Topographic mapping from the retina to the midbrain is controlled by relative but not absolute levels of EphA receptor signaling

Topographic maps are a fundamental feature of sensory representations in ne rvous systems. The formation of one such map, defined by the connection of ganglion cells in the retina to their targets in the superior colliculus of the midbrain, is thought to depend upon an interaction between complementa ry gradients of retinal EphA receptors and collicular ephrin-a ligands. We have tested this hypothesis by using gene targeting to elevate EphA recepto r expression in a subset of mouse ganglion cells, thereby producing two int ermingled ganglion cell populations that express distinct EphA receptor gra dients. We find that these two populations form separate maps in the collic ulus, which can be predicted as a function of the net EphA receptor level t hat a given ganglion cell expresses relative to its neighbors.