Is there a chemical in the brain, that if imbalanced, can cause hallucinations?

I know low seratonin levels can cause anxiety and depression....but what about hallucinations or delusions, lack of concentration, etc. No, I'm not seeing stuff....I used to, but I haven't for awhile. I'm wondering what can cause it though (besides drugs)....I used to see a psychiatrist when I was 10 and I saw things sometimes and they didn't know why....they said something (I was told much later on) that it was likely that I could develop schizophrenia or something....I was diagnosed with ADHD and Asperger's. I haven't been since I was 12 of whatever, but it makes me wonder (I'm going to start seeing someone again though, for other reasons). Also, what does the chemical dopamine in the brain do? I've heard it haveing some relation to addiction or concentration or something...