  Developing networks of neural systems can exhibit spontaneous, synchronous
activities called neural bursts, which can be important in the organization of
functional neural circuits. Before the network matures, the activity level of a
burst can reverberate in repeated rise-and-falls in periods of hundreds of
milliseconds following an initial wave-like propagation of spiking activity,
while the burst itself lasts for seconds. To investigate the spatiotemporal
structure of the reverberatory bursts, we culture dissociated, rat cortical
neurons on a high-density multi-electrode array to record the dynamics of
neural activity over the growth and maturation of the network. We find the
synchrony of the spiking significantly reduced following the initial wave and
the activities become broadly distributed spatially. The synchrony recovers as
the system reverberates until the end of the burst. Using a propagation model
we infer the spreading speed of the spiking activity, which increases as the
culture ages. We perform computer simulations of the system using a
physiological model of spiking networks in two spatial dimensions and find the
parameters that reproduce the observed resynchronization of spiking in the
bursts. An analysis of the simulated dynamics suggests that the depletion of
synaptic resources causes the resynchronization. The spatial propagation
dynamics of the simulations match well with observations over the course of a
burst and point to an interplay of the synaptic efficacy and the noisy neural
self-activation in producing the morphology of the bursts.
