This dynamic synaptic organization may act as a filter for OB inputs to PC that show strong inhalation-coupled temporal patterning.; this prediction might be tested in vivo by comparing PC neuron responses to inhalation-driven, odorant-evoked inputs with responses to brief electrical stimulation of MT axons. There is strong behavioral evidence that a single, inhalation-driven “packet” of activity can encode odor information sufficiently to support odor discrimination. Rodents (and

humans) can discriminate two familiar odors after a single Ion Channel Ligand Library in vivo sniff and in as little as 150–250 ms (Abraham et al., 2004, Laing, 1986, Rinberg et al., 2006b and Uchida and Mainen, 2003). In fact, behavioral measurements of odor perception times in awake rats, performed simultaneous with imaging of ORN inputs to the OB, indicate that a novel odor can be distinguished from a familiar

one before the initial ORN response burst—as inferred from presynaptic calcium imaging—has even finished ( Wesson et al., VX-770 datasheet 2008a). In addition, rats performing a two-choice odor discrimination task tend to make their choice after only a single sniff when that sniff evokes strong neural responses within an optimal time-window after inhalation but not when it evokes activity at later times ( Cury and Uchida, 2010). Thus, the initial onset phase of the inhalation-evoked burst of ORN activity appears particularly important for olfactory processing and odor perception. Rodents require more time—an additional 100–200 ms—to discriminate highly similar odors ( Abraham et al., 2004 and Rinberg et al., 2006b; Adenylyl cyclase Figure 3D). This additional time roughly matches the time-window over which patterns of ORN input and MT cell activity

evolve after an inhalation ( Figure 3A; Cury and Uchida, 2010, Shusterman et al., 2011 and Wesson et al., 2008a). Thus, the dynamics of inhalation-evoked ORN inputs to the OB may set an upper limit on the time window for integrating odor information in the behaving animal ( Schaefer and Margrie, 2007). One longstanding—and still unresolved—question is whether the precise timing of odorant-evoked activity relative to the timing of inhalation plays a role in odor perception. Modeling and experimental data support the idea that spike timing relative to inhalation can robustly represent odor information (Chaput, 1986, Hopfield, 1995, Schaefer and Margrie, 2007 and Shusterman et al., 2011). However, whether animals actually use a sniff-based temporal code remains unclear. Important evidence in support of such a coding strategy comes from a recent study using optogenetics in awake, head-fixed mice to activate the same ORN inputs at different times relative to inhalation or exhalation onset (Smear et al., 2011).