We have investigated whether the relative timing or synchronization of cerebral cortical action potentials has a role in the signalling of basic sensory stimulus features, beside any putative role in feature binding or neuronal grouping. The onsets of sensory stimuli cause large groups of cortical neurons to fire action potentials which take place close together in time and which effectively drive target neuronal populations. These temporally coordinated bursts in firing rate are feature-selective and are believed to represent stimuli. We have found that large neuronal populations in the primate auditory cortex can use relative spike timing to signal stimuli even when they are firing at background rates, by coordinating their action potentials together in time. Auditory cortical coding based on relative spike timing is stimulus-feature tuned, it is topographically mapped, and it can follow the stimulus timecourse even in cases where mean firing rate provides no signal. This suggests that a fundamental cortical signal for a stimulus feature may be the number of temporally coordinated action potentials in a large cortical population.