Showing papers by "University of Cambridge published in 1985"

The evolution of large-scale structure in a universe dominated by cold dark matter

Abstract: The results of numerical simulations of nonlinear gravitational clustering in universes dominated by weakly interacting, 'cold' dark matter are presented. The numerical methods used and the way in which initial conditions were generated are described, and the simulations performed are catalogued. The evolution of the fundamental statistical properties of the models is described and their comparability with observation is discussed. Graphical comparisons of these open models with the observed galaxy distribution in a large redshift survey are made. It is concluded that a model with a cosmological density parameter omega equal to one is quite unacceptable if galaxies trace the mass distribution, and that models with omega of roughly two, while better, still do not provide a fully acceptable match with observation. Finally, a situation in which galaxy formation is suppressed except in sufficiently dense regions is modelled which leads to models which can agree with observation quite well even for omega equal to one.

A new polynomial invariant of knots and links

Abstract: The purpose of this note is to announce a new isotopy invariant of oriented links of tamely embedded circles in 3-space. We represent links by plane projections, using the customary conventions that the image of the link is a union of transversely intersecting immersed curves, each provided with an orientation, and undercrossings are indicated by broken lines. Following Conway [6], we use the symbols L+, Lo, L_ to denote links having plane projections which agree except in a small disk, and inside that disk are represented by the pictures of Figure 1. Conway showed that the one-variable Alexander polynomials of L+, Lo, L_ (when suitably normalized) satisfy the relation

Actions and habits: the development of behavioural autonomy

Abstract: The study of animal behaviour has been dominated by two general models. According to the mechanistic stimulus-response model, a particular behaviour is either an innate or an acquired habit which is simply triggered by the appropriate stimulus. By contrast, the teleological model argues that, at least, some activities are purposive actions controlled by the current value of their goals through knowledge about the instrumental relations between the actions and their consequences. The type of control over any particular behaviour can be determined by a goal revaluation procedure. If the animal's performance changes appropriately following an alteration in the value of the goal or reward without further experience of the instrumental relationship, the behaviour should be regarded as a purposive action. On the other hand, the stimulus-response model is more appropriate for an activity whose performance is autonomous of the current value of the goal. By using this assay, we have found that a simple food-rewarded activity is sensitive to reward devaluation in rats following limited but not extended training. The development of this behavioural autonomy with extended training appears to depend not upon the amount of training per se, but rather upon the fact that the animal no longer experiences the correlation between variations in performance and variations in the associated consequences during overtraining. In agreement with this idea, limited exposure to an instrumental relationship that arranges a low correlation between performance and reward rates also favours the development of behavioural autonomy. Thus, the same activity can be either an action or a habit depending upon the type of training it has received.

Parental investment and sex differences in juvenile mortality in birds and mammals

Abstract: The common finding that juvenile male mammals show higher mortality than females1,2 is usually attributed either to the expression of deleterious recessive alleles on the X chromosome3 or to adaptive manipulation of the postnatal sex ratio by mothers who are unable to rear successful sons4,5. As a general explanation of differential juvenile mortality, the first of these two theories is unsatisfactory because increased male mortality is known to occur in several bird species where females are the heterogametic sex6,7. The second hypothesis was first proposed by Trivers and Willard4 and suggests that differential mortality should occur early in the period of parental investment8. We now show that, although several predictions of the latter explanation are fulfilled, the distribution of differential mortality in red deer and other mammals suggests that higher mortality rates among male juveniles are a consequence of a greater susceptibility of males to food shortage associated with their faster growth rates and increased nutritional requirements.

The anatomical basis of symptomatic hemidystonia

Abstract: Twenty-eight patients with focal (arm or leg) or hemidystonia due to tumour, arteriovenous malformation, infarction, haemorrhage or hemiatrophy are described. All had typical dystonic movements and/or postures, identical to those seen in idiopathic (primary) torsion dystonia. The site(s) of the lesion responsible, as defined by CT (computerized tomography) scan or pathological examination, was in the contralateral caudate nucleus, lentiform nucleus (particularly the putamen) or thalamus, or in a combination of these structures. Review of 13 other patients in the literature with hemidystonia and lesions defined by CT scan, and of 7 other patients with pathologically discrete lesions associated with hemidystonia, also indicated involvement of these structures. Dystonia may be due to abnormal input from thalamus to premotor cortex, due to lesions either of the thalamus itself, or of the striatum projecting by way of the globus pallidus to the thalamus.

A model for the geometries of Van der Waals complexes

Abstract: Structures of Van der Waals complexes are predicted by a simple model based on electrostatic interaction between the monomers. Point multipoles are assigned to the atoms and embedded in hard sphere...

Comparative effects of cholinergic drugs and lesions of nucleus basalis or fimbria-fornix on delayed matching in rats

Abstract: To provide a more specific test of memory impairments following lesions to central cholinergic systems, rats were trained on an operant delayed matching task. Ibotenic acid lesions of the nucleus basalis produced a disruption of performance at all delay intervals (a parallel downward shift in the delay-performance curve). By contrast, fimbriafornix transections had no effects at short delays, but produced a progressively greater impairment as the delays lengthened (an increased downward slope of the delay-performance curve). Scopolamine produced a dose-dependent disruption of performance, apparent at the shortest delays but greater at longer delays, that was similar to the two lesion deficits combined, whereas physostigmine induced a mild but significant enhancement of performance. The results support the hypothesis that disruption of hippocampal circuitries, including cholinergic afferents via the fimbria-fornix, produces short-term or working memory impairments, whereas disruption of the cortical cholinergic system implicates more stable long-term aspects of task performance. Peripherally administered cholinergic drugs produce both types of effect and thus may influence both systems.

Role of fault bends in the initiation and termination of earthquake rupture.

Abstract: Rupture in individual earthquakes apparently is limited to regions between bends in faults. This is illustrated for eight events that have occurred since 1966. A model based on geometric concepts describes why this is so and clarifies earlier ideas of "asperities" and "barriers" used to explain earthquake initiation and termination processes. Because of their importance in the rupture process, bend zones should be monitored for precursory effects.

A common sequence of calcium and pH signals in the mitogenic stimulation of eukaryotic cells

Abstract: When normal quiescent (G0) cells are stimulated by mitogens to enter the cell cycle, the metabolic derepression which occurs1–8 is similar in a variety of cells. The mechanisms initiating these responses and their relationship to subsequent progression through G1 to DNA synthesis in S phase, however, are generally undefined. The clearest evidence has been obtained in sea urchin eggs9, where fertilization by sperm causes a rapid, transient increase in the concentration of free cytoplasmic Ca2+ ([Ca]i), followed by a sustained increase in cytoplasmic pH (pHi). It has been demonstrated clearly that these ionic responses are obligatory for progression to DNA synthesis by the normal pathway after fertilization, although the Ca2+ signal can be bypassed by parthenogenetic agents which elevate directly pHi (for example, NH+4 ions)9. These observations raise the questions of whether other eukaryotic cells show the same sequence of ionic responses when stimulated by mitogens and whether such signals are an obligatory component of their mitogenic pathways. We show here that a common sequence of [Ca]i and pHi responses occurs in both quiescent mouse thymocytes and Swiss 3T3 fibroblasts stimulated by appropriate mitogens. Furthermore, ‘opportunistic’ mitogens (those that do not act on the cells in vivo, such as concanavalin A (Con A)10, the Ca2+ ionophore A2318711 and 12-o-tetradecanoyl phorbol 13-acetate CTPA)12,13) that are mitogenic for both mouse thymocytes and 3T3 fibroblasts14, each produce characteristic ionic responses that are the same in both types of cell.

The migration of neural crest cells and the growth of motor axons through the rostral half of the chick somite.

Abstract: We have studied the pathway of migration of neural crest cells through the somites of the developing chick embryo, using the monoclonal antibodies NC-1 and HNK-1 to stain them. Crest cells, as they migrate ventrally from the dorsal aspect of the neural tube, pass through the lateral part of the sclerotome, but only through that part of the sclerotome which lies in the rostral half of each somite. This migration pathway is almost identical to the path which presumptive motor axons take when they grow out from the neural tube shortly after the onset of neural crest migration. In order to see whether the ventral root axons are guided along this pathway by neural crest cells, we surgically excised the neural crest from a series of embryos, and examined the pattern of axon outgrowth approximately 24 h later. In somites which contained no neural crest cells, ventral root axons were still found only in the rostral half of the somite, although axonal growth was slightly delayed. These axons were surrounded by sheath cells, which had presumably migrated out of the neural tube, to a point about 50 μm proximal to the growth cones. With appropriate antibodies we found that the extracellular matrix components fibronectin and laminin are evenly distributed between the rostral and caudal halves of the somite. Neither of these molecules therefore plays a critical role in determining the specific pathway of neural crest cells or motor axons through the rostral half of the somite.

Evolution of a single wake behind a pair of bluff bodies

Abstract: In this paper the flow behind a pair of bluff bodies placed side by side in a stream is studied using a variety of flow-visualization methods. Above a critical gap size between the bodies, vortex-shedding synchronization occurs, either in phase or in antiphase. It has previously been assumed that such synchronization forms a wake comprising two parallel vortex streets in phase and in antiphase respectively. In the present paper we find that two antiphase streets are indeed formed, although in-phase shedding leads to the development of a single large-scale wake. The vortices which are formed simultaneously at the cylinders rotate around one another downstream, each pair forming a ‘binary vortex’. The combined wake comprises a street of such vortices, which we term a binary vortex street. Below a critical gap size between the bluff bodies the flow becomes asymmetric. We observe in this regime certain harmonic modes of vortex shedding whereby the shedding frequency on one side of the wake is a multiple of that on the other. Again, a large-scale wake is formed downstream. The present observations lead to a new interpretation of hot-wire-frequency data from other studies in terms of the harmonic modes.

Pathophysiology of blepharospasm and oromandibular dystonia

Abstract: The pathophysiology of reflexes mediated by the fifth and seventh cranial nerves has been studied in 16 patients with blepharospasm and oromandibular dystonia compared with normal age-matched subjects. The EMG activity of the dystonic spasms in the periocular and jaw muscles was similar to that described in other muscles in patients with generalized torsion dystonia. The latency of the R1 and R2 components of the blink reflex and of the corneal reflex was normal. However, the amplitude and the duration of the R1 and R2 and the duration of the corneal reflex were increased. In some patients the R1 component was also present on the side contralateral to the stimulus, while in normal subjects it was present only on the ipsilateral side. The excitability cycle of recovery of the R2 component of the blink reflex after a prior conditioning shock was enhanced in the patients. There were no EEG potentials preceding blepharospasms in the patients, although a Bereitschaftspotential was seen beginning some 500 ms prior to voluntary blinks in the same individuals. Exteroceptive suppression in the contracting masseter and orbicularis oculi muscles was absent in 40 to 50 per cent of the patients. The jaw jerk was present in all the patients with normal latency. These results indicate that the neuronal arcs of the facial reflexes in blepharospasm and oromandibular dystonia are normal. However, there is an abnormal excitatory drive, perhaps from the basal ganglia, to the facial motoneurons and the interneurons which mediate the facial reflexes in the brainstem.

Stochastic Models of Computer Communication Systems

Abstract: (Read before the Royal Statistical Society at a meeting organized by the Research Section on Wednesday, May 8th, 1985, Professor J. B. Copas in the Chair) SUMMARY This paper describes some examples of the stochastic models found useful in the design and analysis of advanced computer and communication systems. Our major theme might be termed the control of contention. As illustrations of this theme we discuss concurrency control procedures for databases, dynamic channel assignment for cellular radio, and random access schemes for the control of a broadcast channel. We emphasize asymptotic properties of product-form distributions and we present some new results on the stability of acknowledgement based random access schemes. This paper is intended to describe to the Society some examples of the stochastic models found useful in the design and analysis of advanced computer and communication systems. The examples chosen are broadly concerned with what might be termed the control of contention, and an attempt has been made to provide enough of the technical background to motivate the models considered. In Section 2 we describe a probabilistic model, due to Mitra (1985), for conflicts anlong tran- sactions in a database. Such conflicts can arise in distributed computer systems, where to ensure the consistency of a database it is often necessary to forbid the concurrent execution of transactions involving common items: a transaction must then contend with other transactions for access to the items it requires. Mitra (1985) has shown that his model can be used to answer some important design questions concerning concurrency control procedures which use exclusive and non-exclusive locks. Mitra's results are based upon a product-form solution; we indicate how his asymptotic formulae can be extended beyond the range of light traffic and the assumption of an unstructured database. In Section 3 we discuss one of the many interesting problems which arise in connection with cellular radio. Cellular radio makes efficient use of a limited number of radio channels by allowing the repeated reuse of each channel in sufficiently separated spatial regions. The topic we consider is contention between different regions for the use of dynamically assigned channels. Everitt and Macfadyen (1983) have described an analytically tractable method of dynamic channel assign- ment, which they term the maximum packing strategy. Again a product form solution is involved: from this it is easy to obtain asymptotic formulae applicable in light traffic. These formulae establish the advantage of the strategy over a fixed channel assignment for low enough loss pro- babilities, but the advantage disappears as traffic and the number of channels increase. The real potential of dynamic schemes is their ability to cope automatically with traffic intensities which fluctuate in space and time.

On the Functions of Play and Its Role in Behavioral Development

Abstract: Publisher Summary This chapter describes the limited empirical evidence concerning the functions of play and suggests some possible reasons, both theoretical and methodological, as why evidence in support of functional hypotheses has not been forthcoming. It describes certain assumptions about play, which pervade the biological literature, for example, that early play experience is crucial for the normal development and that play is a very costly activity and so must have major benefits. It is analyzed that these assumptions are mostly without empirical support and reflect an underlying view of behavioral development that is highly questionable. Criticisms of current explanations of play are presented in the chapter in a constructive spirit. Play remains a biological enigma, which demands closer scrutiny. At present, there is no direct evidence that play has any important benefits, with the possible exception of some immediate effects on children's behavior. The problem of confounding variables and the need for multiple, sensitive, and functionally relevant outcome measures in future experiments are discussed.

A Temporal Logic for Multilevel Reasoning about Hardware

Abstract: Because digital systems operate over time, hardware descriptions should be based on formalisms suited to temporal reasoning. One such notation, interval temporal logic, offers a natural basis for the specification of devices and digital signals. As computer systems continue to grow in complexity, the distinction between hardware and software is becoming increasingly blurred. This situation has produced an increasing awareness of the need for behavioral models suited to specifying and reasoning about both digital devices and programs. Contemporary hardware description languages (for example, Barbacci, Parker and Wallace,2 and Su et al. 3) are not sufficient because of various limitations:

The secular variation of Earth's magnetic field

Abstract: Models of the magnetic field at the core–mantle boundary at selected epochs from 1715.0 to 1980.0 reveal novel features in the field at the core. These suggest that the flow of core fluid is coupled to the mantle, and that magnetic diffusion is significant.

Effect of spin fluctuations on the magnetic equation of state of ferromagnetic or nearly ferromagnetic metals

Abstract: The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni3Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (Tc), a nearly linear inverse susceptibility well above Tc, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni3Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature Tc, for the ratio peff/p0 of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T2) variation of the magnetisation with temperature well below Tc, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.

Thermophoretic deposition of particles in gas flowing over cold surfaces

Abstract: The rate at which particles carried in a flowing gas are deposited on a cold surface by thermophoretic movement is studied theoretically. Some exact deductions from the coupled equations for the temperature ( T ) and particle concentration ( C ) in the moving fluid show that, in cases in which the particles come from a region in which T and C are uniform with values T 0 and C 0 , there are strong constraints on the possible values of C / C 0 at a point at which T / T 0 is known. This suggests the hypotheses that C / C 0 is approximately uniform (with value C 1 / C 0 say) over a cold isothermal boundary (at temperature T 1 ), in which event the total particle flux to the boundary is proportional to the total heat flux to the boundary, and that the relation between C 1 / C 0 and T 1 / T 0 is approximately the same for all flow systems. These hypotheses are tested against the available numerical results for a number of steady flow systems, four of them relating to cold bodies in a uniform stream of gas and two relating to flow in a circular tube downstream from a sudden drop in wall temperature, and are found to represent the exact results with adequate accuracy except when both T 1 / T 0 and the thermophoretic coefficient are small. We thus have a means of estimating the total rate of deposition on a cold isothermal boundary from a knowledge of the total heat flux to the boundary in cases in which computation of the distributions of temperature and particle concentration in the fluid is not feasible, although further tests of the hypotheses are desirable.

Cell-mediated damage to helminths.

Abstract: Although it is difficult to draw any sweeping conclusions that would be applicable to all helminth infections, the main features that are emphasized in this review may be summarized briefly. Pathogenic helminths, although extremely diverse in structure and behaviour, have one common feature, namely that they present to the host's defenses large, non-phagocytosable surfaces. Because of this, they are susceptible to a range of effector mechanisms differing either quantitatively or qualitatively from those that are active against other parasites or against normal or abnormal host cells. As an extreme example, the various types of cytotoxic lymphocyte, with one interesting exception, are inactive against helminths. Instead, helminth infections are characterized by high IgE responses and increased numbers of circulating eosinophils. Such eosinophils are activated, and show a marked capacity to kill a variety of target helminths in vitro. Further activation may occur in response to mast cell mediators released as a result of IgE-dependent degranulation; and IgE, as well as IgG and complement, can mediate eosinophil attachment and killing. It may therefore be suggested that the eosinophil/IgE/mast cell axis represents a powerful host defense against helminth infections. IgE can also mediate macrophage-dependent killing of several helminths, a process which involves a functional change in the macrophage, resembling activation. Although eosinophil-mediated and IgE-dependent macrophage-mediated effects are particularly potent, other effector cells are not excluded: in certain circumstances, neutrophils and conventionally activated macrophages may be equally or more effective. Neutrophils appear to act solely by oxidative killing mechanisms, whereas degranulation and the release of toxic granule contents is equally or more important in eosinophil-mediated damage. Different stages of different helminths vary in their degree of susceptibility to different mechanisms. Eosinophils appear to be somewhat less active than neutrophils against ensheathed nematodes, whereas trematodes and exsheathed nematodes are highly susceptible to eosinophil attack. In many experimental helminth infections, studies in vivo suggest a role for antibody-dependent cell-mediated immune effector mechanisms. The identity of the effector cell is difficult to establish because of a lack of techniques for specific manipulation of individual cell types, but histological studies frequently point to a strong eosinophil or macrophage involvement. The development and analysis of in vitro assays allows the study of immune effector mechanisms in man.(ABSTRACT TRUNCATED AT 400 WORDS)

Sinusoidal flow relative to circular cylinders

Abstract: The motions of vortices around single cylinders and around pairs of cylinders in relative sinusoidal flow are investigated in this paper. Using simultaneous flow visualization and force measurements, the vortex motions are related to the fluid-induced lift and in-line forces. For the single cylinder, several repeatable patterns of vortex shedding are identified within particular ranges of flow amplitude. The process of pairing of vortices from a previous half cycle with those in a present half cycle is fundamental to all the patterns. Visualization is shown to be more effective in a reference frame which is fixed with respect to the undisturbed fluid rather than with respect to the cylinders. For this reason, the examples of vortex motions are taken from a rig in which vertical cylinders are oscillated in a tank of fluid. By oscillating a pair of cylinders over a range of gaps, orientations and amplitudes, it is found that the vortex-shedding patterns identified for a single cylinder can synchronize either in phase or in antiphase between the two cylinders. Such observations help to explain how lift and in-line forces are influenced by cylinder proximity and in some cases these forces are significantly magnified. Force coefficients are evaluated for both the single cylinder and the pair of cylinders.