Horizontal and vertical disparity, eye position, and stereoscopic slant perception.
(25/6813)
The slant of a stereoscopically defined surface cannot be determined solely from horizontal disparities or from derived quantities such as horizontal size ratio (HSR). There are four other signals that, in combination with horizontal disparity, could in principle allow an unambiguous estimate of slant: the vergence and version of the eyes, the vertical size ratio (VSR), and the horizontal gradient of VSR. Another useful signal is provided by perspective slant cues. The determination of perceived slant can be modeled as a weighted combination of three estimates based on those signals: a perspective estimate, a stereoscopic estimate based on HSR and VSR, and a stereoscopic estimate based on HSR and sensed eye position. In a series of experiments, we examined human observers' use of the two stereoscopic means of estimation. Perspective cues were rendered uninformative. We found that VSR and sensed eye position are both used to interpret the measured horizontal disparities. When the two are placed in conflict, the visual system usually gives more weight to VSR. However, when VSR is made difficult to measure by using short stimuli or stimuli composed of vertical lines, the visual system relies on sensed eye position. A model in which the observer's slant estimate is a weighted average of the slant estimate based on HSR and VSR and the one based on HSR and eye position accounted well for the data. The weights varied across viewing conditions because the informativeness of the signals they employ vary from one situation to another. (+info)
Contrast dependency of foveal spatial functions: orientation, vernier, separation, blur and displacement discrimination and the tilt and Poggendorff illusions.
(26/6813)
To examine the effect of reducing luminance contrast in human foveal vision, discrimination thresholds were measured in four tasks and also a numerical measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate comparison between them--all featured luminance step edges whose contrast could be varied from near unity down to the detection threshold. Orientation, vernier and blur discrimination thresholds rise on average 5-6-fold when the contrast is reduced from near unity to a Michelson value of 0.03. Jump displacement thresholds are somewhat more robust to contrast reduction, and the curve of separation discrimination versus contrast is much shallower, rising by a factor of about 2. The magnitude of the Poggendorff and tilt illusions changes very little until the inducing contours are barely detectable. (+info)
The role of perspective information in the recovery of 3D structure-from-motion.
(27/6813)
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. (+info)
Comparison of color and luminance contrast: apples versus oranges?
(28/6813)
Using a spatial, forced-choice, matching protocol, we have measured observers' ability to equate the contrasts of sinusoidal gratings which vary along differing directions in a 3-dimensional color space. In a given experiment, the observer obtained a perceptual match between the contrasts of two gratings whose chromaticities or luminances varied along differing chromatic directions which were selected from among five axes: an achromatic luminance axis (lum), an isoluminant axis where only S-cone activation varied (S-axis), an isoluminant axis where L- and M-cone activation varied in a complementary manner (LM-axis), an axis where only L-cone activation varied (L-axis), and an axis where only M-cone activation varied (M-axis). Even though these chromatic axes were chosen to activate independent mechanisms involved in the early stages of spatiochromatic visual processing, and despite the distinctly differing appearance of patterns from variations along differing directions, we find that observers can reliably make such pairwise contrast matches. Furthermore there is reasonable consistency of matching contrasts among observers and the pairwise contrast matches exhibit the properties of homogeneity and transitivity. This observed homogeneity and transitivity allows, for each color direction, the specification of a single scaling factor which relates perceptual contrast to physical contrast. (+info)
Measuring perceived 3D shape at multiple spatial scales.
(29/6813)
We present and test a novel multiscale representation of perceived 3D surface orientation: the orientation path. Using a multiscale probe, we measure perceived surface orientation at multiple spatial scales; linking the measurements for a given surface location yields that location's orientation path. The multiscale data obtained show that observers consistently see different surface orientations at different spatial scales. We demonstrate that such multiscale data can reveal multiscale differences between observers' percepts of a stimulus and the stimulus geometry. We also demonstrate the use of the orientation path in evaluating the multiscale effects of adding a depth cue to a 3D display. (+info)
Models for the description of angioscotomas.
(30/6813)
To describe small scotomas in visual field examinations several statistical models are proposed and applied to the evaluation of angioscotoma in 13 ophthalmologically normal subjects. A special perimetric grid is used where thresholds can be estimated along a line of narrow-spaced test points which crosses the predicted location of the retinal vessel. A two-stage analysis employs single estimations to fit a threshold curve by means of a special parametric description of the luminance difference sensitivity threshold as a function of stimulus position. An alternative model incorporates the threshold as a function of position into the probabilistic description of the binary response (stimulus seen/not seen). (+info)
Cross-sectional comparison of live and interactive voice recognition administration of the SF-12 health status survey.
(31/6813)
OBJECTIVE: To compare interactive voice recognition (IVR) and live telephone methods for administering the SF-12 health status survey (SF-12). STUDY DESIGN: Patients with low back pain received either IVR or live interviews in a cross-sectional design with partial randomization. The interviews consisted of the SF-12 and some additional questions specific to low back pain. PATIENTS AND METHODS: Complete findings were obtainable from 229 patients. Summary scales were compared by using multivariate analysis of variance with mean comparisons for continuously scored items. Response frequencies for categorically scored items were compared by using the chi-square test. RESULTS: The 2 methods produced similar results on the Physical Component Summary scale but not the Mental Component Summary scale. Compared with patients who had a live telephone interview, the patients using IVR acknowledged significantly greater overall mental interference, greater general emotional concerns, and poorer mood and overall health. CONCLUSIONS: Because IVR eliminates the demand characteristics of responding to a personal interviewer, it may be a desirable way to evaluate sensitive topics. It also may reduce costs of data entry, labor, and measurement error. (+info)
Square pegs in round holes: has psychometric testing a place in choosing a surgical career? A preliminary report of work in progress.
(32/6813)
Methods of selection of candidates for training in surgery has long been regarded as lacking explicit criteria and objectivity. Our purpose was to discover the aptitudes and personality types of applicants for surgical posts at the outset, in order to discover which were most likely to result in a satisfactory progression through training and which were associated with career difficulties. This longitudinal predictive validation study has been undertaken in a London Teaching Hospital since 1994. After short-listing, but immediately before interview, all candidates for senior house officer posts in basic surgical training and in geriatric medicine were asked to undertake psychometric tests of numerical (GMA) and spatial (SIT7) reasoning, personality type (MBTI), and self-rating of competency. There were no differences in ability scores between surgeons or geriatricians. Personality differences were revealed between the surgeons and the geriatricians, and between male and female surgeons. This study suggests that while there are no differences in ability between surgeons and geriatricians at the start of training, there are differences in personality. Long-term follow-up of the career development of this cohort of surgical SHOs is required to determine whether the psychometric measures described correlate with achievements of milestones in their surgical careers. (+info)