Relaxation dynamics in the presence of pulse multiplicative noise sources with different correlation properties

The relaxation dynamics of a system described by a Langevin equation with pulse multiplicative noise sources with different correlation properties is considered. The solution of the corresponding Fokker-Planck equation is derived for Gaussian white noise. Moreover, two pulse processes with regulated periodicity are considered as a noise source: the dead-time-distorted Poisson process and the process with fixed time intervals, which is characterized by an infinite correlation time. We find that the steady state of the system is dependent on the correlation properties of the pulse noise. An increase of the noise correlation causes the decrease of the mean value of the solution at the steady state. The analytical results are in good agreement with the numerical ones.

New insights into electron spin dynamics in the presence of correlated noise

The changes in the spin depolarization length in zinc-blende semiconductors when an external component of correlated noise is added to a static driving electric field are analyzed for different values of field strength, noise amplitude and correlation time. Electron dynamics is simulated by a Monte Carlo procedure which takes into account all the possible scattering phenomena of the hot electrons in the medium and includes the evolution of spin polarization. Spin depolarization is studied by examining the decay of the initial spin polarization of the conduction electrons through the D'yakonov-Perel process, the only relevant relaxation mechanism in III-V crystals. Our results show that, for electric field amplitudes lower than the Gunn field, the dephasing length shortens with increasing noise intensity. Moreover, a nonmonotonic behavior of spin depolarization length with the noise correlation time is found, characterized by a maximum variation for values of noise correlation time comparable with the dephasing time. Instead, in high field conditions, we find that, critically depending on the noise correlation time, external fluctuations can positively affect the relaxation length. The influence of the inclusion of the electron-electron scattering mechanism is also shown and discussed.

Regularity of spike trains and harmony perception in a model of the auditory system

Spike train regularity of the noisy neural auditory system model under the influence of two sinusoidal signals with different frequencies is investigated. For the increasing ratio m/n of the input signal frequencies (m, n are natural numbers) the linear growth of the regularity is found at the fixed difference (m - n). It is shown that the spike train regularity in the model is high for harmonious chords of input tones and low for dissonant ones.

Resonant activation in polymer translocation: new insights into the escape dynamics of molecules driven by an oscillating field

The translocation of molecules across cellular membranes or through synthetic nanopores is strongly affected by thermal fluctuations. In this work we study how the dynamics of a polymer in a noisy environment changes when the translocation process is driven by an oscillating electric field. An improved version of the Rouse model for a flexible polymer has been adopted to mimic the molecular dynamics, by taking into account the harmonic interactions between adjacent monomers and the excluded-volume effect by introducing a Lennard-Jones potential between all beads. A bending recoil torque has also been included in our model. The polymer dynamics is simulated in a two-dimensional domain by numerically solving the Langevin equations of motion. Thermal fluctuations are taken into account by introducing a Gaussian uncorrelated noise. The mean first translocation time of the polymer centre of inertia shows a minimum as a function of the frequency of the oscillating forcing field. This finding represents the first evidence of the resonant activation behaviour in the dynamics of polymer translocation.

Dynamics of two competing species in the presence of Lévy noise sources

We consider a Lotka-Volterra system of two competing species subject to multiplicative α-stable Lévy noise. The interaction parameter between the species is a random process which obeys a stochastic differential equation with a generalized bistable potential in the presence both of a periodic driving term and an additive α-stable Lévy noise. We study the species dynamics, which is characterized by two different regimes, exclusion of one species and coexistence of both. We find quasiperiodic oscillations and stochastic resonance phenomenon in the dynamics of the competing species, analyzing the role of the Lévy noise sources.

Stochastic resonance in a trapping overdamped monostable system

The response of a trapping overdamped monostable system to a harmonic perturbation is analyzed, in the context of stochastic resonance phenomenon. We consider the dynamics of a Brownian particle moving in a piecewise linear potential with a white Gaussian noise source. Based on linear-response theory and Laplace transform technique, analytical expressions of signal-to-noise ratio (SNR) and signal power amplification (SPA) are obtained. We find that the SNR is a nonmonotonic function of the noise intensity, while the SPA is monotonic. Theoretical results are compared with numerical simulations.

Epidural analgesia in children: planning, organization and development of a new program

BACKGROUND

The goal of the present work is to describe the development and results of the pediatric epidural analgesia program at the ''Hospital Nacional de Pediatria Prof. Dr. J. P. Garrahan'' in Argentina.

METHODS

Patients with thoracotomy, abdominal surgery, osteotomy, amputations or severe trauma were included in the program. The program provided training to the entire staff, control and record of pain treatment and its consequences, 24 h a day availability of anesthesia staff and standard polices and procedures.

RESULTS

One hundred fifty children under 16 years of age (median age 11 years, median weight 35 kg) were included in the program during the first 18 months. The median of maximum pain reported during activity was 1 (interquartile range 1 to 4 points) using the Visual Analogue Scale (VAS) or Objective Pain Scale (OPS). Eighty seven children (CI 95% 50% to 67%) presented with postoperative nausea and vomiting, urinary retention, itching, motor blockade or sedation. No patient presented with respiratory depression, hypotension, local anaesthetic toxicity, epidural catheter related infection or death during the program evaluation. The postoperative care program enabled a 98-day reduction in treatment in the intensive care unit.

CONCLUSION

The safe use of pediatric epidural analgesia in general wards may require the careful selection of patients, systematic assessment by trained personnel, training of medical and nursing personnel, clear distribution of responsibilities, use of printed indications, systematic record of pain, sedation and complications, information and education of patients and parents, supply of systems for airway resuscitation and management and continuous quality control and revision of the methods.

Signatures of noise-enhanced stability in metastable states

The lifetime of a metastable state in the transient dynamics of an overdamped Brownian particle is analyzed, both in terms of the mean first passage time and by means of the mean growth rate coefficient. Both quantities feature nonmonotonic behaviors as a function of the noise intensity, and are independent signatures of the noise enhanced stability effect. They can therefore be alternatively used to evaluate and estimate the presence of this phenomenon, which characterizes metastability in nonlinear physical systems.

Tryptophan replacement in the nociceptin/orphanin FQ receptor ligand Ac-RYYRWK-NH2

In the present study we describe the in vitro pharmacological characterization of the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) ligand Ac-RYYRWK-NH2 and the synthesis and biological evaluation of 13 Trp5 substituted Ac-RYYRWK-NH2 analogs. Results indicate that Ac-RYYRWK-NH2 behaves as a highly potent and selective partial agonist at the NOP receptors and that the whole indole moiety of the Trp5 side chain is not required, being a phenyl-ethyl side chain already sufficient for maintaining high potency.

Noise in ecosystems: a short review

Noise, through its interaction with the nonlinearity of the living systems, can give rise to counter-intuitive phenomena such as stochastic resonance, noise-delayed extinction, temporal oscillations, and spatial patterns. In this paper we briefly review the noise-induced effects in three different ecosystems: (i) two competing species; (ii) three interacting species, one predator and two preys, and (iii) N-interacting species. The transient dynamics of these ecosystems are analyzed through generalized Lotka-Volterra equations in the presence of multiplicative noise, which models the interaction between the species and the environment. The interaction parameter between the species is random in cases (i) and (iii), and a periodical function, which accounts for the environmental temperature, in case (ii). We find noise-induced phenomena such as quasi-deterministic oscillations, stochastic resonance, noise-delayed extinction, and noise-induced pattern formation with nonmonotonic behaviors of patterns areas and of the density correlation as a function of the multiplicative noise intensity. The asymptotic behavior of the time average of the i(th) population when the ecosystem is composed of a great number of interacting species is obtained and the effect of the noise on the asymptotic probability distributions of the populations is discussed.

Noise-enhanced stability of periodically driven metastable states

We study the effect of noise-enhanced stability of periodically driven metastable states in a system described by piecewise linear potential. We find that the growing of the average escape time with the intensity of the noise is depending on the initial condition of the system. We analytically obtain the condition for the noise enhanced stability effect and verify it by numerical simulations.

Linear and nonlinear experimental regimes of stochastic resonance

We investigate the stochastic resonance phenomenon in a physical system based on a tunnel diode. The experimental control parameters are set to allow the control of the frequency and amplitude of the deterministic modulating signal over an interval of values spanning several orders of magnitude. We observe both a regime described by the linear-response theory and the nonlinear deviation from it. In the nonlinear regime we detect saturation of the power spectral density of the output signal detected at the frequency of the modulating signal and a dip in the noise level of the same spectral density. When these effects are observed we detect a phase and frequency synchronization between the stochastic output and the deterministic input.

Experimental investigation of resonant activation

We experimentally investigate the escape from a metastable state over a fluctuating barrier of a physical system. The system is switching between two states under electronic control of a dichotomous noise. We measure the escape time and its probability density function as a function of the correlation rate of the dichotomous noise in a frequency interval spanning more than four frequency decades. We observe resonant activation, namely a minimum of the average escape time as a function of the correlation rate. We detect two regimes in the study of the shape of the escape time probability distribution: (i) a regime of exponential and (ii) a regime of nonexponential probability distribution.