Prominent lateral spread of imaged evoked activity beyond cortical columns in barrel cortex provides foundation for coding whisker identity.

The posterior medial barrel subfield (PMBSF) of a rat primary somatosensory cortex exquisitely demonstrates topography and columnar organization, defining features of sensory cortices in the mammalian brain. Optical imaging and neuronal recordings in rat PMBSF demonstrate how evoked cortical activity following single whisker stimulation also rapidly spreads laterally into surrounding cortices, disregarding columnar and modality boundaries. The current study quantifies the spatial prominence of such lateral activity spreads by demonstrating that functional connectivity between laterally spaced cortical locations is actually stronger than between vertically spaced cortical locations. Further, the total amount of evoked activity within and beyond single column boundaries was quantified based on intrinsic signal optical imaging, single units and local field potentials recordings, revealing that the vast majority of whisker evoked activity in PMBSF occurs beyond columnar boundaries. Finally, a simple two-layer artificial neural network model of PMBSF demonstrates the capacity of extracolumnar evoked activity spread to provide a foundation for accurate whisker stimulus classification that is robust to random scaling of inputs and local noise. Indeed, classification performance improved when more of the lateral spread was included in the model, providing further evidence for the relevance of the lateral spread.