The flatworms known as planarians are neuroscience darlings. Their centralized brain, complex sensory abilities and rapid regenerative capacities make these nonparasitic worms ideal for studying the mechanisms that regulate stem cell function, neuronal development and limb regrowth. To this repertoire, scientists have now added a new trick: these invertebrates can store memories outside their brain and retrieve them after losing their head and growing a new one.
Researchers at Tufts University tested the worms' recall by leveraging a quirk of planarian behavior: worms that recognize a familiar locale will settle in to feed more quickly than planarians that find themselves in a new environment. Such newcomers typically need time to explore their surroundings to ensure their safety before they eat. So the researchers introduced planarians to a textured petri dish and allowed them to get acquainted with their environs. Next they decapitated the worms and waited two weeks for their heads to grow back.
The scientists then jogged the worms' memory by briefly returning them to the dish and feeding them. The idea was to revive the dormant memory from the body through a short exposure to familiar turf. “For the worm, automatically imprinting the new brain tissue with an old memory that could end up being completely irrelevant would be a waste,” says study co-author Michael Levin, a Tufts developmental biologist. “So the brief exposure tells the brain that the memory is indeed relevant.”
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When the researchers returned the trained flatworms to the same dish, the planarians initiated feeding much more quickly than worms that had gone through the same routine but had not explored the dish prior to decapitation.
The experiment upholds a controversial, decades-old finding by a colorful neuroscientist named James V. McConnell. In the 1950s and 1960s McConnell performed similar tests on planarians—going as far as to feed bits of trained worms to their untrained brethren in an effort to transfer molecules of memory. But some researchers questioned the objectivity of his experiments, many of which were detailed not in mainstream journals but in his own Worm Runner's Digest. The Tufts group aimed to minimize observer bias by using a machine to track and analyze the worms' behavior.
The new research, published in the Journal of Experimental Biology, could have implications for the development of artificial memory and the study of neurodegenerative diseases, which researchers hope to treat by someday replacing damaged brain tissue. “We really don't know what that could do to a patient's personality or memories,” Levin says. “Planarians are the model that might finally enable us to start looking at that.”
