Whatever course of conditioning you take FIRST go to you nieghbors and apologise up front for any disturbance and assure them your working toward a solution. Introduce your dog also. People appreciate communication and see the opposite as unconcern. He needs socialising for future chance and deliberate encounters. So again, introduce him. I had an upstairs nieghbor with a barking dog who ignored even friendly overtures to solve the problem. It was solved. I played a recording of an Alaska Wolf pack howling non stop one day. Trouble is another nieghbor secretly had a wolf puppy and she decided my apartment was a neat place to visit. Fortunately we socialised 'suzy' early on with pets and people ( most of whom were told she was a mallinois) It was always fun hearing her silence the local coyotes at dusk while the collected apartment cats meowed " yea, what suzy said dudes. IN YOUR FACE!"