Time course of ChAT+ cell density (A) and the corresponding cortical thickness (B), during development. A: bars indicate average ChAT+ cell values per unit cortical volume, at the indicated stage. A sharp increase of cholinergic neurons was observed between P6 and P8, followed by a slow progressive decrease during late postnatal development and early adulthood. B: bars give the average cortical thickness at different developing stages. This parameter remained almost constant during the time span that we have studied, which shows that the time course of the cholinergic population illustrated in panel A is genuine.