Fluctuation Induced Instabilities in Front Propagation up a Co-Moving Reaction Gradient in Two Dimensions

We study 2D fronts propagating up a co-moving reaction rate gradient in finite number reaction-diffusion systems. We show that in a 2D rectangular channel, planar solutions to the deterministic mean-field equation are stable with respect to deviations from planarity. We argue that planar fronts in the corresponding stochastic system, on the other hand, are unstable if the channel width exceeds a critical value. Furthermore, the velocity of the stochastic fronts is shown to depend on the channel width in a simple and interesting way, in contrast to fronts in the deterministic MFE. Thus, fluctuations alter the behavior of these fronts in an essential way. These affects are shown to be partially captured by introducing a density cutoff in the reaction rate. Some of the predictions of the cutoff mean-field approach are shown to be in quantitative accord with the stochastic results

The fixation probability of rare mutators in finite asexual populations

A mutator is an allele that increases the mutation rate throughout the genome by disrupting some aspect of DNA replication or repair. Mutators that increase the mutation rate by the order of 100 fold have been observed to spontaneously emerge and achieve high frequencies in natural populations and in long-term laboratory evolution experiments with \textit{E. coli}. In principle, the fixation of mutator alleles is limited by (i) competition with mutations in wild-type backgrounds, (ii) additional deleterious mutational load, and (iii) random genetic drift. Using a multiple locus model and employing both simulation and analytic methods, we investigate the effects of these three factors on the fixation probability $P_{fix}$ of an initially rare mutator as a function of population size $N$, beneficial and deleterious mutation rates, and the strength of mutations $s$. Our diffusion based approximation for $P_{fix}$ successfully captures effects (ii) and (iii) when selection is fast compared to mutation ($\mu/s \ll 1$). This enables us to predict the conditions under which mutators will be evolutionarily favored. Surprisingly, our simulations show that effect (i) is typically small for strong-effect mutators. Our results agree semi-quantitatively with existing laboratory evolution experiments and suggest future experimental directions.Comment: 46 pages, 8 figure

Policy issues and actors

Detention without trial can be a formidable government weapon against political opponents. In South Africa this weapon has been fashioned into a multiple warhead. There are currently seven security law detention provisions on the statute book, of which one is dormant but can be activated by the State President. Non security law detention, for example detention under drug laws, will not be discussed in this paper. While the seven detention laws are of varying severity and serve different purposes, they are best classified and explained under two main categories or groups - preventive detention and pre-trial detention

Application of a urine and hair validated LC-MS/MS method to determine the effect of hair colour on the incorporation of 25B-NBOMe, 25C-NBOMe and 25I-NBOMe into hair in the rat.

NBOMes are a group of new psychoactive substances derived from phenethylamines. Recreational abuse is thought to have begun in 2010 and they are commonly associated with the “club drug” scene. They are administered in liquid form or as blotters due to their high potency. An LC-MS/MS method was validated using SWGTOX parameters for the detection of 25B-, 25C- and 25I-NBOMe using 25B-NBOMe-D3 as internal standard for urine and hair. Calibration graphs with R2 values >0.99 were observed for urine and hair for concentrations ranging from 0.1 -100 ng/mL and 0.025-2.5 ng/mg respectively. Urine LODs ranged from 5-25 pg/mL and had an LOQ of 50 pg/mL. Hair LOD and LOQs ranged from 3-5 pg/mg and 6.25-12.5 pg/mg respectively. Intra and inter-day precision was <20% and accuracy was within ± 20% for both matrices. The method was shown to be selective for both exogenous and endogenous compounds. No matrix effects were observed for either matrix. LLE recovery ranged from 90-103% for urine samples and SPE recovery ranged from 80-107% for hair samples. Long-Evans rats (n=55) were administered 25B-, 25C- or 25I-NBOMe at doses ranging from 30-300 µg/kg over a period of 10 days. Rats were shaved prior to their first dose and re-shaved after the 10-day period. Hair was separated by colour (black: n=55 and white: n=55) and analysed using the validated LC-MS/MS method to assess the impact hair colour has on the incorporation of these drugs. All drugs were successfully detected in black hair. 25B-NBOMe from rats receiving the highest dose and 25C-NBOMe from rats receiving the medium and high doses were quantified in white hair. 25I-NBOMe was detected but fell below the limit of quantification. A dose-dependent concentration increase was observed in the black hair. All pooled urine samples tested positive for their expected NBOMes

To Head or to Heed? Beyond the Surface of Selective Action Inhibition: A Review

To head rather than heed to temptations is easier said than done. Since tempting actions are often contextually inappropriate, selective suppression is invoked to inhibit such actions. Thus far, laboratory tasks have not been very successful in highlighting these processes. We suggest that this is for three reasons. First, it is important to dissociate between an early susceptibility to making stimulus-driven impulsive but erroneous actions, and the subsequent selective suppression of these impulses that facilitates the selection of the correct action. Second, studies have focused on mean or median reaction times (RT), which conceals the temporal dynamics of action control. Third, studies have focused on group means, while considering individual differences as a source of error variance. Here, we present an overview of recent behavioral and imaging studies that overcame these limitations by analyzing RT distributions. As will become clear, this approach has revealed variations in inhibitory control over impulsive actions as a function of task instructions, conflict probability, and between-trial adjustments (following conflict or following an error trial) that are hidden if mean RTs are analyzed. Next, we discuss a selection of behavioral as well as imaging studies to illustrate that individual differences are meaningful and help understand selective suppression during action selection within samples of young and healthy individuals, but also within clinical samples of patients diagnosed with attention deficit/hyperactivity disorder or Parkinson's disease

Assessing School and Student Predictors of Weapons Reporting

School violence and weapons at school are a major concern for community members, school administrators, and policy makers. This research examines both student-level and school-level variables that predict middle school students’ willingness to report a weapon at school under several reporting conditions. Results substantiate previous analyses of these data that student-level variables explain students’ willingness to report a weapon but extend these findings to include school climate variables that affect willingness to report (i.e., collective identity and conflict). School climate variables were also shown to influence reporting under conditions in which there would be consequences for the weapons-carrying student or for the reporting student; however, school climate was not found to influence anonymous reporting conditions. Although policies aimed at improving school climate may increase a student’s willingness to report and are important in their own right, improving a school’s climate may be a daunting task. This research, therefore, suggests that the most efficient way to encourage weapons reporting is to provide students with an anonymous way to report

An empirical approach to modeling ion production rates in Titan's ionosphere II: Ion production rates on the nightside

Ionization of neutrals by precipitating electrons and ions is the main source of Titan's nightside ionosphere. This paper has two goals: (1) characterization of the role of electron impact ionization on the nightside ionosphere for different magnetospheric conditions and (2) presentation of empirical ion production rates determined using densities measured by the Cassini Ion and Neutral Mass Spectrometer on the nightside. The ionosphere between 1000 and 1400 km is emphasized. We adopt electron fluxes measured by the Cassini Plasma Spectrometer-Electron Spectrometer and the Magnetospheric Imaging Instrument as classified by Rymer et al. (2009). The current paper follows an earlier paper (Paper I), in which we investigated sources of Titan's dayside ionosphere and demonstrated that the photoionization process is well understood. The current paper (Paper II) demonstrates that modeled and empirical ionization rates on the nightside are in agreement with an electron precipitation source above 1100 km. Ion production rate profiles appropriate for different Saturnian magnetospheric conditions, as outlined by Rymer et al., are constructed for various magnetic field topologies. Empirical production rate profiles are generated for deep nightside flybys of Titan. The results also suggest that at lower altitudes (below 1100 km) another source, such as ion precipitation, is probably needed

Deep Brain Stimulation of the Subthalamic Nucleus Improves Reward-Based Decision-Learning in Parkinson's Disease

Recently, the subthalamic nucleus (STN) has been shown to be critically involved in decision-making, action selection, and motor control. Here we investigate the effect of deep brain stimulation (DBS) of the STN on reward-based decision-learning in patients diagnosed with Parkinson's disease (PD). We determined computational measures of outcome evaluation and reward prediction from PD patients who performed a probabilistic reward-based decision-learning task. In previous work, these measures covaried with activation in the nucleus caudatus (outcome evaluation during the early phases of learning) and the putamen (reward prediction during later phases of learning). We observed that stimulation of the STN motor regions in PD patients served to improve reward-based decision-learning, probably through its effect on activity in frontostriatal motor loops (prominently involving the putamen and, hence, reward prediction). In a subset of relatively younger patients with relatively shorter disease duration, the effects of DBS appeared to spread to more cognitive regions of the STN, benefiting loops that connect the caudate to various prefrontal areas importantfor outcome evaluation. These results highlight positive effects of STN stimulation on cognitive functions that may benefit PD patients in daily-life association-learning situations

An empirical approach to modeling ion production rates in Titan’s ionosphere I: Ion production rates on the dayside and globally

Titan's ionosphere is created when solar photons, energetic magnetospheric electrons or ions, and cosmic rays ionize the neutral atmosphere. Electron densities generated by current theoretical models are much larger than densities measured by instruments on board the Cassini orbiter. This model density overabundance must result either from overproduction or from insufficient loss of ions. This is the first of two papers that examines ion production rates in Titan's ionosphere, for the dayside and nightside ionosphere, respectively. The first (current) paper focuses on dayside ion production rates which are computed using solar ionization sources (photoionization and electron impact ionization by photoelectrons) between 1000 and 1400 km. In addition to theoretical ion production rates, empirical ion production rates are derived from CH4, CH3+, and CH4+ densities measured by the INMS (Ion Neutral Mass Spectrometer) for many Titan passes. The modeled and empirical production rate profiles from measured densities of N2+ and CH4+ are found to be in good agreement (to within 20%) for solar zenith angles between 15 and 90°. This suggests that the overabundance of electrons in theoretical models of Titan's dayside ionosphere is not due to overproduction but to insufficient ion losses

A Tribute to Charlie Chaplin: Induced Positive Affect Improves Reward-Based Decision-Learning in Parkinson’s Disease

Reward-based decision-learning refers to the process of learning to select those actions that lead to rewards while avoiding actions that lead to punishments. This process, known to rely on dopaminergic activity in striatal brain regions, is compromised in Parkinson’s disease (PD). We hypothesized that such decision-learning deficits are alleviated by induced positive affect, which is thought to incur transient boosts in midbrain and striatal dopaminergic activity. Computational measures of probabilistic reward-based decision-learning were determined for 51 patients diagnosed with PD. Previous work has shown these measures to rely on the nucleus caudatus (outcome evaluation during the early phases of learning) and the putamen (reward prediction during later phases of learning). We observed that induced positive affect facilitated learning, through its effects on reward prediction rather than outcome evaluation. Viewing a few minutes of comedy clips served to remedy dopamine-related problems associated with frontostriatal circuitry and, consequently, learning to predict which actions will yield reward