, 2001 and Pantoja et al , 2007) Potential exceptions are in V1

, 2001 and Pantoja et al., 2007). Potential exceptions are in V1 cortex in the visual system (Shuler and Bear, 2006), the brainstem in the gustatory system (Chang and Scott, 1984), and within the olfactory system, where learning-induced changes occur within the olfactory bulb (OB) one or two synapses away from the sensory neuron (Friedrich et al., 2004, Gao and Strowbridge, 2009, Gray et al., 1986, Kay and Laurent, 1999, Nissant et al., 2009 and Wilson and Leon, 1988). However, it is not clear whether learning-related plasticity in these early circuits represents a modulation in the circuitry to enhance discrimination or whether it plays a more dynamic role and actively contributes to the

encoding of stimulus value (Kay and Laurent, 1999). Please note that when we refer to odor value, we do not exclude Selleck 3-deazaneplanocin A Proteasome inhibitor drugs the possibility that the circuit may carry information on a related reward signal (Wallis and Kennerley, 2010). Olfactory sensory neurons transform information about the chemical structure of an odor into neuronal activity and transmit information synaptically to second-order cells, including the mitral cells (MCs) (Shepherd et al., 2004 and Tan et al., 2010). Interneuron circuits within the OB modulate MC firing and likely provide contrast

enhancement (Aungst et al., 2003, Mori et al., 1999 and Shepherd et al., 2004), and learning modifies activity of MCs through plasticity that is likely caused by feedback from neuromodulatory systems and centrifugal input from the olfactory cortex (OC) back into the OB (Doucette and Restrepo, 2008, Gao and Strowbridge, 2009, Mandairon and Linster, 2009, Restrepo et al., 2009 and Wilson and Mainen, 2006). Interestingly, studies of odor-induced oscillatory field potentials in olfactory discrimination tasks suggest the involvement aminophylline of changes in synchronous firing between neurons in the OB circuit in learning in vertebrates (Gray et al., 1986, Kay and Beshel, 2010 and Martin et al., 2006). In addition, MCs are hypothesized to aid in synthesis of simultaneously detected odor features through

synchronized firing and convergence on neurons in OC (Kashiwadani et al., 1999 and Mori et al., 1999), which has been supported by experiments in invertebrates (Stopfer et al., 1997). Studies in vertebrates are consistent with the claim that synchronous firing of MCs increases the probability of driving target OC neurons (Franks and Isaacson, 2006 and Luna and Schoppa, 2008). However, direct evidence for synchronized firing of MCs in vertebrates is limited to a measurement of synchrony in anesthetized animals (Kashiwadani et al., 1999) that was not replicated (Egaña et al., 2005). Thus, the precise role of synchronized MC firing in transfer of olfactory information, in learning of olfactory stimulus/reward association, or in both is not well understood.

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