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Biomedical subjects

D A Glaser

Publications and source records attributed to D A Glaser.

At least 19 recordsLinked to original sources
PubMed1996

Blue nails and acquired immunodeficiency syndrome: not always associated with azidothymidine use.

Nail dyschromia in patients infected with human immunodeficiency virus (HIV) was first described in 1987 by Furth and Kazakis. It has since been reported in many patients with the acquired immunodeficiency syndrome (AIDS), predominantly in those taking azidothymidine. There have been only three reports of nail pigmentation in HIV-infected patients who had not received azidothymidine. We describe such a case.

Acquired Immunodeficiency Syndrome↗
PubMed1996

Piezogenic pedal papules in two family members.

Piezogenic pedal papules are small, occasionally painful, fat herniations that become apparent when weight is placed on the heel. Although the cause is unknown, hereditary factors may play a role. This is the second report of two family members with this condition.

Adolescent↗
PubMed1995

Depth discrimination of a crowded line is better when it is more luminant than the lines crowding it.

Observers usually cannot discriminate the relative depth of a crowded feature with respect to crowding features about 2 arc min distant if all the features have the same luminance. However, stereo thresholds significantly less than 20 arc sec are obtained when the crowded feature is about twice as luminant as the features crowding it. The thresholds depend only upon the ratio of the luminance of the target feature to the luminance of the crowding features and are independent of the absolute luminance of the features. With further increase in the relative luminance of the target feature, the performance eventually deteriorates and this deterioration is not due to difficulty in seeing the features which were individually clearly visible for all the luminances tested. The closest spacing of local crowded features that still allows good stereo discrimination is about the same as the spatial resolution attainable for many luminance-based non-stereo tasks.

Depth Perception↗
PubMed1994

Some temporal aspects of stereoacuity.

Stereoacuity thresholds improve considerably with practice when measured using three vertical lines 15' apart horizontally and presented briefly. For experienced observers, these thresholds are relatively independent of exposure time for stimulus durations smaller than 100 msec. The thresholds are 2-3 times larger when the outer flanking lines are shown continuously than when they and the central target line are turned on and off simultaneously. When the target and flanking lines are shown sequentially, stereoacuity thresholds can be predicted from the number of times the configuration is presented. Changes in thresholds can be measured for intervals as small as 5 msec between successive presentation of the relative disparity configuration. The underlying mechanism is modeled well by a first order auto-regression process.

Depth Perception↗
PubMed1993

Temporal aspects of depth contrast.

Depth contrast is a contrasting change in the depth of a feature that results from changes in the disparities of other objects in the field of view, even though the disparity of the original feature remains unchanged. Depth contrast effects decrease during continuous viewing of the stimuli and may disappear altogether after several minutes unless the disparities of the inducing features change with time. This fading occurs whenever the inducing features have constant disparity, whether they are stationary or oscillating laterally. Depth contrast effects occur whenever the inducing features are visible within half a second before or after presentation of the test features. When test features are enclosed by a rectangle which is just inside of a circumscribing outer trapezoid, the inner rectangle "shields" the test features from the depth-inducing effects of the outer trapezoid. Surprisingly, this shielding effect persists if the inner rectangle and outer trapezoid have the same slant direction, but fades with time if the slants are opposite in direction.

Contrast Sensitivity↗
PubMed1993

Initial performance, learning and observer variability for hyperacuity tasks.

Psychophysically-experienced and -inexperienced human observers were tested on 34 different non-stereo and 49 stereo hyperacuity stimuli. Performance reached hyperacuity levels within the first five trials for the non-stereo stimuli. For stereoacuity tasks the results were very different. Even extremely experienced observers with very low thresholds for certain stereo tasks required considerable practice to achieve their best performance for slightly different stereo tasks. Performance on both types of tasks showed considerable observer variability. These results suggest that adults do not synthesize new visual modules for hyperacuity tasks early in the visual pathway on a task-driven basis. We also interpret these results to suggest that there can be many equally general models of hyperacuity performance that show only the qualitative general trends of a "standard" observer. Incorporating individual variability might provide sufficient constraints on such models to provide clues about physiological mechanisms.

Depth Perception↗
PubMed1993

Perceived motion of a colored spot in a noisy achromatic background.

It is shown that human observers can use color both for detecting and for discriminating motion. The contributions of chromaticity and luminance to the detection and discrimination of motion are investigated with a high-contrast, nonisoluminant stimulus. The motion stimulus is a rectangular 'particle' defined by its luminance and chromaticity, which moves against a background containing luminance noise. Although the luminance noise is found to make achromatic particles undetectable over a large range of luminances, the addition of color to a particle can render it detectable and also enable accurate speed discriminations to be made. The contributions of both luminance and chromaticity were measured. The effect of changing the hue angle of the particle as it moves was also examined, and it was found that the detectability of motion is low in that circumstance.

Acoustic Stimulation↗
PubMed1993

Comparison of human performance with algorithms for estimating fractal dimension of fractional Brownian statistics.

Five standard methods for estimating fractal dimension were compared by means of one-dimensional fractional Brownian series generated by four different algorithms yielding series with different statistics. The same one-dimensional series were also displayed as jagged lines and as one-dimensional luminance patterns for judgments by human observers. Only the algorithm implementing the maximum-likelihood method, which required that the generation statistic for the fractal series be known, gave better performance than human observers in estimating fractal dimension. However, when the method of generation is not known, one of the four other standard methods for estimating fractal dimension must be used, and these performed significantly worse than human observers.

Algorithms↗
PubMed1992

Depth discrimination of a line is improved by adding other nearby lines.

Depth discrimination thresholds are shown to be lowered by up to a factor of 10 when a few reference lines are added to a stimulus containing a single isolated test line. Four reference lines are better than two, which are better than one, and the improvement in performance is greater when the test line lies between the two reference lines in depth. Stereoacuity for the relative depth of a target line, relative to other nearby reference lines, is shown to be insensitive to changes of disparity of the whole pattern of up to about +/- 5 arc min and is only weakly sensitive to larger displacements of up to +/- 10 arc min.

Adult↗
PubMed1992

Shape analysis and stereopsis for human depth perception.

The perceived relative depth of two isolated short parallel lines in the center of a scene is known to depend on the disparities and positions of other items in the scene, as well as on their own disparities. We demonstrate here that the shapes of these other items also contribute significantly to the perceived depth, and that these non-disparity influences on depth judgements may already be evident when only three dots are presented as stimuli. When two short vertical test lines are surrounded by a trapezoidal "picture frame", the perceived relative depth of the test lines is affected by the shape of the trapezoid as well as by the disparities assigned to its vertical parallel sides. The influence of the trapezoidal frame can be interpreted as an effect of perspective. The induced relative depth of the test lines is measured by recording the amount of "compensating disparity" that must be given to one of the lines in order for observers to judge the two test lines to be equidistant from the observer's viewing position. Surprisingly, for fixed disparities of the vertical edges of the surrounding picture frame, the induced depth of the test lines increases as the difference in the lengths of the vertical sides increases, regardless of whether the perspective interpretation of the difference in the lengths is consistent with or in conflict with the disparity-defined slant. Shape-related apparent depth changes are especially sensitive to the shape of the trapezoid if it is nearly rectangular, and are comparable in magnitude to those resulting from changes in disparity of the surrounding frame. When a pair of short vertical parallel test lines is presented alone, without a surrounding frame or any other items in the scene, excellent relative depth discrimination is displayed by most subjects. However, if the lines are replaced by squares, trapezoids, triangles, single horizontal lines, or other figures of about the same size as the original test lines, the slant discrimination threshold for these plane figures for naïve observers become poorer by a factor of 20-100. By the use of a feedback signal, observers can be trained to use only disparity cues and ignore shape effects. Some observers have difficulty ignoring the shapes of some figures, the "difficult" figures being different for each observer. After training, the relative depth thresholds for most figures approach those of the original unconnected parallel test lines.

Depth Perception↗
PubMed1991

Influence of remote objects on local depth perception.

The perceived relative depth of two test dots displayed within the fovea is shown to be influenced by other features in the surrounding area. These features can be as far apart as 51 deg and can have relative disparities as large as 20 deg, much larger than the disparities of the test dots. Since this effect is seen for stimuli presented for 100 msec or less, changes in direction of gaze cannot play a role. The effect varies inversely as the spatial separation between the test dots and the remote features, and is insensitive to the relative disparities of these remote features when they are greater than 2 deg. Observers sometimes differ significantly from each other in their responses to various configurations of the outlying features. This appears to rule out response mechanisms which depend only on the stimulus; some characteristics of the observer must be involved in determining the response. For these briefly presented stimuli, observers are unable to report accurately the relative depth of the central foveated test items if they are also required to report the depths of distant peripheral features.

Depth Perception↗
PubMed1991

Metastable motion anisotropy.

The phenomenon of apparent motion can arise when two spatially separated visual tokens are presented in temporal sequence. If tokens at opposite corners of a hypothetical square are presented simultaneously followed by simultaneous presentation of tokens at the remaining two corners, an apparent motion percept may occur along either the vertical or horizontal axis. The display is perceptually metastable since most observers will perceive motion along only one axis at a time. The metastable display, however, produces anisotropic results, in that with central fixation, vertical motion is seen more frequently than horizontal motion. The ratio of the vertical to horizontal length of the sides of a rectangle needed to achieve equal frequencies of motion judgments along the respective axes falls in the range of 1.18-1.92 for different observers in our experiments. It appears that signal transmission across the vertical midline is a major determinant of the vertical bias, since the anisotropic effects disappear when the fixation point is sufficiently offset along the horizontal meridian so as to cause a fully homonymous representation of all of the metastable tokens. One of the factors may be signal degradation or delay in callosal transmission which could reduce the strength of the motion signal along the horizontal axis. In addition, there appears to be a strip along the vertical midline with a width of 30-50 min arc within which reduced levels of anisotropy are found. The possibility that this strip is a consequence of a zone of naso-temporal overlap in the projection of the retina to the brain along the vertical meridian will be discussed.

Anisotropy↗
PubMed1989

Motion interference in speed discrimination.

Human speed discrimination can be degraded by additional stimuli in close spatial and temporal proximity to the designated test target. In these experiments, observers judged the relative asynchrony between a pair of briefly flashed dots: speed discrimination for two-dot apparent motion. The addition of two irrelevant (interfering) flashed dots to the stimulus, which produces accelerating apparent motion, impaired speed discrimination. We call this impairment motion interference; adjacent stimuli are not processed independently by the motion system. Motion interference is time selective; interfering dots simultaneous with the target dots do not impair speed discrimination, nor do interfering dots that precede or follow the target by 200 msec or more. Motion interference was observed even when the interfering dots were as far away as 1 deg from the test pair. Similar effects were observed with a smoothly moving test target and with interfering stimuli composed only of high spatial frequencies. A multiple-independent-channel model containing several parallel motion-energy detectors with different receptive-field sizes is considered and rejected. We conclude that speed discrimination depends on a time-selective combination of local motion signals from many detectors. These aggregate detectors combine information from local subunits, degrading information about acceleration.

Discrimination, Psychological↗
PubMed1985

Colony morphology on agar is a sensitive indicator for growth effectors.

Colonies of Chinese hamster ovary (CHO) cells growing on agar exhibit changes in colony morphology and size in response to extremely low doses of hormones and growth factors. Computer-aided densitometric scanning of photographs of these colonies allows the quantitative measurement of these morphological changes. Correlation of these changes with dosage for a variety of growth effectors is a sensitive assay for the effects of these factors, both alone and in combination, and should be useful in comparing effects of agents with similar biological activities and in the search for variant cell types with altered responsiveness. Cells growing attached to solid substrates are often responsive to these low dosages and do not seem to mimic in vivo growth effector phenomena as well as colonies growing in three dimensions on agar.

Animals↗
PubMed1984

A mutagen-testing assay based on heterogeneity in diameter and integrated optical density of mammalian cell colonies.

We investigated the effects of the well-known mutagenic agents ethyl methanesulfonate (EtMes), N-methyl-N-nitro-N-nitrosoguanidine (MNNG), and ICR-191 on colonies of the Chinese hamster ovary line CHO cultured on a semisolid substrate. These agents induced heterogeneity in diameter and integrated optical density of colonies as determined by computer-assisted photography and subsequent analysis of the images of the colonies. When CHO colonies were exposed to agents such as urethane that are not known to be mutagenic in mammalian systems or to activation-requiring mutagens such as cyclophosphamide, there was no noticeable effect on the distribution of colony diameter and volume. Similarly, nonmutagenic agents such as dimethyl sulfoxide (Me2SO) also did not induce heterogeneity in colony diameter and integrated optical density. Our observations recommend the use of agar-grown mammalian cell colonies for predictive testing of chemical mutagens and carcinogens in a simple, in vitro mammalian cell assay. This assay system, unlike other mammalian cell culture assays, allows detection and measurement of the simultaneous effects of chemical mutagens on several genetic and non-genetic targets and, thus, may emulate more closely the potential hazards of these agents in vivo.

Aminacrine↗