Global Coherent Activities in Inhibitory Neural Systems: Chik Tai Wai David.

This dissertation, "Global Coherent Activities in Inhibitory Neural Systems: Chik Tai Wai David." by Tai-wai, David, Chik, 戚大衛, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled GLOBAL COHERENT ACTIVITIES IN INHIBITORY NEURAL SYSTEMS submitted by Chik Tai Wai David for the degree of Doctor of Philosophy at The University of Hong Kong in August 2004 Global coherent activities have been observed in the brains of both human beings and animals. The electrical discharges of a large number of neurons are either synchronized or grouped into a few clusters. These phenomena are suggested to function as information binding, which is essential to various cognitive processes. On the other hand, post-inhibitory rebound is a nonlinear phenomenon present in a variety of nerve cells. Following a period of hyper-polarization, this effect allows a neuron to fire a spike or packet of spikes before returning to rest. Based on suitable mathematical models, global coherent behaviours in a network of inhibitory coupled neurons were studied. First, noise-induced weak synchronized oscillatory activities in a globally in- hibitory coupled Hodgkin-Huxley neuronal network were studied numerically.A kind of intrinsic delay was observed and was found to be important in de- termining the overall frequency of the network. Synchronization occurred in an optimal range of noise intensity with a bell-shaped curve when the inhibitory coupling strength was sufficiently strong. Comparisons with the results for the excitatory coupling were also addressed. Next, it was found that networks of neurons, which did not intrinsically oscillate, might make use of inhibitory synaptic connections to generate large scale coherent rhythms in the form of cluster states. They were classified into two cases: (i) the rebound mechanism was due to anode break excitation and (ii) it was due to a slow T-type calcium current. In the former case, a geometric analysis on a McKean type model was used to obtain expressions for the number of clusters in terms of the speed and strength of synaptic coupling. Results were found to be in good qualitative agreement with numerical simulations of the more detailed Hodgkin-Huxley model. In the second case, a particular firing rate model of a neuron with a slow calcium current was considered, that admitted to an exact analysis. Once again existence regions for cluster states were explicitly calculated. Both mechanisms were shown to prefer globally synchronous states for slow synapses as long as the strength of coupling is sufficiently large. With a decrease in the duration of synaptic inhibition, both systems were found to break into clusters. A major difference between the two mechanisms for cluster generation was that anode break excitation could support clusters with several groups, while slow T-type calcium currents predominantly gave rise to clusters of just two (anti-synchronous) populations. The results agreed with biological experiments and offered a better under- standing on the nonlinear dynamics underlying synchrony and clustering in the nervous system. DOI: 10.5353/th_b3104040 Subjects: Neural networks (Neurobiology) Neurons - Physiology Neurophysiology