  We show that for two identical neuronal oscillators with strictly positive
phase resetting curve, isochronous synchrony is an unstable attractor and
arbitrarily weak noise can destroy entrainment and generate intermittent phase
slips. Small inhomogeneity (mismatch in the intrinsic firing rate of the
neurons) can stabilize the phase locking and lead to more precise relative
spike timing of the two neurons. The results can explain how for a class of
neuronal models, including leaky integrate-fire model, inhomogeneity can
increase correlation of spike trains when the neurons are synaptically
connected.
