Retinotopy (from Greek τόπος, place) is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'retinally mapped'. Visual field maps (retinotopic maps) are found in many amphibian and mammalian species, though the specific size, number, and spatial arrangement of these maps can differ considerably. Sensory topographies can be found throughout the brain and are critical to the understanding of one's external environment. Moreover, the study of sensory topographies and retinotopy in particular has furthered our understanding of how neurons encode and organize sensory signals. Retinal mapping of the visual field is maintained through various points of the visual pathway including but not limited to the retina, the dorsal lateral geniculate nucleus, the optic tectum, the primary visual cortex (V1), and higher visual areas (V2-V4). Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area. For example, in the second visual area (V2), the map is divided along an imaginary horizontal line across the visual field, in such a way that the parts of the retina that respond to the upper half of the visual field are represented in cortical tissue that is separated from those parts that respond the lower half of the visual field. Even more complex maps exist in the third and fourth visual areas V3 and V4, and in the dorsomedial area (V6). In general, these complex maps are referred to as second-order representations of the visual field, as opposed to first-order (continuous) representations such as V1. Additional retinotopic regions include ventral occipital (VO-1, VO-2), lateral occipital (LO-1, LO-2), dorsal occipital (V3A, V3B), and posterior parietal cortex (IPS0, IPS1, IPS2, IPS3, IPS4). History The discovery of visual field maps in humans can be traced to neurological cases, arising from war injuries, described and analyzed independently by Tatsuji Inouye (a Japanese ophthalmologist) and Gordon Holmes (a British neurologist). They observed correlations between the position of the entry wound and visual field loss (see Fishman, 1997 for an historical review). Development Molecular Cues The "Chemoaffinity Hypothesis" was established by Sperry et al in 1963 in which it is thought that molecular gradients in both presynaptic and postsynaptic partners within the optic tectum organize developing axons into