To transmit unbroken images, retinal axons must terminate on their target brain region in the correct relative positions to form a retinotopic map. On p. 2705, David Willshaw presents a new computationally generated model for retinotopic map formation using data from mouse EphA receptor knockin and knockout experiments. Neural map formation is thought to involve two steps: an activity-independent step, which uses position-specific molecular labels to establish a crude map of where retinal axons should migrate, and an activity-dependent mechanism, which refines the map. From his analysis of experimental data, Willshaw concludes that the guiding principle behind retinotopic mapping is that axons carrying similar amounts of Eph receptor terminate near each other on their target and activity-based mechanisms only function later in development. He shows that the 30-year-old marker induction model (in which fixed retinal labels induce labels on tectal cells) can simulate EphA receptor knockin and knockout experiments. Finally, he proposes a refined model - the `retinal induction model' - in which the retinal and tectal labels are Ephs and ephrin ligands, respectively.