To me, there are at least three separate components of "soundness". A dog that is sound is not generally afraid of things in the environment. He may have an occasional fear or startle reaction to something -- a loud, unexpected noise at close proximity, a person acting in a threatening or bizarre manner, or something that for some reason looks strange to the dog -- but he is able to recover and maintain responsibility for guiding the person.
Environmental confidence is one part of soundness. Another part is a little more difficult to define. I guess I could call it stability, or maybe resilience. If a dog is afraid of something, the fear can be broken down into two parts: its fear of the thing itself (the statue, the noisy traffic, the drop-off, whatever it is), and the way that the fear affects its work. If a dog encounters something that scares it, does it shake the fear off, once it realizes that the thing doesn't really present a threat or once it is past the thing? Or does the aftereffect of the fear linger, and cause stress that manifests in poor guidework decisions, or lack of ability to make any decision at all?
Say, for example, a dog is working down a sidewalk in the business district of a town, and the driver of a delivery truck parked right next to the sidewalk where the dog is walking rolls up the back metal door of the truck, and the rattle causes the dog to surge forward in harness or show some other kind of startle reaction. Noticing a sudden noise that could be something dangerous is not necessarily a bad thing. If that rattle was something rolling toward the dog and handler on the sidewalk, you want the dog to notice, so it can keep itself and its handler out of danger. But what happens after the noise is over and the dog realizes there is nothing dangerous happening? Ideally, it should resume guiding at its normal pace and quickly return to a relaxed demeanor. But when a dog is lacking in soundness, little environmental stressors can start to have a cumulative effect. After it hears the truck door rattle open, instead of relaxing, it may continue showing signs of stress, such as heavy panting, excessive pull in harness, overreaction to noises in the environment that do not signify danger (like a car door slamming or a store door opening in its face), or just plain making bad guidework decisions. A dog like this may be trying to do what it has been trained to do, but at the moment where a guidework decision has to be made, the anxiety overrides the ability to decide.
The final component of soundness is the ability to take pressure. I'm not talking about environmental pressure necessarily, but rather the ability to handle the consequences that sometimes come with making decisions. The dog will not be right all the time, just like no human guide is right all the time. Sometimes the dog will be distracted, sometimes the dog will be confused, sometimes the dog will do the right thing and the handler will misread it and give a correction that may not be justified at all. The dog has to be resilient enough to withstand the consequences of guidework errors -- the handler stumbling or bumping into something, a verbal or leash correction, justified or not justified -- as well as the things that commonly happen to guide dogs and are no one's fault, like the dog being stepped on (by pedestrians who aren't looking or by the handler -- I can't tell you how often I step on my dogs when I'm under blindfold) or having something dropped on them or being bumped by a shopping cart or a door opening unexpectedly. A dog that worries unduly about these things is not going to be happy being a guide dog.
Applying pressure at the right times and in the right amounts is one of the most difficult but also most critical things to learn how to do as a trainer, and it's different for every dog. Some dogs thrive under pressure, just like some people do. Give me a deadline, a consequence for non-performance, and someone breathing down my neck to get it right or else, and I am pretty likely to turn out something really good. It's almost like I need the pressure to work at my best. But for other people, the pressure rattles them so much that they can't even think straight to do the task. Dogs are the same way.
I have had any number of dogs throughout my career that were extremely sweet, gentle, willing to please, and eager to learn, with social behavior that is to die for, in every way the kind of guide dog I would want walking around in the world with my name on it... except that they don't handle pressure well. In the old days, those dogs seldom or never made it through, just because traditional training methods were tough on dogs and did not require that trainers made sure the dogs really understood things before they were held accountable for them. I am pretty sure that positive training methods have allowed us to turn some dogs into excellent guides who would never have made it twenty years ago. But there are still some dogs who can't handle pressure no matter how gently it is applied or how much effort we expend on making sure the dog understands before asking it to perform without help from the handler.
I had a dog like this in training not too long ago. He was very sweet, with very low distraction, and enjoyed training. He was always "in the game" with me and paying attention. He had some fear reactions to some things we encountered in early training, but he always made a full recovery from the fear, even if it wasn't the fastest recovery. He did pretty well in training right up to the halfway point, when I gradually started applying more pressure for him to make decisions on his own, without handler input. If we were in a familiar environment that he had been patterned to, and he knew exactly where to go, he was fine. But change anything in the environment, even something as little as putting an obstacle on the sidewalk that wasn't there the day before, and he would freeze up and be unable to do anything at all but stand there and wag his tail. If I gently asked him to "Hup-up", even with a light leash cue that was in no way a leash correction, he winced like I had hit him and dropped his head down and lowered his tail but continued to wag it. This is a dog that I never, ever, as in zero times, had to leash correct or (to the best of my recollection) even verbally correct. He never did anything other than what I asked him to do. But when he had to start experiencing some consequences, like a very minor stumble from me when I was under blindfold and he didn't clear me of an obstacle sufficiently, the risk of making a decision became too great for him, and he couldn't do it. I backed off and did two weeks of no harness work at all, just back-chaining and targeting games on leash, which he loved. These things are confidence-building because they let the dog be right, a lot. But as soon as I picked up the harness again, those same problem behaviors came back instantly. He had to be career changed. There is no environment we can put a guide dog in where we can guarantee that nothing will ever change and someone can always tell him exactly what to do. Believe me, if there were, I would have salvaged this dog and put him in that environment, because I loved everything else about him! But the end result always has to be kept in mind -- that this is a dog who is supposed to make a blind person's life better and easier.
Lack of soundness is a really common reason for career change, and "lack of soundness" is not a phrase that is used a lot in daily life outside of dog training. I really want puppy raisers to understand what it means when a dog is career changed for soundness. When a dog is career changed for distraction or excitability, the raisers are usually not surprised. That dog was usually a handful for them when it was a puppy, and the behaviors that necessitated the career change were usually evident at home too. Sometimes that is also true for soundness. The puppy that Will and I raised, Alaska, was career changed for soundness, and we saw the behaviors appear when she was very young. Of course we hoped she would "grow out" of them, and we did every training- and exposure-related thing we could think of to improve her chances, but at the end we all knew there was no way she was temperamentally suited to be a guide dog. It was obvious with her. But it is less obvious with a lot of dogs, who showed minimal or sometimes even no environmental fears as a puppy. It is hard for the raisers to understand how their dog could be career changed for soundness when they never saw the lack of soundness as a puppy. But sometimes they just won't see it. It is not the puppy raiser's job to put pressure on their puppies when it comes to decision-making. It's the puppy raiser's job to 1) teach the puppy to enjoy working with people, 2) teach the puppy good manners in the house and in public, and 3) expose the puppy to as many things as possible (at a level that particular puppy is able to handle). Applying pressure is best left to the professionals.
So if you are a puppy raiser whose puppy was career changed for soundness, and you never saw any signs of that lack of soundness when it was growing up, and you are feeling like there was something you could have or should have done differently, please don't feel that way. Sometimes it is just the dog's temperament. That dog I wrote about earlier, for example. He loved people and loved working with people. He was extremely well-behaved and showed every sign of being well-exposed. His puppy raiser did a great job. But at a certain point, it comes down to the dog's temperament. Sometimes the dog is not afraid of anything when it is walking around on leash, but when you pick up the harness handle, it sees the environment differently, as full of things that could cause mistakes and have consequences, and the risk of incurring those consequences becomes too great. That's not something that puppy raisers would ever see, and not something they could change if they did see it.
I read a great article on the interaction of genetic and environmental factors on canine development, totally worth reading. I am no scientist, but this article pretty much sums up my feelings about the importance of doing the best you can as a raiser to make sure your puppy reaches its maximum potential, and then not beating yourself up if the outcome isn't everything you hoped for. Genetic factors aren't everything by a long shot, but at the same time, the best raising in the world can't give a puppy temperamental qualities it doesn't have to begin with. That's always part of the gamble you take when you raise a puppy, any puppy.