Environmental stimuli are everywhere; they are everything in the environment that surrounds us, anything that we perceive.
Perceive: to become aware of, know, or identify by means of the senses.
So basically; if your dog is aware of their surroundings then everything has an impact on their brain. Some dogs are much more aware than others; some sort of go through life whistling and wagging, perhaps not taking much in as far as sensory information goes. Then there are the average guys who notice material objects and actions going on around; they take in the information, process it and act accordingly. Accordingly meaning that it will be in accordance to their life experience; how much environmental stimulus they have been subjected to.
Then there are the dogs that don't miss a thing; these are the guys that can be more challenging. They see things that other dogs may pass by. This is all very interesting but the big factor in the level of perception is the end result which is action or reaction. How a dog reacts to stimulus is what we are given as feedback. This is our heads up so to speak. Nothing in life is truly scarey unless you perceive it to be so. Same goes for everything else; this is where fears, likes, dislikes and phobias come from; a basic perception.
So a stimulus is pretty much anything that our dog can hear or see; auditory or visual. Dogs who already have a perception of a certain stimuli can be reprogrammed to change their perception. This is done using counter conditioning which is done by creating a new association to a stimulus. And generally counter conditioning is done slowly to achieve the best results. Flooding is not recommended for canines as it can have negative results.
So by systematically breaking down a reaction by the introduction of a new associated response you can achieve great results. Everything is done by positive application; there is no anger or lack of patience involved. Because both; a lack of patience and growing anger lead to a crack in the building blocks you are stacking towards a new goal. No success can go unrewarded; each and every miniscule step forward is a success and needs to be marked with reward.
Dogs are so much more complicated than many think. Ah; canine psychology, I could go on forever but I must make dinner for the hounds. ;)