PubMed Health⌕ Search

Biomedical subjects

D Buckley

Publications and source records attributed to D Buckley.

At least 37 records · Page 2Linked to original sources
PubMed1999

An orientation anisotropy in the effects of scaling vertical disparities.

Gårding et al. (Vis Res 1995;35:703-722) proposed a two-stage theory of stereopsis. The first uses horizontal disparities for relief computations after they have been subjected to a process called disparity correction that utilises vertical disparities. The second stage, termed disparity normalisation, is concerned with computing metric representations from the output of stage one. It uses vertical disparities to a much lesser extent, if at all, for small field stimuli. We report two psychophysical experiments that tested whether human vision implements this two-stage theory. They tested the prediction that scaling vertical disparities to simulate different viewing distances to the fixation point should affect the perceived amplitudes of vertically but not horizontally oriented ridges. The first used elliptical half-cylinders and the 'apparently circular cylinder' judgement task of Johnston (Vis Res 1991;31:1351-1360). The second experiment used parabolic ridges and the amplitude judgement task of Buckley and Frisby (Vis Res 1993;33:919-934). Both studies broadly confirmed the anisotropy prediction by finding that large scalings of vertical disparities simulating near distances had a strong effect on the perceived amplitudes of the vertically oriented stimuli but little effect on the horizontal ones. When distances > 25 cm were simulated there were no significant differential effects and various methodological reasons are offered for this departure from expectations.

Adult↗
PubMed1998

Crystal structures of Flavobacterium glycosylasparaginase. An N-terminal nucleophile hydrolase activated by intramolecular proteolysis.

Glycosylasparaginase (GA) is a member of a novel family of N-terminal nucleophile hydrolases that catalytically use an N-terminal residue as both a polarizing base and a nucleophile. These enzymes are activated from a single chain precursor by intramolecular autoproteolysis to yield the N-terminal nucleophile. A deficiency of GA results in the human genetic disorder known as aspartylglycosaminuria. In this study, we report the crystal structure of recombinant GA from Flavobacterium meningosepticum. Similar to the human structure, the bacterial GA forms an alphabetabetaalpha sandwich. However, some significant differences are observed between the Flavobacterium and human structures. The active site of Flavobacterium glycosylasparaginase is in an open conformation when compared with the human structure. We also describe the structure of a mutant wherein the N-terminal nucleophile Thr152 is substituted by a cysteine. In the bacterial GA crystals, we observe a heterotetrameric structure similar to that found in the human structure, as well as that observed in solution for eukaryotic glycosylasparaginases. The results confirm the suitability of the bacterial enzyme as a model to study the consequences of mutations in aspartylglycosaminuria patients. They also suggest that further studies are necessary to understand the detail mechanism of this enzyme. The presence of the heterotetrameric structure in the crystals is significant because dimerization of precursors has been suggested in the human enzyme to be a prerequisite to trigger autoproteolysis.

Amino Acid Sequence↗
PubMed1998

Serum from preeclamptic women induces vascular cell adhesion molecule-1 expression on human endothelial cells in vitro: a possible role of increased circulating levels of free fatty acids.

OBJECTIVE: The object was to determine whether serum from preeclamptic women induces expression of vascular cell adhesion molecule-1 on cultured endothelial cells. STUDY DESIGN: Endothelial cells were incubated with medium containing 20% serum (volume/volume) either from women with preeclampsia (n = 15) or from women with normal pregnancies (n = 15) matched for maternal age, gestational age, and parity. A further matched set of samples (n = 10) was exposed to endothelial cells that had previously been incubated in the presence or absence of vitamin E (40 micromol/L final concentration). Free fatty acids were determined in each sample. A mixture of free fatty acids (linoleic, oleic, and palmitic acids, 1:1:1) was added to serum from control subjects in increasing concentrations (70-280 micromol/L final concentration) to emulate preeclamptic serum and the preparation was exposed to endothelial cells. In each experiment vascular cell adhesion molecule-1 expression was determined after 16 hours of exposure by an enzyme-linked immunosorbent assay technique performed on the cell monolayer. RESULTS: Preeclamptic serum had higher levels of free fatty acids than did that of control subjects (0.71 mmol/L, 95% confidence level 0.5-0.93, vs 0.36 mmol/L, 95% confidence level 0.28-0.43). There was a statistically significant increase in vascular cell adhesion molecule-1 expression on the endothelial cells exposed to preeclamptic serum compared with those exposed to control serum (optical density 0.17 vs 0.11). Vitamin E reduced the vascular cell adhesion molecule-1 expression of endothelial cells exposed both to preeclamptic and to control serum samples in a nonspecific manner. Addition of free fatty acids to normal pregnancy serum to mimic the effect of preeclampsia resulted in increased expression of vascular cell adhesion molecule-1 on the cells. CONCLUSION: Preeclamptic serum induces vascular cell adhesion molecule-1 expression on human endothelial cells in vitro, an effect also produced by fatty acids. The elevated level of free fatty acids in women with preeclampsia may contribute to increased vascular cell adhesion molecule expression in vivo.

Adult↗
PubMed1998

Preoperative carcinoembryonic antigen is related to tumour stage and long-term survival in colorectal cancer.

Evidence as to the value of preoperative carcinoembryonic antigen (CEA) in guiding treatment for patients with colorectal cancer is conflicting. The aim of this prospective study was to investigate the value of preoperative CEA in predicting tumour factors of proven prognostic value and long-term survival in patients undergoing surgery for colorectal cancer. Preoperative serum CEA, tumour ploidy, stage and grade were ascertained in 277 patients undergoing colorectal cancer surgery. This cohort of patients were followed up for a minimum of 5 years, or until death, in a dedicated colorectal clinic. Patients with an elevated CEA had a 5 year survival of 39%. This increased to 57% if the CEA was normal (P=0.001). The proportion of patients with a raised CEA increased with a more advanced tumour stage (P < 0.000001) and a poorly differentiated tumour grade (P < 0.005). Once stage had been controlled for, CEA was not a predictor of survival. No relationship between tumour ploidy and CEA was found. In conclusion, a raised preoperative serum CEA is likely to be associated with advanced tumour stage and poor long-term survival, compared with patients with a normal value.

Adult↗
PubMed1998

Pertussis encephalopathy with high cerebrospinal fluid antibody titers to pertussis toxin and filamentous hemagglutinin.

A 7-year-old unimmunized girl with pertussis presented with respiratory failure and electroencephalographic evidence of an encephalopathy. The cerebrospinal fluid (CSF)/serum ratio of antibodies to pertussis toxin and filamentous hemagglutinin were 11- and ninefold higher than the CSF/serum ratio of total immunoglobulin G. The CSF/serum ratio of albumin was normal. These findings indicate production of antibodies in the central nervous system to Bordetella pertussis antigens and imply, therefore, that the pertussis encephalopathy in this girl was associated with the entry of pertussis antigens into the central nervous system.

Antibodies, Viral↗
PubMed1998

Vergence control across saccades in dyslexic adults.

Many aspects of vision have been investigated in developmental dyslexia. Some research suggests deficits in vergence control (e.g. Buzzelli, 1991, Optom. Vision Sci. 68, 842-846), although ability to control vergence across saccades has not yet been investigated. We have explored this question indirectly using Enright's (1996 Vision Res. 36, 307-312.) sequential stereopsis task. The task requires observers to set two adjacent targets (whose textures cannot be resolved simultaneously if either is fixated) to appear equi-distant. Enright has argued that sequential stereopsis stereoacuity thresholds offer an indication of vergence control across saccades. We report two experiments using a total of 17 dyslexic and 18 control adults. Performance was measured on a sequential stereopsis task and an ordinary 'simultaneous' stereopsis task. No significant differences between groups were found. However, whereas practice of the sequential task lowered control group thresholds on the simultaneous task, for the dyslexic group it significantly raised thresholds, suggesting that visual fatigue is especially important in investigations of visual functions in dyslexia. Although the small samples used limit conclusions at this stage, the main sequential stereopsis results suggest that, if Enright is correct, dyslexic adults can show normal vergence control across saccades.

Adult↗
PubMed1997

The role of 3-D surface slope in a lightness/brightness effect.

Adelson has shown how two patches in a 5 by 5 array of grey patches can be perceived to consist of different shades, depending on whether they are represented at a 3-D horizontal or vertical ridge. Adelson interprets the illusion in terms of the orientation of the patches with respect to the inferred illuminant. We investigated: (1) the illusion in the vertical and horizontal stimuli and added a flat (ridgeless) control stimulus; (2) stimuli of varying ridge amplitudes to examine the effect more fully. 3-D renderings of real surfaces were modelled with computer graphics and displayed to observers who used a mouse to alter the brightness of a square to match patches indicated in the stimuli. Five observers were used for the vertical, flat and horizontal stimuli, while a larger group (n = 20) was used for an independent design when varying ridge amplitudes. A significant effect in the flat surface demonstrates that patches lying in the same plane can have their brightness altered without changes in their orientation. When the surface was seen as a 3-D ridge the size of the effect was a function of 3-D slope of the surface. By measuring each patch independently we have shown that the effect changes the brightness of the two patches to differing degrees. We offer an explanation of this based on a proposed qualitative shading rule for identifying reflectance and illumination edges.

Adult↗
PubMed1997

Sequential stereopsis using high-pass spatial frequency filtered textures.

Enright [(1995). Perception, 24 (suppl.), 32-33; (1996). Vision Research, 36, 307-312] described a simple piece of equipment for demonstrating a perceptual mechanism he called sequential stereopsis. The equipment requires an observer to set two textured targets seen behind a pair of small viewing ports to appear equi-distant. The principle upon which the apparatus depends is the use of textures whose elements cannot be resolved in peripheral vision at the eccentricity determined by the target separation. Enright used a fine sandpaper for this purpose. We have conducted two similar experiments using high bandpass filtered textures which eliminate any possibility that the low spatial frequency content of sandpaper textures could play a role. Our results corroborate Enright's general conclusions on sequential stereopsis, while at the same time showing that high-pass textures do not give wholly similar results to sandpaper.

Adult↗
PubMed1996

Effects of the vitamin D3 analog 1 alpha, 25-dihydroxyvitamin D3-3 beta-bromoacetate on rat osteosarcoma cells: comparison with 1 alpha, 25-dihydroxyvitamin D3.

The actions of the hormonal form of vitamin D, 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2 D3], are mediated by both genomic and nongenomic mechanisms. Several vitamin D synthetic analogs have been developed in order to identify and characterize the site(s) of action of 1 alpha, 25-(OH)2D3 in many cell types including osteoblastic cells. We have compared the effects of 1 alpha, 25-(OH)2D3 and a novel 1 alpha, 25-(OH)2D3 bromoester analog (1,25-(OH)2-BE) that covalently binds to vitamin D receptors. Rat osteosarcoma cells that possess (ROS 17/2.8) or lack (ROS 24/1) the classic intracellular vitamin D receptor were studied to investigate genomic and nongenomic actions. In ROS 17/2.8 cells plated at low density, the two vitamin D compounds (1 x 10(-8) M) caused increased cell proliferation, as assessed by DNA synthesis and total cell counts. Northern blot analysis revealed that the mitogenic effect of both agents was accompanied by an increase in steady-state osteocalcin mRNA levels, but neither agent altered alkaline phosphatase mRNA levels in ROS 17/2.8 cells. ROS 17/2.8 cells responded to 1,25-(OH)2-BE but not the natural ligand with a significant increase in osteocalcin secretion after 72, 96, 120, and 144 hr of treatment. Treatment of ROS 17/2.8 cells with the bromoester analog also resulted in a significant decrease in alkaline phosphatase-specific activity. To compare the nongenomic effects of 1 alpha, 25-(OH)2D3 and 1,25-(OH)2-BE intracellular calcium was measured in ROS 24/1 cells loaded with the fluorescent calcium indicator Quin 2. At 2 x 10(-8) M, both 1 alpha,25-(OH)2D3 and 1, 25-(OH)2-BE increased intracellular calcium within 5 min. Both the genomic and nongenomic actions of 1,25-(OH)2-BE are similar to those of 1 alpha,25-(OH)2D3, and since 1,25-(OH)2-BE has more potent effects on osteoblast function than the naturally occurring ligand due to more stable binding, this novel vitamin D analog may be useful in elucidating the structure and function of cellular vitamin D receptors.

Animals↗
PubMed1996

Does the human visual system implement an ideal observer theory of slant from texture?

Texture information about surface shape can be decomposed into three constituents: compression, density and scaling. Blake, Bülthoff and Sheinberg's (1993, Vision Research, 33, 1723-1737) Ideal Observer theory of slant from texture predicts that the relative contribution of compression and density to the percept of planar surface slant should vary with field of view (FOV). The contribution of compression and density should both increase as FOV increases but statistical analysis shows that the reliability, and hence the expected contribution, of density increases at a greater rate than compression with increasing FOV. Specific predictions are that at FOV < 20 deg compression should be more effective than density, at FOV approximately 20 deg, compression and density should be equally effective, and at FOV > 20 deg, compression should be less effective than density. These predictions were tested by pitting these components of texture against one another. The method used was similar to that described in Frisby and Buckley [1992 In Orban, G. & Nagel H.-H. (Eds) Artificial and biological visual systems. Berlin: Springer]: observers judged binocularly the slant of a large table on to which was projected a texture created using computer graphics to contain various texture component cues to slant. The size of the projected texture patch determined the FOV which was by this means set to either 10, 20 or 30 deg. It was found that compression was the dominant cue irrespective of FOV, even if it was perturbed by noise. Possible reasons for this divergence of human observers from predictions of the Ideal Observer theory of slant from texture are discussed.

Adolescent↗
PubMed1996

Evidence for good recovery of lengths of real objects seen with natural stereo viewing.

Six experiments are described in which good performance of the task of matching the lengths of two stationary real objects, gnarled wooden sticks, under a variety of binocular viewing conditions, including variations in viewing distances was demonstrated. Relatively poor matching performance was observed when the sticks were viewed monocularly in oscillatory motion, or monocularly and stationary. The results suggest that stereo can support good representations of metric scene structure when length judgments of natural objects are required under (quasi-)natural viewing. The implications of these results for theories of structure from stereo and structure from motion are discussed.

Adolescent↗
PubMed1996

Pooling of vertical disparities by the human visual system.

Two experiments are described in which the effects of scaling vertical disparities on the perceived amplitudes of dome-shaped surfaces depicted with horizontal disparities were examined. The Mayhew and Longuet-Higgins's theory and the regional-disparity-correction theory of Garding et al predict that scaling should generate a change in perceived depth appropriate to the viewing distance simulated by the scaled vertical disparities. Significant depth changes were observed, by means of a nulling task in which the vertical-disparity-scaling effect was cancelled by the observer choosing a pattern of horizontal disparities that made the dome-shaped surface appear flat. The sizes of the scaling effects were less than those predicted by either theory, suggesting that other cues to fixation distance such as oculomotor information played an appreciable role. In conditions in which 50% of the texture elements were given one value of vertical-disparity scaling and the remaining 50% were left unscaled, the size of the scaling effect on perceived depth could be accounted for by equally weighted pooling of the vertical-disparity information unless the two scalings were very dissimilar, in which case the lower scaling factor tended to dominate. These findings are discussed in terms of a Hough parameter estimation model of the vertical-disparity-pooling process.

Attention↗
PubMed1996

MRI and nonverbal cognitive deficits in children with neurofibromatosis 1.

Magnetic resonance imaging brain scans and neuropsychological assessments of 17 children who met the NIH consensus diagnostic criteria for neurofibromatosis Type 1 were carried out in order to determine if there is a relationship between presence of high intensity signal abnormalities on MRI scans and nonverbal cognitive deficits. Cranial MRI scans in 10 patients (58.8%) demonstrated high intensity signal abnormalities, most frequently in the cerebral peduncles. Fifteen patients had nonverbal cognitive deficits (88.2%), including difficulty judging the orientation of lines, matching complex visual stimulus configurations, recalling pictures of faces, as well as copying and drawing from memory a complex geometric figure. There was not a significant association between nonverbal neuropsychological deficits and presence of high intensity signal abnormalities on MRI scans, possibly because the location of these hyperintense abnormalities was typically below the level of the basal ganglia. These findings suggest that the high intensity signal lesions seen on the MRI scans of children with neurofibromatosis Type 1 do not predict or explain their nonverbal cognitive deficits.

Adolescent↗
PubMed1996

Laparoscopic-assisted rectopexy using a novel hand-access port.

Many surgeons who were ready converts to laparoscopic cholecystectomy did not translate the technique to more advanced procedures. There are several reasons: increased operating times, a steeper learning curve, concern for oncological efficacy, and the loss of manual tactile ability. As shown with this report on laparoscopic assisted rectopexy, many of these difficulties can be overcome using the IntromitTM while still maintaining the benefits of minimally invasive surgery.

Aged↗
PubMed1996

Vigabatrin in childhood epilepsy: comparable efficacy for generalized and partial seizures.

Thirty-one patients, aged 12.6 +/- 5.6 years, with refractory seizures for 8 +/- 4.3 years, were treated with adjunctive vigabatrin. Twenty-four percent had a > 50% reduction in seizure frequency (95% one-sided confidence interval). Generalized myoclonic, atonic, and tonic clonic and partial, with and without secondary generalization, seizures were all reduced at a mean dose of 70 +/- 38 mg/kg/day. Comparison of vigabatrin therapy duration, for partial and generalized seizure groups, utilizing Kaplan-Meier methodology showed similar survival times. Vigabatrin therapy was ineffective in the four children with tuberous sclerosis. Transient somnolence, ataxia and dizziness were the most frequent side effects. A severe aggressive agitation occurred in three patients, and necessitated discontinuation of vigabatrin in one patient. Vigabatrin was as effective in generalized as in partial seizures in this study. Clinical utility may be limited by unacceptable behavioral side effects in some patients.

Adolescent↗
PubMed1996

A comparison of the inactive urinary kallikrein:creatinine ratio and the angiotensin sensitivity test for the prediction of pre-eclampsia.

OBJECTIVE: To determine the relation between the inactive urinary kallikrein: creatinine ratio (IUK:Cr) and the angiotensin sensitivity test (AST) at 28 weeks of gestation and to assess each as a screening test for pre-eclampsia. DESIGN: Prospective interventional study. SUBJECTS: Four hundred and fifty-nine normotensive nulliparous women recruited from hospital antenatal clinics. SETTING: John Radcliffe Maternity Hospital, Oxford, and Queen Charlotte's and Chelsea Hospital, London. INTERVENTIONS: A urine sample for IUK:Cr measurement was provided before performing the AST at 28 weeks of gestation. Those women who demonstrated increased sensitivity to angiotensin II were entered into a randomised placebo controlled trial of low dose aspirin for the prevention of pre-eclampsia (CLASP). MAIN OUTCOME MEASURES: The development of pre-eclampsia. RESULTS: The IUK:Cr ratio was significantly lower in those women who showed increased sensitivity to angiotensin II (P < 0.0001 Student's t test). The sensitivity and specificity for detecting pre-eclampsia were, respectively, 22% and 85% for the AST and 67% and 75% for the IUK:Cr. Low-dose aspirin (60 mg) had no effect on the pregnancy outcome. CONCLUSION: There appears to be some relation between the IUK:Cr and AST tests in pregnancy. However, in this population, the IUK:Cr ratio was a better screening test for pre-eclampsia than the AST, but overall neither test was a powerful predictor for the syndrome.

Angiotensin II↗