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Biomedical subjects

M G Harris

Publications and source records attributed to M G Harris.

At least 91 records · Page 5Linked to original sources
PubMed1986

Sustained and transient temporal integration functions depend on spatial frequency, not grating area.

Contrast sensitivity for sinusoidal gratings was measured by a two-alternative forced-choice method and plotted as a function of duration. Gratings of large or small area and low or high spatial frequency (0.75 or 8.25 c/deg) were used. The temporal integration curve was much flatter in the low spatial frequency conditions than in the high frequency conditions, but its shape did not depend on the area of the grating. The results support the established view that the visual system exhibits transient temporal properties at low spatial frequencies and sustained properties at higher frequencies, but disconfirm the recent proposal that temporal integration depends on the area of the grating (expressed in square-cycles) rather than its spatial frequency.

Fovea Centralis↗
PubMed1986

(delta beta) zero thalassemia of the Southern Italian type. Its geographical origin and interaction with the sickle cell gene.

Hematological phenotypes and molecular defects were compared in 11 examples of heterozygous (delta beta) zero thalassemia. Despite differences in ethnic origins all cases had the gene deletion that is found in (delta beta) zero thalassemia of the Southern Italian type. HbF levels in these patients ranged from 3.6-14.6% with a mean +/- 1 SD of 8.9 +/- 3.1%. Variability in HbF output would suggest that additional factor(s) apart from deletions within the beta globin complex are involved in regulation of gamma gene expression. One individual, a compound-heterozygote for (delta beta) zero thalassemia and HbS, presented with a sickling disorder. Reduced HbF production in family members who are heterozygotes for (delta beta) zero thalassemia may explain the clinical picture in this instance.

Adult↗
PubMed1985

Multiple cerebral thrombosis in Fletcher factor (prekallikrein) deficiency: a case report.

Despite markedly prolonged activated partial thromboplastin times (APTT) patients with Fletcher factor (prekallikrein) deficiency do not clinically bleed. However, there is one reported case of myocardial infarction associated with prekallikrein deficiency. These clinical observations suggest that the contact mechanism has a minor role in normal in vivo hemostatic function. We report a further two cases of prekallikrein deficiency in Caucasian siblings. The propositus presented with multiple cerebral thrombosis. Cautious anticoagulation resulted in massive cerebral hemorrhage and death of this patient. The sibling was investigated in an attempt to establish a possible defective fibrinolytic pathway in vivo. New sensitive tests for fibrinolysis were used. These included radioimmunoassay of fibrin degradation product "D," B beta 15-42, and in vitro 125I-labelled fibrin lysis assays. The plasma half-life of prekallikrein in this patient was calculated to be 58 hr. Family studies of heterozygotes were also performed. Prekallikrein deficiency is found not to be important in normal in vivo fibrinolysis, which possibly operates via tissue or vessel wall fibrinolytic activator release.

Adolescent↗
PubMed1985

Cell culture of rabbit meniscal fibrochondrocytes: proliferative and synthetic response to growth factors and ascorbate.

This study was undertaken to determine whether the cells of the fibrocartilaginous meniscal substance are capable of proliferation and matrix synthesis. Cells were isolated from the fibrocartilaginous menisci of young New Zealand white rabbits, and grown in two alternative culture regimens differing only in the basal nutrient medium used to initiate primary monolayer growth. Under each culture regimen, the cells attached and proliferated both initially and after passage into secondary (2 degrees) culture. Differences were noted in cell morphology and time to reach confluence in primary (1 degrees) culture. Upon passage into 2 degrees culture, the fibrochondrocytes assumed two distinct morphologies depending upon the type of medium used for 1 degree culture. These morphological changes were accompanied by differences in the population doubling time and incorporation of 35SO4 into sulfated proteoglycans. The proliferation of both fibrochondrocyte subtypes was stimulated by the addition of either pituitary fibroblast growth factor (FGF) or human platelet lysate in a dose-dependent manner. Both FGF (10 ng/ml) and ascorbate (40 micrograms/ml) decreased 35-sulfate incorporation, whereas only ascorbate was found to alter the amount of sulfated glycosaminoglycan in the pericellular coat. We conclude that the fibrochondrocytes of the meniscal substance are capable of replication and synthesis of matrix macromolecules if given the proper stimuli. Additionally, there may be two subpopulations of fibrochondrocytes that can be distinguished by their in vitro behavior.

Animals↗
PubMed1984

Spatial selectivity of contrast adaptation: models and data.

Contrast threshold elevation was measured in human observers at a single spatial frequency (4 or 8 c/deg) after adaptation to gratings at different contrasts and spatial frequencies. When plotted against adapting contrast, the threshold elevation functions for different adapting frequencies were neither straight nor parallel, which poses difficulties for the "equivalent contrast transformation". The different functions appear to form a single family, differing by a scaling factor that depends on spatial frequency but is independent of contrast. This is similar to expressing the aftereffect as a proportion of its peak value. A multiple channel, "fatigue" model of adaptation is shown to be consistent with the results, provided the channel shape is appropriate and channels overlap considerably in the frequency domain. Channel bandwidth is estimated at 1.4 octaves, similar to that of cortical cells.

Adaptation, Ocular↗
PubMed1984

The role of pattern and flicker mechanisms in determining the spatiotemporal limits of velocity perception. 1. Upper movement thresholds.

For rapidly moving gratings, the percept of smooth drift is limited by the "coherent drift' threshold. At more rapid rates of movement, the spatial and temporal structure of the stimulus may be individually perceived, but the global spatiotemporal percept of smooth drift breaks down and the stimulus appears to be made up of discrete patches of flickering grating. This coherent drift threshold is strongly dependent upon both spatial frequency and contrast and roughly parallels the "pattern', rather than the "flicker' threshold. The relationships between the three types of threshold suggest that the global percept of smooth drift depends upon mechanisms which are the same as those which underpin the separate percepts of spatial and temporal structure.

Flicker Fusion↗
PubMed1984

The role of pattern and flicker mechanisms in determining the spatiotemporal limits of velocity perception. 2. The lower movement threshold.

For slowly-drifting foveally-presented gratings of 1 cycle deg-1 and above, continuous movement can be perceived at stimulus velocities greater than about 1 min visual angle s-1, irrespective of spatial frequency, providing stimulus contrast is greater than 0.01. For coarser gratings, the minimum perceivable rate of drift is determined by the detectability of the stimulus and is strongly dependent upon both spatial frequency and contrast.

Flicker Fusion↗
PubMed1984

Haemoglobin D Punjab. Interaction with alpha thalassaemia and diagnosis by gene mapping.

A family which illustrates the inheritance of Hb D Punjab (a beta globin chain variant) and alpha thalassaemia is described. Interaction between Hb D Punjab and alpha thalassaemia is observed since levels of Hb D vary according to the number of alpha globin genes deleted. The technique of gene mapping has been utilised in the current study to provide definitive evidence of alpha thalassaemia and also demonstrates a novel way to identify Hb D Punjab.

Adult↗
PubMed1983

Patient response to Polycon II and Silcon contact lenses. A comparative study.

Ten myopic patients with no previous contact lens experience were fitted with similarly designed Silcon and Polycon II contact lenses so that we could evaluate and compare the patients' responses to these lens types in a double masked fashion. None of the patients had any contraindications to contact lens wear. The Silcon lens is a cross-linked silicone resin lens with a reported DK value of 16 X 10(-11). The Polycon II lenses is a combination PMMA-Silicone lens with a reported DK value of 12 X 10(-11). After a thorough contact lens pre-fitting evaluation, each patient was fitted in a random fashion with a Silcon lens on one eye and a Polycon II lens on the other. Patient response was evaluated by the standard of success of Sarver and Harris. No statistically significant differences were found in the patients responses to the two lens types. Nine of the patients were successful with the Silcon lenses while all 10 patients were successful with the Polycon II lenses.

Adolescent↗
PubMed1982

Flexure and residual astigmatism with Polycon and polymethyl methacrylate lenses on toric corneas.

Previous investigators have shown that thin polymethyl methacrylate (PMMA) contact lenses (less than or equal to 0.12 mm) flex on toric corneas, thereby altering the residual astigmatism. The present study was designed to evaluate the flexure and residual astigmatism of gas permeable Polycon contact lenses on toric corneas and compare the results with those obtained with PMMA lenses on the same corneas. Eight subjects (16 eyes) with corneal toricities ranging from 1.25 to 4.25 D wore four pairs of identical Polycon and PMMA lenses. The only lens parameter that varied was center thickness, which ranged from 0.07 to 0.16 mm. Flexure and residual astigmatism were measured on all corneas with all lenses in a double-masked fashion. Our results indicate that Polycon lenses undergo significantly more flexure and alter residual astigmatism more on toric corneas than PMMA lenses at all center thicknesses evaluated. For both materials, flexure and residual astigmatism increased as center thickness decreased. The clinical implications of these findings are discussed.

Adult↗
PubMed1982

Flexure and residual astigmatism with cellulose acetate buterate (CAB) contact lenses on toric corneas.

Thin contact lenses flex on toric corneas, thus changing the residual astigmatism. Our study evaluated the flexure and induced astigmatism of CABCURVE cellulose acetate buterate (CAB) contact lenses on toric corneas and compared the results with those found for polymethyl methacrylate (PMMA) and Polycon lenses. Ten subjects (20 eyes) with corneal toricities ranging from 1.12 to 3.12 D wore six CABCURVE lenses with center thicknesses from 0.10 to 0.20 mm. Flexure and induced residual astigmatism were measured on all corneas with all lenses in a double-masked fashion. We found that all the CABCURVE lenses flexed on the toric corneas and altered residual astigmatism. Both flexure and induced residual astigmatism increased as center thickness decreased, and were greater than with PMMA or Polycon lenses of similar dimensions. These findings should be considered when deciding which lens type to fit on patients with toric corneas.

Adult↗
PubMed1981

Corneal edema with hydrogel lenses and eye closure: time course.

Central corneal thickness changes during the wearing of hydrogel lenses under closed-eye conditions were measured for two subjects. Each wore an investigational ultrathin Bausch & Lomb Soflens contact lens (series X-176) on one eye only under closed-eye conditions for periods ranging from 30 min to 6 hr. The other eye served as a control. Corneal thickness of the control eyes increased gradually and reached a maximum increase of about 4% at the end of the 4-hr wearing trial. Corneal thickness in the experimental eyes also increased with wearing time during the first 4 hr and then remained fairly constant up to a wearing time of 6 hr. The maximum thickness increases for the two subjects were 9.8 and 9.1%, respectively. About 80% of the maximum corneal thickness change was present after 3 hr of closed-eye contact lens wear. The corneal thickness in the control eye returned to baseline 80 min after removal of the contact lens. Corneal thickness of one experimental eye returned to baseline 2 hr after the contact lens was removed.

Adult↗
PubMed1981

Corneal edema with hydrogel lenses and eye closure: effect of oxygen transmissibility.

Several investigators have reported that significant corneal edema develops when daily wear hydrogel lenses are worn with the eyes closed. The amount of edema that develops when extended wear hydrogel lenses are worn with eye closure is not well documented. This study compared the amount of corneal edema that developed when subjects wore one daily wear lens with the edema that developed when they wore three lens types designed for extended wear. The daily wear lens was the U3 series Bausch & Lomb Soflens contact lens. The extended wear lenses were the Cooper Permalens, the Soft Lens Hydrocurve II 55 lens, and the Sauflon PW lens. The five subjects developed only small amounts of corneal edema when they wore the four lens types under opened-eye conditions but they developed significant amounts of corneal edema when they wore the four lens types for 3-hr periods with eye closure. The amount of corneal edema correlated well with the oxygen transmissibility (DK/L) of the study lenses. The results can be used to predict the amount of corneal edema that the average patient will develop initially when wearing a hydrogel lens of known water content or oxygen permeability (DK) and thickness (L) under opened and closed-eye conditions.

Adult↗