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At least 1,405 records · Page 78Linked to original sources
PubMed1996

Recurrent inhibition and clustered connectivity as a basis for Gabor-like receptive fields in the visual cortex.

A continuous-space model of visual cortex interactions which, starting from elongate Gaussian functions, leads to high-selective Gabor-like receptive fields, is proposed. The model is based on intracortical inhibition mechanisms occurring through medium-range clustered connections. The analysis, carried out under the assumption of a linear superposition of geniculate and intracortical contributions, shows how inhibition enhances both spatial and spatial-frequency selectivity. The effects of inhibition strength and of its spatial organization on the properties of the resulting receptive field are investigated. Specifically, the relationships between receptive fields and inhibition schemes are investigated by deriving analytical expressions for their dependence and through a systematic numerical parametric study. The emergence of periodic receptive fields, like the ones observed in neurophysiological measurements, is also pointed out in relation to the clustered nature of the inhibitory scheme.

Animals↗
PubMed1996

Synchronization without oscillatory neurons.

Experimental results suggest that neurons in the cortex synchronize their action potentials on the millisecond time scale. More importantly this binding expresses functional relationships between the neurons. A model of neuronal interactions is proposed in which simultaneous discharges of neurons develop through specialized synaptic circuits. As an important prerequisite for this synchronization it is demonstrated that SynFire chains, generating different levels of excitation, propagate their activity waves at distinct velocities. Two chains were coupled by excitatory synapses and their activity was initiated at different times. Due to synaptic interactions, activity in the earlier-initiated chain accelerates propagation in the other chain until the two activity waves are synchronized. Compared with several neural network models with oscillatory units, physiologically more plausible neurons are simulated. It is still under debate whether neurons in the cortex show oscillatory discharges per se. In particular, a high rate of noise relative to very weak synaptic gains cannot impair our results in the neural network simulations.

Action Potentials↗
PubMed1996

Thermoregulation in homeotherms: central temperature results from optimization of energy transfers.

In contrast to the classical homeostatic concept of the constancy of the central temperature, this study proposes an original model of thermoregulation based on the optimization of energy transfers. Exchange of the energy consumed or produced by the cell between the cell and the external medium has an associated energy cost. The different variables of the internal medium-flows, pressures, concentrations and also temperatures, since heat is but a particular form of energy--are continuously set at optimal values such that this cost is always minimal for the prevailing constraints with which the organism is faced. The proposed thermoregulatory model accounts for the physiological spatial and temporal variability of the body's temperatures. The predictive curves suggest a new approach to experimental studies concerned with thermal regulation and throw new light on their results.

Animals↗
PubMed1996

Extracellular action potentials of skeletal muscle fibre affected by 4-aminopyridine: a model study.

A new specially designed analytical function approximating the intracellular action potentials (ICAPs) for calculation of the extracellular potentials (ECAPs) at various radial and axial distances from the active fibre is proposed. 4-Aminopyridine (4-AP) was used to obtain ICAPs with a prolonged repolarization phase in order to investigate the influence of changes in ICAP shape on the ECAPs. From the experimentally recorded ICAPs before and after treatment of frog skeletal muscle fibres with 4-AP, approximated by the new function, the ECAPs were calculated applying the line-source model in a finite fibre. Using this function allowed calculation of the ECAPs at distances not accessible for the experimental recordings. The total ionic current (Ii) during the action potential was calculated using the cable equation. Our results showed that the ratio of the first positive to the negative phases of the ECAPs of treated fibres increased at large radial distances (3000 microns and more) and the terminal positive phase was asymmetric with an abrupt initial deflection followed by a slow inverse deflection. The calculated ECAPs at various axial distances from the fibre end (cylindrical and conical part) and at radial distances from the fibre membrane ranging from 0 to 5000 microns, corresponded in shape to the experimentally recorded potentials of untreated and 4-AP-treated muscle fibres.

4-Aminopyridine↗
PubMed1996

Structural study of the development of ocularity domains using a neural network model.

We present a model for the development of ocularity domains in the visual cortex of mammals during the embryonic stage. We model the thalamo-cortical pathway with a self-organising neural network with two source layers, each of them serving different retinae, and one target layer, where the connections end. The connectivity between the source layers and the target layer is driven by Hebbian learning. In both the source layers and the target layer we assume excitatory lateral signal diffusion between proximal neurons that causes them to be correlated. According to the developmental state being modelled, we do not consider either correlation or anti-correlation between the signals originated in neurons of different retinae. The basic assumptions made are proved to be sufficient to attain a distribution of connections arranged in ocularity domains. The dependence of the geometry of the ocularity domains on the parameters of the model is analysed and a correlation between the width of the signal diffusion and the extent of the domains is found. The generality of the assumptions made allows an easy translation of the model to explain the development of other elements of the sensory nervous system.

Animals↗
PubMed1996

Modeling motor cortical operations by an attractor network of stochastic neurons.

Understanding the neural computations performed by the motor cortex requires biologically plausible models that account for cell discharge patterns revealed by neurophysiological recordings. In the present study the motor cortical activity underlying movement generation is modeled as the dynamic evolution of a large fully recurrent network of stochastic spiking neurons with noise superimposed on the synaptic transmission. We show that neural representations of the learned movement trajectories can be stored in the connectivity matrix in such a way that, when activated, a particular trajectory evolves in time as a dynamic attractor of the system while individual neurons fire irregularly with large variability in their interspike intervals. Moreover, the encoding of trajectories as attractors ensures high stability of the ensemble dynamics in the presence of synaptic noise. In agreement with neurophysiological findings, the suggested model can provide a wide repertoire of specific motor behaviors, whereas the number of specialized cells and specific connections may be negligibly small if compared with the whole population engaged in trajectory retrieving. To examine the applicability of the model we study quantitatively the relationship between local geometrical and kinematic characteristics of the trajectories generated by the network. The relationship obtained as a result of simulations is close to the '2/3 power law' established by psychophysical and neurophysiological studies.

Animals↗
PubMed1996

Coupled oscillators utilised as gait rhythm generators of a two-legged walking machine.

The gait of current two-legged walking machines differs from that of humans, although the kinematic structures of these machines' legs frequently imitate human limbs. This paper presents a method of generating the trajectories of hip and knee joint angles resulting in a gait pattern similar to that of a human. For this purpose the solutions of coupled van der Pol oscillator equations are utilised. There is much evidence that these equations can be treated as a good model of the central pattern generator generating functional (also locomotional) rhythms in living creatures. The oscillator equations are solved by numerical integration. The method of changing the type of gait by changing appropriate parameter values in the oscillator equations is presented (change of velocity and trajectory of leg-ends). The results obtained enable enhanced control of two-legged walking systems by including gait pattern generators which will assume a similar role to that of biological generators.

Bionics↗
PubMed1996

A columnar model of somatosensory reorganizational plasticity based on Hebbian and non-Hebbian learning rules.

Topographical and functional aspects of neuronal plasticity were studied in the primary somatosensory cortex of adult rats in acute electrophysiological experiments. Under these experimental conditions, we observed short-term reversible reorganization induced by intracortical microstimulation or by an associative pairing of peripheral tactile stimulation. Both types of stimulation generate large-scale and reversible changes of the representational topography and of single cell functional properties. We present a model to simulate the spatial and functional reorganizational aspects of this type of short-term and reversible plasticity. The columnar structure of the network architecture is described and discussed from a biological point of view. The simulated architecture contains three main levels of information processing. The first one is a sensor array corresponding to the sensory surface of the hind paw. The second level, a pre-cortical relay cell array, represents the thalamo-cortical projection with different levels of excitatory and inhibitory relay cells and inhibitory nuclei. The array of cortical columns, the third level, represents stellate, double bouquet, basket and pyramidal cell interactions. The dynamics of the network are ruled by two integro-differential equations of the lateral-inhibition type. In order to implement neuronal plasticity, synaptic weight parameters in those equations are variables. The learning rules are motivated by the original concept of Hebb, but include a combination of both Hebbian and non-Hebbian rules, which modifies different intra- and inter-columnar interactions. We discuss the implications of neuronal plasticity from a behavioral point of view in terms of information processing and computational resources.

Animals↗
PubMed1995

A historical note on "artificial life".

In this paper, I am dealing with some epistemological aspects of what Christopher Langton (1989) and some other scientists have been calling recently "artificial life", whose history, in fact, is far older. I want to take a view on the origin, further developments and latest issues of these models, and try to point out the major philosophical and epistemological problems arising with them.

Algorithms↗
PubMed1991

Experience using the CQI model in health care management.

This article describes practical applications of the Continuous Quality Improvement (CQI) concept applied to management of the Information Services Departments of a health care institution. The article is valuable for two reasons: (1) it is valuable as a successful application of CQI in an award-winning information services division of an award-winning health care institution, and (2) since the work of the information services function is so complex, has major interdependencies with many other functions throughout the entire organization, and is driven by fast-paced change, successful use in that function can be a model for other major functions of the hospital.

Computer User Training↗
PubMed1994

Modeling human motion perception. I. Classical stimuli.

Motion perception is one of the most prominent tasks of the visual system and therefore has been extensively investigated both experimentally and theoretically. A classical model describing the mechanism of motion detection originally developed in the context of insect orientation behavior, the elementary motion detector (EMD) of the correlation type, turned out to be very powerful in explaining many basic aspects of human motion perception. For more complex visual tasks, like the discrimination of a figure from its background by relative motion, on the other hand, further processing of motion information is required. In the first part of this review it will be illustrated by means of a few examples, what kind of motion information can be derived from the mere correlation-type model, and what perceptual phenomena can be accounted for by the EMD. In the second part, more recently developed stimuli will be introduced to answer the question what further processing steps, or more sophisticated mechanisms than the EMD, have to be assumed in order to understand "higher" aspects of human motion perception.

Cybernetics↗