From ancient times, many have asked your question. Greek physician Hippocrates, for example, wondered why trauma on one side of the head caused deficits on the opposite side of the body. Around 100 years ago Spanish neuroanatomist and Nobel laureate Santiago Ramón y Cajal first explained this phenomenon in terms of development of the visual system. Although we now know that animals with rudimentary or no visual systems also show ¿"crossover" neural connections, Ramón y Cajal's explanation did identify important con¿cepts and the stimulus-response arena.

Crossover, or decussation, of neurons within the central nervous system is still not fully understood today. Such a phenomenon arises during embryological development. Recent discoveries indicate that neurons, or nerve cells, get their direction from growth factors with names such as roundabout, commissureless, Sax-3, netrin and sonic hedgehog. And, yes, many animals, including fish, worms, fruit flies and all vertebrates exhibit this decussation of nerve tracts. Where does crossover come from? Scientists are looking for the answers in many places.

This article was originally published with the title Ask the Brains.

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