The inputs into individual glomerulus from receptor neurons are therefore substantially correlated (Koulakov et al., 2007, Lledo et al., 2005, Shepherd et al., 2004 and Wachowiak et al., 2004). This glomerulus-based modularity is preserved further by the mitral cells (MCs),

most of which receive direct excitatory inputs from a single glomerulus only. MCs are a major output class of the olfactory bulb. These cells transmit information about odorants to the olfactory cortex (Figure 1). The representation of odorants by MCs is often described as combinatorial code (Firestein, 2004 and Koulakov et al., 2007). For such a code, both odorant identity and concentration can be derived from the particular combination of active MCs. A

large HSP cancer number of MCs provides enough combinatorial diversity to encode virtually any relevant stimulus. Early studies of the MC code have found that the sustained responses of MCs to odorants are sparse and state dependent (Adrian, 1950, Kay and Laurent, 1999 and Rinberg et al., 2006). Sparseness of combinatorial representation implies that only a small fraction of cells displays detectable responses Cilengitide cell line to odorants. In awake and behaving animals, the odor responses of most MCs vanish on the background of this high spontaneous activity (Adrian, 1950, Kay and Laurent, 1999 and Rinberg et al., 2006). By contrast, in anesthetized animals, the responses are vigorous and dense, at least in the case of ketamine/xylazine anesthesia (Rinberg et al., 2006). Consequently, many MCs lose their responses to odorants when the effects of anesthesia are removed; this suggests that, in awake animals, these cells ignore their odorant-related inputs from the receptor neurons (Rinberg et al., 2006). Thus, in this paper, we ask how MCs can disregard their odorant-related inputs despite receiving substantial inputs from receptor neurons. Another form of odorant representation by MCs is temporal code (Brody and Hopfield, 2003 and Hopfield, 1995). In this coding scheme, MCs respond to odorants by

forming ensembles of cells with transiently synchronized action potentials. The identities of the synchronized cells carry information about the stimulus. Recent observations by Cury and Resminostat Uchida (2010) and Shusterman et al. (2011) demonstrate the potential importance of fine time scales in odor coding. A large fraction of MCs appeared to respond with sharp and temporally precise firing events. These transients are synchronized with the temporal phase of the respiration cycle and occur in a larger fraction of MCs than previously reported on the basis of sustained combinatorial code (Rinberg et al., 2006). It could be argued that the representation of odorants by these cells is temporally sparse; i.e., they respond with transient events that occupy a small part of the respiration cycle.