Implicit Selleck CT99021 experimental evidence for the presence of such regulation already exists. In the anesthetized state, when the efferent signal coming to the bulb from the cortex is minimal, the MCs respond strongly to the odor stimulation. In the awake state, when the cortex is active, the MC code becomes sparser (Adrian, 1950, Kay and Laurent, 1999 and Rinberg et al., 2006). Cutting the lateral olfactory tract and eliminating feedback from the brain in an awake rabbit led to MC responses that were similar to those in an anesthetized rabbit (Moulton, 1963). Therefore, centrifugal projection may indeed regulate the sparseness of the olfactory code. Some evidence also points toward

the possibility of finer network tuning by specific activation or deactivation of subsets of GCs to enhance extraction of relevant information.

selleck inhibitor First, Doucette and Restrepo, 2008 demonstrated that the MC responses to odorants change as animals learn the task. Second, Fuentes et al., 2008 showed that the number of responding MCs depends on the task. When a rat is involved in an odor discrimination task, the average number of MC excitatory responses is less than that in the rat passively smelling an odor. The assumption is that when an animal discriminates odorants, it may be advantageous to suppress redundant MC responses and enhance those that carry the most behaviorally relevant information. Our model proposes the network mechanism for this phenomenon. If the signal first appears in the inputs to MCs, it causes an elevation of the MC firing rates, which, in turn, causes activation of GCs and suppression of MCs by feedback inhibition. The sensory inputs of the MCs are therefore represented initially by transients of activity followed by decay to the steady state as described in this study (Figure 7B). This observation

raises several questions. First, what information about the stimulus is sent to the cortex by transients and the consequent steady state responses? Second, what are the experimental conditions for the observations of such bursts? While the roles of different modes of information transmission are unclear, we can make some predictions Bay 11-7085 about the second question. The MC activity transients are short and stand on top of high levels of spontaneous activity. In order to observe such transients reliably, one needs to synchronize spike trains with stimulus delivery. In mammals, stimulus delivery is controlled by sniffing. In previous studies (Doucette and Restrepo, 2008, Fuentes et al., 2008, Kay and Laurent, 1999 and Rinberg et al., 2006), the authors synchronized their recordings with stimulus onset but not with sniffing/breathing. This approach may lead to smearing of the short bursts and emphasize the steady state responses of the network. In such a regime, the odor responses should be combinatorially sparse as predicted by the model. New evidence by Shusterman et al.