a coarse retinotopic map. This was established after a series of seminal experiments in fish and amphibians showed that retinal ganglion axons were already retinotopically organized within the optic tract and if severed, would regenerate and project back to retinotopically appropriate locations. Later, it was identified that receptor tyrosine kinases family EphA and a related EphA binding molecule referred to as ephrin-A family are expressed in complementary gradients in both the retina and the tectum. More specifically in the mouse, Ephrin A5 is expressed along the rostral-caudal axis of the optic tectum whereas the EphB family is expressed along the medio-lateral axis. This bimodal expression suggests a mechanism for the graded mapping of the temporal-nasal axis and the dorsoventral axis of the retina. Target Space While molecular cues are thought to guide axons into a coarse retinotopic map, the resolution of this map is thought to be influenced by available target space on postsynaptic partners. In wild type mice, it is thought that competition of target space is important for ensuring continuous retinal mapping, and that if perturbed, this competition may lead to the expansion or compression of the map depending on the available space. If the available space is altered, such as lesioning or ablating half of the retina, the healthy axons will expand their arbors in the tectum to fill the space. Similarly, if part of the tectum is ablated, the retinal axons will compress the topography to fit within the available tectal space. Neural Activity While neural activity in the retina is not necessary for the development of retinotopy, it seems to be a critical component for the refinement and stabilization of connectivity. Dark reared animals (no external visual cues) develop a normal retinal map in the tectum with no marked changes in receptive field size or laminar organization. While these animals may not have received external visual cues during development, these experiments suggest that spontaneous activity in the retina may be sufficient for retinotopic organization. In the goldfish, no neural activity (no external visual cues, and no spontaneous activity) did not prevent the formation of the retinal map but the final organization showed signs of lower resolution refinement and more dynamic growth (less stable). Based on Hebbian mechanisms, the thought is that if neurons are sensitive to similar stimuli (similar area of the visual field, similar orientation or direction selectivity) they will likely fire together. This