Biomedical subjects

M S Langer

Publications and source records attributed to M S Langer.

4 recordsLinked to original sources

PubMed2001

A prior for global convexity in local shape-from-shading.

To solve the ill-posed problem of shape-from-shading, the visual system often relies on prior assumptions such as illumination from above or viewpoint from above. Here we demonstrate that a third prior assumption is used--namely that the surface is globally convex. We use complex surface shapes that are realistically rendered with computer graphics, and we find that performance in a local-shape-discrimination task is significantly higher when the shapes are globally convex than when they are globally concave. The results are surprising because the qualitative global shapes of the surfaces are perceptually unambiguous. The results generalise findings such as the hollow-potato illusion (Hill and Bruce 1994 Perception 23 1335-1337) which consider global shape perception only.

Analysis of Variance↗

PubMed2000

Depth discrimination from shading under diffuse lighting.

The human visual system has a remarkable ability to interpret smooth patterns of light on a surface in terms of 3-D surface geometry. Classical studies of shape-from-shading perception have assumed that surface irradiance varies with the angle between the local surface normal and a collimated light source. This model holds, for example, on a sunny day. One common situation in which this model fails to hold, however, is under diffuse lighting such as on a cloudy day. Here we report on the first psychophysical experiments that address shape-from-shading under a uniform diffuse-lighting condition. Our hypothesis was that shape perception can be explained with a perceptual model that "dark means deep". We tested this hypothesis by comparing performance in a depth-discrimination task to performance in a brightness-discrimination task, using identical stimuli. We found a significant correlation between responses in the two tasks, supporting a dark-means-deep model. However, overall performance in the depth-discrimination task was superior to that predicted by a dark-means-deep model. This implies that humans use a more accurate model than dark-means-deep to perceive shape-from-shading under diffuse lighting.

Adolescent↗

PubMed2000

Large-scale failures of f(-alpha) scaling in natural image spectra.

Several studies have demonstrated that the power spectra of natural image ensembles scale as f(-alpha). A stronger claim that has been made is that the power spectra of single natural images typically also scale as f(-alpha). Results are presented that challenge this latter claim. The results are based on a method for estimating large-scale structure in single images that compares aliasing artifacts produced by image windows of different shape. Failures of f(-alpha) scaling are found at large scales in many natural images. These failures cannot be accounted for by f(-alpha) scaling models such as a linear superposition model or a model based on two-dimensional occlusions in the image plane. The results imply that claims about f(-alpha) scaling in single natural images have been exaggerated. The results also offer insight into why such failures of f(-alpha) scaling occur.

Animals↗

PubMed1992

Should all children receive two measles vaccinations? A study of measles susceptibility in a suburban New Jersey private practice.

Because of the rising incidence of rubeola, we tested all our patients who were vaccinated prior to 15 months of age and those vaccinated after 15 months, if requested, for susceptibility to measles (IgG, ELISA). Those found to be susceptible were revaccinated. Of 1,228 tested, 264 (21.5%) were susceptible. In the group vaccinated before 1980, 237 of 901 (26.3%) were susceptible, whereas only 27 of 327 (8.3%) vaccinated after 1980 were not immune. Susceptibility was sharply divided by month of age at vaccination at the 14-month mark. Less than 5% of those vaccinated after age 15 months in the 1980s (one of 22, or 4.5%) were susceptible. Waning immunity (secondary vaccine failure) was not found to be a factor in our patients. Despite outbreaks of measles in surrounding communities and in our area, none of our patients developed measles. Identification of high-risk groups and selective measles revaccination should be considered as an alternative to universal revaccination in populations such as ours, since it is more cost-effective and may prove equally successful.

Analysis of Variance↗