  Mitral cells, the principal neurons in the olfactory bulb, respond to
odorants by firing bursts of action potentials called sharp events. A given
cell produces a sharp event at a fixed phase during the sniff cycle in response
to a given odor; different cells have different phases. The olfactory bulb
response to an odor is thus a sequence of sharp events. Here, we show that
sharp event onset is biased toward certain phases of the ongoing gamma
frequency oscillation. Thus, the signature of an odor is a discrete sequence.
The fact that this sequence is relatively short suggests a new class of "brute
force" solutions to the problem of odor recognition: cortex may contain a small
number of modules, each forming a persistent snapshot of what occurs in a
certain gamma cycle. Towards the end of the sniff, the collection of these
snapshots forms a spatial pattern that could be recognized by standard
attractor-based network mechanisms. We demonstrate the feasibility of this
solution with simulations of simple network architectures having modules that
represent gamma-cycle specific information. Thus "brute force" solutions for
converting a discrete temporal sequence into a spatial pattern (chunking) are
neurally plausible.
