If someone shouts “look behind you,” tadpoles in Michael Levin’s laboratory may be ready. The tadpoles can see out of eyes growing from their tails, even though the organs aren’t directly wired to the animals’ brains, Levin and Douglas Blackiston, both of Tufts University in Medford, Mass., report online February 27 in the Journal of Experimental Biology.
Levin and Blackiston’s findings may help scientists better understand how the brain and body communicate, including in humans, and could be important for regenerative medicine or designing prosthetic devices to replace missing body parts, says Günther Zupanc, a neuroscientist at Northeastern University in Boston.
Researchers have transplanted frog eyes to other body parts for decades, but until now, no one had shown that those oddly placed eyes (called “ectopic” eyes) actually worked. Ectopic eyes on tadpoles’ tails allow the animals to distinguish blue light from red light, the Tufts team found.
Levin wanted to know whether the brain is hardwired to get visual information only from eyes in the head, or whether the brain could use data coming from elsewhere. To find out, he and Blackiston started with African clawed frog tadpoles (Xenopus laevis) and removed the normal eyes. They then transplanted cells that would grow into eyes onto the animals’ tails.
The experiment seemed like a natural to test how well the brain can adapt, Levin says. “There’s no way the tadpole’s brain is expecting an eye on its tail.”
Expected or not, some of the tadpoles managed to detect red and blue light from their tail eyes. The researchers placed tadpoles with transplanted eyes in chambers in which half of the chamber was illuminated in blue light and the other half in red light. A mild electric shock zapped the tadpole when it was in one half of the dish so that the animal learned to associate the color with the shock. The researchers periodically switched the colors in the chamber so that the tadpoles didn’t learn that staying still would save them.
Tadpoles in which nerves from the tail eye had grown to connect to the spinal column were able to learn the color-shock association and swim away from the light that accompanied a shock. Tadpoles whose tail eyes had connected to the stomach or some other part of the body did not learn the association. Neither did tadpoles with no eyes. The finding suggests that visual information from the eye travels up the spinal cord to the brain, which can process it, Levin says.
That result was a surprise, Zupanc says, because previous research had suggested eyes need to be directly connected to the brain to transmit visual information. Somehow the brain is able to distinguish the color messages from other data travelling through the spinal cord. All of those messages arrive as electrical signals that look alike to the experimenters, but Levin and Blackiston’s study suggests the brain can tell the difference.
Learning how the brain sorts visual information from other types of data may be important, Levin suggests, in designing artificial eyes or correcting some forms of blindness in which the brain doesn’t process visual information correctly.