Stability of Negative Image Equilibria in Spike-Timing Dependent Plasticity

We investigate the stability of negative image equilibria in mean synaptic weight dynamics governed by spike-timing dependent plasticity (STDP). The neural architecture of the model is based on the electrosensory lateral line lobe (ELL) of mormyrid electric fish, which forms a negative image of the reafferent signal from the fish's own electric discharge to optimize detection of external electric fields. We derive a necessary and sufficient condition for stability, for arbitrary postsynaptic potential functions and arbitrary learning rules. We then apply the general result to several examples of biological interest.Comment: 13 pages, revtex4; uses packages: graphicx, subfigure; 9 figures, 16 subfigure

SHRINKiT: Automated measurement of shrinkage limit for clay soils

This report describes the development at the British Geological Survey (BGS) of a new automated apparatus, titled ‘SHRINKiT’, for measuring the shrinkage limit of a clay soil. This has been developed at the BGS’s geotechnical laboratories, in part with NERC Innovations funding. The new method is intended to replace two British Standards (BS1377) methods which have fallen into disuse at BGS, in common with many other laboratories in the UK, and to some extent worldwide, partly as the result of safety concerns in their required use of large quantities of mercury. During the test the cylindrical specimen is allowed to air-dry and shrink. The apparatus carries out repeat measurements of diameter, height and weight; the dimension-measuring component being in effect a simple laser scanner. Each test takes a few days to complete; the duration depending on specimen size, soil type, initial water content and environmental conditions. The output of the test is a plot of water content vs. volume from which the shrinkage limit of the soil can be calculated using the graphical construction method described in BS1377. The shrinkage limit is defined as the water content below which there is little or no further structural shrinkage of the soil. Additional information may also be obtained from the results. The test may be carried out on a wide variety of soil types in an undisturbed, remoulded or compacted state

Continent formation through time

The continental crust is the primary archive of geological history, and is host to most of our natural resources. Thus, the following remain critical questions in Earth Science, and provide an underlying theme to all of the contributions within this volume: when, how and where did the continental crust form? How did it differentiate and evolve through time? How has it has been preserved in the geological record? This introductory review provides a background to these themes, and provides an outline of the contributions contained within this volum

Sleep problems are associated with chronic pain over and above mutual associations with depression and catastrophizing

INTRODUCTION Pain, mood problems and sleeping difficulties are often comorbid and contribute to reduced physical function and quality of life for those suffering from chronic pain. However, the way in which these factors interact is unclear. Until recently it was thought that the effect of sleep on pain and physical function was simply a result of its common association with mood problems. However, a growing body of research suggests that sleep may have a unique contribution. OBJECTIVES The aim of this study was to determine whether sleep disturbances were associated with pain and poor physical function independent of psychological distress in patients attending a tertiary pain medicine unit. METHOD 101 patients with chronic pain completed a set of questionnaires measuring pain, sleep and mood, and also completed a selection of physical assessments conducted by a physiotherapist. RESULTS 75.2% of participants had insomnia and 84.3% reported the presence of at least one sleep problem. Significant positive correlations with pain were detected for depression, catastrophizing, insomnia, short sleep duration and poor sleep quality. Sleep duration had a significant independent association with pain after accounting for depression and catastrophizing. Sleep duration also had an independent association with physical function after accounting for pain and catastrophizing. CONCLUSIONS Given that sleep has an important and unique contribution to pain and physical function, it is important that sleep disturbances are addressed both in the assessment and treatment of chronic pain

Rapid non-contacting resistivity logging of core

We demonstrate a non-contact approach to whole-core and split-core resistivity measurements, imaging a 15 mm-thick, dipping, conductive layer, producing a continuous log of the whole core and enabling the development of a framework to allow representative plugs to be taken, for example. Applications include mapping subtle changes in grain fabric (e.g. grain shape) caused by variable sedimentation rates, for example, as well as the well-known dependencies on porosity and water saturation. The method operates at relatively low frequencies (i.e. low induction numbers), needing highly sensitive coil pairs to provide resistivity measurements at the desired resolution. A four-coil arrangement of two pairs of transmitter and receiver coils is used to stabilize the measurement. One ‘coil pair’ acts as a control, enabling the effects of local environmental variations, which can be considerable, to be removed from the measurement at source. Comparing our non-contact approach and independent traditional ‘galvanic’ resistivity measurements indicates that the non-contact measurements are directly proportional to the reciprocal of the sample resistivity (i.e. conductivity). The depth of investigation is discussed in terms of both theory and practical measurements, and the response of the technique to a variety of synthetic ‘structures’ is presented. We demonstrate the potential of the technique for rapid electrical imaging of core and present a whole-core image of a dipping layer with azimuthal discrimination at a resolution of the order of 10 mm. Consequently, the technique could be used to investigate different depths within the core, in agreement with theoretical predictions

Mineral exploration in the Lower Palaeozoic rocks of south-west Cumbria. Part 1, regional surveys

The results of geochemical, geological and geophysical surveys over Lower Palaeozoic rocks in the south-western part of Cumbria are given in two reports. This report (Part 1) describes the results of a geochemical drainage survey and an examination of mineralised sites, and relates them to information from new geological mapping and an assessment of regional geophysical data. Part 2 contains details of follow-up surveys in the Black Combe inlier. The geochemical drainage survey, involving the collection and analysis of heavy mineral concentrates and stream sediment samples from 119 sites, found substantial antimony, arsenic, barium, bismuth, copper, iron, lead, tin, tungsten and zinc anomalies. Gold was reported for the first time from this part of the Lake District: small amounts were noted in panned concentrates from five sites. Other minerals identified in panned concentrates included arsenopyrite, baryte, bismutite, bismuthinite, cassiterite, chalcopyrite, cerussite, pyrite, pyromorphite, scheelite, sphalerite, stolzite and wolfram&e. The examination of old workings and outcrops revealed many undocumented occurrences of quartzsulphide vein-style mineralisation. The chemical analysis of samples taken from old workings and other occurrences confirmed field observations that locally, particularly in the Black Combe area, this mineralisation is polymetallic with variable amounts of arsenic, gold, bismuth, copper, lead, zinc and in a few cases antimony, barium, cobalt, nickel, tungsten and tin. Iron mineralisation occurs both as oxide (hematite) and sulphide deposits. Mercury was present in appreciable amounts in samples from the High Brow pyrite mine. The distribution of panned concentrate anomalies suggests that the vein-style mineralisation is polyphase and that individual phases may be zoned. Highest zinc anomalies occur near Torver and the highest lead on the west side of Black Combe. Tin and tungsten are restricted largely to the central part of Black Combe, and the most prominent arsenic and bismuth anomalies are found in the same area. Copper anomalies are widespread over the Skiddaw Group and the Borrowdale Volcanic Group. Barium anomalies indicate that baryte mineralisation is weak and localised, occurring principally within the Black Combe area and close to the Windermere Supergroup basal unconformity. Iron oxides from host rocks and hematite mineralisation are responsible for local enrichments of iron, antimony, arsenic and molybdenum in panned concentrates

Synthesis of accelerograms compatible with the Chinese GB 50011-2001 design spectrum via harmonic wavelets: artificial and historic records

A versatile approach is employed to generate artificial accelerograms which satisfy the compatibility criteria prescribed by the Chinese aseismic code provisions GB 50011-2001. In particular, a frequency dependent peak factor derived by means of appropriate Monte Carlo analyses is introduced to relate the GB 50011-2001 design spectrum to a parametrically defined evolutionary power spectrum (EPS). Special attention is given to the definition of the frequency content of the EPS in order to accommodate the mathematical form of the aforementioned design spectrum. Further, a one-to-one relationship is established between the parameter controlling the time-varying intensity of the EPS and the effective strong ground motion duration. Subsequently, an efficient auto-regressive moving-average (ARMA) filtering technique is utilized to generate ensembles of non-stationary artificial accelerograms whose average response spectrum is in a close agreement with the considered design spectrum. Furthermore, a harmonic wavelet based iterative scheme is adopted to modify these artificial signals so that a close matching of the signals’ response spectra with the GB 50011-2001 design spectrum is achieved on an individual basis. This is also done for field recorded accelerograms pertaining to the May, 2008 Wenchuan seismic event. In the process, zero-phase high-pass filtering is performed to accomplish proper baseline correction of the acquired spectrum compatible artificial and field accelerograms. Numerical results are given in a tabulated format to expedite their use in practice

Bose-Einstein condensate collapse: a comparison between theory and experiment

We solve the Gross-Pitaevskii equation numerically for the collapse induced by a switch from positive to negative scattering lengths. We compare our results with experiments performed at JILA with Bose-Einstein condensates of Rb-85, in which the scattering length was controlled using a Feshbach resonance. Building on previous theoretical work we identify quantitative differences between the predictions of mean-field theory and the results of the experiments. Besides the previously reported difference between the predicted and observed critical atom number for collapse, we also find that the predicted collapse times systematically exceed those observed experimentally. Quantum field effects, such as fragmentation, that might account for these discrepancies are discussed.Comment: 4 pages, 2 figure

Plasticity and learning in a network of coupled phase oscillators

A generalized Kuramoto model of coupled phase oscillators with slowly varying coupling matrix is studied. The dynamics of the coupling coefficients is driven by the phase difference of pairs of oscillators in such a way that the coupling strengthens for synchronized oscillators and weakens for non-synchronized pairs. The system possesses a family of stable solutions corresponding to synchronized clusters of different sizes. A particular cluster can be formed by applying external driving at a given frequency to a group of oscillators. Once established, the synchronized state is robust against noise and small variations in natural frequencies. The phase differences between oscillators within the synchronized cluster can be used for information storage and retrieval.Comment: 10 page

Diluted neural networks with adapting and correlated synapses

We consider the dynamics of diluted neural networks with clipped and adapting synapses. Unlike previous studies, the learning rate is kept constant as the connectivity tends to infinity: the synapses evolve on a time scale intermediate between the quenched and annealing limits and all orders of synaptic correlations must be taken into account. The dynamics is solved by mean-field theory, the order parameter for synapses being a function. We describe the effects, in the double dynamics, due to synaptic correlations.Comment: 6 pages, 3 figures. Accepted for publication in PR