Decapitated worms regrow their heads with old memories intact

It’s hard to let go of some memories, even if your head has been chopped off. Well, at least if you’re a flatworm. When these tiny critters are decapitated, their heads and brain eventually grow back. But more remarkable than that, so too do their previous memories.

Scientist who study planarian flatworms have known for quite some time that these organisms can regenerate their entire body, including the brain. They also know that these worms are capable of storing long-term memory, which, for these little guys, translates to about two weeks. And work done in the 1960s suggested that certain memories might come back after regeneration, but the results were clunky and inconclusive.

Working with more advanced techniques (i.e. computerized tracking), biologists Tal Shomrat and Michael Levin got to thinking: What would happen to a flatworm that was trained on a specific task, and then had its head chopped off? Would its memory of the learned behavior also regenerate? Conveniently, it takes less than 14 days for the flatworm to regenerate its new brain, which is the maximum length of time for its long-term memory to persist.

So, to embed a specific memory, the researchers spent 10 days teaching the flatworms that food could be found at the center of a petri dish with a light shining on it. This was particularly challenging for the worms because they like to hang out at the peripheries of petri dishes and they have an aversion to light. But, given enough positive association, they learned to overcome their natural tendencies. Hence, learned behavior.

Then their heads were chopped off. Brain bits and all.

Once their heads and brains grew back, the scientists threw them back into the petri dish. But the memory wasn’t there right away. And in fact, the biologists had to give the worms a refresher course on the matter of finding food — but one quick lesson was all it took; the lesson basically allowed the worms to refresh their memories. Subsequent trials showed that the previously decapitated worms reached the food significantly faster than those (previously undecapitated) worms who were also given the benefit of one training session.

Shomrat and Levin aren’t entirely sure how the flatworm is able to do this, though they suspect epigenetics plays a role — the idea that the expression of DNA has been modified by the learned behavior. But they say this alone cannot explain it.

Further work will look into how a chemical signal somewhere outside of a worm’s brain translates into information.

The study appears in the Journal of Experimental Biology: “An automated training paradigm reveals long-term memory in planaria and its persistence through head regeneration.

Image: Shomrat and Levin.