Complexity of spatiotemporal activity of a neural network model which depends on the degree of synchronization

Spatiotemporal activity of a hippocampal CA3 model and its dynamic features were investigated. The CA3 model consists of 256 pyramidal cells and 25 inhibitory interneurons. Each pyramidal cell is a single-compartment model which was reduced from the 19-compartment cable model of the CA3 pyramidal cell developed by [Traub et al. (1991)]. Each interneuron is a model which causes tonic responses to constant depolarizing currents. The hippocampal model spontaneously causes four kinds of rhythms, A-D, which depend on the degree of synchronization of neuronal activity. The rhythm A (about 2Hz) which occurs in a range of strong mutual excitation is spatially coherent, though epileptiform bursts of pyramidal cells propagate from one end of the network to the other in a short period of time. The rhythm B (about 3Hz) occurs in an intermediate range of the strength of mutual excitation; synchronization of bursts is incomplete and the spatiotemporal pattern is complex. When the mutual excitation is relatively weak, the rhythm C (about 6Hz) occurs. Burst propagation is not uniform in direction, and the spatiotemporal activity is irregular. The rhythm D (10-35Hz) occurs in a range of weak mutual excitation when the recurrent inhibition is relatively strong. In this parameter region, pyramidal cells do not cause bursting discharges but irregular beating discharges. The hippocampal model causes phase-lockings and irregular responses to periodic synaptic stimulation depending on its own rhythmic activity and stimulus parameters. Bursting discharges of pyramidal cells are well synchronized in phase-locked responses. Several irregular responses of the rhythms A and B are evidently chaotic; each one-dimensional strobomap of chaotic responses is a non-invertible function with an unstable fixed point. Attractors reconstructed from chaotic responses demonstrate the stretching and folding mechanism.