Fig. 8. The effects of introducing fixed counter-gradients of EphA over the rostrocaudal axis of the colliculus. (A) The simulation of Fig. 7A was repeated with a weaker initial concentration gradient over the rostrocaudal axis of 0.4x + 0.25II together with a weak counter-gradient of EphA. The contribution from the counter-gradient is assumed to have the same functional form as the EphA gradients in the retina. At a distance x this is 0.005 + 0.013e^-2.3x + 0.005II. (B) As A but with a four times stronger counter-gradient resulting in a distorted map.