Bidirectional Cycling Dynamics of Living Neuronal Networks in Vitro

Phenomena of synchronization, rhythmogenesis and coherence found in brain networks are believed to be a dynamical substrate for cognitive functions such as learning and memory. However, it is still debated whether the rhythmic activity emerges from network morphology developed in neurogenesis or as a result of neuronal dynamics realized under certain conditions. In this research we found, that in neural networks formed in mature hippocampal cultures with high cellular density the spiking activity self-organized and converged to long, complex and rhythmically repeated superbursts. The superburst lasted tens of seconds and consisted of hundreds of short (50-100 ms) small bursts with a high spiking rate of 139.0 {+/-} 78.6 Hz that can be associated with high-frequency oscillations in the hippocampus. In turn, the interval between peak burst activities in the range of 100-150 ms can be treated as a theta rhythm (11.2 {+/-} 1.5 Hz). Distribution of spikes within the bursts was non-random, representing a set of well-defined space-time base patterns or motifs. We found that the long superburst can be classified into two types. Each type was associated with a unique direction of spike propagation and, hence, can be encoded by a binary sequence with random switching between the two \"functional\" states. Such precisely structured bidirectional rhythmic activity developed in self-organizing cultured networks were quite similar to what observed in the in vivo experiments.