Development and plasticity of the cerebral cortex: From molecules to maps

ABSTRACT: The role played by environmental in-fluences in the development of the nervous system has been subject to intense study for the last three decades. Many laboratories are currently engaged in character-izing the exact contributions of activity-dependent or-independent processes to the development of the mam-malian neocortex. Here we introduce a special issue devoted to the topic and briefly review recent progress in this exciting field. At the systems level, many investiga-tors are now distinguishing between an “establishment” phase of cortical connections, where activity-dependent and independent mechanisms could operate, and a later “maintenance ” phase, which appears to be controlled by neuronal activity. A particularly interesting recent ex-ample of the role of top-down vs. bottom-up influences in the development of cortical connections is the emergence of orientation selectivity in visual cortex: we propose a synthetic view highlighting the role of the thalamo-cor-tical reciprocal projection in this process. Finally, at the cellular level, NMDA receptors, neurotrophins and many other molecules contribute to activity-dependent rearrangement of cortical connections during appropri-ate critical periods of development. © 1999 John Wiley &amp