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S Coombes

Publications and source records attributed to S Coombes.

At least 37 records · Page 2Linked to original sources
PubMed2000

Dynamics of strongly-coupled spiking neurons.

We present a dynamical theory of integrate-and-fire neurons with strong synaptic coupling. We show how phase-locked states that are stable in the weak coupling regime can destabilize as the coupling is increased, leading to states characterized by spatiotemporal variations in the interspike intervals (ISIs). The dynamics is compared with that of a corresponding network of analog neurons in which the outputs of the neurons are taken to be mean firing rates. A fundamental result is that for slow interactions, there is good agreement between the two models (on an appropriately defined timescale). Various examples of desynchronization in the strong coupling regime are presented. First, a globally coupled network of identical neurons with strong inhibitory coupling is shown to exhibit oscillator death in which some of the neurons suppress the activity of others. However, the stability of the synchronous state persists for very large networks and fast synapses. Second, an asymmetric network with a mixture of excitation and inhibition is shown to exhibit periodic bursting patterns. Finally, a one-dimensional network of neurons with long-range interactions is shown to desynchronize to a state with a spatially periodic pattern of mean firing rates across the network. This is modulated by deterministic fluctuations of the instantaneous firing rate whose size is an increasing function of the speed of synaptic response.

Animals↗
PubMed1999

Mode locking and Arnold tongues in integrate-and-fire neural oscillators.

An analysis of mode-locked solutions that may arise in periodically forced integrate-and-fire (IF) neural oscillators is introduced based upon a firing map formulation of the dynamics. A q:p mode-locked solution is identified with a spike train in which p firing events occur in a period qDelta, where Delta is the forcing period. A linear stability analysis of the map of firing times around such solutions allows the determination of the Arnold tongue structure for regions in parameter space where stable solutions exist. The analysis is verified against direct numerical simulations for both a sinusoidally forced IF system and one in which a periodic sequence of spikes is used to induce a biologically realistic synaptic input current. This approach is extended to the case of two synaptically coupled IF oscillators, showing that mode-locked states can exist for some self-consistently determined common period of repetitive firing. Numerical simulations show that such solutions have a bursting structure where regions of spiking activity are interspersed with quiescent periods before repeating. The influence of the synaptic current upon the Arnold tongue structure is explored in the regime of weak coupling.

Action Potentials↗
PubMed1999

Identification of antigens on porcine pulmonary microvascular endothelial cells recognized by human xenoreactive natural antibodies.

Transplantation of organs between species is prevented in part by humoral immune responses triggered by xenoreactive natural antibodies. Although the immune barrier to xenotransplantation of the lung is thought to be qualitatively and quantitatively different than the immune barrier to xenotransplantation of the kidney or heart, the antibody-antigen reactions responsible for rejection of pulmonary xenografts have not been characterized. To begin to address this issue for porcine lungs transplanted into humans, we analyzed the porcine pulmonary endothelial antigens recognized by human xenoreactive natural antibodies. Human and baboon natural antibodies recognized glycoprotein and glycolipid antigens isolated from the membranes of porcine pulmonary microvascular endothelial cells. The antigens included the integrin chains alpha1, alpha2, alpha3, alpha5, alpha(v), beta1, beta 3, the von Willebrand Factor, and fibronectin. These glycoproteins seemed to be recognized by the same antibodies that bind to porcine kidney or cardiac xenografts. Natural antibodies also recognized at least four glycolipids containing from one to five sugar residues, although at a lower level per unit number of cells than glycoprotein antigens. The epitope recognized by natural antibodies was predominantly Gal alpha1-3Gal, a structure expressed by lower mammals but not by humans and baboons. The antigens recognized by human antibodies in the porcine lung may provide insight into the pathogenesis of the rejection reaction. Moreover, the similarity of porcine lung antigens to porcine kidney and heart antigens suggests that differences in the rejection reactions between these organs reflects the distinct responses of the organs to humoral immunity.

Animals↗
PubMed1996

Using Features for the Storage of Patterns in a Fully Connected Net.

One of the many possible conditions for pattern storage in a Hopfield net is to demand that the local field vector be a pattern reconstruction. We use this criterion to derive a set of weights for the storage of correlated biased patterns in a fully connected net. The connections are built from the eigenvectors or principal components of the pattern correlation matrix. Since these are often identified with the features of a pattern set we have named this particular set of weights as the feature matrix. We present simulation results that show the feature matrix to be capable of storing up to N random patterns in a network of N spins. Basins of attraction are also investigated via simulation and we compare them with both our theoretical analysis and those of the pseudo-inverse rule. A statistical mechanical investigation using the replica trick confirms the result for storage capacity. Finally we discuss a biologicaly plausible learning rule capable of realising the feature matrix in a fully connected net. Copyright 1996 Elsevier Science Ltd

Journal Article↗
PubMed1993

A uniform with uniformity.

Clothing is a powerful form of non-verbal communication, conveying socio-economic status, occupational role and even the authority and power vested in that role. The traditional hospital nursing uniform reflects its historic military and religious foundations, and, many would say, the servile rank of the wearer.

Clothing↗
PubMed1982

Drug states as discriminative stimuli in a flavor-aversion learning experiment.

Injection of poison into rats after they drank in the presence of stimulus compounds of a drug state and a flavor resulted in little stimulus control by the drug state. In Experiment 1, half of the rats were poisoned after drinking salt water while stimulated with amphetamine and after drinking sugar water while sedated with pentobarbital, but they were not poisoned after salt-pentobarbital or sugar-amphetamine combinations. The other half were subjected to counterbalanced procedures. In abstract language, poisoning occurred after AX and BY stimulus combinations but did not occur after AY and BX combinations. In Experiment 2A, rats were poisoned only after consuming a particular flavored solution (salt or vinegar) in a particular state (pentobarbital or undrugged); that is, if AX was poisoned, BX, BY, and AY were experienced without poisoning. There was complete counterbalancing of flavors and drug states. Experiment 2B was similar except that amphetamine was used instead of pentobarbital. In both experiments, there was some discrimination learning based on the drug state, gut it was extremely weak.

Animals↗
PubMed1982

Long-delay associations between drug states produced in rats by injecting two drugs in sequence.

Injection of pentobarbital after a rat has consumed saccharin solution usually produces a mild aversion to the saccharin. However, the pentobarbital-produced aversion is eliminated or attenuated by prior pairings of pentobarbital injections with lithium injections. This is called the Avfail (aversion failure) effect. The present experiments dealt with the effect of the temporal relation of the pentobarbital injection to the lithium injection. After forward pairings (pentobarbital before lithium) with delays between the two injections varying among groups from 2.5 min to 320 min, Avfail was invariably obtained. There was little effect of the length of the forward delay, although the Avfail effect appeared to be slightly stronger at 30-40 min or so. When the two drugs were injected simultaneously or in a backward sequence, there was a weakening of the flavor aversion produced by pentobarbital, but this is attributable to habituation to the drugs, not to Avfail.

Animals↗
PubMed1976

A tangibly reinforced speech reception threshold procedure for use with small children.

Forty normal-hearing children between 17 and 56 months were tested using a new method for determining speech reception thresholds. The words employed were names for parts of a device that was in the form of a colorful clown. The device automatically rewarded the child with a small piece of candy each time he pressed the part of the clown that had been announced through the sound field system of a speech audiometer. Results showed that operant conditioning speech audiometry using tangible reinforcers is feasible for small children. Both boys and girls can be tested accurately by this method down to age two and one-half years.

Audiometry↗
PubMedDate not supplied

Receptors, sparks and waves in a fire-diffuse-fire framework for calcium release.

Calcium ions are an important second messenger in living cells. Indeed calcium signals in the form of waves have been the subject of much recent experimental interest. It is now well established that these waves are composed of elementary stochastic release events (calcium puffs or sparks) from spatially localised calcium stores. The aim of this paper is to analyse how the stochastic nature of individual receptors within these stores combines to create stochastic behaviour on long time-scales that may ultimately lead to waves of activity in a spatially extended cell model. Techniques from asymptotic analysis and stochastic phase-plane analysis are used to show that a large cluster of receptor channels leads to a release probability with a sigmoidal dependence on calcium density. This release probability is incorporated into a computationally inexpensive model of calcium release based upon a stochastic generalisation of the fire-diffuse-fire (FDF) threshold model. Numerical simulations of the model in one and two dimensions (with stores arranged on both regular and disordered lattices) illustrate that stochastic calcium release leads to the spontaneous production of calcium sparks that may merge to form saltatory waves. Illustrations of spreading circular waves, spirals and more irregular waves are presented. Furthermore, receptor noise is shown to generate a form of array enhanced coherence resonance whereby all calcium stores release periodically and simultaneously.

Animals↗