VOICES Voices Icon Ideas and Insight From Explorers


Tadpole Sees Through Eyeball on Its Tail

A rose by any other name would smell as sweet—but does an eye in any other place see as sharply? The answer may be yes—a finding that could help people with blindness or other eye disorders, a new study says.

Recent experiments show that tadpoles bred with eyes surgically implanted in their tails instead of their heads still see—the first discovery of its kind.

tadpole picture
A tadpole with an eyeball growing on its tail. Image courtesy D. Blackiston and M. Levin


Remarkably, despite their placement far from the brain, these out-of-place eyeballs—or “ectopic eyes”—still worked by sending signals to the brain via the spinal cord. (See Sea Urchin Body Is One Big Eye.”)

“Not all the tadpoles with the ectopic eyes could see,” study leader Michael Levin, director of the Tufts Center for Regenerative and Developmental Biology, said in an email.

“But of the ones that could, their performance was very similar to those with normal eyes.”

More Than Meets the Eye

Because of their structural similarities with human eyes, frog eyes are often used for modeling eye disorders.

For the experiment, the team surgically grafted “eye primordia” from donor tadpole embryos onto host embryos, 95 percent of which grew eyes on their tails.

The team then used red and blue LEDs to condition the tadpoles to associate red light with a mild electric shock.

Consequently, both tadpoles with normal eyes and those with ectopic eyes developed an aversion to red light and learned to avoid red-lit areas—meaning that the tadpoles with tail “eyes” could see. In contrast, a control group of eyeless tadpoles did not learn to avoid red light.

The spine is known to transmit all kinds of sensory information throughout the body, but the 64,000-eyeball question is how the brain recognizes the signals from the far-flung ectopic eyes for what they are. (Also read: “Eyes Made of Rock Really Can See, Study Says.”)

“What is really interesting is how the brain knows it is visual data,” said Levin, whose study appeared February 27 in the Journal of Experimental Biology.

“My hypothesis, although we have no proof of it yet, is that these—and probably many other organs’—signals are tagged with ‘metadata’—there is something about these signals that reveals the type of organ they come from.” (Watch a video of a tadpole developing.)

Eye to the Future

The study has far-reaching implications for the treatment of human blindness and beyond, the authors say.

“In the short term, it might mean that ectopic organs could be connected to [a person’s] spinal cord,” said Levin, so the person wouldn’t “need brain surgery to make [the organs] functional.” (Read “See-Through Vision Invented.”)

The research could even expand to include “cybernetic hybrid devices” like electronic eyes or hands, he said.

“Not only is this relevant for biomedical treatment of injuries and birth defects where the normal organs are damaged [or] missing, but [it’s also relevant] for sensory augmentation with new technologies,” said Levin.

For instance, recent breakthroughs have allowed quadraplegic patients to control robotic limbs with their thoughts alone.

Overall, the future for altering and repairing our bodies in new ways looks bright, he said.

“Beyond all this, understanding whether and how the brain builds a map and communicates with body organs will really revolutionize our understanding,” he said, “as well as impact our ability to induce bodies to regenerate missing organs themselves.”

It’s exciting, futuristic stuff—and may even finally provide hope for those who’d like eyes in the back of their heads.


  1. Jacob
    February 21, 2016, 1:20 am

    I agree with “Unclever title”. No “metadata” explanation is needed. If red light happens to feel like one kind of tail itch and blue light feels like a different kind of tail itch, that’s enough to have useful information to base learning on. The devices to aid the blind, that Douglas Blackiston describes, are able to work without access to neurons that have any special “im a visual receptor” metadata reporting hardcoded in. Especially given such evidence of flexibility, where input gets processed to some degree by vision-associated structures as long as it provides vision-like patterns of input, there seems little need to hypothesize any explicit “metadata” mechanism. Anyway, very cool work.

  2. david psalm
    July 18, 2013, 7:39 am

    how could this tadpoles look like if they grow into frogs , would there eyes be also in their tale

  3. Douglas Blackiston
    Boston MA, USA
    April 17, 2013, 4:20 pm

    Hi Unclever title,
    I’m one of the authors of the paper and wanted to respond to your question. We are currently doing some of the exact studies you proposed (visualizing direct neural activity in the brains of tadpoles with ectopic eyes) and hope to report on the results soon.

    On a broader scale, the central nervous system seems to have a remarkable ability to process spatial sensory data as visual. For example, there are blind patients who wear a camera that translates the captured image into a small electrode array which stimulates the skin, or in some cases the tongue (referred to as the electric lollipop). After using this device for a period of time, the patient’s visual hemisphere begins to show activity while navigating even though this device is delivering tactile information.

    Some similar fascinating findings have been observed in blind individuals who echolocate using sound. While navigating, the visual cortex shows activity, which is absent during normal speech (or other non-spatial tasks).

    While were still working out the exact connections between the ectopic eyes and the brain, it’s possible they may be sending precise visual signals just like the native eyes (much like what happens in praying mantis species which have evolved ears on their thorax), or they may act in a more indirect fashion, like the electric lollipop (or other cross modality devices).

    As with most findings in science, each answer only leads to many more questions, but it makes each day interesting and exciting.

  4. Unclever title
    April 15, 2013, 11:28 am

    “What is really interesting is how the brain knows it is visual data.”

    But… does it?

    It’s clear that visual information is being sent to the tadpole’s brain since it’s able to avoid the red light. So it can interpret between red and blue light. But why should the brain know that that information fits the category “visual”?

    Did they do scans and studies of the tadpoles brains while they did the red/blue light experiments? Did they see activity in the same area of the brain that visual activity happens in normal tadpoles?

    All that this article mentions is that the tadpoles were able to perceive the difference between red light and blue light and react rationally. Which makes perfect sense since the eye sends a different pattern (or whatever) for red light than blue light. With electric shock as the difference the tadpole learns that one pattern is benign while the other is hostile.

    There is no inherent need for the visual data to be recognized as “visual data.” If some of the tadpoles had synesthesia and instead considered that visual data to be auditory, they probably would have reacted in much the same way.

    In short:

    Stimulus A produces neural signal A. Neural signal A prompts response A.

    Stimulus B produces neural signal B. Neural signal B prompts response B.

    All we’ve really learned here is that you can use the spinal cord to transmit visual information, which is extremely fascinating and useful knowledge to be sure.

    Sensory organs sending metadeta or the brain recognizing sensory organs by information type isn’t necessarily a part of these results.

  5. ikejoe
    April 12, 2013, 1:57 am

    Is it Fringe Season 10? (^_^)

  6. David
    April 6, 2013, 9:29 pm

    That’s a terrible poem.

  7. t
    April 6, 2013, 9:26 am

    Why would you ever even complete this study? I cant see that any time soon people are going to want to have an eyeball surgically attached to their butt,..as will be the case with the tadpoles once the tail is gone.

  8. Hailish
    April 6, 2013, 9:23 am

    I don’t know what science and genetics engineering will do to humanities for the future. Hurray to science !!!

  9. Hailish
    April 6, 2013, 9:18 am

    I don’t know what science and genetics engineering will do to humanities. Hurray to science !!!

  10. Christian Duerig
    April 6, 2013, 7:16 am

    Sir John Gurdon noticed, that the frequency of impulses defines what type of muscle will develop. So the brain sees only what the neurons tell the brain. Very simple.

  11. George Karapiperis BA (Hons)
    Blackpool, UK
    April 6, 2013, 5:15 am

    This one more of the latest science advancements in vision discoveries by which people like me with vision problems could one day benefit should the concluded part of this research finds the way the brain connects to a remotely placed instrument.

    This is reminiscent of one of the documentary episodes of an old popular science discoveries television program, QED, in which the extraordinary puzzle for the scientists involved was to find out where in this Scandinavian man the brain cells where located… as there was no brain in the scull cavity of this gentleman.

    There is a lot more we do not know about how our bio system works.

  12. Jovan Janevski
    April 6, 2013, 1:54 am

    What a poem…

  13. Ima Ryma
    April 5, 2013, 1:59 am

    “Out of sight, out of mind,” we’ve heard
    As limitation on the brain.
    If eyes see only dark or blurred,
    There’s no memory to retain.
    But think of dreams by night or day,
    Visuals where the eyes have not
    Been how the brain has seen the way.
    Here’s where vision could see a lot.
    Science will find the connect right,
    So that the brain can tune in on,
    And see the world by satellite.
    The need for glasses will be gone.

    All eyes one day will wonders do.
    Programmed, all eyes will be on you.