Sensory experience determines the way we perceive the world and ourselves. It is a lifelong functionality through which we develop our personality and which allows us during the various phases of our live, to act in accordance with what we learned and what we like to express. It turned out that there is a neuronal basis that allows the brain to adapt to alterations in sensory activity. We study the properties of this adaptation mechanism in a sensory system of the mouse that treats the information gathered by the whiskers on the snout. By the nature of its organization, this system allows modifying sensory activity without major perturbation for the animal and guides the observer's eye to the brain region where alteration can be analyzed in a very precise manner. The part of the brain we particularly study is the cerebral cortex. It is the region where sensory activity is transformed into a perception and where memory traces are induced. In the cortical area receiving the information from the whiskers, multi-neuronal arrangements (named "barrels) can be identified that correspond to single whiskers. We alter sensory activity using an electromagnetic device that induces movements of one or several whiskers for a period up to several days. Mice adapt to this stimulation rapidly, and, for example, maintain a normal sleep cycle during the period of stimulation. We have studied the effect of the stimulation in the cerebral cortex of adult mouse using morphological, neurophysiological, biochemical and molecular techniques. May be the most dramatic result is the demonstration that 24 hours of whisker stimulation induces the formation of synapses in the barrel, resulting in a 30% increase in the density of these connections between neurons. After stopping the stimulation, a part of the newly formed synapses remains forming a lasting trace of the period of modified sensory experience.Although the stimulation paradigm we use does not have a behavioral benefit for the animal, we propose that similar modifications may occur in a natural setting, Using molecular analyses we have identified a number of molecules that are up- or down regulated in the barrel cortex during the period of increased sensory stimulation. In the current series of experiments we will try to find out which type of neuron or glial cell is expressing these identified molecules.This series of investigations will identify who is doing what and may help identifying target cells for pharmacological intervention in diseases where increased neuronal activity leads to neuronal destruction - such as epilepsy or neuronal diseases where neuronal activity is decreased.