  Eytan and Marom recently showed that the spontaneous burst activity of rat
neuron cultures includes `first to fire' cells that consistently fire earlier
than others. Here we analyze the behavior of these neurons in long term
recordings of spontaneous activity of rat hippocampal and rat cortical neuron
cultures from three different laboratories. We identify precursor events that
may either subside (`small events') or can lead to a full-blown burst
(`pre-bursts'). We find that the activation in the pre-burst typically has a
first neuron (`leader'), followed by a localized response in its neighborhood.
Locality is diminished in the bursts themselves. The long term dynamics of the
leaders is relatively robust, evolving with a half-life of 23-34 hours.
Stimulation of the culture can temporarily alter the leader distribution, but
it returns to the previous distribution within about 1 hour. We show that the
leaders carry information about the identity of the burst, as measured by the
signature of the number of spikes per neuron in a burst. The number of spikes
from leaders in the first few spikes of a precursor event is furthermore shown
to be predictive with regard to the transition into a burst (pre-burst versus
small event). We conclude that the leaders play a r\^ole in the development of
the bursts, and conjecture that they are part of an underlying sub-network that
is excited first and then act as nucleation centers for the burst.
