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Biomedical subjects

S T Chung

Publications and source records attributed to S T Chung.

At least 37 records · Page 2Linked to original sources
PubMed1997

Congenital nystagmus image motion: influence on visual acuity at different luminances.

In persons with congenital nystagmus (CN), the ability to integrate visual information over time can be limited by two factors--the duration of foveation periods and the temporal integration period of the visual system. The purpose of this study was to assess the relative importance of these two factors for visual acuity for targets of different luminances. We measured visual acuity using Landolt C targets at 5 luminance levels (50 to 0.005 cd/m2) in 6 observers with CN, and in 6 normal observers with comparable motion of the retinal image. To allow comparison, normal observers viewed the targets during image motion simulating jerk CN, with "foveation durations" ranging from 20 to 160 ms. In the normal observers, acuity improves as a function of the simulated foveation duration at all luminance levels. However, this improvement is larger and occurs at a faster rate at high than at low luminances. The more gradual improvement in acuity with simulated foveation durations at low luminances is consistent with a prolongation of the temporal integration period, which we estimate to range from approximately 140 to 380 ms over the 4 log unit range in luminance that we tested. In observers with CN, the change in acuity with luminance is similar, but not identical, to that in normal observers when the duration of the foveation periods is matched. We conclude that the integration of visual information may be limited by either the period of temporal integration or the duration of the foveation period in persons with CN, depending upon which is shorter at the luminance level under consideration.

Female↗
PubMed1996

Enzymatic formation of potential anticancer and antiviral inosine analogues.

Theoretically, inosine analogues should act as effective inhibitors of tumor cell proliferation and viral replication. To acquire a broad spectrum of new candidate inosine analogues, a rapid, facile, quantitative and stereoselective method for deaminating potential antitumor and antiviral adenine analogues previously synthesized in our laboratory was developed. A novel 5'-adenylic acid deaminase, with relaxed substrate requirements, from Aspergillus species was utilized to deaminate four hexofuranosyladenine nucleosides and five adenine nucleoside dialdehydes to their corresponding inosine analogues. The fastest rates of deamination for the hexofuranosyl nucleosides were for the compounds where the vicinal hydroxyl groups on the sugars are oriented in the erythro configuration. For rapid deamination of the adenine nucleoside dialdehydes, the R configuration at the proximal carbon atom is preferred, while the nature of the group on the distal carbon atom has no significant effect on the rate or extent of deamination.

AMP Deaminase↗
PubMed1996

Vernier in motion: what accounts for the threshold elevation?

Vernier acuity is susceptible to degradation by image motion. The purpose of this study was to determine to what extent vernier thresholds are elevated in the presence of image motion because of reduced stimulus visibility, due to contrast smearing, or to a shift in the spatial scale of analysis. To test the visibility hypothesis, we measured vernier thresholds as a function of stimulus velocity (0-6 deg/sec), for various levels of stimulus visibility, each normalized to the detection threshold at the respective velocity. Contrary to the prediction of the visibility hypothesis, vernier thresholds worsen as the velocity increases, even when the stimuli are equally visible. To test the shift in spatial scale hypothesis, we determined spatial frequency tuning functions for vernier discrimination and line detection tasks, using a masking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak masking for both vernier discrimination and line detection, which indicates the most sensitive band of spatial frequencies for each task, shifts systematically toward lower spatial frequencies as the velocity increases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences between peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediate the two tasks, at least at low velocities. The masking results are consistent with previous findings that directionally selective motion detectors mediate detection of moving stimuli, but suggest that these detectors do not analyze vernier offsets. We conclude that the elevation of vernier threshold for a moving stimulus is accounted for primarily by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility.

Contrast Sensitivity↗
PubMed1996

Velocity criteria for "foveation periods" determined from image motions simulating congenital nystagmus.

Foveation periods are brief intervals in the congenital nystagmus (CN) waveform when the image is on or near the fovea and eye velocity is relatively slow. The purpose of this study was to determine how visual acuity depends on image velocity for foveation periods of different durations. Visual acuity was measured using high-contrast, single Landolt Cs in four normal observers during image motion simulating that in jerk nystagmus. The "simulated foveation periods" ranged from 20 to 100 ms in duration and 0 to 16 degrees approximately in velocity. The "critical velocity" was defined as the velocity during simulated foveation periods that produced a just-noticeable worsening of acuity (0.05 and 0.1 logMAR) from that in the zero-velocity condition. Critical velocity increased from approximately 3 degrees approximately for simulated foveation durations between 100 and 40 ms to approximately 5 degrees approximately for a simulated foveation duration of 20 ms. Critical velocities increased further when the targets were presented peripherally or with optical blur, to introduce an additional acuity loss. A consequence of these findings is that less recovery of acuity should be expected when retinal image motion is reduced in individuals with CN if a sensory acuity deficit coexists.

Eye Movements↗
PubMed1996

Ricco's diameter for line detection increases with stimulus velocity.

The purpose of this study was to determine whether Ricco's diameter, the spatial extent within which sensitivity demonstrates a perfect reciprocity between contrast and area, enlarges as the stimulus velocity increases. Detection thresholds were measured for a single line of length 10 arcmin as a function of linewidth that varied between 0.31 and 21.7 arcmin and for velocity ranging from 0 to 6 deg/s. We fitted the detection threshold versus linewidth data with two power functions of slope 0 and 1 and defined the intersection of these two functions as Ricco's diameter. For an increase in velocity from 0 to 6 deg/s, Ricco's diameter increases in dimension by approximately a factor of 4. Similar results were obtained when Ricco's diameter was estimated by comparing detection threshold of a thin line to that of an edge. The increase in Ricco's diameter with stimulus velocity suggests that the spatial-frequency mechanism that mediates line detection shifts progressively toward lower spatial frequencies for faster moving stimuli.

Contrast Sensitivity↗
PubMed1995

"Dumping" of rebound nystagmus and optokinetic afternystagmus in humans.

Rebound nystagmus (RN) is an involuntary movement of the eyes, characterized by slow-phase eye velocity in the direction of previously maintained eccentric gaze. The purpose of this study was to clarify the neural or neuromuscular events that are responsible for the generation of RN. To do so, we examined whether a briefly presented visual stimulus during RN reduces (i.e., "dumps") subsequent eye velocity, compared with the velocity of slow-phase eye movements when no visual stimulus was presented. For comparison, "dumping" was examined also for optokinetic afternystagmus (OKAN), which is generally believed to result from eye-velocity signals stored in a central neural integrator as a consequence of optokinetic stimulation. Results obtained from ten normal observers showed that RN and OKAN both exhibit "dumping": average slow-phase eye velocities were reliably slower after fixation of a 0.6 deg stationary target than on trials when no fixation target was presented. Although RN decayed faster than OKAN in darkness, the magnitude of "dumping" increased similarly with the duration of the visual stimulus (25 ms to 4 s) for both types of eye movement. The results imply that signals from a central velocity-storage mechanism contribute to the generation of RN.

Adult↗
PubMed1995

Effect of retinal image motion on visual acuity and contour interaction in congenital nystagmus.

This study determined how contour interaction (the degradation of visual acuity by the presence of nearby contours) is affected by the incessant retinal image motion that occurs in observers with congenital nystagmus (CN). Visual acuity was measured for single, high-contrast, black Landolt Cs, presented without and with flanking bars (contour-to-C separation = 1, 2, 5, or 10 multiples of the gap width of the C). Stimuli were presented against either a white or a black surround. For comparison, acuity was also determined in normal observers, with and without motion of the stimulus to simulate the retinal image motion in jerk CN. The results show that the peak magnitude of contour interaction (the maximal degradation in acuity attributable to contour interaction) is significantly larger in the observers with CN than in normals. When acuity targets are presented against a black surround, contour interaction also occurs over a wider spatial extent in the observers with CN. Imposed image motion increases the extent of contour interaction in normal observers, but not sufficiently to account fully for the results of the observers with CN. We suggest that the additional contour interaction found in observers with CN may be attributable to the presence of amblyopia. For a small contour-to-C separation, contour interaction is significantly greater when stimuli are presented against a black rather than a white surround. Consequently, single-letter acuity may be appreciably underestimated clinically when an adjustable window is used to isolate letters on a projected acuity chart.

Form Perception↗
PubMed1992

Effects of methylguanidine on sodium and organic ion transport.

We studied the acute effect of methylguanidine (MG), a suspected uremic toxin that accumulates in renal failure, on p-aminohippurate (PAH) and tetraethylammonium (TEA) uptake in rabbit kidney slices, on Na+,K+ ATPase activity in the microsomal fraction of rabbit kidneys, and on transepithelial active Na transport across toad skin. MG at concentrations ranging from 0.05 (similar to that reported in uremic patients) to 1.0 mM does not affect the organic anion (PAH) uptake, although it exhibits a concentration-dependent inhibition of organic cation (TEA) uptake. MG at concentrations from 0.05 to 5 mM had no effect on kidney Na+,K+ ATPase activity or on active transepithelial Na transport across toad skins when applied to the outside bathing solution; however, MG (greater than 1 mM) stimulated Na transport when applied to the inside bathing solution. These results are not consistent with the hypothesis that MG is a potential uremic toxin that causes the natriuresis and other toxic effects. However, long-term toxic effects of MG on the kidney were not assessed in the present study.

Analysis of Variance↗
PubMed1992

Isolation and identification of 3-propylidene-delta 1-pyrroline-5-carboxylic acid, a biosynthetic precursor of lincomycin.

An accumulated lincomycin intermediate in UC 8292, a lincomycin nonproducing strain of Streptomyces lincolnensis, has been isolated and purified by employing an assay system based on complementation of UC 11066, another lincomycin nonproducing strain of S. lincolnensis. The structure of the purified intermediate is shown to be 3-propylidene-delta 1-pyrroline-5-carboxylic acid, or 1, 2, 3, 6-tetradehydro-propylproline by mass spectrometry and NMR spectroscopic studies. Based on the structure of this newly found intermediate, a biosynthetic pathway for propylproline is proposed as tyrosine-->L-3-hydroxytyrosine (Dopa)-->-->-->-->3-propylidene-delta 1-pyrroline-5-carboxylic acid-->3-propyl-delta 2-pyrroline-5-carboxylic acid-->propylproline.

Fermentation↗
PubMed1991

Length-tension recording system for strabismus surgery.

To meet the need for both scientific information and a clinical means for measurement of the mechanical parameters of the most difficult individual strabismus cases we present a technique for directly measuring and plotting the length-tension characteristics of the tissues supporting the eye. Semiconductor strain gauges mounted on the shanks of a custom machined eye forceps and an ultrasonic method of making continuous duction measurements of the eye have proved feasible. When the forceps are interfaced with a dedicated microcomputer, the system provides a permanent, quantitative, length-tension record displayed in real-time. The instrumented length-tension forceps system has provided a noninvasive means for quickly and simply assessing the mechanical underlying determinants of strabismus pathology in the office, the laboratory or in the operating room, and can aid in the planning and immediate intraoperative alteration of strabismus surgery. Under operator coordination, measurements can be made which precisely define the mechanical load which the eye muscles must move. The resulting objectively determined tissue stiffness asymmetries and muscle restrictions limiting ocular motion indicate the purely mechanical contributions to a patient's strabismus. Measurements of active force indicate the magnitudes and patterns of innervation over the entire range of gaze. By comparison of these active force and passive stiffness records, nerve signal imbalances may be quantitatively distinguished from mechanical imbalances in strabismus. It is the detailed interaction of these nonlinear muscle forces and mechanical elements which determines the position of each eye in strabismus and therefore the proper surgical treatment. A brief description of actual use and a few examples of clinical results are included from over 200 human records.

Calibration↗
PubMed1990

Nucleotide sequence of Streptomyces fradiae transposable element Tn4556: a class-II transposon related to Tn3.

The first transposable element to be isolated from Streptomyces fradiae, Tn4556, was completely sequenced; the total of 6625 bp have an overall G + C composition of 68%. Computer-aided analysis of this sequence reveals the location of nine open reading frames (ORFs). Several of these ORFs, numbers 1, 2, and 7, contain ribosome-binding sites (RBS) near their putative translation-initiation sites, which share identity with the consensus RBS sequences of Escherichia coli and Bacillus subtilis. ORF1 potentially encodes an 892-amino acid (aa) protein and this deduced aa sequence shares 61% identity with that of the transposase encoded by the tnpA gene of Tn3. Three other ORFs, 2, 3 and 5, potentially encode proteins which are similar in size to the resolvase protein encoded by the Tn3 gene tnpR; however, none of the protein products deduced from these ORF share extensive aa sequence identity with other resolvase proteins.

Amino Acid Sequence↗
PubMed1987

Tn4556, a 6.8-kilobase-pair transposable element of Streptomyces fradiae.

A 6.8-kilobase-pair (kbp) transposable element (Tn4556) was found in a neomycin-producing strain of Streptomyces fradiae. This element was first observed in two 30.3-kbp plasmids (pUC1123 and pUC1124) which arose when a thiostrepton resistance gene (1 kbp) was ligated with the BclI-2 fragment (22.5 kbp) that contains the origin of replication of phage SF1. The Tn4556 segment was deleted when these plasmids were transduced into another S. fradiae host with phage SF1. These deletion plasmids (pUC1210 and pUC1211) had copy numbers of less than 1 per chromosome and were unstable. In contrast, pUC1123 and pUC1124, with copy numbers of 12 to 15 per chromosome, respectively, were relatively stable. When pUC1210 and pUC1211 were reintroduced into S. fradiae by protoplast transformation, the Tn4556 element transposed again to the plasmids at numerous new locations in either of two orientations. A copy of Tn4556 was found in the S. fradiae chromosome by hybridization studies. It appears that Tn4556 originated from the chromosome, transposed into unstable pUC1210 and pUC1211, and made stable plasmids. A temperature-sensitive hybrid plasmid carrying a viomycin resistance derivative of Tn4556 (pMT660::Tn4556::vph) was constructed. When Streptomyces lividans UC8390 containing the hybrid plasmid was grown at 39 degrees C, Tn4556::vph (Tn4560) transposed to random positions in the host chromosome.

Cloning, Molecular↗
PubMed1983

[Antibiotic susceptibility of Pseudomonas aeruginosa].

The susceptibility of 128 clinical isolates of Pseudomonas aeruginosa against amikacin, kanamycin, gentamycin, minocycline, colistin, chloramphenicol, claforan , piperacillin and carbenicillin were tested. Piperacillin, carbenicillin, amikacin and claforan are effective against at least 54% of strains tested. The susceptibilities of microorganisms to antibiotics were evaluated in different points and discussed in details.

Anti-Bacterial Agents↗
PubMed1982

Isolation and characterization of Streptomyces fradiae plasmids which are prophage of the actinophage phiSF1.

Two plasmids, designated pUC1 and pUC13, have identified as different prophage states of the Streptomyces fradiae actinophage phiSF1. Plasmids pUC1 and pUC13, 51.1 Md, are identical as determined by restriction endonuclease analysis and other criteria. They differ in that while pUC1 specifies "type A" plaques, characterized by a narrow zone of growth inhibition around a transformant colony, pUC13 specifies "type B" plaques, characterized by concentric rings of growth and growth inhibition around a transformant colony. Additionally cultures carrying pUC13 release antinophage phiSF1 at a frequency about 10 3-10 4 times that pUC1 containing cultures. Restriction enzyme digestion, transfection, and transformation analyses show that phiSF1 DNA, 55.7 Md, is about 8-10% larger than pUC13 and appears to be circularly permuted and terminally redundant. The pUC1 and pUC13 prophage are interconvertible.

Bacteriophages↗
PubMed1982

Metabolic accumulation of hydrocarbons by Acinetobacter species.

Acinetobacter HO1-N grown on hexadecane as a sole source of carbon and energy was demonstrated to cytoplasmically sequester this hydrocarbon. Cultures grown with hexadecane and heptadecane accumulated intracytoplasmically both hydrocarbons. The accumulation of specific alkanes was non-specific while the oxidation of the pooled hydrocarbon was highly specific. Naphthalene was highly inhibitory to the growth of this microorganism but slowly co-oxidized in the presence of hexadecane.

Acetates↗
PubMed1974

Apparent mutagenic effect of induction of lambda prophage inserted between lysA and thyA.

Induction of a heat-inducible abnormal lambda prophage inserted between lysA and thyA in Escherichia coli resulted in a number of auxotrophic mutants in the surviving cured-cell populations. These mutants could not be accounted for by deletions arising on formation of lambda hybrid particles carrying regions adjacent to the insertion site. The properties of these mutants, which were almost all spontaneously revertable, have been described and mapped by F' episome complementation. Tentatively, it was suggested that induction of the lambda lysogen leads to a mutagenic state.

Arginine↗
PubMed1973

Loss of an essential function of Escherichia coli by deletions in the thyA region.

In an attempt to obtain deletions in the thyA gene, an abnormal lysogen of lambda having the prophage inserted between the thyA and lysA genes was induced, and the surviving cured cells were examined for Thy(-) and Lys(-) mutants. In nearly 10,000 cured cells, 184 Lys(-) but no Thy(-) mutants were found. At the same time, the induced lambda phage contained an approximately equivalent number of lambdathyA(+) and lambdalysA(+) transducing particles. By contrast, in a strain with the genotype F' thyA(-)lysA(+)/ thyA(+)lysA(+), induction of the abnormal lambda lysogen gave rise to many Thy(-) mutants in the cells cured of the prophage. In these Thy(-) mutants it was not possible to eliminate the episome with acridine orange, although the episome could be removed in control cultures with a thyA(+) allele in the resident gene. Therefore, it was suggested that deletion of a gene in the region of the chromosome from the position of the insertion of the lambda prophage through the thyA gene caused loss of an essential and diffusible function.

Acridines↗