Transition to chaos in random neuronal networks

Firing patterns in the central nervous system often exhibit strong temporal irregularity and heterogeneity in their time averaged response properties. Previous studies suggested that these properties are outcome of an intrinsic chaotic dynamics. Indeed, simplified rate-based large neuronal networks with random synaptic connections are known to exhibit sharp transition from fixed point to chaotic dynamics when the synaptic gain is increased. However, the existence of a similar transition in neuronal circuit models with more realistic architectures and firing dynamics has not been established. In this work we investigate rate based dynamics of neuronal circuits composed of several subpopulations and random connectivity. Nonzero connections are either positive-for excitatory neurons, or negative for inhibitory ones, while single neuron output is strictly positive; in line with known constraints in many biological systems. Using Dynamic Mean Field Theory, we find the phase diagram depicting the regimes of stable fixed point, unstable dynamic and chaotic rate fluctuations. We characterize the properties of systems near the chaotic transition and show that dilute excitatory-inhibitory architectures exhibit the same onset to chaos as a network with Gaussian connectivity. Interestingly, the critical properties near transition depend on the shape of the single- neuron input-output transfer function near firing threshold. Finally, we investigate network models with spiking dynamics. When synaptic time constants are slow relative to the mean inverse firing rates, the network undergoes a sharp transition from fast spiking fluctuations and static firing rates to a state with slow chaotic rate fluctuations. When the synaptic time constants are finite, the transition becomes smooth and obeys scaling properties, similar to crossover phenomena in statistical mechanicsComment: 28 Pages, 12 Figures, 5 Appendice

Seed mass diversity along resource gradients: the role of allometric growth rate and size-asymmetric competition

The large variation in seed mass among species inspired a vast array of theoretical and empirical research attempting to explain this variation. So far, seed mass variation was investigated by two classes of studies: one class focuses on species varying in seed mass within communities, while the second focuses on variation between communities, most often with respect to resource gradients. Here, we develop a model capable of simultaneously explaining variation in seed mass within and between communities. The model describes resource competition (for both soil and light resources) in annual communities and incorporates two fundamental aspects: light asymmetry (higher light acquisition per unit biomass for larger individuals) and growth allometry (negative dependency of relative growth rate on plant biomass). Results show that both factors are critical in determining patterns of seed mass variation. In general, growth allometry increases the reproductive success of small-seeded species while light asymmetry increases the reproductive success of large-seeded species. Increasing availability of soil resources increases light competition, thereby increasing the reproductive success of large-seeded species and ultimately the community (weighted) mean seed mass. An unexpected prediction of the model is that maximum variation in community seed mass (a measure of functional diversity) occurs under intermediate levels of soil resources. Extensions of the model incorporating size-dependent seed survival and disturbance also show patterns consistent with empirical observations. These overall results suggest that the mechanisms captured by the model are important in determining patterns of species and functional diversity

Without \u27Focus\u27

It is widely accepted that a notion of \u27focus\u27, more or less as conceived of in Jackendoff (1972), must be incorporated into our theory of grammar, as a means of accounting for certain observed correlations between prosodic facts and semantic/pragmatic facts. In this paper, we put forth the somewhat radical idea that the time has come to give up this customary view, and eliminate \u27focus\u27 from our theory of grammar. We argue that such a move is both economical and fruitful. Research over the years has revealed that the correlations between prosody, \u27focus\u27, and the alleged semantic/pragmatic effects of focus are much less clear and systematic than we may have initially hoped. First we argue that this state of affairs detracts significantly from the utility of our notion of \u27focus\u27, to the point of calling into question the very motivation for including it in the grammar. Then we look at some of the central data, and show how they might be analyzed without recourse to a notion of \u27focus\u27. We concentrate on (i) the effect of pitch accent placement on discourse congruence, and (ii) the choice of \u27associate\u27 for the so-called \u27focus sensitive\u27 adverb only. We argue that our focus-free approach to the data improves empirical coverage, and begins to reveal patterns that have previously been obscured by preconceptions about \u27focus\u27

Integrative vs. Traditional Learning from the Student Perspective

Background: The interdisciplinary surgery block of the reformed undergraduate curriculum HeiCuMed includes daily cycles of interactive case-based seminars, problem-based tutorials, case presentation by students, skills and communication training, and bedside teaching. The teaching doctors receive didactic training. In contrast, the previous traditional course was based on lectures with only two weekly hours of bedside teaching. Didactic training was not available

A Model for Persistent Improvement of Medical Education as Illustrated by the Surgical Reform Curriculum HeiCuMed

Background: Heidelberg Medical School underwent a major curricular change with the implementation of the reform curriculum HeiCuMed (Heidelberg Curriculum Medicinale) in October 2001. It is based on rotational modules with daily cycles of interactive, case-based small-group seminars, PBL tutorials and training of sensomotor and communication skills. For surgical undergraduate training an organisational structure was developed that ensures continuity of medical teachers for student groups and enables their unimpaired engagement for defined periods of time while accounting for the daily clinical routine in a large surgery department of a university hospital. It includes obligatory didactic training, standardising teaching material on the basis of learning objectives and releasing teaching doctors from clinical duties for the duration of a module