Rodents navigate in their natural environment via signals from their facial whiskers. Under experimental conditions the movement and deflection of even a single whisker can guide the motor behavior of an animal. We have determined how the deflection of a single whisker in rats is represented in identified neurons located in different layers of the somatosensory cortex at the level of inputs to a layer postsynaptic potentials (PSPs) and at their outputs (APs). We found that the representation of a whisker deflection is layer and cell-type specific and depends on whether representational maps are constructed for input signals (PSPs) of a layer or for their output signals (APs). The PSP maps are broader and less dynamic than AP maps, suggesting "dense coding" at the PSP level meaning that a large fraction of cells in a column is activated at the subthreshold level.

The narrower and more dynamic output (AP) maps suggest that only a small percentage of cells in a column are generating APs, indicating "sparse coding" in all layers of the cortex. The difference in AP coding is particularly large between supragranular and infragranular layers of the cortex suggesting that of all layers the cells of L5B represent a deflection most reliably and specifically. It may be hypothesized that the sluggish and delayed AP output from L2/3 generated by a whisker deflection is mostly modulating the excitability of neighboring columns and brain areas adjacent to the somatosensory area, e.g. the association areas. The brisk and reliable AP output of L5B pyramids primarily drives motor reactions in response to whisker deflections.