ch, as well as motor systems—but the visual system has been the predominant model system for studying the development of such maps and the gradients of guidance molecules that control their formation. "Topographic maps are a very common wiring strategy in our brains," Zou says.

Research has been on into these gradients, the global positioning system of the brain, over than forty years.

In 1963, neurobiologist Roger Sperry (Nobel Prize winner in 1981) proposed the `chemoaffinity hypothesis,’ where chemical signals, probably present in concentration gradients, serve as positional landmarks within the brain. These landmarks are then recognized by growth cones at the tip of axons - the projections that grow out from the retinal neurons and into the brain. The wandering axons use these signals to locate their destinations in the map.

Computational models of this process were developed twenty years later, in 1983, by Alfred Gierer. His models indicated that at least two counterbalancing signaling systems were required to push and pull the growing and branching axons as they searched for their ultimate topographic positions within the brain.

Scientists found the first of those signaling systems in 1990s, a family of ephrin proteins that are present in graded fashion in the brain and are involved in axon guidance. The A-class ephrins are required for map formation along the anterior-posterior (front-to-back) axis and the B-class for median-lateral (side-to-side) axis.

For several years, Zou's lab has been studying a different family of proteins, known at the Wnts. Although Wnts were better known because of their role as morphogens –-proteins that pattern bodily structures and determine cell fates—Zou recently showed how members of the Wnt family served to guide path-finding axons up and down the spinal cord by attracting or repelling receptors on the growth cones of sensory or motor nerves.

In this paper, Zou and c
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