Insights into activity-dependent map formation from the retinotectal system: a middle-of-the-brain perspective

The development of orderly topographic maps in the central nervous system (CNS) results from a collaboration of chemoaffinity cues that establish the coarse organization of the projection and activity-dependent mechanisms that fine-tune the map. Using the retinotectal projection as a model system, we describe evidence that biochemical tags and patterned neural activity work in parallel to produce topographically ordered axonal projections. Finally, we review recent experiments in other CNS projections that support the proposition that cooperation between molecular guidance cues and activity-dependent processes constitutes a general paradigm for CNS map formation.