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Biomedical subjects

T L Simpson

Publications and source records attributed to T L Simpson.

At least 19 recordsLinked to original sources
PubMed1998

The Cyclops effect in adults: sighting without visual feedback.

When asked to look through a tube, younger children place it at the bridge of the nose, and not over one eye: the Cyclops effect. This is a natural response to a median plane egocenter. With maturity, the Cyclops effect disappears as we learn to overcome the consequences of an egocenter between the two eyes, and instead, to use the "preferred" eye. We videotaped adults (n = 14) and children with normal vision (n = 30), children with comitant strabismus (n = 14), and adults and children (n = 14) with one eye enucleated as they attempted to look through a plastic tube. Immediately in front of the face was a liquid crystal window that could be either transparent or opaque. As the tube was raised, the window was made opaque--blocking sight of the target, their hands, and the tube. Most binocular observers placed the tube approximately at the bridge of the nose. This was significantly different from the response of the enucleated observers who put the tube 75% of the way to the remaining eye (P = 0.0001). All observers align, on average, with the measured location of their egocenter when asked to perform a monocular task without visual cues. Deprived of visual feedback, binocular observers show the Cyclops effect, regardless of age.

Adolescent↗
PubMed1995

Test-retest variability and correlations between tests of texture processing, motion processing, visual acuity, and contrast sensitivity.

PURPOSE: To compare the test-retest variability (reliability) and the relations among clinical tests of texture and motion processing, visual acuity for high- and low-contrast letters, and the Pelli-Robson contrast sensitivity test. METHODS: In 20 normally sighted subjects, monocular visual acuity for letters of 96% and 11% contrast, Pelli-Robson contrast sensitivity, and motion-defined and texture-defined letter recognition thresholds were measured on each of two different days. RESULTS: Test-retest correlation coefficients were 0.75, 0.91, 0.61, 0.90, and 0.84 and bivariate test-retest regression slopes were 1.0, 1.1, 0.8, 1.0, and 1.2 for high- and low-contrast acuity, contrast sensitivity, and motion and texture processing, respectively. The inter-test correlations with both test and retest significant were as follows: visual acuity for high-contrast letters vs. visual acuity for low-contrast letters; and recognition threshold for texture-defined letters vs. acuity for letters of both high and low contrast. CONCLUSION: Test-retest variability for the tests of motion and texture processing were at least as low as for established clinical tests of high and low contrast acuity and contrast sensitivity. We conclude that these new tests offer a reliable means of obtaining clinical information which complements that provided by conventional tests with luminance-defined letters.

Adolescent↗
PubMed1995

Vision thresholds from psychometric analyses: alternatives to probit analysis.

Vision scientists and clinicians regularly obtain measures of vision and determine a person's vision threshold by choosing one of numerous methods of analysis. Analytical methods differ in their presumptions about the measures, in their complexity and in the ease of obtaining the threshold estimate. In light of these differences among so-called methods of psychometric analysis, the question is, "Do various analytical methods provide essentially equivalent vision threshold estimates or are there consequential differences that require consideration?" Measures of visual acuity, motion processing, and texture processing were obtained from 20 subjects. Each set of measures was analyzed by five psychometric methods: Logit Analysis, Normit Analysis, and linear regression of z-score transformed, logit transformed and untransformed probabilities vs. stimulus strength. The resulting thresholds were compared to the threshold obtained from Probit Analysis, which was used as a reference or "gold standard." Thresholds from the procedures were remarkably similar to those from Probit Analysis. In addition, examination of the speed of the procedures revealed that Probit Analysis was up to 10 times slower than some of the others. Considering the similarity of thresholds, the speed of computation, and the ease of implementation, Logit and Normit Analyses especially provide effective alternatives to the current gold standard, Probit Analysis, for the estimation of psychometric thresholds. In addition, z-score, logit, and linear regressions also produced unbiased threshold estimates under many circumstances, but the latter method should be applied with some caution.

Algorithms↗
PubMed1993

Effect of asymmetry on calculations of convergence in prism diopters.

Calculations of convergence in units of prism diopters are based on an assumption that convergence is symmetrical, that is, the visual axes intersect on the median plane. The effect of departure from symmetrical convergence is shown to produce systematic errors in calculations of convergence distance, if the convergence angle is constrained to be constant. On the other hand, if the convergence distance is constrained to be constant, departure from symmetry results in a systematic reduction in the convergence angle. Because these errors are disregarded in the specification of convergence in prism diopters, care should be taken in using these units for larger amounts of asymmetrical convergence.

Convergence, Ocular↗
PubMed1991

Ultrastructural study of the mature egg of the marine sponge Stelletta grubii (Porifera Demospongiae).

Stelletta grubii is an oviparous demosponge, which, during its reproductive period from summer to autumn, has small eggs (80-90 microns) dispersed uniformly in the mesohyl. The nucleolated nucleus is surrounded by dictyosomes containing small vesicles, which contribute to form reserve material. Vesicles, numerous food vacuoles, and groups of mitochondria are observed in the granular cytoplasm. Electron-dense yolk inclusions and lipids are found peripherally. The cortical portion of the egg cytoplasm possesses vacuoles with fibrillar contents. The egg forms pseudopodia, which could permit the capture of numerous bacteria present in the surrounding mesohyl. A thick layer of collagen fibrils, including lophocytes, separates the egg from the surrounding sponge mesohyl. Ultrastructural analysis has demonstrated the presence both of cellular components capable of autosynthetic activity (nutrient vesicles) and of phagocytosis mechanisms (pseudopod capture of bacteria) for the storage of nutrients by the egg.

Animals↗
PubMed1991

Should level of measurement considerations affect the choice of statistic?

The belief that the level of measurement (nominal, ordinal, interval, ratio) achieved in the data constrains the type of statistic that may be used legitimately for analysis is examined in general, and in an optometric-vision science context. Theoretical considerations indicate that statistical statements about the data may be made independently of the level of measurement, and that although researchers must be concerned about the quality of their measurement, the role of measurement theory is in the interpretation of the meaning of the investigation's results as a whole not in the governance of the choice of statistic. Empirical studies indicate that measurement considerations can be ignored for the purposes of testing the null hypothesis with little or no resultant error. Finally, adherence to the belief that level of measurement considerations limits statistical choice would result in the use of generally less powerful statistical tests--an undesirable and, all things considered, an unwarranted consequence.

Optics and Photonics↗
PubMed1991

The relationship between eye position and egocentric visual direction.

Hering's model of egocentric visual direction assumes implicitly that the effect of eye position on direction is both linear and equal for the two eyes; these two assumptions were evaluated in the present experiment. Five subjects pointed (open-loop) to the apparent direction of a target seen under conditions in which the position of one eye was systematically varied while the position of the other eye was held constant. The data were analyzed through examination of the relationship between the variations in perceived egocentric direction and variations in expected egocentric direction based on the positions of the varying eye. The data revealed that the relationship between eye position and egocentric direction is indeed linear. Further, the data showed that, for some subjects, variations in the positions of the two eyes do not have equal effects on egocentric direction. Both the between-eye differences and the linear relationship may be understood in terms of individual differences in the location of the cyclopean eye, an unequal weighting of the positions of the eyes in the processing of egocentric direction, or some combination of these two factors.

Attention↗
PubMed1990

Appreciation of metaphoric alternative word meanings by left and right brain-damaged patients.

Appreciation of metaphoric and nonmetaphoric alternative word meanings was assessed in 19 aphasic, left (LHD) and 15 non-aphasic, right (RHD) hemisphere brain-damaged stroke patients. With the one exception in the aphasic group, all patients were male. In an unspeeded sorting task, subjects responded on the basis of less frequent, alternative meanings of polysemous target words. Targets were either polysemous adjectives (e.g. "warm") having metaphoric alternative meanings (loving) or polysemous nouns (e.g., "pen") having non-metaphoric alternative meanings (writing implement, cage). Both patient groups performed worse overall than a group of nonbrain-damaged control subjects. Relative to the RHD patients, LHD patients showed a spared appreciation of metaphoric alternative meanings. In addition, LHD, but not RHD, patients performed better on metaphoric adjective trials when there was high similarity between a word's dominant and (metaphoric) alternative meaning. The results suggest a pervasive insensitivity of RHD patients to alternative interpretations of linguistic units, and a special role for the intact right hemisphere in lexical-semantic processes related to metaphor comprehension.

Aged↗
PubMed1987

Effects of luminance on the visual acuity of strabismic and anisometropic amblyopes and optically blurred normals.

Evidence is accumulating to suggest fundamental differences between strabismic and anisometropic amblyopia. We explored differences in these amblyopes by assessing the effects of luminance on optotype acuity, using targets that ranged from photopic to low mesopic levels. Our results show that reducing luminance has less of a detrimental effect on the acuity of strabismic than anisometropic amblyopic eyes. Furthermore, the effect of decreasing luminance on the acuity of optically blurred normal eyes mimicked the effect we found for anisometropic amblyopic eyes. These results are consistent with the hypothesis that the fundamental deficit in anisometropic amblyopia is impaired resolution while in strabismic amblyopia the fundamental deficit is impaired spatial directionalization.

Amblyopia↗
PubMed1986

The effect of optical blur on visual acuity for targets of different luminances.

We studied the effect of 1.25 D of optical blur on visual acuity at luminances ranging from photopic to low mesopic levels. Optical blur reduces acuity at all luminances tested, this reduction being fourfold at a high photopic level (170 cd m-2) and twofold in low mesopia (0.017 cd m-2). The reduction of visual acuity with blur at low luminances is greater than might be expected from a spatial frequency analysis of vision.

Humans↗
PubMed1985

Posttranslational processing of the insulin proreceptor.

Following translation of the insulin proreceptor by 3T3-L1 adipocytes, about 1.5 hours is required for its conversion into active receptor; an additional 1.5 hours is needed for the active receptor to reach the plasma membrane. During this 3-hour period the proreceptor undergoes a complex series of processing events, glycosylation being an essential processing step. Thus, treatment of 3T3-L1 adipocytes with tunicamycin causes the depletion of cellular insulin binding activity and the accumulation of an inactive aglyco proreceptor. To investigate posttranslational processing of normal proreceptor and the role of glycosylation in active receptor formation, metabolic labeling experiments were conducted. The first 35S-labeled intermediate detected is a 190-kDa polypeptide (proreceptor) which is rapidly (t1/2 = 15 minutes) processed into a 210-kDa species. Both polypeptides contain N-linked core oligosaccharide chains, but in the latter case these chains appear to contain terminal N-acetylglucosamine. The 210-kDa precursor is converted slowly (t1/2 = 2 hours) by proteolytic processing into a 125-kDa (alpha') and 83-kDa (beta') species. Immediately prior to insertion into the plasma membrane, 3 hours after its synthesis, the alpha' and beta' precursors are converted to mature receptor composed of alpha (135 kDa) and beta (95 kDa) subunits. The 125-kDa alpha' and 83-kDa beta' precursors are endoglycosidase H-sensitive and their oligosaccharide chains do not contain terminal sialic acid. Just prior to insertion into the plasma membrane the alpha' and beta' precursors are sialylated, giving rise to the 135-kDa alpha and 95-kDa beta receptor subunits and becoming Endo H resistant and neuraminidase sensitive. In the presence of tunicamycin, a 180-kDa aglyco receptor polypeptide accumulates which is not further processed and does not reach the cell surface. It is concluded that N-linked oligosaccharide chains on the proreceptor are required either for its intracellular translocation to the proteolytic cleavage site or for its identification as a target of the cleavage enzyme. Thus, glycosylation of the insulin proreceptor is crucial for proper processing and formation of functional receptor.

Cell Membrane↗
PubMed1984

Role of glycosylation in the processing of newly translated insulin proreceptor in 3T3-L1 adipocytes.

A procedure was developed for the immunoprecipitation of glycosylated and nonglycosylated forms of the insulin receptor and its precursors without prior purification using lectins. 3T3-L1 adipocytes were labeled with [35S]methionine after which 35S-labeled receptor polypeptides were specifically immunoprecipitated and characterized by sodium dodecyl sulfatepolyacrylamide gel electrophoresis. The first 35S-polypeptide detected was a 190-kDa glycosylated proreceptor which was rapidly (t1/2 approximately equal to 15 min) processed to a 210-kDa intermediate. The latter precursor was more slowly (t1/2 approximately equal to 2 h) proteolytically processed to 125-kDa (alpha') and 83-kDa (beta') precursors of the mature alpha- and beta-receptor subunits. Immediately prior to insertion into the plasma membrane, i.e. about 3 h after translation, the alpha'- and beta'-precursor polypeptides were converted to the mature 135-kDa alpha- and 95-kDa beta-receptor subunits. The characteristics of the oligosaccharide moieties of the receptor precursors and products were investigated. The 210-kDa precursor and its two products, the 125-kDa alpha'- and 83-kDa beta'-species, and the mature alpha- and beta-receptor subunits bind tightly to wheat germ lectin, whereas the 190-kDa proreceptor species is not bound. Upon incubation with endoglycosidase H, both the 210- and 190-kDa species are converted to a 180-kDa species. The 125-kDa alpha'- and 83-kDa beta'-species are also cleaved by endoglycosidase H, being reduced in size to 97 and 79 kDa, respectively. Based on their sensitivity to endoglycosidase H and insensitivity to neuraminidase, the oligosaccharide chains of the receptor precursors (190, 210, 125, and 83 kDa) do not contain terminal sialic acid (or other capping sugars). However, near the time of insertion into the plasma membrane, capping of the alpha'- and beta'-species by sialic acid occurs, giving rise to the mature 135-kDa alpha- and 95-kDa beta-receptor subunits, which are partially endoglycosidase H-resistant and neuraminidase-sensitive. When 3T3-L1 adipocytes are treated with tunicamycin, a 180-kDa proreceptor aglycopolypeptide is synthesized which is incapable of undergoing further processing and proteolytic cleavage to the alpha- and beta (or alpha'- and beta'-)-subunits. The 180-kDa species, which appears to be the aglyco-form of hte 190-kDa proreceptor generated by endoglycosidase H, is resistant to trypsin in the intact cell and apparently has not reached the cell surface. Thus, the oligosaccharide moieties of the insulin receptor precursor are crucial for proper processing, intracellular translocation, and formation of functionally competent insulin re

Adipose Tissue↗
PubMed1983

Interaction of germanium (Ge) with biosilicification in the freshwater sponge Ephydatia mülleri: evidence of localized membrane domains in the silicalemma.

In the presence of germanium (Ge) the needle-shaped silica spicules of the freshwater sponge Ephydatia m ulleri are very short and thin and possess bulbs with large spines. SEM-coupled X-ray analyses confirm the incorporation of Ge into the silica. A small number of bulbs are susceptible to erosion by HNO3 and hypochlorite and although the chemical basis of such erosion is presently unknown it suggests the presence of an organic matrix within the bulbs and/or an incomplete polymerization of the silica. Addition of Ge to control media in which silicification is newly initiated increases the incidence of erosion and results in centrally located eroded areas of the silica and discontinuities in its deposition. Removal of Ge from such newly forming structures results in a partial recovery of normal morphology (spine development and thickening of the silica) but only in the central region surrounding the bulbs. Both results establish the presence of a central, active region for silicification and further support the view that there is a distal spreading, away from this center, of transported forms of silica. Secondary centers may also be present. The newly assembled organic core of control structures is associated with tubular elements possibly derived from the surrounding membrane. In such newly silicifying structures the spicule tips contain oriented material in the form of "rays." Both of these new observations increase the likelihood of the presence of an organic matrix within the silica.

Animals↗
PubMed1980

Fluorescent antibody localization of Microciona prolifera aggregation factor and its baseplate component.

Specific rabbit antisera were prepared against purified aggregation factor and its membrane-associated receptor, baseplate, derived from the marine sponge. Microciona prolifera. They were utilized in conjunction with fluorescent-labeled goat anti-rabbit IgG in an assay to demonstrate the surface localizations of both components. The specificity of antibody preparations for AF and BP was demonstrated through inhibition of the rotation-mediated assay by homotypic antibody. This study confirms the presence of aggregation factor on the surface of disaggregated sponge cells maintained in the presence of the divalent cations, Ca++ and Mg++, and its absence when cells are maintained in Ca++ and Mg++-free seawater. The location of BP could also be demonstrated on the cell surface. Aggregation factors and baseplate appear to be heavily distributed on archeocytes and choanocytes, but are localized less intensely on gray cells. Gray cells are typified by yellowish autofluorescence of their intracellular granules in stained and control preparations. The reaction of anti-Microciona aggregation factor with its homotypic factor appeared to be species specificity judged by immunofluorescence assays and by inhibition of rotation-mediated assay by anti-homotypic AF since antibodies prepared against heterotypic AF preparations were unreactive.

Animals↗
PubMed1976

Sex reversal in a freshwater sponge.

Spongilla lacustris exhibits a type of alternative hermaphroditism, new to the phylum, in which a sponge may be exclusively male or female during the period of sexual reproduction one year and the opposite sex the next year. This form of sexuality may facilitate larva production and thus dispersal following colonization of a new habitat. Gametogenesis occurs shortly after gemmule hatching in both males and females but slightly later in males.

Animals↗