Biomedical subjects

I E Dammasch

Publications and source records attributed to I E Dammasch.

4 recordsLinked to original sources

PubMed1989

Compensation type algorithms for neural nets: stability and convergence.

Plasticity of synaptic connections plays an important role in the temporal development of neural networks which are the basis of memory and behavior. The conditions for successful functional performance of these nerve nets have to be either guaranteed genetically or developed during ontogenesis. In the latter case, a general law of this development may be the successive compensation of disturbances. A compensation type algorithm is analyzed here that changes the connectivity of a given network such that deviations from each neuron's equilibrium state are reduced. The existence of compensated networks is proven, the convergence and stability of simulations are investigated, and implications for cognitive systems are discussed.

Algorithms↗

PubMed1988

Self-stabilization of neuronal networks. II. Stability conditions for synaptogenesis.

This study is concerned with synaptic reorganization in local neuronal networks. Within networks of 30 neurons, an initial disequilibrium in connectivity has to be compensated by reorganization of synapses. Such plasticity is not a genetically determined process, but depends on results of neuronal interaction. Neurobiological experiments have lead to a model of the behavior of individual neurons during neuroplastic reorganization, formalized as a "synaptogenetic rule" that governs changes in the amount of synaptic elements on each neuron. When this synaptogenetic rule is applied to a system of neurons, there is some freedom left to the choice of further conditions. In this study it is examined, which assumptions additional to the synaptogenetic rule are essential in order to obtain morphogenetic stability. By explicating these assumptions, their plausibility can be tested. It is analysed, in which respect these conditions are important, in which part of the model they exert their influence, and what kind of instability and degeneration happens if the assumptions are violated. Our essentials for reaching morphogenetic stability are: (1) A network structure that guarantees the possibility of oscillations, (2) a compensation algorithm that guarantees a smooth morphogenesis, (3) kinetic parameters that guarantee convergence in the synaptic elements' change, and (4) a synaptic modification rule that prohibits Hebb-like as well as anti-Hebb-like synaptic changes. It is concluded that many structural features of the mammalian cerebral cortex are in accordance with the requirements of the model.

Algorithms↗

PubMed1986

GABA receptor binding in rat cerebral cortex and superior cervical ganglion in the absence of GABAergic synapses.

High affinity binding of [3H]gamma-aminobutyric acid (GABA) to membrane-containing homogenates prepared from rat cerebral cortex (neonatal or adult) or from adult rat superior cervical ganglion (SCG) was tested for sensitivity to baclofen, bicuculline and several other analogues of GABA. Since no major differences were found between the tissues free of GABAergic synapses (SCG and neonatal cortex) and GABAergic synapse-rich adult rat cortex, it is suggested that the presence of GABAergic synapses does not play a decisive role in the determination of pharmacological sensitivities of high-affinity GABA receptors.

Aging↗

PubMed1986

Self-stabilization of neuronal networks. I. The compensation algorithm for synaptogenesis.

Between the extreme views concerning ontogenesis (genetic vs. environmental determination), we use a moderate approach: a somehow pre-established neuronal model network reacts to activity deviations (reflecting input to be compensated), and stabilizes itself during a complex feed-back process. Morphogenesis is based on an algorithm formalizing the compensation theory of synaptogenesis (Wolff and Wagner 1983). This algorithm is applied to randomly connected McCulloch-Pitts networks that are able to maintain oscillations of their activity patterns over time. The algorithm can lead to networks which are morphogenetically stable but preserve self-maintained oscillations in activity. This is in contrast to most of the current models of synaptogenesis and synaptic modification based on Hebbian rules of plasticity. Hebbian networks are morphogenetically unstable without additional assumptions. The effects of compensation on structural and functional properties of the networks are described. It is concluded that the compensation theory of synaptogenesis can account for the development of morphogenetically stable neuronal networks out of randomly connected networks via selective stabilization and elimination of synapses. The logic of the compensation algorithm is based on experimental results. The present paper shows that the compensation theory can not only predict the behavior of synaptic populations (Wagner and Wolff, in preparation), but it can also describe the behavior of neurons interconnected in a network, with the resulting additional system properties. The neuronal interactions--leading to equilibrium in certain cases--are a self-organizing process in the sense that all decisions are performed on the individual cell level without knowing the overall network situation or goal.

Animals↗