Dogs with strong nerves do not perceive a small noise as a threat so they have a higher threshold for defense. It takes much more stimulation to get the strong-nerved dogs interested in barking. Strong nerved dogs can be trained to get more interested in outside noises by showing them that a small noise outside is often followed by an attack on the dog or handler. This changes their perception of what that noise means. Here is a point where many people get lost in understanding drives and nerves.
Some dogs can have strong drives but weak nerves.
I have seen this with Malinoise. Some have intense prey drive, they bite like alligators, but the same dog has thin nerves. When they get around something a little strange they shy away from it. These dogs can be difficult to train, new trainers identify the problem as a dog that is easily distracted or a dog that hates slippery floors and other strange things in the environment. This problem often shows up in tracking, these dogs have a difficult time concentrating on a track because their nerves are weak.