The role of neuronal death during the development of topographically ordered projections: a computational approach

At the time of synaptogenesis typically 50% of the neurons die. The biologi cal role of this is still unclear, but there is evidence in the visual syst em that many neurons projecting to topographically inappropriate parts of t heir target are eliminated to improve the accuracy of the mapping. The sign aling that determines neuronal survival involves electrical activity and tr ophic factors. Based on these observations, we have elaborated a computatio nal model for the self-organization of a two-layered neural network. We obs erve changes in the topographical organization between the two layers. In l ayer 1, a traveling wave of electrical activity is used as input. Activity transmission to layer 2 can generate, according to a Hebbian rule, a retrog rade death signal that is compensated by a trophic survival signal generate d by the target cells. Approximately 50% of the neurons die, and we observe refinement in the topography between the two layers. In alternative versio ns of the model, we show that an equivalent reorganization can occur throug h Hebbian synaptic modification alone, but with less precision and efficien cy. When the two mechanisms are combined, synaptic modification provides no further improvement over that produced by neuronal death alone. This compu tational study supports the hypothesis that neuronal death during developme nt can play a role in the refinement of topographical projections in the ne rvous system.