Reanalysis of EphA3 knockin double maps in mouse suggests that stochasticity in topographic map formation acts at the retina rather than between competing mechanisms at the colliculus

It has been suggested that stochasticity can act in the formation of topographically ordered maps in the visual system through the opposing chemoaffinity and neural activity forces acting on the innervating nerve fibres being held in an unstable equilibrium. Evidence comes from the Islet2-EphA3 knockin mouse, in which approximately 50% of the retinal ganglion cells, distributed across the retina, acquire the EphA3 receptor, thus having an enhanced density of EphA which specifies retinotopic order along one axis of the retinocollicular map. Sampling EphA3 knockin maps in heterozygotes at different positions along the mediolateral extent of the colliculus had found single 1D maps (as in wild-types), double maps (as in homozygous knockins) or both single and double maps. We constructed full 2D maps from the same dataset. We found either single maps or maps where the visual field projects rostrally, with a part-projection more caudally to form a double map, the extent and location of this duplication varying considerably. Contrary to previous analyses, there was no strict demarcation between heterozygous and homozygous maps. These maps were replicated in a computational model where, as the level of EphA3 was increased, there was a smooth transition from single to double maps. Our results suggest that the diversity in these retinotopic maps has its origin in a variability over the retina in the effective amount of EphA3, such as through variability in gene expression or the proportion of EphA3+ retinal ganglion cells, rather than the result of competing mechanisms acting at the colliculus