Who dominates the retinotectal mapping?

Retina ganglia cells (RGCs) connect to the tectum neurons in the midbrain through 1-1 strict topic mapping, which benefits the precise information transmission and is called retinotopic mapping. It´s significant to understand the underlying mechanisms that contributing to the topic mapping. Inspired by the experimental observations given by [Nicol et al 2007 Nat. Neurosci. 10(3):340], we divide the retinotectal mapping into three almost independent stages, axon overshooting, axon retraction, and the final refinement of axons and arborization. In order to analyze the roles that forward signaling, reverse signaling and neural activity plays in different stages, an axon retraction model and a refinement procedure have been proposed to simulate the retinotopic mapping process. We testify the asymmetry of gradients between forward signaling and reverse signaling, and find that it is the forward signaling that dominates the axon retraction. Also, the refinement procedure demonstrates that the reverse-signaling-induced gradient branching and the synapse-dependent neural activity lead the RGC axons projecting to the topic target tectum neurons.