A particularity of neurons in sensory cortex is to be tuned to specific stimulation features ; for example, in the primary visual cortex, some neurons are selective for the orientation of the stimulus. The large dendritic trees of pyramidal cortical neurons extend over hundreds of microns, allowing neurons to receive thousands of synaptic inputs. Neurons integrate and compute these inputs to produce an action potential output that will excite target cells. The dendritic tree likely serves as a substrate for these computations, through the interaction between synaptic inputs, the tree topology, and the distributions of intrinsic channels in dendrites. Elucidating the mechanisms by which dendrites implement these computations in response to functional stimuli is crucial to the understanding of how stimulus-specificity is performed in sensory neurons.Thin dendrites are not accessible to patch-clamp, but two-photon microscopy allows the measurement of [Ca2+] and voltage fluctuations using fluorescent reporters.We will investigate these questions in rat visual cortical neurons in vitro (in response to photoactivation of group of synapses), and in vivo (in response to visual stimulation), by measuring optically the dynamics of voltage and Ca2+ in single dendrites, in combination with somatic electrophysiological recordings and mathematical modeling of dendritic computations.