The ability of Eph/ephrin signaling to direct migrating axons along Eph/ephrin expression gradients is evidenced in the formation of the retinotopic map in the visual system, with graded expression levels of both Eph receptors and ephrin ligands leading to the development of a resolved neuronal map (for a more detailed description of Eph/ephrin signaling see "Formation of the Retinotopic Map" in ephrin). Further studies then showed the role of Eph’s in topographic mapping in other regions of the central nervous system, such as learning and memory via the formation of projections between the septum and hippocampus.