[1] Ecole Normale Super, Dept Biol, Paris, France
[2] McGill Univ, Montreal Neurol Inst, Dept Neurol & Neurosurg, Montreal, PQ H3A 2B4, Canada
[3] McGill Univ, Montreal Neurol Inst, Dept Physiol, Montreal, PQ, Canada
[4] Univ Roma La Sapienza, Rome, Italy