Model of topographic map development guided by a transiently expressed repulsion molecule.

The projection from the retina develops into a precise map of the visual world on the surface of the tectum. The search for molecular position cues that mediate map formation has recently yielded a tectal molecule that exerts a repulsion to fibers from the entire temporal half retina. This molecule appears not to function in the generally accepted gradient manner but instead provides only binary position information, and it is only expressed transiently during early development. Here we describe modeling results that compare the efficacy of binary versus graded position cues in topographic map formation; the model also includes an activity dependent process. We find that binary repulsion is more efficient than graded chemoaffinity in the rapid establishment of map polarity, and transient expression of either cue provides sufficient guidance for precise map formation.