Cluster Derivation of the Parisi Scheme for Disordered Systems

We propose a general quantitative scheme in which systems are given the freedom to sacrifice energy equi-partitioning on the relevant time-scales of observation, and have phase transitions by separating autonomously into ergodic sub-systems (clusters) with different characteristic time-scales and temperatures. The details of the break-up follow uniquely from the requirement of zero entropy for the slower cluster. Complex systems, such as the Sherrington-Kirkpatrick model, are found to minimise their free energy by spontaneously decomposing into a hierarchy of ergodically equilibrating degrees of freedom at different (effective) temperatures. This leads exactly and uniquely to Parisi's replica symmetry breaking scheme. Our approach, which is somewhat akin to an earlier one by Sompolinsky, gives new insight into the physical interpretation of the Parisi scheme and its relations with other approaches, numerical experiments, and short range models. Furthermore, our approach shows that the Parisi scheme can be derived quantitatively and uniquely from plausible physical principles.Comment: 6 pages, 3 figures, proceedings of international conference on "Disordered And Complex Systems", 10-14 July 2000 King's College Londo

Solvable Lattice Gas Models of Random Heteropolymers at Finite Density: II. Dynamics and Transitions to Compact States

In this paper we analyse both the dynamics and the high density physics of the infinite dimensional lattice gas model for random heteropolymers recently introduced in \cite{jort}. Restricting ourselves to site-disordered heteropolymers, we derive exact closed deterministic evolution equations for a suitable set of dynamic order parameters (in the thermodynamic limit), and use these to study the dynamics of the system for different choices of the monomer polarity parameters. We also study the equilibrium properties of the system in the high density limit, which leads to a phase diagram exhibiting transitions between swollen states, compact states, and regions with partial compactification. Our results find excellent verification in numerical simulations, and have a natural and appealing interpretation in terms of real heteropolymers.Comment: 12 pages, 8 eps figures, revised version (to be published in EPJ

The Psycho-Social Sequelae of a Termination of Pregnancy for Fetal Abnormality

Although all couples who embark on a pregnancy hope for a normal child, about 2% will have an abnormal outcome. For a proportion of these couples the abnormality will be detected prenatally and they may elect for termination of pregnancy. In Scotland there are about 200 terminations of pregnancy (TOP) for fetal abnormality (FA) each year and for each couple this represents one of the most difficult times of their lives. This project undertook to examine the current management of TOP for fetal abnormality to identify if there was scope for improvement. The study consisted of two parts; management aspects of detection and termination for neural tube defects (NTD), and a detailed assessment of sequelae of TOP for various FA. In the NTD study, women who had elected for TOP were identified from the West of Scotland Regional Genetics Service. Although the majority (137, 82%) of consultands were satisfied with the way that prenatal diagnosis was handled, more than one third of the women felt ill-prepared for the TOP and entered hospital with no conception of the procedure or timescale of the termination. Care during the termination procedure was perceived as very good by most (126, 76%) and 95% (158) found the staff attending them kind and sympathetic. After the TOP, many (52, 31%) were nursed in the post-natal ward, often in a side room but usually within hearing distance of the newborn nursery and found this very upsetting. On leaving hospital, the post-termination sequelae were discussed or mentioned to only 25 (15%) patients. This left 135 (81%) confused and bewildered by the postpartum reactions of their bodies. The second study was intended; to monitor sequelae, to investigate in more detail why couples perceived aftercare as inadequate, to consider if this is related to specific factors, to ascertain which groups had particular difficulties in coming to terms with the experience and finally, to assess the effect of the TOP for FA on family planning and further reproduction. Common difficulties were encountered in several areas; pain and fear during the termination procedure, the decision whether to see the fetus; the lack of medical and psychosocial advice and support in the post-termination time on leaving hospital; the perceived lack of understanding of the sequelae; and the consultands' own reticence to voice their needs, fears and worries. 24 (28%) women experienced physical pain and 38 (45%) felt very frightened during the termination procedure. 16 (19%) mothers and 18 (26% of 68) husbands (partners) saw the fetus after the delivery and after two years none regretted the decision. This was in contrast to 38 mothers (55% of 68) who did not look. Consultands who had TOP for Trisomy 21, Cystic Fibrosis, or Duchenne Muscular Dystrophy preferred to maintain an abstract notion of the fetus even on reflection two years later. Of the women who had been too frightened to look at the fetus after the termination because there were obvious malformations, 99% regretted the decision. Reassurance and detailed information were given as the most helpful aspects of genetic counselling by 45 (56% of 80). The ideal time for such a consultation was given as 4 to 6 weeks. For 36 (42%) of consultands there had been doubt before, during and just after the TOP about their decision to abort, and after two years 2 women and 3 men still regretted the decision. 81 (96%) would in principle accept prenatal diagnosis in a subsequent pregnancy, and in fact all 55 who had another pregnancy, had prenatal diagnosis. 82% achieved or were trying to achieve their wished for family size, those who were not sure about PND gave up progeny rather than having a pregnancy without testing for fetal disorders. The 15 couples (18%) who experienced reproductive conflict included several unexpected diagnoses (which differed from the one for which they had sought PND), and the older age-group. All these patients had surviving children. Both studies demonstrate scope for improvement in patient management following TOP for FA. The recurring theme which was present in all sections we examined was impaired communication at all levels and a lack of understanding of the psychological sequelae after this traumatic event. Improved education of those involved in the care and support of these families and more information for those in close contact with the couple could help to reduce excessive anxieties and this might well be an area in which a self-help group will be of particular benefit. (Abstract shortened by ProQuest.)

Disentangling causal webs in the brain using functional Magnetic Resonance Imaging: A review of current approaches

In the past two decades, functional Magnetic Resonance Imaging has been used to relate neuronal network activity to cognitive processing and behaviour. Recently this approach has been augmented by algorithms that allow us to infer causal links between component populations of neuronal networks. Multiple inference procedures have been proposed to approach this research question but so far, each method has limitations when it comes to establishing whole-brain connectivity patterns. In this work, we discuss eight ways to infer causality in fMRI research: Bayesian Nets, Dynamical Causal Modelling, Granger Causality, Likelihood Ratios, LiNGAM, Patel's Tau, Structural Equation Modelling, and Transfer Entropy. We finish with formulating some recommendations for the future directions in this area

A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers

We propose and solve a simple model describing secondary structure formation in random hetero-polymers. It describes monomers with a combination of one-dimensional short-range interactions (representing steric forces and hydrogen bonds) and infinite range interactions (representing polarity forces). We solve our model using a combination of mean field and random field techniques, leading to phase diagrams exhibiting second-order transitions between folded, partially folded and unfolded states, including regions where folding depends on initial conditions. Our theoretical results, which are in excellent agreement with numerical simulations, lead to an appealing physical picture of the folding process: the polarity forces drive the transition to a collapsed state, the steric forces introduce monomer specificity, and the hydrogen bonds stabilise the conformation by damping the frustration-induced multiplicity of states.Comment: 24 pages, 14 figure

Restoration of former agricultural fields on acid sandy soils:Conversion to heathland, rangeland or forest?

Three restoration strategies on agricultural fields with acid sandy soil were evaluated after 18 and 25 years: conversion to heathland, rangeland and forest. Changes in soil microstructure, chemical characteristics, availability of N and P, and vegetation composition were analyzed in agricultural soils, three undisturbed reference sites and five types of restored former agricultural fields. Agricultural soils were characterized by organic slurry without much soil life. Soil nutrients were especially high for P, mostly in mineral form, and P-desorption rates were high. Partial and complete topsoil removal, aiming at heathland restoration, led to (much) lower soil organic matter and nutrients, but not to recovery of soil life, nor to P-limited soils. Heather was accompanied by many grassland species, even with complete topsoil removal. Conversion to rangeland did not decrease nutrient stocks, but led to improved soil life, although different from reference grasslands due to the higher pH. P-availability remained high, but net N-mineralization and plant N-content were clearly lower after 25 than after 18 years. Plant diversity was relatively high, and cover of eutrophic grasses decreased to 8–39% in intermediate and productive rangelands. Nutrient-poor species remained absent, but the slightly higher pH improved conditions for many grassland herbs. Afforestation did also not lead to nutrient-poor conditions, but soil life clearly increased and nutrients were used for rapid tree growth. Undergrowth species however remained eutrophic. It was impossible to retrieve the P-limited reference ecosystems within 25 years of restoration, not even with complete topsoil removal. Differences in plant diversity between expensive topsoil removal and much cheaper conversion to rangeland were also relatively small. For restoration on a landscape scale, it may thus be better to focus on conversion to semi-natural grasslands and afforestation. The half-open, nutrient-, mineral-, and species-rich landscape offers opportunities for large grazers, but also for many insects and birds

Cluster derivation of Parisi's RSB solution for disordered systems

We propose a general scheme in which disordered systems are allowed to sacrifice energy equi-partitioning and separate into a hierarchy of ergodic sub-systems (clusters) with different characteristic time-scales and temperatures. The details of the break-up follow from the requirement of stationarity of the entropy of the slower cluster, at every level in the hierarchy. We apply our ideas to the Sherrington-Kirkpatrick model, and show how the Parisi solution can be {\it derived} quantitatively from plausible physical principles. Our approach gives new insight into the physics behind Parisi's solution and its relations with other theories, numerical experiments, and short range models.Comment: 7 pages 5 figure

Inducing a mental context for associative memory formation with real-time fMRI neurofeedback

Memory, one of the hallmarks of human cognition, can be modified when humans voluntarily modulate neural population activity using neurofeedback. However, it is currently unknown whether neurofeedback can influence the integration of memories, and whether memory is facilitated or impaired after such neural perturbation. In this study, participants memorized objects while we provided them with abstract neurofeedback based on their brain activity patterns in the ventral visual stream. This neurofeedback created an implicit face or house context in the brain while memorizing the objects. The results revealed that participants created associations between each memorized object and its implicit context solely due to the neurofeedback manipulation. Our findings shed light onto how memory formation can be influenced by synthetic memory tags with neurofeedback and advance our understanding of mnemonic processing

Photo-induced dynamics of the heme centers in cytochrome bc 1

The ultrafast response of cytochrome bc1 is investigated for the first time, via transient absorption spectroscopy. The distinct redox potentials of both c1- and b-hemes allow for a clear differentiation of their respective signals. We find that while the c1-heme photo-product exhibits the characteristics of a 5-coordinated species, the b-hemes presumably undergo photo-oxidation at a remarkably high quantum yield. The c1-heme iron–ligand recombination time is 5.4 ps, in agreement with previous reports on homologous cytochromes. The suggested photo-oxidized state of the b-hemes has a lifetime of 6.8 ps. From this short life-time we infer that the electron acceptor must be within van der Walls contact with the heme, which points to the fact that the axial histidine residue is the electron acceptor. The different heme-responses illustrate the flexibility of the c1-heme ligation in contrast to the more rigid b-heme binding, as well as the higher electronic reactivity of the b-hemes within the bc1 complex. This study also demonstrates the remarkable connection between the heme local environment and its dynamics and, therefore, biological functio