Connectivity reflects coding: A model of voltage-based spike-timing-dependent-plasticity with homeostasis

Electrophysiological connectivity patterns in cortex often show a few strong connections in a sea of weak connections. In some brain areas a large fraction of strong connections are bidirectional, in others they are mainly unidirectional. In order to explain these connectivity patterns, we use a model of Spike-Timing-Dependent Plasticity where synaptic changes depend on presynaptic spike arrival and the postsynaptic membrane potential. The model describes several nonlinear effects in STDP experiments, as well as the voltage dependence of plasticity under voltage clamp and classical paradigms of LTP/LTD induction. We show that in a simulated recurrent network of spiking neurons our plasticity rule leads not only to receptive field development, but also to connectivity patterns that reflect the neural code: for temporal coding paradigms strong connections are predominantly unidirectional, whereas they are bidirectional under rate coding. Thus variable connectivity patterns in the brain could reflect different coding principles across brain areas

Corrected phase-type approximations of heavy-tailed queueing models in a Markovian environment

Significant correlations between arrivals of load-generating events make the numerical evaluation of the workload of a system a challenging problem. In this paper, we construct highly accurate approximations of the workload distribution of the MAP/G/1 queue that capture the tail behavior of the exact workload distribution and provide a bounded relative error. Motivated by statistical analysis, we consider the service times as a mixture of a phase-type and a heavy-tailed distribution. With the aid of perturbation analysis, we derive our approximations as a sum of the workload distribution of the MAP/PH/1 queue and a heavy-tailed component that depends on the perturbation parameter. We refer to our approximations as corrected phase-type approximations, and we exhibit their performance with a numerical study.Comment: Received the Marcel Neuts Student Paper Award at the 8th International Conference on Matrix Analytic Methods in Stochastic Models 201

On the accuracy of phase-type approximations of heavy-tailed risk models

Numerical evaluation of ruin probabilities in the classical risk model is an important problem. If claim sizes are heavy-tailed, then such evaluations are challenging. To overcome this, an attractive way is to approximate the claim sizes with a phase-type distribution. What is not clear though is how many phases are enough in order to achieve a specific accuracy in the approximation of the ruin probability. The goals of this paper are to investigate the number of phases required so that we can achieve a pre-specified accuracy for the ruin probability and to provide error bounds. Also, in the special case of a completely monotone claim size distribution we develop an algorithm to estimate the ruin probability by approximating the excess claim size distribution with a hyperexponential one. Finally, we compare our approximation with the heavy traffic and heavy tail approximations.Comment: 24 pages, 13 figures, 8 tables, 38 reference

Technological change and price effects in agriculture

This paper addresses the conceptual issues around the negative price effects of technological change on agricultural producers, explores price policy options vis-à-vis this problem, and reviews and compares experiences across Asian countries as they transformed their rural economies. It then draws implications for the challenge of achieving a smallholder-led agricultural revolution in Africa in the context of market liberalization.Small farmers ,technicological change ,market prices ,

ICT and farmers : lessons learned and future developments

Information and Communication Technologies (ICT) evolution is well advancing Moore?s Law prediction of geometric progression of computer performance indexes. Indeed, these technologies are not only fast developed but, in addition, are giving birth to newer ones nicely branching existing “old fashion” ICT systems and tools. These innovations of ICT are not only regenerating traditional sciences, like Agriculture, and practices, like farming, but also, awake well neglected human sensitiveness and indifference for poverty, environmental protection, climatic deterioration issues and the future of our planet as a whole. To refer to a few examples of these innovations affecting Agriculture and Environmental Sciences: Cloud Computing provides equality in resources management and exploitability to small budget farms against the big ones. Web2 browser allows, as a platform, effective runtime environment and considerably easy access to applications by farmers lacking proper education and training. Parallel Computing brings exponentially increased core processing to low-end computers facilitating the use of huge computer power by small agricultural research units. Never the less agricultural and farming communities, in their majority, do not adopt new ICT tools and systems to the degree required for substantial agricultural development. In this paper, experience gained over the years is used to evaluate and reason poor performance in the area of applicability of ICT innovations and tools by the vast majority of farmers throughout the world.</jats:p

Monthly average daily global and diffuse solar radiation based on sunshine duration and clearness index for Brasov, Romania

The main objective of this study is to develop single location appropriate models for the estimation of the monthly average daily global and diffuse horizontal solar radiation for Brasov, Romania. The study focuses particularly on models based on the sunshine duration and clearness index. The data used for the calibration of the models were collected during a period of 4 yr, between November 2008 and October 2012, at the Transilvania University of Brasov. The testing and validation of the models was carried out using data from the online SoDa database for Brasov for the year 2005. Different statistical error tests were applied to evaluate the accuracy of the models. The predicted values are also compared with values from three other known models concerning the global and diffuse solar radiation. A new mixed model was developed for the estimation of monthly average daily global horizontal solar radiation. The data processing was performed by means of a real-time interface developed with LabVIEW graphical programming language. The parameters taken into account were the relative sunshine, the clearness index, the extraterrestrial radiation, the latitude and the longitude. The methodology is simple and effective and may be applied for any region. Its effectiveness was proven through comparison with global models