Sperry versus Hebb: Topographic mapping in Isl2/EphA3 mutant mice
  In wild-type mice axons of retinal ganglion cells establish topographically precise projection to the superior colliculus of the midbrain. This implies that axons of neighboring retinal ganglion cells project to the proximal locations in the target. The precision of topographic projection is a result of combined effects of molecular labels, such as Eph receptors and ephrins, and correlated electric activity. In the Isl2/EphA3 mutant mice the expression levels of molecular labels is changed. As a result the topographic projection is rewired so that the neighborhood relationships between retinal cell axons are disrupted. Here we argue that the effects of correlated activity presenting themselves in the form of Hebbian learning rules can facilitate the restoration of the topographic connectivity even when the molecular labels carry conflicting instructions. This occurs because the correlations in electric activity carry information about retinal cells' spatial location that is independent on molecular labels. We argue therefore that experiments in Isl2/EphA3 knock-in mice directly test the interaction between effects of molecular labels and correlated activity during the development of neural connectivity. 
Dmitry Tsigankov and Alexei Koulakov