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D R Chialvo

Publications and source records attributed to D R Chialvo.

17 recordsLinked to original sources
PubMed2001

Adaptive learning by extremal dynamics and negative feedback.

We describe a mechanism for biological learning and adaptation based on two simple principles: (i) Neuronal activity propagates only through the network's strongest synaptic connections (extremal dynamics), and (ii) the strengths of active synapses are reduced if mistakes are made, otherwise no changes occur (negative feedback). The balancing of those two tendencies typically shapes a synaptic landscape with configurations which are barely stable, and therefore highly flexible. This allows for swift adaptation to new situations. Recollection of past successes is achieved by punishing synapses which have once participated in activity associated with successful outputs much less than neurons that have never been successful. Despite its simplicity, the model can readily learn to solve complicated nonlinear tasks, even in the presence of noise. In particular, the learning time for the benchmark parity problem scales algebraically with the problem size N, with an exponent k approximately 1.4.

Action Potentials↗
PubMed2000

Noise in neurons is message dependent.

Neuronal responses are conspicuously variable. We focus on one particular aspect of that variability: the precision of action potential timing. We show that for common models of noisy spike generation, elementary considerations imply that such variability is a function of the input, and can be made arbitrarily large or small by a suitable choice of inputs. Our considerations are expected to extend to virtually any mechanism of spike generation, and we illustrate them with data from the visual pathway. Thus, a simplification usually made in the application of information theory to neural processing is violated: noise is not independent of the message. However, we also show the existence of error-correcting topologies, which can achieve better timing reliability than their components.

Brain↗
PubMed2000

Noise-induced memory in extended excitable systems.

We describe a form of memory exhibited by extended excitable systems driven by stochastic fluctuations. Under such conditions, the system self-organizes into a state characterized by power-law correlations, thus retaining long-term memory of previous states. The exponents are robust and model independent. We discuss implications of these results for the functioning of cortical neurons as well as for networks of neurons.

Acoustic Stimulation↗
PubMed1999

Learning from mistakes.

We re-examine the commonly held view that learning and memory necessarily require potentiation of synapses. A simple neuronal model of self-organized learning with no positive reinforcement is presented. The strongest synapses are selected for propagation of activity. Active synaptic connections are temporarily "tagged" and subsequently depressed if the resulting output turns out to be unsuccessful. Thus, all learning occurs by mistakes. The model operates at a highly adaptive state with low activity. Previously stored patterns may be swiftly retrieved when the environment and the demands of the brain change. The combined process of: (i) activity selection by extremal "winner-take-all" dynamics; and (ii) the subsequent weeding out of synapses may be viewed as synaptic Darwinism. We argue that all the features of the model are biologically plausible and discuss our results in light of recent experiments by Fitzsimonds et al. on back-propagation of long-term depression, by Xu et al. on facilitation of long-term depression in the hippocampus by behavioural stress, and by Frey and Morris on synaptic tagging.

Brain↗
PubMed1998

Noise-induced tuning curve changes in mechanoreceptors.

Fibers from the tibial nerve of rat were isolated and spike activity recorded using monopolar hook electrodes. The receptive field (RF) of each recorded unit on the glabrous skin of the foot was mechanically stimulated with waveforms comprised of various frequency sine waves in addition to increasing levels of white noise. Single-unit responses were recorded for both rapidly adapting (RA) and slowly adapting (SA) units. Signal-to-noise ratio (SNR) of the output was quantified by the correlation coefficient (C1) between the input sine wave and the nerve responses. The addition of noise enhanced signal transmission in both RA and SA fibers. With increasing noise, the initially inverted "V"-shaped, zero-noise tuning curves for RA fibers broadened and eventually inverted. There was a large expansion of the frequencies that the RA receptor responded to with increasing noise input. On the other hand, the typical shape of the SA fiber tuning curves remained invariant, at all noise levels tested. C1 values continued to increase with larger noise input for higher frequencies, but did not do so at the lowest frequencies. For both RA and SA fibers the responses with added noise tended to be rate modulated at the low-frequency end, and followed nonlinear stochastic resonance (SR) properties at the higher frequencies. The changes in the tuning properties due to noise found here, as well as preliminary psychophysics data, imply that external noise is relevant for sensing small periodic signals in the environment. All current models of sensory perception assume that the tuning properties of receptors determined in the absence of noise are preserved during everyday tasks. Our results indicate that this is not true in a noisy environment.

Adaptation, Physiological↗
PubMed1995

General relation between variance-time curve and power spectral density for point processes exhibiting 1/f beta-fluctuations, with special reference to heart rate variability.

Counting statistics in the form of the variance-time curve provides an alternative to spectral analysis for point processes exhibiting 1/f beta-fluctuations, such as the heart beat. However, this is true only for beta < 1. Here, the case of general beta is considered. To that end, the mathematical relation between the variance-time curve and power spectral density in the presence of 1/f beta-noise is worked out in detail. A modified version of the variance-time curve is presented, which allows us to deal also with the case beta > or = 1. Some applications to the analysis of heart rate variability are given.

Animals↗
PubMed1990

Low dimensional chaos in cardiac tissue.

Chaos is a term used to characterize aperiodic activity arising in a dynamical system, or in a set of equations describing the system's temporal evolution as a result of a deterministic mechanism that has sensitive dependence on initial conditions. Chaos, in that sense, has been proposed to make an important contribution to normal and abnormal cardiac rhythms. To date, however, descriptions of chaos in heart tissue have been limited primarily to periodically forced cardiac pacemakers. Because many cardiac rhythm disturbances, particularly those initiated or perpetuated by re-entrant excitation, originate from within non-pacemaker cardiac tissues, demonstrations of chaos in non-pacemaker tissue might provide a deterministic explanation for a wide variety of complex dysrhythmias. Here we report experimental evidence for chaotic patterns of activation and action potential characteristics in externally driven, non-spontaneously active Purkinje fibres and ventricular muscle. The results indicate that there is an apparent link between the mechanism of low dimensional chaos and the occurrence of reflected responses which could lead to more spatially disorganized phenomena. A detailed mechanism for the low dimensional chaos observed experimentally is pursued using a difference equation model. Critical features of the model include a non-monotonic relationship between recovery time during rhythmic stimulation and the state of membrane properties, and a steeply sloped recovery of membrane properties over certain ranges of recovery times. Besides explaining our results, the analytical model may pertain to irregular dynamics in other excitable systems, particularly the intact dysrhythmic heart.

Animals↗
PubMed1990

Sustained vortex-like waves in normal isolated ventricular muscle.

Sustained reentrant excitation may be initiated in small (20 x 20 x less than 0.6 mm) preparations of normal ventricular muscle. A single appropriately timed premature electrical stimulus applied perpendicularly to the wake of a propagating quasiplanar wavefront gives rise to circulation of self-sustaining excitation waves, which pivot at high frequency (5-7 Hz) around a relatively small "phaseless" region. Such a region develops only very low amplitude depolarizations. Once initiated, most episodes of reentrant activity last indefinitely but can be interrupted by the application of an appropriately timed electrical stimulus. The entire course of the electrical activity is visualized with high temporal and spatial resolution, as well as high signal-to-noise ratio, using voltage-sensitive dyes and optical mapping. Two- and three-dimensional graphics of the fluorescence changes recorded by a 10 x 10 photodiode array from a surface of 12 x 12 mm provide sequential images (every msec) of voltage distribution during a reentrant vortex. The results suggest that two-dimensional vortex-like reentry in cardiac muscle is analogous to spiral waves in other biological and chemical excitable media.

Animals↗
PubMed1990

Irregular dynamics of excitation in biologic and mathematical models of cardiac cells.

Excitation and impulse propagation in cardiac tissues are dependent on the heart rate and can occur in extremely complex patterns. In this chapter we present the results of Purkinje fiber experiments and of computer simulations using an ionic (Beeler & Reuter) model for the ventricular cell. We have studied the global rate-dependent behavior of cardiac cells through a systematic analysis of their response to single as well as repetitive depolarizing stimuli, and determined the role of nonlinearity in the mechanism(s) of their behaviors. To this end, we devised an analytical difference equation model of cardiac cell excitation which could be used to predict simple as well as chaotic behavior of both the Purkinje fiber and the Beeler & Reuter cell, depending on the stimulation rate. Both experimental and modeling results suggest that the presence of supernormal recovery in cell excitability establishes sufficient nonlinearity so that, during repetitive stimulation, the dynamics of cell response may be regular and predictable when the stimulus magnitude is either very small or very large, or they may be chaotic and very unpredictable when the stimulus magnitude is intermediate. The overall results suggest that the application of nonlinear systems theory to electrophysiology may have importance in the understanding of cardiac rhythm and conduction disturbances, and may have clinical implications as well.

Animals↗
PubMed1990

Supernormal excitability as a mechanism of chaotic dynamics of activation in cardiac Purkinje fibers.

Supernormality, which can be defined as greater than normal excitability during or immediately after action potential repolarization, has been observed in a variety of cardiac preparations. However, as yet, no description of the dynamics of tissue response to repetitive stimulation in the presence of supernormal or relatively supernormal excitability has appeared. Isolated sheep cardiac Purkinje fibers (2-5 mm in length) were superfused with Tyrode's solution and stimulated with depolarizing current pulses through a suction pipette. Recovery of excitability, restitution of the action potential duration, and response patterns were measured in each fiber for a wide range of current amplitudes and stimulation frequencies. When the potassium chloride concentration of the Tyrode's solution was decreased from 7 to 4 mM, the excitability recovery function consistently changed from monophasic ("normal") to triphasic ("supernormal'). During repetitive stimulation at increasing rates, normal preparations responded only with gradual changes in the activation ratio, expressed as periodic phase-locked responses (i.e., Wenckebach-like patterns, etc.). Supernormal preparations showed a nonmonotonic change in the activation ratio, as well as complex aperiodic response patterns. Numerical results from an analytical model gave a quantitative basis for the relation between nonmonotonicity in the excitability function and the development of complex rhythms in cardiac Purkinje fibers. Both our experimental and theoretical results indicate that the presence of supernormality and the slope of the action potential duration restitution curve at short diastolic intervals are responsible for the development of chaotic dynamics. Moreover, our results give an accurate description of the supernormality phenomenon, predict the behavior expected under such conditions, and provide insight about the role of membrane recovery in determining regular and irregular frequency-dependent rhythm and conduction disturbances.

Animals↗
PubMed1990

Directional differences in excitability and margin of safety for propagation in sheep ventricular epicardial muscle.

Computer simulations and isolated tissue experiments were used to characterize the relation between excitability and margin of safety for propagation in anisotropic ventricular myocardium. Longitudinal, uniform transverse, and nonuniform transverse tissue directions were modeled in a one-dimensional Beeler-Reuter based cable. Stimulation threshold was smallest in the nonuniform transverse direction. The safety factor for propagation was determined in the model as the total axial charge that was available for depolarizing downstream tissue divided by the threshold charge that was just sufficient for continued propagation and was largest in the longitudinal direction. The strength-interval plot for the junction between simulated longitudinal and nonuniform transverse directions identified a range of stimulus strengths and intervals that resulted in nonuniform transverse but not longitudinal propagation. When high values of transverse resistance were used, higher stimulus strengths during premature stimulation resulted in longitudinal but not nonuniform transverse propagation. The experimental strength interval plots from 17 L-shaped preparations of isolated sheep epicardial muscles had similar characteristics. In nine additional L-shaped tissue experiments, changing extracellular K+ concentrations from 4 to 20 mM resulted in progressive membrane depolarization and conduction impairment in both directions. However, in eight of nine experiments, complete block occurred first in the transverse direction. In one experiment, block was simultaneous in both directions. We conclude that, under normal conditions, threshold requirements for active propagation are lower for transverse than for longitudinal propagation. In addition, when active membrane properties are impaired, the safety factor for propagation is larger in the direction along the longitudinal axis of the cells.

Adult↗
PubMed1990

Nonlinear dynamics of rate-dependent activation in models of single cardiac cells.

Recent studies in isolated cardiac tissue preparations have demonstrated the applicability of a one-dimensional difference equation model describing the global behavior of a driven nonpacemaker cell to the understanding of rate-dependent cardiac excitation. As a first approximation to providing an ionic basis to complex excitation patterns in cardiac cells, we have compared the predictions of the one-dimensional model with those of numerical simulations using a modified high-dimensional ionic model of the space-clamped myocyte. Stimulus-response ratios were recorded at various stimulus magnitudes, durations, and frequencies. Iteration of the difference equation model reproduced all important features of the ionic model results, including a wide spectrum of stimulus-response locking patterns, period doubling, and irregular (chaotic) dynamics. In addition, in the parameter plane, both models predict that the bifurcation structure of the cardiac cell must change as a function of stimulus duration, because stimulus duration modifies the type of supernormal excitability present at short diastolic interval. We conclude that, to a large extent, the bifurcation structure of the ionic model under repetitive stimulation can be understood by two functions: excitability and action potential duration. The characteristics of these functions depend on the stimulus duration.

Action Potentials↗
PubMed1989

Chaotic activity in a mathematical model of the vagally driven sinoatrial node.

Phase-locking behavior and irregular dynamics were studied in a mathematical model of the sinus node driven with repetitive vagal input. The central region of the sinus node was simulated as a 15 x 15 array of resistively coupled pacemakers with each cell randomly assigned one of 10 intrinsic cycle lengths (range 290-390 msec). Coupling of the pacemakers resulted in their mutual entrainment to a common frequency and the emergence of a dominant pacemaker region. Repetitive acetylcholine (ACh; vagal) pulses were applied to a randomly selected 60% of the cells. Over a wide range of stimulus intensities and basic cycle lengths, such perturbations resulted in a large variety of stimulus/response patterns, including phase locking (1:1, 3:2, 2:1, etc.) and irregular (i.e., chaotic) dynamics. At a low ACh concentration (1 microM), the patterns followed the typical Farey sequence of phase-locked behavior. At a higher concentration (5 microM), period doubling and aperiodic patterns were found. When a single pacemaker cell was perturbed with repetitive ACh pulses, qualitatively similar results were obtained. In both types of simulation, chaotic behavior was investigated using phase-plane (orbital) plots, Poincaré mapping, and return mapping. Period-doubling bifurcations (2:2, 4:4, and 8:8) were found temporally and spatially within the array. Under certain conditions of stimulation, the attractor in the return map during chaotic activity of the single cell resembled the Lorenz tent map. However, when electrical coupling between cells was allowed, the interactions with neighboring cells exhibiting chaotic dynamics resulted in characteristic alterations of the attractor geometry. Our results suggest that irregular dynamics obeying the rules derived from other chaotic systems are present during vagal stimulation of the sinus node. In addition, application of the same analytical tools to the analysis of simulation of reflex vagal control of sinus rate suggests that chaotic dynamics can be obtained in the physiologically relevant case of the baroreceptor reflex loop. These results may provide insight into the mechanisms of dynamic vagal control of heart rate and may help to provide insights into clinically relevant disturbances of cardiac rate and rhythm.

Acetylcholine↗
PubMedDate not supplied

Non-linear dynamics of cardiac excitation and impulse propagation.

Extremely complex frequency-dependent patterns of excitation and impulse propagation can be shown in cardiac tissues. Such complex behaviour can be analysed using methods derived from chaos theory, which is concerned with the non-linear dynamics of deterministic systems that have irregular periodicities as well as an exquisite sensitivity to the initial conditions. We report here that the general response patterns of non-oscillatory cardiac conducting tissues, when driven rhythmically by repetitive stimuli from their surroundings, are similar to those of other deterministic systems showing chaotic dynamics. Such patterns include phase locking, period-doubling bifurcation and irregular activity. We have used electrophysiological techniques and analytical arguments to explain this unforeseen behaviour and to provide some key information about its mechanisms. The study of these dynamics is of general application to the understanding of disordered phenomena in excitable media, and may provide new insight about the origin of fatal cardiac arrhythmias.

Animals↗