on the activity at NORDITA

by Andrius Bernotas

a NORDITA Baltic / NW Russia fellow 1998

    My presence at NORDITA was split into two periods, one month in May-June, and a month in October-November. All this time I have been guided by Professor John Hertz,  and worked in his laboratory. I collaborated with Thomas Z. Lauritzen, a Professor's M.Sc. student from Copenhagen University during the first period of my stay, and with Andrea Fazzini and Sergio Solinas, the M.Sc. students from the Bologna University, during the last visit.
    The problem that we have investigated is the emergence of spontaneous asynchronous chaotic activity in neural networks comprising populations of excitatory and inhibitory neurons. And we have tried to model this activity via the computer simulations of networks consisting of "simple neurons" (with the key parameters chosen to be biologically plausible). Specifically, I am working with the synfiren code simulating the network of "integrate & fire" leaky neurons. (The code was originally developed by a former NORDITA fellow Adam Pruegel-Bennett for simulating the synchronous firing chain, hence its name synfire). In the course of work I modified the code to perform the proper network connections, and to display the spikes emitted by neurons as its output. Also, the afferent excitatory and inhibitory potentials are displayed now. A number of new input parameters were introduced, too.
    The theoretical background for this work are the publications by W. Gerstner, D. Amit, C. van Vreeswijk and H. Sompolinsky, and their co-workers. Although in the mean-field theory framework that they are considering, their models are predictive, we have found that the computer simulations we make often are not exactly in the thermodynamic limit, i. e. our networks are not big enough. The open questions that may arise are: 1) should we attempt to stretch these limits (anyway, even the biological networks aren't infinite); 2) if yes, how far can we do that in simulations; 3) what corrections are expected in the "not-clean-limit" case? These are just few of the problems that need further consideration. Still, the asynchronous spontaneous chaotic activity of neurons in our simulated networks demonstrates itself.
    I would like to thank NORDITA for the support, Professor John Hertz for his guidance, and NORDITA's staff for the courses, seminars, discussions and other activities that I had a pleasure to participate in here.
Andrius Bernotas
Institute of Theoretical Physics and Astronomy
A. Gostauto 12, 2600 Vilnius, Lithuania