THE ROLE OF INHIBITION IN AN ASSOCIATIVE MEMORY MODEL OF THE OLFACTORY-BULB

The external plexiform layer is where the interactions between the mit ral (excitatory) and granule (inhibitory) cells of the olfactory bulb (OB) take place. Two outstanding features of these interactions are th at they are dendrodendritic and that there seem to be none between exc itatory cells. The latter are usually credited with the role of formin g Hebbian cell assemblies. Hence, it would seem that this structure la cks the necessary ingredients for an associative memory system. In thi s article we show that in spite of these two properties this system ca n serve as an associative memory. Our model incorporates the essential anatomical characteristics of the OB. The memories in our system, def ined by Hebbian mitral assemblies, are activated via the interactions with the inhibitory granule cells. The nonlinearity is introduced in o ur model via a sigmoid function that describes neurotransmitter releas e in reciprocal dendrodendritic synapses. The capacity (maximal number of odors that can be memorized) depends on the sparseness of coding t hat is being used. For very low memory activities, the capacity grows as a fractional power of the number of neurons. We validate the theore tical results by numerical simulations. An interesting result of our m odel is that its capacity increases as a function of the ratio of inhi bitory to excitatory populations. This may provide an explanation for the dominance of inhibitory cells in the olfactory bulb.