 degrees, using perspective and orthographic projection. The results show that perspective effects can be important even for relatively small stimuli (8 degrees) and that accelerations contribute to perceived shape. In all conditions, large biases were found. These are well accounted for by a model in which all relevant flow measurements (first-order, perspective, and acceleration) are used together with estimates of the noise in each. The model has no built-in bias toward particular 3D shapes. Instead, the visual system may act as an optimal estimator of 3D structure-from-motion.

Bayes Theorem↗

PubMed1999

Does the visual system exploit projective geometry to help solve the motion correspondence problem?

Projective geometry determines how the retinal image of an object deforms as it moves through three-dimensional space. Does the visual system use constraints derived from this information, such as rigidity, to aid the tracking of moving objects? A novel psychophysical technique is introduced for assessing which of two competing motion transformations is 'preferred' by the visual system, in a two-frame sequence. In the first experiment, relative preference strengths for translations parallel and perpendicular to the major axis of a wire-frame object were measured by pitting the two against each other. It was found that parallel translations were preferred to perpendicular ones. On the basis of these data a proximity measure for normalising different transformations, independent of any effects of figural similarity, was developed. In the second experiment, two wire-frame planar structures were used to pit one of five transformations (rotation, expansion, vertical expansion, shear and random jitter) against a translation. Preference strength was measured as the translation distance at which the transformation and the translation were perceived with equal frequency. The PSEs were found to collapse on to a single line when plotted against the proximity magnitude, with the exception of a residual preference for pure translation over all other transformations. In general, these results suggest that preference strength for moving wire-frame figures is determined primarily by the proximity of local features on the displacing contour, with little regard for the projective shape transformation.

Form Perception↗

PubMed1999

The role of perspective information in the recovery of 3D structure-from-motion.

When investigating the recovery of three-dimensional structure-from-motion (SFM), vision scientists often assume that scaled-orthographic projection, which removes effects due to depth variations across the object, is an adequate approximation to full perspective projection. This is so even though SFM judgements can, in principle, be improved by exploiting perspective projection of scenes on to the retina. In an experiment, pairs of rotating hinged planes (open books) were simulated on a computer monitor, under either perspective or orthographic projection, and human observers were asked to indicate which they perceived had the larger dihedral angle. For small displays (4.6 x 6.0 degrees) discrimination thresholds were found to be similar under the two conditions, but diverged for all larger stimuli. In particular, as stimulus size was increased, performance under orthographic projection declined and by a stimulus size of 32 x 41 degrees performance was at chance for all subjects. In contrast, thresholds decreased under perspective projection as stimulus size was increased. These results show that human observers can use the information gained from perspective projection to recover SFM and that scaled-orthographic projection becomes an unacceptable approximation even at quite modest stimulus sizes. A model of SFM that incorporates measurement errors on the retinal motions accounts for performance under both projection systems, suggesting that this early noise forms the primary limitation on 3D discrimination performance.

Depth Perception↗

PubMed1998

Biases in three-dimensional structure-from-motion arise from noise in the early visual system.

The projected pattern of retinal-image motion supplies the human visual system with valuable information about properties of the three-dimensional environment. How well three-dimensional properties can be recovered depends both on the accuracy with which the early motion system estimates retinal motion, and on the way later processes interpret this retinal motion. Here we combine both early and late stages of the computational process to account for the hitherto puzzling phenomenon of systematic biases in three-dimensional shape perception. We present data showing how the perceived depth of a hinged plane ('an open book') can be systematically biased by the extent over which it rotates. We then present a Bayesian model that combines early measurement noise with geometric reconstruction of the three-dimensional scene. Although this model has no in-built bias towards particular three-dimensional shapes, it accounts for the data well. Our analysis suggests that the biases stem largely from the geometric constraints imposed on what three-dimensional scenes are compatible with the (noisy) early motion measurements. Given these findings, we suggest that the visual system may act as an optimal estimator of three-dimensional structure-from-motion.

Computer Simulation↗

PubMed1997

Monocular discrimination of rigidly and nonrigidly moving objects.

We measured thresholds for the monocular discrimination of rigidly and nonrigidly moving objects defined by motion parallax. The retinal projections of rigidly moving objects are subject to certain constraints. By applying smooth 2-D transformations to the projections of rigidly moving objects, we created stimuli in which these constraints were affected. Thresholds for (generic) nonrigid transformations that in theory can be detected from rigid ones by processing pairs of views depended not only on the extent to which the rigidity constraints were affected, but also on the structure and the movement of the simulated object. Nonrigid transformations under which every three successive views had a rigid interpretation were not discriminable from rigid transformations, except in cases where the distortions were very large. Under the rigidity assumption, this would mean that a large class of nonrigidly moving objects is erroneously perceived as rigidly moving.

Adult↗

PubMed1996

Structure from motion: a tolerance analysis.

We present a tolerance analysis that is applicable to a large group of stimuli used in structure-from-motion tasks. Human performance in structure-from-motion tasks reflects the fact that the visual system deals with projections of a 3-D world on the retina. A tolerance analysis reveals the relationship between the projections and the 3-D world. Any realistic model of the visual system should incorporate a tolerance analysis as a complete description of the stimulus. By way of example we apply the tolerance analysis to the stimuli used in two widely known experiments in which different properties of structure were tested--that is, perceived nonrigidity (Norman & Todd, 1993) and ordering in depth (Hildreth, Grzywacz, Adelson, & Inada, 1990). The analysis explains qualitatively the results of these experiments, illustrating that the results are to a large extent due to stimulus limitations rather than to mechanistic properties of the visual system. From our analysis it follows that far more sensitive measurements of the optic information are needed to obtain metric structure than affine structure.

Depth Perception↗