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W A Simpson

Publications and source records attributed to W A Simpson.

At least 19 recordsLinked to original sources
PubMed1991

Depth-coded motion signals in plaid perception and optokinetic nystagmus.

When two sine-wave gratings drift in different directions at the same speed behind a circular window, a single coherent plaid is seen rather than one grating sliding over the other. We find that as the stereo depth separation of the two component gratings increases, the probability of seeing a plaid declines. The gain of the slow phase of vertical optokinetic nystagmus (OKN) also falls as the separation of the components increases. When the two grating components are in the same depth plane, the vertical eye velocity is greater than that of either component. This shows that the OKN is being driven by the plaid, whose vertical speed is roughly twice as fast as the components. We conclude that both perception and OKN are fed by the same motion signal, which arises after binocular combination and after plaid synthesis.

Depth Perception↗
PubMed1990

The use of different features by the matching process in short-range motion.

The perception of the direction of motion in luminance kinematograms breaks down when the displacement exceeds a few element widths (this limit is called Dmax). When kinematograms whose elements differ only, in hue are used, motion can be seen but performance declines at smaller displacements. The short-range process, once thought to be only a luminance correlator, is thus able to use hue. If the size of Dmax indicates how well a feature stimulates the motion sensors, hue might be said to have an especially weak input to motion sensors. In order to find the relative potency of luminance and hue as bases of the short-range matching process, Dmax for these features was compared to those obtained for kinematograms whose elements differed in phase (T-phase or L-phase) or orientation (/or/). The matching process could use all the features tested. Luminance seems to be the preferred basis for motion matching, with hue and phase (a tie) yielding smaller Dmax and orientation the smallest.

Color Perception↗
PubMed1990

Colonial morphology of staphylococci on Memphis agar: phase variation of slime production, resistance to beta-lactam antibiotics, and virulence.

The growth of Staphylococcus epidermidis sensu stricto and Staphylococcus saprophyticus on Memphis agar yielded up to 6 morphotypes with each strain. With S. epidermidis, one morphotype produced slime (rho) but became non-slime-producing (epsilon) at a high frequency. The slime-producing rho variants were methicillin-resistant and more virulent than methicillin-susceptible epsilon variants in an endocarditis model. With S. saprophyticus, phase variation was of higher frequency. Nitrosoguanidine mutagenesis produced a stable blue epsilon form that was more virulent than the parent in a mouse model of urinary tract infection. Mutants with the blue epsilon phenotype differed from gold epsilon parents in a variety of phenotypic properties, including increased resistance to oxacillin. These staphylococcal species have a high frequency of phase variation: Phase variants differ in antibiotic resistance and virulence, which is only partially correlated with suggested virulence factors such as slime production.

Animals↗
PubMed1990

Identification of an antigenic marker of slime production for Staphylococcus epidermidis.

The pathogenic Staphylococcus epidermidis strain RP62A (ATCC 35984) adheres to smooth surfaces by forming a tenacious bacterial film known as slime. The mechanism of slime production is not known; however, workers in the laboratory of G. Pier (Harvard Medical School, Boston, Mass.) have isolated from RP62A a galactose-rich capsular polysaccharide adhesin (CPA) which mediates the attachment of the organism to smooth surfaces. We have obtained two daughter strains from RP62A that no longer produce slime. One daughter strain, H4A, was obtained by selection for a spontaneous variant; the other strain, HAM892, was obtained by treating growing cultures of RP62A with acriflavin. Using an antiserum generated against whole cells of RP62A, we have examined lysozyme-lysostaphin digests of RP62A, H4A, and HAM892 by double immunodiffusion. The two strains that no longer produced slime no longer produced a particular antigen, which we refer to as the slime-associated antigen (SAA). SAA was also produced by unrelated strains of slime-producing S. epidermidis. SAA was heat and protease stable, had a molecular weight of greater than 50,000, and could be partially purified by chromatographing trypsin-digested material over a Sephadex G-200 column. Chemical analysis of partially purified SAA by gas-liquid chromatography found SAA to be glucose rich (59%) and galactose poor (1.4%). This analysis chemically distinguished SAA from CPA. When tested together by double immunodiffusion with anti-RP62A and anti-CPA antisera, partially purified SAA did not cross-react with CPA. Kinetic studies suggested that SAA is a marker for surface accumulation whereas CPA mediates initial adherence.

Adhesins, Bacterial↗
PubMed1990

Differential production of slime under aerobic and anaerobic conditions.

A series of 37 clinical isolates of coagulase-negative staphylococci previously identified as negative for slime production by the tube test were reexamined by the tissue culture plate test under aerobic and anaerobic conditions. None of the strains produced slime under anaerobic conditions; however, five strains (13%) produced slime under aerobic conditions.

Aerobiosis↗
PubMed1990

Phenotypic variation of Staphylococcus epidermidis in infection of transvenous endocardial pacemaker electrodes.

Coagulase-negative staphylococci isolated from a patient with a pacemaker electrode infection were extensively evaluated by phenotypic and genotypic characterization. Findings from this evaluation were striking because different colony morphologic subtypes were recovered from blood and resected pacemaker electrodes. Staphylococci from each colony subtype (LBL, LBV, LBP, LBS) were identified as slime-producing strains of Staphylococcus epidermidis sensu stricto. Direct plating of isolates from a restricted electrode revealed a mixture of colony phenotypes when examined on a high-salt, low-glucose medium, Memphis agar. Bacteriophage typing employing 17 different phages and plasmid profile analysis were largely unsuccessful in further characterizing bacterial cells of each of the four colony morphotypes. On the other hand, restriction endonuclease analysis by EcoRI digestion of the chromosomal DNA demonstrated the probable common clonal origin of the four colony phenotypes.

Aged↗
PubMed1990

Placental hypoechoic-anechoic areas and infarction: sonographic-pathologic correlation.

High-resolution ultrasound (US) and pathologic analysis were used to define the relationship between placental hypoechoic-anechoic areas, frequently seen in the third trimester, and the clinically significant entity of placental infarction. Placentas were obtained from three groups of patients: those prospectively demonstrating one or more placental hypoechoic-anechoic areas greater than or equal to 1 cm in diameter on third-trimester sonograms (n = 14), those with risk factors for vascular disease (n = 12), and control patients without risk factors (n = 16). Pathologic analysis demonstrated significantly more infarcts in patients with risk factors than in control patients (17 vs three, P = .047). Of a total of 22 infarcts from all three groups, 19 (86%) were isoechoic to viable placenta and therefore not detected with US. The three infarcts identified with US contained hypoechoic or anechoic foci of fibrin or hemorrhage. Of 26 placental hypoechoic-anechoic areas 23 (88%) were decidual septal cysts or intervillous thrombosis without infarction. The authors conclude that nonhemorrhagic placental infarction cannot be identified with ex utero US and, by inference, that prenatal US is probably insensitive for detection of placental infarction.

Female↗
PubMed1990

The role of fibronectin binding in the rat model of experimental endocarditis caused by Streptococcus sanguis.

Inactivation of fibronectin (Fn) binding by insertional mutagenesis of Streptococcus sanguis with Tn916 reduces virulence of this bacterium in the rat model of infective endocarditis (IE). Transconjugants were screened for Fn adherence using an ELISA adherence test. One transconjugant had a decreased adherence to immobilized Fn. Southern hybridization demonstrated that the insertion occurred only once in this mutant. The parent strain and mutant strain JL113 were used as challenge strains in a rat endocarditis model. These experiments demonstrated that the mutant had a reduced ability (P less than 0.05) to produce IE. Spontaneous excision of Tn916 from JL113 produced strains identical to both the parental and mutant phenotypes. One strain (JLR-19) that retained the mutant phenotype and one (JLR-15) that regained the parental phenotype for Fn binding were tested for their ability to produce IE. These strains demonstrated that the ability to bind Fn and to produce IE were correlated after Tn916 excision. The reduced virulence of the mutant suggested that adherence of S. sanguis to immobilized Fn plays an important role in the production of IE.

Animals↗
PubMed1989

The ultrastructural pathological features of congenital microcoria. A case report.

We describe the light microscopic, immunohistochemical, and electron microscopic findings in a sporadic case of congenital microcoria in a 72-year-old man with senile cataract. We demonstrated a lack of myofilaments and desmin in the stromal cytoplasmic processes of the anterior pigmented cells of the iris, although other features of muscle differentiation were present in these few surviving cell processes that normally form the pupil dilator muscle. Degenerative changes in anterior pigment cells and iris stromal atrophy were thought to be late secondary features of microcoria. The findings suggest that congenital microcoria results from a defect of intermediate filaments in the terminal fetal stages of differentiation of the anterior pigmented epithelial cell of the iris, with absence of myofilaments and consequent failure of development of a functional dilator pupil muscle.

Aged↗
PubMed1989

Human optokinetic nystagmus is linked to the stereoscopic system.

It was previously proposed that a linkage between the optokinetic system and the stereoscopic system in higher mammals serves to allow these animals to selectively stabilize those parts of the visual scene which lie in the plane of convergence as the animals move forward in a three-dimensional world (Howard and Ohmi, 1984). A new procedure is now described by which OKN gain can be measured as a function of the binocular disparity of the stimulus. With vergence locked on a vertical line, the gain of the slow phase of vertical optokinetic nystagmus (OKN) was recorded in four human subjects as the binocular disparity (stereo depth) of the moving display was changed from -3 degrees to +3 degrees. The gain of OKN was found to be inversely proportional to binocular disparity. Evidence for cells in the visual cortex, MT and MST that are sensitive both to visual motion and binocular disparity is reviewed. It is argued that the activity of cells responsive to direction of motion and zero disparity selectively augments OKN and that this enables humans to stabilize the images of parts of the scene in the plane of regard while ignoring competing motion signals arising from other distances.

Functional Laterality↗
PubMed1989

Molecular epidemiology of nosocomial, multiply aminoglycoside resistant Enterococcus faecalis.

The prevalence of high-level aminoglycoside resistance among Enterococcus faecalis at the Memphis VA Medical Center was 23.6% (59 of 250 isolates) from October to December 1986. Hybridization to a probe cloned from Ent. faecalis pIP1800 for 6' acetyltransferase-2" phosphotransferase (AAC6'-APH2") was observed in 55 (93.2%) of the resistant isolates and was associated with gentamicin resistance. Hybridization to a probe cloned from Ent. faecalis pJH1 for 3', 5" phosphotransferase type III (APH3', 5" III) was observed in 28 (47.4%) and was associated with streptomycin resistance. Twenty-five of the 32 isolates which were resistant to both gentamicin and streptomycin hybridized to both probes. Cell mating in conjunction with hybridization indicated that the AAC6'-APH2" gene is transferred separately from that for APH3', 5" (III), and the streptomycin resistant gene is cotransferred with the latter. The gentamicin-streptomycin resistant isolates therefore contain genes from two Ent. faecalis plasmids, and resistance to these two antibiotics appears to transfer separately. The genetic homogeneity of these isolates suggests nosocomial transmission of enterococci.

Aminoglycosides↗
PubMed1989

Unusual complication of strabismus and lid surgery.

A patient with longstanding dysthyroid eye disease developed a broad adhesion between the globe and the upper eyelid following simultaneous surgery to recess the levator palpebrae superioris and lateral rectus muscles. This unusual complication may have developed as a consequence of failure to suture the conjuctiva of the upper fornix to the tarsus and also due to the prolonged static contact between the resulting adjacent raw areas due to postoperative eyelid traction and pressure dressings. Periods of such contact may have been further prolonged by the action of diazepam, which reduces the peak velocity of saccadic eye movements and suppresses rapid eye movement of sleep. Eyelid surgery should be performed as a separate procedure, under local anesthetic after healing of areas bare of conjunctiva from strabismus surgery utilizing the adjustable suture technique.

Conjunctiva↗
PubMed1989

The step method: a new adaptive psychophysical procedure.

A new adaptive psychophysical method, the step method, is introduced. Simulations show the method to be less biased and more efficient than constant stimuli or Pentland's adaptive method for fewer than 40 trials. An experiment using discrimination of dot number, however, failed to find any differences among the three methods in either bias or efficiency.

Algorithms↗
PubMed1988

Interaction of fibronectin with DNA/anti-DNA complexes from systemic lupus erythematosus: role of activated complement Cl in modulation of the interactions.

Fibronectin (Fn) is an integral constituent of the endothelial cell surface and the basement membrane. The mechanism for binding DNA/anti-DNA complexes to Fn was examined in a solid-phase assay. In physiological buffer, a low-affinity binding of DNA was observed with Fn and optimal binding was seen at pH 6.5 and in the absence of Ca2+. Further, the interaction of DNA to Fn was inhibited when DNA was complexed to anti-DNA antibody. However, complement Clq mediated the binding of complexes to Fn at pH 7.4 and it was proportional to the extent of the dissociation of Cl. Cl inactivator (Cl-In) appeared to play a modulating role; whereas at low concentrations (Cl:Cl-In::4: less than 1) it enhanced the binding of complexes to Fn, higher concentrations inhibited the binding. Further, sera from patients with active systemic lupus erythematosus reacted with Fn, which was shown to be dependent on the presence of Clq and was minimally affected by DNase treatment of sera, indicating a relatively minor role of DNA in the direct binding of DNA to Fn. These findings support "circulating immune complex" hypothesis in the pathogenesis of lupus glomerular immune complex deposition disease.

Antibodies, Antinuclear↗
PubMed1988

Depth discrimination from optic flow.

A simple scheme for deriving relative depth (time-to-collision, or TTC) from optic flow is developed in which the total flow is first sensed by unconnected motion (imperfect filter) sensors and then the rotational component is subtracted to yield the translational component. Only the latter component yields depth information. This scheme is contrasted with one where the TTC sensors respond only to the translational component at the initial registration of the flow (perfect filter sensors or looming detectors). The simple scheme predicts the results of three experiments on discrimination of TTC: discrimination thresholds are elevated if the objects withdraw from rather than approach the observer, thresholds are elevated if a rotational component is added to the flow, and the amount of threshold elevation resulting from the addition of a rotational component is reduced by prior adaptation to a pure rotational flow. These results confirm the simple model and disconfirm predictions based on the looming detector scheme.

Attention↗
PubMed1988

Characteristics of adherence to plastic tissue culture plates of coagulase-negative staphylococci exposed to subinhibitory concentrations of antimicrobial agents.

Adherence and hydrophobic properties characteristic of 6-h cultures of coagulase-negative staphylococci on plastic tissue culture plates were determined after the organisms had been treated for up to an additional 6 h with subinhibitory concentrations of antimicrobial agents. Clindamycin, erythromycin, norfloxacin, tunicamycin, and vancomycin had no effect. Cephalothin, chloramphenicol, gentamicin, imipenem, methicillin, novobiocin, polymyxin B, rifampin, and tobramycin had varied effects. Most changes induced by these agents resulted in differing degrees of inhibition of adherence. However, an increase in adherence of 65% was observed for one strain, RP14 (ATCC 35981), with rifampin treatment. Hydrophobicity of coagulase-negative staphylococci treated with antimicrobial agents measured by using a biphasic system with hexadecane showed excellent correlation with adherence (r = .958, P less than .0005), a result suggesting that adherence of these bacteria in this system depends heavily on hydrophobic surface moieties. Transmission electron microscopy of drug-treated coagulase-negative staphylococci revealed only an increase in cell-wall thickness, regardless of whether adherence was increased or decreased.

Anti-Bacterial Agents↗