Our results indicate that the effect of recurrent input on the ability of olfactory bulb input to drive spiking is highly dependent on the relative timing of the two sets of KRX 0401 inputs. When piriform axons are activated simultaneously with or slightly after stimulation of the LOT, the firing of piriform neurons is significantly enhanced. However, when piriform is activated prior to stimulation of the LOT, the firing of piriform neurons in response to LOT inputs is suppressed. This dynamic circuitry is poised to generate a homogenous, associative network that can potentially explain a number of features of olfactory processing

observed in the piriform. For example, the number of odor-responsive neurons in the piriform is only weakly dependent on odorant concentration (Stettler and Axel, PD0332991 solubility dmso 2009), even though both the number of activated glomeruli (Rubin and Katz, 1999) and the amount of excitatory input to individual piriform pyramidal cells (Poo and Isaacson, 2009) increases with odorant concentration. A diffuse recurrent cortical network with scaled inhibition

affords a normalization mechanism that can maintain a constant level of piriform activation. The recurrent piriform network may also explain the observation that the number of piriform neurons activated by a mixture of odorants is far less than the sum of the neurons activated by individual odorant components. Rather, odorant mixtures tend to suppress activity in cells Endonuclease responsive to individual odorants presented

alone (Stettler and Axel, 2009). Thus, the pattern of active neurons in response to a mixture of odorants differs from the representation of individual components. A highly interconnected recurrent network might accommodate these computations (Barkai et al., 1994, Haberly, 2001, Haberly and Bower, 1989 and Wilson and Bower, 1992). We find that the recurrent circuitry in the piriform cortex exhibits organizational properties that are different from those of sensory neocortices. In vision, touch, and hearing, spatial information in the peripheral sense organ is maintained in the cortex. In sensory neocortices, cells responsive to similar stimulus features tend to be clustered. In these cortices, recurrent circuitry is primarily local and serves to connect cells with similar receptive fields (Braitenberg and Schüz, 1998 and Ko et al., 2011). As a consequence, this circuitry is thought to increase signal-to-noise ratio (Douglas et al., 1995) and sharpen the tuning of neurons to specific features of the stimulus (Anderson et al., 2000, Murphy and Miller, 2009, Wehr and Zador, 2003 and Wilent and Contreras, 2005). Longer-range parasagittal connections in the neocortices are specific and connect areas that respond to similar features (Gilbert, 1992).