Showing papers by "University of Oxford published in 1991"

Some Tests of Specification for Panel Data: Monte Carlo Evidence and an Application to Employment Equations.

Abstract: This paper presents specification tests that are applicable after estimating a dynamic model from panel data by the generalized method of moments (GMM), and studies the practical performance of these procedures using both generated and real data. Our GMM estimator optimally exploits all the linear moment restrictions that follow from the assumption of no serial correlation in the errors, in an equation which contains individual effects, lagged dependent variables and no strictly exogenous variables. We propose a test of serial correlation based on the GMM residuals and compare this with Sargan tests of over-identifying restrictions and Hausman specification tests.

Quantum cryptography based on Bell's theorem.

Abstract: Practical application of the generalized Bells theorem in the so-called key distribution process in cryptography is reported. The proposed scheme is based on the Bohms version of the Einstein-Podolsky-Rosen gedanken experiment and Bells theorem is used to test for eavesdropping. © 1991 The American Physical Society.

Excursion set mass functions for hierarchical Gaussian fluctuations

Abstract: It is pointed out that most schemes for determining the mass function of virialized objects from the statistics of the initial density perturbation field suffer from the cloud-in-cloud problem of miscounting the number of low-mass clumps, many of which would have been subsumed into larger objects. The paper proposes a solution based on the theory of the excursion sets of F(r, R sub f), the four-dimensional initial density perturbation field smoothed with a continuous hierarchy of filters of radii R sub f.

Mobile robot localization by tracking geometric beacons

Abstract: The application of the extended Kaman filter to the problem of mobile robot navigation in a known environment is presented. An algorithm for, model-based localization that relies on the concept of a geometric beacon, a naturally occurring environment feature that can be reliably observed in successive sensor measurements and can be accurately described in terms of a concise geometric parameterization, is developed. The algorithm is based on an extended Kalman filter that utilizes matches between observed geometric beacons and an a priori map of beacon locations. Two implementations of this navigation algorithm, both of which use sonar, are described. The first implementation uses a simple vehicle with point kinematics equipped with a single rotating sonar. The second implementation uses a 'Robuter' mobile robot and six static sonar transducers to provide localization information while the vehicle moves at typical speeds of 30 cm/s. >

The neuropsychology of schizophrenia.

Abstract: A model is proposed for integrating the neural and cognitive aspects of the positive symptoms of acute schizophrenia, using evidence from postmortem neuropathology and neurochemistry, clinical and preclinical studies of dopaminergic neurotransmission, anatomical connections between the limbic system and basal ganglia, attentional and other cognitive abnormalities underlying the positive symptoms of schizophrenia, specific animal models of some of these abnormalities, and previous attempts to model the cognitive functions of the septohippocampal system and the motor functions of the basal ganglia Anatomically, the model emphasises the projections from the septohippocampal system, via the subiculum, and the amygdala to nucleus accumbens, and their interaction with the ascending dopaminergic projection to the accumbens Psychologically, the model emphasises a failure in acute schizophrenia to integrate stored memories of past regularities of perceptual input with ongoing motor programs in the control of current perception A number of recent experiments that offer support for the model are briefly described, including anatomical studies of limbic-striatal connections, studies in the rat of the effects of damage to these connections, and of the effects of amphetamine and neuroleptics, on the partial reinforcement extinction effect, latent inhibition and the Kamin blocking effect; and studies of the latter two phenomena in acute and chronic schizophrenics

A Re-Examination of the Basic Postulates of Thermomechanics

Abstract: This paper is mainly concerned with a re-examination of the basic postulates and the consequent procedure for the construction of the constitutive equations of material behaviour in thermomechanics. However, the implication of the basic postulates and the significance of the related procedure for the development of the constitutive equations is also illustrated in some detail in the context of flow of heat in a rigid solid with particular reference to the propagation of thermal waves at finite speed. More specifically, after briefly examining the nature of the basic equations of motion for a system of particles within the scope of the classical newtonian mechanics, the basic postulates of the purely mechanical theory for a continuum (including its specialization for a rigid body) is re-examined. This includes some differences from the usual procedure on the subject. Next, thermal variables are introduced and after observing a useful analogy between the thermal and mechanical variables, a discussion of a theory of heat (or a purely thermal theory) is provided which differs from the usual development in the classical thermodynamics. A detailed application of the latter development is then made to the problem of heat flow in a stationary rigid solid using several different and well-motivated constitutive equations. Special cases of these include linearized theories of the classical heat flow by conduction and of heat flow transmitted as thermal waves. The remainder of the paper is concerned with thermal mechanical theory of deformable media along with discussions of a number of related issues on the subject.

A report--chronic fatigue syndrome: guidelines for research.

Abstract: Authors Dr M C Sharpe MRCPsych Clinical Lecturer, Department of Psychiatry, University of Oxford Dr L C Archard PhD Senior Lecturer, Department of Biochemistry, Charing Cross and Westminster Medical School, London Professor J E Banatvala MD Professor of Virology, Department of Virology, St Thomas' Hospital Medical School, London Dr L K Borysiewicz FRCP Wellcome Trust Senior Lecturer, Department of Medicine, University of Cambridge Professor A W Clare MD Clinical Professor of Psychiatry, Trinity College, Dublin Dr A David MRCPsych MRC Training Fellow, Institute of Psychiatry, London Professor R H T Edwards FRCP Professor of Medicine, Department of Medicine, University of Liverpool Dr K E H Hawton DM Consultant Psychiatrist, Department of Psychiatry, University of Oxford Professor H P Lambert MD Emeritus Professor of Microbial Disease, St George's Hospital Medical School, London Dr R J M Lane MD Consultant Neurologist, Regional Neurosciences Centre, Charing Cross Hospital, London Dr E M McDonald MRCPsych Research Psychiatrist, Institute of Psychiatry, London Professor J F Mowbray FRCP Immunopathology, Department oflmmunopathology, St Mary's Hospital Medical School, London Dr D J Pearson FRCP Senior Lecturer, Department of Medicine, University of Manchester Dr TEA Peto MRCP Consultant Physician, Department of Infectious Diseases, John Radcliffe Hospital, Oxford Dr V R Preedy PhD Research Biochemist, Department of Biochemistry, King's College Hospital, London Dr A P Smith PhD Research Psychologist, Department of Psychology, University of Sussex Dr D G Smith MB General Practitioner, Horndonon-the-Hill, Essex Dr D J Taylor DPhil MRC Staff Scientist, MRC Biochemical and Clinical Magnetic Resonance Unit, John Radcliffe Hospital, Oxford Dr D A J Tyrrell MD Director, MRC Common Cold Unit, Harvard Hospital, Salisbury Dr S Wessely MRCPsych Wellcome Training Fellow in Epidemiology, Institute of Psychiatry, London Dr P D White MRCPsych Senior Lecturer, Department of Psychological Medicine, St Bartholomew's Hospital, London Other signatories who contributed to the guidelines but who were unable to attend the meeting Professor P 0 Behan MD Professor of Neurology, Department of Neurology, University of Glasgow Dr F Clifford Rose FRCP Director, Academic Unit of Neuroscience, Charing Cross and Westminster Medical School, London Professor T J Peters FRCP Professor of Clinical Biochemistry, Department of Biochemistry, King's College Hospital, London Dr P G Wallace MRCGP Head of Research Unit, Department of General Practice, St Mary's Hospital Medical School, London Professor D A Warrell FRCP Professor of Tropical Medicine and Infectious Diseases, Nuffield Department of Medicine, John Radcliffe Hospital, Oxford Dr D J M Wright MD Consultant Microbiologist, Department of Microbiology, Charing Cross Hospital, London

On the “Downward Control” of Extratropical Diabatic Circulations by Eddy-Induced Mean Zonal Forces

Abstract: The situation considered is that of a zonally symmetric model of the middle atmosphere subject to a given quasi-steady zonal force F, conceived to be the result of irreversible angular momentum transfer due to the upward propagation and breaking of Rossby and gravity waves together with any other dissipative eddy effects that may be relevant. The model's diabatic heating is assumed to have the qualitative character of a relaxation toward some radiatively determined temperature field. To the extent that the force F may be regarded as given, and the extratropical angular momentum distribution is realistic, the extratropical diabatic mass flow across a given isentropic surface may be regarded as controlled exclusively by the F distribution above that surface (implying control by the eddy dissipation above that surface and not, for instance, by the frequency of tropopause folding below). This “downward control” principle expresses a critical part of the dynamical chain of cause and effect governin...

The selection-mutation-drift theory of synonymous codon usage.

Abstract: It is argued that the bias in synonymous codon usage observed in unicellular organisms is due to a balance between the forces of selection and mutation in a finite population, with greater bias in highly expressed genes reflecting stronger selection for efficiency of translation. A population genetic model is developed taking into account population size and selective differences between synonymous codons. A biochemical model is then developed to predict the magnitude of selective differences between synonymous codons in unicellular organisms in which growth rate (or possibly growth yield) can be equated with fitness. Selection can arise from differences in either the speed or the accuracy of translation. A model for the effect of speed of translation on fitness is considered in detail, a similar model for accuracy more briefly. The model is successful in predicting a difference in the degree of bias at the beginning than in the rest of the gene under some circumstances, as observed in Escherichia coli, but grossly overestimates the amount of bias expected. Possible reasons for this discrepancy are discussed.

Cortical areas and the selection of movement: a study with positron emission tomography.

Abstract: Regional cerebral blood flow was measured in normal subjects with positron emission tomography (PET) while they performed five different motor tasks. In all tasks they had to moved a joystick on hearing a tone. In the control task they always pushed it forwards (fixed condition), and in four other experimental tasks the subjects had to select between four possible directions of movement. These four tasks differed in the basis for movement selection. A comparison was made between the regional blood flow for the four tasks involving movement selection and the fixed condition in which no selection was required. When selection of a movement was made, significant increases in regional cerebral blood flow were found in the premotor cortex, supplementary motor cortex, and superior parietal association cortex. A comparison was also made between the blood flow maps generated when subjects performed tasks based on internal or external cues. In the tasks with internal cues the subjects could prepare their movement before the trigger stimulus, whereas in the tasks with external cues they could not. There was greater activation in the supplementary motor cortex for the tasks with internal cues. Finally a comparison was made between each of the selection conditions and the fixed condition; the greatest and most widespread changes in regional activity were generated by the task on which the subjects themselves made a random selection between the four movements.

Economic Perspectives on Privatization

Abstract: Despite being one of the most fundamental issues in political economy, the question of the appropriate boundary between public and private enterprise received relatively little attention in mainstream economic analysis until quite recently. In the 1980s, however, programs of ownership reform were started in many developed and developing countries. Dramatic though some of these policies have been, they are likely to be overshadowed in the 1990s by even greater privatization in the reforming socialist economies. The opening sections of this paper are organized around three broad and interrelated questions. How does ownership matter for the efficiency of enterprise performance? What is the role for privatization in financing public debts and deficits? What are the distributional and political implications of privatization? Finally we examine privatization in practice in three countries: Britain, Chile, and Poland.

Lysosomal storage diseases

Abstract: Lysosomal storage diseases (LSDs) are a group of over 70 diseases that are characterized by lysosomal dysfunction, most of which are inherited as autosomal recessive traits. These disorders are individually rare but collectively affect 1 in 5,000 live births. LSDs typically present in infancy and childhood, although adult-onset forms also occur. Most LSDs have a progressive neurodegenerative clinical course, although symptoms in other organ systems are frequent. LSD-associated genes encode different lysosomal proteins, including lysosomal enzymes and lysosomal membrane proteins. The lysosome is the key cellular hub for macromolecule catabolism, recycling and signalling, and defects that impair any of these functions cause the accumulation of undigested or partially digested macromolecules in lysosomes (that is, 'storage') or impair the transport of molecules, which can result in cellular damage. Consequently, the cellular pathogenesis of these diseases is complex and is currently incompletely understood. Several LSDs can be treated with approved, disease-specific therapies that are mostly based on enzyme replacement. However, small-molecule therapies, including substrate reduction and chaperone therapies, have also been developed and are approved for some LSDs, whereas gene therapy and genome editing are at advanced preclinical stages and, for a few disorders, have already progressed to the clinic.

Spatial structure and chaos in insect population dynamics

Abstract: MOST environments are spatially subdivided, or patchy, and there has been much interest in the relationship between the dynamics of populations at the local and regional (metapopulation) scales1 Here we study mathematical models for host-parasitoid interactions, where in each generation specified fractions (µN and µp, respectively) of the host and parasitoid subpopulations in each patch move to adjacent patches; in most previous work, the movement is not localized but is to any other patch2 These simple and biologically sensible models with limited diffusive dispersal exhibit a remarkable range of dynamic behaviour: the density of the host and parasitoid subpopulations in a two-dimensional array of patches may exhibit complex patterns of spiral waves or spatially chaotic variation, they may show static 'crystal lattice' patterns, or they may become extinct This range of behaviour is obtained with the local dynamics being deterministically unstable, with a constant host reproductive rate and no density dependence in the movement patterns The dynamics depend on the host reproductive rate, and on the values of the parameters µN and µp The results are relatively insensitive to the details of the interactions; we get essentially the same results from the mathematically-explicit Nicholon–Bailey model of host-parasitoid interactions, and from a very general 'cellular automaton' model in which only qualitative rules are specified We conclude that local movement in a patchy environment can help otherwise unstable host and parasitoid populations to persist together, but that the deterministically generated spatial patterns in population density can be exceedingly complex (and sometimes indistinguishable from random environmental fluctuations)

Partial oxidation of methane to synthesis gas using carbon dioxide

Abstract: INCREASING concern about world dependence on petroleum oil has generated interest in the more efficient use of natural gas1–4. The conversion of methane to the common feedstock synthesis gas (carbon monoxide and hydrogen) by steam reforming is already well established5, and we have shown recently that yields of syn-thesis gas in excess of 90% can be obtained at moderate tem-peratures and ambient pressure by partial oxidation, with air or oxygen, over supported transition-metal catalysts6,7. The use of carbon dioxide as an oxidant for conversion of natural gas to synthesis gas is well established in steam reforming5, and is also known in CO2 reforming (for example, the Calcor process8,9), in which the use of excess CO2 yields mainly CO. In the present work, we describe an alternative catalytic strategy for CO2 reform-ing which gives excellent yields (90%) from a stoichiometric (1:1) feed of CO2 and CH4. Carbon deposition ('coking'), which is a hazard of CO2-reforming routes, is suppressed here by the use of catalysts based on platinum-group metals. We show that the exothermic partial oxidation of CH2 and the endothermic CO2-reforming reaction can be carried out simultaneously, thus introducing the possibility of tuning the thermodynamics of the process.

The evolution of costly mate preferences ii. the "handicap" principle.

Abstract: We use a general additive quantitative genetic model to study the evolution of costly female mate choice by the "handicap" principle. Two necessary conditions must be satisfied for costly preference to evolve. The conditions are (i) biased mutation pressure on viability and (ii) a direct relationship between the degree of expression of the male mating character and viability. These two conditions explain the success and failure of previous models of the "handicap" principle. Our model also applies to other sources of fitness variation like migration and host-parasite coevolution, which cause effects equivalent to biased mutation.

The Importance of Behaviour in the Maintenance of Anxiety and Panic: A Cognitive Account

Abstract: The theoretical and empirical basis of commonly accepted propositions concerning the role of behaviour in the practice of behavioural psychotherapy for anxiety problems is considered. A number of problems are identified, and an alternative, more explicitly cognitive hypothesis is described. According to this cognitive account, there is both a close relationship and specific interactions between “threat cognitions” and “safety seeking behaviour”. For any individual, safety seeking behaviour arises out of, and is logically linked to, the perception of serious threat. Such behaviour may be anticipatory (avoidant) or consequent (escape). Because safety seeking behaviour is perceived to be preventative, and focused on especially negative consequences (e.g. death, illness, humiliation), spontaneous disconfirmation of threat is made particularly unlikely by such safety seeking behaviours. By preventing disconfirmation of threat-related cognitions, safety seeking behaviour may be a crucial factor in the maintenance of anxiety disorders. The implications of this view for the understanding and treatment of anxiety disorders are discussed.

A functional microcircuit for cat visual cortex.

Abstract: 1. We have studied in vivo the intracellular responses of neurones in cat visual cortex to electrical pulse stimulation of the cortical afferents and have developed a microcircuit that simulates much of the experimental data. 2. Inhibition and excitation are not separable events, because individual neurones are embedded in microcircuits that contribute strong population effects. Synchronous electrical activation of the cortex inevitably set in motion a sequence of excitation and inhibition in every neurone we recorded. The temporal form of this response depends on the cortical layer in which the neurone is located. Superficial layer (layers 2+3) pyramidal neurones show a more marked polysynaptic excitatory phase than the pyramids of the deep layers (layers 5+6). 3. Excitatory effects on pyramidal neurones, particularly the superficial layer pyramids, are in general not due to monosynaptic input from thalamus, but polysynaptic input from cortical pyramids. Since the thalamic input is transient it does not provide the major, sustained excitation arriving at any cortical neurone. Instead the intracortical excitatory connections provide the major component of the excitation. 4. The polysynaptic excitatory response would be sustained well after the stimulus, were it not for the suppressive effect of intracortical inhibition induced by the pulse stimulation. 5. Intracellular recording combined with ionophoresis of gamma-aminobutyric acid (GABA) agonists and antagonists showed that intracortical inhibition is mediated by GABAA and GABAB receptors. The GABAA component occurs in the early phase of the impulse response. It is reflected in the strong hyperpolarization that follows the excitatory response and lasts about 50 ms. The GABAB component occurs in the late phase of the response, and is reflected in a sustained hyperpolarization that lasts some 200-300 ms. Both components are seen in all cortical pyramidal neurones. However, the GABAA component appears more powerful in deep layer pyramids than superficial layer pyramids. 6. The microcircuit simulates with good fidelity the above data from experiments in vivo and provides a novel explantation for the apparent lack of significant inhibition during visual stimulation. The basic circuit may be common to all cortical areas studied and thus the microcircuit may be a 'canonical' microcircuit for neocortex.

Surveys of patients satisfaction: I--Important general considerations.

Abstract: was 46%, then introducing the protocol saved the radiology department almost £500 a year. The protocol also resulted in an increase in the number of patients reviewed and hence the work of the accident and emergency department. In terms of the patients reviewed in the department, however, the total numbers (an extra 36 over the eight weeks) were small, amounting to an extra four or five patients a week in a department with an average new-patient attendance each week of about 800. Eleven of the patients reviewed in the department were referred to the soft tissue clinic, six of whom required physiotherapy. This suggests that, as had been suspected, before the protocol was introduced patients with ligamentous injuries had been undertreated; this seems to justify the small increase in the number of patients reviewed. After introducing the protocol the number of patients reattending of their own accord fell from five to one, supporting the view that the protocol improved the treatment of patients with ligamentous injuries. The protocol also resulted in a reduction of the work of the fracture clinics as inappropriate referrals were reduced by 53%. The two patients who were referred by their general practitioners for radiography show the importance of informing local general practitioners of changes in treatment policy. At the end of the study no changes were required in the protocol, and the algorithm is now included in the notes of any patient attending with an ankle injury. The algorithm proved an effective means of improving treatment of ankle injuries and algorithms may improve treatment of other conditions in accident and emergency departments.

Serum cholesterol concentration and coronary heart disease in population with low cholesterol concentrations.

Abstract: OBJECTIVE--To examine the relation between serum cholesterol concentration and mortality (from coronary heart disease and from other causes) below the range of cholesterol values generally seen in Western populations. DESIGN--Prospective observational study based on 8-13 years of follow up of subjects in a population with low cholesterol concentrations. SETTING--Urban Shanghai, China. SUBJECTS--9021 Chinese men and women aged 35-64 at baseline. MAIN OUTCOME MEASURE--Death from coronary heart disease and other causes. RESULTS--The average serum cholesterol concentration was 4.2 mmol/l at baseline examination, and only 43 (7%) of the deaths that occurred during 8-13 years of follow up were attributed to coronary heart disease. There was a strongly positive, and apparently independent, relation between serum cholesterol concentration and death from coronary heart disease (z = 3.47, p less than 0.001), and within the range of usual serum cholesterol concentration studied (3.8-4.7 mmol/l) there was no evidence of any threshold. After appropriate adjustment for the regression dilution bias, a 4 (SD 1)% difference in usual cholesterol concentration was associated with a 21 (SD 6)% (95% confidence interval 9% to 35%) difference in mortality from coronary heart disease. There was no significant relation between serum cholesterol concentration and death from stroke or all types of cancer. The 79 deaths due to liver cancer or other chronic liver disease were inversely related to cholesterol concentration at baseline. CONCLUSION--Blood cholesterol concentration was directly related to mortality from coronary heart disease even in those with what was, by Western standards, a "low" cholesterol concentration. There was no good evidence of an adverse effect of cholesterol on other causes of death.

Left neglect for near but not far space in man

Abstract: It has been suggested that, among the many visual areas of the human brain, there might be one set of spatial maps specialized for 'near' (peripersonal) and another for 'far' (extrapersonal) space. A distinction between 'grasping distance' and 'walking distance', or between a 'reaching field' and a pointing or throwing field has commonly been made. Evidence for such a division has been found in monkeys. Unilateral ablation of the frontal eye field (area 8) produces a more prominent inattention (or 'neglect') for objects in contralesional far space than in near space; by contrast, unilateral ablation of frontal area 6, which receives direct projections from area 7b (the rostral part of the inferior parietal lobules) results in inattention to visual stimuli limited to contralesional near space. Despite predictions that comparable dissociations should be found in man, there has been no convincing evidence. We report here such evidence in a patient with a unilateral right hemisphere stroke. Within peripersonal space, he showed severe left visuo-spatial neglect on conventional tests, including the highly sensitive task of line bisection. When line bisection was performed in extrapersonal space, neglect was abolished or attenuated.

Collective decision-making in honey bees: how colonies choose among nectar sources

Abstract: A honey bee colony can skillfully choose among nectar sources. It will selectively exploit the most profitable source in an array and will rapidly shift its foraging efforts following changes in the array. How does this colony-level ability emerge from the behavior of individual bees? The answer lies in understanding how bees modulate their colony's rates of recruitment and abandonment for nectar sources in accordance with the profitability of each source. A forager modulates its behavior in relation to nectar source profitability: as profitability increases, the tempo of foraging increases, the intensity of dancing increases, and the probability of abandoning the source decreases. How does a forager assess the profitability of its nectar source? Bees accomplish this without making comparisons among nectar sources. Neither do the foragers compare different nectar sources to determine the relative profitability of any one source, nor do the food storers compare different nectar loads and indicate the relative profitability of each load to the foragers. Instead, each forager knows only about its particular nectar source and independently calculates the absolute profitability of its source. Even though each of a colony's foragers operates with extremely limited information about the colony's food sources, together they will generate a coherent colonylevel response to different food sources in which better ones are heavily exploited and poorer ones are abandoned. This is shown by a computer simulation of nectar-source selection by a colony in which foragers behave as described above. Nectar-source selection by honey bee colonies is a process of natural selection among alternative nectar sources as foragers from more profitable sources “survive” (continue visiting their source) longer and “reproduce” (recruit other foragers) better than do foragers from less profitable sources. Hence this colonial decision-making is based on decentralized control. We suggest that honey bee colonies possess decentralized decision-making because it combines effectiveness with simplicity of communication and computation within a colony.