Firing rate and firing synchrony distinguish dynamic from steady state sound.

The role of firing-rate and firing synchrony within and between three auditory cortical areas in coding complex sounds was investigated. In cortex, steady-state sound rarely caused any difference in firing rate over the spontaneous rate, but induced greater neural synchrony within and between cortical areas than were present during silence. Dynamic changes, other than onsets, produced similar synchrony as steady-state stimulation, but the firing rates were significantly larger in each cortical area than for steady-state sound. This suggests that the firing rate signals the difference between dynamic and steady-state stimulation. Because firing rates are high for stimulus onsets, the combination of firing rate and intercortical synchrony may distinguish between dynamic and steady-state stimuli and silence.