Showing papers by "University of Oxford published in 1986"

Markets with consumer switching costs

Abstract: Ex ante homogeneous products may, after the purchase of one of them, be ex post differentiated by switching costs including learning costs, transaction costs, or "artificial" costs imposed by firms, such as repeat-purchase discounts. The nonco-operative equilibrium in an oligopoly with switching costs may be the same as the collusive outcome in an otherwise identical market without switching costs. However, the prospect of future collusive profits leads to vigorous competition for market share in the early stages of a market's development. The model thus explains the emphasis placed on market share as a goal of corporate strategy.

Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks

Abstract: Subduction-accretion complexes can be approximated as wedge-shaped continua with a rigid buttress behind and a subducting litho-spheric slab beneath. Thick wedges undergoing prograde metamorphism have a negligible long-term yield strength and are likely to exhibit a complex nonlinear viscous rheology. Such a wedge will tend to deform internally until it reaches a stable configuration, in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab. Accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry. This shortening will be expressed as late (out-of-sequence) thrusting, backthrusting, and folding. Conversely, underplating of sediment or crustal slices will thicken the wedge, which may need to extend internally to regain stability. Extension will cause listric normal faults that may merge downward into zones of ductile extension. Continued underplating at depth and compensating extension above provides a mechanism for bringing high-P/low-T metamorphic rocks to upper levels in the rear of the wedge, where they are commonly observed. Many major tectonic boundaries in convergent orogens (such as the Coast Range thrust in the Franciscan Complex, major nappe contacts in the Alps, and the contact between the Nevado-Filabride and Higher Betic nappe complexes in the Betic Cordillera) show abrupt increases in metamorphic grade downward across them. This is consistent with their origin or reactivation as uplift-related, extensional structures.

Hypothalamic control of adrenocorticotropin secretion: advances since the discovery of 41-residue corticotropin-releasing factor.

Abstract: IN THE 25 yrs after the first demonstration of CRF in hypothalamic tissue (1, 2), research on CRF progressed along two major lines of experimentation. In the context of a functional anatomical approach, researchers sought to elucidate the hypothalamic organization of neurons that produce CRF and control the secretion of ACTH (for reviews see Refs. 3–5). The second major trend was the quest for the isolation and chemical characterization of the hypothalamic CRF principle (for reviews see Refs. 6–8). It was the discovery of 41-amino acid residue CRF (CRF-41) by Vale and co-workers (9) which provided the missing link between the two major paths of investigation. Aided by decades of expertise accumulated in laboratories throughout the world, workers finally having the right stuff at hand rapidly established CRF-41 as a major physiological mediator of the hypothalamic control of ACTH secretion (for reviews see Refs. 10–12). However, it soon became evident that CRF-41 is not the sole hypothalamic neurohormone i...

Shortening of continental lithosphere: the neotectonics of Eastern Anatolia — a young collision zone

Abstract: Summary We use the tectonics of Eastern Anatolia to exemplify many of the different aspects of collision tectonics, namely the formation of plateaux, thrust belts, foreland flexures, widespread foreland/hinterland deformation zones and orogenic collapse/distension zones. Eastern Anatolia is a 2 km high plateau bounded to the S by the southward-verging Bitlis Thrust Zone and to the N by the Pontide/Minor Caucasus Zone. It has developed as the surface expression of a zone of progressively thickening crust beginning about 12 Ma in the medial Miocene and has resulted from the squeezing and shortening of Eastern Anatolia between the Arabian and European Plates following the Serravallian demise of the last oceanic or quasioceanic tract between Arabia and Eurasia. Thickening of the crust to about 52 km has been accompanied by major strike-slip faulting on the right-lateral N Anatolian Transform Fault (NATF) and the left-lateral E Anatolian Transform Fault (EATF) which approximately bound an Anatolian Wedge that is being driven westwards to override the oceanic lithosphere of the Mediterranean along subduction zones from Cephalonia to Crete, and Rhodes to Cyprus. This neotectonic regime began about 12 Ma in Late Serravallian times with uplift from wide-spread littoral/neritic marine conditions to open seasonal wooded savanna with colluvial, fluvial and limnic environments, and the deposition of the thick Tortonian Kythrean Flysch in the Eastern Mediterranean. Earthquake hypocentres are scattered throughout the region but large earthquakes are concentrated mainly on the major faults and are mostly shallow, supporting the idea of a brittle elastic lid with hypocentres concentrated towards its base with more ductile deformation in the middle and lower crust. Neotectonic magmatic suites are nepheline-hypersthene normative alkali basalts of mantle origin, and silicic/intermediate/mafic calcalkaline suites, both suites occurring in pull-apart basins in strike-slip regimes and along N-S extensional fissures, and both suites showing a strong change to central activity in the Pliocene. Upper-crustal strains appear to be discontinuous in space and time, with zones of strong shortening representing shoaling of crustal detachment zones flattening between 5 and 10 km. Approximately NW- (dextral) and NE- (sinistral) trending lineaments bound less deformed wedges (low relief seismically ‘dead’ areas) and vary from simple strike-slip faults to complicated braided transform-flake boundaries with pull-apart and compressional segments (N and E Anatolian Transform Faults). Volcanoes lie in grabens on N-S ‘cracks’ that extend into the Arabian Foreland and in transcurrent pull-aparts. Major extensional basins lie at plate (Adana) and flake (Karliova) triple junctions and result from compatibility problems.

The Curvature Primal Sketch

Abstract: In this paper we introduce a novel representation of the significant changes in curvature along the bounding contour of planar shape. We call the representation the Curvature Primal Sketch because of the close analogy to the primal sketch representation advocated by Marr for describing significant intensity changes. We define a set of primitive parameterized curvature discontinuities, and derive expressions for their convolutions with the first and second derivatives of a Gaussian. We describe an implemented algorithm that computes the Curvature Primal Sketch by matching the multiscale convolutions of a shape, and illustrate its performance on a set of tool shapes. Several applications of the representation are sketched.

Self-cleavage of plus and minus RNA transcripts of avocado sunblotch viroid

Abstract: Self-cleavage of both plus and minus RNA transcripts of the 247-residue avocado sunblotch viroid (ASBV), prepared from tandem dimeric cDNA clones, occurs specifically at two sites in each transcript to give monomeric plus and minus species. The cleavage reaction occurs both during transcription and on incubation of purified transcripts at pH 8 and 37 degrees C in the presence of magnesium ions to give a 3'-terminal 2',3'-cyclic phosphate and a 5'-terminal hydroxyl group. Although the self-cleavage occurs at different sites in the ASBV molecule for the plus and minus species, very similar secondary structures with high sequence homology can be drawn at each site. The results are considered to provide further evidence that ASBV is replicated in vivo by a rolling circle mechanism involving non-enzymic cleavage of high molecular weight RNA precursors of ASBV.

Vacuum Noise and Stress Induced by Uniform Acceleration Hawking-Unruh Effect in Rindler Manifold of Arbitrary Dimension

Abstract: This is a comprehensive account of a particular recent development concerning the thermal character inherent in the quantum field as viewed from a uniformly accelerated frame of reference. That is, the power spectrum of the vacuum noise (or the detector-response function) seen by a uniformly accelerated observer in flat spacetimes of arbitrary dimensions is investigated and is shown to exhibit the phenomenon of the apparent inversion of statistics in odd dimensions. Its relation to the thermalization theorem is clarified. Also discussed are the closely related phenomena occurring in the vacuum stress of the Rindler manifold and in the noise seen by a comoving observer in the de Sitter spacetime, as well as those associated with the circular motion in the flat spacetime.

Bibliography and the sociology of texts

Abstract: In Bibliography and the Sociology of Texts, D. F. McKenzie shows how the material form of texts crucially determines their meanings. He unifies the principal interests of both critical theory and textual scholarship to demonstrate that, as all works of lasting value are reproduced, re-edited and re-read, they take on different forms and meanings. By witnessing the new needs of their new readers these new forms constitute vital evidence for any history of reading. McKenzie shows this is true of all forms of recorded information, including sound, graphics, films, representations of landscape and the new electronic media. The bibliographical skills first developed for manuscripts and books can, he shows, be applied to a wide range of cultural documents. This book, which incorporates McKenzie's classic work on orality and literacy in early New Zealand, offers a unifying concept of texts that seeks to acknowledge their variety and the complexity of their relationships.

Privatization in theory and practice

Abstract: Privatization George Yarrow This paper examines the theoretical and empirical case for private versus public ownership. Privatization usually leads managers to place greater emphasis on the pursuit of profits. However, whether this is beneficial to society depends on the trade-off between possible market failures due to a lack of competition and deficiencies in government control of public firms. The competitive and regulatory environment is more important than the question of ownership per se . In competitive markets there is a presumption in favour of private ownership. Where there is a natural monopoly, vigorous regulatory action is required. The evidence suggests that privatization has led to improved performance by firms such as the National Freight Corporation and Cable and Wireless which operate in a relatively competitive environment. The benefits are less clear in the case of firms like BP, Britoil and British Aerospace. However, the most worrying case is that of British Telecom and the projected privatization of British Gas where little thought has been given to limiting the abuse of monopoly power. Rather the regulatory environment has been tailored to the needs of the existing management and to ensuring a successful share flotation. Privatization is also advanced as a weapon for reducing trade union power, encouraging wider share ownership, redistributing wealth and improving the public finances. However, there are other policy instruments better suited to achieving these objectives. The existing privatization programme has not led to a marked widening of share ownership and has resulted in windfall gains to a small group of investors at the expense of taxpayers in general.

Role of the cerebellum in the visual guidance of movement.

Abstract: Mathematicians, control engineers and information technologists are beginning to take a greater interest in neuroscience They are perhaps starting to realize that they may be able to learn a few tricks from nature with which to improve their machines At the same time there is a good chance that neuroscientists will benefit from their input of fresh ideas and techniques with which to attack the problems of understanding neural processing One area of the brain which seems particularly promising in these respects is the cerebellum

High frequencies of alpha-thalassaemia are the result of natural selection by malaria.

Abstract: The frequency of alpha+-thalassaemia, but not other unlinked DNA polymorphisms, exhibits an altitude- and latitude-dependent correlation with malaria endemicity throughout Melanesia, supporting the hypothesis that protection against this parasitic disease is the major factor responsible for the high frequencies of haemoglobinopathies in many parts of the world.

Implicit and explicit knowledge in the control of complex systems

Abstract: These experiments deal with the relation between the ability of a person to carry out efficient action in a situation, and that of the same person to answer questions about the situation. Existing knowledge on the topic has various gaps. In particular it is relatively common to show performance changing without change in verbal knowledge, but relatively rare to show changes in verbal knowledge without change in performance. A second major gap is in defining the kinds of task on which discrepancies may be expected to occur, since in everyday life we do quite frequently expect people to know what they are doing. Accordingly, seven groups of subjects were studied controlling two artificial systems, one concerned with transportation and one with a model of the economy. The discrepancy between verbal knowledge and performance was changed by altering the number or salience of the relationships being learned, and this is at least one factor in making tasks show the discrepancy. Several instances were found of change in verbal question answering opposed to or in the absence of any change of decision performance; thus, the discrepancy is not asymmetric affecting only one measure of learning. This implies that all the variables examined affect processes specific to one or other measure, rather than changing a common database. The results challenge a common view of the discrepancy between performance and verbal accounts, and suggest rather that there are alternative modes of processing in human decision making, each mode having its own advantages.

A geography of dust storms in South-West Asia

Abstract: Mean annual dust storm frequencies are mapped for south-west Asia (Iran, Afghanistan, Pakistan and India) showing areas of greatest activity in two regions: an area at the convergence of the borders of Iran, Afghanistan and Pakistan that comprises the Seistan Basin, Registan and north-western Baluchistan; and the plains of Afghan Turkestan The highest average annual number of dust storm days is 807 at Zabol in Iranian Seistan Little dust-raising activity occurs on the Indian peninsula Dust storms occur principally during the dry season months of spring and summer Local and regional meteorological conditions associated with dust storms are described, ranging from the short-lived dust devil measuring tens of metres across to dust plumes visible on satellite imagery Dust is transported from the region north to Asian states of the USSR, south over the Arabian Sea and east over south-east Asia The importance of soils in the wind erosion system is indicated Dust storms commonly occur on fine-grained material, particularly loess, alluvium, silt, clay and other outwash sediments Dust storms have received a variety of names locally in the region Dust storm frequency during the peak season in northern India is found to be poorly related to mean rainfall, mean wind speed and a climatic wind erosion factor, C developed by Chepil et al (1962)

On the Spatial Spread of Rabies among Foxes

Abstract: We present a simple model for the spatial spread of rabies among foxes and use it to quantify its progress in England if rabies were introduced. The model is based on the known ecology of fox behaviour and on the assumption that the main vector for the spread of the disease is the rabid fox. Known data and facts are used to determine real parameter values involved in the model. We calculate the speed of propagation of the epizootic front, the threshold for the existence of an epidemic, the period and distance apart of the subsequent cyclical epidemics which follow the main front, and finally we quantify a means for control of the spatial spread of the disease. By way of illustration we use the model to determine the progress of rabies up through the southern part of England if it were introduced near Southampton. Estimates for the current fox density in England were used in the simulations. These suggest that the disease would reach Manchester within about 3.5 years, moving at speeds as high as 100 km per year in the central region. The model further indicates that although it might seem that the disease had disappeared after the wave had passed it would reappear in the south of England after just over 6 years and at periodic times after that. We consider the possibility of stopping the spread of the disease by creating a rabies `break' ahead of the front through vaccination to reduce the population to a level below the threshold for an epidemic to exist. Based on parameter values relevant to England, we estimate its minimum width to be about 15 km. The model suggests that vaccination has considerable advantages over severe culling.

Energetics of human muscle: exercise-induced ATP depletion.

Abstract: The energetics of human muscle have been investigated in vivo during and after fatiguing aerobic, dynamic exercise. Changes in cytoplasmic pH and concentrations of phosphocreatine, ATP and Pi were followed using 31P nuclear magnetic resonance spectroscopy. ATP was significantly depleted in 6 out of 12 experiments and in these 6 experiments decreased to 55 +/- 5% of the pre-exercise concentration. Depleted muscle had a lower phosphocreatine concentration (17 +/- 5% of resting value) and lower pH (6.12 +/- 0.04) than fatigued muscle in which ATP loss was not observed (26 +/- 5% for phosphocreatine and 6.37 +/- 0.09 for pH). The free energy of hydrolysis of ATP was not significantly different in the two groups and was also similar in exhausted and nonexhausted muscle. Loss of ATP was associated with altered recovery of the muscle: [phosphocreatine], [Pi], and pH returned more slowly to their pre-exercise values and the initial rate of oxidative phosphorylation was diminished. The restitution of [ATP] to its pre-exercise value was much slower than that of the other metabolites.

Transcriptional interference and termination between duplicated α -globin gene constructs suggests a novel mechanism for gene regulation

Abstract: The interesting possibility that transcriptional interference can occur between eukaryotic genes was raised by studies on the avian leukosis retrovirus (ALV)1 which showed that deletion of the promoter in the 5′ long terminal repeat (LTR) activates the 3′ LTR promoter, linked to a downstream gene. These observations provide a molecular explanation for the fact that insertional oncogenesis by the ALV promoter is invariably associated with either a rearranged or deleted 5′ LTR sequence2‐4. This letter extends these findings to chromosomal RNA polymerase II genes by studying transcriptional interference between duplicated α-globin gene constructions. I demonstrate that transcriptional interference causes substantial inhibition of the downstream α gene by transcription of the upstream α gene. Furthermore, this inhibition is alleviated by placing transcriptional termination signals between the two α genes. Because many eukaryotic genes may be arranged in tandem on a chromosome, these observations suggest that transcriptional termination is an important mechanism for preventing interference between adjacent genes. The selective use of termination signals may provide a novel way of regulating the activity of eukaryotic genes.

The three types of high-mass X-ray pulsator

Abstract: High mass X-ray pulsators are shown to be divisible into three distinct groups on the basis of their observational properties: 1) the Be star systems, 2) non-Be star systems with long pulse periods, (~102 — 103 s), and 3) non-Be star systems with short pulse periods (~100 — 101 s). Differences in mass transfer processes between the groups are invoked to account for the different properties. The separation of high mass pulsators into three clearly defined groups enables some of the properties of a pulsator to be predicted if only a few of its parameters are known. Because of this the optical counterparts of the X-ray sources H0850-42, OAO1653-40 and 1E1048.1-5937 are predicted to be either Be stars or, much less likely, low mass binaries. The orbital (Po) and spin (Ps) periods of the short period systems may be anti-correlated, a phenomenon which is also exhibited by the intermediate polar white dwarf binaries. For the long period systems there is no strong relationship between Po and Ps as there is for the Be star systems. It is shown that for the Be star systems this relationship cannot be accounted for by accretion from a stellar wind if there is equality between the Alfven and corotation radii of the neutron stars in these systems. However, accretion from a non-expanding atmosphere is not excluded. The lack of any correlation between Ps and Po for the supergiant systems is attributed to the low angular momentum of accreted matter. The Be star systems, however, probably accrete significant angular momentum which can result in an accretion disk.

Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic

Abstract: Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian-Toarcian boundary with resolution to the zonal level. The oxygen-isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ13C value of 4·52%PDB is attained. These intervals are known to be favoured periods of organic-rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian-Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’. Some Italian sections spanning this Lower Jurassic interval contain organic-rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDB and, in one section examined in detail, correlate with the calcium-carbonate content of the shales; they may reflect a partial change to a non-calcified planktonic biota during deposition of this lime-poor interval, possibly responding to upwelling and increased fertility of near-surface waters. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.

Allocentric spatial learning by hippocampectomised rats: a further test of the "spatial mapping" and "working memory" theories of hippocampal function.

Abstract: This paper reports a series of three experiments that tested the “spatial-mapping” and “working-memory” theories of hippocampal function. The experimental designs incorporate separate reference- an...

Size and contrast have only small effects on the responses to faces of neurons in the cortex of the superior temporal sulcus of the monkey.

Abstract: There is a population of neurons in the cortex in the middle and anterior part of the superior temporal sulcus (STS) of the monkey with responses which are selective for faces. To investigate whether the responses of these neurons show some of the perceptual properties of face recognition such as tolerance to changes in the size and contrast of the face, the effects of alteration of the size and contrast of an effective face stimulus on the responses of these neurons were analysed quantitatively in macaque monkeys. First, it was shown that the majority of these neurons had responses which were relatively invariant with respect to the size of the stimulus. The median size change tolerated with a response of greater than half the maximal response was 12 times. Second, it was found that for a few of these neurons, the size of the face did affect the neuronal response. For most of these neurons, it was found that when the size of the image and its distance were altered, the neuronal response was related to the retinal angle subtended by the image. But for four neurons the absolute size of the image determined the magnitude of the neuronal response, independently of the distance of the image. Thus these four neurons showed size constancy. It is suggested that these neurons would be useful as part of a face recognition system, because only objects in a certain absolute size range should normally be classified as faces. Third, the responses of the neurons were relatively invariant with respect to the contrast of the face. The mean contrast at which the neurons still responded with more than half the maximal response was 0.26. Fourth, the responses of the neurons were relatively invariant with respect to the sign of the contrast of the face, that is the neurons responded to negative as well as to positive images of faces. Fifth, the neurons typically responded to a face when the information in it had been reduced from 3D to a 2D representation in gray on a monitor, with a response which was on average 0.5 that to a real face. These results show that the responses of these neurons have some of the invariant properties with respect to size and contrast alteration shown by face perception, and show that their processing is at a level which would be useful in face recognition. The results also show that the responses of these neurons are not simply to local contour information.

Evidence from the Parana of south Brazil for a continental contribution to Dupal basalts

Abstract: It has been suggested1,2 that the oceanic upper mantle preserves a large scale isotope anomaly, Dupal2, which may be thousands of millions of years old. New Nd-, Pb- and Sr-isotope data on the continental volcanic rocks of the Parana, south Brazil, reveal ‘enriched’isotopic ratios, with 87Sr/86Sr = 0.705−0.716 and end = −2.5 to −8, similar to those from other continental flood basalt provinces. Their 207Pb/204Pb ratios are higher than those of mid-ocean-ridge basalts (MORB) at comparable 206Pb/204Pb, and even though the basalts appear to have been derived from lithospheric sources within the sub-continental mantle, they preserve isotope and trace element ratios similar to those in oceanic basalts with the Dupal signature in the South Atlantic. The implied link between these continental flood basalts and Dupal oceanic volcanics raises the possibility that in some areas the Dupal anomaly marks a comparatively shallow-level feature in the Earth's mantle.