Showing papers by "University of Cambridge published in 1993"

Inositol trisphosphate and calcium signalling

Abstract: Inositol trisphosphate is a second messenger that controls many cellular processes by generating internal calcium signals. It operates through receptors whose molecular and physiological properties closely resemble the calcium-mobilizing ryanodine receptors of muscle. This family of intracellular calcium channels displays the regenerative process of calcium-induced calcium release responsible for the complex spatiotemporal patterns of calcium waves and oscillations. Such a dynamic signalling pathway controls many cellular processes, including fertilization, cell growth, transformation, secretion, smooth muscle contraction, sensory perception and neuronal signalling.

Instability and decay of the primary structure of DNA

Abstract: Although DNA is the carrier of genetic information, it has limited chemical stability. Hydrolysis, oxidation and nonenzymatic methylation of DNA occur at significant rates in vivo, and are counteracted by specific DNA repair processes. The spontaneous decay of DNA is likely to be a major factor in mutagenesis, carcinogenesis and ageing, and also sets limits for the recovery of DNA fragments from fossils.

High confidence visual recognition of persons by a test of statistical independence

Abstract: A method for rapid visual recognition of personal identity is described, based on the failure of a statistical test of independence. The most unique phenotypic feature visible in a person's face is the detailed texture of each eye's iris. The visible texture of a person's iris in a real-time video image is encoded into a compact sequence of multi-scale quadrature 2-D Gabor wavelet coefficients, whose most-significant bits comprise a 256-byte "iris code". Statistical decision theory generates identification decisions from Exclusive-OR comparisons of complete iris codes at the rate of 4000 per second, including calculation of decision confidence levels. The distributions observed empirically in such comparisons imply a theoretical "cross-over" error rate of one in 131000 when a decision criterion is adopted that would equalize the false accept and false reject error rates. In the typical recognition case, given the mean observed degree of iris code agreement, the decision confidence levels correspond formally to a conditional false accept probability of one in about 10/sup 31/. >

Type 2 (non-insulin-dependent) diabetes mellitus, hypertension and hyperlipidaemia (syndrome X): relation to reduced fetal growth

Abstract: Two follow-up studies were carried out to determine whether lower birthweight is related to the occurrence of syndrome X-Type 2 (non-insulin-dependent) diabetes mellitus, hypertension and hyperlipidaemia. The first study included 407 men born in Hertfordshire, England between 1920 and 1930 whose weights at birth and at 1 year of age had been recorded by health visitors. The second study included 266 men and women born in Preston, UK, between 1935 and 1943 whose size at birth had been measured in detail. The prevalence of syndrome X fell progressively in both men and women, from those who had the lowest to those who had the highest birthweights. Of 64-year-old men whose birthweights were 2.95 kg (6.5 pounds) or less, 22% had syndrome X. Their risk of developing syndrome X was more than 10 times greater than that of men whose birthweights were more than 4.31 kg (9.5 pounds). The association between syndrome X and low birthweight was independent of duration of gestation and of possible confounding variables including cigarette smoking, alcohol consumption and social class currently or at birth. In addition to low birthweight, subjects with syndrome X had small head circumference and low ponderal index at birth, and low weight and below-average dental eruption at 1 year of age. It is concluded that Type 2 diabetes and hypertension have a common origin in sub-optimal development in utero, and that syndrome X should perhaps be re-named "the small-baby syndrome".

The formation and evolution of galaxies within merging dark matter haloes

Abstract: We construct semi-analytic models for galaxy formation within the framework of a hierarchical clustering scenario for structure formation in the Universe. We use the algorithm of Kauffmann & White to generate ensembles of merging histories for present-day dark matter haloes with a wide range of circular velocities. A galaxy is assumed to form from gas which cools and turns into stars at the centre of a halo until that halo merges with a more massive object. At this time the galaxy loses its source of new gas and becomes a non-dominant object within a larger group or cluster. Our methods thus enable us tolook insidepresent dark matter haloes and investigate the formation, evolution and merging of the galaxies that they contain

An introduction to metal matrix composites

Abstract: Preface 1. General introduction 2. Basic composite models 3. The Eshelby approach to modelling composites 4. Plastic deformation 5. Thermal effects and high temperature behaviour 6. The interfacial region 7. Fracture processes and failure mechanisms 8. Transport properties and environmental performance 9. Fabrication processes 10. Development of matrix microstructure 11. Testing and characterisation techniques 12. Applications Appendix 1. Nomenclature Appendix 2. Matrices and reinforcements - selected thermophysical properties Appendix 3. The basic Eshelby S-tensors Appendix 4. Listing of a program for an Eshelby calculation.

Germ-line mutations of the RET proto-oncogene in multiple endocrine neoplasia type 2A

Abstract: Multiple endocrine neoplasia type 2A (MEN 2A) is a dominantly inherited cancer syndrome that affects tissues derived from neural ectoderm. It is characterized by medullary thyroid carcinoma (MTC) and phaeochromocytoma. The MEN2A gene has recently been localized by a combination of genetic and physical mapping techniques to a 480-kilobase region in chromosome 10q11.2 (refs 2,3). The DNA segment encompasses the RET proto-oncogene, a receptor tyrosine kinase gene expressed in MTC and phaeochromocytoma and at lower levels in normal human thyroid. This suggested RET as a candidate for the MEN2A gene. We have identified missense mutations of the RET proto-oncogene in 20 of 23 apparently distinct MEN 2A families, but not in 23 normal controls. Further, 19 of these 20 mutations affect the same conserved cysteine residue at the boundary of the RET extracellular and transmembrane domains.

The distribution of low-mass stars in the Galactic disc

Abstract: We quantify the complex interdependence of stellar binarity, the stellar mass-luminosity relation, the mass function, the colour-magnitude relation and Galactic disc structure, all of which must be understood when analysing star-count data and stellar luminosity functions. We derive a mass-M V relation and a model for the change of stellar luminosity with changes in chemical abundance and age. Combination of this with detailed modelling of all astrophysical and observational contributions to the Malmquist scatter allows us to model star-count data without approximating Malmquist corrections. We show for the first time that a single mass function and normalization explain the stellar distribution towards both Galactic poles, as well as the distribution of stars within a distance of 5.2 pc of the Sun

Efficient light-emitting diodes based on polymers with high electron affinities

Abstract: CONJUGATED polymers have been incorporated as active materials into several kinds of electronic device, such as diodes, transistors1 and light-emitting diodes2. The first polymer light-emitting diodes were based on poly(p-phenylene vinylene) (PPV), which is robust and has a readily processible precursor polymer. Electroluminescence in this material is achieved by injection of electrons into the conduction band and holes into the valence band, which capture one another with emission of visible radiation. Efficient injection of electrons has previously required the use of metal electrodes with low work functions, primarily calcium; but this reactive metal presents problems for device stability. Here we report the fabrication of electroluminescent devices using a new family of processible poly(cyanoterephthalylidene)s. As the lowest unoccupied orbitals of these polymers (from which the conduction band is formed) lie at lower energies than those of PPV, electrodes made from stable metals such as aluminium can be used for electron injection. For hole injection, we use indium tin oxide coated with a PPV layer; this helps to localize charge at the interface between the PPV and the new polymer, increasing the efficiency of recombination. In this way, we are able to achieve high internal efficiencies (photons emitted per electrons injected) of up to 4% in these devices.

Bayesian Computation Via the Gibbs Sampler and Related Markov Chain Monte Carlo Methods

Abstract: The use of the Gibbs sampler for Bayesian computation is reviewed and illustrated in the context of some canonical examples Other Markov chain Monte Carlo simulation methods are also briefly described, and comments are made on the advantages of sample-based approaches for Bayesian inference summaries

A phenomenological theory for strain gradient effects in plasticity

Abstract: A Strain Gradient Theory of plasticity is introduced, based on the notion of statistically stored and geometrically necessary dislocations. The strain gradient theory fits within the general framework of couple stress theory and involves a single material length scale l. Minimum principles are developed for both deformation and flow theory versions of the theory which in the limit of vanishing l, reduce to their conventional counterparts: J2 deformation and J2 flow theory. The strain gradient theory is used to calculate the size effect associated with macroscopic strengthening due to a dilute concentration of bonded rigid particles; similarly, predictions are given for the effect of void size upon the macroscopibic softening due to a dilute concentration of voids. Constitutive potentials are derived for this purpose.

The 1993 IGTI Scholar Lecture: Loss Mechanisms in Turbomachines

Abstract: The origins and effects of loss in turbomachines are discussed with the emphasis on trying to understand the physical origins of loss rather than on reviewing the available prediction methods. Loss is defined in terms of entropy increase and the relationship of this to the more familiar loss coefficients is derived and discussed. The sources of entropy are, in general: viscous effects in boundary layers, viscous effects in mixing processes, shock waves, and heat transfer across temperature differences. These are first discussed in general and then the results are applied to turbomachinery flows. Understanding of the loss due to heat transfer requires some discussion of cycle thermodynamics

Particle Creation by Black Holes

Abstract: In the classical theory black holes can only absorb and not emit particles. However it is shown that quantum mechanical effects cause black holes to create and emit particles as if they were hot bodies with temperature\(\frac{{h\kappa }}{{2\pi k}} \approx 10^{ - 6} \left( {\frac{{M_ \odot }}{M}} \right){}^ \circ K\) where κ is the surface gravity of the black hole. This thermal emission leads to a slow decrease in the mass of the black hole and to its eventual disappearance: any primordial black hole of mass less than about 1015 g would have evaporated by now. Although these quantum effects violate the classical law that the area of the event horizon of a black hole cannot decrease, there remains a Generalized Second Law:S+1/4A never decreases whereS is the entropy of matter outside black holes andA is the sum of the surface areas of the event horizons. This shows that gravitational collapse converts the baryons and leptons in the collapsing body into entropy. It is tempting to speculate that this might be the reason why the Universe contains so much entropy per baryon.

"Infectious" transplantation tolerance

Abstract: The maintenance of transplantation tolerance induced in adult mice after short-term treatment with nonlytic monoclonal antibodies to CD4 and CD8 was investigated. CD4+ T cells from tolerant mice disabled naive lymphocytes so that they too could not reject the graft. The naive lymphocytes that had been so disabled also became tolerant and, in turn, developed the capacity to specifically disable other naive lymphocytes. This process of "infectious" tolerance explains why no further immunosuppression was needed to maintain long-term transplantation tolerance.

The baryon content of galaxy clusters: a challenge to cosmological orthodoxy

Abstract: Baryonic matter constitutes a larger fraction of the total mass of rich galaxy clusters than is predicted by a combination of cosmic nucleosynthesis considerations (light-element formation during the Big Bang) and standard inflationary cosmology. This cannot be accounted for by gravitational and dissipative effects during cluster formation. Either the density of the Universe is less than that required for closure, or there is an error in the standard interpretation of element abundances.

Properties of filtered-ion-beam-deposited diamondlike carbon as a function of ion energy

Abstract: A highly tetrahedrally bonded form on nonhydrogenated amorphous carbon (a-C) is produced by deposition from filtered medium-energy ion beams. A range of such films was grown and the ${\mathit{sp}}^{3}$-bonded fractions, plasmon energies, compressive stresses, and resistivities were measured as a function of ion energy. These properties are found to be strongly correlated and each to pass through a maximum at an ion energy of about 140 eV. The optimum ion energy is observed to depend on the type of carbon ions deposited and, possibly, on the deposition flux rate. The data are found to support deposition models in which the ${\mathit{sp}}^{3}$ bonding arises from the subplantation of incident ions, giving rise to a quenched increase in density and strain.

Two paradigms of baroclinic‐wave life‐cycle behaviour

Abstract: Two idealized baroclinic wave-6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic-scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave-theoretic viewpoints, using the ‘saturation-propagation-saturation’ (SPS) picture of wave-6 life-cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher-resolution versions of the Simmons-Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper-air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward-tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary-scale mid-stratospheric ‘Rossby-wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward-tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut-off cyclones in high latitudes. These morphologies are visualized by upper-air maps of potential temperature on the nominal tropopause, defined as a constant-potential-vorticity surface. Some atmospheric mid-latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases. The SPS picture is re-examined, using Eliassen-palm and refractive-index cross-sections. It is shown, in particular, by reference to a wave-activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large-amplitude counterpart of the nonlinear reflection scenario of Rossby-wave critical-layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical-layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby-wave activity, has largely ceased. This accounts for the more persistent EKEE in the LC2 case.

Statistical tests of models of DNA substitution

Abstract: Penny et al. have written that "The most fundamental criterion for a scientific method is that the data must, in principle, be able to reject the model. Hardly any [phylogenetic] tree-reconstruction methods meet this simple requirement." The ability to reject models is of such great importance because the results of all phylogenetic analyses depend on their underlying models--to have confidence in the inferences, it is necessary to have confidence in the models. In this paper, a test statistic suggested by Cox is employed to test the adequacy of some statistical models of DNA sequence evolution used in the phylogenetic inference method introduced by Felsenstein. Monte Carlo simulations are used to assess significance levels. The resulting statistical tests provide an objective and very general assessment of all the components of a DNA substitution model; more specific versions of the test are devised to test individual components of a model. In all cases, the new analyses have the additional advantage that values of phylogenetic parameters do not have to be assumed in order to perform the tests.

Compressive failure of fibre composites

Abstract: A review of experimental data and elementary theoretical formulas for compressive failure of polymer matrix fibre composites indicates that the dominant failure mode is by plastic kinking. Initial local fibre misalignment plays a central role in the plastic kinking process. Theoretical analyses and numerical results for compressive kinking are presented, encompassing effects of strain-hardening, kink inclination, and applied shear stress. The assumption of rigid fibres is assessed critically, and the legitimacy of its use for polymer matrix composites is established.

Temporal phase-unwrapping algorithm for automated interferogram analysis.

Abstract: A new algorithm is proposed for unwrapping interferometric phase maps. Existing algorithms search the two-dimensional spatial domain for 2π discontinuities: only one phase map is required, but phase errors can propagate outward from regions of high noise, corrupting the rest of the image. An alternative approach based on one-dimensional unwrapping along the time axis is proposed. It is applicable to an important subclass of interferometry applications, in which a sequence of incremental phase maps can be obtained leading up to the final phase-difference map of interest. A particular example is quasi-static deformation analysis. The main advantages are (i) it is inherently simple, (ii) phase errors are constrained within the high-noise regions, and (iii) phase maps containing global discontinuities are unwrapped correctly, provided the positions of the discontinuities remain fixed with time. The possibility of real-time phase unwrapping is also discussed.

The concept and definition of dominance in animal behaviour.

Abstract: The concept of dominance has contributed greatly to our understanding of social structure in animals. Over the past three decades, however, a variety of concepts and definitions of dominance have been introduced, leading to an ongoing debate about the usefulness and meaning of the concept. Criticisms aimed at one definition of dominance do not necessarilly apply to other definitions. Existing definitions can be structural or functional, refer to roles or to agonistic behaviour, regard dominance as a property of individuals or as an attribute of dyadic encounters, concentrate on aggression or on the lack of it, and be based either on theoretical constructs or on observable behaviour. Thirteen definitions of dominance are reviewed, and their usefulness assessed with respect to their descriptive value. The predictive and explanatory values of definitions are specific to the questions asked in each particular study and are not considered as criteria to judge the usefulness of the dominance concept. By virtue of its high descriptive value, the original definition of dominance by SCHJELDERUPP-EBBE (1922, Z.Psychol. 88: 226-252) emerged as the basis to formulate a structural definition with wide applicability and which reflects the essence of the concept: Dominance is an attribute of the pattern of repeated, agonistic interactions between two individuals, characterized by a consistent outcome in favour of the same dyad member and a default yielding response of its opponent rather than escalation. The status of the consistent winner is dominant and that of the loser subordinate. Dominance status refers to dyads while dominance rank, high or low, refers to the position in a hierarchy and, thus, depends on group composition. Dominance is a relative measure and not an absolute property of individuals. The discussion includes reference to the heritability of dominance, application of dominance to groups rather than individuals, and the role of individual recognition and memory during agonistic encounters.

Loss Mechanisms in Turbomachines

Abstract: The origins and effects of loss in turbomachines are discussed with the emphasis on trying to understand the physical origins of loss rather than on reviewing the available prediction methods. Loss is defined in terms of entropy increase and the relationship of this to the more familiar loss coefficients is derived and discussed. The sources of entropy are in general: Viscous effects in boundary layers, viscous effects in mixing processes, shock waves and heat transfer across temperature differences. These are first discussed in general and then the results are applied to turbomachinery flows. Understanding of the loss due to heat transfer requires some discussion of cycle thermodynamics. Sections are devoted to discussing: Blade boundary layer and trailing edge loss, tip leakage loss, endwall loss, effects of heat transfer and miscellaneous losses. The loss arising from boundary layer separation is particularly difficult to quantify. Most of the discussion is based on axial flow machines but a separate section is devoted to the special problems of radial flow machines.In some cases, eg attached blade boundary layers, the loss mechanisms are well understood, but even so the loss can seldom be predicted with great accuracy. In many other cases, eg endwall loss, the loss mechanisms are still not clearly understood and prediction methods remain very dependent on correlations. The paper emphasises that the use of correlations should not be a substitute for trying to understand the origins of loss and suggests that a good physical understanding of the latter may be more valuable than a quantitative prediction.Copyright © 1993 by ASME

Contrasting mechanisms of impaired attentional set-shifting in patients with frontal lobe damage or Parkinson's disease

Abstract: Tests which assess the ability to shift cognitive set modelled after the Wisconsin Card Sorting Test are particularly sensitive to impairments in patients with Parkinson's disease as well as in patients with frontal lobe damage. However, the underlying mechanisms responsible for the similar deficits observed in the two patient groups are not well understood and may not be identical. For example, an apparent deficit in set-shifting ability may reflect either an impairment in the ability to shift from a perceptual dimension which has previously commanded attention (i.e. 'perseveration'), or in the ability to shift to an alternative perceptual dimension which has previously been irrelevant (i.e. 'learned irrelevance'). In this study, the performance of both medicated and non-medicated patients with Parkinson's disease were compared with a group of neurosurgical patients with localized excisions of the frontal lobes on a novel task designed to assess the relative contribution of 'perseveration' and 'learned irrelevance' to impaired set-shifting ability. Patients with frontal lobe damage were worse than controls in their ability to shift attention from a previously relevant stimulus dimension. Medicated patients with Parkinson's disease were worse at shifting to a previously irrelevant dimension. In contrast to both groups, nonmedicated patients with Parkinson's disease were impaired in both conditions. These results suggest that the gross set-shifting deficits reported in both frontal lobe patients and patients with Parkinson's disease may involve fundamentally different, though related, cognitive processes, and that these may be differentially affected by medication. Specifically, L-dopa therapy may protect Parkinson's disease patients from preservation of attention to a formerly relevant stimulus dimension.