  Spike synchrony, which occurs in various cortical areas in response to
specific perception, action and memory tasks, has sparked a long-standing
debate on the nature of temporal organization in cortex. One prominent view is
that this type of synchrony facilitates the binding or grouping of separate
stimulus components. We argue instead for a more general function: A measure of
the prior probability of incoming stimuli, implemented by long-range,
horizontal, intra-cortical connections. We show that networks of this kind --
pulse-coupled excitatory spiking networks in a noisy environment -- can provide
a sufficient substrate for stimulus-dependent spike synchrony. This allows a
quick (few spikes) estimate of the match between inputs and the input history
as encoded in the network structure. Given the ubiquity of small, strongly
excitatory subnetworks in cortex, we thus propose that many experimental
observations of spike synchrony can be viewed as signs of input patterns that
resemble long-term experience, i.e. patterns of high prior probability.
