Formation of topographic maps.
The catalogue of data presented here form many systems demonstrates that multiple mechanisms are involved in the formation of topographic maps. We are not yet in a position to explain why a particular mechanism appears to dominate in some situations and not in others. Certain generalizations can be made, however. First, at least some form of chemospecificity can be invoked to help explain connectivity in all of the experiments we have cited. Often, the differential identities of a population of neurons can be reflected in an orderly pattern of axon outgrowth and in the actively maintained preservation of neighbor relations as the axons grow toward their targets; such orderly arrangements are not obligatory, but, where present, they facilitate the speedy establishment of orderly maps when the axons reach their target nuclei. Within a terminal zone, chemospecific cues may dominate and constrain a given axon to terminate in a specific location, but axon-axon interactions commonly supercede chemospecific matching. At least two forms of axon-axon interaction occur, one based on some sort of biochemical properties related to the axon's embryological identity and another based on the axons' electrical activity. Tasks for the future are to identify the cellular bases of each of these mechanisms and to understand the situations in which each is manifested.