Your horse spooks at the plastic bag in the barn aisle…unless it has his treats in it. He doesn’t want to go past a driveway where someone has put out a garbage can. He has suddenly started to avoid going into that one corner of the arena. You’ve worked at desensitizing him to all kinds of objects, but he still spooks occasionally at a new object or situation (stimuli).
There’s a perfectly valid reason for his behaviour. It’s called self-preservation. Being fearful and avoiding unfamiliar objects (or even familiar objects in a different place), is how horses stay alive in the wild. Your horse reacts first (as in “get out of here now!”), and gets himself to a distance where he feels he can safely check out that thing. He may become curious about what it is and then slowly and cautiously move toward it. But until he knows it’s harmless, he is alert and ready to flee again.
You may have been taught to make your horse face his fears by pushing him forward when he baulks and making him go right up to whatever is scaring him. You make him stand still – preventing him from escaping – while you flap or touch him with different objects. You’ve been taught to keep putting the horse in the fearful situation until he stops reacting. This technique is called flooding.