Several neural codes have been proposed in order to explain how neurons encode sensory information. Here we tested the hypothesis that different codes might be employed concurrently and provide complementary stimulus information. Quantifying the information encoded about natural sounds in the auditory cortex of alert animals, we found that temporal spike-train patterns and spatial populations were both highly informative. However, the relative phase of slow ongoing rhythms at which these (temporal or population) responses occurred provided much additional and complementary information. Such nested codes combining spike-train patterns with the phase of firing were not only most informative, but also most robust to sensory noise added to the stimulus. Our findings suggest that processing in sensory cortices could rely on the concurrent use of several codes that combine information across different spatiotemporal scales. In addition, they propose a role of slow cortical rhythms in stabilizing sensory representations by reducing effects of noise.
