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Abstract

In neuronal populations of the sensory cortex, stimulus responses are shaped by the cortical architecture on anatomical scales from tens of microns to millimeters. In particular, in L2/3 rodent vibrissal cortex we previously observed that whisker deflection evokes pairwise correlations that decrease both with distance between neurons and distance to the center of the whisker-associated column (Kerr, de Kock, Greenberg, Bruno, Sakmann, and Helmchen. (2007). J. Neurosci. 27: 13316-28). One possible explanation for this finding is that these correlations arise from anatomically structured common inputs. L4 spiny stellate (SS) cells send vertical axon fibers to L2/3 that are confined within the borders of the whisker-associated column and neuronal pairs closer together could exhibit greater dendritic overlap. Therefore, for pairs closer to the column center more of this overlap will intersect with SS projections. We tested this hypothesis using 2-photon targeted patching of L2/3 pyramidal pairs in anaesthetized rats to record sub- and suprathreshold stimulus responses followed by anatomic reconstruction of the neurons and barrel field. We found a positive and statistically significant association between correlated AP firing and dendritic overlay inside the whisker-associated column. This effect was strongest for suprathreshold activity evoked shortly after whisker deflection (~20 ms), and decayed rapidly thereafter. It was also robust with respect to the voxel size, determined by the L4 axon reconstructions, used to quantify dendritic overlap. No relationship was detectable for offset responses or spontaneous activity. These results support the notion that the spatially structured correlations observed for short-latency stimulus-evoked spiking arise from anatomically structured feed-forward projections.