Functional independence of layer IV barrels.

Our aim was to investigate the patterns of functional inputs and outputs from individual barrels in the mouse somatosensory cortex, and to test the hypothesis that individual barrels in layer IV are functionally independent of direct inputs from neighboring barrels. In a mouse in vitro slice preparation of the barrel cortex, we recorded voltage-sensitive dye signals evoked in response to microstimulation of a single barrel. Activity propagated from the stimulated barrel to the supragranular layers, where it spread to activate several barrel columns. However, in no instance did activity propagate directly from the stimulated barrel to neighboring barrels. Neither suppression of GABAergic inhibition, nor activation of N-methyl-D-aspartate receptors, revealed direct interbarrel interactions. By contrast, microstimulation in the supra- or infragranular layers resulted in direct propagation of activity to neighboring barrel columns. We conclude that the neurons within individual barrels are functionally independent of direct inputs from neighboring barrels. This suggests that the response properties of layer IV barrel neurons are shaped primarily by their presynaptic thalamic afferents and by intrabarrel interactions, and that these responses are independent of direct inputs from neighboring barrels.