One barrel column in vS1 projects to a band of vM1, with its long axis in the anterior/posterior (A/P) direction (Aronoff et al., 2010). vM1 projects diffusely to vS1, covering most of the barrel field and adjacent areas (Veinante and Deschênes, 2003). Reciprocal cortical connections have also been detected in neurophysiological recordings in vivo. Following the deflection of a whisker, excitation first ascends into vS1 and then rapidly propagates to vM1 (Farkas et al., 1999, Ferezou et al., 2007 and Kleinfeld et al., 2002). Neuronal activity in vS1 is modulated

by whisking (Curtis and Kleinfeld, 2009, de Kock and Sakmann, 2009, Fee et al., 1997 and O’Connor et al., 2010b), mediated in 5-Fluoracil solubility dmso part by an efference copy-like signal originating in vM1 (Ahrens and Kleinfeld, 2004 and O’Connor et al., 2002). Integrating signals related to whisking and whisker deflection might underlie object localization (Curtis and Kleinfeld, 2009 and Diamond et al., 2008). The detailed neural circuits underlying the vS1 ←→ vM1 loop are poorly understood. A circuit

diagram, based on functional connections between defined cell types, might reveal the primary loci where sensorimotor associations are formed. In addition to the connectivity between cell types, the interactions between neurons in vS1 and vM1 depend on the locations of synapses within the dendritic arbors of the postsynaptic neurons (Larkum et al., 2004 and London and Häusser, 2005). Anatomical methods, relying on visualizing axons and dendrites with light microscopy, have often been used to predict circuits (Binzegger et al., click here 2004, Meyer et al., 2010a and Shepherd et al., 2005). However, axodendritic overlap is not necessarily a good predictor of functional connection strength (Callaway, 2002, Dantzker and Callaway, 2000, Petreanu et al.,

Carnitine dehydrogenase 2009, Shepherd et al., 2005 and White, 2002). Alternatively, electrophysiological methods that detect functional synapses, including paired recordings and glutamate uncaging-based methods, have been applied to map local circuits within vS1 (Bureau et al., 2006, Hooks et al., 2011, Lefort et al., 2009, Lübke and Feldmeyer, 2007, Schubert et al., 2003, Schubert et al., 2006, Shepherd et al., 2003, Shepherd et al., 2005 and Shepherd and Svoboda, 2005) and vM1 (Hooks et al., 2011). These techniques require the preservation of pre- and postsynaptic neurons and their axonal processes within a brain slice and are thus mostly limited to local circuits (Luo et al., 2008). Although a subset of long-range connections between vS1 and vM1 can be preserved in brain slices (Rocco and Brumberg, 2007), it is unclear how complete the preserved circuit is. We previously applied subcellular Channelrhodopsin-2-assisted circuit mapping (sCRACM) to chart the connections made by long-range projections onto vS1 neurons (Petreanu et al., 2009).