Repeating triplets of spikes and oscillations in the mitral cell discharges of freely breathing rats.

The olfactory bulb responses to odours display evident temporal organization, both in the form of high-frequency oscillations and precisely replicating triplets of spikes. In this study, the frequency of replicating triplets in a sample of 118 individual responses from 45 cells was compared with that in simulations of non-homogeneous Poisson processes, constructed from the experimental post-stimulus time histograms (PSTHs). In a large majority of the records, replicating triplets (to a precision of 0.5 ms) are found to be more numerous in the physiological records; in some of them, they are approximately 10 times more abundant. An excess of precisely replicating triplets is also found in records where no oscillations are apparent in the autocorrelograms. Triplet replication thus seems a more robust phenomenon than transient oscillation. Not unlike fast oscillations observed in other preparations, replicating triplets produced by a given mitral cell are generally observed only during a restricted period of time of the respiratory cycle (at least in the case of the responses under olfactory stimulation). No relation was found, however, between the nature and strength of the olfactory stimulus and the frequency of replicating patterns. In the absence of olfactory stimulation, some mitral cell discharges also contain more replicating triplets than the non-homogeneous Poisson simulations. Thus, replicating triplets in single-cell discharges seem to play only an indirect role in the coding of olfactory information at the mitral cell output level.