When I was just beginning 7th grade, I had moved into a new house in Hong Kong and was very unfamiliar with the local wildlife. Upon getting out of the shower one morning and putting on a towel, I looked at myself in the mirror and noticed that something alive was clinging on to the outside. While it was actually a gecko, my mind immediately assumed that it was a giant bug and I reflexively started swinging my body around trying to get it off. The gecko dropped off my towel and ran away, somehow severing its tail and leaving it writhing on my bathroom floor while it made its escape.
Regeneration is a fairly common biological phenomenon. Starfish can regenerate limbs and some strains of mice possess the ability to regrow damaged tissue, cartilage and skin. But perhaps the most impressive regenerators are the flatworms, some of which actually reproduces by means of fragmenting. Planaria in particular is a species of flatworm that has the dubious honor of being the single most primitive animal with a centralized brain in its head (as opposed to many other flatworms that have a more distributed nervous system). But if you chop off the head of one of these worms, completely removing all brain tissue, the worm will grow a completely functional and entirely new head and brain within as little as four days. While research in the early 2000’s began documenting the processes and mechanisms behind this process, it was unclear what happened to the memory of the worms after their regeneration. Did they retain the memories stored in their old brain before they were decapitated? A 2013 study decided to tackle this question.
Of course, when we talk about a flatworm’s memory, we’re discussing a very different concept from human memory. It’s difficult to compare the mental processes of a species with 302 neurons in their brain to a species with 86 billion. Flatworms don’t have a declarative memory that can store and recall experiences and knowledge, but they do have rudimentary memory and can learn. Planaria are able to make and remember the basic characteristics of different environments. They can be trained to be less fearful in an environment that is safe and consistently provides food. Experimenters demonstrated this by training a group of planaria for 10 days by keeping them in a smooth-bottomed petri dish, while consistently moving them to feed in a rough-bottomed dish, creating an association between the rough bottom and food. Finally, they attempted to feed the worms by moving them into a rough-bottomed dish that was well-lit, an extremely aversive stimulus for the darkness-loving planaria. The planaria that had been previously conditioned to the rough-bottomed dish felt more comfortable in this environment and were willing to ignore the light and focus on eating far faster than planaria that had not been trained. These associations between the rough-bottomed dish and safety and food were resilient and lasted for weeks.
The experimenters then decapitated both trained and untrained worms and waited for them to regrow their heads and brains. After regrowth, the worms were fed in their smooth-bottomed dish for a few days until they were tested again in the aversive well-lit rough bottomed dish. The regenerated worms, regardless of whether they had been trained or untrained before decapitation, all avoided the food for a similarly long amount of time before finally deciding that it was safe to feed, indicating that the trained worms did not remember the association made by their old brain.
However, even if the worms clearly didn’t have immediate access to their old training, does this mean that those old memories had completely gone? The experimenters subjected all the decapitated worms to a single day of rough-bottomed dish feeding training. For the worms that had been untrained before decapitation, this was the first time they had not been fed in their smooth-bottomed homes. It took 10 days for the worms to originally began to associate the rough-bottomed dish with food and safety, so this one day training course was not meant to have significant effects on the untrained worms, but was meant as a way of ‘jogging’ the memories of the trained worms and give them a chance to reconnect with the associations they learned before they grew a new brain.
The day after this training session, the worms were once again placed in the well-lit rough-bottomed dish. The one day of training had a large effect on the previously-trained worms, making them far more comfortable and quick to feed despite the bright lights. The untrained worms, however, remained extremely cautious and still took a long period of time before feeding.
These findings suggest that old memories are present in worms that undergo brain regeneration, they just aren’t easily accessible. The worms were unable to immediately remember whether the rough bottomed environment was safe despite being well-lit, but were able to re-learn this concept far faster than worms that hadn’t received the original training. The mechanism behind this is still very unclear, but epigenetic factors are a prime suspect. This research also makes planaria the only organism that can be used to study brain regeneration and memory, and there is potential that further study will yield a better understanding of epigenetics, brain development and memory formation.