The cortex has been described to display synaptic plasticity, which is determinant for the function of cortical microcircuits. Although there is a large literature on neocortical plasticity, most of the experiments addressed the connections of pyramidal cells located in particular cortical layers, so the majority of intracortical connections are yet unexplored with respect to their plasticity. Moreover, these studies are contradictory in that apparently synaptic plasticity rules like spike-timing dependent plasticity (STDP) could be different in different neurons. Therefore, to understand the cortical microcircuit it appears necessary to characterize the different types of plasticity that these circuits are implementing and examine precisely how work the cell-specificity of the synaptic plasticity. The aim of this project is to characterize the plastic rules present among different subtypes of cortical neurons involved in a same microcircuit. I will focus on cortical microcircuit of pyramidal layer 5 cells which are the output cells of the cortex. Using calcium imaging coupled with two-photon photostimulation, I will identify connected cells and determine the extent of a cortical microcircuit that controls layer 5 cells. When connected cells will be identified, electrophysiological recordings and morphological reconstruction will allow me to characterize the cells involved in this microcircuit. I will characterize the plasticity forms which can occur between different connected cells using STDP protocols. The induced plasticity will be correlated with the nature and morphology of the considered connected cells to characterize the cellspecificity of the plasticity in a cortical microcircuit and to describe for the first time the rules of plasticity of cortical microcircuits. Furthermore, the return phase of the project will allow me to use these techniques developed by Pr Yuste to explore the dynamics of basal ganglia neural networks in my return host laboratory.