  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.
