  We study a role of EphB receptors and their ligand ephrin-B1 in
dorsal-ventral retinotopic mapping. Earlier studies suggested that ephrin-B1
acts as an attractant for EphB expressing axons. We address the results of the
recent experiment in chick tectum (McLaughlin et al., 2003b) in which axons of
retinal ganglion cells were shown to be repelled by high ephrin-B1 density.
Thus it was proposed that ephrin-B1 might act as both attractant and repellent.
We show that the same axonal behavior may follow from attraction to ephrin-B1
density and axonal competition for space. Therefore, we show how apparent
repulsive interaction can be induced by a combination of attraction to the
target and competitive interactions between axons. We suggest an experimental
test that may distinguish repulsive interaction with the target from repulsion
induced by attraction and competition.
