Biomedical subjects

F E Pollick

Publications and source records attributed to F E Pollick.

8 recordsLinked to original sources

PubMed1997

Constant affine velocity predicts the 1/3 power law of planar motion perception and generation.

Numerous studies have shown that the power of 1/3 is important in relating Euclidean velocity to radius of curvature (R) in the generation and perception of planar movement. Although the relation between velocity and curvature is clear and very intuitive, no valid explanation for the specific 1/3 value has yet been found. We show that if instead of computing the Euclidean velocity we compute the affine one, a velocity which is invariant to affine transformations, then we obtain that the unique function of R which will give (constant) affine invariant velocity is precisely R1/3. This means that the 1/3 power law, experimentally found in the studies of hand-drawing and planar motion perception, implies motion at constant affine velocity. Since drawing/perceiving at constant affine velocity implies that curves of equal affine length will be drawn in equal time, we performed an experiment to further support this result. Results showed agreement between the 1/3 power law and drawing at constant affine velocity. Possible reasons for the appearance of affine transformations in the generation and perception of planar movement are discussed.

Analysis of Variance↗

PubMed1997

The perception of motion and structure in structure-from-motion: comparisons of affine and Euclidean formulations.

I investigated the discrimination of rigid from nonrigid structure and the perception of affine stretches along the line of sight [Norman & Todd (1993). Perception and Psychophysics, 53, pp. 279-291]. Investigations of performance at discriminating rigid from nonrigid structure showed that performance improved when number of views and amount of simulated three-dimensional nonrigidity increased. Investigations of rotations about the vertical which include affine stretches along the line of sight compared Euclidean interpretations of affine-stretching stimuli to human perception. These Euclidean interpretations were obtained from a simple algorithm which recovered structure and motion from this limited class of stimuli under the assumption that distances to the axis of rotation did not change. The algorithm predicted that stretches along the line of sight would be perceived as nearly rigid and have variable angular velocity. These predictions were supported by subjects' reports of occurrences of nonrigidity and minima of angular velocity. The Euclidean algorithm also provided measures of nonrigidity and motion coherence, and experimental results were consistent with a prediction of when perception of nonrigidity would be independent of perception of coherence. The results are discussed relative to the advantages and shortcomings of both the affine and Euclidean approaches to structure-from-motion.

Algorithms↗

PubMed1997

Correspondence in pictorial space.

We have investigated psychophysically determined image correspondences between pairs of photographs of a single three-dimensional (3-D) object in various poses. These correspondences were obtained by presenting the pictures simultaneously, side by side, and letting the subject match a marker in one picture with a marker (under manual control) in the other picture. Between poses, the object was rotated about a fixed vertical axis; thus, the shifts of the veridical correspondences (with respect to the surface of the object) were very nearly horizontal. In fact, the subjects produced appreciable scatter in both horizontal and vertical directions. The scatter in repeated sessions and between data depends on the local (landmarks) and global (interpolation) structure of the pictures. Since the object was fairly smooth (white semigloss finish) and nontextured, the only way to establish the correspondence is by way of the "pictorial relief." The relief is some largely unknown function of the image structure and the observer. Apparently, more immediate entities (e.g., the shading or the contour) cannot be used as such, since they vary with the pose. We compare these data with results obtained with a surface attitude probe on a single picture. We studied various measures of consistency both within a single method and between methods. We found that subjects were confident in establishing correspondences, but results scattered appreciably in a way that depended on both global and local image structure. Correspondence results for various pose angles were mutually very consistent, but only to a minor extent with results of attitude measurements. The main finding was that subjects could establish correspondence on the basis of their 3-D interpretation (pictorial relief), even if the 2-D graytone distributions are quite different.

Form Perception↗

PubMed1996

Perception of local orientation from shaded images.

The perception of local orientation from shaded images was examined. In Experiment 1, subjects viewed a boundaryless Gaussian hill and judged local orientation using both a gauge figure and a pointing method. One subject reported an internally consistent surface which was incompatible with the judged light-source direction and model used to generate the image. The remaining subjects reported a surface similar to the generating one, and analysis of their results indicated a contour of zero difference between response and generating slants. This contour of zero slant difference was explored in three subsequent experiments using the pointing technique. These experiments investigated possible influences of luminance artifact (Experiment 2), perception of global orientation (Experiment 3), and self-occluding contours (Experiment 4). All three of these experiments yielded results similar to those of Experiment 1, with distinct contours of zero slant difference. This contour was explored for relationships with the simulated slant of the generating surface and the differential structure of image intensity. This analysis indicated that the contour of zero slant difference was approximately a line of constant slant which shared large regions of adjacency to the zero crossings of the second directional derivative of image intensity.

Adult↗

PubMed1994

Perceiving shape from profiles.

Four experiments related human perception of shape from profiles to current theoretical predictions. In Experiment 1, judgments of structure and motion were obtained for single- and dual-ellipsoid displays rotating about various axes. Ratings were highest when the axis of rotation was in the image plane and were influenced by the number of ellipsoids and the orientation of a single ellipsoid. The subsequent experiments explored the effect of orientation on shape judgments of a single ellipsoid. The results of Experiments 2 and 3 suggested that the effect of orientation found in Experiment 1 was not due to either the inability of certain orientations to be perceived as three-dimensional objects or to two-dimensional artifacts. It was thus argued that this effect of orientation was due to points of correspondence in relative motion that arise when the major axis is not perpendicular to the axis of rotation. In Experiment 4, subjects provided judgments of both shape and angular velocity. The elevated ellipsoids that were judged as larger were also judged as rotating more slowly. The inverse relationship between size and angular velocity is consistent with current theories. The connection between theory and data was further demonstrated by applying a shape-recovery algorithm to the stimuli used in Experiment 4 and finding a similar tradeoff between angular velocity and shape.

Form Perception↗

PubMed1994

Perceived motion in structure from motion: pointing responses to the axis of rotation.

We investigated the ability to match finger orientation to the direction of the axis of rotation in structure-from-motion displays. Preliminary experiments verified that subjects could accurately use the index finger to report direction. The remainder of the experiments studied the perception of the axis of rotation from full rotations of a group of discrete points, the profiles of a rotating ellipsoid, and two views of a group of discrete points. Subjects' responses were analyzed by decomposing the pointing responses into their slant and tilt components. Overall, the results indicated that subjects were sensitive to both slant and tilt. However, when the axis of rotation was near the viewing direction, subjects had difficulty reporting tilt with profiles and two views and showed a large bias in their slant judgments with two views and full rotations. These results are not entirely consistent with theoretical predictions. The results, particularly for two views, suggest that additional constraints are used by humans in the recovery of structure from motion.

Attention↗

PubMed1990

Discriminating rigid from nonrigid motion: minimum points and views.

Theoretical investigations of structure from motion have demonstrated that an ideal observer can discriminate rigid from nonrigid motion from two views of as few as four points. We report three experiments that demonstrate similar abilities in human observers: In one experiment, 4 of 6 subjects made this discrimination from two views of four points; the remaining subjects required five points. Accuracy in discriminating rigid from nonrigid motion depended on the amount of nonrigidity (variance of the interpoint distances over views) in the nonrigid structure. The ability to detect a rigid group dropped sharply as noise points (points not part of the rigid group) were added to the display. We conclude that human observers do extremely well in discriminating between nonrigid and fully rigid motion, but that they do quite poorly at segregating points in a display on the basis of rigidity.

Adult↗

PubMed1987

Effect of area postrema lesion on low-frequency arterial pressure oscillations in dogs.

To explore the possibility that chronic inactivation of the area postrema (AP) may alter the frequency distribution of oscillations in blood pressure, the power spectra for mean arterial pressure (MAP) were evaluated in conscious dogs before and after heat coagulation (n = 4) or sham lesions (n = 6) of the AP. No significant changes in MAP were observed in either group of dogs after surgery. Tachycardia was seen in AP-lesioned animals after surgery; no consistent changes in heart rate were found in sham-lesioned dogs. Spectra were averaged to provide a group spectral estimate for the AP-lesioned and sham-lesioned groups, respectively, for each experimental period. In the sham-lesioned group a variance peak was observed at approximately 0.03 Hz both before and after surgery. The same peak was seen in the AP-lesioned group during the control period but disappeared following AP lesion, apparently because a greater proportion of the variance was shifted toward frequencies below 0.03 Hz. In addition, a peak related to respiratory rate was present in both groups before surgery but was selectively abolished by AP lesion. AP lesion also substantially reduced the power associated with frequencies between 0.1 and 0.4 Hz. The use of spectral analysis has allowed us to demonstrate that a low-frequency oscillation of MAP in conscious, resting dogs requires the integrity of the AP and that the 0.1- to 0.4-Hz components of the variability of MAP are attenuated after removal of the AP.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗