My most vivid stimulus I would have to say is the smell of a pipe. Growing up I spent a lot of time with my poppy (grandfather) and he was always smoking a pipe. When he was not smoking it he smelled of it. When he was not around and I would smell a pipe I thought of him and thinks that we talked about. I would say this would be classical conditioning. The smell of a pipe triggers feelings and emotions about my past. Just with substance abuse I feel it works the same way. A certain smell, or even a feeling can be a trigger. This can cause a relapse. I know I have been substance free for a long time, however, certain smells today will trigger that feeling for me back when I was using. I learned to reverse the trigger.