Differential origin of projections from SI barrel cortex to the whisker representations in SII and MI

We have previously shown that projections from SI barrel cortex to the MI whisker representation originate primarily from columns of neurons that are aligned with the layer IV septa. SI barrel cortex also projects to SII cortex, but the origin of these projections has not been characterized with respect to the barrel and septal compartments. To address this issue, we injected retrograde tracers into the SII whisker representation and then reconstructed the location of the labeled neurons in SI with respect to the layer IV barrels. In some animals, two different tracers were injected into the whisker representations of SII and MI to detect double‐labeled neurons that would indicate that some SI neurons project to both of these cortical areas. We found that the projections to SII cortex originate from sites that are uniformly distributed throughout the extragranular layers of barrel cortex. In cases in which different tracers were injected in SII and MI, double‐labeled neurons appeared above and below the layer IV septal compartment and at sites aligned with the boundaries of the layer IV barrels. To the extent that the columns of neurons aligned with the barrel and septal compartments represent functionally distinct circuits, these results indicate that SII receives information from both circuits, whereas MI receives inputs primarily from the septal circuits. J. Comp. Neurol. 498:624–636, 2006. © 2006 Wiley‐Liss, Inc.