How do features of sensory representations develop?

Summary form only given, as follows. Advanced mammals are characterized by complex sensory systems with a number of interconnected sensory representations. These representations are highly similar in internal features for a given area across individuals, but differ across areas and across species. The development of these systems likely reflects both position-dependent gene expression and resulting distributions of signalling molecules, as well as information relayed from the receptor sheet. The precise roles of these two factors have been difficult to determine, but an often neglected source of relevant information comes from comparative studies of how sensory representations are similar and different. For example, disruptions and anatomically apparent septa in representations often reflect actual discontinuities in the receptor sheet that are variable in some individuals or different across species. Such variable features of sensory representations seem most compatible with theories of development that involve instruction by neural activity patterns originating in the receptor sheet.