The following memoir is presented in the form of two principal scientific articles. One of them is submitted for publication in Journal of Physiology and the other one is submitted to the Journal of Neurophysiology. The articles are precedes by a general introduction, which describes the theoretical context and experimental strategies of the studies described in the articles including one part of an article to which I contributed as coauthor and which is published in European Journal of Neuroscience titled: Modulation of synaptic transmission in neocortex by network activities. A general conclusion finalizes the memoir with the presentation of a communes concepts linked the results of these articles. The bibliography used for both introduction and conclusion, is presented at the very end of the memoir. Since the articles have been submitted to different scientifics journals, the format of bibliographical citations changes from one to another.

I would like to use this occasion to express my gratitude to my memoir supervisor Dr. Igor Timofeev for the opportunity of working in his laboratory. Without his critics, discussions, corrections and support the present study would not have been possible.

I also would like to thank my colleagues Sylvain Crochet, who in parallel with my superviser, teached me to do electrophysiological experiments and analysis, Sylvan Chauvette for his unvaluable contribution to our common publications and Josée Seigneur for her general help.

Finally, I would like to thank Mr. Pierre Guiguère, Denis Drolet and all my colleagues for their technical and moral support in every moment.

© Soufiane Boucetta, 2005