When I was working for Guide Dogs there was the most handsome GSD called Valiant. He was a huge dog, very laid back and relaxed. He was a bit slow on the uptake but suddenly seemed to click and became an exceptional working dog -responsive, confident and safe. As he was so large you felt very safe being guided by him.
He had one major problem though. His recall was so unreliable it was impossible for him to qualify. Every time we thought we had cracked the problem we ended up with very red faces running across miles of woodland, farm land or, even worse for the self image, public parks full of people.
As a last resort we took him to the Animal Health Trust with his litter sister, Vanda, for brain scans.
Vanda's responses were totally normal and she went on to become a very good Guide Dog.
Valliant, however, was far from normal. He had no hearing at all! It was only his poor recall that gave us any clue that he had a problem as he behaved normally otherwise and had succesfully trained as an exceptionally good guide dog. An amazing animal with a great intelligence, he was given a home with one of the instructors and went on to compete in agilty and obedience.
(Hand signals and body postures are an integral part of guide dog training.)