Are you sure there really aren't actual things the horse is scared of on the trail? Even small things? For instance, my mare does much of the same (stops, stares, sometimes spooks in place, snorts, etc.) and about 1/2 the time it is a result of an animal, such as an elk, deer, or even a small bird. Or a piece of something blowing in the wind. She is just sensitive like that. The other 1/2 the time I may not even figure out what is bothering her, I just push her on and keep riding.
She is just a sensitive horse to stimuli, especially when riding alone. As she was used as a broodmare I sometimes wonder how much riding experience she actually has. But as long as I can push her forward and she doesn't do any major spooks, I just ride it out. She gets better the more I ride her. If I let her sit for a while, she is sensitive and spooky all over again.
I would check with a vet just to rule out eyesight problems- that is a good idea. That would be a reason for a horse to see boogers in every bush as well. Can't comment on the neurologic stuff as I have no experience with it.
PS. I also used to own a Paint that would stop and tremble when he encountered elk. I couldn't always see or hear them myself, but I knew they were there because of his reaction to them.