A trigger is similar to hallucination. It can come in the form of sight or a sound. It is a "stimulus", meaning an external factor that makes an organ or a tissue (tissues are a group of cells that make an organ) have a response (i.e., a functional reaction.) An example of a stimulus is food when you're hungry (a conditioned stimulus), it makes your mouth water. The evolutionary significance of a stimulus is to make the body ready for a particular condition. Another well-known example is the jerking of the leg when it is hit by something on the knee. Stimulus must always be an external thing (factor.)


Now a trigger too is a stimulus. It evokes a particular group of cells in the brain that arouses a particular sensation or a memory. For example, a retired army soldier watching a film on combat that is remarkably accurate and vivid, will have his traumatic experience as a soldier triggered. Now it's not in my understanding why and how triggers are formed, but from what I get, a trigger must always be linked to a previous psychological trauma (i.e., participating in a war, seeing someone close to you die in a horrible way, or drowning in a lake or a swimming pool.)

Firstly, a psychological trauma is anything horrible that has affected your mind throughout. It can occur at any age but the most serious psychological traumas are those that occur in childhood (like a child drowning in a river or lake or a swimming pool) and will almost always lead to mental disorders in future. As opposed to a neurological trauma, the brain isn't directly affected. A neurological trauma is something similar to stroke or a head injury. It should be horrible enough to cause a post traumatic stress disorder in near future.

Now it's where it really gets complicated and I don't have the authority to discuss about complex medical conditions (due to my own psychological trauma that damaged my hippocampus....) but triggers cannot "come out of nowhere." Those telling you it can, should argue with the Ph.D level neuropsychologists that have researched upon the issue. For a trigger to be a trigger (be a stimulus that transports you back to the horrible event that you witnessed which is attributed as the origin of your PTSD), you need a psychological trauma. A person who hasn't witnessed childhood abuse cannot possibly get dissociation triggered after seeing someone that didn't abuse them.

Any questions? My speech is slightly disorganized so it may hard for you to follow up but I will try to explain what you did not get in my post.