When I watch my horse in the paddock, she does the same thing...if she hears something in the distance, her head goes up and she freezes. Usually, after about 10 seconds or so, she'll take off running in the opposite direction of the noise. I think of it as kind of a "before flight" warning. Which in your case, at least she gives you enough time to be prepared. I would say like its akin to an adrenaline rush for us, different chemicals start being released (acetylcholine, etc) and motor synapses stop firing. People may mistakenly think that a "wooden horse" is just being an unwilling horse. However, when a horse freezes, it often holds it breath, and when we ask it to go forward, it will do so with a burst of built up energy and may buck. When a horse's head is high, it just makes it easier for all those chemicals to flow from the brain to the body, so I would concentrate on getting him/her to lower his head and begin to breathe before asking him to continue any work. I don't ride my horse just yet, but when she does this on the ground when I am leading her, I rub the underside of her neck and her legs and it usually encourages her to drop her head and distracts her. She responds really well to a couple of low drawn out eaaaaasssy's. Its like she hears that and knows it means slow down and relax. :)
You may also consider using those little ear puffs to block out any distant concurrent noise.
Last edited by Seahorseys; 11-22-2009 at 12:07 PM.