Abstract
[Introduction] Planarians are well known for its abilities of bidirectional regeneration and basic learning such as habituation and conditioning. Our pilot observations suggest that (a) UV light is more stimulating than non-UV light, sensed by UV opsins distributed across the entire body, and (b) response to UV light (wiggling) and that to an electric shock (immediate contraction) are qualitatively different. Based on these, we reexamine Pavlovian conditioning to see if a tail half of a conditioned planarian can retain learned behavior after dissection. [Method] 20 planarians in the experimental group were instilled with a Pavlovian conditioning procedure, in which weak UV light was paired with an electric shock with a 3 second onset delay (but overlap). The weak UV, a neutral stimulus, intrinsically leads to a wiggling/swimming response but typically after 10-15 seconds. The electric shock, the unconditioned stimulus, leads to an immediate contortion/constriction (Unconditioned Response). After training (weak UV followed by electric shock) 20 times x 7 days, they started contorting (Conditioned Response) to the UV stimulus alone, at which point, they were dissected. After 4 days (to regenerate motor functions post-dissection), they were tested twice a day with the same UV light alone. The control group underwent the same procedures as the experimental group except for conditioning (UV+electric shock). [Results] A total of 16 out of 20 planarians showed the CR, i.e. contortion/constriction to the UV. The tail halves in the control group continued to show slow wiggling responses but not CR. [Discussion] The results suggest that planarians have a structure other than its ganglia that retains learned effects, which is consistent with the latest evidence with habituation (Shomrat et al., 2013). Since UV sensitivity distributes in the entire body, it opens up the possibility of retention and execution without the ganglia.