If the lives of nematodes are rich enough to experience a PTSD-analogue, young honeybees can play with balls and fruit flies enclosed in a glass box can vary their flying behavior in a way that is inconsistent with confusion and consistent with boredom, who knows where the limit is. I once saw a paper that argued the middle germ layer of placozoans might support a rudimentary neural network encoded in glutamate-GABA gradients instead of neurons (since they don't have neurons) that could carry out aversion behavior coordination across the entire placozoan body.