title: Competition is a driving force in topographic mapping.
abstract: Topographic maps are the primary means of relaying spatial information in the brain. Understanding the mechanisms by which they form has been a goal of experimental and theoretical neuroscientists for decades. The projection of the retina to the superior colliculus (SC)/tectum has been an important model used to show that graded molecular cues and patterned retinal activity are required for topographic map formation. Additionally, interaxon competition has been suggested to play a role in topographic map formation; however, this view has been recently challenged. Here we present experimental and computational evidence demonstrating that interaxon competition for target space is necessary to establish topography. To test this hypothesis experimentally, we determined the nature of the retinocollicular projection in Math5 (Atoh7) mutant mice, which have severely reduced numbers of retinal ganglion cell inputs into the SC. We find that in these mice, retinal axons project to the anteromedialj portion of the SC where repulsion from ephrin-A ligands is minimized and where their attraction to the midline is maximized. This observation is consistent with the chemoaffinity model that relies on axon-axon competition as a mapping mechanism. We conclude that chemical labels plus neural activity cannot alone specify the retinocollicular projection; instead axon-axon competition is necessary to create a map. Finally, we present a mathematical model for topographic mapping that incorporates molecular labels, neural activity, and axon competition.
authors: Department of Molecular, Cell, and Developmental Biology, Santa Cruz Institute of Particle Physics, University of California, Santa Cruz, CA 95064, USA.
authors: Jason W
authors: JW
authors: Triplett
authors: Cory
authors: C
authors: Pfeiffenberger
authors: Jena
authors: J
authors: Yamada
authors: Ben K
authors: BK
authors: Stafford
authors: Neal T
authors: NT
authors: Sweeney
authors: Alan M
authors: AM
authors: Litke
authors: Alexander
authors: A
authors: Sher
authors: Alexei A
authors: AA
authors: Koulakov
authors: David A
authors: DA
authors: Feldheim
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics; D005453:Fluorescence; D006801:Humans; D051379:Mice; D008817:Mice, Mutant Strains; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000235:genetics; D012160:Retina / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D018709:Statistics, Nonparametric; D013477:Superior Colliculi / Q000502:physiology; D014795:Visual Pathways / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D009419:Nerve Tissue Proteins; C117638:Atoh7 protein, mouse