I've only ever had one flashback, with mushrooms. The trip was the most intense one I have ever had and it was 100% life transforming. I first had the most horrible part of a trip ever, so unbearable that I wanted to be free, to be gone... and that caused me to completely lose my ego for hours. Pure beauty and heaven, pure terror and hell at the same time, and then: nothing and everything. Oneness.
When I came out of the trip I knew one thing: that I needed to hear a particular old trance track, I guess to ground myself again and knew that I was in fact not dead, and had not moved to the beyond. That my life had not necessarily been a dream or a lie (although not in the extreme sense I had experienced it, I still think in a buddhist sense it is an illusion of sorts).
Some 10 months later perhaps, I heard that exact same trance track again for the first time since that trip and for 5 minutes I was completely back there again, I was very much suprised to be deeply dissociated like that without having taken any drug, but it was not especially bad or good at all. Instead, it was an intense reminder of what I have gone through triggered by a hard wired cue. The trigger faded after this, since I have tried to accept it and listened to the track again after that it gradually lost it meaning and it has not made me flash back again.
Music and smell are two powerful triggers that can be hard-wired deeply to be associated and connected to primal emotions. It must be because they are more unique to be recognized than a sight, unless you are talking about a very special visual cue.
Basically anything can potentially associated to be a trigger for a flashback, but it has to be able to bear a special meaning. Some triggers just have a better potential for that than others.
For more on this please visit the Big & Dandy Flashback thread which is listed in the Index, there is a link in my signature here VVV
