Learning-dependent potentiation in the vibrissal motor cortex is closely related to the acquisition of conditioned whisker responses in behaving mice

The role of the primary motor cortex in the acquisition of new motor skills was evaluated during classical conditioning of vibrissal protraction responses in behaving mice, using a trace paradigm. Conditioned stimulus (CS) presentation elicited a characteristic field potential in the vibrissal motor cortex, which was dependent on the synchronized firing of layer V pyramidal cells. CS-evoked and other event-related potentials were particular cases of a motor cortex oscillatory state related to the increased firing of pyramidal neurons and to vibrissal activities. Along conditioning sessions, but not during pseudoconditioning, CS-evoked field potentials and unitary pyramidal cell responses grew with a time-course similar to the percentage of vibrissal conditioned responses (CRs), and correlated significantly with CR parameters. High-frequency stimulation of barrel cortex afferents to the vibrissal motor cortex mimicked CS-related potentials growth, suggesting that the latter process was due to a learning-dependent potentiation of cortico-cortical synaptic inputs. This potentiation seemed to enhance the efficiency of cortical commands to whisker-pad intrinsic muscles, enabling the generation of acquired motor responses.