Probing the Limits of Memory: Can a Learned Response Persist Through Decapitation and Regeneration in Planaria?

<p><strong>Research stemming from the invertebrate literature is forcing us to question some of our assumptions about the nature of memory. Planaria, a flatworm with a centralised brain and incredible regenerative capabilities, have demonstrated retention of simple associative memories, such as associating a texture with a reward, in brainless tail fragments after decapitation. This suggests basic memories may be stored outside the brain but leaves open the question of whether complex goal-oriented memories share this capacity. To address this, we performed a series of experiments to determine whether planaria can acquire an operantly conditioned response that persists for at least two weeks, and whether this can be retained in the brainless tail halves of decapitated planaria. Using a Y-maze paradigm, we established baseline arm preferences and then rewarded treatment subjects with either cocaine or methamphetamine for entering their non-preferred arm during conditioning. Control subjects received vehicle only (distilled water). For the key experiments, subjects were then cut into head and tail fragments and allowed to regenerate for 14 days before we tested for memory retention. The next day, subjects were exposed to the rewarding compounds to identify whether the memory could be brought back or strengthened with a reinstatement procedure. Our results regarding whether planaria can learn and retain an operantly conditioned response were mixed. Experiments 3 and 4 provided preliminary evidence for learning, as treatment subjects entered the active arm more often at the end of conditioning. However, experiments 2 and 5 failed to show a significant change in behaviour compared to control subjects. We found no conclusive evidence that learned responses were retained in the brainless tail fragments, although the relatively weak learning may have limited the likelihood of successful retention. Future experiments will require more robust training methods to conclusively test the hypothesis of complex memory retention outside of the brain.</strong></p>