Synaptic shot noise and conductance fluctuations affect the membrane voltage with equal significance

The subthresholdmembranevoltage of a neuron in active cortical tissue is a fluctuating quantity with a distribution that reflects the firing statistics of the presynaptic population. It was recently found that conductancebased synaptic drive can lead to distributions with a significant skew. Here it is demonstrated that the underlying shot noise caused by Poissonian spike arrival also skews the membrane distribution, but in the opposite sense. Using a perturbative method, we analyze the effects of shot noise on the distribution of synaptic conductances and calculate the consequent voltage distribution. To first order in the perturbation theory, the voltage distribution is a gaussian modulated by a prefactor that captures the skew. The gaussian component is identical to distributions derived using current-based models with an effective membrane time constant. The well-known effective-time-constant approximation can therefore be identified as the leading-order solution to the full conductance-based model. The higher-order modulatory prefactor containing the skew comprises terms due to both shot noise and conductance fluctuations. The diffusion approximation misses these shot-noise effects implying that analytical approaches such as the Fokker-Planck equation or simulation with filtered white noise cannot be used to improve on the gaussian approximation. It is further demonstrated that quantities used for fitting theory to experiment, such as the voltage mean and variance, are robust against these non-Gaussian effects. The effective-time-constant approximation is therefore relevant to experiment and provides a simple analytic base on which other pertinent biological details may be added

Unearthing learners’ conceptions of reflection to innovate business education for the 21st century

The development of learners’ capacities for critical reflection is an important learning outcome for 21st century business education. Theories suggest that a learner holds a particular orientation to reflection, and that this perspective will be influenced by his or her underlying beliefs. This, coupled with an increased focus on the student experience, personal development, and self-regulation in higher education, offers scope for considering instructional design from a second-order perspective, or in other words, from the student’s point of view. This study sought to understand: 1) the ways that business students orientate to reflection, 2) the different conceptions they hold of reflection, and 3) whether there is a relationship between the two. Reflective learning questionnaires were completed by 112 business students studying at the University of Northampton. Survey results showed that while the research instrument was a good fit for investigating orientations to and conceptions of reflection, there did not appear to be a correlation between the two. Learning analytics such as these will be useful for considering how the University can design more meaningful business curricula. However, the disconnect between conceptions of and orientations to reflection needs to be explored through further research

Micro-mechanical analysis of damage growth and fracture in discontinuous fiber reinforced metal matrix composites

The near-crack-tip stresses in any planar coupon of arbitrary geometry subjected to mode 1 loading may be equated to those in an infinite center-cracked panel subjected to the appropriate equivalent remote biaxial stresses (ERBS). Since this process can be done for all such mode 1 coupons, attention may be focused on the behavior of the equivalent infinite cracked panel. To calculate the ERBS, the constant term in the series expansion of the crack-tip stress must be retained. It is proposed that the ERBS may be used quantitatively to explain different fracture phenomena such as crack branching

Recent U.S. Trade Policy and its Global Implications

The purpose of this paper is to describe United States trade policy since World War II, and to assess the possibility for ongoing U.S.trade-policy leadership. U.S. trade policy has shown remarkable consistency since World War II. It has never been as purely free-trade-focussed as some commentators suggest, but it has not recently shifted toward isolationism as dramatically as alarmists fear. It has almost always been best described as "open, but fair," with injury to import competitors being the measure of "fairness." The general consistency of U.S. trade policy over time is quite remarkable given the frequent change of political party in power, especially in the executive branch, but also in the Congress. U.S. trade-policy leadership seems still potentially strong despite a decline in U.S. hegemony. It is clearly strong in a protectionist direction.Any shift toward aggressive insularity justifies parallel trade-policy aggression in the eyes of trading partners. It is arguably strong ina liberalizing direction as well. The U.S. seems ideally poised for aggressive trade-policy peacemaking; perhaps multilaterally, but perhaps also bilaterally; perhaps with its traditional industrial trading partners, but perhaps also with Japan and newly industrializing Asian countries that play so importanta role in U.S. trade, and that, on many matters,may be closer in spirit to U.S. economic philosophy than Europe, Canada, or Latin America.

A fast and robust numerical scheme for solving models of charge carrier transport and ion vacancy motion in perovskite solar cells

Drift-diffusion models that account for the motion of both electronic and ionic charges are important tools for explaining the hysteretic behaviour and guiding the development of metal halide perovskite solar cells. Furnishing numerical solutions to such models for realistic operating conditions is challenging owing to the extreme values of some of the parameters. In particular, those characterising (i) the short Debye lengths (giving rise to rapid changes in the solutions across narrow layers), (ii) the relatively large potential differences across devices and (iii) the disparity in timescales between the motion of the electronic and ionic species give rise to significant stiffness. We present a finite difference scheme with an adaptive time step that is posed on a non-uniform staggered grid that provides second order accuracy in the mesh spacing. The method is able to cope with the stiffness of the system for realistic parameters values whilst providing high accuracy and maintaining modest computational costs. For example, a transient sweep of a current-voltage curve can be computed in only a few minutes on a standard desktop computer.Comment: 22 pages, 8 figure