I thought that it would be nice to write down here a part of the AND clinic. It's about the history of classical dressage, and why it proves that dressage can be done without bridle too.
The history of dressage
Dressage is about as old as the relationship between humans and horses: as soon as the humans began to ride horses, they wanted the horses to become stronger and more impressive and beautiful - most of all to show off to other people, but still.
Xenophon already writes centuries BC on how to make the horse appear more wild and impressive so that your onlookers would be in awe of your capabilities of controlling such a wild creature. His method to reach that though consisted of pushing with your legs while pulling hard on the bridle so that the horse feels claustrophobic, and then release both pressures so that the horse is so releaved to be able to move again that he moves as free and elegant as a dancer. The funny thing is that in most citations of Xenophon only the part that's quote is the part in which he tells about the horse moving as elegant as a dancer once you let go of the reins - people forget the lines before those in which he advices you to first push and pull in order to get the horse agitated. It was a search for collection, but without a real system of training behind it that focused on this high collection.
It's from the fourteenth century onwards that people spend more time at thinking over the system behind horsetraining and achieving collection. From then on also the highly stylized collected exercises are developed, up to the airs above the ground, or the High School/Haute Ecole jumps as they are called. A lot of riders, like Frederico Grisone, use the most awful methods to get a horse to do these exercises, like using straps with nails in them to wound the horse so that he moves very fierce, or beating him with sticks and sharp poles to make his actions bigger or repress unwillingness. The bits used were shank-bits with not only long shanks, but also complex mouth pieces with curbs, spikes and bulging rollers lying on the tongue of the horse. Over the years though, methods became softer and trainers like Gueriniere and Pluvinel realised that you could reach more with rewards than force too. Horsemen started to think about how the horse learns, and which tools to use in order to teach the horse collection.
The bit and bridle as tool
The interesting thing about this period, is that the bit is seen as a tool, a means to an end, not a prerequisite of maintaining collection. If you look at old pictures of dressage riders, you see that they still use hughe bits, but with the reins hanging through:
Francois de la Gueriniere
The interesting thing is that this is the start of modern dressage, and that the bit was seen as a means to an end. When you started training a horse, you used a bitless cavesson or halter, and then when you progressed and wanted finer means of communication and a higher collection, you needed more precise tools that you could use with small gestions that wouldn't effect your own balance as rider - and which would still effect the horses as much. Thats where the shank-bit came in. But this bit wasn't meant to steer the horse around with. The reins were used as a tool to communicate, but the bit itself was left alone as much as possible. It was there to serve as a kind of border for the horse: go to deep or too high or too much forward with your head, and you will bump into it. Apart from that the horse was directed through neck-reining - with the bit as a consequence if the horse ignored the cues with the reins on the neck.
Some classical/baroque dressage trainers in the 20th en 21th century still use the bit like this: it's there not to be used, but it's there as a reminder of the horse, an active correction that sets in as soon as the horse does something wrong, with a very good timing because even though the reins are hanging through, there still is only that much room for error. The horse is given the chance to avoid contact with the bit: as opposed to modern dressage riders since Steinbrecht at the end of the 19th century, who teach the horse to 'lean' into the bit by focussing more on the forwards quality of movement. Instead of first teaching collection on a loose rein and then going more forwards like in the past, these riders now take forwards riding with a stronger contact towards more collected movement. But the horse is always 'on the bit'. Not real leaning into it with their whole body, but always taking up contact, always following the bit when the reins give more room into that steady contact between the hands of the rider and the mouth of the horse. The horse learns to accept this constant contact/pressure as normal, and more pressure as a correction.
Francois Baucher kind of went back to the baroque dressage: not only did he introduce a lot of flexions in halt as a preliminary stage to riding forwards, but he also went back to the horse staying off the bit, avoiding coming in contact with it as much as possible. This got him a lot of critique from the 'ride your horse forwards' camp, but also a few famous followers, like Nuno Oliveira, who was a advocate of holding nothing but the weight of the reins in his hands.
The interesting thing is that all these 'hanging rein' riders saw the bit as a teaching tool for the horse, not as a tool to maintain collection. As long as the horse was collected, the bit wasn't there are the reins were hanging through. As soon as the horse left the frame of collection that the rider had chosen for him, he would get a corrections from the bit that he bumped into. And real classical/baroque trainers still follow this line of thought in that they regularly check the state of collection by dropping the reins to check if the horse hasn't become too dependant on the bit - the horse shouldn't follow the reins down like many riders nowadays want, but he should maintain the same collected frame in the same exercise:
Some classical dressage riders, like Alfons Dietz shows in his book, even every now and then ride without any bridle on the head at all in order to check the same principle.
Cognitive psychology through the centuries: how do horses learn?
This all shows that the bit was never seen as an important part of the collection itself: it has been a tool to teach the horse the difference between the right and the wrong posture, and a mean of correction the horse for the wrong behavior. This type of teaching with the bit as corrector is based on the way people thought about learning in the past centuries: you correct the wrong, and reward the right by not correcting it but instead of accepting it as normal. That was what people knew about learning at that time: that was how dogs, children and pets were raised and taught alike. If your kid finally knows how to add up 2+2 well, don't reward him too much, or he'd be spoiled and will think that he has done something amazing. He shouldn't have been doing it wrong in the first place!
Since the 1950s a lot of research has been done on how horses, people and animals in general learn, and what the best way of learning is. Researchers like Skinner discovered that the most important thing about teaching an animal, is the timing of your response to the behavior: Your response should come within three seconds of the wrong/right behavior. That is why the bit with hanging reins has been so succesfull in training: as soon as the horse collected and raised his neck, the pressure was off. As soon as he leaned forwards, the pressure was on. All the time the rider doesn't have to do anything more than just keeping his hand in place all the time. He doesn't have to consciously respond to the actions of the horse: as long as his hand is still, the horse will correct and reward himself through the reins.
Another thing biologists discovered however, is that corrections also have a big negative impact on the self-esteem and that there are a lot of negative side-effects for the learner.
When you look really close at the history of dressage, the question isn't really if collection can be trained without bridle or not, but if there are other learning tools available to teach horses with. The answer to that is really quite simple: since the 1950's the answer is 'yes'. Every biologist knows that when you couple rewards to a reward-signal that gives a good timing to the horse, you can teach him anything that you can teach with corrections too - even more and more easy because now the horse is willing to work along with you. In baroque dressage the bit wasn't meant to steer the horse with: they had the reins in one hand and used neck-reining for that. The bit also wasn't used for holding the head and neck up in a collected frame: if the horse had the right neck-set, the reins were actually hanging through and were in no contact with the bit at all. The bit was simply used as a teaching-tool to guide the horse towards collection by telling him what's wrong - the real collection came from the exercises the horses were strengthened with, like shoulder-in, piaffe and passage. And now, centuries later, we know that there are a lot more and a lot better tools for teaching horses. If ancient and modern dressage riders had the goal to ride the horse collected with the reins hanging through, using exercises to teach the horse how to collect, and using the bridle only to correct the horse when he was wrong - then there is no reason at all why the other way round shouldn't work either: not to use a bridle to correct the horse, using exercises to teach him how to collect and using rewards to reward the horse when he gets the collection right.
The question of training collection without bridle
If you look at the history of dressage, the question isn't if training collection without bridle can be done. The question is if you as trainer stick to the old ideas on how learn horses and other animals (and children!) learn, or if you follow the new insights of cognitive psychology on how you can make sure that your horses (and children) learn in a better, more productive and safe way.