Hebbian rules lead to an effective attraction in the target between axons with correlated activity. (A) The origin of axons in the retina is color-coded. A small subset of axons in the area representing the middle of the retina is selected for tracing (red). (B) In the absence of correlated activity E
= 0 the topographic projection is imprecise and the set of axons selected in (A) forms a diffuse cloud in the target. (C) When E
≠ 0 the axons neighboring in retina (red) carry correlated activity they are attracted to each other. This effective attraction leads to the condensation of the axons into an almost precise image of the circle in the retina shown in (A). Thus Hebbian contribution E
leads to the sharpening of topographic projection.