In Hebbs theory of the cell assembly, short-term memory traces can be stored in closed loop neuronal circuits in form of persistent reverberatory activity.In previous studies, we have observed persistent network reverberation in cultured hippocampal neurons (Lau and Bi, PNAS, 2005).Here, we investigated the spatiotemporal dynamics of such collective activity of neurons grown on multi-electrode arrays (MEAs).In this preparation, a reverberation episode consisted of many distinct population bursts defined as reverberation event, each involved the spiking activity of many neurons within a time span of tens of milliseconds.Here, we developed a robust method to define the similarity between two events and use it to find conserved patterns among all events.We found that there existed conserved spatiotemporal patterns with high temporal precision in cultured neuronal networks.Furthermore, multiple patterns could co-exist in the same network during spontaneous reverberation and could occur intermittently within the same reverberation episode.In smaller networks, reverberation could only be evoked by stimulating an input channel, and sometimes only one conserved pattern was identified.These results suggested that precise spatiotemporal dynamics in neural circuits might allow for more efficient information coding in the brain.