Showing papers by "University of Sussex published in 1983"

Path integral approach to quantum Brownian motion

Abstract: We apply the influence-functional method of Feynman and Vernon to the study of Brownian motion at arbitrary temperature. By choosing a specific model for the dissipative interaction of the system of interest with its environment, we are able to evaluate the influence functional in closed form and express it in terms of a few parameters such as the phenomenological viscosity coefficient. We show that in the limit h→0 the results obtained from the influence functional formalism reduce to the classical Fokker-Planck equation. In the case of a simple harmonic oscillator with arbitrarily strong damping and at arbitrary temperature, we obtain an explicit expression for the time evolution of the complete density matrix ϱ(x, x′, t) when the system starts in a particular kind of pure state. We compare our results with those of other approaches to the problem of dissipation in quantum mechanics.

Landmark learning in bees

Abstract: 1. The experiments described here were undertaken to discover how bees use nearby landmarks to guide their way to a food source. Two major questions are raised. First, what do bees learn about the spatial layout of landmarks and food source? Secondly, how might this information help them reach their destination? 2. Single, marked bees were trained to collect sugar solution from a small and inconspicuous reservoir in a room in which extraneous visual cues had been reduced to a minimum. The position of the reservoir was defined by an array of one or more matt black landmarks. After bees had been trained, their flight path was recorded on videotape when the landmarks were present, but the food source absent. During such tests bees spent most of their time searching where the food source should have been. 3. Thus, if bees were trained to a reservoir whose position was specified by a single cylindrical landmark and tested with the same landmark, they searched at the expected site of the reservoir. However, when the size of the landmark was changed between training and testing, the area in which bees searched was displaced to one where the landmark appeared roughly the same size as the training landmark when viewed from the reservoir. These experiments suggest that bees learn no more than the apparent size and bearing of the landmark as seen from the food source, and that to return there they move to a position where their retinal image matches their remembered image of the landmark. 4. Experiments with more complex arrays of landmarks support the same hypothesis. A simple rule predicts a bee's search area when it is trained to a food source defined by the position of three landmarks and tested either with the same array, or with landmarks of different sizes, or with landmarks placed at different distances from the reservoir. The bee then always searches where the compass bearings of the landmarks on its retina were the same as they had been when it was stationed at the food source. 5. Tests with bees trained to either one or three landmarks suggest that the bearings of landmarks on the retina are learnt with respect to external compass bearings. Thus, a single, cylindrical landmark does not define direction. Nonetheless, bees searched in one location and not in a circle centred on the landmark. Bees trained to three landmarks only learnt the site of the reservoir if the array was kept in a constant orientation during training. 6. Computer models were devised to discover how bees might use a remembered image of the landmark array to direct their flight path to their destination. The models simulated a situation in which a bee takes a 2-dimensional snapshot of its surroundings from the position it wishes to retrieve and continuously compares this with its current retinal image. It then uses the difference between the two to guide its way. Different models of increasing complexity were explored until one was found which closely mimicked the bee's behaviour.

The Economic Analysis of Technological Change

Abstract: This book presents a comprehensive view of the economic approach to the analysis of technical change which is an increasingly important sector of economic theory. It is in three parts: the first on the generation of new technology, th second on its spread or diffusion, and the third on the impact of new technology.

Human visual pigments: microspectrophotometric results from the eyes of seven persons.

Abstract: The material for this work was obtained from seven eyes removed because of malignant growths. Foveal and parafoveal samples of the retinas were taken and transverse measurements were made of the absorbance spectra of the outer segments of the rods and cones, using a Liebman microspectrophotometer. Four kinds of spectra were obtained with absorbance peaks at the following wavelengths: rods, 496.3 +/- 2.3 nm (n = 39); red cones, 558.4 +/- 5.2 nm (n = 58); green cones, 530.8 +/- 3.5 nm (n = 45); blue cones, 419.0 +/- 3.6 nm (n = 5). The distribution of the peaks was unimodal for the rods. For the red and green cones, however, there was evidence for bimodal distributions, with sub-population maxima at 563.2 +/- 3.1 nm (n = 27) and 554.2 +/- 2.3 nm (n = 31) for the reds and at 533.7 +/- 2.1 nm (n = 23) and 527.8 +/- 1.8 nm (n = 22) for the greens. A substantial difference in mean spectral location of the red cones was observed between patient 1 (561 nm) and patient 4 (553 nm). Both patients were classified as normal trichromats by all clinical tests of colour vision but there was a clear difference in their relative sensitivities to long-wave fields. In both direction and magnitude, this difference proved to be that required by the microspectrophotometric results.

The population dynamics of transposable elements

Abstract: This paper describes analytical and simulation models of the population dynamics of transposable elements in randomly mating populations. The models assume a finite number of chromosomal sites that are occupable by members of a given family of elements. Element frequencies can change as a result of replicative transposition, loss of elements from occupied sites, selection on copy number per individual, and genetic drift. It is shown that, in an infinite population, an equilibrium can be set up such that not all sites in all individuals are occupied, allowing variation between individuals in both copy number and identity of occupied sites, as has been observed for several element families in Drosophila melanogaster. Such an equilibrium requires either regulation of transposition rate in response to copy number per genome, a sufficiently strongly downwardly curved dependence of individual fitness on copy number, or both. The probability distributions of element frequencies, generated by the effects of finite population size, are derived on the assumption of independence between different loci, and compared with simulation results. Despite some discrepancies due to violation of the independence assumption, the general pattern seen in the simulations agrees quite well with theory.Data from Drosophila population studies are compared with the theoretical models, and methods of estimating the relevant parameters are discussed.

Varietal differences in the toxicity of sodium ions in rice leaves

Abstract: Visual damage is commonly used as the criterion for assessment of salinity resistance in rice breeding programmes. The use of other indicators, such as sodium uptake, is being evaluated: a correlation between initial sodium uptake to the third leaf and varietal survival was established and the physiological basis of this examined. Chlorophyll was used as an indicator of metabolic status and the relationship between leaf sodium and chlorophyll concentrations investigated for nine varieties differing in their resistance to salinity. By sampling a population of leaves having a wide range of salt concentrations, inverse relationships between chlorophyll and sodium concentrations were established. The salt concentration in the leaf tissue which causes equivalent toxicity (50% loss of chlorophyll) differed 3-fold amongst these varieties. Varieties showing the greatest tolerance to salt within the leaves were not necessarily those showing the greatest overall phenotypic resistance to salinity. The enzymes malate dehydrogenase and nitrate reductase were activated equally by sodium and potassium ions at 60–80 mol m−3 in both control and saline grown plants and severely inhibited at higher concentrations. If all the salt in the tissue of leaves with 50% chlorophyll remaining was in solution and uniformly distributed the concentration would be 135–500 mol m−3. This is improbable and some level of compartmentation is likely. It is concluded that while initial sodium uptake has predictive value for salinity resistance of varieties, there are other characteristics which are masked by excess salt entry and require independent assessment; no single factor confers resistance.

The use of foreign patenting as an internationally comparable science and technology output indicator

Abstract: Foreign patenting activity in some of the world major patent systems is being compared between countries and industries and is found to be, with a few notable exceptions, relatively unbiased. Furthermore, a brief dynamic analysis of the foreign patenting activity in the USA of a number of OECD-countries in 41 industrial sectors in terms of Revealed Technological Advantage indices suggests that foreign patent data might provide a very useful addition to the arsenal of Science and Technology Output Indicators.

Sexual Selection In A Hermaphroditic Plant

Abstract: The ornaments and weapons of male animals are among the showiest and seemingly most wasteful of nature's productions. Darwin's theory of sexual selection showed how such traits could be selected through competition for mates even if they were otherwise detrimental1. Flowering displays of plants often show a comparable degree of gaudiness and profligacy, but exploration of the role of sexual selection in plants has only just begun2–5. Bateman6 argued that sexual selection is caused by the greater ability of males than females to increase fitness by mating repeatedly, due to the females' greater energetic commitment to gametes or parental care. Similar reasoning applies to hermaphrodites2,3. In hermaphroditic milkweeds, most young fruits are aborted7–10 and female reproduction (seeds) is limited more by resources than by pollination11. Sexual selection theory therefore predicts that traits increasing mating success will have evolved because they increase male success through pollen. Here I report that a suite of floral traits of a hermaphroditic plant is best interpreted as having evolved through the male competition component of sexual selection, a result with important implications for evolutionary studies of pollination systems.

The Chemistry of Nitrogen Fixation and Models for The Reactions of Nitrogenase

Abstract: Publisher Summary This chapter discusses the chemistry of nitrogen fixation and models for the reactions of nitrogenase The interest in nitrogen fixation for the inorganic chemist is to try to understand, using simple compounds, what nature does so comparatively effortlessly within the enzyme, nitrogenase This is of particular value considering the increasing demand for nitrogenous fertilizers, and the vast industrial expenditure of energy in producing ammonia The literature of chemical nitrogen fixation, even excluding that related to the Haber process, is now considerable The mechanisms of reaction of coordinated dinitrogen are a matter of dispute There are several proposals extant, chemical and biological, and these are espoused by their progenitors with varying degrees of fervor However, there is now sufficient chemistry available for us to make preliminary judgments concerning the validity of the various proposals

Partial bivoltinism may cause alternating sex-ratio biases that favour eusociality

Abstract: Fisher1showed that frequency-dependent selection will stabilize balanced sex ratios of parental investment2, under a wide range of mating systems and patterns of mortality. But many actual ratios of investment are biased. Some of these can be explained by models3–14in which the life history lacks certain kinds of symmetry that are present in Fisher's model. For example, where brothers compete directly for matings, female biases are expected and found3,9. Here I describe a model for the evolution of sex ratios in species having two generations per year with an asymmetrical pattern of overlap between the generations. Many insects have partially bivoltine life histories of this kind. Two predictions emerge from an analysis of the model. First, one of the two generations produced each season will be female-biased, and the other will be male-biased. Second, eusociality will arise more often in haplodiploid species that overwinter as inseminated females than it will in those that overwinter as larvae or as unmated adults. Data on the sex ratios of bivoltine solitary Hymenoptera and on the phylogenetic distribution of eusociality are consistent with these predictions. Thus the model shows one particular way in which ecological and genetic variables may interact to encourage the evolution of eusociality in some species and to discourage it in others.

Positive control and autogenous regulation of the nifLA promoter in Klebsiella pneumoniae.

Abstract: The nitrogen fixation (nif) genes of Klebsiella pneumoniae are specifically regulated by the products of the nifLA operon. We have located the promoter of this operon, and identified sequences required for nifLA transcription. Transcription from this promoter is shown to be positively regulated by the ntrC gene product (which coordinates the expression of many operons required for nitrogen assimilation) and also autogenously by the product of the nifA gene.

Multi-oscillatory control of circadian rhythms in human performance

Abstract: Circadian rhythms are known to exist in many measures of human performance efficiency as well as in physiological processes1. The demands of a task, and in particular its ‘working memory’2 load, play a large part in determining the time of day at which it is best performed3. Furthermore, task demands may affect the speed with which performance rhythms adjust to the altered sleep/wake schedules occasioned by shift-work and rapid time-zone transitions4. These differences in rate of adjustment may be explained by a similar multi-oscillatory model to those proposed for physiological rhythms5–10. These assume any given circadian rhythm to be jointly controlled by two endogenous oscillators. The first is thought to be relatively immune to exogenous factors and to control the temperature rhythm, while the second is thought to be more influenced by exogenous factors and to have the major role in governing the sleep/wake cycle. Normally, the pronounced 24-h time cues, or ‘Zeitgebers’, in our environment result in both oscillators, and hence all circadian rhythms, running with a period of 24 h. However, under altered sleep/wake schedules, and in conditions of temporal isolation, the temperature rhythm and sleep/wake cycle may separate from one another and run with distinctly different periods. When such ‘internal desynchronization’ occurs, other physiological rhythms have been found to run in synchrony with one or other of these two functions. This finding forms the basis of current multi-oscillatory models. However, studies of abnormal sleep/wake schedules suggest that the rhythm in working memory performance may sometimes separate from both the sleep/wake cycle and temperature rhythm by running with a period of less than 24 h11. We have investigated this possibility here and our results indicate control of working memory performance rhythms by a previously unidentified oscillator with an autonomous period of about 21 h.

Models of evolution

Abstract: Different ways in which evolution can be modelled will be reviewed. Two asexual models -- \`Muller's ratchet' and a model due to Eigen and Schuster -- both lead to the conclusion that the accuracy of replication must reach a limiting value, but the details are different. In classic population genetics models, difficulties arise if fitnesses depend on interactions with others. Two approaches -- \`trait group' methods, and game theory -- are discussed. If the interacting individuals are relatives, there is again a choice between the exact \`neighbour-modulated fitness' approach and the more intuitive \`inclusive fitness' method. A more drastic change in the nature of the model arises if the units of the evolving system are not individual organisms, but either genes or species. There are conceptual difficulties which must be clarified before species selection can be analysed mathematically.

Chaos in the mixmaster universe

Abstract: A four-dimensional system of nonlinear difference equations describing the evolution of the general relativistic "mixmaster" cosmological model is studied. The evolution splits into two parts: one given by a (chaotic) generalized Baker's transformation and the other by a (systematic) change of scales. The chaotic degrees of freedom are studied in detail: Their evolution may be described by a two-sided shift and the invariant measure for these variables is found. Strong ergodic properties (mixing, nonzero entropy) are deduced.

Prosodic marking in speech repair

Abstract: Spontaneous self-corrections in speech pose a communication problem; the speaker must make clear to the listener not only that the original Utterance was faulty, but where it was faulty and how the fault is to be corrected Prosodic marking of corrections making the prosody of the repair noticeably different from that of the original utterance offers a resource which the speaker can exploit to provide the listener with such information A corpus of more than 400 spontaneous speech repairs was analysed, and the prosodic characteristics compared with the syntactic and semantic characteristics of each repair Prosodic marking showed no relationship at all with the syntactic characteristics of repairs Instead, marking was associated with certain semantic factors: repairs were marked when the original utterance had been actually erroneous, rather than simply less appropriate than the repair; and repairs tended to be marked more often when the set of items encompassing the error and the repair was small rather than when it was large These findings lend further weight to the characterization of accent as essentially semantic in function 1 Some determinants of intonational marking in self-corrections At least two people are in trouble when a speaker interrupts the flow of speech in order to make a self-correction The first person is the speaker himself* who apparently became aware of some unclarity or error in what he just said The second person is a listener who is confronted with an abrupt break, and with the task to find out whether what is going to follow is just a continuation, as after a mere hesitation, or whether it is a repair of something said previously In the latter case, moreover, she has to find out what the reparandum is, and to replace it by the appropriate items in the correction This will be called the listener's continuation problem * For ease of reference we will in the following treat the speaker, ie the trouble maker, as male and the listener, ie the victim, as female JOURNAL OF SEMANTICS, vol 2, no 2, pp 205-217 205

Origin of the receptor potential in inner hair cells of the mammalian cochlea--evidence for Davis' theory.

Abstract: Origin of the receptor potential in inner hair cells of the mammalian cochlea—evidence for Davis' theory

Nitrogen control of the nif regulon in Klebsiella pneumoniae: involvement of the ntrA gene and analogies between ntrC and nifA.

Abstract: The ntrC and nifA gene products of Klebsiella pneumoniae are transcriptional activators involved in general nitrogen control and nif-specific regulation, respectively. Multicopy plasmids expressing either ntrC or nifA from a foreign promoter were used to study the relationship between these two genes and ntrA. The nifA product substituted for ntrC product in activation of a number of genes including nifLA, hutUH and genes for arginine and proline utilisation. NtrC could not substitute for nifA in transcriptional activation of the nifHDKY operon. In ntrA- strains, neither the ntrC nor the nifA product functioned to activate transcription of nif promoters. In vitro transcription/translation studies with plasmid clones demonstrated similar levels of expression of ntrC and nifA in ntr+ and ntrA- S-30 extracts. Hence, lack of activator function in an ntrA mutant indicates that both the ntrC and nifA products require a functional ntrA for activity. When expressed from foreign promoters, both the ntrC and nifA products were active in conditions which would normally repress nif expression. Hence, the ntrA product was apparently not limiting in these conditions.

Frequency tuning in a frog vestibular organ.

Abstract: Several distinct mechanisms have evolved in the auditory periphery to extract frequency information from a sound. In the mammalian cochlea, a travelling wave on the basilar membrane enhanced by a physiologically vulnerable neuromechanical interaction performs the primary frequency separation. In lizards, tuning is likely to depend on structures in the papilla other than the basilar membrane, and tuning in the auditory nerve is correlated with the length of the stereocilia. In turtles and possibly some bird species, an electrical resonance in the receptor cells is responsible for frequency selectivity. In addition to those organs obviously specialized to detect acoustic stimuli, afferents of the vestibular system can exhibit tuning to low-frequency airborne sounds, despite the absence of mechanical frequency separation by accessory structures. I report here that in the frog saccule, a vestibular organ apparently constructed for the detection of vibratory accelerations, frequency tuning may arise from an electrical resonance intrinsic to the hair cells. The mechanism is similar to that found in turtle and ensures that a stimulus with frequency corresponding to the membrane resonant frequency will produce the largest signal in the cell. This type of tuning may thus be quite widespread. Oscillatory mechanisms have been reported in sensory cells of other modalities in several lower vertebrates, and may even contribute to their sensitivity, although such mechanisms do imply that the signal-to-noise ratio is degraded near threshold.

The development of the 'participant observation' method in sociology: origin myth and history

Abstract: Common understandings about the ancestry of “participant observation” as now defined are historically misleading. In the prewar texts taken as ancestors, access to meanings was not associated with participation per se, and where participation was used data might be quantified and were not distinguished from data from other sources; the term “participant observation” was not widely current, and only gradually developed its present meanings. Methods are defined in relation to the perceived alternatives, and it was only in the 1940s that the current set of alternatives emerged. To understand earlier writers' conceptions, their work must be related to its context.

Vibration control of multi-mode rotor-bearing systems

Abstract: This paper presents a computationally fast and efficient least-squares method to minimize the vibration of any general rotor-bearing system by the application of external control forces. The D-optimality concept is used to optimize the force locations. The proposed method provides a wide range of statistical information, and the sensitivity of the optimum response to changes in the control forces. Magnetic bearings can be applied to implement the open-loop adaptive vibration control strategies outlined in the paper. These components can also be used to inject a multi-frequency test signal as required for identi­fication studies.

Applicability of Weibull analysis for brittle materials

Abstract: Weibull analysis for the interpretation of strengths of brittle materials was previously justified for a particular flaw size distribution. The present results show that the Weibull distribution provides a close approximation to the distribution of failure stress for all the flaw size distributions considered. However certain reservations are noted in the interpretation of the Weibull modulus.