There’s an episode of J.J. Abram's cult favorite Fringe where Olivia chugs a beaker full of chopped up worms. Walter, the stereotypical absent minded professor-slash-mad scientist wonders, "Can memories be transferred by eating them?" Walter remembers an experiment that transferred memories between worms by chopping them up and feeding them to each other. In the context of the show, Walter’s worm shake seems like one of many implausible examples of “fringe” science, like pyrokinesis or interdimensional travel. But 50 years ago these experiments actually happened, blurring the lines between fact and fiction and igniting one of the fiercest controversies in the history of neuroscience.
Planarian flatworms first wiggled their way into behavioral neuroscience labs in the early 1950s. Among the simplest animals with a central nervous system, researchers hoped that planaria could provide clues to the earliest evolution of the brain. Although they are only half an inch long and use the same orifice to eat and excrete, planaria have a rudimentary mind: behind their eyespots sits a cerebral ganglion, a bundle of nerves at the evolutionary cusp of brain-ness.
To study how the planarian brain could learn and remember, James McConnell, in 1956 a young psychology professor just starting out at the University of Michigan, started electrocuting worms. McConnell and his team of “worm runners” trained planaria to respond to light through a process of Pavlovian conditioning. They would shine a bright light over a worm and then immediately shock it with an electric current, causing the worm to contract and scrunch up its body. After about 150 cycles of light-then-shock, the worms learned to scrunch once the light turned on, even without being shocked.
Shock The Worm
Planaria hold some unique advantages over more common and more complicated laboratory creatures such as maze-running rats. They are small and easy to care for—you can store them in the refrigerator when you want to take a day off from the lab—and they can be chopped up into pieces, regenerating new planaria from each segment. This remarkable power of regeneration is part of the reproductive life cycle of planaria: the worms can reproduce asexually by splitting themselves in half across the belly. The head will regenerate a new tail in a few days, while the tail will catch up, growing a new head—brain and all—within a couple weeks.
McConnell and his worm runners wondered: what happens to the worm’s memories and training after its head splits from its tail? Might the head still remember its past lessons, even after such a trauma? And what of the tail, once it regrows a new brain? To test the worms’ ability to remember after regenerating, McConnell trained a large batch of worms to scrunch—and then he cut them in half. A lucky control group of trained worms didn’t get chopped and waited for the others to regenerate.
After the tails grew new heads and the heads new tails, McConnell put the worms back in the shock tank, and tested their long-term memories. Shockingly, all of the worms retained their training—even the ones that had grown new brains. How could this be? How could a tail remember?
Many experiments followed, each one weirder than the last. Was the brain necessary for learning at all? The team surgically sliced worms’ heads in half and waited for each half to regenerate. The resulting two-brained worms could learn faster than their one-brained colleagues (worms without a brain couldn’t learn at all). How long would the memory last? Chopping up worms for a second re-generation, the team found that worms retained memories even when they didn’t retain any body parts from the trained animals.
McConnell was an unorthodox thinker—he riled his colleagues with provocative hypotheses, and even published a satirical series of scientific articles called "The Worm Runner's Digest," bound upside-down in the Journal of Biological Psychology. And so, catalyzed by his strange findings, McConnell’s next hypothesis drove even further into the surreal: If a worm’s memories were not locked in its brain, then could memories be transferred between worms? McConnell hypothesized that planarian memories might be stored as molecules rather than neural signatures in the brain. If memories were molecules, then one worm might be able to literally eat the memories of another.
This is science fiction territory: two-headed creatures, brain regeneration, and cannibalistic memory transplants are the stuff of Fringe episodes and mad scientists, not friendly mid-century psychology professors. But new science always represents a leap into the unknown, the unfamiliar, and the potentially fictional. Hypotheses are the science fictions at the heart of science, asking us to consider what if this were true?
The worm runners took two groups of worms, one that had been shock-trained and one that hadn’t, and chopped them up. Then, they fed these wormshakes to naive, untrained worms (planaria are, conveniently, cannibals). Would the worms that ate the “trained” food learn from it? When McConnell began to shock-train the new set of worms, he observed a surprising fact: the group that ate trained worms learned to scrunch much faster than the one fed untrained worms—it seemed that the memory was indeed edible. (In the introduction to "The Worm Re-turns," a compilation of stories from McConnell’s "Worm Runner’s Digest," Arthur Koestler writes of the edible memory experiments: “In the jargon of communication engineering, information is always ‘fed’ into a computer or an organism; here the metaphor became flesh.”)
Early press for McConnell’s findings created a furor of skepticism and new waves of speculation. Could this result hint at how learning worked in human brains? Would we someday be able to learn by taking a pill instead of studying? Had the worms really “learned” or were the experiments somehow flawed?
The controversy raged for 15 years, as other scientists tried to reproduce parts of the original experiments in other labs and with other animals. (The history of the conflict is detailed in Harry Collins and Trevor Pinch’s book about experimental controversies and the practice of science, The Golem: What Everyone Should Know About Science.) In experiments with goldfish and rats, scientists trained animals to be afraid of the dark, blended up their brains, and then injected the brain juice into untrained animals to see if the fear could be somehow transferred.
Arguments flared over how to measure fear of the dark, how to define statistical significance in these experiments, and how to design rigorous experiments to test such strange hypotheses. Personalities clashed, integrity was questioned: science was getting done.
Eventually, neuroscientists shifted their attention to other aspects of brain structure and function. McConnell and the scientists who had explored the chemical transfer of memories retired or passed away, and the controversy eventually died with them. With the worm runners’ results never satisfyingly verified, the field moved on.
Today the story of the planaria and their edible memories is largely forgotten—a strange episode from the history of science. The idea that memories might be edible is now the stuff of television fantasy, not scientific inquiry. But the story of the worm runners reminds us how science happens in the real world: between the facts of observation and the science fictions of hypothesis, at the edge of plausibility.