Silver,
I was once asked by a writer what I thought caused a trout to take a fly. He published the article in Fly Fisherman and quoted/paraphrased me in the article.

What I told him was the story of a practice in my Air Force ROTC classes back in undergraduate school back in the mid 60's.
The practice was to show us outlines of Soviet aircraft. Slowly at first, and then faster and faster in the slides projected on the screen. We learned to make split second decisions as to which plane silhouette was friendly and which was Soviet.

So, we were developing "trigger" points about the outlines flashed on the screen. We looked for "positive" points about the plane's outline which told us it was a "target aircraft". I believe trout operate the same way. Their instinct helps them quickly identify positives in a floating object which cause it to determine it is a familiar food source and attack it.