  Synchronization across long neural distances is a functionally important
phenomenon. In order to access the mechanistic basis of long-range synchrony,
we constructed an experimental model that enables monitoring of spiking
activities over centimeter scale distances in large random networks of cortical
neutrons. We show that the mode of synchrony over these distances depends upon
a length scale, $\lambda$, which is the minimal path that activity should
travel through before meeting its point of origin ready for reactivation. When
$\lambda$ is experimentally made larger than the physical dimension of the
network, distant neuronal populations operate synchronously, giving rise to
irregularly occurring network-wide events that last hundreds of milliseconds to
couple of seconds. In contrast, when $\lambda$ approaches the dimension of the
network, a continuous self-sustained reentry propagation emerges, a regular
dynamical mode that is marked by precise spatiotemporal patterns (`synfire
chains') that may last many minutes. These results contribute to discussions on
the origin of different modes of neural synchrony in normal and pathological
conditions
