-_- I just stated that the signals in the brain WERE real, and that in the case of an hallucination, do not convey messages that represent reality.
Any lasting influence a hallucination has on a person that doesn't demand that they fail to realize that they were hallucinating is pointless, and any that is the result of considering the hallucination to represent reality is more likely to do harm than not.
Consider this: if you had a stroke that made you forget cacti exist, would cacti cease to exist? No. But, if that same stroke removed all memories of any dream or hallucination you had ever had, would they cease to exist? Yes, because they only existed as ideas, and since they weren't based in actual events that occurred, there's no way for them to come into existence again.
Anything for which its existence depends solely upon the memories of those currently alive isn't real to me. Quantum physics could be rediscovered. Math could be made anew. But your personal hallucinations die with you and anyone else you tell them to. This is because YOU were their physical form. The connections in your brain that made that memory were the only aspect of it that was real.