Epaminondas Rosa

On the role of subthreshold currents in the Huber–Braun cold receptor model

We study the role of the strength of subthreshold currents in a four-dimensional Hodgkin-Huxley-type model of mammalian cold receptors. Since a total diminution of subthreshold activity corresponds to a decomposition of the model into a slow, subthreshold, and a fast, spiking subsystem, we first elucidate their respective dynamics separately and draw conclusions about their role for the generation of different spiking patterns. These results motivate a numerical bifurcation analysis of the effect of varying the strength of subthreshold currents, which is done by varying a suitable control parameter. We work out the key mechanisms which can be attributed to subthreshold activity and furthermore elucidate the dynamical backbone of different activity patterns generated by this model.

Neurones and synapses for systemic models of psychiatric disorders.

We propose a mechanism-based modelling approach which brings together the most relevant features of neural dynamics and synaptic transmission for clinically valuable simulations of psychiatric disorders and their pharmaceutical treatment. It is based on a minimal, but physiologically justified concept, which allows to account for a great diversity of neuronal dynamics and synaptic mechanisms. It can simulate ionotropic as well as metabotropic receptors in addition to the effects of eventual co-transmitters and external neuromodulators. The proposed model can mimic the clinically most important aspects of synaptic disturbances, such as impaired transmitter availability or reduced number of postsynaptic receptors, for example due to their internalization as a function of transmitter concentration. It also allows evaluation of the effects of drugs with specific actions such as receptor agonists and antagonists or reuptake inhibitors. It is a major advantage of this physiologically based approach that it can be adjusted to different types of neurons and synapses, and also can be extended to more elaborate physiological situations, e. g. by including additional receptors or ion channels, whenever this is indicated by clinical or experimental data.

Communication-Based on Topology Preservation of Chaotic Dynamics

By using the Chua circuit we present experimental results for the feasibility of a chaotic communication scheme in which large parameter variations are allowed. The parameters are varied along special codimension one directions, on which the topology of chaotic attractors remains roughly invariant.

Temperature-dependent stochastic dynamics of the Huber-Braun neuron model

The response of a four-dimensional mammalian cold receptor model to different implementations of noise is studied across a wide temperature range. It is observed that for noisy activation kinetics, the parameter range decomposes into two regions in which the system reacts qualitatively completely different to small perturbations through noise, and these regions are separated by a homoclinic bifurcation. Noise implemented as an additional current yields a substantially different system response at low temperature values, while the response at high temperatures is comparable to activation-kinetic noise. We elucidate how this phenomenon can be understood in terms of state space dynamics and gives quantitative results on the statistics of interspike interval distributions across the relevant parameter range.

Phase Oscillatory Network and Visual Pattern Recognition

We explore a properly interconnected set of Kuramoto type oscillators that results in a new associative-memory network configuration, which includes second- and third-order additional terms in the Fourier expansion of the networks coupling. Investigation of the response of the network to different external stimuli indicates an increase in the network capability for coding and information retrieval. Comparison of the network output with that of an equivalent experiment with subjects, for recognizing perturbed binary patterns, shows comparable results between the two approaches. We also discuss the enhanced storage capacity of the network.

DRIVING PHASE SYNCHRONOUS PLASMA DISCHARGES WITH SUPERIMPOSED SIGNALS

Numerical simulations and experimental measurements performed on a plasma discharge, simultaneously paced with two independent sinusoidal functions, show the plasma moving from one synchronous state to another, back and forth, between the two independent pacers. Different approaches for studying this competition for phase synchronization indicate the relevance of the relative frequency values of the two sinusoidal functions with respect to the predominant frequency characteristic of the unpaced plasma discharge.

Effects of reciprocal inhibitory coupling in model neurons

Central pattern generators are neuron networks that produce vital rhythmic motor outputs such as those observed in mastication, walking and breathing. Their activity patterns depend on the tuning of their intrinsic ionic conductances, their synaptic interconnectivity and entrainment by extrinsic neurons. The influence of two commonly found synaptic connectivities--reciprocal inhibition and electrical coupling--are investigated here using a neuron model with subthreshold oscillation capability, in different firing and entrainment regimes. We study the dynamics displayed by a network of a pair of neurons with various firing regimes, coupled by either (i) only reciprocal inhibition or by (ii) electrical coupling first and then reciprocal inhibition. In both scenarios a range of coupling strengths for the reciprocal inhibition is tested, and in general the neuron with the lower firing rate stops spiking for strong enough inhibitory coupling, while the faster neuron remains active. However, in scenario (ii) the originally slower neuron stops spiking at weaker inhibitory coupling strength, suggesting that the electrical coupling introduces an element of instability to the two-neuron network.

Pacing a chaotic plasma with a music signal

Abstract An experimental plasma system is driven with a music signal output from a cd player. With the plasma in the periodic regime, optical detection of the light discharge allows the music to be heard from a loudspeaker. That does not happen when the plasma is chaotic, in which case we hear a noise-like hiss typical of chaos. Modulation is observed in the periodic paced plasma, but is not so obvious when the plasma is chaotic.

Chaotic moving striations in inert gas plasmas

Abstract Three distinct data acquisition systems (photo-diode, hybrid video camera/rotating mirror, and high-speed video camera) are utilized to study the behavior of a contemporary geissler plasma discharge tube. The geometry of the tube is responsible for the large changes that occur in the plasma current density over a short distance. The three techniques utilized reveal details of the plasma behavior in a complementary manner. The value of the fractal capacity dimension of the attractor depends on the voltage applied across the tube.

Experimental issues in the observation of water drop dynamics

Abstract The dripping faucet experiment is reexamined. High-speed video sequences reveal interesting dynamics, and expose the limitations of the traditional emitter-detector analysis technique. Refinements to the traditional technique are suggested, based on the video data. A novel data visualization and reduction technique for use with high-speed video is described.