Imagine the part of your brain which governs memories & thoughts is a page filled with words, and all your other brain functions are more pages filled with more words, large and complicated words, and all toghether these pages full of words comprise a very large and heavy book.
Now imagine someone has picked a pretty plant from a field, a plant with yellow flowers, and decides they want to dry it out and press it in the book. They don't have wax paper or anything, so they open the book at random and stick the plant in, close it and forget about it. It just so happens that the plant was placed, quite by accident, on the page filled with words that govern your memories and thoughts.
They forget about the plant for years. During that time the plant becomes part of your memories and thoughts - it is very real. You can touch it, feel it, smell it because it is there, pressed into your thoughts by the weight of the entire book. It is part of your existance.
Years later someone pulls the book off the shelf and stumbles across this dried, pressed plant, and thinks it's beautiful. They remove it so they can display it so that others may enjoy it.
But the outline of the plant is still there, on that page full of words. Maybe particles from the plant remain too. And even though the plant is no longer there, is no longer real, it remains in your thoughts. Even though it is physically gone, the ghost of it is still there. Like when you close your eyes after staring at something bright, you can still see a shadow of what you were looking at.
It has become imprinted on your brain. It may cause obsessive thinking, perhaps rumination or even hallucinations.
The moral: always use wax paper when pressing plants, cause you never know whose brains you're messing with.
I made all this up, just now. I know nothing about obsessions, ruminations or hallucinations, no matter what Patti Anne says.