Failure to classically condition planarian flatworms

Planarian flatworms represent one of the most evolutionarily informative nervous systems for an account of ancient bilaterian brains. Likewise, the unparalleled regenerative ability of planarians makes possible certain investigations of neural development, memory, and behavior that are simply impossible with other model organisms. Despite these facts, learning and memory are today underexplored in planarians, likely due in part to the shadow of controversial 20th-century experiments on the transfer of memories between individual flatworms. Here, we attempted to replicate and extend the classical conditioning experiments in planarians that were the basis of the later memory transfer work. We failed to find evidence for classical conditioning in any of our procedural variations and obtained similar results using computer vision methods to avoid subjectivity in manual video annotation. Our results cast doubt on the suitability of planarian flatworms for studying primitive learning processes and the molecular basis of memory using classical conditioning.